32 NATURE OF BIOLOGY BOOK 1
2 Membranes and cell organelles
Figure 2.1 This image shows a transverse section of a mouse
tail. Look at the incredible range of different kinds of cells
present: cartilage, connective tissue, nerve, muscle, epithelial
cells and others. The nucleus of each cell contains the same
DNA. Although some proteins are made by all cells, others are
different and give each kind of cell its uniqueness. These are
eukaryotic cells and all share the characteristic of an internal
structure of membranous chambers called organelles. In this
chapter we consider the structure and function of organelles.
We also consider the transport of material within cells and the
passage of material across plasma membranes.
KEY KNOWLEDGEThis chapter is designed to enable students to:
• understand the extent of the plasma membrane in forming a series of membranous channels for the packaging and transport of biomolecules throughout eukaryote cells
• enhance their knowledge and understanding of the structure and function of cell organelles
• distinguish the different ways in which biomolecules enter or leave cells
• develop their knowledge and understanding of connections between cells
• extend their understanding of apoptosis.
MEMBRANES AND CELL ORGANELLES 33
Life or death for a cell?Groups of similar cells form tissues and groups of tissues come together to form
organs. The death of cells is a natural feature of healthy tissue. This ‘programmed
cell death’ was fi rst noted in 1972 by Andrew Wyllie and is called apoptosis
(from Greek, meaning ‘shedding of leaves in autumn’).
In fully formed tissue, cell death and cell reproduction are generally balanced.
If this balance is not regulated, an uncontrolled increase in cells can occur and a
tumour develops. If a tumour continues to grow and invades healthy tissue, it is
said to be malignant. A malignant tumour is called cancer. Too little apoptosis can
lead to cancer and too much can cause degenerative diseases such as Alzheimer
disease.
Figure 2.2 The frequency of
different types of cancer in adult
females
Uterus (4%)
Lung (7%)Non-Hodgkin’s lymphoma (4%)
Colo-rectal (15%)
Other (19%)
Pancreas (2%)
Cervix (4%)
Ovary (4%)
Melanoma of skin (10%)
Breast (26%)
Unknown primary (5%)
Cancer is the second highest cause of death after heart disease in Australia and
breast cancer is the most common cause affecting adult females (see fi gure 2.2).
Although there has been improvement in the treatment of cancers in recent years,
30 per cent of women diagnosed with breast cancer will die from it. Researchers
in the Cancer Division at the Walter and Eliza Hall Institute of Medical Research
(WEHI) in Melbourne are investigating how breast cancer develops. This involves
identifying regulator proteins within cells and investigating the interactions of
these proteins that ultimately decide whether a cell lives or dies. Special stains,
such as those used on the cells in fi gure 2.3, assist in pinpointing the positions
of regulator proteins within cells. Other experiments are aimed at establishing
the physiological role of these proteins. If we have better information about
the control and development of cancers, there is an increased chance that better
treatments can be developed.
Read about Sue Macaulay’s work as a radiographer with St Vincent’s Breast-
Screen service, on page 36.
Figure 2.3 Mouse fi broblast cells.
Fibroblasts are common in areolar
tissue, a connective tissue found
below the skin, around blood vessels
and nerves, and fi lling the spaces
between organs. When images of
Bim proteins (stained red) and Bcl-2
proteins (stained green) within a cell
are superimposed, a yellow colour
results. This indicates that the two
proteins, which are both associated
with programmed cell death
(apoptosis), are bound to the same
membranes within the cytoplasm of
the cell.
10 µm 10 µm 10 µm
34 NATURE OF BIOLOGY BOOK 2
• The cell is the basic unit of structure in living organisms.• Programmed cell death and reproduction of new cells are balanced in
fully formed tissues.
KEY IDEAS
Apoptosis, or programmed cell death, is self- destruction
by cells for the good of the whole organism. What is
the difference between this type of cell death and the
type that we call necrosis? Necrosis occurs if a cell is
seriously damaged by some mechanical or chemical
trauma and this causes general damage to the plasma
membrane of the cell. the plasma membrane can no
longer control what enters or leaves the cell, the cell
swells then bursts and the contents spread out over
nearby cells, causing infl ammation of those tissues.
In apoptosis, cells respond to signals. There are two
main pathways of signals that initiate apoptosis: the
mitochondrial pathway and the death receptor pathway.
Signals from inside a cell — the mitochondrial pathwayIf serious damage occurs inside a cell, for example,
severe DNA damage or malfunction of an oxidative
enzyme, proteins on the surface of mitochondria are
activated and the mitochondrial membrane breaks.
This starts a series of events in the cell, including the
action of caspases (special enzymes that cleave specifi c
proteins at the amino acid aspartite) which enter the
nuclear pores and break DNA into small pieces. Events
after this are similar to those described below for signals
from outside a cell.
Another situation in which a cell may initiate death
itself is if a cell is infected with a virus. The cell identi-
fi es the infection and kills itself before the virus has had
time to replicate and spread to other cells.
Signals from outside a cell — the death receptor pathwayWhy would a perfectly healthy cell receive a message
to self-destruct? There are different reasons. The signal
that a cell may receive could be:
• You haven’t developed fully. This occurs in the
embryonic brain when billions of cells are formed
but some fail to be incorporated accurately into the
brain network. These ‘stray’ cells die by apoptosis.
• There are more of you than are needed. It ‘costs’ an
organism energy and materials to keep unneeded
cells alive. Some immune system cells are produced
in larger numbers than required. These excess cells
die by apoptosis.
APOPTOSIS
Figure 2.4 (a) Scanning electron micrograph (SEM) of
lymphocytes undergoing apoptosis. Note the small bumps,
also called ‘blebs’, on the lymphocyte surfaces.
1 Why are cells known as the basic building blocks of living organisms? 2 How might an examination of cells help diagnose disease?
QUICK-CHECK
(a)
MEMBRANES AND CELL ORGANELLES 35
Death signalsinstruct cell to die
Death signalreceptors
Signal recognisedand self-destruct
program activated
Apoptoticcell
• Caspase enzymes activated• Contact with neighbour cells lost• DNA and proteins fragmented• Cell fragments packaged
• Phagocytosis of parts• Cytokines secreted• Components recycled• Organelles recycled
Figure 2.4 (b) Summary of the stages of apoptosis
• You have outlived your usefulness. Fingers and toes
(digits) develop within pads of cells (as illustrated in
Nature of Biology Book 1 Third Edition, page 36 and
on page 49 of this chapter). Cells remaining between
the digits are no longer required. Also, after you
recover from a disease, your body no longer requires
all the T and B cells that have been produced. Cells
no longer useful to an organism die by apoptosis.
Cell membranes have death receptors that receive
the messages referred to above. When such a message
is received, a cascade of events occurs.
1. Many different caspases are activated within the
cell and at the same time a message goes out to
phagocytes in the area.
2. All cells that have received the death signal begin
to shrink and develop small bumps (blebs) on their
surface (see fi gure 2.4a).
3. Caspases enter through the nuclear membrane pores,
the DNA and proteins in the nucleus are degraded
and mitochondria break down.
4. Organelles other than the nucleus and mitochondria
are generally preserved as the cell breaks into small
membrane-enclosed fragments.
5. The small fragments bind to receptors on phagocytic
cells that have responded to messages from the dying
cell. These phagocytes then engulf the fragments.
They also secrete cytokines which are compounds
that inhibit infl ammation so that surrounding cells
are not damaged in the way that neighbouring cells
can be with necrosis.
The process is summarised in fi gure 2.4b.
Disease and apoptosisApoptosis is an essential feature of development. We
have noted that a healthy state relies on a balance
between cell production and cell loss in an organism.
An increasing number of diseases are now known to be
caused by a defect in apoptosis, for example:
• too much apoptosis can lead to neuro-degenerative
diseases such as Alzheimer and Huntington diseases
• too little apoptosis can lead to the production of
cancers and autoimmune diseases.
Refer also to page 405 in chapter 11. You will need to
understand apoptosis for your studies later in the year.
(b)
36 NATURE OF BIOLOGY BOOK 2
In 2003 in Victoria, 185 000 people were screened for breast cancer. More than 42 000 of these were seen by St Vincent’s BreastScreen service. I decided on a career in radiology after working at a
private radiology practice for my Year 10 work experi-ence placement and I qualifi ed with a Diploma of Applied Science and Medical Radiations at RMIT in 1983 (now a degree course at Monash University and RMIT). The course involves theoretical and clinical components at a rural, metropolitan, private or public practice. Gaining supervised, practical experience in the fi eld is important because it helps students to decide if radiography is something they really want to do.My third-year clinicals were undertaken at St Vin-
cent’s Public Hospital where I was lucky to secure a position after completing my diploma. St Vincent’s offers a wide range of modalities, including magnetic resonance imaging (MRI), angiography (imaging blood vessels), ultrasound, general radiography, and mammography (breast imaging). Because most women prefer it, mammograms are mainly done by women.I became the chief radiographer when St Vincent’s
BreastScreen was established in the early 1990s and it now incorporates eight satellite metropolitan and rural screening-services. BreastScreen is a free service, from screening to diagnosis. Women in the target 50–69 age group are identifi ed through the electoral roll and actively recruited for the program. However, women over 40 can also access the early-detection service. People may be advised to have a mammogram if there is a family history of breast cancer, or to investigate causes of pain, lumps or nipple discharge. These may be symptoms of breast cancer or benign causes, such as cysts. Mammography is used to determine the cause of symptoms and assist with diagnosis. The breast is positioned on an imaging cassette and a perspex plate is then lowered to fi rmly compress the breast. Com-pression is required to spread all the breast tissue out to avoid structures overlying one another and also to reduce radiation exposure.BreastScreen differs from diagnostic mammography
in that the radiographer takes two projections of each breast and checks that the fi lms are technically adequate, ensuring all the tissue is shown and the patient has not moved and distorted the image. Films are read by two independent radiologists, and the results are sent to the client within two weeks of their screening. Those given the all-clear are advised to have another examination in two years. Clients needing further assessment are asked by
a counsellor to return for further examination. This
usually includes more X-rays and depending on the results, an ultrasound, physical examination or biopsy may be required. A fi ne needle biopsy is undertaken to obtain cells, or a core biopsy to extract tissue. These are carried out with ultrasound or X-ray guidance. If a lesion cannot be seen under ultrasound, then X-ray guidance is used with a prone stereo-tactic biopsy table. The radiographer takes a series of pictures from different angles and the images are acquired digitally and fed into a computer for determination of the exact position of the lesion. This digital radiography is very expensive and is a new technique in Australia.BreastScreen Victoria has established a Radiogra -
pher Training Centre at Monash BreastScreen. This allows qualifi ed radiographers to train for their Certifi cate of Competency in Mammography (CCPM). This certifi cate is required to work in the Breast-Screen program. I oversee Quality Assurance for all of St Vincent’s BreastScreen sites to ensure a high quality of mammography is being produced. I also work at Monash University teaching fi rst-year students positioning for general radiography. I also look after the radiographic aspects of the two mobile Breast-Screen Victoria vans, which take the service to women in isolated communities.For me, radiography has provided lots of challenges.
With BreastScreen in particular, I very much feel part of a team, where radiologists, surgeons, radiographers, counsellors, pathologists and clerical staff work together to bring a free and highly specialised service to women in Victoria. It is exciting to be involved in bringing devel-oping digital technology to mammography.
Sue Macaulay — Chief Radiographer, St Vincent’s BreastScreen
BIOLOGY IN THE WORKPLACE
Figure 2.5 Sue Macaulay and the BreastScreen equipment
MEMBRANES AND CELL ORGANELLES 37
Looking at eukaryotic cellsExamining cells using various microscopes can reveal a great deal about their
internal environment. You will have learned about and perhaps used a number of
different types of microscopes in your previous studies, including various light
microscopes (for example, fi gure 2.6) and electron microscopes. We have also
outlined the capabilities of the synchrotron (see pages 3–4). In this chapter, we
consider structures that, for the most part, require confocal and electron micro-
scopes for observation. Typical sizes of cells and some parts are shown on a
logarithmic scale in fi gure 2.7 (page 38).
Figure 2.6 A scientist using a
confocal microscope. Note how the
vertical segment of the microscope
can be rotated away from the stage
to make it easier to position the
specimen on the stage.
Compartments within cellsEach living cell is a small compartment with an outer boundary, the plasma
membrane. Within this one compartment that makes up a living eukaryotic cell
is a fl uid, called cytosol, that consists mainly of water containing many dissolved
substances (see table 2.1, page 38). There is a labyrinth of membranes within
the cytosol that, in effect, create large numbers of smaller, functionally distinct
compartments within the cell itself (see table 2.2). These membrane-bound com-
partments are called organelles (fi gure 2.8, page 39).
• Signals initiating apoptosis may come from either inside or outside a
cell.
• A defect in apoptosis can lead to a disease, such as Alzheimer disease,
cancer and autoimmune diseases.
KEY IDEAS
3 List three possible death signals a cell might receive to initiate
apoptosis.
QUICK-CHECK
38 NATURE OF BIOLOGY BOOK 2
Organelles are held in place by a network of fi ne protein fi laments within a
cell, collectively known as the cytoskeleton (see page 52). Prokaryotic cells such
as bacteria lack these internal membranes.
In the following sections, we examine the plasma membrane and cell organelles
of eukaryotes and discuss their functions.
Table 2.1 Relative volumes of the major compartments within a liver cell
Intracellular compartment Percentage of total cell volume
Cytosol 54
Mitochondria 22
Rough endoplasmic reticulum 9
Smooth endoplasmic reticulum 6
Nucleus 6
Lysosomes, peroxisomes, endosomes 3
Table 2.2 Relative amounts of membrane associated with some of the organelles in two
different kinds of cells
Percentage of total cell membrane
Membrane type Liver cell Pancreatic cell
Plasma membrane 2 5
Rough endoplasmic reticulum 35 60
Smooth endoplasmic reticulum 16 less than 1
Golgi complex membrane 7 10
Mitochondria
Outer membrane
Inner membrane
7
32
4
17
Nucleus inner membrane 0.2 0.7
Volume of cell (approx.) 5000 µm3 1 000 µm3
Area of cell membranes (estimate) 110 000 µm2 13 000 µm2
Figure 2.7 Typical sizes of
cells and some of their parts,
shown against a logarithmic
scale. A logarithmic scale is one
in which each unit of measure is
one-tenth of the preceding unit.
In this example, on the left-hand
side, 1 mm is the fi rst measure,
the next is 100 µm which is
one-tenth of 1 mm, and so on
along the scale.
1 m
m
10
0 µ
m
10
µm
1 µ
m
0.1
µm
0.0
1 µ
m
0.0
01
µm
0.0
00
1 µ
m
Human
vision
Frog egg
Animal cell Mitochondrion
Ribosome Molecules
Plasmamembrane
Lightmicroscope
Electron
microscope
Cytoplasm = cytosol + organelles
except the nucleus
Protoplasm = cytosol + all
organelles
MEMBRANES AND CELL ORGANELLES 39
The plasma membrane boundary
The boundary of all living cells is a plasma membrane which controls entry of
dissolved substances into and out of the cell. A plasma membrane is an ultra thin
and pliable layer with an average thickness of less than 0.01 µm (0.000 01 mm).
A plasma membrane is too thin to be resolved with a light microscope but it can
be seen using an electron microscope (see fi gure 2.9 below, image at top right).
A plasma membrane comprises a phospholipid bilayer into which proteins and
glycoproteins protrude (see fi gure 2.9). Some of the proteins embedded in this
layer form channels that allow certain substances to pass across the membrane in
either direction. This is known as the fl uid mosaic model.
Figure 2.8 The outer plasma
membrane of a typical animal
cell contains a network of inner
membranes that create smaller
compartments within the cell, known
as organelles. We will discuss the
plasma membrane and the organelles
labelled in red in the following
sections.
Figure 2.9 The plasma membrane
of all cells has the same basic
structure. Note the phospholipid
bilayer. Proteins penetrate into or
through the phospholipid bilayer
and carbohydrate chains bond to
many of these. A few carbohydrate
chains bond directly to the outer
phospholipid layer. Note the pores
in the nuclear membrane.
Cytosol
Proteinfilament
Plasma membrane
Nucleus
Mitochondrion
Ribosome
Endoplasmicreticulum
Lysosome
Centriole
Endosome
Peroxisome
Protein
microtubule
Golgi apparatus
Vesicle
Outside cell
Cytoplasm
Nucleus
Glycolipid
Phospholipidbilayer
Membraneglycoprotein
Some cells also have a cellulose
cell wall exterior to the plasma
membrane (refer to page 42).
40 NATURE OF BIOLOGY BOOK 2
Recognising cells: self or non-selfOn its outer surface, a plasma membrane has substances, often called antigens,
that ‘label’ or identify a cell as belonging to one particular organism. Antigens
usually consist of proteins combined with carbohydrates. When various mammals
of the same species are compared, the antigens on their plasma membranes are
found to differ.
If cells from one organism are introduced into the body of a different
organism from the same species, the immune system of the recipient recognises
the introduced cells as ‘foreign’ or ‘non-self’. The immune system responds with
chemical and cellular attacks which kill the ‘non-self’ cells. The immune system
does not normally attack its own cells because it recognises these cells as ‘self’.
This ability to recognise foreign cells and attack them is an important defence
mechanism against bacterial infection.
Crossing the membraneAll cells must be able to take in and expel various substances in order to survive,
grow and reproduce. Generally these substances are in solution but, in some
cases, they may be tiny solid particles.
Because a plasma membrane allows only some dissolved materials to cross
it, the membrane is said to be a partially permeable boundary. Dissolved sub-
stances that are able to cross a plasma membrane — from outside a cell to the
inside or from inside to the outside — do so by various processes, including
diffusion and active transport.
Free passageDiffusion is the net movement of a substance, typically in a solution, from a
region of high concentration of the substance to a region of low concentration
(see fi gure 2.10a). The process of diffusion does not require energy.
At all times, molecules are in random movement. If a substance is more con-
centrated outside the cell than inside, molecules move from outside to inside the
cell. Diffusion stops at the stage when the concentration of substance X is equal
on the two sides of the membrane.
Substances that can dissolve readily in water are termed hydrophilic, or
‘water-loving’. Some substances that have low water solubility or do not dissolve
in water are able to dissolve in or mix uniformly with lipid. These substances
are termed lipophilic (sometimes called hydrophobic). Examples of lipophilic
substances include alcohol and ether. Lipophilic substances can cross plasma
membrane boundaries readily.
Channel mediated
Some substances that are unable to carry out simple diffusion through the phos-
pholipid bilayer gain free passage across a membrane with the assistance of
protein channels (see fi gure 2.10b). Molecules move from a high concentration
to a low concentration without requiring energy.
Carrier mediated
Sometimes a protein channel alone is insuffi cient and a carrier molecule is
required to move molecules down the concentration gradient through a protein
channel (see fi gure 2.10c). When a specifi c carrier molecule is required, this kind
of movement is also called facilitated diffusion.
Movement of substances by facilitated diffusion mainly involves substances
that cannot diffuse across the plasma membrane by dissolving in the phospholipid
bilayer of the membrane. For example, the movement of glucose molecules across
the plasma membrane of red blood cells involves a specifi c carrier molecule.
All three methods of passive transport (fi gure 2.10a–c) result in molecules
moving from a region of high concentration to a region of low concentration
without the expenditure of energy.
ODD FACT
The fi rst donor transplants of kidneys, and
later, hearts, failed because the immune system of the recipients
recognised the transplanted organs as ‘non-self’ and reacted,
causing them to be rejected. Drugs were developed to
suppress the body’s normal immune reaction.
You may wish to revise the
topic of movement across cell
membranes by reading
Nature of Biology Book 1,
Third Edition, pages 27–30.
‘Partially permeable’ is also known
as selectively or differentially or
semi-permeable.
ODD FACT
One special case of diffusion is known as
osmosis. The process of osmosis occurs when a net movement of water molecules occurs by
diffusion across a cell membrane either into or out of a cell.
MEMBRANES AND CELL ORGANELLES 41
Paid passage: active transportActive transport is the net movement of dissolved substances into or out of
cells against a concentration gradient (see fi gure 2.10d above). Because the net
movement is against a concentration gradient, active transport is an energy-
requiring process. The process involves a carrier protein for each substance that
is actively transported.
Active transport enables cells to maintain stable internal conditions in spite of
extreme variation in the external surroundings.
Bulk transportSolid particles can be taken into a cell. For example, one kind of white blood cell
is able to engulf a disease-causing bacterial cell and enclose it within a lysosome
sac where it is destroyed. Unicellular protists, such as Amoeba and Paramecium,
obtain their energy for living in the form of relatively large ‘food’ particles that
they engulf and enclose within a sac where the food is digested. The process of
bulk transport of material into a cell is known as endocytosis (see fi gure 2.11a).
Figure 2.10 Transport of molecules across membranes: (a–c) Three ways in which molecules move from a region
of high concentration, across a plasma membrane, to a region of low concentration without the expenditure of energy.
(d) Movement of molecules from a region of low concentration across a plasma membrane to a region of high concentration
requires the expenditure of energy. Note the movement of molecules against the concentration gradient.
Figure 2.11 (a) Endocytosis (bulk
transport into cells) occurs when part
of the plasma membrane forms around
a particle to form a vesicle, which moves
into the cytosol. (b) Exocytosis (bulk
transport out of cells) occurs when
vesicles within the cytosol fuse with
the plasma membrane and vesicle
contents are released from the cell.
ODD FACT
When bulk material is taken into a cell as a
solid, the process is termed phagocytosis (from the Greek ‘phagos’ = eating, and ‘cyto’
= cell). When bulk material is taken into a cell as a fl uid, the process is termed pinocytosis
(‘pinos’ = drinking).
Cytosol
Lysosome
Outside cell
Lipid bilayer
Cytosol
Outside cell
Lipid
bilayer
PASSIVE TRANSPORT
FREE PASSAGE — NO ENERGY REQUIRED
ACTIVE TRANSPORT
ENERGY REQUIRED
(a) Simple diffusion
(b) Channel mediated
(c) Carrier mediated
(d) Active transport
Concentration gradient in all
the cases shown
Outsidecell
Outsidecell
Insidecell
Phospholipidbilayer
Insidecell
Energy
(a) (b)
42 NATURE OF BIOLOGY BOOK 2
Bulk transport out of cells (for example, the export of material from the Golgi
complex, see pages 46–7) is called exocytosis. In exocytosis, vesicles formed
within a cell fuse with the plasma membrane before the contents of the vesicles
are released from the cell (see fi gure 2.11b). If the released material is a product
of the cell (for example, the contents of a Golgi vesicle), then ‘secreted from the
cell’ is a phrase generally used. If the released material is a waste product after
digestion of some matter taken into the cell, ‘voided from the cell’ is generally
more appropriate.
PLANTS HAVE CELL WALLS
The plasma membrane forms the exterior of animal cells.
However, in plants, fungi and bacteria, another structure — a
rigid cell wall — lies outside the plasma membrane. The cells of
organisms in the Kingdom Animalia do not have a cell wall.
The original or primary cell wall of a plant cell is made of
— cellulose. In some fl owering plants, the primary cell wall
in certain tissues becomes thickened and strengthened by the
addition of lignin to form secondary cell walls (see fi gure 2.12).
This process provides great elastic strength and support, allowing
certain plants to develop as woody shrubs or trees.
Figure 2.12 The primary cell wall of a plant cell is made of cellulose.
The layers of microfi brils in the secondary walls are laid down in
different directions and give extra strength and support to a plant.
• Each eukaryotic cell contains many membranous structures, called
organelles, suspended in the cytosol.
• Every living cell has a plasma membrane boundary.
• There are several different ways in which materials cross plasma
membranes to enter cells.
• Cell walls lie outside the plasma membrane of plant, fungal and
prokaryotic cells.
KEY IDEAS
4 Make a labelled sketch of a typical plasma membrane.
5 List the different ways in which materials cross plasma membranes.
For each way, indicate whether or not it is energy-requiring.
6 Many plant cells have secondary cell walls as well as primary. Of what
advantage is this to a plant?
QUICK-CHECK
Organelle 1: the nucleus — control centreCells have a complex internal organisation and are able to carry out many func-
tions. The control centre of the cells of animals, plants, algae and fungi is the
nucleus. The nucleus in these cells forms a distinct spherical structure that is
enclosed within a double membrane, known as the nuclear envelope (see fi gure
2.13). Cells that have a membrane-bound nucleus are called eukaryote cells.
Nucleus
Layers ofsecondarycell walls
Adjacentcells
Primarycell wall
MEMBRANES AND CELL ORGANELLES 43
Figure 2.13 Coloured freeze-
fracture transmission electron
micrograph (TEM) of part of the
nuclear membrane of a liver cell.
The inner membrane (top blue) and
the outer membrane (brown) are both
visible. The rounded pores on the
membrane allow large molecules to
exit the nucleus and move into
the cytosol.
Refer to Nature of Biology
Book 1, Third Edition,
page 24 for more information
about prokaryotes such as
bacteria.
• Nucleoli contain the nucleic acid RNA.• The nucleus contains the nucleic acid DNA, which is the genetic
material within a cell.• The nucleus of eukaryote cells is enclosed within a nuclear envelope.
KEY IDEAS
7 State whether the following are true or false and briefl y explain your answer.a A nucleus from a plant cell would be expected to have a double
nuclear membrane.b Chromosomes are always visible in a eukaryotic cell.
8 Suggest why the nucleus is sometimes called the ‘control centre’ of a cell.
QUICK-CHECK
Cells of organisms from the Kingdom Monera, such as bacteria, contain the
genetic material (deoxyribonucleic acid (DNA)), but it is not enclosed within a
distinct nucleus. Cells that lack a nuclear envelope are called prokaryote cells.
A light microscope view reveals that the nucleus contains many granules that
are made of the genetic material (DNA). The DNA is usually dispersed within
the nucleus. During the process of cell reproduction, however, the DNA granules
become organised into a number of rod-shaped chromosomes.
The nucleus also contains one or more large inclusions known as nucleoli
which are an aggregation of ribonucleic acid (RNA) molecules.
44 NATURE OF BIOLOGY BOOK 2
Organelle 2: mitochondrion — energy-supplying organelleLiving cells use energy all the time. The useable energy supply for cells is
chemical energy present in a compound known as adenosine triphosphate
(ATP) (see fi gure 2.14). The ATP supplies in living cells are continually being
used up and must be replaced.
Mitochondrion
Figure 2.14 Chemical structure of
adenosine triphosphate (ATP), which
has three phosphate groups and so is
adenosine tri(= 3) phosphate
The role of mitochondria
in respiration is discussed in
chapter 3, pages 82–4.
ODD FACT
Many biologists agree with the hypothesis
that, thousands of millions of years ago, mitochondria were free-living organisms,
like bacteria. This hypothesis suggests that these organisms
became associated with larger cells to form a mutually
benefi cial arrangement. This idea is supported by the fact
that mitochondria contain small amounts of the genetic
material DNA. The size of a mitochondrion is about 1.5 µm
by 0.5 µm. This is similar to the dimensions of a typical
bacterial cell.
HO P O P O P O CH2
O O O
OC
H C
OH
H
C
OH
H C
H
OOO
NC
N
HC
N
C
CN
CH
Adenine
D-ribose
Triphosphate }Adenosine
NH2
ATP is produced during cellular respiration (or just simply respiration). In
eukaryote cells, most of this process occurs in organelles known as mitochondria
(singular = mitochondrion) which form part of the cytoplasm. Mitochondria
cannot be resolved using a light microscope but can be seen with an electron
microscope (see fi gure 2.15). Each mitochondrion has an outer membrane and a
highly folded inner membrane. ATP is produced by reactions that occur on the
inner folded membranes. Prokaryote cells lack mitochondria.
Figure 2.15 (a) Transmission electron micrograph (x 50 000) of mitochondria (circles),
the organelles responsible for producing ATP by cellular respiration (b) Scanning electron
micrograph (SEM) of a section through a mitochondrion (pink) from the cytoplasm of an
epithelial cell. Which is more highly folded — the outer membrane or the inner membrane?
Mitochondria also contain circular molecules of DNA.
(a) (b)
Outer
membrane
Inner
membrane
MEMBRANES AND CELL ORGANELLES 45
Organelle 3: ribosomes — protein factoriesLiving cells make proteins by linking amino acid building blocks into long
chains. For example:
• human red blood cells manufacture haemoglobin, an oxygen-transporting
protein
• pancreas cells manufacture insulin, a small protein which is an important
hormone
• liver cells manufacture many protein enzymes, such as catalase
• stomach cells produce digestive enzymes, such as pepsin
• muscle cells manufacture the contractile proteins, actin and myosin.
Ribosomes are the organelles where protein production occurs. These
organelles, which are part of the cytoplasm, can only be seen through an electron
microscope (see fi gure 2.16).
Ribosomes
Figure 2.16 Scanning electron
micrograph of the rough endoplasmic
reticulum in a pancreatic cell.
The very small ‘bumps’ on the
endoplasmic reticulum membranes
are ribosomes, the site of protein
synthesis. The endoplasmic reticulum
provides a series of channels for
transporting the protein produced by
ribosomes to other parts of the cell.
Ribosomes are not enclosed by a membrane. Although many ribosomes are
attached to membranous internal channels within the cell (the endoplasmic
reticulum, discussed below), they are also found in the cytosol.
The proteins produced by ribosomes on rough endoplasmic reticulum are
transported to other parts of the cell and many are transported away from the
cell. Proteins made by ‘free’ ribosomes unattached to endoplasmic reticulum are
for local use within the cell. Mitochondria and chloroplasts also contain free
ribosomes.
Chemical testing shows that ribosomes are composed of protein and
ribonucleic acid (RNA). Ribosomal RNA (rRNA) comes from the nucleolus in
the cell. Particular parts of the DNA carry the genetic code necessary for the
formation of ribosomal and other RNAs.
46 NATURE OF BIOLOGY BOOK 2
• Living cells use energy all the time, principally as chemical energy present in ATP.
• Mitochondria are the major sites of ATP production in eukaryotic cells.• Mitochondria contain small amounts of DNA.• Ribosomes are tiny organelles where proteins are produced.• Ribosomes are made of rRNA and protein.
KEY IDEAS
9 Of what advantage is a folded inner membrane in mitochondria?10 What is the source of ribosomal RNA (rRNA)?11 Some ribosomes are free in cytosol; some are attached to endoplasmic
reticulum. What is the signifi cance of this difference?
QUICK-CHECK
Organelles 4 and 5: endoplasmic reticulum and Golgi complex
We saw above that the proteins made by some cells are kept inside those cells.
Examples are contractile proteins made by muscle cells and the haemoglobins
made by red blood cells. Other cells produce proteins that are released for use
outside the cells. For example, the digestive enzyme, pepsin, is produced by cells
lining the stomach and released into the stomach cavity; the protein hormone,
insulin, is made by pancreatic cells and released into the bloodstream.
Transport of substances within cells occurs through a system of channels
known as the endoplasmic reticulum (ER). Figure 2.17 shows this system of
channels in a cell (see also fi gure 2.16, page 45). The channel walls are formed
by membranes.
Figure 2.17 Transmission electron
micrograph of rough endoplasmic
reticulum (ER), the thin ‘channels’
coloured green in the centre. What
are the tiny ‘dots’ attached to the
endoplasmic reticulum?
Endoplasmicreticulum
MEMBRANES AND CELL ORGANELLES 47
Golgicomplex
Figure 2.18 Transmission electron
micrograph of a Golgi complex
(fl attened disc-like structure,
coloured orange). This organelle is
a delivery system for the proteins
passing in and out of the cell and is
named after Camillo Golgi who fi rst
identifi ed it in 1898.
Figure 2.19 The secretory pathway
for proteins made at ribosomes. They
are packaged by the endoplasmic
reticulum and transported to the
Golgi complex where they may be
concentrated. Secretory vesicles
formed by the Golgi complex
eventually fuse with the plasma
membrane and the protein contents
are discharged from the cell.
Roughendoplasmicreticulum
Ribosomes
Secretoryvesicle
Golgicomplex
Membranefusion occurring
Transitionvesicle
Cytoplasmof cell
Discharge byexocytosis; for example,a hormone
In the Golgi complex, the proteins are packaged into secretory vesicles and may
be stored in the cytosol before they eventually fuse with the plasma membrane.
The protein is then discharged from the cell by exocytosis into the surrounding
tissue fl uid. The protein may be taken up by other cells close by or may pass into
the bloodstream where it is transported to other tissues around the body.
A structure known as the Golgi complex is prominent in cells that shift
proteins out of cells. This structure consists of several layers of membranes (see
fi gure 2.18). The Golgi complex is also called the Golgi apparatus.
The proteins produced by ribosomes that are destined for secretion diffuse
from the site of their production into the membranous chambers formed by the
layers of endoplasmic reticulum. They are then packaged into membranous
vesicles and transported to the Golgi complex where they may be concentrated
(see fi gure 2.19).
48 NATURE OF BIOLOGY BOOK 2
Organelle 6: lysosomes — controlled destructionAnimal cells have sac-like structures surrounded by a membrane and fi lled with
a fl uid containing dissolved digestive enzymes. These fl uid-fi lled sacs are known
as lysosomes and they are part of the cytoplasm (see fi gure 2.20).
Lysosomes
Figure 2.20 Coloured
high-resolution scanning electron
micrograph (SEM) of two lysosomes
(green) in a pancreatic cell. The
material in each lysosome is probably
undigested material. Note the
membranes of endoplasmic reticulum
nearby (pink) with ribosomes (the
tiny knobs) on the surface.
• The endoplasmic reticulum (ER) is a series of membrane-bound channels.
• The ER functions in the transport of substances within a cell.• The Golgi complex packages substances into vesicles for export.
KEY IDEAS
12 Name three substances that would be produced at the surface of the ER of a cell and transported for use outside the cell.
QUICK-CHECK
MEMBRANES AND CELL ORGANELLES 49
Figure 2.21 In a chicken
embryo, cell death brought about
by lysosomes produces separate
digits. Blue areas are regions where
cell death occurs. In contrast, in
a duck embryo, cells between the
digits do not die but are retained as
webbing.
Separatetoes
Webbetweentoes
Chicken Duck
Footbud1.
2.
3.
Lysosomes use their enzymes to destroy unwanted cell parts or damaged
molecules from within or outside the cell. The unwanted material is enclosed
by a lysosome membrane and is digested. This process of controlled ‘self-
destruction’ of cells is important in development: lysosomes appear to play a role
in the controlled death of zones of cells in the embryonic human hand so that the
fi ngers become separated (see Nature of Biology, Book 1, Third Edition, page 36).
A similar event occurs in a developing chick embryo (see fi gure 2.21).
Lysosomes produce enzymes that digest substances that are no longer needed
within cells. Defects may occur in the enzymes found within lysosomes. When
this happens, the substance may accumulate in the lysosomes and the cells can
no longer function normally. Diseases resulting from these errors in lysosome
enzymes include Tay Sachs disease, in which abnormal accumulation of lipids
occurs, and Hurler syndrome, in which abnormal accumulation of complex
carbohydrates occurs.
Small organelles that have some similarity with lysosomes and occur in
eukaryotic cells are peroxisomes and endosomes.
Peroxisomes
Hydrogen peroxide (H2O2) is a product of many biochemical processes within
cells. If allowed to accumulate, it is a poisonous substance. Peroxisomes are small
membrane-bound organelles rich in the enzymes catalase and urate oxidase. The
accumulation of hydrogen peroxide is prevented by the action of catalase.
2H2O2 catalase 2H2O + O2
Peroxisomes detoxify various toxic materials that enter the bloodstream.
For example, about 25 per cent of any alcohol consumed is detoxifi ed through
oxidation to aetaldehyde. Peroxisomes in different types of cells may contain
different sets of enzymes. Plant and animal cells have peroxisomes.
Endosomes
Endosomes are membrane-bound organelles found in animal cells. When material
enters a cell by endocytosis, endosomes pass on the newly ingested material to
lysosomes for digestion.
• Lysosomes are membrane-bound sacs containing dissolved digestive
enzymes.
• Lysosomes can digest material brought into their sacs.
• Peroxisomes contain enzymes that destroy toxic materials.
• Endosomes, found in animal cells, pass on material to lysosomes for
digestion.
KEY IDEAS
13 Lysosomes are sometimes called ‘suicide bags’. Suggest why this name
is given.
14 How is the hydrogen peroxide produced in cellular metabolism
detoxifi ed?
15 What is the function of endosomes?
QUICK-CHECK
50 NATURE OF BIOLOGY BOOK 2
Grana Inner
membrane
Stroma
Outer
membrane
Plant cell organelle: chloroplasts — sunlight trappersHundreds of millions of years ago, some bacteria and all algae and then land plants
developed the ability to capture the radiant energy of sunlight and transform it
to chemical energy present in organic molecules, such as sugars. The organelles
present in some cells of plants and algae that carry out this function are known
as chloroplasts (see fi gure 2.22). The complex process of converting sunlight
energy to chemical energy present in sugar is known as photosynthesis.
The boundary of each chloroplast is a double membrane (inner and outer). The
inner membrane extends to form a system of membranous sacs called lamella or
thylakoids. When several of these stack together they form grana. Chlorophyll
is located in the grana and it is here that the light-dependent reactions of photo-
synthesis occur (see chapter 3, page 72). The stroma, the semi-fl uid substance
between the grana, contains the enzymes necessary for the light-independent
reactions of photosynthesis.
Photosynthesis is discussed further
in chapter 3, pages 69–77.
Figure 2.22 (a) Transmission
electron micrograph of
chloroplasts from the
leaf of a pea plant
(b) A three-dimensional
representation of a
chloroplast
• Chloroplasts are relatively large organelles found in photosynthetic cells of plants and algae.
• Chloroplasts have an external membrane and layers of folded internal membranes.
• Chlorophyll is located inside the grana of chloroplasts.• Chloroplasts can capture the radiant energy of sunlight and convert it
to chemical energy in sugars.
KEY IDEAS
16 What is the function of chlorophyll?17 What are (a) thylakoids; (b) grana; (c) stroma?
QUICK-CHECK
Prokaryote cells do not have chloroplasts. Some kinds of bacteria, however,
possess pigments that enable them to capture the radiant energy of sunlight and
use that energy to make sugars from simple inorganic material. These are known
as photosynthetic bacteria.
The length of a typical chloroplast is 5 to 10 µm. In comparison, the length of
a mitochondrion is about 1.5 µm. In 1908, the Russian scientist, Mereschkowsky,
suggested that chloroplasts were once free-living bacteria that later ‘took up residence’
in eukaryote cells. Some evidence in support of this suggestion comes from the fact
that a single chloroplast is very similar to a photosynthetic bacterial cell.
Chloroplasts also contain molecules of DNA, free ribosomes, starch grains
and lipid droplets.
Chloroplast
(a)
(b)
MEMBRANES AND CELL ORGANELLES 51
Putting the organelles togetherThe cell is both a unit of structure and a unit of function. Organelles within one
cell do not act in isolation, but interact with each other. The normal functioning
of each kind of cell depends on the combined actions of its various organelles,
including plasma membrane, nucleus, mitochondria, ribosomes, endoplasmic
reticulum, Golgi complex and peroxisomes.
Consider the membranous compartments within a cell that produce a specifi c
protein for use outside the cell. Table 2.3 identifi es the parts of a cell involved in
this process.
Figure 2.23 shows the typical structure and organelles of an animal and a plant
cell, as discussed in the previous pages and in table 2.3.
Table 2.3 Parts of a cell involved in producing a specifi c protein
Structure Function
plasma membrane Structure that controls the entry of raw materials, such as amino acids, into the cell
nucleus Organelle that has coded instructions for making the protein
ribosome Organelle where amino acids are linked, according to instructions, to build the protein
mitochondrion Organelle where ATP is formed; provides an energy source for the protein-manufacturing
activity
endoplasmic reticulum Channels through which the newly made protein is moved within the cell
Golgi complex Organelle which packages the protein into vesicles for transport across the plasma
membrane and out of the cell
peroxisome Organelle that detoxifi es H2O2 produced in many metabolic reactions
Figure 2.23 The structure and organelles of (a) an animal cell, and (b) a plant cell
Cytosol
Proteinfilament
Plasma membrane
Nucleus
Nucleolus
Mitochondrion
Nuclear envelope
Ribosome
Endoplasmicreticulum
Endosome
Peroxisome
Lysosome
Centriole
Proteinmicrotubule
Golgi apparatus
Vesicle
Cell wall
Vacuole
Filament
Peroxisome
Cytosol
Plasma membrane
Nucleus
Nucleolus
Mitochondrion
Nuclear envelope
RibosomeLysosome
Golgiapparatus
Vesicle
Endoplasmicreticulum
Microtubule
Chloroplast
(a) (b)
52 NATURE OF BIOLOGY BOOK 2
The cell skeletonEach cell has an internal framework of protein microtubules, microfi laments and
intermediate fi laments (see fi gure 2.24). These supply strength and support for
the cell. This supporting structure is called the cytoskeleton.
Microtubules are hollow and are made of subunits of the protein tubulin (see
fi gure 1.24, page 21). Microfi laments are solid, thinner and more fl exible than
microtubules. They are made of actin. Intermediate fi laments are made of a variety
of proteins, depending on the particular cell, and are very tough. They often tie
into the cytoskeleton of other cells (refer to the following section ‘Connections
between cells’).
These three structures combine to assist in:
• maintaining the shape of a cell
• providing a support structure for other components within a cell
• the movement of materials within a cell
• movement of the cell itself if required.
You will recall from your earlier studies of mitosis that microtubules play an
important role in the movement of chromosomes during reproduction of cells.
Connections between cells: animal cellsAlthough some cells, for example blood cells, are free to move as individuals
around the body, most cells remain as members of a group. What connections, if
any, exist between such cells? Look at the epithelial tissue in fi gure 1.1 (page 2).
What holds the cells together so that they form an integral layer even when the
body moves around and pressure may be placed on different groups of cells? Do
they communicate with each other in any way?
There are three different types of junctions in animal cells: occluding,
communicating and anchoring junctions (see fi gure 2.25).
Occluding junctionsOccluding junctions involve cell membranes coming together in contact with
each other (fi gure 2.25). There is no movement of material between cells.
Figure 2.24 Three structures that
make up the cytoskeleton of a cell
25 nm
ODD FACT
Occluding junctions between brain cells and brain
capillary cells prevent the passage of some materials, for
example certain drugs, from the blood into the brain.
7–10 nm6 nm
(a) Microtubule (b) Microfi lament (c) Intermediate
fi lament
Cell 1
Cytoplasm
Lipid bilayer
Nonjunctionalmembraneproteins
Pipeline betweenadjacent cells
Extracellularspace
Intercellular ‘gap’of 15 nm
Solute molecules
Membraneprotein
Intercellularspace
Cell 2 Anchoringjunctions
Occludingjunction
Communicatingjunction
MEMBRANES AND CELL ORGANELLES 53
Figure 2.26 Communicating
junction of animal cells. Note the
pore formed by protein molecules
aligned as if on the circumference
of a circle.
Figure 2.25 Diagram of the three
types of intercellular junctions found
in epithelial cells
Cell 1
Plasma
membrane
Plasma
membrane
Cell 2
Communicating junctionsCommunicating junctions are also called gap junctions. They consist of protein-
lined pores in the membranes of adjacent cells. The proteins are aligned rather
like a series of rods in a circle with a gap down the centre (see fi gure 2.26)
Communicating junctions permit the passage of salt ions, sugars, amino acids
and other small molecules as well as electrical signals from one cell to another.
One example of the latter is the control of the beating of the heart. A small area of
your heart, called the pace maker, receives an electrical impulse. This electrical
impulse is transmitted to all cells of the heart through communication junctions
so that the heart ‘beats as one’.
54 NATURE OF BIOLOGY BOOK 2
Anchoring junctionsAnchoring junctions are the most common form of junction between epithelial
cells in areas such as the skin or uterus. They are also called desmosomes. Dense
plaques of protein exist at the junction between two cells (see fi gure 2.27). Fine
fi brils extend from each side of these plaques and into the cytosol of the two
cells involved. These are intermediate fi laments (as represented in fi gure 2.24c,
page 52) that use the plaques as anchoring sites. This structure has great tensile
strength and acts throughout a group of cells because of the connections from
one cell to another.
Figure 2.27 Transmission electron
micrograph (TEM) showing the most
common type of junction, called
desmosomes (green), between two
epithelial cells. Dense plaques (red)
are at the junction, lying immediately
beneath the membranes. Fine fi brils
(red) extend from plaques into the
cell cytoplasm on each side of the
junction.
Connections between cells: plant cellsPlants have rigid cell walls. In addition, the primary walls of adjacent cells are
held together tightly by a layer of pectin, a sticky polysaccharide. Hence, plant
cells have no need for a structure such as the anchoring junctions of animal cells.
Secondary walls are laid down in each cell on the cytosol side of the primary
wall so that the structure across two cells is relatively wide, at least 0.1 µm thick.
The junctions that exist in plant cells to allow communication between adjacent
cells in spite of the thick wall are plasmodesmata (singular: plasmodesma) (see
fi gure 2.28).
Because of the way in which plant cell walls are built up, the gap or pore
between two cells is lined with plasma membrane so that the plasma membrane
of the two cells is continuous. A structure that bridges the ‘gap’ is also
continuous with the smooth endoplasmic reticulum of each cell.
MEMBRANES AND CELL ORGANELLES 55
Plasmodesmata exist in virtually all plants and hence cell-to-cell communi-
cation can occur between large numbers of cells that are in effect connected via
their cytoplasm.
We have considered the connections between plant cells through which
material can move from one cell to another. Some animal cells have the same
characteristic. Cells do connect with each other and the transfer of material and
messages can occur through some of these connections. How important is such
a feature in the overall functioning of an organism? Cell communication and cell
signalling is considered in greater detail in chapters 5 and 6.
Figure 2.28 Plasmodesmata, the junctions between plant cells. Note the relative
thickness of the section containing the walls of two plant cells, the continuation of the
cell membrane from one cell to another and the connections between smooth endoplasmic
reticulum of adjacent cells.
Cytoplasm
Plasmodesmata
Plasma membrane liningplasmodesma, connectingtwo adjacent cells
Smoothendoplasmicreticulum Desmotubule
Cytosol
100 nm
Cell wallsof adjacentplant cells
• Organelles interact to facilitate the production of proteins and the
transport of these and other compounds throughout a cell.
• Cells have an internal support system called the cytoskeleton.
• In multicellular animals, some cells have connections that allow
communication with adjacent cells.
• In multicellular plants, all cells have connections that allow
communication with adjacent cells.
KEY IDEAS
18 Name the different structures that make up the cytoskeleton of a cell.
19 List the three types of connections possible between two animal cells
and name a characteristic of each.
20 What are the connections between two plant cells called?
QUICK-CHECK
BIOCHALLENGE
56 NATURE OF BIOLOGY BOOK 2
1
This image shows a portion of a cell and some
of its organelles.
a Name the structures labelled A, B, C and D.
b Name the material in which organelles are
suspended.
c Name the compound found in structure C.
d Where else in a cell would you fi nd the
compound found in structure C?
2
This image shows plasmodesmata connections
between two cells. A number of cell organelles
are also visible.
a Is this an image of animal or plant tissue?
b Name the structures labelled A, B, C,
D and E.
c What is the function of structure F?
d What is the function of plasmodesmata?
Explain their importance.
3
This image shows a portion of a cell and some
of its organelles.
a Name the structures labelled A and B.
b Name the structure labelled C. What is its
function?
c Structures C and D are the same kind of
organelle yet their appearance is quite
different. Explain why they look so different
from each other.
A
A
B
D
C
A
B C
F
ED
A B
CD
BIOCHALLENGE
MEMBRANES AND CELL ORGANELLES 57
CHAPTER REVIEW
Key words
Questions
active transport
adenosine triphosphate
(ATP)
antigens
apoptosis
cancer
cellular respiration
chloroplasts
chromosomes
cytoskeleton
cytosol
deoxyribonucleic acid
(DNA)
desmosomes
diffusion
endocytosis
endoplasmic reticulum
eukaryote
exocytosis
Golgi apparatus
Golgi complex
grana
hydrophilic
lamella
lipophilic
lysosomes
mitochondria
nuclear envelope
nucleus
organelles
osmosis
partially permeable
phagocytosis
photosynthesis
pinocytosis
plasma membrane
plasmodesmata
primary cell wall
prokaryote
protein fi laments
proteins
ribosomes
secondary cell walls
stroma
thylakoids
CROSSWORD
1 Making connections between concepts � Use at least six of the key words
from this chapter to construct a concept map.
2 Analysing information and drawing conclusions � Figure 2.29 is a coloured
transmission electron micrograph of a plasma cell. One function of plasma
cells is to secrete antibodies during an immune response. Note the extensive
network of endoplasmic reticulum.
a Explain whether you would expect the ER to be rough or smooth.
b Given the function of plasma cells, what other organelle would you expect
to be rather prominent in parts of this cell?
c What is the darkly stained material in the nucleus?
3 Making connections between concepts � Mitochondria and chloroplasts both
contain circular molecules of DNA and free ribosomes. What conclusions
can reasonably be made on the basis of the presence of such structures?
4 Applying knowledge and understanding � Examine table 2.2 on page 38.
a What is the difference in structure between rough and smooth endoplasmic
reticulum?
b Which kind of cell shown in the table has the greater percentage of rough
endoplasmic reticulum? Which has the greater percentage of smooth
endoplasmic reticulum?
c As a result of this difference, what would you conclude about the fate of
the majority of protein produced by each cell? Explain your conclusion.
5 Analysing information and drawing conclusions � The folded internal
membranes of mitochondria have many stalked particles on their innermost
surfaces (see figure 2.30). Given the function of mitochondria and where
most of the reactions occur, of what advantage might the presence of these
particles be for the production of ATP in the organelle?
Figure 2.29 Transmission electron
micrograph of a plasma cell
Figure 2.30 Internal membrane of
mitochondria
Fold of inner membrane
Stalkedparticle
Holes in membrane
Outer membrane
Inner membrane
58 NATURE OF BIOLOGY BOOK 2
6 Analysing information and drawing conclusions � In figure 2.30, you may
have noted the holes in the folds of the inner membrane of mitochondria.
Explain a possible function for these holes.
7 Applying knowledge and understanding � Examine figure 2.31 which is
a coloured, high-resolution scanning electron micrograph of a portion of
cell.
a Explain whether you can distinguish if the cell involved came from an
animal or a plant.
b What is the name of the structure shown?
c What is its function?
Figure 2.31 Coloured,
high-resolution scanning electron
micrograph of a portion of cell
Figure 2.32
Y
X
W
Z
8 Analysing information and drawing conclusions � Figure 2.32 shows a
portion of an animal cell.
a From what part of the cell has the structure been taken?
b Name the kind of organic molecule labelled X and Y and Z.
c Explain the function of the structure labelled W.
MEMBRANES AND CELL ORGANELLES 59
9 Analysing information and applying knowledge and understanding � Fats
are generally transported in the blood in the form of small particles, called
chylomicrons. Examine the three examples given in figure 2.33. Note the
compounds that make up these particles. Explain why the components of
the particles aggregate in the way they do, ending up as spherical.Figure 2.33
10 Applying knowledge and understanding � Examine figure 2.34 which shows
a coloured scanning electron micrograph of a portion of cell.
a Name structure X and state its function.
b Given the density of the X structures, what could you reasonably deduce
about the metabolic rate of this cell?
c Name structure Y and state its function.
11 Using the web � Go to www.jaconline.com.au/natureofbiology/natbiol2-3e
and click on the ‘Cytoskeleton’ weblink for this chapter. Select ‘Cell
biology’ at the left-hand side. Scroll down and click on ‘The cytoskeleton’.
Then select ‘Microtubules, microfilaments and intermediate filaments’.
a What is the role of the cytoskeleton?
b i What is the main protein found in microfilaments? Name two
properties of this protein.
ii Which protein is associated with muscle contraction?
c i Which protein is found in microtubules?
ii Name two functions of microtubules.
12 Using the web � Go to www.jaconline.com.au/natureofbiology/natbiol2-3e
and click on the ‘Cell structure animation’ weblink for this chapter. Select
the option ‘Cell Structure’.
a Explore the animations to test your knowledge and understanding of the
structural characteristics of prokaryotic, animal and plant cells.
b Design two cells, one animal and one plant. Use these two designed cells
to test the knowledge of your biology practical work partner.Figure 2.34 Scanning electron
micrograph of part of a cell
X
Y
Phospholipid (4%)
Triacylglycerol (90%)
Cholesterol (5%)
Protein (1%)
(a) Chylomicron
Phospholipid (20%)
Triacylglycerol (10%)
Cholesterol (45%)
Protein (25%)
(b) Low-density lipoprotein (LDL) (c) High-density lipoprotein (HDL)
Phospholipid (30%)
Triacylglycerol (5%)
Cholesterol (20%)
Protein (45%)