Membranes and cell organelles - Wikispacesgleneaglesbio2010.wikispaces.com/file/view/Nature of Biology...32 NATURE OF BIOLOGY BOOK 1 2 Membranes and cell organelles Figure 2.1 This

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


• 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)


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)


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


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


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


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).


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.


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)


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


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.


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


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.


• 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?



Golgicomplex


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).


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


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


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)


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


Endosome

Peroxisome

Lysosome

Centriole

Proteinmicrotubule

Golgi apparatus

Vesicle

Cell wall

Vacuole

Filament

Peroxisome

Cytosol

Plasma membrane

Nucleus

Nucleolus

Mitochondrion

Nuclear envelope

RibosomeLysosome

Golgiapparatus

Vesicle


Microtubule

Chloroplast

(a) (b)


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


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’.


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.


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.


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


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


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?


micrograph of a plasma cell

Figure 2.30 Internal membrane of

mitochondria

Fold of inner membrane

Stalkedparticle

Holes in membrane

Outer membrane

Inner membrane


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.


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%)


Cholesterol (45%)

Protein (25%)

(b) Low-density lipoprotein (LDL) (c) High-density lipoprotein (HDL)

Phospholipid (30%)


Cholesterol (20%)

Protein (45%)

http://www.jaconline.com.au/natureofbiology/natbiol2-3e

http://www.jaconline.com.au/natureofbiology/natbiol2-3e

Documents

Membranes and cell organelles - Wikispacesgleneaglesbio2010.wikispaces.com/file/view/Nature of Biology...32 NATURE OF BIOLOGY BOOK 1 2 Membranes and cell organelles Figure 2.1 This