STPM Text
STPM Scheme of Assessment
Term of Paper Code Theme / Title Type of Test Mark Duration Administration
Study and Name (Weighting)
First 964/1 Biological Written test 60
Term Biology Molecules and (26.67%)
Paper 1 Metabolism Section A 15
15 compulsory
multiple-choice
questions to be
answered.
Section B 15
2 compulsory short 1 Central
structured questions to 1— hours assessment
2
be answered.
Section C 30
2 out of 3 essay
questions to be
answered.
All questions are based
on topics 1 to 6.
Second 964/2 Physiology Written test 60
Term Biology (26.67%)
Paper 2 Section A 15
15 compulsory
multiple-choice
questions to be
answered.
Section B 15
2 compulsory short 1 Central
structured questions to 1— hours assessment
2
be answered.
Section C 30
2 out of 3 essay
questions to be
answered.
All questions are based
on topics 7 to 13.
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Term of Paper Code Theme / Title Type of Test Mark Duration Administration
Study and Name (Weighting)
Third 964/3 Ecology and Written test 60
Term Biology Genetics (26.67%)
Paper 3 Section A 15
15 compulsory
multiple-choice
questions to be
answered.
Section B 15
2 compulsory short 1 Central
structured questions to 1— hours assessment
2
be answered.
Section C 30
2 out of 3 essay
questions to be
answered.
All questions are based
on topics 14 to 19.
964/5 Written Practical Test 45
Biology (20%) Central
1
Paper 5 3 structured questions 1— hours assessment
with diagram/graph/ 2
table to be answered.
First, 964/4 Biology School-based 25
Second Biology Practical Assessment of (20%)
and Third Paper 4 Practical Through- School-based
out the
Terms three assessment
15 compulsory terms
experiments to be
carried out.
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CONTENTS
Chapter 4.4 Inhibitors 146
1 BIOLOGICAL MOLECULES 1 4.5 Classification of Enzymes 150
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
4.6 Enzyme Technology 152
1.1 Water 2 STPM Practice 4 156
1.2 Carbohydrates 6 Answers 160
1.3 Lipids 15
1.4 Proteins 22
1.5 Nucleic Acids 33 Chapter CELLULAR RESPIRATION 163
5
1.6 Analytical Techniques 39 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
STPM Practice 1 42 5.1 The Need for Energy in Living
Answers 48 Organisms 164
5.2 Aerobic Respiration 168
5.3 Anerobic Respiration 177
Chapter STRUCTURE OF CELLS AND
2 ORGANELLES 51 STPM Practice 5 180
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Answers 185
2.1 Prokaryotic and Eukaryotic Cells 52
2.2 Cellular Components 59
2.3 Specialised Cells 84 Chapter PHOTOSYNTHESIS 187
6
STPM Practice 2 102 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
Answers 107 6.1 Autotrophs 188
6.2 Light-dependent Reactions 193
6.3 Light-independent Reactions 197
Chapter MEMBRANE STRUCTURE AND
3 TRANSPORT 111 6.4 Limiting Factors 206
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • STPM Practice 6 209
3.1 Fluid Mosaic Model 112 Answers 214
3.2 Movement of Substances across
Membrane 116
STPM Practice 3 128 STPM Model Paper (964/1) 217
Answers 131
Answers 223
Chapter
4 ENZYMES 133
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Glossary 225
4.1 Catalysis and Activation Energy 134
4.2 Mechanism of Action and Kinetics 136
4.3 Cofactors 144
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CHAPTER STRUCTURE OF CELLS
2 AND ORGANELLES
Concept Map
Cells Prokaryotic cells Bacterial cells
comparison
Plant cells
Eukaryotic cells comparison
Animal cells
Principles of light and
electron microscopes
Typical plant and animal cell
Cellular components Structures and functions of organelles
Differential centrifugation
Unspecialised cells
Structures, functions
Specialised cells Specialised plant cells
and distributions
Specialised animal cells
Bilingual Keywords
Prokaryotic cell – Sel prokariot Secretion – Rembesan
Eukaryotic cell – Sel eukariot Fluid – Bendalir
Cyanobacteria – Sianobakteria Sieve tube – Tiub tapis
Capsule – Kapsul Spindle fibre – Gentian gelendung
Nitrogen fixing – Pengikatan nitrogen Centromere – Sentromer
Chromosome – Kromosom Digestion – Pencernaan
Cell wall – Dinding sel Gland – Kelenjar
Cytoplasm – Sitoplasma Pith – Empulur
Fungi – Kulat Muscle – Otot
Cilium – Silium Nerve – Saraf
Lysosome – Lisosom Cartilage – Rawan
02[STPM Bio T1].indd 51 3/29/18 5:07 PM
Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
Learning Outcomes 2.1 Prokaryotic and Eukaryotic
Students should be able to: Cells
(a) state the cell theory;
(b) compare the structures
of prokaryotic and Cell Theory
eukaryotic cells;
(c) compare typical animal 1. A cell is the basic unit of the structure and function of all living
2 and plant cells as organisms. This is according to the cell theory, the result of studies
seen under electron
microscopes; by Schleiden, a botanist, and Schwann, a zoologist.
(d) describe the basic 2. According to Rudolp Virchow, all cells arise from pre-existing cells
principles of light and
electron microscopy. by cell division.
3. A cell is made up of protoplasm surrounded by a selectively permeable
lipoprotein membrane. Within the cell, the basic chemistry of life
Summary exists.
4. Cells are divided into two types, the prokaryotic cells and the
Cell theory eukaryotic cells.
1. A cell is a basic unit of
structure and function
of all living things. Cell Theory
2. Cells arise from pre-
Language Check VIDEO
Language Check
existing cells by cell
division.
Comparison Between Structures of Prokaryotic and
Eukaryotic Cells
2013, 2015
1. Prokaryotic cells are primitive cells. They are not true cells. They do
not have nucleus and are found in bacteria and cyanobacteria, which
belong to the kingdom Prokaryotae.
Language Check 2. A generalised prokaryotic cell is as shown in Figure 2.1.
Language Check
• Bacterium – singular Cell wall Capsule
Bacteria – plural Plasma membrane
• Flagellum – singular
Flagella – plural Cytoplasm
Ribosomes
Plasmid
Pili
Prokaryotic and
Eukaryotic Cell
VIDEO
Bacterial flagellum
Nucleoid (circular DNA)
Figure 2.1 Structure of a generalised prokaryotic cell
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
3. Eukaryotic cells are true cells with nucleus and are more advanced Exam Tips
cells as found in plants, animals, fungi and protoctists (Figure 2.2). Remember the differences
between the prokaryotic
and the eukaryotic cells
including their examples
2
Plasma membrane Microtubule
Centriole
Nuclear envelope
Nucleus
Ribosome
Rough endoplasmic
reticulum
Lysosome
Golgi body
Mitochondrion
Cytoplasm
Figure 2.2 Structure of a generalised eukaryotic cell
4. Table 2.1 summarises the comparison between the structures of Summary
prokaryotic and eukaryotic cells.
Table 2.1 Comparison between prokaryotic and eukaryotic cells Comparison of prokaryotic
and eukaryotic cells
Characteristic Prokaryotic cell Eukaryotic cell 1. Both have cytoplasm
and plasma membrane.
1 Cell size Usually small Usually bigger (as big as 2. Differences on cell
Language Check
Language Check
(diameter 0.5-5 µm) 40 µm) surface structures, wall
material, membraneous
organelles, ribosomes
2 Form Unicellular / Unicellular, filamentous / and DNA.
filamentous multicellular
3 Cell wall Peptidoglycan Cellulose, chitinised in
(murein), not of fungi Language Check
Language Check
cellulose
• Nucleus – singular
4 Flagellum type Fine, simple and Complex, with ‘9+2’ Nuclei – plural
only consists of one pattern of triplet • Nucleolus – singular
microtubule microtubules Nucleoli – plural
• Mitochondrion – singular
5 Capsule May be present, of Usually absent Mitochondria – plural
glycoprotein
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
Characteristic Prokaryotic cell Eukaryotic cell
2010 6 Pillus May be present for No such structure present
attachment
7 Nucleus Absent, with no Present, with nuclear
nuclear membrane envelope
2
8 Nucleolus Absent, ribosome Present, for ribosome
formed in cytoplasm synthesis
2011 9 DNA/ Circular, ‘naked’ Linear, combined with
Chromosomes DNA with no histone histone to form X or
(contained in the inverted V shapes
nucleiod region)
10 Organelles Few, none with Many, three with
envelope envelopes
11 Ribosomes Smaller types, 70S (18 Bigger types, 80S (22 nm)
nm)
12 Mesosome or Mesosome or plasma Mitochondria for
mitochondria membrane for respiration
respiration
13 Spindle No spindle during cell Spindle present during
division mitosis and meiosis
14 Vesicles for Foldings of plasma Chloroplasts involved in
photosynthesis membrane for photosynthesis
photosynthesis
15 Vesicles for Vesicles or plasma No structure is capable of
nitrogen membrane can fix such ability
fixation nitrogen
16 Cytoplasm Present Present
and plasma
membrane
Quick Check 1
1. Why are prokaryotic cells considered as primitive?
2. What are the different forms of prokaryotic organisms?
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
Comparison Between Typical Animal and Plant Cells
1. The structure of a typical plant cell e.g. mesophyll cell is as shown in
Figure 2.3.
Middle lamella
Cell wall 2
Smooth Lipid globule
endoplasmic Animal Cells
reticulum and Plant
Golgi body Cells
Nucleus VIDEO
Ribosomes
Rough
Sap vacuole endoplasmic
reticulum
Mitochondrion
Chloroplast
Plasmodesma
Tonoplast
Figure 2.3 A mesophyll cell
Exam Tips
2. The structure of a typical animal cell e.g. intestinal epithelial cell is as Remember the generalised
shown in Figure 2.4. ultra structure of animal
cell and plant cell including
a function each of their
organelles (STPM 2000
Desmosome essay).
Golgi body
Microtubule
Smooth endoplasmic
Centriole reticulum Summary
Nucleus
Nucleolus
Mitochondrion Nucleoplasm Comparison between
Rough endoplasmic plant and animal cell
Lysosome reticulum 1. Both are eukaryotic
Language Check
with basic similarities in
Language Check
Microfilament Lipid globule organelles.
2. Differences in cell
Ribosome
Glycogen surface structures, cell
granule wall, chloroplast, food
reserves and centrioles.
A intestinal epithelid cell
Figure 2.4 A intestinal epithelial cell
Table 2.2: Comparison between animal and plant cell
Language Check
Characteristic Animal cell Plant cell Language Check
1 Shape Not restricted, can be Restricted by cell wall • Plasmodesma – singular
altered Plasmodesmata – plural
• Lamella – singular
2 Cell wall and middle Absent Surrounding the cell Lamellae – plural
lamella
3 Plasmodesmata No such pore Present
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
Characteristic Animal cell Plant cell
4 Microvillus, cilium and Present in certain Absent except sperm of
flagellum specialised cells fern has flagellum
5 Sap vacuole and Small vesicles may be Big, centrally located
tonoplast found instead
2 6 Cytoplasm Throughout cell At the periphery
7 Nucleus position Centrally located At one side
8 Lysosome Usually present Usually absent
9 Centriole Present, to organise Not found, still can
spindle fibre formation form spindle fibre
10 Chloroplast and other Absent Present for
plastids photosynthesis and
storage
11 Carbohydrate food Glycogen Starch
reserves
12 Intermediate filaments Present Usually absent
13 Mitosis In all cells In meristem only
14 Secretory vesicles May be present Usually absent
15 Plasma membrane, Present Present
nucleus, mitochondria,
endoplasma reticulum
Quick Check 2
1. List the structural similarities between plant and animal cells. Briefly explain why each of the
structures you have listed is essential.
Light and Electron Microscopes
Light Microscope
Light microscope
VIDEO
1. Light microscopes are the ones that make use of light to form an
image.
2. There are compound microscopes and phase contrast microscopes.
(a) Compound microscopes
(i) Compound microscopes contain two sets of lenses i.e.
the objective lens to form an enlarge image and an ocular
(eyepiece) lens to further enlarge the image.
(ii) Compound microscopes in the laboratory normally have
three magnification powers.
• Low power: 10 × 4 = 40 times.
• Medium power: 10 × 10 = 100 times.
• High power: 10 × 40 = 400 times.
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
(iii) The uses of compound microscopes are as follows:
• Compound microscopes can be used to observe objects
smaller than 0.1 mm.
• The invention of microscopes led to the formulation of
the cell theory, which states that all organisms are made
of cells.
• Observation of microscopes also proves that 2
microorganisms cause diseases.
(iv) They have the following limitations:
• Objects have to be sectioned into thin slices, fixed and
stained before they can be observed. This makes the
study of living cells difficult as they have to be killed.
• The resolution limit of light microscope is 0.2 µm,
meaning that any object smaller than that cannot be seen
clearly.
(b) Phase contrast microscopes
(i) These are compound microscopes that can adjust the
contrast of the object against the background, either
darkened or lightened. Living cells can be observed without
staining.
(ii) These microscopes are fitted with an annular diaphragm
to form a cone of light passing through the object.
A phase plate is then used to change the phase of the
object relative to the background before the final image is
formed. By using a phase plate of suitable thickness, the
background light can be darkened or lightened. This is due
to the difference in refractive indices of the object and its
surrounding that causes the light passing through them to
differ in phases, which can then be enhanced (lightened) or
cancelled (darkened).
Annular diaphragm Phase plate
Figure 2.5 An annular diaphragm and a phase plate of a phase contrast microscope
(iii) Its advantage is that freshly prepared living cells can be
studied without being killed by dye. Activities such as
mitosis, meiosis, phagocytosis and movement especially
that of zooplanktons can be observed.
(iv) Its limitation is that the adjusting of phase plate requires
experience. The microscopes have a resolution limit of
0.2 µm.
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
Exam Tips Electron microscope
Remember the basic 1. Electron microscope is a microscope that uses beams of electrons to
principles of phase-contrast
microscope, transmission form an image.
and scanning electron
microscopes including 2. They are divided into transmission electron microscope and
examples of their uses. scanning electron microscope.
2 (a) Transmission electron microscope (TEM)
(i) It is a microscope that makes use of beams of electrons to
pass through a very thin section of an object to form an
image on a monitor screen.
2014
(ii) The principle involved is the same as the compound light
microscope in which the light is replaced by beams of
electrons; electromagnet is used instead of lens to focus
the beams of electrons as shown in Figure 2.6.
Electron
Light source
source
Electromagnetic
Condenser
lenses condenser
lens
Specimen Specimen
Objective
Objective
lens lens
Ocular Ocular
lens lens
Image
Image
Light microscope Electron microscope
Figure 2.6 Comparison of light and electron microscopes
(iii) Its uses are as follows:
• The limit of resolution is very small which is around 0.5
nm. Therefore, the structure of virus and ultrastructure
of cell including organelles can be studied.
• The final magnification produced in the form of
micrograph can be more than 250,000 times.
(iv) It has the following limitations:
• Experience is required to produce electro-micrograph of
cell structures.
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
• Object has to be fixed by chemicals to prevent Summary
deterioration, impregnated with heavy metallic ions to
produce clearer image and hardened by plastic so that it Basic principles of light and
can be cut to less than 1 µm thick. This procedure will electron microscopes
add in artefacts. Light Electron
• The instrument is very sensitive to external interference microscope microscope
such as magnetic and electric fields. Getting clear images Light wave Electron beam 2
may take time to learn.
• It is not portable. It is expensive and costly to maintain. Glass lenses Electromagnetic
lenses
(b) Scanning electron microscope (SEM)
(i) It makes use of a beam of electrons to irradiate the surface Lamp as light Cathode tube
as electron
source
of an object and form a three dimensional image of the fine source
structure of the object surface. Light needs Electron beam
(ii) The beam of electrons being reflected by the surface of fine to be needs to be
object is projected and magnified on a screen. condensed condensed
(iii) It has the following uses: Whole Very thin section
• The surface of fine structure such as that of insect specimen used
possible
antenna, inner surface of uterus and even cell surface
can be observed in great detail. Live Dead specimen
• The three dimensional structure such as the internal specimen
structure of organelles like granal system of chloroplasts, Coloured Black and white
cristal system in mitochondria and the outer structure image image
of virus can be photographed or shown on the monitor Low High resolution
screen. resolution
• Living process as such as movement of cilia and other Low High
cell surface activities such as the budding of HIV from magnification magnification
lymphocyte can also be observed.
(iv) It has the following limitations:
• Experience is essential, so time has to be devoted to learn
in order to get the best picture.
• Images are easily distorted.
2.2 Cellular Components Learning Outcomes
Students should be able to:
Cellular Components of Typical Plant and Animal Cells (a) identify the cellular
components of typical
plant and animal cells;
Cell membrane (b) describe the structures
of organelles and state
1. Cell membrane is a lipoprotein layer that surrounds the cell and their functions;
organelles such as the nucleus, mitochondria, chloroplast, vacuole (c) explain the basic
principles of differential
and lysosome. centrifugation used
to fractionate cellular
2. The structure of the membrane based on Singer and Nicolson’s components (g and S
values).
fluid-mosaic model is as shown in Figure 2.7.
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
Carbohydrate
Glycoprotein
Glycolipid
Globular Hydrophilic
protein heads
Protein
channel
(Transport
2 protein) Phospholipid
bilayer
Cholesterol
Phospholipid
molecule
Peripherial protein
Globular protein Surface protein Alpha-Helix protein
(integral protein) Filaments of cytoskeleton (Intergral protein)
Figure 2.7 The fluid-mosaic model of the cell membrane
Exam Tips 3. The basic membrane structure consists of a bimolecular
phospholipid fluid layer with globular protein units floating in it,
Remember the structure,
functions of membrane forming a mosaic pattern.
based on the fluid-mosaic
model of Singer and 4. The heads of the phospholipids are hydrophilic, pointing outwards
Nicholson and the roles into the aqueous medium on both sides of the membrane.
of component molecules.
(STPM 2009 essay on the 5. The tails of phospholipids are hydrophobic, facing each other
role of carbohydrate)
forming a non-polar interior in the middle of the membrane.
6. The structure is dynamic where each lipid molecule can move within
its own monolayer and so is each of the protein unit. Some protein
units however, are immobilised by microfilaments in the interior of
the cell.
7. The fluidity of the membrane depends on the length of the fatty
acid chains, their saturation and the amount of cholesterol in them.
Fluidity affects permeability, membrane enzyme activities, reception
to molecules and with which membranes it will fuse.
8. Cholesterol with its hydrophilic head and hydrophobic tail fits neatly
within the phospholipid layer. It functions to control mechanical
stability, flexibility and permeability of the membrane, especially in
reducing leakage of small polar molecules.
9. The proteins are embedded in the phospholipid layer like mosaic,
either in only one monolayer or span the bilayer. These are integral
or intrinsic proteins, fitted neatly because of their corresponding
non-polar properties of their surfaces. The peripheral or extrinsic
ones are attached on the outer polar layers of phospholipid.
10. The proteins function as carriers or channels for polar molecules
to cross the membrane, as structural components, enzymes,
receptors and electron carriers for respiratory or photosynthetic
phosphorylations.
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
11. The carbohydrates exist as short branched chain of sugars attached
to proteins (glycoproteins) or lipids (glycolipids) on the outer
surface of the membrane. They function as receptors for chemicals
like hormones, adhesion to neighbouring cells and for immune
responses.
12. Functions of cell membrane: 2
(a) The membrane protects the cell. Thus, any chemical or reaction
happening outside the cell will not harm it.
(b) It serves as a boundary between the cell and its environment.
Therefore, ions outside the cell cannot easily enter the cell.
(c) It regulates or controls the passage of substances in and out of
the cells. This happens especially through the protein channels,
which allow only specific polar molecules to move in or out.
(d) It acts as receptor sites in recognising external stimuli such as
hormone and antigen molecules. This also enables the cells to
recognise other cells and so to behave in an organised manner
during the formation of tissues in the embryo.
(e) Within the cells, membranes allow compartmentalisation and
division of labour especially within membrane-bound organelles
like nucleus, mitochondrion and chloroplast.
(f) Certain membranes can perform special functions such as light
reaction in the internal membranes of chloroplast and oxidative
phosphorylation in the inner membrane of mitochondria.
(g) It helps in cell mobility such as in the white blood cells where
the membrane can carry out amoeboid movement.
Cell wall
1. The cell wall is a carbohydrate layer of cellulose found outside the
plasma membrane of plant cells.
2. There are two types of cell wall, the primary cell wall and the
secondary cell wall.
(a) Primary cell wall
The primary cell wall is found in young cells and cells that
are not highly differentiated such as meristem, parenchyma
and collenchyma. The primary cell wall has the following
characteristics:
(i) It is a thin layer, found just outside the plasma membrane
of most plant cells. It is also found on the outer layer of
cells with secondary cell wall.
(ii) It consists of randomly arranged microfibrils of cellulose
in an amorphous matrix as in Figure 2.8.
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
Microfibril
Matrix
2
Figure 2.8 Microfibrils arranged in an amorphals matrix
(iii) Each microfibril consists of about 100 chains of cellulose
molecule of 5-20 µm and a diameter of 1-2 µm.
(iv) The matrix is made up of complex polysaccharides such
as pectins and hemicellulose with long and branched
molecules.
(v) On the outer layer of the wall, there is a middle lamella
layer that consists of magnesium and calcium pectates for
cementing adjacent cells.
(vi) The primary cell wall is porous. It enables water to be
transported apoplastically along it.
(vii) It is elastic and strong. It enables parenchyma cells to
become turgid and support the whole plant especially in
herbaceous plants.
(viii) The wall is usually perforated with plasmodesmata for the
transport of substances between cells.
(b) Secondary cell wall
The secondary cell wall is a harder and usually thicker layer
of cell wall, formed between the plasma membrane and the
primary cell wall. It has the following characteristics:
(i) The wall is made up of regularly arranged microfibrils or
bigger macrofibrils. The fibrils are arranged in layers of
parallel rows, which are perpendicular to those of upper or
lower layers (Figure 2.9).
Microfibrils
Figure 2.9 Fibrils arranged in layers of parallel rows
(ii) The matrix in the secondary wall is impregnated with
lignin, forming a hard and impervious layer.
(iii) The deposition of lignin in xylem vessels is not uniform
but in patterns like rings, helices or networks.
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
3. The functions of cell wall are as follows:
(a) It protects the cell from physical injuries and haemolysis.
(b) It supports the plant through cell turgidity or mechanical
strength for tall woody trees.
(c) It controls growth by limiting individual cell size and shape of
the cell through orientation of the fibrils in the wall.
(d) It forms a system of transport pathways for water and mineral 2
ions. Water can both be transported along the porous cell wall
in apoplast way and through the plasmodesmata of cells in
symplast way.
(e) It controls excessive loss of water from epidermal cells of the
leaves and stems by having a waxy cuticle on the surface of the cell
wall. Cork cells have suberised cell wall for the same purpose.
(f) Cell wall of the xylem vessels forms empty tubes for water
transport from the roots to the leaves. Tracheids form a water
transport system with a lot of pits so that water can be laterally
transported. Sieve tubes with a thinner wall but with sieve plates
(a sieve-like lignified cross wall) will give extra strength for
transport of organic compounds.
(g) It provides food storage in the form of hemicellulose in some
seeds.
(h) It provides large surface area to volume ratio in root hair cells
where absorption can take place.
(i) It controls passage of water and dissolved mineral ions into the
plant by having lignified Casparian strip or passage cells in the
endodermis of the root.
Cytoplasm
1. Cytoplasm is the protoplasmic part of the cell, which is outside the
nucleus and is surrounded by the plasma membrane. It is the aqueous
part of the cell, after all organelles are removed by centrifugation.
2. The cytoplasm of plant cell is usually referred to as protoplast,
excluding the sap vacuole.
3. The pH of the cytoplasm is 6.8 ± 0.2.
4. It has a considerable high density with a variety of solutes.
5. Cytoplasm can be divided into cytosol (ground substance) and
cytoskeleton (cell inclusion).
Cytosol (ground substance)
1. Cytosol or the ground substance is the soluble part of the cytoplasm.
2. The solutes can be divided into three groups.
(a) True solutes or crystalloids. These consist of:
(i) Micromolecules such as gases (O , CO and N ), mineral
2
2
2
2+
+
ions which include Na , K , Cl , Ca , Mg , Mn and Fe 2/3+ .
2+
–
+
2+
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
(ii) Mesomolecules such as monosaccharides which include
glucose and fructose, disaccharides which include maltose,
sucrose (plant cells), amino acids, organic acids, nucleotides
and vitamins.
(b) Colloids. These include macromolecules such as proteins
(enzymes, hormones and structural proteins), glycogen in liver
2 cells and muscle tissues.
(c) Particles, droplets and vesicles. These include glycogen granules
in liver cells and muscle tissues, starch granules in plant cells,
fine fat droplets and minute vesicles, which contain liquid.
3. The functions of cytosol are as follows:
(a) It stores vital chemicals including fats.
(b) It is the site for certain metabolic pathways such as glycolysis,
synthesis of fatty acids, amino acids and proteins.
(c) It enables organelles to move about in it. These organelles include
mitochondria, chloroplasts, ribosomes, lysosomes and vacuoles.
Cytoskeleton
1. The cytoskeleton determines the three dimensional shape of the
animal cells and give certain firmness in the plant cells.
2. The fine fibrils can be divided into three types i.e. microtubules,
microfilaments and intermediate filaments.
(a) Microtubules
2010 (i) The microtubules are fine, unbranched tubules with a
diameter of 25 nm, a wall of 5 nm thick and vary in length.
(ii) The wall is composed of 13 rows of globular protein
subunits called tubulin, which are arranged helically.
(iii) Tubulin can grow from a certain organisation centre, which
is made of dense protein. Tubulin can be added at the
base or at one end of microtubule causing it to increase in
length or be removed, causing it to decrease in length.
(iv) The microtubules in cells are usually stable. However, some
may be unstable as they can change their length suddenly.
(v) Other tubulin subunits are able to attach to the base of cilia
and flagella, participating in their growth and movement.
(vi) The spindle fibres during cell divisions are microtubules.
In animal cells, they are organised by the centrioles. Their
formation can be inhibited by colchicine, causing non-
disjunctions (chromatids not separating) and mutation in
the number of chromosomes.
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Intermediate
filament
2
Microfilament
Microtubule
Arrangement of tubulin
Cytoskeleton
Figure 2.10 Cytoskeleton and arrangement of tubulin in microtubule
(vii) The functions of microtubules are as follows:
• They form the cytoskeleton that determines the shape of
the cell.
• They can divide the cytoplasm of the cell into
compartments so that specialised enzymes can be
isolated from others to function better.
• They can contract, causing movement in the cilia and
flagella.
• They can pull chromosomes or chromatids during
mitosis or meiosis.
• They can cause the movement of organelles, including
mitochondria, lysosomes and vesicles to move along like
railway tracks.
(b) Microfilaments
(i) The microfilaments are fine filaments made of protein with
a diameters of 7 nm and lengths of several µm.
(ii) They are composed of one or two types of protein including
actin and myosin.
(iii) They are dynamic which means they can change their
length very quickly depending on their locations and
functions.
(iv) Each type of protein forms subunits that are arranged
helically.
(v) The subunits can slide over one another causing the
microfilaments to contract.
(vi) Microfilaments exist in bundles and they are normally
found in layers in the cytoplasm.
Two strands of
actin subunits
Microfilament composed of two Intermediate filament with eight
strands of protein subunits strands of protein subunits
Figure 2.11 Microfilament and intermediate filament
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Exam Tips (vii) They cause membrane invagination and evagination during
endocytosis and exocytosis.
Remember: membrane,
cell wall and cytoplasm (viii) They cause the protrusion of pseudopodium during
are cell components, not amoeboid movement in the white blood cells.
organelles but microtubules
and microfilaments are (ix) They assist in the cleavage process during cytokinesis of
considered as organelles. animal cells after nuclear division.
2 (c) Intermediate filaments
(i) The intermediate filaments are filaments bigger than the
Summary microfilaments but smaller than the microtubules, with
diameter between 8 to 12 nm and are only found in animal
Cellular components of cells.
typical plant and animal cell (ii) The filaments are made up of 4 long strands of α-helix
1. Plasma membrane coiled fibrous proteins, each consisting of only secondary
• Phospholipid
• Cholesterol coiled polypeptide.
• Protein (iii) There are several types of intermediate filaments with each
• Carbohydrate composing of only one type of protein, including one with
2. Cell wall
• Primary – celulose keratin.
• Secondary + lignin (iv) They are very stable and branched, forming a network of
3. Cytoplasm
• Cytosol – water, gases, cytoskeleton in the cytoplasm and nucleus.
ions, carbohydrates, (v) Each type of cell has its own arrangement and types of
amino acids, protein. For example, the cells in the skin epithelium or
nucleotides, vitamins
and proteins forming the nail, hair and horn have keratin filaments,
• Cytoskeleton – which are different from those in the muscle and nerve
microfilament, cells.
microtubule and
intermediate filament (vi) They maintain the shape of the cell including the nucleus.
(vii) They distribute the organelles and support them in the
cytoplasm.
(viii) They help some specialised cells to perform their functions.
Some examples include the nail-producing cells to form
nails, and neurone to transmit impulses.
Structure and Functions of Organelles
Nucleus
1. The nucleus is the largest organelle in the eukaryotic cell and
functions to control all activities of the cell.
2. It is found in all cells, except in the red blood cells of mammals and
the sieve tubes of phloem in flowering plants.
3. The nucleus is normally found in the centre of the cell but in matured
plant cells, it is pushed to one side of the protoplast by the big sap
vacuole.
4. There is one nucleus per cell, however, in some exceptional cases, two
are found in Paramecium and abnormal liver cells.
5. It is normally spherical or oval in shape, but it may be cylindrical or
lobed in the white blood cells. The shape can later be changed.
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6. Nucleus has a diameter of 10 – 20 μm and it occupies 75% of the cell
volume in the meristem.
7. The nucleus can be divided into
(a) Nuclear envelope
(b) Nucleoplasm
(c) Nucleolus
(d) Chromosome 2
(a) Nuclear envelope
(i) Nuclear envelope is the double lipoprotein membrane that
surrounds the nucleus.
(ii) The outer membrane is smooth, may have ribosomes attached
to it and is possibly continuous with the membrane of the
endoplasmic reticulum. Sometimes, it may be continuous right
to the plasma membrane.
Ribosomes
Smooth endoplasmic reticulum
Exocytosis
Nuclear pole
Golgi apparatus
Chromatin
Nucleolus Rough endoplasmic reticulum
Figure 2.12 Relationship of outer nuclear membrane with other membranes
(iii) The inner membrane is smooth; no ribosome is attached to it
and it is not folded.
(iv) This nuclear envelope disappears at prophase of cell division and
reappears at the end of telophase. Therefore, the components of
the membrane can be hydrated and reorganised into its original
state.
(v) There are nuclear pores in the envelope. The pores are relatively
big, 40 – 150 nm and cover a surface of 8% of the envelope.
However, the passage of substances is very well controlled.
The bigger pores are specifically for the transport of RNA and
ribosome subunits from the nucleus to the cytoplasm. Steroid
hormones may enter the nucleus through the phospholipid
layers by diffusion.
(vi) There is a perinuclear space of about 10 – 40 nm wide between
the outer and inner membrane of the envelope.
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(vii) The functions of the nuclear envelope are as follows:
• It protects the inner structure of the nucleus especially the
chromatin.
• It separates the nucleus from the cytoplasm so that reactions
occur in the nucleus are not affected by those of the cytoplasm.
• It controls the shape of the nucleus. This provides a three
2 dimensional space for processes such as the synthesis of DNA
and RNA.
• It controls the passage of substances like ribosomes and RNA
from the nucleus to the cytoplasm.
(b) Nucleoplasm (nuclear sap / karyoplasm)
(i) Nucleoplasm is the part of protoplasm inside the nucleus,
separated by the nuclear envelope.
(ii) Its composition is the same as that of the cytoplasm, consisting
mainly of water with crystalloids and dissolved colloids. It has
DNA, histone and pentoses that are not found in the cytoplasm.
(iii) The crystalloids are monosaccharides including glucose, ribose
and deoxyribose; amino acids, organic acids, nucleotides and
+
mineral ions like phosphates and K ions.
(iv) The colloids are DNA, RNA and proteins particularly histone
that mix with DNA forming chromatin.
(v) Chromatin is made of DNA and histone protein. Eight molecules
of histone, wound by DNA strand, form a nucleosome unit.
Chromatin is divided into two types, the euchromatin and the
heterochromatin.
(vi) Euchromatin composes of more DNA that is less wound on
histone protein. It is more lightly stained. It contains more
genes that are active and is found in the centre of the nucleus.
(vii) Heterochromatin found in the periphery of the nucleus
composes of DNA that is more wound with histone forming
more nucleosomes. The genes in it are not active.
(viii) The chromatin is easily stained with acidic eosin to form a purple
colour. This makes nucleoplasm different from cytoplasm.
(ix) It is common to find foreign structures that are not supposed
to be there. Such structures include mitochondria and parts of
the endoplasmic reticulum.
(x) Nucleoplasm performs various functions. It contains various
enzymes for metabolism including that for glycolysis, Krebs
cycle, phosphorylation and the synthesis of NAD, replication
and transcription of DNA.
(c) Nucleolus
(i) Nucleolus is a spherical structure that is the site for ribosome
synthesis in the interphase of nucleus.
(ii) Its location is not fixed, it is usually found in the centre or on
one side of the nucleus.
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(iii) The shape of the nucleolus is usually spherical, but it can be
oval and it can change its shape.
(iv) There is usually one nucleolus per nucleus, but in rare cases,
there can be two per nucleus.
(v) The structure of nucleolus as interpreted from the electron
micrograph is shown in Figure 2.13.
2
Region with chromosome/DNA
Region with fibrils
Region with granules
Figure 2.13 Schematic interpretation of the nucleolus structure
(vi) It has a region with chromosome where one or two chromosomes
or DNA are found. In fact, it is the genes (organisers) found in
the DNA that will start the process of nucleolus formation.
The genes code the rRNA and protein of the ribosome. The
nucleolus is the structure that is involved in the process of
making ribosomes.
(vii) It has another region with fibrils where the transcription of
genes forms rRNA. Some of the RNA act like mRNA, move
out into the cytoplasm and are translated into proteins by
ribosomes there. The others are rRNA and combined with the
proteins moved in from cytoplasm to form coarser fibrils before
they coil to form the ribosome subunits.
(viii) It has a third region with granules where rRNA and protein
interact, coil and fold to form two types of ribosome subunits.
One type is larger than the other. The larger is the 60S type and
the smaller is the 40S type. These granular ribosome subunits
will move away from the nucleus through the nuclear pores
into the cytoplasm.
(ix) There is a cyclic change for the nucleolus like the nuclear
membrane. It disappears during prophase of cell division and
reappears later at telophase. This is because RNA and protein
can be hydrated at prophase. The cells have to form ribosomes
after cell division.
(d) Chromosomes (and its organisation)
(i) Chromosomes are structures that are formed from DNA and
histone during metaphase of mitosis. The DNA of the nucleus
at other times can also be called chromosomes.
(ii) Chromosomes have no shape and they are not organised during
interphase. They exist as chromatin, long DNA molecules with
certain parts attached with histone. The part of DNA that is not
coiled around histone contains active genes. The genes can later
be transcribed to form proteins.
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(iii) The number of chromosomes per cell ranges from 8 – 100. The
chromosomes in diploid organisms exist in pairs. For example,
each human somatic (non-gametic) cell has 46 chromosomes,
in 23 pairs. In gametes, the number is halved.
(iv) Homologous chromosomes are identical in structure, though
not necessarily in allelic constitution.
2 (v) Maternal chromosomes are the set of chromosomes which
originates from the mother through the ovum. Paternal
chromosomes are the set that originates from the father through
the spermatozoon.
(vi) Sex chromosomes determine the sex of the organism. For
example, females have a pair of homologous X chromosomes
whereas the males have an X chromosome and a non-
homologous and much smaller Y chromosome. A gene called
the t factor in the Y chromosome determines the formation
of testes during the formation of the sex organ in the foetus.
Otherwise, the foetus will be a female.
(vii) Chromosomes other than the sex chromosomes are called
autosomes. They are usually in larger numbers. For example,
the human cells have 22 pairs of autosomes and one pair of sex
chromosomes.
(viii) The size of the chromosomes varies between species. The
average size of the human chromosome is 6 µm. The largest
human chromosome is labelled as chromosome 1 and the
smallest as chromosome 22 and chromosome Y.
Plants usually have larger chromosomes than animals. Birds and
fungi are among the organisms with the smallest chromosomes.
(ix) The chromosome shapes vary during the cell cycle. When
we refer to the chromosome shapes, we refer to their shapes
during metaphase of mitosis. Chromosomes consist of two
strands of cylindrical chromatids, which are attached together
by a structure called centromere. Therefore, the shapes of
chromosomes are determined by the position of the centromere,
which can be metacentric, telocentric or acrocentric, as shown
below.
Metacentric Telocentric Acrosentric
Figure 2.14 Types of chromosomes
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The centromere is the primary constriction of the chromosome.
Besides the centromere, there may be another constriction which
is called secondary constriction. Some chromosomes may have
secondary constriction. Other chromosomes may be of longer or
shorter length. All these determine the shape of chromosomes.
2
Chromosome with Chromosome with
secondary constriction extra length
Figure 2.15 Two types of abnormal chromosomes
(x) The structural organisation of chromosome is as follows:
• Each chromosome is made up of two sister chromatids which are
identical. Each chromatid is made up of one molecule of DNA as
a result of replication. Each of the molecules becomes a chromatid
and they are attached together by the centromere.
• During prophase, each DNA molecule winds around a group of
8 histone molecules, forming a complex unit called nucleosome.
During interphase, a certain amount of DNA forms nucleosomes,
and the genes are inactivated.
• 6 such nucleosomes may coil regularly to form a secondary
structure, which may be folded or coiled to become the compact
chromatid.
• Such regularity may not be present in all species of organisms
especially in bees, whose chromosomes with DNA and nucleosomes
are folded, rather than neatly coiled.
• The centromere is a constricted portion of the chromatid where
protein keeps the two chromatids together. At the end of mitotic
metaphase, the centromere divides, chromatids separate and are
pulled by spindle fibres to the opposite poles.
Secondary
coiled
DNA Histone Nucleosome
Chromatin
fibre
Tertiary coiled Coiling
Figure 2.16 Organisation of chromosomal structure
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Exam Tips (xi) The chromosomes perform two major functions as follows:
• Chromosomes that contain genes, control the production of
Remember that organisation
of chromosomes includes RNA and proteins in cells. Through these proteins, especially the
not only structural enzymes, chromosomes control all the activities of the cell and
organisation but also that inheritable characters of an organism.
during interphase, their
organisation into pairs and • The compact chromosomes formed during metaphase enables
2 sex chromosomes. mitosis and meiosis to take place. These chromosomes can
move easily compared to the untidy long slender DNA. Hence,
chromosomes enable genes to be passed down from one mother cell
to daughter cells and thus, one generation to the next generation.
Endoplasmic reticulum (ER)
1. ER is a network of flattened sacs and tubules that interconnect to
form a complex structure in the cytoplasm for internal transport of
substances.
2. Each flattened sac or tubule is called cisterna. These interconnecting
cisternae form the basic units of function for ER.
3. The membrane of the ER is the typical lipoprotein type. The
membrane is not folded and the proteins on both sides are of
different types.
4. The content of the cisternae is a sol called matrix. The matrix varies
in content between different cells and contains a mixture of proteins.
5. The outside of the cisternae form a complex network of inter-
cisternal space. Its composition is the same as the cytoplasm but with
microfilaments attached on its outer membrane to maintain the ER’s
shape.
6. The membrane of ER may connect to the outer membrane of the
nucleus, which may continue to expand to form more ER membrane.
The ER itself will bud off to form the Golgi apparatus. Certain parts
of the ER may connect to the plasma membrane through the tubules.
7. The size of ER depends on the type of cell. In glandular cells and
liver cells, the ER is very big and complex.
8. ER can be divided into two types: the rough ER and the smooth ER.
The smooth ER is formed from the rough type.
(a) Rough ER
(i) The rough ER is the type with a lot of ribosomes attached
to its outer surface. It is found in glandular cells that
produce a lot of protein for secretion, such as the glandular
or goblet cells of the digestive system including pancreas,
stomach and small intestine.
(ii) These ribosomes produce proteins for export in the
cytoplasm attach themselves on the surface of the ER. Such
proteins have signal sequence to attach to the surface of
ER. The protein formed then enters the matrix of cisterna
through special pores. The protein is later moved to the
Golgi apparatus, packed into vesicles and exported through
exocytosis.
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Polysome
Cisterna
Fenestration 2
(perforation) in
reticulum sheet
Ribosomes
Lamela of reticulum
node made up of Smooth endoplasmic reticulum
double membrane with branching tubes
Rough endoplasmic reticulum
Figure 2.17 Structure of rough and smooth endoplasmic reticulum
(iii) The two major functions of the rough ER are as follows:
• Rough ER produces proteins such as digestive enzymes
found in the glandular cells of the pancreas, stomach,
small intestine and liver.
• Rough ER transports proteins to smooth ER or to the
Golgi apparatus through sacs pinched off from its surface
membrane. Protein like mucus has its carbohydrate
component added in the smooth ER or the Golgi
apparatus.
(b) Smooth ER
(i) Smooth ER is the type with little or no ribosome on its
surface. Embedded on the inner surface of the membrane,
there are a lot of enzymes catalysing the synthesis of
carbohydarates and lipids. Vesicles and larger sacs bud off
to fuse with the cisternae of the Golgi apparatus.
(ii) The functions of the smooth ER are as follows:
• In animal cells, the smooth ER produces and transports 2011
lipids, including oils and phospholipids, sex hormones
such as in the testes and ovaries, and in the brain cells.
• In the liver cells, smooth ER detoxifies drugs and toxins
in our body, with the help of enzymes.
• In the striated muscles, smooth ER, called sarcoplasmic
reticulum is involved in the storage and transport of
calcium ions.
• In the meristem cells, smooth ER forms cellulose,
hemicellulose and pectin. The smooth ER transports
them to the central plate where they are used to form
new cross walls after mitosis.
• Smooth ER forms lysosomes which are vesicles that are
used for internal transport and reactions.
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Language Check
Language Check
Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
2015 Mitochondria
1. Mitochondria are the ‘power houses’ of the cell, where energy in the
form of ATP is formed. The energy comes from cellular respiration
Language Check where ATP is formed from the bonding of phosphate to ADP.
Language Check
• Mitochondrian – singular 2. Mitochondria are found in every eukaryotic cell. Their location
Mitochondria – plural inside the cell is not fixed as they can move around.
2
3. Protozoa and yeasts have only one mitochondrion per cell. In the
liver cells, there are between 500-1,400 mitochondria per cell whereas
in the root cap cell of maize, they vary between 100-3,000 per cell.
The number can be increased when a cell becomes more active and
needs more energy.
4. Each mitochondrion can divide to form two mitochondria. This
happens when the cells become active or just before cell division.
However, such division cannot occur outside the cells, as it requires
enzymes coded by the nucleus.
5. Usually they are spherical, oval or sausage-shaped. It can be like a
rod or a spiral. The shape is changeable.
6. Mitochondria are considered medium size organelles in the cell.
They can hardly be observed under light microscope. Their diameter
is 0.25-1 µm and length 3.5-10 µm.
7. They have 65-75% protein, 25-35% lipid and about 0.5% of nucleic
acids.
8. They have an envelope with liquid matrix within as shown below.
Outer membrane
Cristae
Inner membrane
Matrix
Stalked granule
Figure 2.18 A mitochondrion
9. Its envelope is made up of two layers of lipoprotein membranes with
an inter-membrane space in between.
10. The outer membrane is smooth with no granule attached. It has a lot
of pores with diameters between 2.5-3.0 nm. Such pores are part of
the channel proteins or translocase for the passage of ADP or ATP
+
and NAD or NADH.
11. The inner membrane is folded to form cristae that are tube-like
in plant cells or folding in animal cells. The cristae will increase in
number when the respiration rate inside the cell increases. There are
a lot of stalked granules embedded on the inner membrane.
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12. The size of these granules vary. These are the ATP synthase enzyme
that performs oxidative phosphorylation to produce ATP from ADP
and phosphate, in the presence of oxygen.
13. The colloidal interior of the mitochondria contains ribosomes, DNA,
RNA and a lot of enzymes, which are involved in the Krebs cycle and
the oxidation of fatty acids.
2
14. The functions of mitochondria are as follows:
(a) Mitochondria carry out Krebs cycle, part of cellular respiration
within their matrices.
(b) They carry out oxidation and complete breakdown of fatty acids
into carbon dioxide and water to produce ATP.
(c) They carry out oxidation and complete breakdown of amino
acids.
(d) They carry out oxidative phosphorylation, which produces ATP
from ADP and phosphate.
(e) They produce their own proteins from DNA with the help of
RNA. The proteins are those required for the oxidative process.
Golgi apparatus (Dictyosome in plant cells)
1. Golgi apparatus is an organelle consisting of a stack of flattened sacs, 2014
which produce vesicles full of secretion for internal or external uses.
2018
2. They are found in large numbers in glandular cells, neurones, muscle
tissues, root cap cells and meristems of plants. Their locations within
the cell are not fixed. They are formed from ER.
3. There is usually one of them per cell. However, there are many
in glandular cells and their number can increase as the secretory
activities increase. Meristem has more of them per cell.
4. Each consists of a stack of flattened sacs called cisternae, which are
rough and circular with a network of tubules around their periphery
as shown in Figure 2.19.
Vesicle budding off and moving
towards cell membrane
Cisterna
Inter–cisternal space
Network of
tubules
Sac from ER added to the
convex face
Figure 2.19 Golgi apparatus
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5. Each of the cisternae is of different sizes of 1-3 µm in diameter and
0.05 µm in thickness. There are channels connecting one cisterna
to another. Usually the whole stack is curved with its convex cis face
facing the nucleus. Sacs are added onto the convex surface from ER
for the transport of protein, lipid or carbohydrate.
6. Vesicles can bud off carrying secretion of protein, glycoprotein
2 or lipid. The whole cisterna of the outermost trans face can be
completely budded off as vesicles.
7. The membrane is of lipoprotein type. The membrane can be added
on to form new cisterna at one side and budded off completely on
the other side.
8. There are microfilaments that bind the cisternae to keep them in a
stack.
9. The functions of Golgi apparatus are as follows:
(a) It forms lysosomes through the budding of larger vesicles or
fusion of several smaller ones.
(b) It processes proteins transported from ER to form glycoprotein
before it is packaged into vesicles to be exported from the cell.
(c) It packs digestive enzymes and export them as in the pancreatic
glandular cells.
(d) It produces cell wall materials in vesicles, which are directed
to the cell plate where new cell wall is formed after mitosis in
meristem.
(e) It can process lipids to form glycolipids, package them, transport
and store them within the cell.
(f) It exerts some forms of control over internal transport of vesicles
from one part of the cytoplasm to another part.
(g) It also exerts control over the turn over of the plasma membrane
as each time exocytosis takes place, a certain amount of plasma
membrane is added.
Exam Tips Lysosomes
Remember the process of 1. Lysosomes are spherical vesicles that contain digestive hydrolases.
lysosome action, which
includes its membrane 2. They are found in cells that carry out endocytosis such as phagocytes
fusing with the membrane and protozoa. They are found in most animal cells as well as cells of
of organelle or plasma
membrane to release its metamorphorsising insects and tadpoles. Usually they are absent in
content of hydrolases. The plant cells except in immature xylem cells and sieve tubes.
hydrolases will hydrolyse
complex biochemicals to 3. There may be only one lysosome per cell. A lot of them are usually
their simple and absorbable present in the phagocytes and cells of the tadpole tail.
products.
Remember that nucleus, 4. It is spherical in shape, bound by a layer of lipoprotein membrane.
ribosomes, ER, the Golgi
apparatus and lysosome 5. The size varies from 0.1 to 0.5 µm.
are inter-related.
6. Its membrane is the usual single layer of lipoprotein but the enzymes
it carry do not digest it.
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7. Their content is acidic, homogenous in nature and contain many
types of hydrolases. They include proteinase, lipase, carbohydratase,
acid nuclease (DNase and RNase) and acid phosphatase.
8. The functions of lysosomes are as follows:
(a) It can digest foreign substances or cells that are endocytosised.
(b) It clips certain bond such as that of thyroglobulin. Thyroglobulin
is formed within the follicle of the thyroid gland. When it passes 2
through cells lining the follicle, thyroxin is released from the
globulin and emptied into the capillary.
(c) It can carry out autophagy i.e. old or worn out organelles are
digested by their digestive enzymes. Red blood cells have their
nuclei digested during their course of development in bone
marrows.
(d) It exports their enzymes by exocytosis such as in the cartilage by
osteoclasts during its development to form bones.
(e) It can carry out autolysis in which the whole cells are digested for
rebuilding of new tissue during metamorphorsis. This happens
in the tail of the tadpole where the digestion products are used
for building lungs and adult skin.
Ribosomes
1. Ribosomes are small granules where synthesis of proteins occurs.
2. They are found in all cells particularly cells that produce a lot of
proteins such as the glandular cells of pancreas and liver. Ribosomes
are found in the nucleus, free in cytoplasm or in cytoplasm attached
to ER, mitochondria and chloroplasts.
3. Their number is not fixed. It is found in large numbers in the
glandular cell that produces a lot of proteins and its numbers can
increase.
4. They are spheroid in shape, consisting of two subunits in which one
is larger than the other such as shown in Figure 2.20.
Stalk
Central ridge
Large
subunit
Wing
Platform 20 nm Small subunit Large subunit
Small Cleft
subunit
Head
Front view Side view
Figure 2.20 Ribosome structure
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5. They are very small, about 22 nm in diameter for the eukaryotic 80S
types and 18 nm diameter for the prokaryotic 70S types which are
found in prokaryotic cells as well as mitochondria and chloroplasts.
6. The subunits can be attached to form bigger functional units in the
presence of magnesium ion.
For eukaryote: 60S + 40S 80S
2 For prokaryote: 50S + 30S 70S
7. They are made of RNA and protein synthesised in the nucleolus. As
for the 80S types, the 60S subunits contain about 3 types of RNA and
equal number of types of proteins. The 40S smaller subunits contain
1 type of RNA and protein.
8. As for the 70S types, the larger 50S subunits contain 3 types each of
different RNA and proteins. The smaller 30S subunits contain only a
single type each of RNA and protein.
9. The function of ribosomes is to provide the site for the formation of
peptide bonds in which amino acids are joined to form polypeptide
or protein. The subunits can form a complex with mRNA. Two sites
are found on the surface where two tRNAs will each bring an amino
acid to the corresponding site, matching the codons of mRNA to that
of the anti-codon of the tRNA. Therefore, ribosomes can ‘read’ the
codons on the mRNA and join specific sequence of amino acids to
form specific protein.
Chloroplasts
1. Chloroplasts are plastids, organelles that contain chlorophyll and
carry out photosynthesis.
2. Chloroplasts are found in the part of the plant that is green in
colour, especially in the mesophyll cells of leaves, parenchyma of
young stems, fruits, sepals and even aerial green roots.
3. Their locations in cells are not fixed, they can move and orientate
themselves with their larger surface towards the sunlight. This is to
enable them to obtain the maximum amount of sunlight.
4. There are about 100 of them in a palisade mesophyll cell of flowering
plants. The number can be increased when light intensity increases,
or decrease if the light intensity decreases.
5. They have biconvex circular shapes like that of lenses.
6. They are reasonably big to be observed under a light microscope.
They are about 3-10 µm in diameter and 2-3 µm thick.
7. The envelope consists of two layers of lipoprotein membranes that
are smooth, with no foldings or granules.
8. There is an internal membrane system inside the chloroplast called
the thylakoid system within a liquid called stroma. The internal
structure is shown in Figure 2.21.
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Chloroplast envelope Ribosomes
Oil droplet
Thylakoid
Stroma
Granum 2
Language Check
Starch grain Language Check
Chloroplast DNA
Intergranal lamella
(a)
Intergranal lamellae
Language Check
Language Check
Thylakoid
• granum – singular
grana – plural
• lamella – singular
lamellae – plural
(b)
Figure 2.21 Structure of chloroplasts
9. The thylakoid membrane forms circular discs that are stacked like
shillings called granum.
Exam Tips
10. Each granum is made up of 10-100 thylakoids stacked together.
There are 40-60 grana per chloroplast. Remember the structure,
functions and distribution
11. There are channels called inter-granal lamella connecting one of chloroplast of flowering
plants.
thylakoid of a granum to another granum. These inter-granal Remember that three
lamellae form a network between the grana. The membrane of organelles i.e. nucleus,
the tylakoids and lamellae is lipoprotein in nature, but there are mitochondria and
chloroplasts are surrounded
photosynthetic pigments forming photosystems that are studded in by envelope of double
the membrane of both the lamellae and grana. Each photosystem lipoprotein membrane
consists about a mixture of 300 various chlorophyll, carotenoid and and they have several
similarities.
protein molecules to form a complex.
12. Stroma contains a colloidal sol where enzymatic reactions that
require no light to take place. It contains the followings:
(a) Enzymes, especially those involved in the Calvin cycle where
reactions forming carbohydrates and other organic compounds
take place.
(b) End products of photosynthesis such as sucrose, starch and fat
droplets, which are usually attached to the lamellae.
(c) Intermediate compounds, such as organic acids, phosphorylated
monosaccharides and their acids.
(d) Ribosomes of the 70S types.
(e) A circular ring of DNA and RNA.
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13. The functions of chloroplasts are as follows:
(a) The major function of chloroplast is to carry out photosynthesis
producing organic compounds especially carbohydrates.
(b) Chloroplasts carry out their functions by using the membranes
of thylakoids and lamellae to trap lights and convert them to
chemical energy mainly in the form of ATP. They carry out
2 photoactivation and photophosphorylation through Calvin cycle.
(c) The ATP is then used to perform the fixation of carbon dioxide
to become organic compounds in the stroma.
(d) The DNA and the protein synthetic system in the chloroplasts
produces some of the specific proteins used in photosynthesis.
The chloroplasts still depend on the nucleus to obtain most
proteins within.
(e) The chloroplast can divide especially in their premature
protoplastid stage in the meristem. Mature chloroplasts do divide.
Centrioles (Centrosome)
1. Centrioles are organelles that assemble spindle fibres in animal cells
but are not found in plant cells.
2. They are found in all animal cells except the nerve cells. They are also
found in fungal and algal cells.
3. One pair of centrioles is usually located beside the nucleus.
4. The two centrioles are cylindrical in shape, arranged perpendicular
to one another as shown in Figure 2.22.
Figure 2.22 The structure of centrioles
5. They are small and can be observed as a dot under a light microscope.
Their length is about 0.3-0.4 µm with a diameter of about 0.2 µm.
6. Each is made up of 9 triplets of microtubules, which are attached
lengthwise together as shown as a cross-section in Figure 2.23.
Centriole cylinder
consisting of 9 sets of
microtubules. Each 1 set (3 microtubules)
set has 3 microtubules
Third microtubule
Second microtubule
First microtubule
Figure 2.23 9 triplets of microtubules
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7. Centrioles divide during prophase of mitosis and each pair can move Summary
to opposite poles as shown in Figure 2.24.
Structures and functions of
organelles
1. Nucleus (nuclear
envelope, nucleolus,
nucleoplasm and
chromosomes) – to 2
control all activities of
cell through the genes.
2. Endoplasmic reticulum:
RER (with ribosomes) –
to produce proteins for
export out of cell
SER (with no ribosome)
– to produce lipids
– to detoxify toxins
3. Mitochondrion (double
membranes with matrix)
Figure 2.24 Division of centrioles and formation of spindle fibres
– to carry out respiration
8. The functions of centrioles are as follows: and production of ATP
(a) Centrioles organise the formation of spindle fibres, which are 4. Golgi apparatus (stack
of flattened sacs) – to
attached to the centromeres of chromosomes during metaphase. package substance for
export out of cell
During anaphase, the chromosomes or the chromatids will 5. Lysosome (vesicles of
separate and are pulled by the fibres to opposite poles in meiosis hydrolases) – to digest
or mitosis respectively. internal substances and
(b) Centrioles organise the formation of cilia and flagella, which external substances
6. Ribosome (2 combined
also have a “9 + 2” pattern. units of protein and
RNA) – to produce
Vacuoles protein
7. Chloroplast (double
1. Vacuoles are sacs with lipoprotein membrane, which are usually membranes + thylakoid
spherical in shape. system) – to carry out
photosynthesis
2. There are three types of vacuoles: 8. Centrioles (2
(a) Sap or central vacuole short cylinders of
microtubules) – to
(b) Food vacuole organise formation of
(c) Contractile vacuole spindle fibres in animal
cell
3. Sap or central vacuole 9. Vacuoles (spherical
(a) Sap vacuole is found in plant cells. sacs):
Sap vacuole – stores
(b) Sap vacuole is small and numerous in young plant cells but is water
big and can occupy 90% of the volume of matured plant cells. Food vacuole – digests
It contains water, sucrose, amino acids and some mineral ions food
Contractile vacuole – to
especially those in excess or wastes such as silicates. expel excess water
(c) Sap vacuole stores water and mineral ions. It can balance water
potential when required and acts as a store for waste products.
(d) Sap vacuoles in mesophyll cells of leaves push the chloroplasts to
the edges so that the chloroplasts can receive maximum amount Cell Structures
of light.
VIDEO
4. Food vacuole
(a) Food vacuole is found in cells that perform endocytosis such as
phagocytic white blood cells and protozoans.
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(b) Food vacuoles or phagosomes in phagocytes are small. They
contain bacteria, organic particles and dissolved proteins.
(c) Food vacuole is a place for food digestion. Any undigested food
is egested through the plasma membrane.
5. Contractile vacuole
(a) Contractile vacuole is found in freshwater protozoans.
2 (b) Contractile vacuole is spherical in shape, able to absorb water
and contract, forcing water out through the membrane. Those
found in Euglena and Paramecium have feeding smaller sacs or
tubules to empty water into the main vacuole.
(c) Contractile vacuole acts as an osmoregulatory mechanism to get
rid of excess water in freshwater protozoans. If not, the cell may
burst due to excessive water absorbed through osmosis.
Quick Check 3
1. Mitochondria and chloroplasts seem to have evolved from endosymbiosis of prokaryotic cells.
Suggest reasons for this.
2. Export of substances from cells requires concerted actions of nucleus, ER, the Golgi apparatus and
lysosomes. Why?
Differential Centrifugation
1. Differential centrifugation is a technique of separating cell components,
including macromolecules using a centrifuge. Centrifuge uses
centrifuging force equivalent to many times that of gravitational force
(g) to spin down cell components of different S values (sedimentation
units) step by step. 2017
2. The procedure to fractionate cellular components is as follows:
(a) Tissue is first homogenated with a homogeniser, which can be
very sophisticated using ultrasound to break up cells to the level
required.
(b) Normally the tissue has to be chilled and added with buffer to
keep the enzymes functional and added with isotonic solution to
prevent the organelles from breaking.
(c) The homogenate is centrifuged at 600 times gravity for 10
minutes for animal tissues. Nuclei and unbroken cells are spun
down.
(d) Then, the supernatant is centrifuged at 10,000 times gravity for
20 minutes. Mitochondria, cisternae of endoplasmic reticulum
and Golgi apparatus are spun down.
(e) Further centrifuge at 100,000 times gravity for 60 minutes will
spin down ribosomes, microtubules and microfilaments. The
supernatant will then contain macromolecules such as nucleic
acids and proteins.
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Animal tissues Exam Tips
Remember the basic
Homogenisation principles of centrifugation.
Remember the examples
Homogenate of uses in the isolation of
cellular components.
Centrifugation at 600 g
for 10 minutes 2
Nuclei and Supernatant Summary
unbroken cells
Centrifugation at
10,000 g for 20 minutes Differential centrifugation
1. Fractionation by
homogenisation
2. Centrifuge 600 g for 10
min to obtain nuclei
Mitochondria, ER Supernatant 3. Centrifuge 10, 000 g
and Golgi bodies for 20 min to obtain
Centrifugation at mitochondria and
100,000 g for 60 minutes chloroplasts
4. Centrifuge 100,000 g
for 60 min to obtain
ribosomes
Ribosomes, microtubules Nucleic acids and 5. Ultra-centrifuge with gel
in vacuum to separate
and microfilaments proteins component of ribosome
proteins, RNA and DNA
Figure 2.25 Step by step cell fractionation of different S values
3. Further differential centrifuge is ultra-centrifugation using force
with more than 100,000 times gravity.
4. This technique is to separate mixture of macromolecules of different
molecular weights or S values. S value is a scale of sedimentation Info Bio
units for molecule to move down in gel used in ultra-centrifugation.
Higher S values are heavier and more stream-lined. For examples, 80S ribosomes can be
separated into 60s and 40s
DNA can be separated from RNA, nucleic acids can be separated from sub-units. Why 60s + 40s
not equal to 100s? This is
proteins and radioactive DNA can be separated from normal ones. due to 80S ribosomes with
one big and one small units
5. The method and precautions are as follows: are not so stream-lined. So,
(a) The space within the ultracentrifuge should be vacuumed to they have lower s units.
avoid any friction between the tubes and the air.
(b) The temperature has to be lowered.
(c) Gel is added to stop the molecule at certain levels in the tube.
(d) Dye is added to the mixture to detect separation.
Quick Check 4
1. How can radioactive DNA and normal DNA be separated by ultra centrifugation?
2. How can the background of different refractive indexes with the object be lightened or darkened?
3. Why can electron beam and not light beam be used to resolve smaller objects?
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Learning Outcomes 2.3 Specialised Cells
Students should be able to:
(a) outline the structures,
functions and 1. Specialised cells are cells that have undergone specialisation or
distributions of differentiation in the course of embryonic development. All cells
unspecialised cells
found in plants of a multicellular organism develop from a single zygotic cell after
(meristematic cells); fertilisation. Since then, mitosis increases the cell number and
2 (b) describe the
structures, functions ensures exact genetic makeup in all the cells formed later. However,
and distributions of in the course of development, cells acquire different structures and
specialised plant cells
found in epidermal, functions.
ground and vascular
tissue; 2. Specialisation is a result of genes being ‘switched on’ or ‘switched
(c) describe the off’ during development, even though the cells have the same genetic
structures, functions
and distributions of content for a particular individual.
specialised animal cells
found in connective, 3. Red blood cells have their haemoglobin genes ‘switched on’ when
nervous, muscular they were developed in the bone marrow, but their melanin genes
and epithelial tissues,
including the formation are ‘switched off’. Conversely, skin cells have their haemoglobin
of endocrine and genes ‘switched off’ whereas their melanin genes ‘switched on’ in the
exocrine glands.
course of development.
Unspecialised Cells Found in Plants
Meristematic cells
Exam Tips 1. Meristematic cells are cells found in the meristem, a localised tissue
Remember the eight basic that can divide by mitosis.
plant tissues.
2. The structures of meristematic cells are as follows:
(a) All the cells are young and have not undergone differentiation.
They look the same and have the same size.
Summary (b) Apical meristematic cells have isodiametric prismatic shapes.
They are all regular in shape and almost look spherical.
Unspecialised cells in Cambium cells are brick-like and some are very thin.
plants (meristematic cells) (c) The cell wall is thin, only made up of primary cell wall. The cells
Thin wall, little cytoplasm
and compact can be easily damaged and nutrients can easily diffuse into them.
1. Apical meristem – found (d) The nucleus is large, relative to the volume of the cell. All the
at tips of shoot and root nuclei are ready to start mitosis.
to form primary tissues
2. Vascular cambium (e) There is no intercellular space; the cells are compact and are
– found in vascular close together.
bundle woody plants to (f) The cytoplasm is dense, with few organelles which are small.
form secondary xylem
and phloem All the organelles are young. Chloroplasts if present, are in the
3. Cork cambium – found proto-plastid stage.
beneath epidermis
of woody dicots to 3. The distributions and functions of meristem are as follows:
produce cork
4. Intercalary cambium (a) Apical meristem. It is found in the shoot and root tips. Its
– found in grasses to function is to produce primary tissues, such as tissues found in a
produce cells above herbaceous plant.
nodes
(b) Vascular cambium. It is found mainly in woody stems and
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roots. Its function is to produce secondary xylem (wood) and
phloem.
(c) Cork cambium (phellogen). It is found on the outer layer of
dicotyledonous woody stem and root. Its function is to produce
cork cells (phellem) on the outside and secondary cortex
(phelloderm) on the inside.
(d) Intercalary cambium. It is only found in monocotyledonous 2
plants, especially Gramineae. It is a thin layer of cells above the
node of the stem when it is young. Its function is to produce
more cells for the internode and later, it disappears.
Specialised Plant Cells
Parenchyma
1. Parenchyma is a tissue with the least differentiation, with a thin wall
and contains living protoplast and nucleus. 2017
2. It has the following structural features.
(a) The cells are spherical, isodiametric and may be oval in shape.
(b) The cells are the least differentiated i.e. with very few specialities.
(c) The cells are alive, with protoplast and nucleus. All the enzymes
within are active.
(d) Their cell walls are thin, which are made up of only primary cell
walls.
(e) The sap vacuole is big and centrally located. It stores water and
soluble mineral ions.
(f) Their protoplast may store starch and they have chloroplasts.
Some parenchyma are full of starch grains especially is storage
organs.
(g) The cells usually have large amounts of intercellular space. The air
spaces allow easy exchange of gases especially for photosynthesis.
(h) The cells can divide if stimulated by hormone such as auxin.
3. Its distributions are shown in all the eight basic organs (Figures 2.26
– 2.33).
(a) In the cortices (singular cortex, layer beneath epidermis) of
stems and roots.
(b) In the pith (centre of root or stem) of dicotyledenous stem and
monocotyledenous root.
(c) In the mesophyll of leaves.
(d) In the medullary rays of the secondary xylem and phloem.
(e) Around vascular bundles in stems and leaf stalk.
(f) In the epidermis with thickened wall and cuticle.
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Epidermis Epidermis
Cork cambium
Collenchyma
Secondary
Sclerenchyma cortex
Cortex
Phloem
Cambium
Cambium
Vascular Phloem
2 bundle Pith
Xylem Xylem
Pith
Figure 2.26 Dicot herbaceous stem Figure 2.27 Dicot stem with secondary thickening
Piliferous layer
Epidermis
(epidermis)
Sclerenchyma Cortex
Xylem
Vascular
bundle Phloem
Endodermis
Xylem
Phloem
Figure 2.28 Monocot stem Figure 2.29 Dicot herbaceous root
Cork Epidermis
Cork cambium
Sclerenchyma
Secondary cortex
Cortex
Phloem Pith
Endodermis
Cambium Xylem
Phloem
Xylem
Figure 2.30 Dicot root with secondary thickening Figure 2.31 Monocot root
Upper epidermis
Collenchyma
Lower epidermis
Palisade Mesophyll cells
mesophyll
Spongy
mesophyll Xylem Xylem
Phloem
Sclerenchyma
Parenchyma
Collenchyma Phloem
Figure 2.32 Dicot leaf Figure 2.33 Monocot leaf (Zea mays)
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4. The functions of parenchyma are as follows:
(a) It functions as a photosynthetic tissue as in the mesophyll of the
leaves or in young stems.
(b) It functions as a storage tissue as in the cortices or the pith of
stems and roots. Some fruits and seeds have parenchyma cells as
storage tissue.
(c) It functions as packing tissue around the vascular bundles of 2
stems and leaf stalks.
(d) As epidermis, it protects the cells beneath physically and from
desiccation.
Collenchyma
1. Collenchyma is a tissue where the cells have thickened non-uniform
primary cell walls.
2. The structure of collenchyma are as follows:
(a) The cells are alive with protoplast and nucleus.
(b) The shape of the cells is isodiametric or like an elongated prism.
(c) The cell wall is not uniformly thickened. The collenchyma
is divided into two types: angular (thickened at corners) and
lamella (thickened at tangential wall) types.
Figure 2.34 Collenchyma
(d) The cell wall is of primary type and not lignified.
(e) The cells are compact with no intercellular space.
3. Its distributions are shown in Figures 2.26 and 2.32.
(a) It is found below the epidermis of dicotyledonous stems.
(b) It is found beneath the epidermis of the main vein of
dicotyledonous leaves.
4. The functions of collenchyma are listed as follows:
(a) It functions to support the stem or the leaves of dicotyledonous
plants.
(b) It becomes meristematic and produces cork cambium in the
dicotyledonous stem that undergoes secondary thickening.
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Sclerenchyma
1. Sclerenchyma is a simple tissue consisting of fibre cells or stone cells
(sclereids), which have thick walls impregnated with lignin.
2. The structural features are listed as follows:
(a) When sclerenchyma cells mature, the cells are dead and have no
2 protoplast.
(b) They have thick secondary walls impregnated with lignin. Lignin
is a branched polymer which makes the wall very hard and
impervious to water. The walls have many pits.
(c) Their lumens are very small and empty.
(d) The shapes of the cells depend on the types as shown in Figure
2.35.
Thick wall Thick wall
Small lumen
Small lumen Pits
A single fibre A stone cell
Figure 2.35 Two types of sclerenchyma
(i) Fibres
They are like fibres i.e. long and thin with two sharp ends.
They exist in bundles or layers.
(ii) Stone cells
They have a stone shape, usually exist as layers or scattered
stone cells.
3. Their distributions are as follows:
(a) Fibres are found beneath the epidermis and in the bundle sheaths
of monocotyledonous stems or leaves as shown in Figure 2.28
and Figure 2.33.
(b) Fibres are also found on the outside (Figure 2.26) or inside
(Figure 2.27) the phloem of dicotyledonous stems.
(c) Stone cells are found in fresh pears and ciku fruits, which give
them their gritty texture. They are found as hard protective
layers of plums, olive seeds and coconut shells.
4. Their functions are as follows:
(a) Fibres support and protect the plants especially in stems and
leaves.
(b) Stone cells protect seeds and prevent germination so that the
seeds can be dispersed further.
Xylem
1. Xylem is a complex vascular tissue, which is used to transport water
and support plants.
2. Xylem is divided into primary and secondary types. Primary xylem is
formed from procambium at the shoot or root tips. Secondary xylem
is formed from vascular cambium during secondary thickening or
wood formation in the stem or root.
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3. Xylem consists of four types of cells, namely vessels, tracheids,
parenchyma and sclerenchyma.
(a) Vessels
(i) These are the largest cells and are shaped like vessels.
(ii) Their ends slant, open and connect to one another to form
long pipes.
(iii) They are dead cells with hollow lumen and no cross walls 2
when mature.
(iv) They have secondary wall impregnated with lignin.
(v) The wall is specially thickened with patterns depending on 2015
their age or location as shown in Figure 2.36.
Annular Spiral Scalariform Reticulate Pitted
Figure 2.36 Different types of lignin deposition in xylem vessels
(b) Tracheids
(i) They are long, thin cells with polygonal cross sections.
(ii) Their lumens are small and hollow when mature.
(iii) Both ends are tapering, fitting those on the top and bottom
like tiny tubes.
(iv) Their walls are thick, lignified, and covered with a lot of
pits.
(c) Parenchyma
(i) They are just like other parenchyma, only slightly smaller.
(ii) They are young undifferentiated xylem, newly formed from
meristems. They also form medullary rays in wood.
(iii) Their walls are usually not thickened or lignified.
(iv) They store starch in the form of granules.
(d) Sclerenchyma
(i) They originate from old tracheids.
(ii) They look more or less like tracheids.
4. Their functions are as follows:
(a) Vessels and tracheids
(i) They transport water and mineral ions.
(ii) They support the plants especially tall woody plants in
which the wood of the plants consist of xylem.
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(b) Parenchyma
(i) They differentiate to form vessels and tracheids.
(ii) They store food in the form of starch.
(c) Sclerenchyma
Their functions are mainly for support.
5. Xylem is found
2 (a) in the stem, towards the inside of the vascular bundles, as in
Figures 2.26, 2.27 and 2.28.
(b) in the roots, towards the inside of the vascular tissue, as in
Figures 2.29, 2.30 and 2.31.
(c) in the leaves, on the upper part of the vascular bundles, as in
Figures 2.32 and 2.33.
Phloem
1. Phloem is a complex vascular tissue that translocates organic food
especially sucrose and amino acid formed after photosynthesis.
2. Like xylem, phloem is divided into primary and secondary types.
3. Phloem consists of sieve elements, companion cells, parenchyma
and sclerenchyma.
(a) Sieve elements
(i) Sieve elements consist of sieve cells and sieve tubes.
(ii) Sieve cells are young cells with nuclei but with no defined
sieve plates.
(iii) Sieve tubes are mature cells, thin walled with no nucleus
but with protoplast and sap vacuoles.
(iv) Sieve tubes are long and slender cells connecting end to
end to form tubes for translocation.
(v) Their cross sections may be rectangular, round or polygonal.
(vi) The cross walls of sieve tubes form lignified sieve plates
with holes, allowing protoplast to flow from one tube to
another. These plates have supporting functions as they
prevent breakage of these thin-walled tubes under pressure.
(b) Companion cells
(i) They are small slender cells fitting neatly end to end, at
least one beside a sieve tube.
(ii) Their walls are thin and usually square in cross sections.
(iii) They have nuclei, compact protoplast with high organic
content, no sap vacuole but with a lot of mitochondria.
(iv) Their cytoplasm is connected by many plasmodesmata to
the sieve tubes. Sucrose and amino acid can diffuse to and
fro between companion cells and sieve tube.
(c) Parenchyma
(i) They exist as undifferentiated phloem or medullary rays
extended from xylem.
(ii) Their structures are the same as ordinary parenchyma but
smaller in size.
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(iii) They have thin primary wall. Summary
(iv) They are usually elongated prisms.
(v) They are living cells with protoplasts and nuclei. They store Specialised plant cells
starch. 1. Parenchyma (thin wall,
(d) Sclerenchyma living with intercellular
(i) It is found in dicotyledonous stems, either on the outside spaces) found in
epidermis, mesophyll,
or inside of the phloem. cortices, pith and 2
(ii) The structure is the same as an ordinary one. ground tissue – for
packing, storage of food
4. The functions are as follows: and photosynthesis
(a) Sieve elements 2. Collenchyma (non-
uniformly thickened
They translocate organic food substances especially sucrose, wall, living and
amino acids, organic acids and proteins. compact) found
(b) Companion cells beneath dicot stem
and leaf epidermis –
They provide energy in the form of ATP and their membranes for strengthening and
have proton pumping-system for the loading of sucrose into the support
3. Sclerenchyma (lignified
sieve tubes from neighbouring mesophyll cells. thick wall fibres or stone
(c) Parenchyma cells) found in phloem
They differentiate to form phloem cells and some remain as of dicot stem, beneath
epidermis in bundle
storage cells. sheath of monocot
(d) Sclerenchyma and in husk or stone of
They protect and support the thin-walled phloem cells. seeds – for protection
and support
4. Xylem (mainly vessel
5. They are found in stems, roots and leaves as shown in Figures 2.26 – element and tracheid of
2.33. thick lignified wall) – for
transport of water and
soluble mineral ions
Specialised Animal Cells 5. Phloem (mainly sieve
tubes and companion
1. Animal cells are classified into four types, based on four fundamental cells of thin wall) – for
tissues i.e. epithelial, nervous, muscle and connective tissues. transport of sucrose
and amino acids.
Epithelial tissues
1. Epithelial tissues are covering or glandular cells.
2. They are divided into covering epithelia and glandular epithelia.
(a) Covering epithelia
(i) Covering epithelia are layers of cells that line the external or
internal surfaces of organs.
(ii) Their structural features are as follows:
• The cells are arranged in a single layer called simple
epithelium or in more than one layer called stratified
epithelium.
• The shape of the cells depends on the types; scale-like
called squamous epithelium, cube-like called cuboidal
epithelium and column-like called columnar epithelium.
• The cells are attached to a thin layer of fine connective
tissues at the bottom called basement membrane. This
helps to attach it to other tissues.
• At the top, the cells are exposed to air such as in the skin,
or liquid such as those of the stomach.
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• There are no capillaries in the cells as the cell layers are
too thin for capillaries to enter.
• Mitosis can take place to replace dead or worn out cells
of the skin epidermis.
(iii) Their functions are as follows:
• The epithelia help to protect tissues or organs below
2 them.
• They help to absorb substances and allow substances to
cross them.
• Some of the epithelial cells are modified to form special
receptors for stimuli.
(iv) They are classified based on their cell arrangement or the
shape of the cells.
• Based on the arrangement of cells, they are classified
into three groups:
Simple epithelium – The cells are arranged in one layer.
Stratified epithelium – The cells are arranged in more
than one layer.
Pseudo-stratified epithelium – The cells seemed to be
arranged in layers but each is attached to the basement
membrane.
• Based on their cell shapes, they are classified into four
groups:
Squamous epithelium – The cells are flattened like scales
or tiles.
Cuboidal epithelium – The cells are more or less shaped
like cubes.
Columnar epithelium – The cells look like pillars, their
height is longer than their base width.
Transitional epithelium – The cells can change shape
when stretched.
(v) There are eight types of covering epithelia and the examples
are as follows:
• Simple squamous epithelium. This is found in the outer
layer of Bowman capsule, endothelium of blood vessels
and alveolar walls.
Top view
• Bowman capsule channels filtrate into proximal
convoluted tubule. Endothelium allow exchange of gases
and small molecule within the blood capillaries and
Vertical section
body fluid. Alveolar walls allow gaseous exchange in the
Figure 2.37 Simple squamous
epithelium capillaries and the lungs.
• Simple cuboidal epithelium. This epithelium is found in
the collecting ducts and tubules of the nephron.
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
• The proximal convoluted tubules allows absorption of
filtrate molecules back into blood capillaries. The distal
convoluted tubules and collecting ducts are affected by
ADH to reabsorb back water in the kidneys.
Basement membrane
2
Longitudinal section
Cross section
Figure 2.38 Simple cuboidal epithelium
• Simple columnar epithelium. This epithelium is found
lining the innermost layer of the intestines and stomachs.
• The epithelia in stomach and small intestine can be
modified to form glands to produce mucus and digestive
enzymes.
• The epithelia in small intestine can absorb digested food
molecules. In the large intestine, the epithelium can
absorb water beside producing mucus.
Epithelial lining
Vilus
Capillary
Lacteal
Vertical section of a simple
columnar epithelium
Crypt of
Lieberkuhn
Venule
Arteriole
Lymph vessel
Figure 2.39 Simple columnar epithelium
• Stratified squamous epitelium. This epithelium is found
in the epidermis of skin, lining the innermost layer of the
oesophagus. The epithelium top layer keeps worning off
while the cells at the bottom divide and grow to replace
the loss. It’s function therefore is more for protection.
• Stratified cuboidal epithelium. This epithelium is found Vertical section
in the excretory ducts of sweat glands. Therefore, the Figure 2.40 Stratified
ducts are to channel sweat out. squamous epithelium
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
Summary
Epithelial tissues
Covering epithelia
1. Simple squamous
• for absorption in
endothelium and alveoli
2 2. Simple cuboidal Vertical section
• for absorption in Figure 2.41 Stratified cuboidal epithelium
collecting duct of
nephron • Stratified columnar epithelium. This is found in the
3. Simple columnar secretory ducts of the mammary glands.
• for absorption in small
intestine • The duct is to channel the milkout.
4. Stratified squamous
• for protection in skin
5. Stratified cuboidal
• for transport in sweat
ducts
6. Stratified columnar
• for transport in milk
ducts
7. Pseudo-stratified Vertical section
• for transport in airways Figure 2.42 Stratified columnar epithelium
of lungs
8. Transitional • Pseudo-stratified epithelium. This is found in the
• for stretching in wall of innermost layers of the trachea, bronchi and bronchioles.
urinary bladder
• The function of its cilia is to sweep the dust out.
Glandular epithelia
1. Exocrine glands
• Formed by invagination
with ducts for transport
2. Endocrine glands
• Formed by detachment
with no ducts and Vertical section
produce hormones-into
capillaries Figure 2.43 Pseudo-stratified epithelium
• Transitional epithelium. This is found in walls of
urinary bladders.
• The epithelium allows the bladder to stretch when filled
with water.
Wall relaxes Wall stretches
Figure 2.44 Transitional epithelium
(b) Glandular epithelia
(i) Glandular epithelia are gland cells derived from epithelia and
can secrete liquid containing mucus, hormones or enzymes.
(ii) There are two types of glandular epithelia i.e. exocrine
glands and endocrine glands.
(iii) Exocrine glands
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Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
• They have ducts that secrete substances.
• There are little capillaries in them.
• They produce liquid with proteins or enzymes.
• They are formed by invagination of epithelia as shown in
Figure 2.45.
2
Duct
Gland
Figure 2.45 Formation of exocrine gland
• Examples include tubular glands of the large intestine
and the sweat glands.
(iv) Endocrine glands
• They do not have ducts, thus the secretion is directly
emptied into the blood capillaries around them.
• There are a lot of capillaries in them.
• They produce hormones.
• They are formed is by detachment from the epithelia as
shown below in Figure 2.46.
Detached from epithelium
Gland with capillaries
Figure 2.46 Formation of endocrine gland
• Examples include the thyroid and the adrenal gland.
Nervous tissue 2013
Exam Tips
1. Nervous tissue is a group of nerve cells or neurones together with
Remember the formation
neuroglia or supporting cells, which transmits electro-chemical of endocrine and exocrine
messages called impulses along their membrane. glands.
2. Neurone consists of two parts, the cell body and the nerve process.
3. The cell body has different shapes, depending on the types of Summary
neurone. It is surrounded by a plasma membrane and contains a
nucleus like any normal cell. It has a lot of mitochondria, Golgi Nervous tissues
apparatus, endoplasmic reticulum, ribosomes (previously called 1. Motor neurone
Nissl’s granules) but no centriole. • Short dendron, long
axon for transmitting
4. The nerve process is the thin slender structure attached to the cell impulse from CNS to
muscles
body. It includes the axon, dendron and dendrites. Dendron transmits 2. Sensory neurone
impulse towards the cell body whereas axon transmits impulse away • Long dendron, short
from the cell body. The length of dendron and axon varies. It can axon for transmitting
be very long, up to over a metre in our body. Both are protected impulses from receptors
to CNS
by myelin sheath. The end of the axon or dendron is branched to 3. Interneurone
form smaller dendrites. Axon dendrites end with little knobs called • Dendron and axon same
length for transmitting
impulses from sensory
to motor neurones
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