CHAPTER 9 93 Biology Term 2 STPM Chapter 9 Control and Regulation Steroid hormone Non-steroid hormone 4. No cAMP is produced. cAMP is produced. 5. No cascading effect occurs. Cascading effect occurs and a quick result is effected. 6. The complex enters into the nucleus. The complex does not enter the nucleus. 7. Certain genes are activated by the complex. Genes are not activated directly by the complex. 8. Transcription takes place followed by production of proteins. Transcription does not take place and protein is not produced. 9. Effects produced through the proteins formed Effects produced through the enzymes activated and their products. 10. Usually produced long term effect such as testosterone produces muscular body. Short term effects such as adrenaline increases blood glucose level. Quick Check 4 1. How do steroid hormones affect the genes? 2. Why do only certain cells respond to a hormone, even though the hormone flows throughout the body? 3. List all the endocrine glands and the hormones produced. Roles of Plant Hormones in Growth and Development 1. Plant hormones are also known as phytohormones or growth substances. They are organic compounds that act as chemical signals bringing about responses that regulate growth and development. 2. Plant hormones have the following characteristics: (a) Plant hormones are effective in extremely small concentrations, less than 10–6 mol/L (b) Plant hormones are produced by young cells at the apices of shoot and root, ovary, embryo, seed and injured cells. (c) A hormone production is stimulated by changes of season when the light period increases or decreases; temperature increases or decreases after a cold night. (d) Transport of the hormones is by diffusion and a small amount may be transported in the phloem or xylem. (e) Cells that response to hormones have receptors for them and are usually developing young cells. (f) Plant hormones act by signal transduction, an external signal brings about an internal signal that causes one or more responses in the cell. (g) Some hormones respond to signals from other hormones. These responses bring about interactions that enhance or antagonise each other. (h) Each group is related chemically and there is no chemical similarity between one group with another. 2011 INFO Plant Hormones VIDEO Plant Hormones
CHAPTER 9 94 Biology Term 2 STPM Chapter 9 Control and Regulation 3. They control growth and development processes, either promote or inhibit growth, at both vegetative and reproductive stages. 4. They are divided into five clases, namely auxins, gibberellins, cytokinins, abscisic acids and ethylene. Auxins 2008, 2014/P2/Q18(a) 1. Auxins are a group of plant hormones, including the natural form, indoleacetic acid (IAA), its derivatives and analogues such as naphthalene acetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D). 2. The structure of IAA is as shown in Figure 9.30. CH2 COOH N H Indole Acetic acid (ethanoic acid) Figure 9.30 Structure of indoleacetic acid (IAA) 3. The parts of plant that produce IAA are apices of shoot and root, floral buds, young ovaries, embryos and injured tissues. 4. Auxin has the following characteristics: 2017 (a) Its movement is polar i.e. basipetal (apex to base) in the shoot and acropetal (upward) in the root. (b) It is more sensitive to root cells than to stem cells i.e. a certain concentration that causes shoot growth is inhibiting root growth. This is as shown in Figure 9.31. 100 Auxin concentration/ parts per million Roots Shoots 0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000 50 Percentage inhibition in growth Percentage stimulation in growth 50 100 150 200 Figure 9.31 ,ɈLJ[ZVMH\_PUVUNYV^[O
CHAPTER 9 95 Biology Term 2 STPM Chapter 9 Control and Regulation (c) It moves laterally away from light. This explains why plant shoot responds by growing towards light in phototropism. (d) It moves downwards due to gravity if seedling is placed horizontally. More auxin in lower side causes shoot to bend upwards but causes root to bend downwards because auxin inhibit growth in root above a certain concentration. (e) It can be broken down by strong light, pH and enzyme IAA oxidase. (f) Its reactions can be inhibited by chemicals, such as coumarin, ascorbic acid and antibiotics. 5. IAA has the following roles: (a) Auxin induces growth by cell expansion, resulting in elongation of organs such as the stem, coleoptile, root, leaf and flower. It is produced in very dilute concentration in the tips of shoot, coleoptile, root and cells of leaf and floral primordia. IAA binds to receptor at the membrane and stimulates proton pump to transport hydrogen ions out into the cell wall. There, the hydrogen ions activate the enzyme hemicellulase that breaks the bonds of branched hemicellulose molecules. The cellulose microfibrils are thus loosen allowing the wall to expand after water entered the cell by osmosis. This is called the acid growth hypothesis as shown in Figure 9.32. (b) It causes tropism especially phototropism, when a unidirectional light brings about bending of the shoot tip toward the light. Auxin moves laterally away from light, resulting in more of it accumulated on the dark side as shown in Figure 9.33. Cytoplasm Auxin Cross linkage 1. Auxin stimulates cells to transport H+ actively out to cell walls. 2. H+ ions activate hemicellulase to break or hydrolyse hemicellulose. 3. Wall fibres are loosened to allow the cell to expand when osmosis occurs. Enzyme (inactive) Cellulose fibre in cell wall Active enzyme H+ H+ H+ H H + + H+ Turgor Figure 9.32 ,_WSHUH[PVUVMHJPKNYV^[OO`WV[OLZPZ * VSLVW[PSL Sheath in monocotyledonous seed or the cylindrical shoot structure growing up from NLYTPUH[PUNNYHPUSPRLTHPaL 7YPTVPKP\T¶ZPUN\SHY Primoidia – plural Primodium is a very young bud or outgrowth * VSLVW[PSL Language Check Sheath in monocotyledo seed or the cylindrical s structure growing up fro NLYTPUH[PUNNYHPUSPRLT 7YPTVPKP\T¶ZPUN\SH Primoidia – plural Primodium is a very you bud or outgrowth
CHAPTER 9 96 Biology Term 2 STPM Chapter 9 Control and Regulation Language Check Light More auxin in dark side Darkness The tip is placed on an agar block for some time Tip is removed The block is then placed on one side of a decapitated coleoptile Coleoptile bends because of more auxin in one side Auxin move to dark side Went's experiments to prove movement of auxin Auxin is involved in phototropism as shown in Darwin's experiment Light Normal Capped Coleoptiles before experiment Decapitated Light causes auxin to move to the dark side and stimulated bending after 1 day + IAA Agar block + IAA Figure 9.33 :VTLLHYS`L_WLYPTLU[ZVUWOV[V[YVWPZT\ZPUNVH[JVSLVW[PSLZ Intact Decapitated Agar with IAA Side shoots develop Shoots of papaya intially Shoots of papaya after a week of treatment Figure 9.34 ,ɈLJ[ VM H\_PU VU apical dominance (c) Auxin stimulates production of meristem and activates meristem. When cells are injured, auxin causes them to become meristematic and form callus tissue, which is a lump of parenchyma cells. It activates cell division in the cambium, especially after winter, during spring when the temperature rises. Thus, it also promotes secondary thickening i.e. the stem or root forms more vascular tissues. (d) Auxin inhibits development of lateral buds. Therefore, it promotes apical dominance, a phenomenon in certain plant such as papaya. Only when the tip of the shoot is decapitated does the papaya produce side shoots. However, treating the decapitated shoot with dilute solution of auxin stops the production of the side shoot (Figure 9.34). (e) Auxin induces root initiation from cuttings and calluses. Auxin can be used to promote root growth in cuttings by immersing them in its very dilute solution (10–5 gm/litre). Usually fibrous roots would develop from the base of the cutting. (f) Auxin promotes fruit growth and inhibits abscission of young fruits. It is naturally produced by developing embryo in the ovary and causes the ovary wall to become fruit. Farmers use auxin to promote more fruiting and even parthenocarpy. Parthenocarpy is a phenomenon of fruit development without fertilisation. This occurs in seedless fruits like guava and watermelon, where the ovary wall cells can produce auxin. (g) Auxin inhibits abscission of leaves and flowers. Therefore, it promotes leaf and flower growth in certain plants. Only old leaves and old petals fall off when no auxin is produced. Gibberellins 2008 1. Gibberelins are plant hormones that include gibberellic acid and the most active one is called GA3 , its analogues and derivatives. IZJPZZPVU ( +YVWWPUNSLH]LZHUKÅV^LYZ after a layer of dead cells MVYTLKH[[OLZ[HSR
CHAPTER 9 97 Biology Term 2 STPM Chapter 9 Control and Regulation 2. The structure of gibberellic acid is as shown in Figure 9.35. HO CO CH3 COOH CH2 Gibberelic acid (GA3) OH O Figure 9.35 Structure of gibberellic acid 3. They are produced in young organs such as apical leaves, developing buds, root tips and germinating seeds, especially from Gramineae family. 4. Gibberellins transport is not polar or unidirectional, but usually by diffusion and may be transported in the xylem and phloem. 5. Gibberellins roles are as follows: (a) Gibberellins promote stem elongation and vegetative growth in plants in the presence of auxin. Plants that do not produce it naturally are dwarfs, which are mutants. If dwarfs are treated with gibberellin, they become normal. Cabbage plants, when treated with gibberellin, causes ‘bolting’; the stem lengthens like ordinary plant. Its mechanism of action involves the activation of certain genes, resulting in growth. One of them is amylase that causes the hydrolysis of starch in the seed so that the seedling can get enough food for growth. Normal cabbage 'Bolted' cabbage Figure 9.36 ,ɈLJ[VMNPIILYLSSPJHJPKVUJHIIHNL (b) Gibberellins break dormancy in seeds and buds, especially after winter. The production of gibberellins is a response of increase in the environmental temperature. In cereal seed, the embryo secretes gibberellin after imbibition with water. The gibberellin diffuses into the aleurone layer. The aleurone layer then produces hydrolases, which are used to digest stored food and the products of digestion are used for the germinating process. (c) Gibberellins promote flowering in long day plants. Long day plants kept in a short day cycle do not flower, but when treated with gibberellin, they do. Similarly, plants that require cold treatment or vernalisation before flowering would flower if treated with gibberellins. 2014/P2/Q18(b)
CHAPTER 9 98 Biology Term 2 STPM Chapter 9 Control and Regulation (d) Gibberellins can promote fruiting and parthenocarpy in certain plants. (e) Gibberellin promotes apical dominance and inhibits the growth of side shoots, especially in the presence of auxin. Cytokinins 1. Cytokinins (Figure 9.37) are plant hormones including zeatin (a natural one), kinetin (a synthetic one) and adenine that is related to the natural one. 2. Zeatin is produced mostly in cells of embryos, which are actively dividing, especially in fruits and seeds. Some are produced in the root tips and transported to the leaves in the xylem and possibly from the leaves through phloem to other parts of a plant. 3. Zeatin has the following roles: (a) Zeatin promotes cell division and cell differentiation in meristems, injured cells and callus. They function only in the presence of auxin or gibberellin. (b) Zeatin delays the process of senescense (aging), especially in leaves. Leaves that are plucked and treated with cytokinin retain their green colour even after a few days. A plucked leaf treated with a drop of cytokinin retains a green spot while other parts become yellow due to loss of chlorophyll. Nutrients are found to be transported to the green spot. (c) Zeatin inhibits apical dominance by promoting development of lateral buds into branches. The cytokinin is produced at root tip and transported up into the shoots where it promotes developments of them. (d) Cytokinin inhibits the formation of lateral roots. Zeatin promotes apical dominance in roots. It works together with auxin to determine the form of root and shoot. (e) Cytokinin is used to prolong the storage of leafy vegetables such as cabbage and lettuce. Cut flowers are kept fresh by treating with cytokinin. It can be used to promote germination in certain seeds. Abscisic acid (ABA) 1. Abscisic acid, also known as abscisin or dormin, is an example in the group of hormones that promote dormancy in plants. 2. The structure of abscisic acid is as shown in Figure 9.38. 3. Abscisic acid is synthesised in the leaves, stems, roots, fruits and seeds, especially in later part of autumn. It is synthesised in the chloroplasts, dormant and abscission organs. It can be transported in the phloem and by simple diffusion in the root from root cap. Zeatin N H N N NH CH2 CH C CH2OH CH3 N Kinetin N H N N NH2 Figure 9.37 Structure of cytokinin O CH3 OH CH3 CH3 CH3 H H C C COOH C H C Figure 9.38 Structure of abscisic acid 2014/P2/Q18(a) STPM 2016 STPM
CHAPTER 9 99 Biology Term 2 STPM Chapter 9 Control and Regulation 4. Abscisic acid has the following roles: (a) It causes leaf fall in autumn when it is synthesised in the leaves. It inhibits the synthesis of DNA and RNA, where it induces senescence and yellowing of leaves. Nutrients are drained out and stored in the roots during winter. It induces the formation of abscission layers in the leaf stalks causing leaves (defoliation) to drop all at the same time. (b) Abscisic acid promotes dormancy in buds and seeds. It is produced in autumn, stimulated by cold and disappeared in spring when the temperature rises. Such effect of abscisic acid can be blocked by treatment with gibberellic acid or auxin. (c) Abscisic acid causes closure of stomata. When plant is under stress of no water and at night, abscisic acid is produced. It causes water to diffuse out of the guard cells and closes the stoma. (d) Abscisic acid antagonises the effects of auxin, gibberellin and cytokinin. This is particularly so in the promotion of growth and cell division. (e) Abscisic acid can cause dropping of fruits, especially durian. When the fruits are ripen, they produce abscisic acid that diffuses into the stalk. An abscission layer of dead cell is formed at the base and the fruit just drop. Ethylene 1. Ethylene is a gas that induces fruit ripening. 2. The chemical structure of ethylene is as shown in Figure 9.39. 3. Ethylene is produced in ripening fruits, injured parts and in very low concentration in almost every part of the plant. 4. Ethylene has the following roles: (a) Ethylene promotes ripening of fruits. Fruit ripening is a process in which the chlorophyll of its epidermis break down and yellow and red colors carotenoids formed are shown. Besides that, cell walls become soft, the starch and acid contents are converted into sugar and amount of alkaloids is reduced, making the fruit tastes sweet and gives out aromatic smell. (b) Ethylene promotes defoliation or abscission of leaves and petals of flowers. Before abscission, a special abscission layer is formed by the break down of the middle lamellae and cell walls at the base of the stalk. The cells there secrete enzymes that cause the production of suberin to cover the exposed surface after the leaf or organ drops. (c) Ethylene promotes etiolation, yellowing of plants as if kept in prolonged darkness. This is a response when the plant is shaded and stimulated to extend the shoot without production of chlorophyll to reach light as fast as possible. H H C=C H H Figure 9.39 Structure of ethylene Exam Tips 9LTLTILYH[SLHZ[[OYLL M\UJ[PVUZMVYLHJOVM[OLÄ]L KPɈLYLU[OVYTVULZ Exam Tips 9LTLTILYOV^WSHU[ OVYTVULZ^VYR[VNL[OLY to control growth, apical dominance (STPM 2009 essay question), dormancy, ÅV^LYPUNZLULZJLUJL fruit ripening, defoliation, stomatal closing and parthenocarpy. Closure of stoma +HYRVYSHJRVM^H[LY()(PZ synthesised in guard cells Ca2+ are transferred from vacuole to cytoplasm. 5LNH[P]LPVUZSPRL*S– are transported out -VSSV^LKI`2+ and water .\HYKJLSSZILJVTLÅHJJPKHUK stoma closes 6WLUPUNVMZ[VTH 3PNO[WYVTV[L()(IYLHRKV^U Ca2+ ions are transferred into vacuole 5LNH[P]LPVUZSPRL*S– KPɈ\ZLPU -VSSV^LKI`2+ and water Guard cells become turgid and stoma opens Info Bio
CHAPTER 9 100 Biology Term 2 STPM Chapter 9 Control and Regulation (d) Ethylene inhibits growth of the roots. Table 9.7 Summary on the roles of plant hormones Hormone Roles Effects Auxin Promotes cell elongation Phototropic responses Shoots bend towards light Geotropic responses Roots grow down into the soil Stimulates cell division Promotes development of roots More roots grow from stem Stimulates cambial activity Callus development at the site of wounds Stimulates development of fruits Fertilisation causes embryos to produce auxin and ovary will not fall Maintains cell wall function Inhibits leaf abscission If the supply of auxin from leaves exceeds that from the stem, the leaf remains intact Inhibits fruit abscission If the supply of auxin from the fruit exceeds that from the stem, the fruit remains intact Inhibits growth in high concentration Causes apical dominance Lateral buds remain dormant under the influence of auxin from the apical bud Gibberellin Stimulates genes for growth Mutated genes in cabbage produce dwarfism Promotes cell elongation Increases the length of internodes Breaks dormancy of buds when temperature rises Buds develop into shoots in spring Breaks dormancy of seeds Stimulates synthesis of hydrolases to hydrolyse stored food for germination Stimulates fruit development Embryos and ovaries of many plants produce GA for fruit development Stimulates growing in some plants Carrots produce GA after a cold period to stimulate flowering Affects flowering Promotes flowering in some long-day plants Cytokinin Promotes cell division Increases growth rate in many plants, e.g. sunflower (Helianthus) Delays leaf senescence Maintains leaves for some time once detached from plant Stimulates bud development Promotes development of internal shoot so it inhibits apical dominance Breaks dormancy Break dormancy in both seeds and buds for some plants
CHAPTER 9 101 Biology Term 2 STPM Chapter 9 Control and Regulation Hormone Roles Effects Abscisic acid Inhibits growth Retards growth in most plant parts Promotes abscission Causes the formation of an abscission layer in the stalks of leaves, flowers and fruits Induces dormancy Promotes dormancy in the seeds and buds of many plants, e.g. birch (Betula) and sycamore (Acer) Closes stomata Promotes stomatal closure under conditions of water stress Ethylene Inhibits DNA and protein synthesis Ripens fruits Breaks dormancy Ends dormancy of buds in some plants Induces flowering Promotes flowering in pineapples Quick Check 5 1. How is phototropism explained? 2. How do hormones interact in the control of growth and apical dominance? Mechanism of Phytochrome Action and Their Roles in Photoperiodism and Flowering Phytochrome 1. Phytochrome is a blue green pigment which acts as photoreceptor i.e. it can absorb a red or far-red light to promote or inhibit growth for a variety of plant parts. Thus, it can stimulate or inhibit flowering in certain plants. 2. Phytochrome is a lipoprotein, a conjugated protein with lipid as the prosthetic group. The lipid part is smaller and sensitive to light. 3. Phytochrome is found in all plants, some green algae but not in fungi and bacteria. It is synthesised in the leaves, seeds and seedlings. 4. Phytochrome can exist in two forms. (a) PFR or P730 is a form that absorbs far-red light of wavelength 730 nm. (b) PR or P660 is a form that absorbs red light of wavelength 660 nm. 5. The two forms of phytochrome are inter-convertible after absorbing light of certain wavelengths. Such inter-conversion is as shown below. PR PFR Biochemical process Red light (660 nm) Stimulates /inhibits Inactive Active form Growth of plant part is promoted or inhibited Far-red light (730 nm)/slowly at night
CHAPTER 9 102 Biology Term 2 STPM Chapter 9 Control and Regulation (a) Phytochrome PR is converted to PFR after absorbing red light or sunlight, which consists of large amount of red light. (b) Phytochrome PFR is converted to PR after absorbing far-red light or slowly at night or in the dark when there is more far-red light. 6. After phytochrome is produced in the inactive form of PR, it is quickly converted to the active PFR form and predominates at the end of the day. The PFR form activates or inhibits the production of enzymes that are required to form certain hormones from their precursors. One of the enzymes is required for the formation of a flowering hormone called florigen. Another is required for the formation of a hormone that induces germination. In both cases, the hormone involved may be gibberellic acid, but the concentration is too minute for positive identification. 7. Phytochrome also acts to control daily changes in activities such as rhythmic movements of leaves and responses to light, including phototropism. This is mediated by inducing Ca2+ ions to be transported into the cells to combine with certain proteins that bring about the activities. Therefore, it is part of the internal timing mechanisms called biological clocks in flowering plants. Photoperiodism and flowering 2009, 2016 1. Photoperiodism is a phenomenon in which organisms, especially plants, response to changes in day length relative to dark period in a 24-hour cycle. 2. Examples of processes that are influenced by photoperiodism are flowering, germination, stem elongation, stem branching, development of leaves, fruits and seeds. 3. Plants are divided into three types, based on their responses to photoperiodism. (a) Short-day plants (SDPs). They flower when exposed to a critical day length of not more than a certain number of hours in a 24-hour cycle. In fact the dark period is more important. For example, cocklebur only flowers when the dark period is more than 8.5 hours without any interruption. Other examples of SDP are chrysanthemums and spinach. SDP would flower in nature in late summer or early autumn when the day is getting shorter. (b) Long-day plants (LDPs). They flower when the day length is longer (or the dark period is shorter) than a certain critical number of hours in a 24-hour cycle. For example, cabbage only flowers when the dark period is shorter than 13 hour period. Other examples are tulips and LDPs would flower in spring when the day length is getting longer. (c) Day-neutral plants. They flower independent of light period, they flower regardless the length of light period, once matured. Dayneutral plants originally are from tropical regions of the world.
CHAPTER 9 103 Biology Term 2 STPM Chapter 9 Control and Regulation 4. Photoperiodism and flowering are both controlled by phytochrome as follows: (a) One of the functions of phytochrome is to control flowering in plants that are affected by photoperiod. (b) When phytochrome is synthesised, it is in the inactive PR form. It is easily converted in the presence of light during day or by red light to the active PFR form. (c) In the presence of far-red light, phytochrome is converted back to PR form. Similarly, at night or in the shade, in predominantly more far-red light, it is also slowly converted back to the inactive form. (d) Phytochrome PFR in certain concentration stimulates flowering in LDPs by activating genes for the synthesis of enzymes to produce a flowering hormone called florigen from its precursor before flowering can take place. (e) This is because after several relatively long days in late spring or summer, the PFR form is sufficiently accumulated to trigger the flowering process. (f) LDPs do not flower in relatively short days as there is insufficient PFR form when the days are shorter in late summer or early autumn. In fact, the relatively long night converts it back to the inactive form. (g) Phytochrome PFR inhibits flowering in SDPs. (h) Phytochrome PFR in very small concentration or none stimulates flowering in SDPs. Therefore, a SDP needs several long nights to get rid of PFR form before a mature SDP can flower. (i) Phytochrome PFR does not affect flowering in day-neutral plants. (j) The roles of phytochrome in flowering is as shown in Figure 9.39. PR inactive PFR active Precursor Precursor Florigen Flowering Day/ red light far-red LDPs (cabbage) SDPs (Chrysanthemums) Stimulates Slowly at night Inhibits Figure 9.39 9VSLVMWO`[VJOYVTLVUÅV^LYPUN 5. From Figure 9.40, it can be explained that flashing of red light in the dark can promote flowering in LDP but not in SDP. Similarly, flashing of far-red light can inhibit flowering in LDP but not in SDP. The flashing of red light can neutralise the far-red light if one follows the other and of equal strength.
CHAPTER 9 104 Biology Term 2 STPM Chapter 9 Control and Regulation (R) (FR) (R) Night Flowering White light Red Light(R) Far-red Light(FR) (R) (FR) Day Flowering Flowering No flowering No flowering Flowering (R) (FR) (R) Night White light Red Light(R) Far-red Light(FR) (R) (FR) Day Flowering Flowering Flowering No flowering No flowering No flowering Long day (short night) plant e.g. Cocklebur Short day (long night) plant e.g. Cabbage Figure 9.40 Long day and short day plants 6. Because of photoperiodism, flower farmers can control the timing of flowering to sell flowers at higher price at certain festivals. Chysanthemum farmers just have to switch on lights in their farms for one minute at mid-night to stop the flowers from flowering. This is due to chrysanthemums are SDPs. They need long nights to get rid of PR before they can flower. Flashing of white light or red light will cause the increase in PFR that will inhibit flowering. Application of Plant Growth Regulators in Agriculture Synthetic Auxins 1. Synthetic auxins are used as analogues in agriculture as they have the same effects of natural IAA in the plants and are cheaply produced. They are resistant to degradation by plant enzymes and the effects can last longer. There is an advantage of very dilute concentration is used and the effect can be enhanced with chloride substitution in the structure. 2. Synthetic auxins are used in the promotion of rooting of cuttings to increase the propagation of plants asexually. This is to maintain the qualities of both ornamental and farm plants. Examples of synthetic auxins are α-naphthalene acetic acid (NAA) and indole-butyric acid (IBA) that are sold as rooting powder. The base of the cutting is dipped in the powder prior to planting. This will enable a fast emergence of roots even for some cuttings that are not propagated this way.
CHAPTER 9 105 Biology Term 2 STPM Chapter 9 Control and Regulation 3. Synthetic auxins are used to increase fruit setting and induction of the ovary or pericarp even in the absence of fertilisation. This is used by fruit farmers to maintain the maximum number of fruits. The fruit produced have the advantage of being seedless as well. This is also used later when the fruits are bigger to prevent pre-harvest fruit drop. The fruits produced also exhibit size bigger than normal. All fruits can be set and increased in sizes including tomato, cherry, apple, watermelon and banana. Figure 9.41 )HUHUHZ[YLH[LK^P[OZ`U[OL[PJH\_PU 4. Phenoxyacetic acids can be used as auxin analogues to act as herbicides e.g., 2, 4-D (2, 4-dichlorophenoxyacetic acid). 2, 4-D kills dicotyledonous plant selectively making it a good weed killer. The spraying of 2, 4-D for cereal and lawn plants which are monocotyledonous will kill only the dicotyledonous weeds. Another more powerful 2, 4-T (2,4-trichlorophenoxyacetic acid) was used by the US army to kill forest plants in Vietnam during the war. However, 2, 4-T is always contaminated with dioxin and when left in the ecosystem bringing direct ill effects like foetal abnormalities and cancer to humans after taken into the body through the food eaten. 5. NAA is also employed for the induction of flowering in pineapple (actually caused by the auxin-induced production of ethylene). This will bring about early flowering and sucessful fruiting. Synthetic gibberellins 1. Commercially, gibberellin is produced as gibberellic acid (GA3 ) by fungal cultures. GA3 is also the natural form produced by all plants. It is the only gibberellin obtainable in commercial quantities, although an expensive mixture of GA4 and GA7 is now commercially available for specific purposes. 2. Synthetic gibberellins are used for enhanced production of seedless grapes. Bigger, more uniform branches with larger fruits are produced. Among other effects, the gibberellin causes lengthening of the stalk
CHAPTER 9 106 Biology Term 2 STPM Chapter 9 Control and Regulation attaching each grape to the cluster, thus permitting larger grapes to form. Almost all the grapes that are commercially produced are now treated with synthetic gibberellin. 3. Oranges are treated with GA3 to prevent rind senescence, to permit longer storage on the tree, and thus to extend the marketing period. This will ensure better marketing price as they need not have to be harvested in a short period of time. 4. GA3 is used for enhancement of flower bud formation and improvement of fruit quality in cherries. The trees are sprayed with a very dilute solution of suitable concentration. 5. GA3 can be used in the improvement of fruit setting in apples and pears, particularly under weather conditions poor for setting. This may be in unusual hot weather condition or rainy days. 6. GA3 is also used as substitute for a chilling requirement in instances such as: (a) flower induction for seed production in radish (b) increased elongation of celery and rhubarb (c) earlier flower production of artichokes 7. GA3 can be used in the production of hybrid cucumber seed. Most high-producing cucumbers are F1 hybrids. GA sprays induce the production of male flowers on cucumber plants that normally produce only female flowers. The seed from neighbouring all-female plants of a different strain is then exclusively hybrid. 8. GA3 is used to increase sugarcane yield. GA3 promotes the elongation of sugarcane stalks with no change in the sugar concentration, so that the net yield of sugar is increased. Synthetic ethylene 1. Ethylene is a gaseous hormone with limited usesin gaseousstate. The synthetic acetylene produced by carbides was used before as ethylene analogue. Recent discovery that the natural ethylene is produced in all ripening fruits though slowly dispenses the use of the gas. The fruits are just covered with plastic sheet will just produce the gas themselves. 2. Ethylene can be used to enhance the ripening of fruits such as bananas in storage following their shipment in an unripe condition. This is of great benefit, since the green bananas are rugged and do not bruise or spoil easily. The tender ripe bananas can then be carried safely to market from the nearby warehouse. Besides, ethylene in high concentration can produce uniform yellow colour in bananas and mangoes. 3. Recently, an ethylene-producing liquid chemical, 2-chloroethylphosphonic acid (commercially called Ethrel or Ethephon) has been introduced into commerce. This compound is Figure 9.42 Bigger grapes produced compared with untreated smaller ones
CHAPTER 9 107 Biology Term 2 STPM Chapter 9 Control and Regulation Test 1 Light period Dark period Keys: 660 nm – red light 735 nm – Far-red light Critical period Test 2 Test 3 Test 4 Test 5 0 4 8 12 16 20 24 28 32 660 nm 735 nm 660 nm 36 735 nm sprayed onto the plant at a slightly acidic pH. When it enters the cells and encounters the cytoplasm at about neutral pH, it breaks down to release gaseous ethylene. Numerous commercial applications for this compound have appeared, mostly in relation to the natural effects of ethylene. 4. The most important commercial use involves enhancing latex flow in rubber trees in Southeast Asia. When a rubber tree is “tapped,” the latex flows for a certain period before the cut seals and the flow stops. Ethephon delays the healing of the cut so that the latex flow continues for a longer period, thus yielding more latex with less tapping. 5. Enhancement of uniform fruit ripening and coloration. This has been shown to be of particular value in field tomatoes picked at a single time by machine. Ethephon in high concentration can produce uniform yellow colour in bananas and mangoes. 6. Ethephon is used to accelerate of fruit abscission for mechanical harvesting. Fruits such as grapes, cherries, and citrus when sprayed with Ethephon will ripen fast and drop (Figure 9.43). Netting or mechanical devices can be left at the ground to collect all the fruits at the same time. 7. Ethephon is used to promote female flower production in cucurbits (cucumber, squash and melon) so as to increase the number of fruits produced per plant. These plants are especially sensitive to ethylene in the production of female flowers. 8. Ethephon is used for promotion of flower initiation and controlled ripening in pineapples. When the plants are matured and sprayed, they will flower uniformly. Later when the fruits are big, they can ripe uniformly. Figure 9.43 Cherry will ripen and drop uniformly after treated with ethephon Quick Check 6 1. Cocklebur would flower if it is given a non-interrupted dark hours of at least 8 1 —2 hours. During the dark period, flashlight of different wavelengths is shone to it as follows. (a) Is this a long-day plant or a short-day plant? Explain. (b) What form of phytochrome is required for its flowering? (c) State which test results in flowering and which does not. (d) Give a reason for your answer in (c).
CHAPTER 9 108 Biology Term 2 STPM Chapter 9 Control and Regulation Objective Questions 1. Which combination about axon and synapse is correct? A Axon has Schwann cell, synapse has acetylcholine B Axon has Schwann cell, synapse has no acetylcholine C Axon has no Schwann cell, synapse has acetylcholine D Both axon and synapse have acetylcholine 2. The table below shows the concentration of main ions during resting potential. Ions Concentration within cell/ mole dm–1 Concentration outside cell/ mole dm–1 K+ 150 5 Na+ 15 150 Cl– 10 110 What happens during action potential? A Na+ ions leave the cell B K+ ions enter the cell C Na+ ions enter the cell D K+ ions leave the cell 3. Which event occurs during depolarisation of neurone membrane? A Sodium ions diffuses out from the neurone B Resting potential becomes action potential C Action potential approaches zero D Membrane potential becomes more negative 4. Which statement is true of a resting neurone? I Sodium-potassium pumps stop II Facilitated diffusions occur III Three sodium ions are pumped in for every two potassium ions pumped out IV Three sodium ions are pumped out for every two potassium ions pumped in A I and III B I and IV C II and III D II and IV 5. Arrange the following events during skeletal muscle contraction in a correct sequence. I Troponin changes shape exposing the actin binding site II Calcium is released and binds to the troponin complex III Transverse tubules depolarise the sarcoplasmic reticulum IV Bending of the myosin heads exerts force on the thin filaments V Production of action potential across the post-synaptic membrane A II, I, III, V, IV B II, III, IV, I, V C V, III, I, II, IV D V, III, II, I, IV 6. Which diagram is true of the changes in polarity along an axon during the propagation of impulse? A – – – + + + – – – + + + – – – + + + B + + + – – – + + + – – – + + + – – – C – – – + + + – – – – – – + + + – – – D + + + – – – + + + + + + – – – + + + 7. The diagram below shows formation of an action potential when a nerve is stimulated by electric current at S1 , S2 , S3 and S4 . STPM PRACTICE 9
CHAPTER 9 109 Biology Term 2 STPM Chapter 9 Control and Regulation –120 –100 –80 –60 –40 Threshold value Potential difference/ mV –20 0 A1 A2 S1 S2 S3 S4 A3 A4 20 40 A1 and A2 are membrane depolarisation values A3 and A4 are action potentials Which statement explains the diagram? I The stimulus value at S1 and S2 are lower than –40 mV II This nerve has a threshold value of –40 mV III At the depolarised membrane, the charge on the surface is +40 mV IV Below the threshold value, the total stimulus is directly proportional to the depolarisations of membrane A I and II B I, II and III C I, III and IV D I, II, III and IV 8. The structure of a neuron is shown in the diagram below. P Q R S Which structure speeds up the transmission of impulses? A P B Q C R D S 9. Which of the following are the functions of the synapse? I To connect two neurones II To act as the impulse inhibition site III To increase the rate of propagation of impulse IV To prevent excessive stimulation through the fatigue process A I and II B I and IV C II and III D III and IV 10. The events that occur during muscle contraction as explained by the sliding filament theory are as follows: I The releasing of ADP + Pi from the myosin head II The binding of the myosin head to the actin filament forming a cross-bridge III The stimulation by the nerve impulse releasing calcium ions into sarcoplasm IV The sliding of the actin filament over myosin filament towards the centre of sarcomere V The binding of a new molecule of ATP releasing the myosin head from its binding site Which is the correct sequence of events during muscle contraction? A I, IV, V, II, III B I, V, IV, II, III C III, I, II, IV, V D III, II, IV, I, V 11. Which is the role of tropomyosin in skeletal muscle? A To catalyse hydrolysis of ATP B To hold calcium ions in place C To block myosin binding site on actin D To form an actin-myosin crossbridge
CHAPTER 9 110 Biology Term 2 STPM Chapter 9 Control and Regulation 12. The differences of the effects of the sympathetic nervous system and parasympathetic nervous system on organ are shown in the table below. Sympathetic nervous system Parasympathetic nervous system I Dilates the pupils of the eyes Constricts the pupils of the eyes II Decreases the heart rate Increases the heart rate III Dilates the bronchioles in the lungs Constricts the bronchioles in the lungs IV Stimulates the activity of the stomach Inhibits the activity of the stomach Which are true of the above comparisons? A I and III C II and III B I and IV D II and IV 13. What is the advantage of using curare in surgical operation? A To induce hallucination B To relax a muscle in the operated region of a body C To reduce pain in a patient during the operation D To make a patient unconscious during the operation 14. Which of the followings hormone must have its gene expression upregulated to be more resistant to drought? A Auxin B Abscisic acid C Cytokinin D Gibberellins 15. A plant has its apical bud removed, what would happen? A The plant undergoes lateral bud dominancy B A new apical bud forms C The lateral buds grow into branches D The stem of the plant increases in length 16. Which of the followings hormone correspond to its function? Hormone Function A Auxin Inhibits elongation of cell B Cytokinin Initiates programmed death of cell C Ethylene Promotes the elongation of cell D Gibberellin Stimulates seed germination 17. Which statement is correct about the mechanism of action of steroid hormones? A The hormones bind to the receptor on the plasma membrane. B The action involves the transcription and translation of a gene. C Adenyl cyclase is activated D ATP is converted to cAMP. 18. Which is true of phytochromes? I Regulates flowering in some plants II Converted to active form by red light III Regulates the absorption of light IV Red pigment acts as a photoreceptor A I and II C II and III B I and III D III and IV 19. The modes of action of hormones are described as follows: I It triggers a cascade effect. II It activates gene expression. III It binds to a receptor protein on the surface of the target cell. IV It penetrates the plasma membrane and binds to a receptor inside the target cell. Which descriptions are true of a steroid hormone? A I and II C II and III B I and IV D II and IV 20. A short-day plant will not flower when placed in a long dark period if it is interrupted by a flash or red light. Why the plant does not flower? A A lot of Pr is converted to Pfr B A lot of Pfr is converted to Pr C No Pr is converted to Pfr D No Pfr is converted to Pr
CHAPTER 9 111 Biology Term 2 STPM Chapter 9 Control and Regulation Structured Questions 1. The diagram shows the changes in the potential difference across the axon membrane during an action potential. Potential difference / mV –50 50 0 –100 Time / ms 0 1 Y X Z 2 (a) State the function and the importance of sodium-potassium pump at phase X. [2] (b) What happens at the axon membrane during phase Y? [2] (c) (i) Name phase Z. [1] (ii) Give two reasons why are the membrane potential of phase Z is less than that of phase X? [2] 2. The diagram below shows the vertical section of a neuromuscular junction. T tubule Axon terminal Synaptic vesicles Presynaptic membrane Y Z X W Sarcolemma Synaptic cleft (a) Explain how ATP produced by structure W are utilised. [2] (b) State where structure X comes from and the name of the substance within it. [2] (c) Explain the role of the substance found in structure X. [2] (d) Explain the importance of structure Y. [2] (e) Name and explain the basic unit found in structure of Z. [2]
CHAPTER 9 112 Biology Term 2 STPM Chapter 9 Control and Regulation 3. The diagram below shows the mechanism of action of a hormone. ATP cAMP Active kinase A Phosphorylase Enzyme Y Glucose phosphate Glycogen Capillary Hormone X Binds to a receptor Plasma membrane of target cell (a) Label hormone X and enzyme Y. [2] (b) State two differences between a target cell and non-target cell. [2] (c) Name two examples of target cells or tissues. [2] (d) Explain why hormone X cannot get across the plasma membrane easily. [2] (e) Explain how a fast production of glycogen can be accomplished. [2] 4. Five tests were carried out on a species of plant to induce flowering by varying the period of light and flashing different types of light during the dark period. The results are as shown below. Non-flowering Non-flowering Non-flowering Period / hour 0 4 8 12 16 R R FR R FR R 20 24 Critical night length R : Red light FR : Far-red light A Flowering Flowering Flowering B C D E R FR R FR F (a) Is the plant a short- or long-day plant? Explain. [2] (b) Describe the effects of flashing different types of light during the dark period. [3] (c) State the effect of flashing white light during the dark period of a long-day plant. [1] (d) Which form of phytochrome is involved in the plant used in this test? Explain. [2] (e) How does phytochrome explain the effects of the flashing of various lights in the above test? [2] Essay Questions 1. (a) Explain the structure of a sarcomere. [7] (b) Explain the sliding filament theory in muscle contraction. [8]
CHAPTER 9 113 Biology Term 2 STPM Chapter 9 Control and Regulation 2. Explain the roles of any five of the following in muscular contraction. (a) myofibril (e) actin (b) T tubule (f) myosin (c) sarcoplasmic reticulum (g) troponin (d) calcium ion [5 = 3 = 15] 3. (a) Upon returning from a vacation, you noticed that your potted plant had wilted. Explain this observation by relating to the cohesion-tension theory of water movement in plant. [10] (b) Explain how curare causes paralysis and death to victim. [5] 4. Explain three physiological roles each for auxin, gibberellin, cytokinin, abscisin and ethylene in the flowering plants. [15] 5. Explain how hormones control the following processes: [15] (a) Dormancy and germination in seeds (b) Shoot and root induction (c) Flowering (d) Parthenocarpy and fruit ripening (e) Senescence and defoliation 6. (a) Where are the following hormones produces? State their function. [7] (i) Auxin (ii) Cytokinins (b) Explain how gibberellin controls seed germination. [8] 7. (a) (i) Give a definition of photoperiodism. [1] (ii) How are the responses of short-day and long-day plants in photoperiodism for flowering? [8] (b) State the similarities and differences of the roles of auxin and cytokinin in affecting the growth and development of plants. [6]
CHAPTER 9 114 Biology Term 2 STPM Chapter 9 Control and Regulation ANSWERS Quick Check 1 1. The transport proteins within the plasma membrane in the epidermal cells are different from those of nerve cells and they are not meant for transmitting impulse. The epidermal cells do not have a resting potential of –70 mV, neither can they generate an action potential to transmit impulse. 2. Memory involves transmission of impulses across specific number of synapses in specific circuit within a larger part of the forebrain. The circuit when ‘switched’ on many times reinforces to a more long-term memory. Special neurotransmitter is involved at the synapses. Each time the chemical is discharged, it facilitates the next discharge and finally brought about structural changes on the postsynaptic membrane. This results in long-term memory. 3. It contracts by thick filaments sliding into thin filaments, just as in striated muscles. However, the filaments are not supported at the centre in an orderly manner by membrane but connected in a disorderly manner, forming no dark and light bands. Quick Check 2 1. Autonomic system controls reflex actions for internal organs. It is divided into sympathetic and parasympathetic systems. The sympathetic system prepares the body for unusual or emergency conditions, such as exercising that automatically results in an increase in heartbeat and breathing rate. The parasympathetic system then sets it back to the normal condition after the exercise. Besides that, it also controls normal reflex; for example when food enters the mouth, the saliva is automatically released. 2. One example is when something is reaching close to our eyes, we blink. Another example is when there is a strong light, our pupils constrict. Quick Check 3 1. This is because impulse requires neurotransmitters or chemicals to cross synapses and neuromuscular junctions. Certain drugs like amphetamine (pep pill) and cocaine are similar in structure with norepinephrine therefore produce stimulating effects like those of the sympathetic system. Others like heroine bind to opioid receptors in the brain postsynaptic membrane, cutting down pain sensation and producing pleasure sensation. Curare produces paralysis as it blocks the acetylcholine receptors in the neuromuscular junctions. Quick Check 4 1. Steroid hormones affect genes through their easy passage into the nucleus through the plasma and nuclear membranes with the help of receptors. Once inside the nucleus the hormone-receptor complex binds to specific site, the regulator gene. It stimulates the transcription of genes, followed by the synthesis of proteins, notably enzymes that bring about effects of the hormones. For example, oestrogen goes into the lining of menstruated uterus, binds to certain genes and produce enzymes and other proteins to repair and thicken the uterine wall. 2. Only cells with corresponding receptors can respond to certain hormone. Non-steroid hormones cannot enter the cells; the hormones bind to specific receptors found on the surface of the plasma membrane before they trigger response. Steroid hormones enter cell easily and bind with receptors either in the cytoplasm or nucleus before they activate specific genes to bring about the response. 3. Endocrine glands Hormones produced 1. Pituitary glands ADH, FSH, LH, ICSH, oxytocin, prolactin, growth hormone, thyroidstimulating hormone, ACTH 2. Thyroid gland Thyroxine 3. Parathyroid gland Parathyroid hormone 4. Adrenal gland Epinephrine (adrenaline), aldosterone, cortisol 5. Pancreas Insulin, glucagon 6. Ovary Oestrogen, progesterone 7. Testis Testosterone Quick Check 5 1. Shoot responds positively towards light source. Light causes auxin to diffuse from the lighted side to the dark side of the shoot. The accumulation of auxin in the lighted side induces more growth there, causing the shoot to curve. However, the accumulation of it in the dark side of the root inhibits growth in the dark side. Thus, it causes root to bend away from light. 2. Auxin, gibberellic acid and cytokinin work synergistically to promote growth. Gibberellic acid works better in the presence of auxin to promote growth in treated shoot, especially in naturally dwarf plants. Similarly, cytokinin works better in the presence of auxin or gibberellic acid to promote
CHAPTER 9 115 Biology Term 2 STPM Chapter 9 Control and Regulation growth by cell division. Conversely, abscisic acid and ethane work antagonistically with auxin, gibberellic acid and cytokinin to promote growth. However, auxin and gibberellic acid work antagonistically to promote apical dominance. Auxin inhibits growth of lateral shoots whereas gibberellic acid and cytokinin promote their growth. Quick Check 6 1. (a) It is a short-day plant because it requires more than 8 1 —2 hours of darkness to flower i.e. it flowers when the day is shorter than 12 hours in a 24- hour cycle. (b) Phytochrome PFR inhibits flowering in this shortday plant. Therefore, it will flower when there is little or none of this phytochrome. (c) Flowering: Test 3, 4 and 5 Non-flowering: Tests 1 and 2 (d) Test 1 does not result in flowering because the dark period is 6 hours, short of the critical period. The dark period is required to get rid of PFR to stimulate flowering. Test 2 does not result in flowering because the dark period is interrupted with red light that converts PR to PPR, which inhibits flowering. Test 3 results in flowering because the dark period is 9 hours. Test 4 results in flowering because the last exposure is far-red light that gets rid of cytochrome PFR. Test 5 results in flowering because the last exposure is far-red light that gets rid of cytochrome PFR, even though the dark period is only 5 hours. STPM Practice 9 Objective Questions 1. A 2. C 3. B 4. D 5. D 6. B 7. B 8. B 9. B 10. D 11. C 12. A 13. B 14. B 15. C 16. D 17. B 18. B 19. D 20. A Structured Questions 1. (a) Function: to maintain resting potential of –70 mV Importance: so that action potential and impulse can be generated (b) t 4PEJVN HBUFE DIBOOFMT PQFO TPEJVN JPOT rush inside the axon t Depolarisation occurs with generation of action potential of +70 mV (c) (i) Hypopolarisation (ii) t 5PPNVDIQPUBTTJVNJPOTSVTIPVUXIFO gated potassium channels open. t The potential difference slightly overshoots the resting potential of –70 mV 2. (a) ATP is required by the sodium-potassium pump to maintain the resting potential. It is required for anabolism, especially for the synthesis of acetylcholine. It is required to exocytosise the acetylcholine. (b) It comes from Golgi apparatus and the substance is acetylcholine. (c) Acetylcholine acts as a neurotransmitter and bind to the receptors found in the postsynaptic membrane. This causes the opening of the sodium channels where sodium ions rush in to produce the postsynaptic excitory potential and enables the impulse to cross the synapse. (d) Structure Y increases the surface area of postsynaptic membrane, thus increases the number of receptors for the neurotransmitter and sodium channels. (e) The basic unit is called sarcomere. It is the structure between two Z membranes of adjacent light band, consisting of a dark band with two half light bands on either side. 3. (a) Hormone X is insulin and enzyme Y is adenyl cyclase. (b) Target cell has the receptor for the hormone whereas non-target cell does not. Target cell responds to the hormone whereas non-target cell does not. Target cell brings about an effect of the hormone whereas non-target cell does not. (c) The two examples are liver and muscle tissues. (d) Hormone X is insulin, a protein too large to be transported across the plasma membrane. Insulin, being a protein, is polar in nature and cannot pass through non-polar plasma membrane easily. There is no transport protein present in the plasma membrane to transport the hormone. (e) This is through a cascade effect of amplification (or positive feedback). The hormone acting as a first messenger brings about the production of a lot of second messenger cAMP. This in turn triggers the activation of more enzyme, one after the other, to accomplish the fast production of glycogen. 4. (a) Short-day plant. When the day length is shorter than the critical period (13 hours), it flowers as indicated by test B. (b) Flashing red light during the dark period causes the plant not to flower as seen in C. t 'MBTIJOHGBSSFEMJHIUDBVTFTUIFQMBOUUPĘPXFS as seen in D. t 3FEMJHIUDBODBODFMUIFFČFDUPGGBSSFEMJHIUPG equal intensity and vice versa, so the last light flashed determine the outcome of flowering as seen in E and F. (c) The plant would not flower.
CHAPTER 9 116 Biology Term 2 STPM Chapter 9 Control and Regulation (d) Phytochrome PFR. This is the active form that inhibits the production of florigen in short-day plant; only when it is in low concentration that the plant would flower. (e) Flashing red light converts PR form into the active PFR that inhibits flower, so no flowering occurs. Flashing far-red light removes the active PFR and converts it into PR form, so flowering occurs. Essay Questions 1. B t *UDPOTJTUTPGBEBSLCBOEXJUI UXPIBMGMJHIU bands at either side and equals to the portion of myofibril between two adjacent Z membranes. One sarcomere One myofibril with four sarcomeres t ćF EBSL CBOE PS " CBOE DPOTJTUT PG UIJDL filaments supported in the centre by M membrane. t ćF MJHIU CBOE PS * CBOE DPOTJTUT PG UIJO filaments supported in the centre by Z membrane. t ćFUIJDLBOEUIJOĕMBNFOUTDBOTMJEFJOUPPOF another as each thick filament is surrounded by six thin filaments and each thin filament is surrounded by three thick filaments. t ćJDLĕMBNFOUJTNBEFVQPGCVOEMFPGQSPUFJO myosin that has many tiny cross bridges with heads, arranged in a helical pattern on its surface. t &BDI PG UIFTF CSJEHFT DBO FYUFOE CJOET JUT head to a site on the thin filament; bends its head and contracts. t ćJOĕMBNFOUJTNBEFVQPGGPVSTUSBOET UXP inner strands of actin, two outer strands of tropomyosin with troponin in between the different strands at regular distance along them. The four strands are twisted like string. t ćFSFBSFCJOEJOHTJUFTMPDBUFEBMPOHUIFXIPMF length of the actin strands for the myosin heads to bind. t 8IFO UIF NVTDMF JT BU SFTU UIF TJUFT BSF covered by the two outer tropomyosin strands. When troponin is attached by calcium ion, it causes realignment in the four strands and the binding sites along the actin strands are exposed. t 8IFO UIF TJUFT BSF FYQPTFE UIF IFBET PG the bridges can bind to the sites, pulling the filaments into each other, and causing the contraction of the myofibrils and muscle fibre as a whole. (b) t ćJTUIFPSZFYQMBJOTUIBUUIFDPOUSBDUJPOPGUIF muscle fibre is due to the sliding of the thick and thin filaments into each other, causing the myofibril to shorten. t 8IFOUIFNVTDMFJTSFMBYFE FMFDUSPNJDSPHSBQI shows that the filaments are as shown in the following diagram. Dark band (A) Light band (I) M membrane Z membrane Thick filament (myosin) Thin filament (actin) One sarcomere t "ęFS UIFNVTDMFIBTDPOUSBDUFE UIF QPTJUJPO of the thick and thin filaments are as shown in the following diagram. Dark band Light band Z membrane Thick filament (myosin) Thin filament (actin) One sarcomere t ćFTBSDPNFSFCFDPNFTTIPSUFS*UJTBMTPUSVF for the light band but the dark band remains the same. t ćJT JT UIF SFTVMU PG UIF TMJEJOH PG UIF UIJO filaments into those of the thick filaments. t 4VDITMJEJOHJTEVFUPUIFSBDIFUNFDIBOJTNPG the cross bridges which are present all over the surfaces of the thick filaments, pulling the thin filaments towards them. t ćF XPSLJOH PG UIF SBDIFU NFDIBOJTN JT BT shown in the following diagram. ATP required (a) Head released (d) Head released ATP required (b) Attached ATP splits to ADP + P (e) Attached to more distal site
CHAPTER 9 117 Biology Term 2 STPM Chapter 9 Control and Regulation (c) Pulling ADP and P released (f) Pulled and the process is repeated t ćF NZPTJO IFBE DBO CJOE QVMM SFMFBTF BOE the process is repeated about 50 times per second. t 8JUI TPNBOZ PG UIFTFDSPTT CSJEHFTBOE UIF rate of reactions so fast, this results in the shortening of the myofibrils and the muscle fibres. 2. (a) The role of myofibril in muscular contraction is its ability to shorten that brings about the muscle fibre to contract as it is made up of several hundred myofibrils within. The myofibril in turn is made up of alternating sections of thick filaments and thin filaments that can slide into one another, causing the myofibril to contract. The sliding is caused by cross bridges on the thick filaments working in a very fast pace to attach on the thin filaments and pulling them towards the thick filaments. Therefore, the role of myofibril is to shorten, thus shortening the muscle fibre and in turn shortening the muscle bundle i.e. causing the muscle to contract. (b) T tubule increases the surface area of the postsynaptic membrane i.e. a small part of the sarcolemma. This increases the number of receptors on its surface and also the number of gated sodium channels across the membrane. More sodium ions can cross into the muscle in the shortest possible time. This enables a faster generating of excitatory postsynaptic potential and also the ability to amplify the effect of weak impulse arriving at the neuromuscular junction. (c) Sarcoplasmic reticulum transports calcium ions from the outside of the muscle fibres to the inside. It also releases and distributes the calcium ions fast when necessary i.e. after the sarcolemma is depolarised. This enables the calcium to enter deep into the thin filaments of the myofibril to bring about contraction of the muscle. After the contraction, it helps to absorb the calcium ions and store them for subsequent contraction. (d) Calcium ions bind to troponin, a protein found along the thin filament of the myofibril. After binding, the troponin causes realignment of tropomyosin and actin strands within the thin filament. Such realignment exposes the binding sites along the whole length of the actin strands so that the myosin heads can bind. Calcium ions are required to start the process of filament sliding, thus causing the muscle to contract. (e) Actin is the protein that is used for making two of the strands of proteins making up the thin filament. The actin strands provide special sites along the whole of their length so that the myosin heads can bind. This enables the ‘ratchet’ mechanism to work, causing the sliding of filaments to bring about muscle contraction. (f) Myosin is the protein that is used to make the thick filament with its cross bridges within the myofibril. The whole filament is covered with cross bridges which are arranged helically on its surface. The myosin heads binds on the binding sites on the thin filaments, pull, release and rebind very fast to cause sliding of the filaments to bring about muscle contraction. (g) Troponin is the protein found along the whole length of the thin filament between the tropomyosin and the actin strands. It can bind calcium ions and bring about the realignment of the component strands of the thin filament to expose the binding sites found along them. The sites then enable the myosin heads to attach and pull the filaments into each other to bring about muscle contraction. 3. B t ćF QPUUFE QMBOUXJMUFE EVF UP MBDL PGXBUFS caused by no replacement of water after transpiration. t %VSJOH UIF EBZ XBUFS WBQPVST FWBQPSBUFE from mesophyll cell wall diffuse out from the leaves creating low water potential in the mesophyll cells. t 0TNPTJT PDDVST XIFSF XBUFS NPWFT GSPN xylem vessels into the mesophyll cells. t ćJT JO UVSOT DBVTFT USBOTQJSBUJPO TUSFBNT within the xylem from the roots to the leaves as roots have higher water potential then the leaves. t )FODF UIFSF JT B HSFBU GPSDF DSFBUFE CZ UIJT transpiration pull from the leaves aided by cohesion-tension within the tiny xylem vessels. t ćFGPSDFPGDPIFTJPOJTDBVTFECZUIFBUUSBDUJPO among water molecules by hydrogen bonding. t ćFGPSDFPGBEIFTJPOCFUXFFOUIFWFTTFMXBMM molecules and water supports the tiny column of water t ćJTDSFBUFTMPXXBUFSQPUFOUJBMJOUIFYZMFNPG the root centre t 8BUFS EJČVTFT EPXO DPODFOUSBUJPO HSBEJFOU from the soil through the cortex cells of the roots due to higher water potential of the soil water. t 'JOBMMZ XIFOUIFSFJTOPXBUFSJOUIFTPJMPGUIF pot, the cells in the plant become flaccid and wilted.
CHAPTER 9 118 Biology Term 2 STPM Chapter 9 Control and Regulation C t $VSBSFCJOET UPBDFUZMDIPMJOFSFDFQUPSTQPTU synaptic membrane including that of end plates. t 4P XIFO DVSBSF HFUT JOUP UIF CPEZ acetylcholine is blocked from binding to the receptors. t /P EFQPMBSJTBUJPO BOE OP USBOTNJTTJPO PG impulses can occur across synapses including going into muscles t ćFSFGPSF NVTDMFT DBOOPU DPOUSBDU UIVT paralysis of all muscles occur t 8IFO UIF CSFBUIJOH NVTDMFT EJBQISBHN and external intercostal) cannot contract, breathing stops and the person dies. 4. Auxin t "VYJOJOEVDFTHSPXUICZFMPOHBUJPOPGPSHBOTTVDI as the stem, coleoptile, root, leaf and floral parts. t "VYJO TUJNVMBUFT QSPEVDUJPO PG NFSJTUFN BOE activates meristem. This happens when cells are injured or in the cambium, especially in spring. t "VYJO JOIJCJUT EFWFMPQNFOU PG MBUFSBM CVET Therefore, it promotes apical dominance, a phenomenon in certain plant such as papaya. Gibberellin t (JCCFSFMMJO QSPNPUFT TUFN FMPOHBUJPO BOE vegetative growth in plants. Plants that are dwarfs when treated with gibberellin becomes normal. t (JCCFSFMMJO CSFBLT UIF EPSNBODZ JO TFFET BOE buds, especially after winter. The production of gibberellin is a response of an increase in environmental temperature. t (JCCFSFMMJODBOQSPNPUFGSVJUJOHBOEQBSUIFOPDBSQZ Cytokinin t $ZUPLJOJO QSPNPUFT DFMM EJWJTJPO JO NFSJTUFNT injured cells and callus. They function only in the presence of auxin or gibberellin. t $ZUPLJOJOEFMBZTUIFQSPDFTTPGTFOFTDFOTF BHJOH especially in leaves. t $ZUPLJOJO QSPNPUFT TZOUIFTJT PG %/" 3/" BOE proteins especially in the leaves. Abscisin t *UDBVTFTMFBGGBMMJOBVUVNOXIFOJUJTTZOUIFTJTFE in the leaves. It inhibits the synthesis of DNA and RNA where it induces senescence and yellowing of leaves. Nutrients are drained out and stored in the roots during winter. t *UJOEVDFTUIFGPSNBUJPOPGBCTDJTTJPOMBZFSTJOUIF leaf stalks, causing leaves to drop all at the same time. t *U QSPNPUFT EPSNBODZ JO CVET BOE TFFET *U JT produced in autumn, stimulated by cold and disappeared in spring when the temperature rises. Ethene t *UQSPNPUFTSJQFOJOHPGGSVJUT t *U QSPNPUFT BCTDJTTJPO PG MFBWFT BOE QFUBMT PG flowers. t *UJOIJCJUTHSPXUIPGUIFSPPUT 5. (a) In most temperate plants, the seeds mature in autumn with cold as a stimulus for the production of abscisic acid, causing them to fall. The seeds does not germinate even with favourable conditions, unless after winter or stimulated by cold for the production of gibberellic acid. Gibberellic acid is a hormone that breaks dormancy and stimulates germination. During germination, auxin and cytokinin stimulate the growth of the embryo. (b) Auxin, gibberellic acid and cytokinin are involved in the shoot and root induction for growth. Depending on the species of plants, either auxin or gibberellic or both, are involved in shoot and root elongation. Root requires much lesser concentration of auxin for growth and too much of auxin results in inhibition. Cytokinin requires the presence of either auxin or gibberellic acid to induce growth by cell division. (c) Auxin, gibberellic acid or a hormone known as florigen induces flowering in different species of plants. Such hormone is produced as a result of physical maturity of the plant, usually stimulated by raining season with abundance of water. Stress or decrease in water supply too can stimulate flowering. However, temperate plants flower as a result of day length changes during different seasons, due to hormone production. (d) Auxin or gibberellic acid produced by the embryo can cause development of the ovary to form fruit. Pineapple and banana produce either hormone in the ovary walls even without fertilisation, a process called parthenocarpy to form fruit. The fruit later produces ethene that in turn causes ripening by converting starch to form sugar and yellowing of the skin. Abscisic acid may also be involved in the induction of ripening of the fruit by inhibiting the synthesis processes there. (e) Abscisic acid stimulates aging process called senescence and abscission process, resulting in defoliation and falling of other plant organs. It inhibits synthesis of DNA and synthesis of protein and induces nutrient withdrawal from the leaves and other organs. Auxin, gibberellic acid and cytokinin antagonise the action of abscisic acid. Cytokinin, in the presence of auxin or gibberellic acid, stimulates cell division and synthesis of protein and promotes the production of chlorophyll. 6. B J t "VYJOJT QSPEVDFE BU UIF UJQT PG ZPVOH shoot or root. t Auxin is also produced in embryos or injured tissues. t Auxin induces growth of stem and root by cell expansion. t Auxin causes phototropism when light comes from one direction, by bending
CHAPTER 9 119 Biology Term 2 STPM Chapter 9 Control and Regulation the shoot towards the light. t Auxin activates meristem especially in spring. t Auxin inhibits development of lateral shoots, thus controls apical dominance. (ii) t $ZUPLJOJOJTQSPEVDFEJOUIFFNCSZPT t Cytokinin is also produced at the root tips of some plants. t Cytokinin promotes cell division in meristem together with auxin or gibberellin. t Cytokinin inhibits senescence of the leaves by stimulating synthesis of chlorophyll. t Cytokinin inhibits apical dominance of the shoot by promoting lateral bud growth. C t (JCCFSFMMJDBDJE ("TUJNVMBUFTUIFQSPDFTTPG germination when released from seed embryo t This occurs when soaked in water as in cereal seeds t Temperate plant seeds release GA when temperature increases in spring t Some plant seeds release GA when exposed to light especially red light t This is due to the presence of phytochrome mechanism that causes the synthesis of GA t Some plant seeds produce GA after cold stimulation, for example after putting in fridge overnight. 7. (a) (i) Photoperiodism is a phenomenon in which organisms especially plants, response to changes in day length relative to dark period in 24-hour cycles. (ii) t 4IPSUEBZ QMBOUT SFTQPOTFT UP MJHIU period shorter than a critical period in 24-hour cycles by flowering when reproductively matured. t An example of short-day plant is cocklebur which will only flower when the critical period is shorter than 15.5 hours. t The dark period is more important as short-day plants depend on phytochrome, PFR, to be converted into an inactive form in the dark so as not to inhibit the formation of flowering hormone. t If the dark period is interrupted by flash light or red light, the active form of PFR formed is enough to inhibit flowering t Long-day plants response to light period longer than a critical period in 24-hour cycles for flowering t An example of long-day plant is cabbage which only flowers when the critical period is longer than 13 hours. t Similarly, the dark period is more important as long-day plants depend on the active form of PFR to stimulate the formation of flowering hormone. t If the dark period is interrupted by farred light, the active form is converted into inactive form and no flowering occurs. C t #PUIBVYJOBOEDZUPLJOJOTUJNVMBUFHSPXUIJO plants especially increase in length. t "VYJO TUJNVMBUFT HSPXUI CZ DFMM FYQBOTJPO whereas cytokinin stimulates growth by cell divisions. t #PUI TUJNVMBUF EFWFMPQNFOU PG QMBOUT especially plant shapes. t "VYJO TUJNVMBUFT BQJDBM EPNJOBODF XJUI shorter lateral branches whereas cytokinin stimulates lateral branches to grow taller than main shoots. t #PUIBMTPTUJNVMBUF UIFEFWFMPQNFOUPGQMBOU roots. t "VYJO TUJNVMBUFT SPPU GPSNBUJPO FTQFDJBMMZ cutting in a very dilute concentration whereas cytokinin stimulates only the main root inhibiting lateral roots. t #PUINBJOUBJOUIFEFWFMPQNFOUPGMFBWFTBOE prevent senescence. t "VYJO BMTP TUJNVMBUFT EFWFMPQNFOU PG GSVJUT whereas cytokinin does not.
CHAPTER REPRODUCTION, DEVELOPMENT AND GROWTH Concept Map 10 Reproduction, Development and Growth Reproduction Seed germination Growth curves and patterns of growth In humans In plants Stages in embryonic development Process of parturition Roles of hormones in menstrual cycle and pregnancy Spermatogenesis and oogenesis Double fertilisation External factors affecting germination The embryonic development in seed and formation of fruit Mobilisation of nutrients after imbibition Fertilisation and implantation The roles of placenta, chorion, amniotic fluid and allantois in foetal development Passage and development of sperm from the testis Types of growth curves Pattern of growth Ecdysis and metamorphosis Bilingual Keywords Sexual reproduction – Pembiakan seks Fertilisation – Persenyawaan Menstrual cycle – Kitar haid Pregnancy – Kehamilan Ecdysis – Ekdisis Development – Perkembangan Growth – Pertumbuhan Germination – Percambahan Growth curve – Lengkung pertumbuhan Metamorphosis – Metamorfosis
Learning Outcomes CHAPTER 10 121 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Language Check Students should be able to: (a) outline spermatogenesis and oogenesis; (b) describe the passage and development of sperms from the testis to the oviduct for fertilisation; (c) describe the process of fertilisation and implantation; (d) describe the roles of hormones in menstrual cycle and pregnancy; L KLZJYPILIYPLÅ`[OL stages in embryonic development; (f) explain the roles of placenta, chorion, HTUPV[PJÅ\PKHUK allantois in foetal development; (g) explain the process of parturition. 10.1 Sexual Reproduction in Humans 10.1 Sexual Reproduction in Humans 1. Human sexual reproduction involves the production of spermatozoa from man (spermatogenesis) and oocytes from woman (oogenesis). 2. Spermatogenesis occurs in the testes of man while oogenesis occurs in the ovaries of woman. The male and female reproductive organs are as shown in Figure 10.1. Prostate Urethra Glans Scrotum Testicles Uro-genital orifice Erectile bodies Vas deferens Seminal vesicle Vas deferens Epididymis Lobule Seminiferous tubules Testis Male reproductive organs Fallopian tube Ovary Cervix Vagina Uterus Female reproductive organs Figure 10.1 Male and female reproductive organs 3. During sexual intercourse, the semen is deposited in the vagina. The spermatozoa will swim up to the Fallopian tube where a spermatozoon will fertilise the oocyte. 4. Fertilisation will result in pregnancy and the development of the foetus. After forty weeks of gestation, childbirth (parturition) will occur. A new human being is then brought to this world. ; LZ[PZ¶ZPUN\SHY ;LZ[LZ¶WS\YHS INFO Sexual Reproduction in Humans
CHAPTER 10 122 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Language Check Spermatogenesis and Oogenesis Spermatogenesis 1. Spermatogenesis occurs in the testes within the wall of seminiferous tubules (Figure 10.2). There are 1, 000 very fine coiled tubes called seminiferous tubules in each testis. Each is about 50 cm long and 20 μm in diameter. A bundle of the tubules packed within one small section of the testis called lobule. Each lobule has one common tubule called vase efferential that leads out and joins with those from in other lobules. 10 to 20 vasa efferentia join to form larger vase efferential in the epididymis i.e. a small structure attached beside the testis. Spermatid Spermatocyte Cell of Sertoli Spermatogonium Spermatozoon Figure 10.2 Cross section of seminiferous tubule showing spermatogenesis 2. Spermatogenesis begins between the ages of 11 and 15 years old and will continue for life. This coincides with the start of male puberty under the influence of the male sex hormone, testosterone. The hormone is produced in the testis by the Sertoli cell and the Leydig (interstitial) cells outside the seminiferous tubules. 3. Normally, between 100 and 200 million spermatozoa will be made every day. This starts from the blind end of each seminiferous tubule with cells from outer wall that divide first by mitosis, grow to bigger cells, then divide by meiosis and finally differentiate to form the spermatozoa. 4. Diploid cells of the epithelial (outermost) layer of the seminiferous tubules divide by mitosis to form spermatogonia. This layer of cells is also known as the primordial germ cells. Mitosis will ensure that the number of chromosome is maintained and all the daughter cells are genetically identical. 5. The spermatogonia move towards the lumen of the seminiferous tubule and increase in size to form primary spermatocytes. These primary spermatocytes will synthesise more proteins and more organelles especially mitochondria. 6. The primary spermatocytes divide by meiosis I to form two haploid cells called secondary spermatocytes. The secondary spermatocytes will rapidly continue to start the second meiotic division. It is difficult to identify the secondary spermatocytes in the section of seminiferous tubules. : WLYTH[VNVUP\T¶ singular Spermatogonia – plural : WLYTH[VaVVU¶ZPUN\SHY :WLYTH[VaVH¶WS\YHS 4P[VJOVUKYPVU¶ZPUN\SHY Mitochondria – plural
CHAPTER 10 123 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth 7. Then, second meiotic division occurs to form four haploid spermatids from each primary spermatocyte. The spermatids formed are smaller. They are now at the inner part nearest to the lumen of the seminiferous tubules. 8. Each spermatid will mature into a spermatozoon. During the maturation, each spermatid will undergo a process of differentiation. The spermatozoon is a specialised cell with the shape like a tadpole as shown in Figure 10.3. The tip of the head has a large lysosome called acrosome which contains hydrolytic enzymes. The neck has a pair of centrioles and the mid-piece is packed with mitochondria. The tail is a flagellum made up of microfilaments covered with tail sheath. The tail enables the spermatozoon to swim. Midpiece Tail Acrosome Nucleus Head Centrioles Mitochondria Flagellum Tail sheath Figure 10.3 Structure of a spermatozoon Growth & differentiation 2 secondary spermatocytes MEIOSIS I MEIOSIS II 4 spermatids Spermatogenesis 4 spermatozoa lumen of tubule Mitosis Spermatogonium Seminiferous tubule (in the testis) Daughter cell type A spermatogonium Daughter cell type B spermatogonium Primary spermatocyte 2n 2n 2n 2n 1n 1n 1n 1n 1n 1n Figure 10.4 The process of spermatogenesis
CHAPTER 10 124 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Language Check 9. The developing spermatozoa are nourished by Sertoli cells. The Sertoli cells are long cone-shaped cells extending from the wall to the lumen of seminiferous tubule. A group of developing spermatozoa are attached to the tip of each Sertoli cell to obtain nourishment. The Sertoli cells secrete testosterone and also the liquid within the lumen. The whole process is summarised in Figure 10.4. 10. The whole process takes approximately 64 days. The mature spermatozoa are not active and are moved along the seminiferous tubule out of the testis into the efferent duct (vasa efferentia). The efferent duct is the only duct that leads the spermatozoa out of each testis. It has a thicker wall and coils to form the epididymis just outside the testis. The wall of the efferent duct has a thick layer of smooth muscle that contract to force the spermatozoa out through the vas deferens during ejaculation. Oogenesis 1. Oogenesis occurs in the ovaries. It starts in the outermost epithelial layer of the ovary where the primordial germ cells will divide by mitosis. 2. Oogenesis begins very early in life i.e. in embryonic stage of the female. The process will continue through the foetal stage. This is in great contrast with spermatogenesis in which the start of puberty is when the epithelial germ cells divide. 3. About five weeks after the formation of a female embryo, diploid oogonia are also formed in the tiny developing ovaries. This is the multiplication phase same as in spermatogenesis. 4. When the embryo is 24 weeks old, there will be millions of oogonia in the ovaries. The ovaries have rapidly increased in size. 5. About 6 months after birth, the oogonia will begin the first division of meiosis but the process remains at prophase I to form primary oocytes. However, not all the oogonia would undergo meiosis. So, only around 400,000 primary oocytes are produced at puberty. 6. Some cells surround each primary oocyte to form a primordial follicle and later each develops into a primary follicle. Initially, there is only one layer of cell so the follicle is called primordial follicle. Later, more layers of cell are added to form a primary follicle. 7. At the onset of puberty, every month hormones stimulate the formation of secondary follicles which develop with fluid-filled cavity. Initially, there are many small fluid-filled cavities but slowly they fuse to form one big cavity called anthrum with an outer protective layer called theca. 6VNVUP\T¶ZPUN\SHY 6VNVUPH¶WS\YHS 6VNVUPH ¶ WS\YHS
CHAPTER 10 125 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth 8. Then, every month from either one of the ovaries, one primary oocyte begins to grow rapidly, forming a Graafian or mature follicle. The primary oocyte is surrounded by layers of cell called zona pellucida in the centre of a fluid-filled anthrum connected with the middle layer granulosa cells by a stalk of cells as shown in Figure 10.5. Growing follicle Vesicular follicle OVARY Oocyte(egg) Ovulation Mature corpus luteum Corpus albicans (regressed corpus luteum) Primordial follicle Ovarian blood vessels Mature Grafiaan follicle Figure 10.5 Schematic diagram of ovary with various stages of follicles 9. The primary oocyte undergoes first meiotic division, producing one large haploid cell, the secondary oocyte and one tiny cell, called the polar body. 10. The large secondary oocyte continues straight into the second meiotic division but stops at the stage of metaphase II. So, in human the ‘egg’ released later is actually the secondary oocyte that has not reached the ovum stage as shown in Figure 10.6. 11. The follicle ruptures and releases the secondary oocyte, surrounded by granulosa cells during ovulation. Ovulation occurs alternatively between the two ovaries. Only one ovulation will occur every 28 days at the 14th day of the menstrual cycle. 12. The oocyte is drawn into the oviduct (Fallopian tube), mainly by peristalsis, although there are cilia present in addition. 13. In the oviduct, if fertilisation takes place, it will complete the second division of meiosis. If fertilisation does not occur, the secondary oocyte will die around 24 hours. * PSP\T¶ZPUN\SHY *PSPH¶WS\YHS 6]\T¶ZPUN\SHY 6]H¶WS\YHS * PSP\T ¶ ZPUN\SHY *PSP S S Language Check *PSPH ¶ WS\YHS 6]\T ¶ ZPUN\SHY 6]H ¶ WS\YHS
CHAPTER 10 126 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth 14. The follicle transforms into a corpus luteum. This is as a result of the granulosa cells left in the rupture follicle divide to repair to form this spherical ball of cells. The corpus luteum produces the hormone progesterone as well as oestrogen. 1st polar body (may or may not divide, but products degenerate) MEIOSIS I MEIOSIS II Primary before birth oocyte after birth 2n 1n 1n 2n 2n 2n 1n 1n 2nd polar body (also degenerates) Ovum Secondary oocyte Hours before ovulation Growth and differentiation Mitosis Many oogonia Oogonium Figure 10.6 The process of oogenesis Corona radiata Cytoplasm Nucleus Nucleolus Plasma membrane Zona pellucida Figure 10.7 Structure of human occyte
CHAPTER 10 127 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth 15. The differences between spermatogenesis and oogenesis are summarised in the table below: Spermatogenesis Oogenesis 1. Continuous after puberty In cycles after puberty 2. Millions produced One/few per cycle 3. Occurs from 12 years old to 90 9 years to menopause around 50 4. Four spermatozoa per meiosis One ovum per meiosis 5. No polar bodies Polar bodies are produced 6. All mitotic products used Many mitotic products degenerate 7. Complete meiosis on release Completes meiosis after fertilisation 8. Primary spermatocyte smaller Primary oocyte bigger 9. Products need to differentiate No differentiation of products 10. Requires testosterone Requires oestrogen Passage of Spermatozoa from Testis to Oviduct 1. The sperms from the testis are first moved into the epididymis. The epididymis is a coiled tube, about 6 m, pressed against the testis. The sperms take several days to pass through this tube. The sperms are concentrated here to about 5000 million per cm3 by reabsorption of fluid secreted originally by the seminiferous tubule. The sperms are passed to the end of the tube for a short period of storage. Chemicals produced by the lining of the tube are essential for maturation of the sperm. 2. Then, the sperms enter the vas deferens for storage. This is a straight tube about 40 cm long that join from left and right sides of the testes to the urethra at the base of the urinary bladder. 3. During sexual intercourse, the stimulation in the penis results in orgasm causing the parasympathetic system to stimulate the smooth muscles in the wall of vasa differentia and other muscles to contract resulting in ejaculation. More liquid is added to the spermatozoa to form semen on the way the spermatozoa are ejaculated out. This is the result of addition of liquid from the various glands on the journey out. 4. The first gland that adds liquid is the seminal vesicles. The seminal vesicles secrete mucus and a watery alkaline fluid that contains nutrients, including fructose that supplies energy source for the sperms. 5. At the junction of the urethra, there is the prostate gland. This gland secretes mucus and a slightly alkaline fluid that neutralises the acidity in the vagina and makes the sperms more active. 6. Next to the prostate gland is the small Cowper’s gland. The fluid released from Cowper’s gland is first released when the male is sexually exited to neutralise the acidity of any urinary remain in the urethra.
CHAPTER 10 128 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth 7. The sperm have to pass through the urethra in the penis before ejaculated into the vagina during sexual intercourse. The urethra is also where the urine from the urinary bladder has to pass through. The penis serves as the intromittory organ to deposit the sperm in the vagina. 8. From the vagina, the sperms which are activated will swim through the cervix, through the uterus into the oviduct. The cervix is usually plugged with thicker mucus but after the period, the mucus is thinner for the easy passage of the sperms. 9. The journey through the uterus into the oviduct may take just 5 minutes. It may be too fast a rate for the sperm to swim so the journey may be helped by peristaltic movements of the uterus and oviduct. The cilia present in the oviduct might also help in the passage. It takes for most sperms 4 – 8 hours to reach the oviduct from the vagina. The sperm can survive there for 1 – 3 days but are only fertile for 12 – 24 hours. Fertilisation and Implantation 1. Fertilisation is the fusion of the spermatozoon nucleus with the ovum nucleus to form a diploid cell known as the zygote. The process occurs in the oviduct where the ovum travels along the oviduct and is fertilised by spermatozoon to become a zygote. 2. Before fertilisation can take place a process known as capacitation must occur first. This involves the removal of a layer of glycoprotein and plasma protein from the outer surface of the sperm by enzymes in the uterus. The glycoprotein is added in epididymis while the plasma protein is added by the seminal fluid. Cholesterol is also lost from the surface membrane around the sperm head, weakening the membrane. 3. Before a sperm cell penetrates through the ovum’s hard shell to fuse with nucleus of the ovum, the sperm cell goes through a process known as acrosome reaction. Acrosome reaction is the reaction that occurs in the acrosome of the sperm as it approaches the egg (Figure 10.8). 4. As the sperm cell approaches the zona pèllucida of the ovum, which is necessary for initiating the acrosome reaction, the membrane surrounding the acrosome fuses with the plasma membrane of the oocyte, exposing the contents of the acrosome. The contents include surface antigens and numerous enzymes which are responsible for breaking through the ovum’s tough coating of glycoprotein and allowing fertilisation to occur. 5. Immediately after the spermatozoon has penetrated, cortical reaction takes place. The cortical granules which are the lysosomes at the outer region released their contents to cause the zona pellucida to thicken and harden forming the ‘fertilisation membrane’ to prevent the entry of another spermatozoon. The enzymes also destroy the sperm receptor sites so no sperm can bind to the zona pellucida.
CHAPTER 10 129 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Ovum plasma membrane Ovum cytoplasm Cortical granule Cortical granule content Fused plasma Acrosome reaction membrane Protein receptor Only one sperm can fuse with one ovum Figure 10.8 Acrosome reaction 6. The secondary oocyte completes the second meiotic division after the entry of the spermatozoon that stimulates it. A second polar body is released. The tail of the sperm within the cytoplasm degenerates. 7. The chromatin of the two nuclei fuse to form the diploid zygote. The zygote has two sets of chromosomes. The cell will start its first mitotic division. 8. After the first mitosis is completed, it will be followed by the first cleavage. Further mitosis and cleavages will produce a ball of 32 cells called morula. By that time, the morula already moved from the oviduct to the uterus helped by coordinated beating of the cilia. That takes about four days. 9. After another day, the ball of cell is called blastocyst. It is a hollow ball of cells with one cell thick and the cells are called blastomeres and the central fluid-filled cavity is called blastocoel. 10. Further development results in some cells moving inwards forming the inner cell mass. The outer layer is called trophoblast which later thickens to form the chorion. The chorion produces a hormone called human chorionic gonadotrophin (HCG). HCG stimulates the development of the corpus luteum that continues the secretion of progesterone and oestrogen to maintain pregnancy. Through the testing of HCG in the urine, it is a way to confirm pregnancy too. 11. Then implantation occurs after 6 to 9 days. The blastocyst then becomes embedded into the endometrium of the uterus. The chorion will absorb nutrients from the endometrium. Later, the chorion develops chorionic villi to increase the surface area to absorb nutrients from the endometrium wall of the uterus. The process of implantation is summarised in Figure 10.9.
CHAPTER 10 130 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Day 4 Day 5 Day 6 Day 3 Day 2 Fertilised Day 1 egg(zygote) Nucleus sperm(n) Nucleus ovum(n) Follicle cell Secondary oocyte Ovary Implantation of the blastocyst Ovulation Fertilisation Figure 10.9 The process of implantation Roles of Hormones in Menstrual Cycle and Pregnancy Menstrual Cycle 2011 1. The control centres for producing gametes are the hypothalamus and the pituitary gland. The hypothalamus secretes gonadotrophin releasing hormone (GnRH) which stimulates release of follicle stimulating hormone (FSH) and luteinising hormone (LH). FSH and LH are referred as gonadotrophic hormones because they stimulate the gonads i.e. the ovaries. The hormones are secreted cyclically in every 28 days called menstrual cycle. 2. During the first few days of the cycle, secretion of FSH and LH increases slightly. This causes a number of primary follicles to develop, though only one in one ovary continues to develop. This is the result of FSH molecules binding to the receptor sites of the primordial follicles thus stimulating the follicles to develop. 3. The granulosa cells of the developing follicle secrete oestrogen, the levels of which start to rise sharply during the first half of the cycle. Oestrogen stimulates the repair and development of the lining of the uterus, the endometrium. 4. The increase in the level of oestrogen inhibits any further secretion of FSH and LH and their levels start to fall. 5. On day 12, the oestrogen causes the endometrium to thicken. The endometrium is 3 – 4 mm thick. This is in preparation for the possibility of pregnancy when an embryo will be implanted in the endometrium. 6. On day 14 i.e. half way through the cycle, LH peaks and causes ovulation, a secondary oocyte detaches from the Graafian (mature) follicle. The released oocyte is moved into the oviduct (Fallopian Summary Hormonal changes during menstrual cycle Estrogen and progesterone Hypothalamus Anterior pituitary Ovary Uterus GnRH LH/FSH Keys: Stimulate Inhibit
CHAPTER 10 131 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth tube). Usually only one oocyte is released each month by one of the ovaries so that ovulation alternates between the pair of ovaries. 7. When the secondary oocyte has been released, the remaining granulosa cells enlarge and fill up with a yellow substance. It is now called a corpus luteum or yellow body, which continues to secrete large amounts of progesterone and smaller amounts of oestrogen. 8. The progesterone brings about further development of the endometrium, increasing its supply of blood and the glandular activities with further increase in the amount of glycogen and lipids in its cells. The endometrium thickness reaches 5 – 6 mm and it is prepared for implantation, if the egg has been fertilised. Pituitary hormones in blood FSH LH FSH LH LH LH peak triggers ovulation and corpus luteum formation Ovarian cycle Ovarian hormones in blood Menstrual cycle Estrogen Progesterone and estrogen Estrogen Progesterone and estrogen Progesterone Corpus luteum Degenerating corpus luteum Ovulation Mature follicle Growing follicle Endometrium 1 5 14 28 Menstruation Days in menstrual cycle FSH Control by hypothalamus Anterior pituitary Hypothalamus Releasing hormone ,QKLELWHGE\FRPELQDWLRQRI estrogen and progesterone 6WLPXODWHGE\KLJKOHYHOVRI estrogen Figure 10.10 Summary of menstrual cycle
CHAPTER 10 132 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth 9. If fertilisation has not occurred, the oestrogen and progesterone inhibit the secretion of FSH and LH from the anterior pituitary gland. As a result, the corpus luteum degenerates. 10. All four hormones now fall to a low level and, once the oestrogen and progesterone have dropped far enough, the hypothalamus detects the changes. GnRH is released and the anterior pituitary again begins to secrete FSH and LH and the cycle begins again. 11. The fall in oestrogen and progesterone causes the endometrium to break down and menstruation occurs during the next four to seven days of the cycle. Hormones Where produced Roles in menstrual cycle 1. Gonadotrophin releasing hormone (GnRH) Hypothalamus It stimulates the anterior pituitary gland to secrete FSH and LH. 2. Follicle stimulating hormone (FSH) Anterior pituitary gland (a) It promotes development of several primordial follicles but only one becomes Graafian follicle (b) It promotes secretion of oestrogen from granulosa cells of the developing follicles in the ovary by positive feedback 3. Oestrogen Follicles & corpus luteum (a) It stimulates repair and thickening of endometrium of uterus (b) It inhibits the anterior pituitary gland to secrete FSH by negative feedback (c) It promotes secretion of LH 4. Luteinising hormone (LH) Anterior pituitary gland (a) It causes ovulation, the release of secondary oocyte from ovary (b) It stimulates the formation of corpus luteum, which continues to secrete oestrogen and progesterone 5. Progesterone Corpus luteum (a) It stimulates thickening and glandular activity of the endometrium (b) It inhibits secretion of FSH and LH by negative feedback (c) It starts the menstrual cycle when the corpus luteum degenerates, as there is no progesterone to inhibit the production of FSH Pregnancy 1. If pregnancy occurs, the blastocyst formed will secrete chorionic gonadotrophin (HCG) from the trophoplast or outermost chorion. HCG will stimulate the continual release of progesterone and oestrogen from the corpus luteum. This will continue for the next three months before the corpus luteum degenerate. 2. The level of progesterone increases to maximum around the eighth month and decreases to minimum at the end of pregnancy. The roles of progesterone are as follows:
CHAPTER 10 133 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth (a) Together with oestrogen, it maintains the lining of the uterus and controls the growth of the uterus. (b) It stimulates the development of milk glands in breasts ready for lactation. (c) It inhibits release of FSH and prevents menstruation. (d) It inhibits the release of prolactin and thus inhibits lactation. (e) It inhibits the contraction of the uterus myometrium so the muscle relaxes and helps to prevent miscarriage. 3. The level of oestrogen increases to maximum at the end of pregnancy. The roles of oestrogen are as follows. (a) Together with progesterone, it controls the growth of the uterus especially the myometrium muscles. (b) It stimulates the development of milk duct system of the breasts. (c) It inhibits release of FSH and prevents menstruation. (d) It inhibits the release of prolactin and thus inhibits lactation. (e) It increases the sensitivity of uterus myometrium to oxytocin. 4. The secretion of progesterone and oestrogen from corpus luteum is slowly replaced from the placenta as it develops. By the end of the third month, the secretion of progesterone and oestrogen is completely done by placenta when it is fully developed. 5. Human placenta lactogen (HPL) is secreted from placenta about 2 months into pregnancy to maximum at the end of it. Its roles are as follows: (a) It stimulates growth and development of the mammary glands in preparation for lactation. (b) It is needed before oestrogen and progesterone can have their effect on the breasts. (c) It maintains glucose and fat metabolism of mother to the advantage of foetus. 6. Prostaglandins are released from the uterus before birth caused by foetal corticoids that crossed into mother’s blood. They increase the power of contraction of the uterus muscles. 7. Oxytocin is produced by the posterior lobe of pituitary gland. It is secreted before birth. It starts the contraction of the uterus for the birth process. 8. Prolactin is produced by anterior lobe of pituitary gland. It is produced after birth of the baby. It stimulates the flow of milk from the mammary glands. 9. Summary in terms of graph for hormonal changes during pregnancy is as shown in Figure 10.11.
CHAPTER 10 134 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Weeks 20 100 200 Progesterone binds to LH-receptors Human placental lactogen hCG from syncytiotroblasts 6000 ng.ml–1 50 I.U.ml–1 12 28 42 Corpus luteum Placenta 0 14 Concentrations ng.ml–1 200 100 Prolactin Oestradiol Figure 10.11 Hormonal changes during pregnancy 10. The roles of hormones in pregnancy are as shown in Figure 10.12. Embryonic chorion Corpus luteum Mammary glands Placenta Uterus Chorionic gonadotrophin human placenta lactogen Oestrogen, progesterone Parturition (birth) First 3 months After 3 months Inhibition Prolactin Hypothalamus Milk Oxytocin Growth Growth Contraction inhibition Control Pituitary gland Anterior Posterior Figure 10.12 Hormonal control during pregnancy
CHAPTER 10 135 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Hormones Where produced Roles in pregnancy 1. Chorionic gonadotrophin (HCG) Chorion It maintains the function of corpus luteum and its production of oestrogen and progesterone 2. Human placenta lactogen (HPL) Placenta (a) It stimulates growth and development of the breasts in preparation for lactation before oestrogen and progesterone have their effect on them (b) It maintains glucose and fat metabolism of mother to the advantage of foetus 3. Progesterone Corpus luteum & placenta (a) Together with oestrogen, it controls the growth of the uterus. (b) It stimulates the development of milk glands in breasts. (c) It inhibits release of FSH and prevents menstruation. (d) It inhibits the release of prolactin and thus inhibits lactation. (e) It inhibits the contraction of the uterus myometrium. 4. Oestrogen Ovary & placenta (a) It controls the growth of the uterus especially the muscles (b) It stimulates the development of milk ducts in breasts (c) It inhibits release of FSH and prevents menstruation (d) It inhibits the release of prolactin and thus inhibits lactation (e) It increases the sensitivity of uterus myometrium to oxytocin 5. Prostaglandins Uterus They increase the power of contraction of the uterus muscles 6. Oxytocin Posterior Pituitary It starts the contraction of the uterus for the birth process 7. Prolactin Anterior Pituitary It stimulates the flow of milk from the mammary gland Stages in Embryonic Development 1. The process of embryonic development starts immediately after fertilisation when a spermatozoon enters successfully into the secondary oocyte. This stimulates the second meiotic division of the oocyte and the formation of a fertilisation membrane. Thus, it forms the zygote. 2. The first mitosis occurs and is followed by first cleavage after about 30 hours. Subsequent cleavages occur to form a ball of cells, as it is swept by cilia down into the uterus. This is the cleavage stage. 3. When it reaches the uterus in four days, it is called a morula of 32 cells. The morula forms a bigger ball of cells called blastocyst on the fifth day. The blastocyst consists of an inner cell mass that forms the embryo, and an outer layer of cells called trophoblast as shown in Figure 10.13. 2013/P2/Q7 VIDEO Embryonic Development
CHAPTER 10 136 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Inner cell mass Blastocoel Trophoblast Endometrium Embryonic disc Amnion Yolk sac Chorionic cavity Embryo Allantois Placenta Amniotic fluid Foetus Umbilical cord Embryonic sac Figure 10.13 Embryonic development in human 1. Blastocyst 4. Neurulation 2. Implantation 5. Formation of foetus 3. Gastrulation 6. Growth in size 4. On the sixth day, the blastocyst digests its way into the endometrium wall of the uterus and implantation takes place. This is aided by enzymes produced by the trophoblast (chorion) and the loose endometrium that is rich in blood capillaries. 5. The blastocyst produces a hormone called human chorionic gonadotrophin (HCG), which stimulates the corpus luteum continual production of progesterone. Trophoblastic villi develop to absorb nutrients from the uterine wall. 6. A space with fluid called amniotic cavity is formed within the inner cell mass. It is surrounded by an outer layer of cells called amnion. This is gastrulation and later forms the three primary layers: ectoderm, mesoderm and endoderm. The inner cell mass develops to form the embryonic disc. 7. At the same time, a layer of cell surrounds the blastocoel forming the yolk sac. The yolk sac cannot supply nutrients to the developing embryo as in reptile and bird as there is no yolk. In humans, part of it becomes the site of blood cell formation and part of it gives rise to germ cell, which later form gametes. 8. Later, another cavity is formed surrounding the amnion and the yolk sac. This is called the chorionic cavity or extra-embryonic coelom. 2014/P2/Q19(a) STPM
CHAPTER 10 137 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth 9. The trophoplast layer is reinforced with the chorion, which becomes spongy placenta. Later the placenta becomes a circular disc from which the nutrients for the foetus are obtained. 10. Another sac grows out from the interior of the developing embryo forming the allantois. It becomes part of the umbilical cord and placenta. 11. Folding of ectoderm cells of the embryonic disc occurs during the next process known as neurulation. This will form the embryo and later the foetus. 12. The developing foetus is connected to the placenta by the umbilical cord. The umbilical cord carries blood vessels to and from the placenta. 13. The organs in the foetus are well formed by the fourth month. Further development of the foetus involves the formation of the heart, sexual organs and other minor changes. More pronounced changes are mainly involved in the increase in size until birth at about forty weeks. 14. The whole foetal development can be also divided into three threemonth trimesters. The first trimester is when the foetus forms most of its organs while the second and third trimesters are characterised by growth and enlargement of the foetus. Time lapse Stages 30 hours First cleavage is completed 4 days Morula of 32 cells reaches the uterus 5 days Blastocyst is formed 6 days Implantation 3 months (first trimester) 1. Amniotic cavity is formed within the inner cell mass 2. Embryonic disc undergoes gastrulation 3. Yolk sac surrounds the blastocoel 4. Chorionic cavity is formed 5. Allantois is formed from the inner embryo 6. Placenta is formed 7. Yolk sac & allantois form umbilical cord 8. Neurulation changes embryo to foetus 9. Most organs are formed 4 - 9 months (2nd and 3rd trimester) 1. Sex organs developed 2. Heart fully developed 3. Growth in size The Roles of Chorion, Amniotic Fluid, Allantois and Placenta 1. The chorion is the outer trophoblastic layer. (a) The chorion produces the HCG, the hormone that stimulates the development corpus luteum to produce progesterone and oestrogen before the placenta is formed. 2010 2014/P2/Q7
CHAPTER 10 138 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth (b) The chorion absorbs nutrients from the endometrium. Hydrolytic enzymes are released to digest food in the endometrium. (c) The chorion also helps in the process of implantation. This is also helped by the hydrolytic enzymes released by it. (d) Later many chorionic villi are formed to increase the surface area for absorption of nutrients. Each villus is bathed in a small pool of blood of the endometrium. This reduces the diffusion distance for exchange of nutrients and wastes between the embryonic and mother blood. (e) The chorion forms the placenta from which the embryo is attached to the uterus and also obtains the nutrients. (f) The chorion later fuses with the amnion to form a protective layer for the developing foetus. This is the embryonic sac which is tough but thin transparent pair of membranes. 2. The amniotic fluid is the fluid found in the cavity first formed in the inner cell mass. The cavity becomes much bigger later. (a) The amniotic fluid enables the foetus to grow and move freely. (b) The fluid protects the foetus from abrupt temperature changes. (c) The fluid can cushion any impact on the foetus from outside the body. (d) The fluid also serves as lubricant so that the wall of the uterus does not rub against the foetus or the baby during birth. 3. In birds and reptiles, allantois serves as a sac for disposal of nitrogenous wastes such as uric acid and gaseous exchange. In humans, it forms the urinary bladder and blood vessels of the placenta. 4. The placenta is the spongy layer with trophoblastic villi for exchange of substances between the embryo and the mother’s blood as shown in Figure 10.14. Umbilical arteries Umbilical vein Maternal portion of placenta Foetal capillaries Foetal portion of placenta (chorion) Maternal arteriole Maternal venule Umbilical cord Maternal blood pools Figure 10.14 Structure of placenta
CHAPTER 10 139 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Later it becomes a circular disc joined to the foetus through the umbilical cord. Roles of the placenta are as follow: (a) The major function of the placenta is for mutual exchange of substances between the mother and the foetus without the mixing of the blood. (b) Oxygen, water, amino acids, glucose, mineral ions and vitamins are allowed to diffuse from the mother’s blood to that of foetus. (c) Carbon dioxide, urea, uric acid and other wastes are allowed to diffuse from the foetal blood to that of mother’s. (d) Antibodies from mother’s blood are allowed to diffuse into that of foetus. This would allow the baby born with passive natural immunity against antigens that the mother already immune to. (e) The placenta disallows pathogens and toxins to cross into the foetal blood. There are exceptions such as HIV and Rubella viruses can cross the placenta into the foetal blood. Alcohol and nicotine from mother’s drinking and smoking can affect the foetal development. (f) The placenta prevents sex hormones from mother diffusing into the foetal blood. However, sometimes certain amount of them does diffuse causing enlargement of baby breast and even induce the production of milk. (g) The placenta produces progesterone and oestrogen. Both control the development of the uterus wall, the mammary glands and prevent ovulation and menstruation. It also produces human placenta lactogen for the development of the mammary glands. (h) The placenta avoids blood pressure of mother affecting that of foetus. The blood of foetus is not mixed with that of the mother. The Process of Parturition 1. Parturition is the process of birth, occurs after the full term of gestation of about 40 weeks in normal birth. 2. There are many factors causing the start of birth process. (a) During the final month of pregnancy the uterus becomes more and more sensitive to oxytocin. Oxytocin is a peptide hormone that causes contraction of the smooth muscle of the myometrium, the middle muscular layer of the uterus. The increase in sensitivity is due partly to the synthesis of more and more oxytocin receptors in the myometrium, possibly a result of high levels of oestrogen. Oxytocin levels also rise as a result of the level of progesterone decreasing late in pregnancy. (b) As the foetus gows, it produces more foetal corticosteroids from the adrenal cortex as a result of the foetal hypothalamus causes the release more ACTH from the anterior pituitary. The corticosteroids cross the placenta and enter mother’s circulation, causing a decrease of progesterone production and an increase in secretion of prostaglandins. 2013/P2/Q19(a) 2014/P2/Q19(b)
CHAPTER 10 140 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth (c) Prostaglandins are secreted by the uterus and stimulate contraction of the uterus. (d) The reduction in progesterone level also removes the inhibitory effect of progesterone on contraction of the myometrium. (e) Oxytocin causes contraction of the smooth muscle of the myometrium and the prostaglandins increase the power of contractions. Oxytocin is released from the posterior lobe of the pituitary gland. (f) The release of oxytocin occurs in ‘waves’ that start the labour. The onset of contractions of the myometrium marks the beginning of ‘labour pains’. There are 3 stages of labour. 3. The first stage is when the cervix dilates. Before the cervix dilates, the sequence of events occurs as follows: (a) Labour starts with very mild contractions. Then, contractions gradually get stronger and more frequent due to positive feedback control of oxytocin production. The more the uterus contracts, the greater the stimulation of stretch receptors in the uterus and cervix. These receptors send more impulses via the autonomic nervous system to the myometrium, which contracts even more and so on. (b) Besides, the autonomic nervous system sends impulse to the hypothalamus stimulating the release of more oxytocin from the posterior pituitary gland. More prostaglandins are secreted by the uterus and result in more powerful contractions. (c) Contractions spread down the uterus and are strongest from top to bottom, so pushing the baby downwards. Then, such waves of contraction become regular and when they come in 10 – 15 minute intervals, the plug of mucus that blocks the cervix during pregnancy will come away. This is called a ‘show’ consisting of a mass of sticky pinkish mucus that comes out of the vagina. (d) The amniotic ‘bag’ bursts (the ‘water break’) at around this stage releasing the amniotic fluid. This comes out of the vagina in drips or a gush. (e) The cervix will gradually dilate during the contractions. When the cervix is fully dilated to about 10 cm wide, it is wide enough for the baby head to pass through. 4. The second stage is when the baby is born. The baby is pushed out of the uterus and down the vagina, usually head first. Once the head of the baby comes out, the rest of the body can be easily slipped out with the amniotic fluid as lubricant. (a) The baby would take in the first breath of air usually is accompanied and aided by crying to make use of the lungs for the first time. (b) The umbilical cord is clamped at two places and cut between the clamps. Then, it comes the third stage.
CHAPTER 10 141 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth Learning Outcomes Students should be able to: (a) outline double fertilisation; (b) describe the embryonic development in seed and formation of fruit. 5. The third stage is the release of the placenta, umbilical cord and the embryonic sac. These structures are called the ‘afterbirth’. The last few powerful contractions of the uterus cause the placenta to detach from the uterus wall and to come out through the vagina. It may take a few minutes to half an hour for the ‘afterbirth’ to come out. A bleeding of 350 cm3 of blood is normal during and after the birth. The bleeding would stop quickly as the muscle fibres around the blood vessels of placenta would contract. 10.2 Sexual Reproduction in Flowering Plants owering Plants 1. Sexual reproduction in flowering plants involves the production of flowers. 2. Peacock flower is monoecious, having both male and female parts (Figure 10.15). Petal Style Anther Filament Ovary ‘Standard’ Ovule Pedicel Sepal Stigma Figure 10.15 :[Y\J[\YLVMHWLHJVJRÅV^LY 3. The male part is called androecium consisting of ten free staments. Each stament consists of a long filament with an anther at the tip. Endotesium Vascular bundle Pollen sac Tapetum Spore mother cell (2n) Figure 10.16 Cross section of an anther 7LHJVJRÅV^LYCaesalpinia, belongs to the family *HLZHSWPUVPKLHL Info Bio INFO VIDEO Sexual Reproduction in Flowering Plants Sexual Reproduction in Flowering Plants
CHAPTER 10 142 Biology Term 2 STPM Chapter 10 Reproduction, Development and Growth During the development of pollen, it is called microgametogenesis. The spore mother cells in the pollen sac undergo meiosis to form tetrads of haploid cells. Each haploid cell develops into a pollen grain with thick sculptured wall with one nucleus. Later, mitosis takes place to form a tube nucleus and a generative nucleus i.e. two nuclei, as shown in Figure 10.17. Young ovule Matured ovule Embryo sac mother cell (2n) Two celled stage (n) Antipodal cell Polar nuclei (n + n) Ovum (n) Synergid cells Matured embryo sac Eight nucleus stage Four nucleus stage Two nucleus stage Mitosis Mitosis Primary embryo sac cell (n) Tetrad (n) Meiosis II Mitosis Die Meiosis I Second male nucleus First male nucleus Figure 10.18 Development of an embryo sac and double fertilisation Spore mother cell (2n) Two-celled stage Tetrad Matured pollen Pollen grains Tube nucleus Generative nucleus Separate Mitosis Meiosis I Meiosis II Figure 10.17 Microgametogenesis 4. The female part is called gynaecium, consisting of only one carpel or pistil. (a) Each carpel consists of a bean shaped ovary, a slender style ending to form an enlarged stigma. (b) The ovule develops by a process called megagametogenesis, as shown in Figure 10.18. (c) A diploid embryo sac mother cell in the ovule divides by meiosis to form a row of four haploid cells.