yr 12 bio chapter 2

Chapter 2MeMbranes and Cell organellesFigure 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 structures and functions 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:• understandtheextentoftheplasmamembraneinformingaseriesof

membranous channels for the packaging and transport of biomolecules throughout eukaryote cells

• enhancetheirknowledgeandunderstandingofthestructuresandfunctions of cell organelles

• distinguishthedifferentwaysinwhichbiomoleculesenterorleavecells• developtheirknowledgeandunderstandingofconnectionsbetweencells• extendtheirunderstandingofapoptosis.

36 NATURE OF BIOLOGY BOOK 2

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 � 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 bal-

anced. 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 frequencies of

different types of cancer in adult

females

Uterus (4%)

Lung (7%)Non-Hodgkin’s lymphoma (4%)

Colorectal (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 � 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 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 � gure 2.3, assist in pinpointing the positions

of regulator proteins within cells. Other experiments are aimed at establishing

the physiological roles 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

BreastScreen service, on page 39.

See more Apoptosis

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FIGURE 2.3 Mouse � broblast cells.

Fibroblasts are common in areolar

tissue, a connective tissue found

below the skin, around blood vessels

and nerves, and � 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

5_61_89158_NOB_BK2_4E_02.indd 36 26/03/13 9:33 AM

MeMbraNes aNd cell orgaNelles 37

Key ideas• 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.

QuiCK-CheCK 1 Why are cells known as the basic building blocks of living

organisms? 2 Howmightanexaminationofcellshelpdiagnosedisease?

apoptosisApoptosis, 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 inflammation 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, such as 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 specific 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 at right 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 iden-tifies 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 mes-sage 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 aremore 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.

Figure 2.4 (a) Scanning electron micrograph (SEM) of lymphocytes undergoing apoptosis. Note the small bumps, also called ‘blebs’, on the lymphocyte surfaces.

(a)

38 Nature of biology book 2

• Youhaveoutlivedyourusefulness. Fingers and toes (digits) develop within pads of cells (as illustrated in NatureofBiology,Book1,FourthEdition, page 39, and on page 52 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 figure 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 phago-cytic 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 inflammation, so that sur-rounding cells are not damaged in the way that neighbouring cells can be with necrosis. The process is summarised in figure 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 neurodegenerative dis-

eases such as Alzheimer and Huntington’s diseases• too little apoptosis can lead to the production of

cancers and autoimmune diseases.Refer also to page 404 in chapter 11. You will need to

understand apoptosis for your studies later in the year.

(b) Death signalsinstruct cell to die

Death signalreceptors

Signal recognisedand self-destructprogram 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


biology in the worKplaCeSue Macaulay — chief radiographer, St Vincent’s BreastScreenIn 2011 in Victoria, 207 655 people were screened for breast cancer. Of these, 44 710 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 experience placement and I qualified 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 field 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 Vincent’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 mag-netic resonance imaging (MRI), angiography (imaging blood vessels), ultrasound, general radiography, and mammography (breast imaging). Because most women prefer it, mammograms are done mainly 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 ser-vice, from screening to diagnosis. Women in the target 50–69-year age group are identified through the electoral roll and actively recruited for the pro-gram. 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 may have 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 firmly compress the breast. Compression 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 mammog-raphy in that the radiographer takes two projections of each breast and checks that the films are techni-cally 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 fine-needle biopsy is undertaken to obtain cells, or a core biopsy is used to extract tissue. These are carried out with ultrasound or X-ray guid-ance. If a lesion cannot be seen under ultrasound, then X-ray guidance is used with a prone stereotactic 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 qualified radiographers to train for their Certificate of Competency in Mammography (CCPM). This certificate is required to work in the BreastScreen program. I oversee quality assurance for all of St Vincent’s BreastScreen sites to ensure that a high quality of mammography is being produced. I also work at Monash University teaching first-year stu-dents positioning for general radiography. I also look after the radiographic aspects of the two mobile BreastScreen Victoria vans, which take the service to women in isolated communities.

For me, radiography has provided lots of chal-lenges. 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 special-ised service to women in Victoria. It is exciting to be involved in bringing developing digital technology to mammography.

Figure 2.5 Sue Macaulay and the BreastScreen equipment


Key ideas• 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.

QuiCK-CheCK 3 List three possible death signals a cell might receive to initiate

apoptosis.

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 (such as figure 2.6) and electron microscopes. We have also outlined the capabilities of the synchrotron (see pages 3–5). 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 figure 2.7 (page 41).

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 fluid, called cytosol, that consists mainly of water containing many dissolved substances (see table 2.1, page 41). There is a labyrinth of mem-branes 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 compartments are called organelles (figure 2.8, page 42).

See more Living organisms are made of cells

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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.Inthisexample,ontheleft-hand side, 1 mm is the first measure,thenextis100µm,which is one-tenth of 1 mm, and so on along the scale.

1 m

m

10

0 m

m

10

mm

1 m

m

0.1

mm

0.0

1 m

m

0.0

01

mm

0.0

00

1 m

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Humanvision

Frog egg

Animal cell Mitochondrion

Ribosome Molecules

Plasmamembrane

Lightmicroscope

Electronmicroscope

Organelles are held in place by a network of fine protein filaments within a cell, collectively known as the cytoskeleton (see page 55). Prokaryotic cells such as bacteria lack these internal membranes.

In the following sections, we examine the plasma membrane and cell orga-nelles 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 cell

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

732

417

Nucleus inner membrane 0.2 0.7

Volume of cell (approx.) 5 000 µm3 1 000 µm3

Area of cell membranes (approx.) 110 000 µm2 13 000 µm2

Cytoplasm = cytosol + organelles except the nucleusProtoplasm = cytosol + all organelles

See more Different shaped bacteria

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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.

CytosolProtein�lament

Plasma membrane

Nucleus

Mitochondrion

Ribosome

Endoplasmicreticulum

Lysosome

Centriole

Endosome

Peroxisome

Proteinmicrotubule

Golgi apparatus

Vesicle

the plasma membrane boundaryThe boundary of all living cells is a plasma membrane that controls the 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 figure 2.9 below, image at top right).

A plasma membrane comprises a phospholipid bilayer into which proteins and glycoproteins protrude (see figure 2.9). Some of the proteins embedded in this layer form channels that allow certain substances to pass across the mem-brane in either direction. This is known as the fluid mosaic model.

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.

Outside cell

Cytoplasm

Nucleus

Glycolipid

Phospholipidbilayer

Membraneglycoprotein

Do more Build an animal cell

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Some cells also have a cellulose cell wall exterior to the plasma membrane (refer to page 45).

Do more Movement across membranes

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recognising cells: ‘self’ or ‘non-self’On 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 mam-mals of the same species are compared, the antigens on their plasma mem-branes 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 that 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 sur-vive, 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 figure 2.10a, page 44). 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 dis-solve in water are able to dissolve in or mix uniformly with lipid. These sub-stances are termed lipophilic (sometimes called hydrophobic). Examples of lipophilic substances include alcohol and ether. Lipophilic substances can cross plasma membrane boundaries readily.

Channel mediatedSome 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 figure 2.10b). Molecules move from a high concentration to a low concentration without requiring energy.

Carrier mediatedSometimes a protein channel alone is insufficient and a carrier molecule is required to move molecules down the concentration gradient through a protein channel (see figure 2.10c). When a specific 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 specific carrier molecule.

All three methods of passive transport (figure 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 FaCtThe first 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, Fourth Edition, pages 30–3.

‘Partially permeable’ is also known as selectively or differentially per-meable or semi-permeable.

odd FaCtOne 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.

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Practice VCAA exam questions


Figure 2.10 Transport of molecules across membranes: (a–c) Three ways in which molecules move from a region of high concentration,acrossaplasmamembrane,toaregionoflowconcentrationwithouttheexpenditureofenergy.(d) Movement of moleculesfromaregionoflowconcentrationacrossaplasmamembranetoaregionofhighconcentrationrequirestheexpenditureof energy. Note the movement of molecules against the concentration gradient.

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

Paid passage: active transportActive transport is the net movement of dissolved substances into or out of cells against a concentration gradient (see figure 2.10d). Because the net movement is against a concentration gradient, active transport is an energy-requiring pro-cess. 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 figure 2.11a).

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(bulktransport out of cells) occurs when vesicles within the cytosol fuse with the plasma membrane and vesicle contents are released from the cell.

Cytosol

Lysosome

Outside cell

Lipid bilayer

Cytosol

Outside cell

Lipidbilayer

(a) (b)

odd FaCtWhen 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 fluid, the process is termed pinocytosis (pinos = ‘drinking’).


Bulk transport out of cells (such as the export of material from the Golgi complex, see pages 49–50) 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 figure 2.11b). If the released material is a product of the cell (such as 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.

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 flowering plants, the primary cell wall in certain tissues becomes thickened and strengthened by the addition of lignin to form secondary cell walls (see figure 2.12). This process provides great elastic strength and support, allowing certain plants to develop as woody shrubs or trees.

plants have Cell walls

Figure 2.12 The primary cell wall of a plant cell is made of cellulose. The layers of microfibrils in the secondary walls are laid down indifferentdirectionsandgiveextrastrengthandsupporttoaplant.

Layers ofsecondarycell walls

Adjacentcells

Primarycell wall

Key ideas• 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 membranes of plant, fungal and

prokaryotic cells.

QuiCK-CheCK 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 it is energy-requiring. 6 Many plant cells have secondary cell walls as well as primary cell

walls. Of what advantage is this to a plant?

organelle 1: 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 figure 2.13). Cells that have a membrane-bound nucleus are called eukaryote cells.

Nucleus


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 gran-ules 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.

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 exitthenucleusandmoveintothecytosol.

Key ideas• 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.

QuiCK-CheCK 7 Statewhetherthefollowingaretrueorfalseandbrieflyexplain

your answers.a Anucleusfromaplantcellisexpectedtohaveadoublenuclear

membrane.b Chromosomes are always visible in a eukaryotic cell.

8 Suggest why the nucleus is sometimes called the ‘control centre’ of a cell.

Refer to Nature of Biology, Book 1, Fourth Edition, page 27 for more information about prokaryotes such as bacteria.


organelle 2: mitochondrion — energy-supplying organelleLiving cells use energy all the time. The usable energy supply for cells is chem-ical energy present in a compound known as adenosine triphosphate (ATP) (see figure 2.14). The ATP supplies in living cells are continually being used up and must be replaced.

Figure 2.14 Chemical structure of adenosine triphosphate (ATP), which has three phosphate groups and so is adenosine tri(= 3) phosphate

HO P O P O P O CH2

O O O

OC

H C

OH

H

C

OH

H C

H

OOO

NC

NHC

NC

CN

CH

Adenine

D-ribose

Triphosphate

Adenosine

NH2

ATP is produced during cellular respiration (or 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 figure 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)Transmissionelectronmicrograph(x50000)ofmitochondria(circular structures), 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

Mitochondrion

The role of mitochondria in respiration is discussed in chapter 3, pages 86–7.

odd FaCtMany 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 beneficial 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 µmby0.5µm.Thisissimilar to the dimensions of a typical bacterial cell.


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 that 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 orga-nelles, which are part of the cytoplasm, can be seen only through an electron microscope (see figure 2.16).

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 sites 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 later), 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.

Ribosomes


Key ideas• 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.

QuiCK-CheCK 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 significance of this difference?

organelles 4 and 5: endoplasmic reticulum and golgi complexWe saw earlier 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 figure 2.16, page 48). The channel walls are formed by membranes.

Figure 2.17 Transmission electron micrograph (TEM) of rough endoplasmic reticulum (ER), the thin ‘channels’ coloured green in the centre. What are the tiny ‘dots’ attached to the endoplasmic reticulum?



A structure known as the Golgi complex is prominent in cells that shift pro-teins out of cells. This structure consists of several layers of membranes (see figure 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 figure 2.19).

Figure 2.18 Transmission electron micrograph (TEM) of a Golgicomplex(flatteneddisc-likestructure, 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 first identified it in 1898.

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 mem-brane. The protein is then discharged from the cell by exocytosis into the sur-rounding tissue fluid. 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.

Figure 2.19 The secretory pathway for proteins made at ribosomes. They are packaged by the endoplasmic reticulum and transportedtotheGolgicomplexwhere they may be concentrated. Secretory vesicles formed by the Golgicomplexeventuallyfusewiththe 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

Golgicomplex


Key ideas• The endoplasmic reticulum (ER) is a series of membrane-bound

channels.• The ER functions in the transport of substances within a cell.• TheGolgicomplexpackagessubstancesintovesiclesforexport.

QuiCK-CheCK12 Name three substances that would be produced at the surface of the

ER of a cell and transported for use outside the cell.

organelle 6: lysosomes — controlled destructionAnimal cells have sac-like structures surrounded by a membrane and filled with a fluid containing dissolved digestive enzymes. These fluid-filled sacs are known as lysosomes and they are part of the cytoplasm (see figure 2.20).

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.

Lysosomes


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 fingers become separated (see Nature of Biology, Book 1, FourthEdition, page 39). A similar event occurs in a developing chick embryo (see figure 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.

PeroxisomesHydrogen 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 detoxified through oxidation to acetaldehyde. Peroxisomes in different types of cells may contain different sets of enzymes. Both plant and animal cells have peroxisomes.

endosomesEndosomes are membrane-bound organelles found in animal cells. When mate-rial enters a cell by endocytosis, endosomes pass on the newly ingested material to lysosomes for digestion.

Key ideas• Lysosomes are membrane-bound sacs containing dissolved digestive

enzymes.• Lysosomes can digest material brought into their sacs.• Peroxisomescontainenzymesthatdestroytoxicmaterials.• Endosomes, found in animal cells, pass on material to lysosomes for

digestion.

QuiCK-CheCK13 Lysosomes are sometimes called ‘suicide bags’. Suggest why this

name is given.14 Howisthehydrogenperoxideproducedincellularmetabolism

detoxified?15 What is the function of endosomes?

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

Webbingbetweentoes

Chicken Duck

Footbud1.

2.

3.


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 trans-form 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 figure 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 photosynthesis occur (see chapter 3, page 77). The stroma, the semi-fluid substance between the grana, contains the enzymes necessary for the light-independent reactions of photosynthesis.

Grana Innermembrane

Stroma

Outermembrane

Figure 2.22 (a) Transmission electron micrograph (TEM) of chloroplasts from the leaf of a pea plant (b) A three-dimensional representation of a chloroplast

(a)(b)

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.

Key ideas• Chloroplasts are relatively large organelles found in photosynthetic

cells of plants and algae.• Chloroplastshaveanexternalmembraneandlayersoffoldedinternal

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.

QuiCK-CheCK16 What is the function of chlorophyll?17 What are (a) thylakoids, (b) grana and (c) stroma?

Chloroplast

Photosynthesis is discussed further in chapter 3, pages 74–81.


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 specific protein for use outside the cell. Table 2.3 identifies 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 specific 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 that packages the protein into vesicles for transport across the plasma membrane and out of the cell

peroxisome Organelle that detoxifies H2O2 produced in many metabolic reactions

Figure 2.23 The structures and organelles of (a) an animal cell and (b) a plant cell

CytosolProtein�lament

Plasma membrane

Nucleus

Nucleolus

Mitochondrion

Nuclear envelope

RibosomeEndoplasmicreticulum

Endosome

Peroxisome

Lysosome

Centriole

Proteinmicrotubule

Golgi apparatusVesicle

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, microfilaments and intermediate filaments (see figure 2.24). These supply strength and support for the cell. This supporting structure is called the cytoskeleton.

Figure 2.24 Three structures that make up the cytoskeleton of a cell

25 nm 7–10 nm6 nm

(a) Microtubule (b) Microfilament (c) intermediate filament

Microtubules are hollow and are made of sub-units of the protein tubulin (see figure 1.24, page 21). Microfilaments are solid, thinner and more flexible than microtubules. They are made of actin. Intermediate filaments 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• 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, such as 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? What holds the cells of epithelial tissue 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 figure 2.25, page 56).

occluding junctionsOccluding junctions involve cell membranes coming together in contact with each other (figure 2.25). There is no movement of material between cells.

odd FaCtOccluding junctions between brain cells and brain capillary cells prevent the passage of some materials, such as certain drugs, from the blood into the brain.


Cell 1

Cytoplasm

Lipid bilayer

Nonjunctionalmembraneproteins

Pipeline betweenadjacent cells

Extracellularspace

Intercellular ‘gap’of 15 nm

Solute molecules

Membraneprotein

Intercellularspace

Cell 2 Anchoringjunctions

Occludingjunction

Communicatingjunction

Cytoplasm

Figure 2.25 Diagram of the three types of intercellular junctions found in epithelial cells

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 figure 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 electrical signals is the control of the beating of the heart. A small area of your heart, called the pacemaker, receives an electrical impulse. This electrical impulse is transmitted to all cells of the heart through communication junctions so that the cells of the heart ‘beat as one’.

Figure 2.26 Communicating junction of animal cells. Note the pore formed by protein molecules aligned as if on the circumference of a circle.

Cell 1

Plasmamembrane

Plasmamembrane

Cell 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 figure 2.27). Fine fibrils extend from each side of these plaques and into the cytosol of the two cells involved. These are intermediate filaments (as represented in figure 2.24c, page 55), which 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. Finefibrils(red)extendfromplaquesinto 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: plas-modesma) (see figure 2.28, page 58).

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.


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

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 are considered in greater detail in chapters 5 and 6.

Key ideas• 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.

QuiCK-CheCK18 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?


Mary — thalassaemia minorI am a secondary school teacher of VCE Biology, Mathematics and Science. When I was a university student in the second year of my course studying Genetics, we considered the disorder thalassaemia, the gene responsible and its physical symptoms. I had been experiencing lethargy, faintness and headaches and, given my Greek–Cypriot background, my doctor suggested a blood test. I was diagnosed as a carrier of thalassaemia; that is, one of my two alleles for the thalassaemia gene was defective and I had what is called thalassaemia minor (thal-minor). People from countries around the Mediterranean have a higher chance of carrying a defective allele for thalassaemia than those from other regions.

An individual who has inherited a defective thal-assaemia allele from each parent has thal-major and, although treatment is available, they generally have a reduced quality of life and life expectancy. My blood test results showed a low haemoglobin count and some unusual-shaped red blood cells. The unusual-shaped red blood cells have a lower oxygen-carrying capacity than normal red blood cells. This would explain why I had low oxygen levels and why I was always tired.

I was also diagnosed as having mild anaemia and was prescribed iron tablets. I took these but later concentrated on eating a healthy diet that included high-iron foods, such as red meat, which is generally sufficient to prevent anaemia in a person with thal-minor. Much more is known now about thalassaemia and its optimal treatment.

Many members of my family have been tested and diagnosed with thal-minor. A cousin and spouse each tested positive to thal-minor. Knowing this meant that they were aware of the chance of having a child with thal-major and could consider all options and plan accordingly rather than be faced with the unexpected.

Given that my parents were both carriers of a defective allele, each child of theirs had a 25 per cent chance of having thalassaemia major. We con-sider ourselves lucky that the odds were in our favour. Naturally I had some concern about being thal-minor. My partner was tested and luckily he was not a car-rier. This made our decision to have children easier as there was no chance of having a thal-major baby. Each pregnancy had a fifty per cent risk of a thal-minor baby. This is really of little concern because mild anaemia that may be present can be readily con-trolled by careful diet.

I have two beautiful children, Stephen and Andrea, who are both healthy. However, I had some concern about their energy levels and therefore their iron and haemoglobin levels, especially my daughter who had a pale complexion and often tired easily. Some doctors suggest that testing of children for thalassaemia status can be delayed until they are thinking of starting a family. We chose to have the children tested to see if they had inherited the defective allele. We believed that, if they did prove to be thal-minor, they would over time come to a better understanding of what it meant, I would have immediate information about whether they needed a higher than average intake of iron, and we could discuss options that might arise. In the future, the options could include testing of carrier status of a partner before starting a family.

I am pleased to be able to share with students my experiences and the knowledge that living with a genetic disorder, or knowing that you can pass on a defective allele to a child, does not have to control a person’s life, especially if they are informed and aware.

personal story

Figure 2.29 Mary and her children, Andrea and Stephen

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 find

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? Explaintheirimportance.

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. Explainwhytheylooksodifferentfrom each other.

AA

B

D

C

A

B C

F

ED

A B

C D


Chapter reviewKey words

active transportadenosine triphosphate (ATP)antigensapoptosiscancercellular respirationchloroplastschromosomescytoskeletoncytosoldeoxyribonucleic acid (DNA)desmosomesdiffusionendocytosisendoplasmic reticulum

eukaryoteexocytosisGolgi apparatusGolgi complexgranahydrophiliclamellalipophiliclysosomesmitochondrianuclear envelopenucleusorganellesosmosispartially permeable

phagocytosisphotosynthesispinocytosisplasma membraneplasmodesmataprimary cell wallprokaryoteprotein filamentsproteinsribosomessecondary cell wallsstromathylakoids

Questions 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 2.30 is a col-oured 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 (ER).

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 con-clusions can reasonably be made on the basis of the presence of such structures?

4 Applying knowledge and understanding ➡ Examine table 2.2 on page 41.

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 2.31). 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?

6 Analysing information and drawing conclusions ➡ In figure 2.31, you may have noted the holes in the folds of the inner membrane of mitochon-dria. Explain a possible function for these holes.

Figure 2.30 Transmission electron micrograph of a plasma cell

Figure 2.31 Internal membrane of mitochondria

Fold of inner membrane

Stalkedparticle

Holes in membrane

Outer membrane

Inner membrane


7 Applying knowledge and understanding ➡ Examine figure 2.32, 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 2.32 Coloured, high-resolution scanning electron micrograph of a portion of cell

8 Analysing information and drawing conclusions ➡ Figure 2.33 shows a portion of an animal cell.

a From what part of the cell has the structure been taken?b Name the kinds of organic molecules labelles X and Y and Z.c Explain the function of the structure labelled W.

Figure 2.33

Y

X

W

Z

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 2.34. 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 2.34

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%)

10 Applying knowledge and understanding ➡ Examine figure 2.35, 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 ➡ Use the Cytoskeleton weblink for this chapter in your eBookPLUS. Select ‘Cell biology’ at the left-hand side. Scroll

down and click on ‘The cytoskeleton’. Then select ‘Microtubules, micro-filaments 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 ➡ Use the Cell structure animation weblink for this chapter in your eBookPLUS. 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 2.35 Scanning electron micrograph of part of a cell

X

Y

yr 12 bio chapter 2

Documents

death of cells

epithelial cells

eukaryotic cells

breast cancer

cell reproduction

cell organellesfigure

kind of cell

groups of similar cells