Strategic STPM Sem 2 (2026) Biology

BIOLOGY• Khoo Wan Teng• Yeap Chee Beng> Info > Video> e-QuizDIGITAL RESOURCESPre-U TextStrategicSTPMSEMESTER2• Purified STPM Syllabus• Scheme of Assessment Starting from Semester 1 STPM Examination 2026New! Based on2022 – 2024

iiStrategic STPM Semester 2 Biology is written based on the purified STPM syllabus introduced by the Malaysian Examinations Council (MEC), which will be implemented starting from the 2026 STPM Semester 1. This book is carefully designed and well-organised with the following features to enhance students' understanding of the concepts.ivSemester of StudyCode and Paper Name Theme / Title Type of Test Mark (Weighting) Duration AdministrationSemester1964/1Biology Paper 1Molecules ofLife andMetabolismWritten testSection A20 multiple-choicequestions.Section B2 structuredquestions.Section C 2 essay questions.60(26.67%)2014261.5 hours Central assessmentSemester2964/2Biology Paper 2Physiology Written testSection A20 multiple-choicequestionsSection B2 structuredquestionsSection C 2 essay questions.60(26.67%)2014261.5 hours Central assessmentSTPM Scheme of AssessmentConcept Map8 GASEOUS EXCHANGECHAPTERGaseous ExchangeGaseous Exchange in HumansBreathing Cycle in HumansRespiratory system structure1Structure of haemoglobin Relation to functionBohr effectHaemoglobinMedulla oblongataTidal volumeMyoglobinChemoreceptorsVital capacityTotal lung capacityResidual volumeFoetal haemoglobinTransport of oxygen and carbon dioxide in bloodMicroscopic structure of the wall of an alveolusOxygen dissociation curvesControl of breathing mechanismLung volumes and capacitiesRelation to oxygen dissociation curveInspiratory reserve volumePotassium and chloride ions movementExpiratory reserve volumeGaseous Exchange in PlantsStructure and function of stomaMechanism of opening and closing of stomata17Chapter8Table 8.1 Summary of lung volumes and capacity in spirogramLung Volume and Capacity Description Approximate Value (mL)Tidal volume (TV) Air inhaled / exhaled in a normal breath ~500Inspiratory reserve volume (IRV) Air forcibly inhaled after normal breath 2000 – 3000Expiratory reserve volume (ERV) Air forcibly exhaled after normal breath 1000 – 1200Residual volume (RV) Air remaining in the lungs after forced exhalation ~1200Vital capacity (VC) Max air exhaled after maximal inhalation 4500 – 5000Total lung capacity (TLC) Total air lungs can hold after maximal inhalation 5800 – 6000Quick Check 5 1. What is a spirogram? 2. What are the main lung volumes shown on a spirogram? 3. What are lung capacities derived from lung volumes?8.3 Gaseous Exchanges in PlantsStructure of StomaStomata (singular: stoma) are microscopic pores found mainly on the lower epidermis of dicot leaves, though they can occasionally be present on the upper epidermis. These pores are crucial in regulating both gas exchange (CO₂ and O₂) and water loss (transpiration) in plants. The structure of stomata is highly specialised to balance the conflicting demands of photosynthesis, respiration and water conservation.Guard Cells: The Key to Stomatal DynamicsEach stoma is flanked by two guard cells that are distinct from the other epidermal cells in both structure and function. The guard cells work in pairs. They can swell or shrink based on cellular turgor pressure. The guard cells regulate the size of stomatal opening (whether the stoma is open or closed) based on the turgidity of the guard cells.• Describe the structure of stoma• Explain the relationship between the structure and function of stomata• Explain the mechanism of opening and closing of stomata based on potassium and chloride ions movementLearning OutcomeStructure of StomaVIDEOBiology Semester 2 STPM Chapter 8 Geseous Exchange PREFACE3Chapter8the pharynx (mucociliary escalator), where it can be swallowed or expelled.(c) Cartilage rings prevent the collapse of the trachea, maintaining an open passage for airflow.Connective tissueSmooth muscleSub-mucosa+ mucous glandsCiliated epitheliumLumenCartilage 3. Bronchi and bronchioles:(a) The bronchi are reinforced with cartilage plates, which diminish as the airway branches into smaller bronchioles.(b) Bronchioles lack cartilage but contain more smooth muscle, allowing for constriction and dilation (bronchoconstriction and bronchodilation) to regulate airflow. This mechanism is controlled by the autonomic nervous system, particularly the sympathetic and parasympathetic systems.(c) The terminal bronchioles end in respiratory bronchioles, which lead to alveolar ducts and eventually alveolar sacs.Connective tissueSmooth muscleCiliated epitheliumBronchusCartilage ConnectivetissueSmoothmuscleBronchioleCiliatedepithelium• The inner wall of alveoli: simple squamous epithelial cells – important in efficient exchange of oxygen and carbon dioxide with the capillaries.• The innermost layer of the trachea (mucosa): ciliated pseudostratified columnar epithelial cell – contains goblet cells that produce and release mucus. (The mucus traps dust and microorganisms, which are then swept out of the airway by the cilia.)Exam Tips 4. Lungs and pleura:(a) The lungs are divided into lobes (three on the right, two on the left), and each lobe is subdivided into lobules. Each lobule contains a bronchiole and its associated alveoli.Biology Semester 2 STPM Chapter 8 Geseous Exchange Concept MapProvides an overall view of the concepts learnt in the chapterQuick CheckProvides short question for students to test their understanding of the concepts learnt in the subtopicsSTPM Scheme of AssessmentLatest STPM Scheme of Assessment starting 2026Learning OutcomesA list of subtopics that students will learn in each chapterExam TipsProvides helpful tips for students in answering exam questions

ivSemester of StudyCode and Paper Name Theme / Title Type of Test Mark (Weighting) Duration AdministrationSemester1964/1Biology Paper 1Molecules ofLife andMetabolismWritten testSection A20 multiple-choicequestions.Section B2 structuredquestions.Section C 2 essay questions.60(26.67%)2014261.5 hours Central assessmentSemester2964/2Biology Paper 2Physiology Written testSection A20 multiple-choicequestions.Section B2 structuredquestions.Section C 2 essay questions.60(26.67%)2014261.5 hours Central assessmentSTPM Scheme of Assessment

viChapter8 Gas Exchange 18.1 Gaseous Exchange in Humans 28.2 Breathing Cycle 138.3 Gaseous Exchange in Plants 17STPM Practice 8 28e-Quiz 1 QR code 31Chapter Transport in Animals and 9 Plants 329.1 Transport System in Humans 339.2 Transport System in Vascular Plants 67STPM Practice 9 80 e-Quiz 2 QR code 83Chapter 84 1010.1 Nervous System 8510.2 Hormones 129STPM Practice 10 162e-Quiz 3 QR code 165Chapter Reproduction, Development 11 and Growth 16611.1 Sexual Reproduction in Humans 16711.2 Reproduction in Flowering Plants 19411.3 Seed Germination 20711.4 Growth 212STPM Practice 11 230e-Quiz 4 QR code 234ChapterHomeostasis 1212.1 Importance of Homeostasis 12.2 Liver 12.3 Osmoregulation in Mammals 12.4 Osmoregulation in Plants STPM Practice 12 e-Quiz 5 QR code ChapterImmunity 1313.1 Immune System 13.2 Development of Immunity 13.3 Concept of Self and Non-self 13.4 Immune Disorder STPM Practice 13 e-Quiz 6 QR code ChapterInfectious Diseases 1414.1 Infectious Disease 14.2 Dengue 14.3 Cholera 14.4 Tuberculosis (TB) 14.5 Malaria 14.6 Hand-Foot-and-Mouth Disease (HFMD) STPM Practice 14 e-Quiz 7 QR code STPM Model Paper (964/2) Answers Glossary CONTENTS

Concept Map11REPRODUCTION, DEVELOPMENT AND GROWTHCHAPTERReproduction, Development and GrowthSexual Reproduction in HumansSexual Reproduction in Flowering PlantsSpermatogenesis and oogenesisHormones CleavageOrganogenesisGastrulationPatterns of growthEmbryonic developmentEmbryonic development in seedFertilisation and implantationAbsolute growth curveMenstrual cycle, prenancy, parturitionAbsolute growth rate curveRelative growth rate curveRoles of placenta in foetal developmentSeed GerminationGrowthTypes of growth curvesTissue culture and grafting in asexual reproductionDouble fertilisationImbibition of waterMobilisation of nutrientsExternal factors affecting germinationAllometric growthLimited growth in humansUnlimited growth in perennial plants166

167Chapter11.1 Sexual Reproduction in Humans1. Human sexual reproduction is a biological process involving the fusion of male and female gametes (sperm and ovum) to form a zygote, which develops into an embryo and ultimately a fully formed human. 2. The process consists of several key stages:(a) Gametogenesis (formation of gametes)• Gametogenesis is the process of producing sperm and ovathrough meiosis.➢ Spermatogenesis (male gamete formation)➢ Oogenesis (female gamete formation)(b) Fertilisation• Fertilisation occurs in the fallopian tube (oviduct).(c) Embryonic development• The zygote undergoes mitotic divisions (cleavage) hiletravelling down the fallopian tube.• By day 5, the zygote becomes a hollow ball of cells called blastocyst.• Around day 6 – 7, the blastocyst undergoes implantation embeds into the endometrium of the uterus.(d) Gestation (pregnancy)• The embryo develops into a foetus approximately months (40 weeks).(e) Parturition (birth or childbirth)Spermatogenesis and Oogenesis, and Passage of Sperms to OviductMale Reproduction System1. The male reproductive system includes the  external genitals (penis, testes and scrotum) and internal parts, including the prostate gland, vas deferens, bulbourethral glands, urethra, epididymis and seminal vesicles.2. Functions of the male reproductive system(a) Produces and transports sperm (male gametes).(b) Delivers sperm into the female reproductive tract.(c) Secretes male sex hormones (testosterone) to regulate reproductive functions.• Outline spermatogenesisand oogenesis, andpassage of sperms tooviduct • Explain the processof fertilisation andimplantation• Explain the roles ofhormones in menstrualcycle, pregnancy andparturition• Explain cleavage,gastrulation, andorganogenesis inembryonic development• Explain the roles ofplacenta in foetaldevelopmentLearning OutcomeBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

168Table 11.1 Main structures and their rolesStructure FunctionTestes Produce sperm via spermatogenesis in the seminiferous tubules and secrete testosterone.Epididymis Stores and matures sperm, allowing them to gain motility.Vas deferens Transports sperm from the epididymis to the urethra.Seminal vesicles Produce seminal fluid rich in fructose, providing energy for sperm.Prostate gland Secretes alkaline fluid to neutralise vaginal acidity and enhance sperm survival.Bulbourethral glands Release lubricating mucus into the urethra before ejaculation.Urethra Carries semen (sperm + seminal fluid) out of the penis.Penis Delivers sperm into the female reproductive tract.Vas deferensRectumBulbourethral glandEjaculatory ductGlans penisProstate glandSeminal vesicleBladderUrethraEpididymisTestisFigure 11.1 Male reproductive organs3. Testis and seminiferous tubule (location of spermatogenesis)(a) A testis (plural: testes) is the primary male reproductive organ responsible for spermatogenesis (sperm production) and testosterone secretion. It is located in the scrotum and consists of the following key structures:• Seminiferous tubules: Coiled tubules where spermatogenesisoccurs.• Sertoli cells: Support and nourish developing sperm cells.• Leydig cells: Located between seminiferous tubules, responsible for testosterone secretion.• Epididymis: Stores and matures sperm before ejaculation.• Vas deferens: Transports mature sperm from the epididymis to the urethra.(b) A radial arrangement of meiotic cells within the seminiferous tubule indicates that spermatogenesis follows a structured process, moving from the outer layer towards the lumen (central cavity) of the tubule.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

169Chapter4. Role of Sertoli cells in spermatogenesis(a) Sertoli cells are nongerminal cells found in the walls of seminiferous tubules.(b) They regulate spermatogenesis by responding to hormonal signals such as testosterone and follicle-stimulating hormone (FSH).(c) They provide nourishment, structural support and regulation for developing sperm cells.(d) They also help in removing excess cytoplasm from developing sperm and regulate the microenvironment necessary for spermatogenesis.5. Spermatogenesis Spermatogenesis is the process of sperm cell development that occurs in the seminiferous tubules of the testes. It involves a series of mitotic and meiotic divisions, as well as cellular differentiation, to produce mature spermatozoa. The key stages of spermatogenesis are:(a) Mitosis of germ cells (spermatogonia)• Diploid germ cells, which are known as spermatogonia,undergo mitotic division to produce more spermatogonia.• Some spermatogonia remain as stem cells while spermatogonia differentiate into primary spermatocytes (2n), which will undergo meiosis.• In the growth phase, primary spermatocytes enlarge prepare for meiosis.(b) Meiosis I• Each primary meiotic division (Meiosis I).• This results in two secondary spermatocytes, each of which is haploid (having half the chromosome number).(c) Meiosis II• Secondary spermatocytes undergo the second meiotic di(Meiosis II).• One spermatocyte produces four haploid spermatids.(d) Spermiogenesis (maturation into spermatozoa)• Spermatids undergo morphological changes, including:➢ Development of a flagellum for motility➢ Condensation of the nucleus➢ Formation of the acrosome, which contains enzymes to penetrate the egg➢ Shedding of excess cytoplasm➢ High number of mitochondria in the midpiece• Mature sperm cells (spermatozoa) are released into the lumen of the seminiferous tubule. • The spermatozoa move from the seminiferous tubules epididymis, where they undergo further maturation and gain motility.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

170• They are stored in the epididymis until ejaculation, wthey travel through the vas deferens (plural: vasa deferentia) and urethra. • The vas deferens extends from the scrotum into the pcavity.• Each vas deferens empties into a short ejaculatory duct, wpasses through the prostate gland and then channels into the urethra. • The urethra, which at different times conducts urine semen, passes through the penis to the outside of the body.Meiosis IGrowth phaseMeiosis IIDifferentiationPrimordialgerm cell, 2nSpermatogonium,2nPrimaryspermatocyte, 2nSecondaryspermatocyte, nMitosis Spermatozoa, nSpermatid, nSertoli cellsSeminiferoustubulesEpididymis Vas deferensTestisFigure 11.2 The process of spermatogenesis. Male germ cells proliferate by mitosis, then produce diploid spermatogonia. Spermatogonia continue to divide by mitosis, producing a steady supply of spermatocytes that divide meiotically to produce haploid spermatids, which differentiate into haploid sperm.6. Hormonal regulation of spermatogenesis(a) Gonadotropin-releasing hormone (GnRH): Stimulates the pituitary gland.(b) Follicle-stimulating hormone (FSH): Activates Sertoli cells the testes to support sperm production.(c) Luteinising hormone (LH): Stimulates Leydig cells to produce testosterone.(d) Testosterone: Essential for spermatogenesis and male secondary sexual characteristics.7. Characteristics of spermatozoa(a) Head: Contains nucleus (genetic material) and acrosome (enzymes for fertilisation).(b) Midpiece: Packed with mitochondria for ATP production.(c) Tail (flagellum): Enables motility for swimming toward the ovum.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

171ChapterMidpieceTailAcrosomeHeadNucleusCentriolesMitochondriaFlagellumTail sheathFigure 11.3 Structure of a spermatozoa8. The sequence of spermatogenesis is as follows. Spermatogonium (diploid) ➞ primary spermatocyte (diploid) ➞ two secondary spermatocytes (haploid) ➞ four spermatids (haploid) ➞four mature sperm (haploid)What is Sperm?VIDEOTable 11.2 Summary of spermatogenesisStage Process ResultMultiplication Spermatogonia undergo mitosis Primary spermatocytes (2n)Growth Primary spermatocytes enlarge Ready for meiosisMeiosis I Primary spermatocytes → secondary spermatocytesTwo haploid (n) secondary spermatocytesMeiosis II Secondary spermatocytes → spermatids Four haploid (n) spermatidsSpermiogenesis Spermatids mature into sperm Functional spermatozoa9. How do accessory glands nourish sperm? These secretions combine with sperm to form semen, which is essential for sperm survival, transport and fertilisation. Accessory Gland Main Secretions FunctionSeminal vesiclesSecretes about 60 – 70% of semen volume.It contains fructose, prostaglandins, fibrinogen andalkaline mucus.Provides energy, promotes uterine contractions, neutralises vaginal acidity to ensure sperm survivalProstate glandProduces 30% of the semen volumeCitrate, prostate-specific antigen(PSA), enzymes, zincProvides nutrients, liquefies semen afterejaculation, stabilises sperm membraneBulbourethral (Cowper’s) glandsMucus, an alkaline fluid Lubricates the urethra, neutralises acidic urine in the urethra to prevent sperm damage and facilitates sperm movementBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

172Female Reproductive System1. The female reproductive system consists of  internal and external organs. 2. It creates hormones and is responsible for fertility, menstruation and sexual intercourse. 3. Hormones secreted by the hypothalamus, pituitary gland and ovaries interact to regulate and coordinate these processes.4. Main functions of the female reproductive system:(a) Produces ova (eggs) through oogenesis.(b) Receives sperm during sexual intercourse.(c) Supports fertilisation and implantation.(d) Nourishes the developing embryo and foetus.(e) Gives birth and produces milk for the newborn.Fallopian tubeOvarian ligamentMuscular wallof uterusOvaryUterusCervixVaginaFigure 11.4 Female reproductive organsTable 11.3 Main structures and roles in female reproductionStructure Function Ovaries produce ova (eggs) and secrete oestrogen and progesterone.Fallopian tubes (oviducts)Fallopian tubes transport the ova to the uterus. It is also the site of fertilisation.Uterus The uterus provides a suitable environment for embryo implantation and foetal development.Endometrium The endometrium is the inner lining of the uterus. It thickens for implantation and sheds during menstruation if no pregnancy occurs.Cervix The cervix is a narrow opening to the uterus; it secretes mucus that changes consistency during ovulation.Vagina Receives the penis and sperm during sexual intercourse. Vagina also serves as the birth canal.Mammary glands (breasts)Mammary glands produce breast milk for newborn nourishment (lactation).Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

173Chapter5. Ovary (Location of oogenesis)(a) The ovary is a paired female reproductive organ responsible for producing ova and secreting hormones essential for the menstrual cycle and pregnancy. It is part of the female reproductive system and plays a vital role in oogenesis, ovulation and hormone regulation.(b) The two ovaries are located on either side of the uterus, within the pelvic cavity.(c) Each ovary is about 3 cm long, 1.5 cm wide and 1 cm thick.(d) It is surrounded by a thin layer of connective tissue (tunica albuginea) and covered by a germinal epithelium.(e) Functions of the ovary(i) Oogenesis (egg production)• Ovaries produce haploid ova through oogenesis.• This process begins before birth, pauses at different stages, and completes only upon fertilisation.(ii) Ovulation• Each month, an ovary releases a mature secondaryoocyte into the fallopian tube.• This process is triggered by a surge in luteinising hormone (LH).(iii) Hormone secretion• Ovaries produce key hormones that regulate menstrual cycle, pregnancy and secondary sexual characteristics:➢ Oestrogen: Oestrogen promotes the development of female secondary sexual characteristics (breast development, widening of hips). It also stimulates the growth of the endometrium (uterine lining) during the menstrual cycle. ➢ Progesterone maintains the uterine lining for implantation of the fertilised egg. It also inhibits uterine contractions during early pregnancy.➢ Inhibin regulates FSH secretion to control follicle development.➢ Relaxin (During pregnancy) softens the cervixrelaxes the pelvic ligaments to prepare for childbirth.6. OogenesisOogenesis occurs in the follicles of an ovary.(a) Step 1: Formation of oogonia (before birth)• Oogenesis begins in the female foetus with diploid germ cells called oogonia (2n).• These cells divide by mitosis to increase in number.• By the third month of foetal development, oogonia differentiate into primary oocytes (2n).Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

174(b) Step 2: Meiosis I (arrest at prophase I)• Primary oocytes enter meiosis I, but the process halts at prophase I.• At birth, a female has about 1 – 2 million primary oocytesall arrested in prophase I of meiosis.• These primary oocytes are surrounded by follicle forming primordial follicles.(c) Step 3: Puberty and completion of meiosis I• At puberty, follicle-stimulating hormone (FSH) triggers primary oocyte per menstrual cycle to resume meiosis.• Meiosis I is completed, forming:➢ One large secondary oocyte (n).➢ One small polar body (which degenerates).(d) Step 4: Meiosis II (arrest at metaphase II, completed upon fertilisation)• The secondary oocyte begins meiosis II, but pauses metaphase II.• Ovulation occurs, releasing the secondary oocyte into fallopian tube.• If fertilisation occurs, Meiosis II completes, forming:➢ One mature ovum (n).➢ A second polar body (which also degenerates).2n2nn nnPrimordialgerm cell, 2nMitosis Growth phasePrimaryoocyte, 2nOogonium, 2nSecondaryoocyte, nOvum, n Second polarbody, nMeiosis IIFirst polarbody, nBeforebirthReachpubertyMeiosis IMaturity phaseOogenesisnFigure 11.5 Oogenesis. Oogonia of human females cease division in the embryo. Primary oocytes remain arrested during Prophase I of meiosis until ovulation and fertilisation. Every oocyte produces one haploid ootid and three polar bodies.7. Characteristics of the ovum The ovum (plural: ova) has specialised structural featuresfertilisation and early embryonic development:(a) Large cell size• The ovum is the largest human cell (~100 µm in diameter).• It contains cytoplasm rich in nutrients (yolk) to support early embryonic growth. and MenstrualVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

175ChapterOVARYGraafianfolliclesVesicularfollicles GrowingfolliclesCorpusluteumRupturedfollicleLigament (holdsovary in place inthe abdomen)Primary oocyte (2n)After roughly one week,usually only one primaryoocyte keeps developing.Just before ovulation, itundergoes meiosis to forma secondary oocyte (n).3The follicle ruptures atovulation, releasing theoocyte.4Following ovulation, the remainingfollicular cells transform into the corpus luteum, responsible forproducing progesterone and oestrogen.5In the absence of pregnancy,the corpus luteum undergoesdegeneration.6The surrounding follicular cells providenourishment to thedeveloping oocyte andsecrete oestrogen.2Approximately once a month between puberty and menopause, 6 - 12 primaryoocytes begin to mature. Eachprimary oocyte, together withits surrounding cells, forms a follicle.1 ovarian cycle. The ovarian cycle progresses from the development of a follicle to ovulation and then to growth and finally to degeneration of the corpus luteum.(b) Haploid nucleus (n)• The mature ovum carries 23 chromosomes (haploid) to combine with sperm during fertilisation.(c) Zona pellucida• A glycoprotein layer surrounding the ovum that plays a role in sperm binding and preventing polyspermy (entry of multiple sperm).(d) Corona radiata• A layer of follicular cells surrounding the ovum that provides protection and nourishment.(e) Cortical granules:• Cortical granules are secretory vesicles poised at the cortex ofan ovum. The exocytosis from cortical granules (after entryby a sperm) can prevent polyspermy. Polyspermy means fertilisation of an ovum by more than one sperm.• Cortical granules contain enzymes that modify and therebyharden the zona pellucida of the ovum.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

176Chapter11CoronaradiataCytoplasmNucleusNucleolusPlasmamembraneZonapellucidaFigure 11.7 Structure of human oocyteTable 11.4 Comparison of spermatogenesis and oogenesisFeature Spermatogenesis OogenesisDefinition Process of sperm formation in the testesProcess of ovum formation in the ovariesLocation Seminiferous tubules of the testes Ovarian follicles of the ovariesTiming of initiation Begins at puberty and continuesthroughout lifeBegins during foetal development pauses until pubertyType of germ cell Spermatogonia (diploid) Oogonia (diploid)Growth phase Spermatogonia grow into primary spermatocytesOogonia grow into primary oocytes before birthMeiotic division Continuous and sequential process Arrested at stages until fertilisationCompletion of meiosisCompleted before sperm is released Completed only if fertilisation occursCytokinesis Equal division of cytoplasm in both meiotic divisionsUnequal division, producing one ovum and three polar bodiesNumber of gametes producedFour haploid spermatozoa from one primary spermatocyteOne ovum and three polar bodies from one primary oocyteDuration ~74 days for full sperm maturation,which occurs continuouslyTakes years; oocytes remain arrested for decadesLifespan of gamete Sperm survive 3 – 5 days in thefemale reproductive tractOvum survives 24 hours afterovulationHormonal control FSH and testosterone regulate spermatogenesisFSH and LH regulate oogenesis; oestrogen and progesterone play rolesMaturation process Spermatids differentiate into motile spermatozoaOocyte is released at ovulation and matures fully upon fertilisationQuick Check 11. How do sperm travel to the oviduct?2. What adaptations in the female reproductive tract facilitate the passage of sperm to the oviduct?Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

177ChapterRoles of Hormones in the Menstrual CycleTable 11.5 Hormonal interactions, feedback mechanisms and their impact on the menstrual cycleStage Days Main Events Hormonal Changes Feedback MechanismMenstrual Phase 1 – 5Shedding of the endometrial lining (menstruation). Progesterone & Oestrogen (corpus luteum degenerates) → Removes negative feedback on GnRH → FSH secretion.Negative feedback removed → FSH rises, stimulating new follicle growth.Follicular Phase (Early & Mid-Follicular Phase)1 – 13Follicle development in the ovary, the endometrial lining thickens. FSH → stimulates follicle growth; follicles secrete oestrogen.Negative feedback: Low oestrogen inhibits GnRH, FSH and LH secretion (prevents multiple follicles from maturing).Late Follicular Phase (PreOvulation)~12 – 13Dominant follicle matures, high oestrogen secretion. Oestrogen (>200 pg/mL for ~48 hours) stimulates the anterior pituitary.Positive feedback: High oestrogen stimulates LH surge → Triggers ovulation.Ovulation ~14LH surge triggers follicle rupture and egg release.LH surge, FSH rises slightly, oestrogen peaks then declines.Positive feedback:Oestrogen causes LH surge, leading to ovulation.Luteal Phase 15 – 28Corpus luteum forms, secretes progesterone & oestrogen. The uterus prepares for implantation. Progesterone & oestrogen from corpus luteum → suppresses GnRH, FSH, LH.Negative feedback:Progesterone & oestrogen inhibit GnRH, FSH and LH → Prevents new follicle development.End of Luteal Phase (If No Fertilisation)~26 – 28Corpus luteum degenerates, and hormone levels drop. Progesterone & Oestrogen → GnRH inhibition removed → FSH rises, triggering a new cycle.Negative feedback removed: FSH rises to stimulate new follicle growth.If Fertilisation Occurs 1The embryo implants in the uterus 6 - 10 daysafter fertilisation,corpus luteum is maintained.Embryo secretes hCG → maintains corpus luteum → progesterone & oestrogen remain high.Negative feedback maintained: High progesterone & oestrogen inhibit FSH & LH, preventing further ovulation.Quick Check 21. How do hormonal feedback mechanisms regulate the menstrual cycle?Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

178ChapterHypothalamusPituitaryglandOestrogensOvariesGnRHFSH and LH++––BloodstreamOestrogens inhibitoversecretion ofgonadotropins(except duringovulation)Release intobloodstreamStimulationInhibitionFSH induces folliculardevelopment and meioticprogression of primaryoocytes to secondaryoocytes.LH triggers ovulation andfacilitates the luteinisationof the ruptured follicle intothe corpus luteum.Promotes breast developmentIncreases skin vascularisationEnlarges accessory reproductiveorgansThickens the uterine endometriumEnhances fat deposition in breasts,thighs and buttocksFigure 11.8 Hormonal regulation of the menstrual cycle, focusing on the interactions between the hypothalamus, anterior pituitary and ovary across different phases. It highlights the role of gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), luteinising hormone (LH), oestrogen and progesterone, in coordinating ovulation and endometrial changes and female characteristic development. Oestrogens inhibit LH and FSH during most of the cycle, except during ovulation. Follicle recruitmentFollicular MaturationHormone levels Ovarian eventsOvarian CycleHormone levels Uterine CyclePrimaryfolliclesLHFSHOestrogenProgesteroneDominant follicle Graafian follicleCorpus luteumCycle Cycle Cycle CycleFollicular phase Ovulation Luteal phase0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 28Cycle Cycle Cycle Cycle Cycle CycleSecondaryfollicleDegenerating ofcorpus luteumPhases ofMenstrual CycleVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

179ChapterOvulationDaysMenstruation Proliferation Secretory Menstruation0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 28Hormone levels EndometriumUterine CycleOestrogenProgesteroneDominant follicle Graafian follicleFollicular phase Ovulation Luteal phase0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 28Figure 11.9 Follicular maturation, ovarian cycle and uterine cycleProcess of Fertilisation and ImplantationFertilisation1. Fertilisation is the process by which a haploid sperm cell (n) fuses with a haploid ovum (n) to form a diploid zygote (2n). This marks the beginning of embryonic development and occurs in the ampullaof the fallopian tube in humans. 2. The process consists of several steps that ensure the successful fusion of sperm and ovum.(a) Step 1: Sperm transport and capacitation• During ejaculation, millions of sperm are deposited in thevagina.• The sperm travel through the cervix, uterus, and into thefallopian tube, assisted by:➢ Muscular contractions of the female reproductive tract➢ Sperm motility (flagellar movement)• Only a few hundred sperm reach the ampulla, the fertilisationsite.Capacitation• Before fertilisation, sperm undergo capacitation, a biochemical process that occurs in the female reproductive tract.• Changes during capacitation:➢ The sperm plasma membrane is altered, increasing its permeability to calcium ions.➢ Sperm become hyperactivated, improving motility.➢ The acrosome membrane is primed for the acrosomereaction.FertilisationVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

(b) Step 2: Sperm penetration of the corona radiata• The ovum is surrounded by a protective layer of follicularcells called corona radiata.• Capacitated sperm release hyaluronidase enzyme, which dissolves the intercellular matrix of the corona radiata.• This allows sperm to reach the zona pellucida, the next protective barrier of the ovum.(c) Step 3: Sperm binding to the zona pellucida• The zona pellucida is a glycoprotein layer surrounding ovum. It contains specific glycoproteins that act as sperm receptors.• When a sperm binds to a receptor, it triggers the acrosomereaction.• This is a crucial step in fertilisation.(d) Step 4: Acrosome reaction and zona pellucida penetration• The acrosome, a cap-like structure on the sperm head, releases hydrolytic enzymes.• Hydrolytic enzymes in the acrosome are divided into types:➢ Acrosin (digests zona pellucida proteins)➢ Hyaluronidase (further degrades corona radiata remnants)• These enzymes hydrolyse the zona pellucida, creating a passage for sperm and allowing sperm to reach the perivitelline space.(e) Step 5: Sperm-ovum membrane fusion• Once a sperm reaches the perivitelline space (the space between the zona pellucida and ovum plasma membrane), it binds to integrin receptors on the ovum membrane.• The sperm and ovum membranes fuse, leading to:➢ entry of sperm nucleus into the ovum cytoplasm.➢ entry of sperm centriole, which will contribute to the first mitotic spindle formation.(f) Step 6: Cortical reaction (polyspermy prevention)• To prevent polyspermy (entry of multiple sperm), the ovum undergoes the cortical reaction:➢ Cortical granules in the ovum release enzymes into the perivitelline space.➢ These enzymes modify the glycoproteins of the zona pellucida.➢ The zona pellucida hardens, preventing additional sperm from binding.➢ Therefore, only one sperm is allowed to fertilise the ovum.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

181Chapter(g) Step 7: Oocyte activation and completion of meiosis II• Before fertilisation, the ovum was arrested in metaphase II of meiosis.• Sperm entry triggers oocyte activation, causing the ovum to complete meiosis II, producing:➢ a haploid female pronucleus (n).➢ a second polar body, which degenerates.(h) Step 8: Formation of male and female pronuclei• The sperm nucleus decondenses, forming the male pronucleus (n).• The female pronucleus (n) is already present.• Microtubules guide the two pronuclei toward each other.(i) Step 9: Zygote formation (syngamy)• The male and female pronuclei fuse, restoring the diploidchromosome number (2n = 46).• This fusion process is called syngamy.• A zygote is formed, marking the beginning of embryonic development.Ovum plasmamembrane Ovum cytoplasmCorticalgranuleCortical granulecontentFused plasmamembraneAcrosome reactionProtein receptorOnly one sperm can fuse withone ovumFigure 11.10 The process of fertilisation in humans3. Significance of the fertilisation(a) Ensures species-specific fertilisation through receptor-mediatedsperm binding.(b) Avoids polyspermy, which would result in an abnormalchromosome number.(c) Leads to zygote formation, marking the start of embryonic development.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

Day 4Day 5Day 6Day 3Day 2Fertilised Day 1egg (zygote)Nucleussperm (n)Nucleusovum (n)FolliclecellSecondary oocyteOvaryImplantation ofthe blastocystOvulationFertilisationFigure 11.11 The journey of a fertilised ovum from ovulation to implantation in the uterusImplantation1. Implantation is the process by which the blastocyst attaches to and embeds itself into the endometrial lining of the uterus.2. This process occurs approximately 6 – 10 days after fertilisation3. It is essential for establishing pregnancy and ensuring proper nutrient exchange between the mother and embryo.4. Stages of implantation (Figure 11.12) (a) Cleavage and morula formation: The zygote undergoes cleavage divisions, forming a morula cells) by Day 3.(b) Blastocyst formation: The morula develops into a blastocyst (~200 – 300 cells) b 5. The blastocyst consists of:• Trophoblast: The trophoblast is the outer layer of the blastocyst. It plays a crucial role in implantation and later develops into the chorion (chorion is the foetal part of the placenta). • Inner cell mass (ICM): The inner cell mass is a cluster of cells located on one side of the blastocyst. It gives rise to the embryo proper and eventually forms all the tissues and organs of the developing foetus. It also contributes to some extraembryonic structures, such as parts of the amnion and yolk sac.• Blastocoel: The blastocoel is a fluid-filled cavity within blastocyst. It allows cell migration during gastrulation and helps establish the polarity of the embryo.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

183Chapter(c) Zona pellucida degeneration:The zona pellucida degenerates, allowing the blastocyst to implant into the uterine wall later.(d) Movement to the uterus:The blastocyst moves from the fallopian tube to the uterus via ciliary action and muscle contractions.(e) Implantation (Day 6 – 9): Implantation occurs around Day 6 to 9 after fertilisation, whenthe blastocyst becomes embedded into the endometrium of the uterus. • The trophoblast differentiates and releases enzymes to burrowinto the uterine lining. • Blood vessels grow around the implanted blastocyst, formingthe early placenta.(f) Formation of extraembryonic membranes (Day 10 – 11):Following implantation, the trophoblast continues to expand into the endometrium. Four new membranes, called extraembryonicmembranes, begin to form. These membranes arise from the embryo but enclose specialised structures located outside the embryo. As implantation is completed, gastrulation progresses.Gastrulation, the process by which the three germ layers (ectoderm, mesoderm and endoderm) form, begins around Day 13.5. The chorion, one of the extraembryonic membranes, forms from the trophoblast and plays a critical role in nutrient absorption. Initially, the chorion absorbs nutrients directly from the endometrium. Later, the chorion develops chorionic villi, which increase the surface area for nutrient and gas exchange between the maternal blood and the embryo.6. The role of the chorion in nutrient support is taken over by the placenta by the second month, where it delivers oxygen and nutrients to the foetus via the umbilical cord. It also removes carbon dioxide and other waste products from the foetus.Quick Check 31. How does the zona pellucida prevent polyspermy during fertilisation?2. Why is the timing of implantation critical for a successful pregnancy?Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

184Chapter11Roles of Hormones During PregnancyTable 11.6 Six key hormones which play their different roles during pregnancyHormone Source Functions Changes During PregnancyFeedback MechanismhCG (Human Chorionic Gonadotropin)Embryo (trophoblast), then placentaMaintains corpus luteum in early pregnancy → continues progesterone and oestrogen secretionRises sharply in early pregnancy, peaks at ~8 - 10 weeks, then declines as the placenta takes over progesterone productionNegative feedback:Inhibits FSH & LH to prevent ovulationProgesterone Initially corpus luteum, later the placentaMaintains endometrial lining, suppresses uterine contractions, inhibits immune response against the foetusIncreases throughout pregnancy, highest in the third trimesterNegative feedback:Inhibits GnRH, preventing menstrual cyclesOestrogen Placenta Stimulates uterine growth, increases blood flow, enhances myometrium excitabilityGradually increases, peaks near termNegative feedback:Suppresses FSH & LH; Positive feedback near term (prepares for parturition)hPL (Human Placental Lactogen)Placenta Increases maternal insulin resistance, promotes foetal glucose supply, stimulates mammary gland developmentRises steadily throughout pregnancyNo direct feedback on reproductive cycle, but its effects on maternal metabolism help regulate glucose homeostasis.Prolactin Anteriorpituitary glandStimulates milk production (lactogenesis)Increaseprogressively throughout pregnancydue to rising oestrogen levels. Progesterone inhibits lactationdespite high prolactin levelsNegative feedback on GnRH – High prolactin suppresses GnRH, reducing FSH and LH, inhibiting ovulation, leading to temporary infertility.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

185ChapterRelaxin Corpus luteum and placentaRelaxes pelvic ligaments, softens cervix,inhibits uterine contractions in early pregnancyIncreases gradually, highest in late pregnancyWorks with progesterone to maintain pregnancyOxytocin Maternal and foetal posterior pituitaryMinimal role in pregnancy, but increases uterine receptor sensitivity for labourLow during pregnancy, receptors increase near termPositive feedback during parturitionCharacteristics of female hormones during pregnancy and parturition:• During pregnancy:Progesterone and oestrogen dominate, preventing contractions and maintaining the endometrium. hCG maintains the corpus luteum early on.• During parturition:Oestrogen, oxytocin and prostaglandins surge, driving uterine contractions via positive feedback loops.Cleavage, Gastrulation and Organogenesis in Embryonic DevelopmentEmbryonic development occurs in three sequential stages: cleavage, gastrulation and organogenesis. These stages involve the transformation of a single-cell zygote into a complex multicellular structure with distinct tissue layers and developing organ systems.Cleavage (First Stage of Embryonic Development)1. Timeframe: First week after fertilisation2. Key events in cleavage:(a) Formation of the zygote:• After fertilisation, a diploid zygote forms, containing genetic material from both the sperm and the egg.• The zygote is totipotent, meaning it has the potential to develop into any cell type.(b) Rapid mitotic divisions (cleavage):• The zygote undergoes multiple rounds of mitotic cell division without increasing in overall size.• These divisions form progressively smaller cells calledblastomeres.(c) Formation of the morula:• By the third or fourth day, the embryo consists of a solid ball of 16 – 32 blastomeres, known as the morula.HormonalRegulation ofPregnancyVIDEOGastrulation andOrganogenesisVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

• The morula reaches the uterus and begins fluid accumulationinside the cell mass.(d) Formation of the blastocyst:• Around day 5, the morula develops into a hollow structurecalled the blastocyst.• The blastocyst has two key regions:➢ Inner cell mass (ICM): This group of cells will develop into the embryo proper.➢ Trophoblast: This outer layer contributes to the formation of the placenta.(e) Blastocyst implantation (Day 6 – 7):• The blastocyst attaches to the endometrial lining of the uterus.• The trophoblast cellssecrete enzymes that allow implantationinitiating pregnancy.3. Significance of cleavage:(a) Establishes a multicellular structure from a single-celled zygote.(b) Ensures equal distribution of genetic material to all cells.(c) Prepares the embryo for further differentiation.Gastrulation (Second Stage of Embryonic Development)1. Timeframe: Week 2 – 32. Key events in gastrulation:(a) Formation of the primitive streak:• A groove-like blastocyst’s surface.• It establishes the body’s bilateral symmetry a ac as a for cellular movement.(b) Formation of the three germ layers:• Ectoderm: Forms from cells that remain on the outer surface, and develops into the nervous system (brain, spinal cord, peripheral nerves), epidermis, hair, nails and sensory organs.• Mesoderm: Forms from migrating cells between the ectoderm and endoderm, and develops into the muscular system, circulatory system, bones, kidneys, gonads and connective tissues.• Endoderm: Forms from cells that migrate inward and line the primitive gut, and develops into the digestive tract, liver, pancreas, respiratory tract and endocrine glands.(c) Neural plate formation (beginning of neurulation):• The ectoderm thickens, forming the neural plate, which folds to form the neural tube.• This is the first step in nervous system development.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

187Chapter(d) Formation of the notochord:• The notochord develops from mesodermal cells and serves asa structural support for the early embryo.• It later contributes to the formation of the vertebral column.3. Significance of gastrulation:(a) Establishes the basic body plan of the embryo.(b) Creates the three primary germ layers, which give rise to all tissues and organs.(c) Sets up the anterior-posterior and dorsal-ventral axes of the body.CleavageThe zygote undergoes mitotic cell divisions called cleavage. Many stages of cleavage result in the formation of multicellular stage known as blastula. The blastula usually is a hollow ball of cells. (Only one cleavage stage (the eight-cell stage) is shown here)BlastocoelEndodermEctodermBlastoporeZygote Eight-cell stage BlastulaGastrula GastrulationCross sectionof blastulaCleavageBlastocoelGastrulation is the embryonic process in which one region folds inward to fill the blastocoel, forming the primary germ layers of the embryo. The development process from zygote to gastrulaOrganogenesis (Third Stage of Embryonic Development)1. Timeframe: Week 3 – 82. Key events in organogenesis:(a) Neurulation (formation of the nervous system):• The neural plate folds to form the neural tube (precursor to the brain and spinal cord).• Neural crest cells migrate to form peripheral nerves, facialcartilage and pigment cells.(b) Development of the circulatory system:• The heart tube begins to beat by week 4, initiating blood circulation.• The aorta and major blood vessels start forming.(c) Formation of somites (segmented body structure):• Mesodermal cells form somites, which give rise to vertebrae, skeletal muscles and dermis.(d) Development of the digestive system:• The endoderm forms the primitive gut tube.• The liver, pancreas and intestines begin to take shape.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

188(e) Limb bud formation:• Small protrusions appear, which later develop into the and legs.(f) Eye and ear development:• The optic vesicles (primitive eyes) and otic vesicles (inner ear structures) begin forming.(g) Formation of the urogenital system:• The kidneys, gonads and reproductive ducts develop from mesodermal structures.3. Significance of organogenesis:(a) Establishes the foundation for all major organ systems.(b) The embryo begins to resemble a human-like structure.(c) Marks the end of the embryonic stage, transitioning into the foetal stage by week 9.Table 11.7 Three stages in early embryonic developmentStage Period Major EventsCleavage Week 1 Rapid mitotic divisions, formation of the morula and blastocyst, implantation in the uterus.Gastrulation Week 2 – 3 Formation of the primitive streak, establishment of three germ layers (ectoderm, mesoderm, endoderm).Organogenesis Week 3 – 8 Formation of neural tube, heart, digestive organs, limbs and sensory structures.Quick Understanding of Embryonic Development1. Cleavage: Rapid cell division forms a multicellular blastocyst.2. Gastrulation: Formation of three germ layers that establish the body plan.3. Organogenesis: Development of vital organs and systems.By the end of the eighth week, the embryo is now termed a foetusmarking the transition to the foetal stage, where organs mature, and the body grows rapidly.Quick Check 41. How does gastrulation establish the foundation for organogenesis?2. Why is the timing of organogenesis critical for foetal development?Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

189Chapter(a)ChorionAmnionExtraembryoniccavityAllantoisUmbilicalcordYolksacEndometriumDevelopingplacentaMaternalblood vesselsAmnioticcavity(b) Umbilical cordMyometrium Umbilical arteriesUmbilical veinEmbryonicblood vesselsLacunaVilli(embryonicportion of placenta)Decidua basalis(Maternal portionof placenta)Maternal bloodvesselsFigure 11.13 (a) As the amnion develops, it surrounds the embryo with amniotic fluid within the amniotic sac. An umbilical cord also starts to develop from the embryo towards the placenta. It is a rope-like cord connecting the foetus to the placenta. (b) The umbilical cord contains two arteries and one vein. Key functions of the placenta include efficient exchange of gases (O2 and CO2) and nutrients between maternal and foetal blood; prevention of direct mixing of maternal and foetal blood; hormone secretion to maintain pregnancy; and removal of foetal metabolic wastes through maternal circulation.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

190Roles of the Placenta in Foetal DevelopmentThe placenta is a vital organ that forms during pregnancy to support the developing foetus. Its main functions include:1. Gas exchange(a) Allows oxygen to diffuse from the maternal blood to the foetal blood.(b) Facilitates carbon dioxide removal from foetal blood to maternal circulation.2. Nutrient supply(a) Transfers essential nutrients (such as glucose, amino acidsacids, vitamins and minerals) from the mother to the foetus.3. Waste removal(a) Eliminates foetal metabolic wastes (such as urea, uric acid andcreatinine) by passing them into maternal blood for excretion.4. Hormone production(a) Secretes hormones such as:• Human chorionic gonadotropin (hCG): Maintains the corpus luteum and progesterone production.• Progesterone: Maintains pregnancy by preventing uterine contractions.• Oestrogen: Stimulates uterine growth and breast development for lactation.5. Immune protection(a) Acts as a (b) Transfers maternal to the foetus.6. Prevention of blood mixing(a) Separates maternal and foetal blood to prevent immune rejection (as the foetus has different antigens from the mother).PlacentaVIDEOQuick Check 51. How does the placenta adapt to meet the changing needs of the growing foetus?2. What might happen if placental function is impaired during pregnancy?Roles of Hormones During Parturition (Childbirth)Parturition is the process of childbirth, which involves hormonal and physiological changes that lead to uterine contractions, cervical dilation and delivery of the baby. The regulation of parturition primarily involves oestrogen, progesterone, oxytocin and prostaglandins, controlled by a positive feedback mechanism.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

191Chapter1. Preparatory hormonal changes before labour(a) Changes in oestrogen and progesterone levels• During pregnancy, progesterone maintains the uterine and prevents contractions.• In the final trimester, oestrogen levels rise, while progesteronelevels decline.• Increased oestrogen levels:➢ Stimulate the formation of oxytocin receptors in the myometrium (uterine muscle).➢ Increase gap junctions between uterine smooth muscle cells for coordinated contractions.➢ Promote the release of prostaglandins, which help the cervix (cervical ripening).2. Initiation of labour(a) Role of foetal signals• The foetus plays an active role in triggering labour.• The foetal hypothalamus signals the foetal adrenal gto secrete cortisol, which helps in lung maturation and may contribute to labour initiation.(b) Myometrial activation and prostaglandin secretion• Oestrogen stimulates the release of prostaglandins from placenta.• Prostaglandins:➢ Induce cervical dilation and softening (cervical ripening).➢ Enhance uterine contractions by increasing calcium levels in the myometrial cells.3. Positive feedback loop of oxytocin during labour(a) Stretching of the cervix (Ferguson reflex)• As the foetus moves downward, the foetal head apressure on the cervix.• Stretch receptors in the cervix send nerve impulses t hypothalamus.(b) Oxytocin release from the posterior pituitary• The hypothalamus signals the posterior pituitary to roxytocin.• Oxytocin binds to oxytocin receptors on the myometrium,stimulating strong uterine contractions.• Increased contractions push the baby further down, increasingcervical stretch, which stimulates more oxytocin release.• This creates a positive feedback loop, amplifying ucontractions until birth occurs.4. Expulsion of the baby and placenta(a) Delivery of the baby• Strong, rhythmic uterine contractions push the baby throughthe cervical canal and vagina.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

192Chapter11• The baby is expelled, and the umbilical cord is clamped cut.(b) Expulsion of the placenta (afterbirth)• After the baby is born, the uterus continues contracting to oxytocin, helping detach and expel the placenta.• The blood vessels in the uterus constrict, preventing excessivebleeding.Table 11.8 Hormonal regulation during parturitionHormone Source Functions Changes Before/During Labour Feedback MechanismOestrogenPlacenta Increases oxytocin receptors in the uterus, stimulates gap junction formation in the myometriumHigh before labour, primes the uterus for contractionsPositive feedback:Increases oxytocin sensitivityOxytocinMaternal and foetal posterior pituitaryStimulates uterine contractions, promotes prostaglandin releaseRises at labour onset, peaks during deliveryPositive feedback:Contractions trigger more oxytocin releaseProstaglandinsUterus and placentaStimulates uterine contractions, cervical ripeningRises sharply before labourPositive feedback:Enhances contractions, further oxytocin releaseRelaxin Placenta and Softens cervix, relaxespelvic ligamentsPeaks before Helps prepare for childbirthCortisol (Foetal Hormone)Foetal adrenal glandsMatures foetal lungs, may contribute to labour initiationIncreases before birthPossible positive feedback with oestrogen0Plasma hormone concentrationMonths since start of last menstrual period Delivery1Human chorionicgonadotropin (hCG)OestrogenProgesterone2 3 4 5 6 7 8 9 10Figure 11.14 Three hormonal changes (hCG, oestrogen and progesterone) which regulate pregnancy maintenance and initiate labour at full termBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

193ChapterAn increase in theoestrogen-to-progesteroneratio occurs at the endof the third trimester.Increased contractilityof uterine muscleIncreased uterinecontractionsGrowth offoetusIncreased stretchof uterusIncreased pressure offoetal head on cervixThe nervous systemtransmits signals of uterinestretching to thehypothalamus.Increased oxytocinsecretionOxytocin intensifiesuterine contractionsduring labor and birththrough a positivefeedback mechanism. The hormonal regulation of labour and childbirth, specifically focusing on the positive feedback mechanism of oxytocin. Unlike negative feedback, positive feedback amplifies a process. Labour contractions increase in intensity and frequency due to oxytocin release. The cycle continues until delivery of the baby, at which point the feedback loop stops.Stages of Childbirth (Parturition) 1. Stage 1: Dilation of the cervix(a) The foetus is positioned head-down in preparation for birth.(b) The amniotic sac is intact, surrounding the baby.(c) The cervix begins to dilate (widen) due to uterine contractions.(d) The placenta remains attached to the uterine wall.2. Stage 2: Rupture of the amniotic sac(a) The amniotic sac ruptures, releasing amniotic fluid (“water breaking”).(b) This helps lubricate the birth canal for easier passage of the baby.(c) The foetus moves downward, exerting pressure on the cervix.3. Stage 3: Expulsion of the baby(a) Strong uterine contractions push the baby through the cervix and vagina.(b) The head emerges first (typically in a normal delivery).(c) The placenta is still attached to the uterus at this stage. VIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

194Chapter114. Stage 4: Delivery of the placenta(a) After the baby is born, the placenta and umbilical cord expelled from the uterus.(b) The uterus continues contracting to detach the placenta from the uterine wall.(c) This prevents excessive bleeding and marks the completion of childbirth.11.2 Reproduction In Flowering Plants1. Structure of a flower(a) A flower is the reproductive organ of angiosperms, specificallyadapted for sexual reproduction.• It consists of reproductive and non-reproductive structuresthat facilitate pollination and fertilisation.• Flowers are typically composed of four main whorls: spetals, stamens and carpels.(b) The calyx consists of sepals, which are usually green and function to protect the developing flower bud.(c) The corolla consists of petals, which are often brightly coloured,scented or nectar-producing to attract pollinators.StigmaCarpelStyleOvuleOvaryPeduncleSepalAll stamens = androeciumAll carpels = gynoeciumAll petals = corollaAll sepals = calyxPetalAntherFilamentFigure 11.16 A flower morphology of a flowering plant(d) The androecium is the male reproductive structure, consisting of stamens.Quick Check 61. What roles do oxytocin and prostaglandins play in parturition?• Explain doublefertilisation in sexualreproduction• Explain the embryonicdevelopment in seed formation of fruit• Describe tissue cultureand grafting in asexualreproductionLearning OutcomeBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

195Chapter• Each stamen comprises an anther, which contains pollen sacs where pollen grains develop, and a filament, which positions the anther for effective pollen dispersal.(e) The gynoecium is the female reproductive structure, consisting of one or more carpels (pistils).• Each carpel includes a stigma, which is sticky to trap pollen, a style, which connects the stigma to the ovary, and an ovarywhich contains ovules.• The ovules develop into seeds after fertilisation, while ovary often develops into a fruit.2. Development of pollen (microsporogenesis and microgametogenesis)(a) Microsporogenesis• Pollen development occurs inside the anther, within pollen sacs (microsporangia).• Each pollen sac contains diploid microsporocytes (microspore mother cells) that undergo meiosis.• Each microsporocyte produces four haploid microsporesarranged in a tetrad.• These microspores eventually separate and develop individual pollen grains.(b) Microgametogenesis• Each microspore undergoes mitotic division to form two cellsa large vegetative (tube) cell and a smaller generative • The vegetative cell forms the pollen tube during germination,while the generative cell later divides into two sperm cells.• A mature pollen grain consists of a tough exine (outer wall), a thin intine (inner wall), and cytoplasm containing the two cells.• When the pollen is mature, the anther splits open (dehiscence)to release the pollen grains.• The released pollen grains are dispersed by wind, water, pollinators to reach the stigma of a compatible flower.3. Development of the embryo sac (megasporogenesis and megagametogenesis)(a) Megasporogenesis• The embryo sac (female gametophyte) develops within ovule, which is located in the ovary.• Each ovule contains a diploid megaspore mother cell (megasporocyte) that undergoes meiosis.• This process produces four haploid megaspores, but only one survives, while the other three degenerate.(b) Megagametogenesis• The functional megaspore undergoes three rounds of mitoticdivision, producing eight haploid nuclei.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

196Chapter11AntherOvaryPollensacPollensacMicrosporocyte(2n)NucleusMicrosporocyte 4 microspores (n) Single microsporesTube cellGenerative cellPollen grain(immature male gametophyte)(a)AntherOvary(b)OvuleIntegumentMegasporocyte(2n)Megaspores (n)Disintegratingmegaspores Survivingmegaspore Eightnucleiwithincytoplasm Eggcell (n)Femalegametophyte (embryo sac)SynergidcellPolarnucleiAntipodal cellsFigure 11.17 The formation of male and female gametophytes in flowering plants, specifically pollen grain (male gametophyte) development and embryo sac (female gametophyte) development. These processes are part of sexual reproduction in flowering plants and occur within the anther (male organ) and ovule (female organ), respectively.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

197Chapter• These nuclei arrange themselves within the embryo sac asfollows:➢ Three antipodal cells are located at the opposite end of the sac.➢ Two polar nuclei are positioned in the central cell.➢ One egg cell is located at the micropylar end, ready for fertilisation.➢ Two synergids flank the egg cell and assist in guiding the pollen tube to the embryo sac.• At maturity, the embryo sac is fully developed and ready forfertilisation.Double Fertilisation in Sexual ReproductionThe double fertilisation process is a defining characteristic of angiosperms, distinguishing them from other plant groups such as gymnosperms, which lack this mechanism.Steps in the Double Fertilisation1. Step 1: Pollen tube formation(a) The pollen grain germinates on the stigma of the flower and forms a pollen tube that grows down through the style towards the ovule.(b) Inside the pollen tube, there are two haploid cells:• The tube cell, which directs the growth of the pollen tube.• The generative cell, which will later divide to form two sperm cells.2. Step 2: Generative cell division(a) The generative cell undergoes mitosis, producing two haploidsperm cells.(b) The pollen tube reaches the micropyle (an opening in the ovule) and enters the embryo sac.(c) One of the synergid cells degenerates upon the arrival of the pollen tube, helping guide the sperm cells into the embryo sac.3. Step 3: Release of sperm cells(a) The pollen tube ruptures, and the two sperm cells are released into the embryo sac.(b) These sperm cells are now ready to fertilise different cells within the embryo sac.4. Step 4: First fertilisation (zygote formation)(a) One sperm cell fuses with the egg cell, forming a diploid (2n) zygote.(b) The zygote will undergo mitotic divisions and develop into an embryo, which will become the next sporophyte generation.DoubleFertilisationVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

1985. Step 5: Second fertilisation (endosperm formation)(a) The other sperm cell fuses with the central cell, which contains two polar nuclei.(b) This fusion forms a triploid (3n) nucleus, which develops into the endosperm.(c) The endosperm provides nutrients for the developing embryoThree antipodalcellPolarnucleiCentralcellSynergidsEgg cellTube cellnucleusTube cellGenerativecellThe pollen tube containstwo haploid cells: the generative cell and the tube cell.1 The generative cell formstwo haploid sperm cellsvia mitosis, and onesynergid cell degenerateswhen the pollen tubearrives.2Both sperm cells arereleased from the pollentube.3 One sperm cell fertilise the egg cell, forming thezygote (2n). The other sperm cell fuses with thecentral cell, forming triploid (3n) nucleus.4 process of double fertilisation in flowering plants (angiosperms), a unique mechanism where two sperm cells fertilise different cells within the ovule. Significance of Double Fertilisation1. Efficient resource use – The endosperm develops only if fertilisationoccurs, ensuring that nutrient storage is not wasted on unfertilised ovules.2. Increased genetic variation – The fertilisation of both the egg the central cell allows for more genetic diversity.3. Nutrient storage – The triploid endosperm provides a rich source for the developing embryo, supporting seed germination.Quick Check 71. How does double fertilisation benefit flowering plants?2. What evolutionary advantages does double fertilisation provide?Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

199ChapterEmbryonic Development in Seed and Formation of FruitAfter double fertilisation in flowering plants, the zygote develops in anembryo, while the ovule matures into a seed. Simultaneously, the ovary develops into a fruit. The following sections provide a detailed description of these processes.Embryogenesis in Plants: From Fertilisation to Dormancy1. Definition and overview(a) Embryogenesis in plants is the process of embryo development that begins after fertilisation and continues until the seed tersdormancy. • During this stage, the basic body plan of the sporophyte isestablished. • This body plan includes fundamental structures such theshoot, root and primary meristems, which will later drive post-germination growth.(b) Even though embryogenesis lays the structural foundation, further development and organ formation occur only fterdormancy is broken, ensuring the embryo remains viable until environmental conditions are suitable for germination.2. Stages of embryogenesis The process of embryogenesis occurs in distinct stages, ensuring the proper development of a viable embryo within the seed:(a) Zygote formation and initial division• After fertilisation, the diploid zygote is theovule.• The zygote undergoes asymmetric cell division, resulting in:➢ A smaller apical cell that develops into the embryo.➢ A larger basal cell that forms the suspensor, hichfunctions in nutrient transport to the developing embryo.(b) Globular stage• The apical cell undergoes multiple rounds of mitotic division,forming a ball-like globular embryo.• During this stage, three primary meristems (dermal issue,ground tissue and vascular tissue) begin differentiating.(c) Heart-shaped stage (only in dicots)• In dicot plants such as red beans, two cotyledons begin toemerge, giving the embryo a heart shape.• The cotyledons serve as nutrient storage organs that willprovide energy during germination.• The radicle (future root) and plumule (future shoot) becomedistinguishable.(d) Torpedo stage• The embryo elongates, forming a torpedo shape.Embyro, Fruitand SeedVIDEOEmbryogenesisVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

200Chapter11• Cell differentiation becomes more defined, with visible and root structures.• Vascular tissues start forming, allowing future transport water and nutrients.(e) Mature embryo stage• The embryo fully develops and enters dormancy, ensurvival in adverse conditions.• In monocots (such as maize), the single cotyledon (scutellum)is distinct, and the embryo remains surrounded by the endosperm, which serves as the primary nutrient reserve.• In dicots (such as red bean), the two cotyledons dominate seed and store nutrients.ZygoteApicaldaughter cellHeartembryo stageTorpedoembryo stageMature seedEarlyembryoCotyledonprimordiumDevelopingshoot apicalmeristemDevelopingroot apicalBasal meristemdaughter cellSuspensorShoot apicalmeristemSeed coatCotyledonsRoot apicalmeristem Plant embryogenesis stages – the development from a zygote to a mature seed.3. Establishment of the sporophyte body plan During embryogenesis, the basic body structure of the sporophyte is formed, including:(a) Radicle – The embryonic root, which grows downward germination.(b) Plumule – The embryonic shoot, which grows upward to the stem and leaves.(c) Cotyledons – In dicots, they store nutrients; in monocots, assist in nutrient absorption from the endosperm.(d) Hypocotyl – The region between the root and cotyledons, aids in seedling emergence.(e) Epicotyl – The stem region above the cotyledons, developinginto the shoot system.(f) Primary meristems – The dermal, ground and vascular tdifferentiate to form the basic tissues of the plant.4. Dormancy and post-dormancy development(a) Once the embryo is fully developed, the seed enters dormancy, a phase where metabolic activities slow down, allowing the seed to withstand harsh environmental conditions. Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

201Chapter• Dormancy is broken when the seed encounters favourableconditions, such as water, oxygen and the right temperature.(b) After germination begins:• The radicle emerges first to anchor the plant and absorb • The plumule emerges later, developing into the shoot sys• The stored nutrients in cotyledons (dicots) or endosperm(monocots) provide energy for early seedling growth.(c) Although the basic body plan was established dembryogenesis, further growth and development continue germination, leading to the formation of leaves, flowers and reproductive structures in the mature plant.Formation of Fruit1. Development of the ovary into a fruit(a) The ovary of the flower develops into a fruit after fertilisation.(b) The ovule develops into a seed, while the ovary wall develops into the pericarp, which forms the fruit wall.2. Structure of a fruitA typical fruit consists of three layers:(a) Exocarp – The outermost layer, which forms the protective (b) Mesocarp – The middle layer, which may be fleshy (such mango) or dry (such as pea pod).(c) Endocarp – The innermost layer, which may be hard and (such as coconut) or soft and 3. Types of fruitsFruits are classified based on their development:(a) Simple fruits – Develop from a single ovary of one flower as mango, tomato).(b) Aggregate fruits – Develop from multiple ovaries of a flower (such as strawberry).(c) Multiple Fruits – Develop from the ovaries of multiple floin an inflorescence (such as pineapple).4. Role of fruits in seed dispersal(a) Fruits aid in seed dispersal through various mechanisms:• Wind dispersal – Light and winged fruits (such as dandelion).• Water dispersal – Floating fruits with air cavities (such coconut).• Animal dispersal – Fleshy, edible fruits (such as mcherry).• Explosive mechanism – Fruits that burst open forcefully(such as balsam).Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

202Chapter11StigmaCarpelOvaryOvuleStamenOvaryCarpel StyleStigmaStamenFlowers Ovary(in receptacle)Ovule SepalPetalStigmaStylePea flower Raspberry flower Pineapple inflorescene Apple flowerSeedCarpel(fruitlet) Remains of stigma andstyleOvaryWitheredstamenEach segmentdevelopsfrom the carpelof one flowerSeedReceptacleSepalsRemains ofstamens andstylesPea fruit Raspberry fruit Pineapple fruit Apple fruit(a) Simple fruit (b) Aggregate fruit (c) Multiple fruit (d) Accessory fruitFigure 11.20 Different types of fruits in flowering plants (pea, raspberry, pineapple and apple). Each represents a different fruit classification based on the floral structures that contribute to the fruit.Quick Check 81. How does the hormonal regulation of fruit formation ensure seed dispersal?2. Why is the development of a seed coat critical for seed survival?Tissue Culture and Grafting in Asexual ReproductionIntroduction to Asexual Reproduction in Plants1. Asexual reproduction in plants refers to the production of offspringwithout the fusion of gametes (no fertilisation). The resulting progeny are typically genetically identical (clones) to the parent plant. This mode of reproduction offers several advantages, such as rapid propagation, uniformity of traits and consistency in yield and quality. Vegetative reproduction can occur naturally (such as runners in strawberries, tubers in potatoes) or be facilitated byhumans, in which case it is termed vegetative propagation.2. Two common asexual propagation techniques in commercial horticulture and research are tissue culture and grafting. VegetationPropagationVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

203ChapterPlant Tissue Culture1. Definition and principle(a) Definition: Plant tissue culture (also known as micropropagation) is an in vitro technique in which smallplant cells, tissues or organs (explants) are grown on a nutrientmedium under sterile (aseptic) conditions.(b) The fundamental concept applied in plant tissue culture is plantcell totipotency. • A single living plant cell has the potential to develop intoa whole plant given the right environmental conditions of nutrients and an appropriate balance of growth regulators (such as auxins and cytokinins).(c) Tissue culture techniques are widely used in:• Micropropagation (mass cloning of plants)Micropropagation is a general term for the large-scale cloningof plants using tissue culture techniques.• Virus elimination (for example, meristem culture)Plant tissue culture is important in eliminating weakly pathogenic viruses from vegetatively propagated varieties. Although the presence of weak viruses may not be obvious,the yield or quality may be substantially reduced due to infection. • Breeding programs (such as anther / pollen culture for haploid plants) Anther or pollen culture is used to produce haploid plantsby culturing immature pollen grains (microspores) or entire anthers.• Genetic engineering (such as protoplast culture and fusion)Protoplasts are plant cells devoid of cell walls, obtained by enzymatic digestion (such as cellulase and pectinase). Protoplasts are isolated from leaf mesophyll or callus. They are cultured in a liquid or semi-solid medium with growth regulators to regenerate cell walls, form callus, and subsequently regenerate whole plants. Protoplast fusion can create somatic hybrids by combining protoplasts from different species (such as potato and tomato to form “pomato”).• Conservation of endangered species (in vitro gene banks)2. Key steps in tissue culture(a) Selection and preparation of explant• An explant (for example, leaf disc, shoot tip, meristem) isselected from a disease-free parent plant.• The explant is surface-sterilised (commonly using alcoholor sodium hypochlorite solutions) to remove microbial contaminants.Both tissue culture (micropropagation) and grafting are asexual plant reproduction methods, but they differ significantly in their application and complexity. Grafting joins parts of different plants, while tissue culture involves culturing plant cells or tissues in a sterile environment to create new plants. Tissue culture is commonly used for rapid propagation, disease-free plants, and genetic modification, whereas grafting is used to combine desirable characteristics of different plants.Exam TipsSteps in TissueCultureVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

(b) Initiation of culture• The sterile explant is placed on a nutrient agar mediumcontaining macronutrients, micronutrients, vitamins and plant growth regulators (such as auxins, cytokinins).• This medium supports callus formation (an unorganised of cells) or direct shoot induction, depending on the balance of hormones.(c) Callus induction and organogenesis• Callus induction: A high auxin-to-cytokinin ratio stimulates callus formation.• Organogenesis: By adjusting the hormone ratio, the differentiates into shoots or roots.• In some protocols, direct shoot formation (without a stage) can occur from meristematic tissue.(d) Shoot multiplication and rooting• The newly formed shoots are transferred to a fresh mediumwith a suitable balance of auxins to induce root formation.• Multiple cycles of shoot proliferation can be performed increase the number of plantlets (micropropagation).(e) Acclimatisation• The in vitro plantlets, once they have adequate sand roots, are transferred to a greenhouse or nursery for gradual adaptation to external conditions (humidity, light, temperature).• This process is critical to ensure a high survival rate when plantlets are moved to soil.3. Advantages of tissue culture(a) Rapid clonal propagation• A large number of genetically identical plantlets c produced in a short period.(b) Production of disease-free plants• Meristem culture can eliminate viruses and other pathogens,producing healthy planting material.• Meristem culture involves the excision and in vitro culture shoot apical meristems or axillary buds. These meristematic regions often remain virus-free even in infected pmaking this method useful for virus elimination.(c) Conservation of rare or endangered species• Tissue culture aids in the ex-situ conservation of species limited seeds or in threatened habitats.(d) Uniformity and quality control• All regenerated plantlets have identical traits, ensuringconsistency in yield and quality.4. Disadvantages of tissue culture(a) High cost and technical expertise• Requires sterile laboratory conditions, specialised equipmentand trained personnel.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

205Chapter(b) Risk of somaclonal variation• Prolonged callus phases may introduce genetic mutations,resulting in unexpected variations.(c) Contamination issues• Fungal or bacterial contamination can rapidly destroy cultures.Grafting1. Definition and principle(a) Grafting is a horticultural technique that joins tissues of two different plants to form a single plant.(b) The two main components are:• Scion – The aerial part or shoot system of the desired variety(providing the fruit or flower).• Rootstock (Stock) – The lower portion that includes theroot system, chosen for its disease resistance, hardiness or adaptability to local soil conditions.ScionIn grafting, the scion is positioned so its vascular cambiumaligns with that of the stock.Figure 11.21 Grafting is a common horticulture technique practised by cutting, joining the parts of two different plants and rooting them in the ground. The “host” root or stem is the stock; the upper grafted piece is the scion. After a few days, the graft tissues become integrated with the tissues of the rooted plant. It develops as a single plant over time. 2. Process of grafting(a) Preparation of plant parts• A healthy rootstock is selected, and a cut is made to exposethe cambium layer.• A scion with at least one or two buds is cut to match the cutsurface of the rootstock.(b) Alignment of vascular cambium• The vascular cambium of the scion must align closely withthat of the rootstock.• Proper alignment ensures successful vascular connection,allowing water and nutrient transport between the two parts.(c) Securing the graft union• The scion and rootstock are held firmly together using graftingtape or a suitable wrap to prevent desiccation and infection.• Sometimes, wax or a sealant is applied to reduce moisture lossand pathogen entry.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

206(d) Healing and callus formation• Over time, a callus forms at the graft union, facilitating connection of vascular tissues.• If successful, the scion will begin to grow and produce lewhile the rootstock provides water, minerals and support.3. Advantages of grafting(a) Combining desirable traits• Growers can combine the high-quality fruit or flower traits the scion with the disease resistance or soil adaptability of the rootstock.(b) Early fruiting• Grafted plants often reach maturity and the fruit-bearingstage faster than seed-grown plants.(c) Repair of damaged plants• Grafting can restore damaged trunks or branches ornamental or fruit trees.(d) Clonal uniformity• Ensures that the scion’s genetic makeup (and thus the characteristics) is maintained.4. Limitations of grafting(a) Compatibility issues• Not all plant species are compatible; closely related susually graft successfully, while distant relatives often fa(b) Skill-dependent• Grafting requires manual expertise; improper technique lead to failure.(c) Possible spread of diseases• If either the scion or rootstock is infected, diseases can transferthrough the graft union.Quick Understanding of Tissue Culture and Grafting and grafting are two critical techniques in asexual reproduction for rapid multiplication and trait enhancement in horticulture and agriculture. • Tissue culture leverages cell totipotency and sterile laboratory conditions to produce diseasefree clones at scale, while grafting unites desirable shoot and root characteristics into a single plant. 2. Both methods play a vital role in ensuring high-quality crops, disease resistance and efficient production, reflecting the importance of asexual propagation in modern plant science and commercial horticulture.Quick Check 91. What is tissue culture and grafting in asexual reproduction?2. How does tissue culture overcome the limitations of traditional plant propagation methods?3. Why is grafting an effective method for combining desirable traits in plants?Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

207Chapter11.3 Seed GerminationProcess of Imbibition in Seed Germination1. Imbibition is the initial step of seed germination, where dry seeds absorb water and swell. 2. This process occurs due to the hydrophilic nature of seed components such as cellulose and proteins.(a) When seeds are immersed in water, they swell due to imbibition. This swelling causes a temporary increase in the volume of the cell and does not require energy as materials are transported passively.(b) Water is transported to different parts of the plants through imbibition, diffusion and osmosis.3. Mechanism of imbibition(a) Water absorption by dry seed – Hydrophilic substances theseed coat and cotyledons attract water molecules.(b) Swelling of seed tissues – The absorbed water causes the seed toincrease in volume and soften the seed coat.(c) Generation of imbibition pressure – Water absorption leads tothe development of high imbibitional pressure, aiding seed coat rupture.(d) Activation of metabolic processes – Water entry activatesenzymes and initiates respiration, preparing the seed for growth.4. Significance of imbibition in seed germination:(a) Imbibition rehydrates dry seed tissues, enabling biochemical reactions.(b) It breaks dormancy, as it signals the transition from dormancy to active metabolism.(c) It activates enzymes which are essential for mobilising the stored food reserves.(d) Water exerts pressure, which facilitates the rupture of the seed coat. (e) Imbibition initiates cellular respiration as water provides the ATP required for growth and development.Mobilisation of Nutrients After Imbibition in Seed Germination1. Once the seed has imbibed water, the stored food reserves are mobilised to provide energy and building materials for the developing embryo. 2. This process involves the activation of hydrolytic enzymes that break down complex macromolecules into simpler, transportable forms for the growing embryo.• Describe the processof imbibition in seedgermination• Explain the mobilisationof nutrients afterimbibition in seedgermination• Explain the externalfactors affectinggerminationLearning OutcomeSeedGerminationVIDEOMobilisation ofNutrientsVIDEOBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

3. Main process of nutrient mobilisation:(a) Activation of hydrolytic enzymes• Water absorption activates enzymes like amylase, proteaseand lipase.• These enzymes hydrolyse the stored macromolecules (sproteins and lipids) in the endosperm or cotyledons.(b) Breakdown of stored nutrients• Carbohydrates: Starch is hydrolysed into maltose and glucose by amylase.• Proteins: Proteins are hydrolysed into amino acids by proteases.• Lipids: Stored lipids in oil-rich seeds are broken down into glycerol and fatty acids by lipase.(c) Transport of nutrients to embryo• The breakdown products (glucose, amino acids, fatty are transported to the embryo via the endosperm (monocots) or cotyledons (dicots).• In monocots (such as maize and wheat), the endosperm the primary storage tissue that contains starch and other nutrients. The scutellum, a modified cotyledon, acts absorptive organ, transferring nutrients from the endosperm to the developing embryo.• In dicots (such as beans and peas), the cotyledons store directly mobilise nutrients for the growing embryo, as the endosperm is usually absorbed during seed development.• Thus, the endosperm (not the scutellum) serves as the ssite in monocots, while cotyledons serve this function in dicots.• These nutrients provide energy through cellular respirationand are used for synthesising new cellular structures.(d) Support for embryo growth• The mobilised nutrients enable the embryo to grow, dethe radicle and plumule, and establish the seedling.• The radicle emerges first, establishing the seedling’s ability absorb water and nutrients from the soil. • The plumule develops into the shoot, initiating photosynthesisonce the first leaves appear.• Once the seedling becomes autotrophic, reliance on reserves decreases, and photosynthesis becomes the primary source of energy.4. Significance of nutrient mobilisation:(a) Provides energy and essential molecules for cell division and elongation.(b) Supports embryo development until the seedling establishes photosynthesis. This process ensures successful seedling emergence and establishment in the early stages of plant growth.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

209ChapterRole of Gibberellins in Seed Germination 1. Imbibition and gibberellin release(a) When a seed imbibes water, it absorbs moisture, causing it to swell and activate metabolic processes.(b) The embryo, a rich source of gibberellins (GA), begins to synthesise and release these plant hormones.(c) GA signals the seed to break dormancy and initiates germination.2. Activation of the aleurone layer (Monocots like cereals)(a) In cereal seeds (such as maize, rice, wheat), the endosperm stores nutrients for germination.(b) The outer layer of the endosperm, called the aleurone layer, responds to GA by synthesisingand secreting digestive enzymes.(c) One key enzyme is α-amylase, which hydrolyses starch into maltose and glucose, making them available for the growing embryo.3. Mobilisation of stored nutrients(a) The breakdown of starch into soluble sugars provides energy (ATP) and carbon skeletonsfor cell division and elongation.(b) Other enzymes digest proteins and lipids, releasing amino acids and fatty acids, essential for biosynthesis and respiration.(c) In monocots, the scutellum (cotyledon) absorbs and transports these nutrients from the endosperm to the embryo.(d) In dicots, the cotyledons function as the main nutrient reservoir and supply food directly to the growing seedling.4. Germination and seedling growth(a) The energy from nutrient mobilisation , allowing the root o emerge (b) Shoot growth follows, leading to seedling establishment. In some seeds, GA treatment can replace environmental triggers (such as light, low temperatures), artificially breaking dormancy and stimulating germination.Table 11.9 Key components and their rolesComponent FunctionGibberellin (GA) Hormone released by the embryo; stimulates enzyme production in the aleurone layer.Aleurone layer Responds to GA by synthesising hydrolytic enzymes likeα-amylase.Endosperm (in monocot)Cotyledons (in dicot)Stores starch and nutrients; broken down by α-amylase into sugars.α-amylase Hydrolyses starch into maltose and glucose for embryo growth.Scutellum in monocot (Cotyledon in dicot)Absorbs nutrients and transports them to the embryo.Radicle Develops into the primary root for anchorage and water uptake.Shoot (Plumule) Develops into the first leaves and stem.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

210Chapter11After imbibition, the embryo releases gibberellin (GA), which signals the aleurone of the endosperm.The aleurone responds to GA by producing digestive enzymes, such as α-amylase, which break down starch and other nutrients in the endsperm.Sugars and nutrients from the endosperm are absorbed by the scutellum (cotyledon) and used to support the embryo’s growth into a seedling.1 2 3AleuroneEndospermWaterScutellum(cotyledon)GARadicleGAα-amylase sugarFigure 11.22 The gibberellin signalling mechanism in seed germination of monocots, where GA induces enzyme production in the aleurone layer, leading to the breakdown of stored starch into sugars that fuel the embryo’s growth into a seedling. RadicleSeed coatColeoptileEmbryoAleurone layerEndospermScutellumEpicotylHypocotylRadicleColeorhizaColeoptileEpicotyl ColeoptileFirst leafLeafRootBiology Semester 2 STPM Chapter 11 Reproduction, Development and Growth

211Chapter(b) DicotPlumuleHipocotyl EmbryoEpicotylRadicleTestaTegmenRadicleSeed coatHipocotylCotyledonSeed coatCotyledonFirst leavesRootFigure 11.23 The internal structures and germination stages of monocotyledonous (monocot) and dicotyledonous (dicot) seeds. It provides a comparative view of their anatomy and how they develop into seedlings.• Monocots (such as maize, rice, wheat): Nutrients are primarily stored in the endosperm. The scutellum isa modified cotyledon that absorbs nutrients from the endosperm and transfers them to the embryo during germination.• Dicots (such as beans, peas, sunflower): Nutrients are stored in the cotyledons, which function as the mainfood reserve. The endosperm is either absent or significantly reduced because the cotyledons absorb and store the nutrients before seed maturity.External Factors Affecting Germination (a) Oxygen: for aerobic respiration (the seed requires large amounts of ATP in order to develop).(b) Water: to metabolically activate the seed (triggers the synthesis of gibberellin)(c) Temperature: seeds require certain temperature conditions in order to sprout (for optimal function of enzymes).(d) pH: seeds require a suitable soil pH in order to sprout (for optimal function of enzymes).2. Additionally, certain plant species may require additional conditionsfor germination:(a) Fire: some seeds will only sprout after exposure to intense heat(for example, after bushfires remove established flora).(b) Freezing: some seeds will only sprout after periods of intensecold (for example, in spring, following the winter snows).(c) Digestion: some seeds require prior animal digestion to erode the seed coat before the seed will sprout.(d) Washing: some seeds may be covered with inhibitors and will only sprout after being washed to remove the inhibitors.(e) Scarification: seeds are more likely to germinate if the seed coatis weakened from physical damage.Why can certain seeds not germinate even under favorable conditions?Innate dormancy may occur in viable seeds. It is the condition of seeds which is incapable of germination even if conditions suitable for seedling growth are supplied. This inability to germinate may be due in certain species to the embryo being immature at the time of dispersal.Biology Semester 2 STPM Chapter 11 Reproduction, Development and Growth