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C h a p t e r

Dynamic cells: molecules on the move



● Earth

● Biosphere

● Biome

● Ecosystem

● Community

● Population

● Organism

● Systems

● Organs

● Tissues

● Cells

● Organelles

● Molecules

● Atoms

2

bioTERMSprokaryotic cellcells that lack a membrane-bound nucleus and other membrane-bound organelles; all bacteria are prokaryotic cells

eukaryotic cella cell with a membrane-bound nucleus and other membrane-bound organelles

40 Unit 3

Key knowledge● The role of the organelles and plasma membranes in the packaging and transport of biomolecules

● Applications of molecular biology in medicine, including the design of drugs and in medical diagnosis

In this chapter we investigate the dynamic nature of cells at the molecular level: how the major biomolecules of life are packaged and transported in cells, and how they interact with others in a controlled and efficient way.

Cells produce substances that need to be modified and stored in special compartments. Prokaryotic organisms – bacteria – have a simple structure and lack these special compartments (Figure 2.1). In eukaryotic cells, however, the internal structure is made up of various organelles (Figure 2.2), which can be viewed as ‘membrane-bound’ compartments – they have membranes that separate them from the rest of the cell and some have membranes that fold within which are sites of chemical activity. These organelles concentrate reactants and some maximise their surface area by the folding and stacking of internal membranes. The membranes of organelles also control the entry and exit of substances.

Every living cell of every part of an organism needs matter and a source of energy to keep it alive. Each kind of organism has its own way of making this happen but there are processes that are common to all. Cells have a variety of strategies for importing and exporting substances that are necessary for cell functioning. The strategies used depend upon the chemical nature of the substances.

Unicellular organisms take in materials (inputs) from their external environment and process these materials inside their single cell. The outputs or products of these activities are biomolecules and inorganic wastes. The biomolecules form the structures that carry out tasks for the organism and they provide it with the energy to perform these tasks. The waste products produced by working cells must be removed. The plasma membrane is the boundary of the unicellular organisms and controls what goes in and what goes out.

Figure 2.1 Prokaryotic cells, such as these Bacillus

bacteria, do not have a nucleus or membrane-

bound organelles.

R e v i e w1 Compare,bymeansofatable,prokaryoticandeukaryoticcells.Suggestareasonforthedifference

instructuralorganisationbetweenthetwocelltypes.

2 Definethetermorganelle.Compareandcontrasttheorganellesfoundineukaryoticplantand

animalcells.

vesicle

cytoskeleton

plasma membranerough endoplasmic

reticulum

Golgi apparatus

nucleus

41

Chapter 2 Molecules on the move

Figure 2.3 Large organisms solve the problem of having a small surface-area-to-volume

ratio by having systems that supply materials to cells and

remove waste products.

Complex multicellular organisms have to do it differently. They cannot rely on a simple exchange of materials over the surface of their body to obtain what their cells need. Even if they are bathed in a watery medium that prevents their surface from drying out, their surface area in relation to their volume is far too small to supply materials to the cells that lie deep in the organism. These substances have to move from cell to cell and through systems that deliver materials to cells and remove wastes (Figure 2.3).

Multicellular organisms, such as humans and plants, have specialised body systems that contribute to the functioning of the organism as a whole: there is an efficient division of labour. Complex plants have transport systems that carry water and minerals in solution in such a way that, under normal circumstances, all cells obtain what they need. The products of activities of cells are distributed in the transport system too, but along different pathways.

Humans have a circulatory system that transports the products of digestion to every living cell. As heterotrophs, our food is essentially composed of organic compounds, many of which are large biomolecules that are too large to pass through cell membranes. These have to be broken down into their smaller subunits. Only then can they pass from the tube that they enter (digestive tract), through the walls of the fine blood vessels next to it, into the fluid that bathes the tissues and into the circulatory system for transport to cells and tissues elsewhere.

Figure 2.2 Eukaryotic cells, such as (a) animal and (b) plant cells, localise specific cell functions in membrane-bound organelles.

a b

A small organism has a large surface-area-to-volume ratio. It obtains its oxygen inputs and removes its carbon dioxide outputs by diffusion across the surface.

A larger organism has a smaller surface-area-to-volume ratio. It may have difficulty obtaining enough oxygen inputs or removing carbon dioxide outputs by diffusion across its surface.

This problem is solved by having special organs that provide exchange surfaces, for example, gills.

gut or lungs

gut gill

gut

lung

mitochondrion

cell wall

plasma membrane

nucleolus

nucleus

endoplasmic reticulum

chloroplast

vacuole

leucoplast

Golgi apparatus

mitochondrion

nucleolus

cytoplasm (cytosol)

nuclear membrane

nuclear membrane

bio B Y T EA new branch of molecular biology has emerged, phenomics, which investigates

how the genes in the human genome coordinate the function and health of the human body. Australian and international researchers will use the Australian Phenomics Facility at the Australian National University to discover more about gene function in relation to health.

42 Unit 3

Figure 2.4 Research using cell cultures helps scientists learn about the way molecules and cell systems behave. This is an islet of Langerhans beta cell of the pancreas that has been grown in cell culture. It contains numerous membrane-bound secretory vesicles (red) containing the hormone insulin

Cells as systemsThe movement of molecules across membranes, the movement of molecules within cells and cell interactions show how cells are systems. They represent systems biology on a molecular scale. Through advances in technology we have come to know more about how cells communicate with each other, how protein chains fold into shapes that give them functionality, and how genes that carry the code of instructions for

what they are and do are regulated.Knowing how a system works can help us understand what causes

a system to fail. Advances in molecular biology have improved the medical diagnosis of malfunctions in cells, tissues, organs

and systems and so have enabled the development of ways of treating them.

Molecular biologists have developed an array of technologies to help understand how cells work as systems. Cells of humans are composed of substances and carry out chemical processes in common with cells of all other organisms. Much of what we know has been derived from the study of simpler organisms. These are referred to as model organisms and include bacteria, yeast, fruit-flies, plants and mice.

Model organisms are easy to maintain in laboratories and there are strict regulations for their care and use.

These organisms allow scientists to carry out investigations under controlled conditions; the organisms are selected and

bred with characteristics that are useful for studying specific biochemical processes.

The development of sophisticated computer technologies has advanced much of our knowledge and has taken the place of much live

modelling. Programs have been designed to model how molecules interact at the cellular level in networks of biochemical pathways. Computer modelling can predict how a small change in one cellular component can affect hundreds of biochemical processes.

By studying the relationships and interactions between various parts of a biological system (e.g. organelles, cells, physiological systems, organisms) we hope to develop an understandable model of the whole biological system.

Bioinformatics is an important tool for much modern biomedicine and biotechnology. It is the application of information technology, statistics and mathematics to biological problems. It involves accumulating large volumes of data and developing the ability to analyse it to understand and predict complex interrelationships.

To fully appreciate cells as interactive systems, we must explore the beauty of their molecular design. Our journey begins at the outer boundary – the plasma membrane.

43


Do you fancy a little robot machine the size of a rice grain buzzing along in your blood, detecting how healthy you

are and reporting to a health computer? This might be the way of the future.

Nanotechnology is a rapidly expanding field in science. Knowledge of how atoms and molecules relate to each

other because of their shape and charge is being used to engineer a variety of materials and ‘nanomachines’.

Nature has already perfected the manufacture of machines from atoms – each cell contains many ‘nanomachines’

that carry out the work of the cell. But now we are entering a new age of science in which we are creating the

nanomachines. Future medicines may contain nanorobots, for example, that will be capable of finding and attacking

cancer cells and viruses, rendering them harmless. Others act as biosensors that detect the condition of the blood or

the balance of chemicals in cells.

Research laboratories are using nanoprobes to find out what is going on in cells. Nanorobots could help doctors

to operate, leaving little or no scarring as less scalpel work is required, and artificial blood cells would eliminate the

need for blood transfusions.

What knowledge does the field of nanotechnology draw from to create ‘nanomachines’ for medicine and

industry?

What are some of the ‘nanomachines’ that are being designed for use in medicine?

What are ‘nanomachines’ or nanorobots constructed from? Would you say these are natural or unnatural entities?

BioBox 2.1RoBots in the Blood?

R e v i e w3 Explainthemeaningofthefollowingterms:

a modelorganism b systemsbiology c bioinformatics

4 Howdoesanunderstandingofhowcellsoperateassystemsimprovemedicine?

the boundary umpire – the plasma membrane

As early as the 1890s, Charles Overton understood that cells had an outer layer that allowed some substances into the cell and kept other substances out. He found that lipid-soluble substances readily entered the cell, which was a clue that lipid molecules may have been part of the structure of the membrane.

It was not until the invention of the electron microscope in the 1950s that we were able to see details of the cell membrane’s structure. Since then, our knowledge of membranes and what they do has grown. The term membrane is often used to describe any thin layer, whether in relation to living things or not. The membrane of cells is properly called the plasma membrane.

student Cd

Plasma membrane

bioTERMplasma membranethe insoluble boundary of the living cell that maintains the contents of the cell and regulates the movement of substances in and out of the cell. All cells have a plasma membrane

In addition to a plasma membrane, some life forms have cell walls. In plants, the cell wall is composed of cellulose and pectin, whereas in fungi it is composed of chitin and in bacteria it is made of peptidoglycan. All these substances are insoluble polysaccharides. Cell walls are tough and add strength and support to the cells. However, they are described as freely permeable as they allow a lot of particles through.

The plasma membrane is described as selectively or differentially permeable – it allows some substances through and not others. This is the key to much of what goes on inside the cell. A knowledge of the structure of the plasma membrane can help us understand cell functioning.

the fluid mosaic modelThe plasma membrane is composed of two layers of phospholipids. Each phospholipid can be represented by a head and two tails. The head (the phosphate group) is hydrophilic and the fatty acid tails are hydrophobic. This means that the head tends to dissolve in water (like dissolves in like), whereas the tails are repelled

and forced to face inwards away from the watery environment and towards each other (Figure 2.6), forming a phospholipid bilayer.

Phospholipid molecules are capable of sideways movement. Specialised protein molecules are also embedded in the bilayer in various patterns or ‘mosaic’. Some of these proteins can move laterally, and others are fixed in position. Proteins and lipids can also flip around in the membrane. As a result of the work of Singer and Nicolson in the 1980s, the structure of the plasma membrane is understood as a ‘fluid mosaic model’ (Figure 2.7).

The lipid structure of the membrane gives it the unique property of being flexible and being able to repair itself if, for example, it is pierced; punctures that are not too extreme can be sealed. This property is made use of in biotechnological procedures when the inside of a cell has to be accessed (Figure 2.8).

Figure 2.5 Phospholipid molecule. The polar hydrophilic heads are attracted to water whereas the hydrophobic hydrocarbon tails repel water.

Figure 2.6 The plasma membrane is a bilayer that is made up of phospholipid molecules.

Figure 2.7 Three-dimensional view of a plasma membrane based on the fluid-mosaic model.

hydrophilic head containing phosphate

hydrophobic tail made of fatty acid side chains

44 Unit 3

hydrophilic head

hydrophobic tail

water

water

phospholipidbilayer

phospholipid bilayer

glycolipidcarbohydrate chain

cholesterolglycoproteinphospholipid

To die or not to die, that is the question. In the human body, around 60 billion cells die each day due to

the process of apoptosis. This is programmed cell death, which is controlled, in part, by receptors on

cell membranes. Apoptosis maintains constant cell numbers within organs and tissues, with cell death

counteracted by cell proliferation (cell production). If the balance is upset, tumours

form or body wasting diseases occur that can be

fatal. Programmed cell death is also important

for the removal of cells with damaged DNA, that

are infected with virus, that are starving or are

unwanted immune system killer T cells.

Messages to trigger cell death can come from

inside or outside a cell. Messages from outside

the cell bind to ‘death receptors’. These membrane

proteins relay messages to the inner workings of

the cell, which stimulate it to activate a group of

‘protein-eating’ enzymes (caspases). The activated

caspases break down structural and functional

components of the cell. The dying cells display

an ‘eat me’ signal that is recognised by a passing

macrophage, which disposes of the cells by engulfing

them.

What is the name of the process responsible for

killing up to 60 billion cells in our bodies each

day?

Why is programmed cell death important for

maintaining life?

When messages to trigger cell death come from outside the cell, how do they

transmit their message to inside the cell?

Draw a simple flow chart to illustrate the steps involved in apoptosis when the

message to trigger cell death comes from outside the cell.

BioBox 2.2Cell death ReCeptoRs

Figure 2.8 Puncturing and resealing the membrane of a cell. Microscopic sequence of removal of the nucleus of an egg.

45


Figure 2.9 Colour scanning electron micrograph of human white blood cells (magnification × 1500). Programmed cell death, or apoptosis, is integral to maintain the balance of cells in multicellular organisms.

healthy white blood cell

apoptotic white

blood cell

R e v i e w

biolink

Plasma membrane

46 Unit 3

Figure 2.10 The relative permeability of a lipid bilayer to different classes of molecules.

The protein molecules embedded in the plasma membrane have particular functions and many of them carry a sugar molecule, giving them their collective name of glycoproteins. Glycoproteins are often receptors and marker molecules that identify the cell as belonging – each kind of organism has it own kind of glycoprotein and even different individuals of the same species can be distinguished by the glycoproteins that they have on the surface of their cell membranes. These marker molecules identify self and are important in cell recognition.

For example, the immune system can recognise invaders by the self or non-self glycoproteins they have. They can recognise, for example, which kind of blood

is necessary for transfusion: is it type A or type B? The difference lies in the glycoproteins that are embedded in the blood cell membranes.

Membrane proteins are important for the regulation of cell behaviour and the organisation of cells in tissues. Proteins are also involved in cellular communication. Receptor sites on the surface of membranes detect hormones and other chemical molecules to control the transmission of messages within and between cells.

Controlling the trafficThe plasma membrane can be thought of as a sieve: small particles can pass through incredibly small openings or pores between the phospholipids, some can be helped through and large ones are held back (Figure 2.10). Molecules of substances such as water, carbon dioxide, oxygen and other small molecules pass through easily. Glucose molecules and various ions can move through channels in the membrane with assistance from transport proteins but large protein molecules cannot pass through.

5 Bymeansofanannotateddiagram,relatethestructureof

theplasmamembranetoitsfunction.

6 Plasmamembranesareselectivelypermeable.Whatdoes

thismean?

7 Whichpartoftheplasmamembraneisdescribedas(a)‘fluid’

and(b)‘mosaic’?Explainwhythesetermsareused.

8 Glycoproteinsarecomponentsoftheplasmamembrane.

Statetwofunctionsofthesemolecules.

Spreading the molecules – defining diffusion

What makes particles move? When you smell something, think about where the smell is coming from and how it reached your nose. The particles that constitute the smell, whether of perfume or something unpleasant, travel from the source to the smell detectors in your nose between the particles of air. This is the process of diffusion.

lipid bilayer

small hydrophobic molecules

ions

Na+, H+, K+, Mg+, Ca2+, Cl–, HCO3

–

larger uncharged polar molecules

small uncharged polar molecules

oxygencarbon dioxidenitrogen

amino acidsnucleotidesglucose

waterethanolglycerol

BioBox 2.3dialysis and the aRtifiCial kidney

BioBox 2.3 continuednextpage

time

water

potassium permanganate

lipid bilayer of plasma membrane

concentration gradient

high concentration

low concentration

47


Diffusion is defined as the net or overall movement of particles (molecules or ions) from a region where they are at a relatively high concentration to a region where they are at a relatively low concentration (Figure 2.11).

The difference in concentration between the two regions provides a concentration gradient or diffusion gradient; the steeper the gradient, the more rapid the rate of diffusion. As the gradient declines, diffusion slows down until it reaches equilibrium. The particles at equilibrium are distributed evenly and move to and fro at the same rate in all directions (Figure 2.12).

Diffusion is a passive process as it does not require additional energy to make it happen, although it can be sped up by making the particles move faster or by increasing the concentration gradient.

Influential Australian businessman, Kerry Packer, was extremely fortunate to have his friend and employee, Nick Ross,

donate a kidney when he was diagnosed with kidney failure. Without a kidney transplant, Packer would have spent

the rest of his days receiving kidney dialysis treatment. Dialysis treatment involves having your blood flow through

a dialysis machine to remove waste products and excess water from the blood, a process normally performed by the

kidneys. The blood returns to the body with toxins and wastes removed and essential factors added.

Willem Kolff developed kidney dialysis in the early 1940s. He searched for an effective method to employ dialysis, the

process where particles pass selectively through a membrane, to treat patients with kidney failure. The key component

for the filtering process was to find a suitable membrane. World War II had brought about a shortage of materials to

use, making his task a difficult one. Eventually he came across cellophane, a thin membrane used as sausage casing. He

found that two solutions of different concentrations that were separated by this membrane would exchange molecules,

that is, they moved from the solution of greater concentration to the solution of lower concentration. He successfully

Figure 2.12 (above) Simple diffusion of molecules through the plasma membrane.

Figure 2.11 (left) Diffusion is dependent on a concentration gradient, as shown by the diffusion of potassium permanganate molecules in water over a period of time.

biolink

diffusion

BioBox 2.3 continued

48 Unit 3

used this membrane to achieve diffusion in the first artificial kidney. Constructed from wood, juice cans and an old

washing machine, it provided life-saving dialysis for end-stage kidney disease patients.

Solutes such as urea and creatinine crossed the semipermeable membrane down a concentration gradient, leaving

the blood and entering the dialysis fluid compartment. Large components of the blood, such as the red blood cells, would

not cross the membrane. Some solutes, such as bicarbonate ions that help buffer the blood, were supplied in the dialysate

solution so they would diffuse into the blood. This process is nowhere near as effective as the glomerulus filtering system

in the kidneys but it serves to remove most wastes and help control fluid levels.

Today’s dialysis machines act as an artificial kidney for more than one million patients worldwide. Research directions

are moving towards improving the dialysis membrane to address problems with blood clotting when the blood contacts

the membrane surface, the amount of pressure that can be placed on the membrane and its selectively permeable nature.

Today’s filter membranes are either cellulose-based or synthetic. Synthetic filters tend to be more biologically compatible

and statistics suggest that they provide a greater survival advantage than their cellulose counterparts.

With an estimated 14 000 Australians currently using dialysis and one in seven Australian adults at risk of developing

kidney disease, current research efforts include developing alternatives to dialysis, such as a true artificial kidney,

xenotransplantation and replacement kidneys developed through tissue engineering.

Researchers in Japan have cloned a human kidney by implanting human stem cells into rat embryos, growing

the embryos in the rat uterus and, after two days, removing them and extracting the cells from the kidney area of the

embryo. These cells then developed into nephrons, the excretory unit of the kidney, in tissue culture. This process will

allow implantation of compatible nephrons into patients with kidney disease.

Describe how the first artificial kidney worked.

How is cellophane used to remove toxins, such as urea, from the blood?

What problems are researchers working with to improve the dialysis membranes of artificial kidneys?

How is tissue engineering promising to help the one in seven Australians who will

develop kidney disease?

Figure 2.13 The first artificial kidney was designed by Willem Kolff in 1942. The cylindrical drum was wrapped with 30 windings of cellophane tubing. As the drum rotated through the tank of fluid, the patient’s blood flowed through the cellophane tubing, allowing the toxic impurities to be ‘dialysed’ away.

R e v i e w

bio B Y T EThe moist surfaces of lungs are necessary for the diffusion of gases across alveoli sacs into the blood of capillaries lining these alveoli. However, the surface

tension of water is so high that it will cause water to form droplets rather than a film. When a baby is born, its lungs produce a mixture of lipids and proteins, called surfactant, to lower the surface tension of water and allow the alveoli to remain open for respiration. Surfactant production begins after gestation has reached 36 weeks. Premature babies often experience respiratory distress syndrome, a breathing disorder in which the alveoli in their lungs do not remain open due to insufficient production of surfactant.

bioTERMfacilitated diffusiona form of diffusion that requires the substance to be attached to a specific carrier molecule for its passage across the cell membrane

49


Size mattersMolecules of substances such as water, carbon dioxide and oxygen are small enough to pass through minute pores in the cell membrane by diffusion. When oxygen is used up in cells in the process of cellular respiration, there are fewer particles of oxygen on one side of the membrane than on the other. The reverse happens with carbon dioxide, a waste product of cellular respiration. To correct this imbalance, the particles jostle against each other and the plasma membrane until they diffuse across it. As long as the concentration gradient is maintained, the particles at a higher concentration will continue to diffuse to where they are at a lower concentration.

One way of maintaining concentration gradients is for the circulatory system to remove the diffused substance, such as carbon dioxide, away from the tissues. Another is to convert the diffused substance into something else, thus lowering the concentration. For example, when glucose molecules actively diffuse into liver cells, some are used up and others converted to glycogen, maintaining the concentration gradient.

Polar water molecules can diffuse across the membrane because of their very small size. However, they do not move quickly but diffuse at a rate 10 000 times more slowly than they would if the membrane were not there.

Ethanol and glycerol are larger polar molecules than water but they too can cross the lipid bilayer by diffusion. Smokers and sniffers get their highs rapidly because the molecules they inhale pass quickly through cell membranes into the blood of the capillaries lining the nose and lungs and move out of the blood (crossing the blood–brain barrier) on reaching the brain cells.

Glucose molecules, however, are about twice the size of ethanol molecules. Large polar and non-polar molecules, need protein molecules to carry them through. This process is described as facilitated diffusion.

9 Explaintheprocessofdiffusionbymeansofsimplelabelleddiagrams.

10 Wateriscomposedofhighlypolarmolecules.Explainwhywatermoleculescandiffusefreelyacross

theplasmamembrane.

11 Howdocellsmaintainaconcentrationgradientforoxygensothatitcontinuestomoveintocells?

Solution (mixture of particles) = solvent particles + solute particles High-concentration solution = low concentration of solvent + high concentration of solute Low-concentration solution = high concentration of solvent + low concentration of solute

PRA

CtiC

Al ACtivity 2.1

bioTERMSsolutea substance dissolved in another substance (the solvent) to create a solution

solventa substance in which other substances (solutes) can be dissolved to create a solution; water, owing to its polar nature, is the universal solvent for living things

biolink

osmosis

solute particles

solvent particles

(H2O)

solute particle

solvent particles (H2O)

50 Unit 3

Osmosis – the diffusion of water molecules

When injured wildlife are received by wildlife recovery staff, they check to see if the animal is dehydrated by pinching the skin and looking at the speed that it returns to its usual position. The skin will stay bunched for longer in a dehydrated animal. The lack of water in the cells and the fluid surrounding them means that the tissues are slack and do not keep their firmness.

The equivalent in plants is wilting but it is usually more obvious. If we fail to water pot plants or the weather is extremely hot, it is likely that the plants droop because of a lack of water in their cells. Without water, we eventually die, although

some organisms are capable of doing with less for longer than we can.

Water is the medium in which biochemical processes take place, water transports materials in solution, it helps keep cells in shape and it forms the fluid that bathes tissues. Water is described as the universal solvent.

Making sense of solutionsIf you add more sugar or salt to water, you are adding more solute to the solvent and making the solution more concentrated. A dilute solution has a relatively high concentration of water molecules compared to the

solute particles dissolved in it, whereas a solution of high concentration has a relatively low concentration of solvent molecules and a relatively high concentration of solute particles.

Solutions and cellsPlasma membranes are permeable to water molecules but selectively permeable to solutes. This means that the plasma membrane allows some solutes through but not others. This is usually based on the size and polarity of the solute particles.

If the concentration of water molecules inside cells is lower than the concentration of water molecules outside the cells, water molecules will diffuse from the outside to the inside until a balance or equilibrium is reached: the water molecules move from a relatively high concentration to a relatively low concentration. Water molecules will diffuse through a cell wall and through the cell membrane.

The diffusion of water molecules from one solution to another of different concentration through a differentially or selectively permeable membrane is called osmosis. It is defined as the net or overall movement of water molecules (molecules of solvent) across a differentially permeable membrane from a dilute solution (high concentration of water molecules and low concentration of solute particles) to a

Figure 2.14 Making solutions: (a) a concentrated solution; (b) a dilute solution.

a b

net movement of water molecules

low concentration of solute molecules

high concentration of water molecules

low osmotic pressure

differentially permeable membrane

dilute sucrose solution

concentrated sucrose solution

key water sucrose

high concentration of solute molecules

low concentration of water molecules

high osmotic pressure

51


Figure 2.16 These diagrams show the net movement of water molecules between solutions separated by a differentially permeable membrane. Diffusion of sucrose cannot occur.

Figure 2.15 Summary of the conditions on the two sides of a differentially permeable membrane.

concentrated solution (low concentration of water molecules and high concentration of solute particles), that is, down the concentration gradient for water molecules.

If the surrounding fluid and internal fluid of cells are of equal concentration, the external solution is said to be isotonic (iso – same) to the cell; water molecules jostle either side of the membrane and particles move in both directions equally.

If cells and tissues are surrounded by a solution of lower concentration than themselves, the external solution is said to be hypotonic to the cells (hypo – lower). Water molecules will diffuse through the membrane into the cells. The reverse applies if the cells are surrounded by a solution of higher concentration: the external solution is hypertonic (hyper – higher) to the cells and water molecules will diffuse out.

Cells and movement of water moleculesThe cells of unicellular eukaryotes and multicellular organisms, such as animals, are surrounded only by a plasma membrane, unlike the cells of plants, fungi and bacteria, in which a cell wall surrounds the plasma membrane. As we have seen, the plasma membrane is fluid, which means that, under hypotonic or hypertonic conditions, the cell will swell or lose volume, respectively.

Animal cells can swell much more than plant cells because they lack a cell wall. However, they seldom do so because they have regulatory mechanisms that control the concentration of tissue fluids. These mechanisms ensure that the concentration of tissue fluids does not become so high as to cause the cell to swell to the point of damage. For instance, the cells of tissues are packed very closely together. Thus, if their volume increases as a result of water uptake, they exert a pressure on each other until no further cell expansion, or swelling, is possible.

Another regulatory mechanism can be seen in the single-celled organisms Amoeba and Paramecium, which live in fresh water. They have a constant intake of water molecules because the concentration gradient for water molecules is higher on the outside of the organism than on the inside. Unless there is some way of removing this excess, these organisms will reach bursting point. Fortunately, they are able to remove

a b

2% sucrose

4% sucrose

3% sucrose

3% sucrose

3% sucrose

3% sucrose

3% sucrose

3% sucrose

Start Finish Start Finish

net movement of water in a is to right net movement of water in b is zero

52 Unit 3

the excess fresh water by forming little bubbles of water in their cytoplasm, in organelles called contractile vacuoles (Figure 2.17). When the membrane of the vacuoles is stretched to a certain point, the vacuole contracts and expels the water.

When red blood cells are placed in an isotonic solution, they maintain their shape as there is no net movement of water molecules (Figure 2.18). However, when red blood cells are placed in a hypotonic solution, there is a net movement of water molecules into the cell by osmosis (endosmosis) and the cells swell and burst.

If red blood cells are placed in a hypertonic solution, in which the water molecules are in a relatively low concentration, then there is a net movement of water molecules from the inside to the outside (exosmosis), and the red blood cells shrink and crinkle.

Cell walls and movement of water moleculesPlant cells, as well as fungi and bacteria, have a firm but flexible porous cell wall that surrounds the plasma membrane. Plant cells contain vacuoles, which are full of cell sap that is rich in solutes – a solution of high concentration. Water molecules diffuse by osmosis into the vacuole from adjacent cells and the fluid between cells through the freely permeable cell wall, the semipermeable plasma membrane and the vacuole tonoplast (membrane). The vacuole swells and pushes the plasma membrane against the cell wall, making the cell turgid (Figure 2.19). The pressure exerted is called turgor pressure and it keeps the cells firm, helping the plant to maintain shape and form.

If the external concentration of water molecules is less than in the vacuole, water molecules diffuse out, reducing the volume of the vacuole. This may happen to the

Figure 2.18 Bursting blood cells! Human red blood cells in solutions of varying concentration.

Figure 2.17 Amoebae are able to remove fresh water by organelles called contractile vacuoles.

nucleus

food vacuole

contractile vacuole moves to edge of cell and water is expelled

when cells are placed in a hypotonic solution (e.g. fresh water)

cells shrink and crinkle (crenation)

when cells are placed in a hypertonic solution, (e.g. sea water)

Normal red blood cells in an isotonic solution (no change in shape of cells, no net movement of water)

cells swell

and burst (haemolysis)

53


Figure 2.19 Cells in the leaves of the herbaceous plant, Elodea, lose turgor and become plasmolysed when fresh water is in low supply. (a) In fresh water, the cells swell and become turgid (magnification × 400). (b) In salt water, plasmolysis occurs and gaps develop between the cytoplasm and the cell wall (magnification × 400).

Many plants regulate the

exchange of gases, including

water vapour, between

their internal and external

environment by means

of small openings called

stomata, which are usually

found on the underside of

leaves.

Each stoma consists

of two guard cells, which

surround the stomatal

pore. Each guard cell has

a thick, rather inelastic cell

wall bordering the inside

BioBox 2.4opening and Closing By osmosis


Figure 2.20 The role of osmosis in controlling stomal aperture. (a) Turgid

guard cells open a pore while (b) the flaccid guard cells close the pore.

extent that the plasma membrane is withdrawn from the cell wall, so that the cells become limp or flaccid. This process is called plasmolysis.

Plasmolysis occurs very rarely but can happen if, for example, land is flooded with sea water or the watertable rises, bringing salts with it. Plants living along seashores do not have this problem as they maintain a high concentration of solutes in their cell sap.

a b

a b

K+

guard cell stoma guard cell

water

waterK+

CO2H2O+O2

R e v i e w

54 Unit 3

of the pore but a thinner and elastic cell wall on the outer side. This difference in thickness

is significant. As water is taken in by endosmosis, the guard cells become turgid and bend to

become rather sausage-shaped, opening the pore. If the guard cells lose water, they become

flaccid and the pore is closed.

Guard cells swell in response to changes in the concentration of solutes within the cells.

During periods of light, the concentration of solutes inside the guard cells increases, causing

water to move into the cells by osmosis so they swell, opening the pore. During periods of

darkness, the solute concentration drops so water moves out of the cells by osmosis and the

pore closes.

The guard cells can take up potassium ions (K+) from adjacent cells in the epidermis or

lose them to the epidermal cells. The mechanism of moving potassium ions is thought to be

triggered by receptors in the cell membrane that are sensitive to blue light. Guard cells are

packed with chloroplasts and mitochondria to provide energy to move potassium ions against

their concentration gradient.

What name is given to the cells that control the opening and closing of stomatal pores?

Use a diagram to explain how the importing of potassium ions by guard cells affects

osmosis, thus resulting in the opening of stomatal pores.

Draw a flow chart to outline the steps that would occur when darkness falls and triggers

a plant to close its stomata.


12 Explain,usingannotateddiagrams,theprocessofosmosis.

13 Explainwhatwouldhappen,intermsofthediffusionofwatermolecules,ifananimalcellwasplacedinahypotonicsolution.

14 Describetheconditionsunderwhichplantcellsloseturgor.

Facilitated diffusion – an assisted passage

Transport proteins in the cell membrane assist the movement of simple sugars, amino acids, nucleotides, charged ions and other nutrient molecules across membranes. These transport proteins form channels or act as carrier molecules. The importance of these membrane proteins becomes apparent when individuals, such as those suffering cystic fibrosis, cannot produce them in adequate amounts (see Biobox 2.5).

biolink

Facilitated diffusion

BioBox 2.5missing memBRane pRotein makes the muCus thiCk

55


Cystic fibrosis is the most common inherited disease in Western society. The everyday problems

suffered by patients with cystic fibrosis occur because of a missing membrane protein, the CFTR

protein, in the glands that secrete body fluids (exocrine glands). This large protein, which is

composed of 1480 amino acids, acts as a chloride channel and regulates the function of many

other proteins.

If the CFTR protein is overactive, as seen in individuals infected by cholera toxin, it induces

diarrhoea, which can kill due to excessive water loss. When the CFTR protein is not working or is

absent from the membrane, chloride ions, and therefore water, will not flow from the bloodstream

into the mucus. This causes secretions to become thick and sticky, blocking pancreatic and

reproductive ducts. Mucus secretions become too thick and the cilia

lining the lungs cannot remove it. The airways become blocked and

this makes it difficult to breathe.

Production of CFTR protein requires a series of steps as it

moves through the endoplasmic reticulum (ER) and the Golgi

apparatus. In the ER, chaperone proteins assist in trafficking

and folding the polypeptide and carbohydrates essential for its

function are added. Generally, only one-third of the CFTR protein

being produced moves from the ER to the Golgi apparatus. Only

one-third will eventually reach the cell membrane. The rest is

destroyed in the ER by proteases, protein-digesting enzymes.

The most common mutant form of the CFTR protein, which

is found in 90% of persons with the inherited disease cystic

fibrosis, is delta-F508 CFTR. This mutation causes the CFTR

polypeptide to get stuck in the ER so that it cannot move to

the cell membrane to function as a chloride channel. Scientists

believe that the delta-F508 CFTR polypeptide adheres too strongly to the chaperone

proteins in the ER or that it does not fold quickly enough. As a result, too many copies are

destroyed by the action of proteases and very few make it to the Golgi apparatus and cell

membrane. The mutant protein is able to function as an ion channel so a dysfunctional

protein does not cause cystic fibrosis. Rather, it’s a problem of not enough CFTR protein

getting to the plasma membranes.

What is the cause of cystic fibrosis?

Refer to the process of osmosis to explain how reducing the flow of chloride ions

into exocrine glands can cause body fluids, such as mucus, to become thick and

sticky.

Describe the effects on the CFTR protein, and hence osmosis and body fluids,

when a person is infected with the cholera toxin.

Why doesn’t enough CFTR reach cell membranes in people with the delta-F508

CFTR mutation?

Could this mutant protein operate as a chloride channel if it reached the

membrane?

Figure 2.21 The CFTR protein forms a pore in cell membranes to pump chloride ions out of cells. This is important for keeping mucus secretions hydrated.

pore

cytoplasm of cell chloride ion

plasma membranecarbohydrate

ATP binding site

56 Unit 3

Figure 2.23 A channel protein forms a water-filled pore across the bilayer through which polar substances can diffuse.

bio B Y T EMedical examiners using high-performance liquid chromatography coupled with mass spectrometry have found traces of plant toxins in homicide cases that might otherwise have

gone unnoticed. Digoxin, a potent heart toxin found in foxglove, works by blocking calcium ion channels in heart muscles, which disrupts the heartbeat and kills the victim. Foxglove poisoning is well-known to Western toxicologists. A similar toxin, cerberin, has been found in the kernels of Cerbera odollam, a widespread plant in India. The poison has a bitter taste that is easily obscured by spices. Murderers beware! New tests will show if the victim consumed cerberin.

Carrier proteinsMoving some molecules across membranes involves giving them a lift. The diffusing molecule binds to its carrier protein and is carried to the other side. The carrier protein is thought to be able to do this by changing its shape to accommodate the charged or polar groups of molecules (Figure 2.22), shielding them from the non-polar interior of the membrane. The relationship between the carrier protein and the transported molecule is specific and the way that they attach is thought to be similar to the way the active site of an enzyme and its substrate (the substance it acts on) bind together.

Channel proteinsChannel proteins form pores in the membrane that fill with water (Figure 2.23). The lining of the channel is hydrophilic so water-soluble substances pass through relatively easily. This is an intriguing way of helping particles that dissolve in water, such as ions, move through the membrane with maximum efficiency. The channels are selective: they allow some ions through and not others. These channels are like gateways: they open only when they receive an appropriate signal.

Figure 2.22 A carrier protein has a shape that allows other substances to bind to it.

bioTERMScarrier proteinsproteins within the plasma membrane that assist with the passage of other molecules across the membrane in facilitated and active transport

channel proteinsproteins that form channels within the plasma membrane to allow for the passage of hydrophilic substances across the membrane

carrier protein takes up particles on one side of the

plasma membrane …… changes shape and releases

them on the other side

high concentration

low concentration


channel protein open

channel protein closed

high concentration

low concentration


R e v i e w

57


15 Whatisfacilitateddiffusion?Whatsubstancesenterthecellbythis

process?

16 Howdoesactivetransportdifferfromsimplediffusion?

17 Comparetheprocessofhow(a)glucosemoleculesand(b)ions

enteracellpassivelyandactively.

Figure 2.24 Active transport via a carrier protein in the cell plasma membrane. The carrier protein is coupled to an energy source so it can move the particles against a concentration gradient.

active transportGlucose molecules that are products of digestion have to be moved out into adjoining cells against the concentration gradient. This is an example of active transport – active because the process requires an input of energy. It has been estimated that while a person sleeps, as much as 40% of the total energy budget is used for active transport. Without active transport the kidneys could not reabsorb useful materials, muscles would not contract and impulses would not be able to travel along nerves.

Cells engaged in active transport have huge numbers of organelles called mitochondria, whose function it is to make energy available to the cell. We investigate their role in cellular activity more closely in Chapter 3.

How does active transport take place? The current view is that it involves carrier proteins that are similar to those responsible for facilitated diffusion but that the carrier protein is coupled to a source of energy. These carrier proteins have binding sites that pick up specific molecules. They function in one direction only, like valves, and require energy to change shape and move the solute particle (Figure 2.24).

Importing and exporting across membranes

At times, very large molecules or clumps of macromolecules have to be moved around in the cell, stored within or even moved out of cells across membranes. Specialised animal cells produce a variety of substances, such as hormones, mucus, milk proteins

…and releases them on the other side

low concentration

high concentration


transport direction

When energy is provided, carrier proteins take up particles on one side of the plasma membrane… outside cell

energy (ATP)

inside cell

student Cd

Phagocytosis

58 Unit 3

and digestive enzymes, that have important functions elsewhere in the organism. So too, in plants, particular cells are specialised to produce certain products that have to be relocated. These include growth regulators, toxins to ward off predators and macromolecules for use elsewhere.

In other circumstances, relatively large particles have to be imported. The size of many particles that have to move from place to place makes simple and facilitated diffusion impossible. In these cases the membrane has an important role to play.

Cells make very small containers or sacs within them from the plasma membrane itself. They are called vesicles and their formation and movement is referred to as cytosis. The vesicles transport such things as solids or liquids across the membrane, outwards in the case of exocytosis and inwards in the case of endocytosis.

During exocytosis, a small membrane-bound vesicle moves through the cytoplasm to the plasma membrane, where it

fuses with it and then releases its contents to the exterior of the cell (Figure 2.25a). During endocytosis, the plasma membrane

sinks inwards and engulfs particles and liquid droplets. It encloses the material within it to form an endocytic vesicle, which then stores or transports the material within the cytoplasm (Figure 2.25b).

The endocytic process that encloses solid material such as food is called phagocytosis and the one that encloses droplets of liquid is called pinocytosis.

Cells of the small intestine engulf fat droplets by pinocytosis. Some white blood cells are referred to as phagocytes (phago – feeding, cyte – cell) because they detect the presence of invading foreign bodies, such as bacteria, flow around them and engulf them.

Phagocytosis, like pinocytosis, can be selective. Cells can discriminate between different kinds of particles. Phagocytic white blood cells will only attack certain types of invading bacteria and an Amoeba will ingest particles of food value but rarely particles that are not (Figure 2.26).

Figure 2.26 A scanning electron micrograph of an amoeba surrounding its prey (Tetrahymena) for ingestion.

bio B Y T ECells in the peel of citrus fruits secrete oils by exocytosis. These oils deter insects from piercing the

peel to eat the fruit. About 95% of the oil in orange peel is a carbon-based compound called limonene. The properties of limonene are used widely in industry. It has been used to fragrance products, as a solvent, as an insect repellent and now it is the building block for making the plastic, polylimonene carbonate.

A team of scientists at Cornell University have produced this plastic using limonene oxide, carbon dioxide and a catalyst to speed up the reaction. The team hopes that sources of carbon dioxide collected to make this plastic will reduce the amounts pumped into the atmosphere.

Figure 2.25 Exporting and importing: (a) exocytosis and (b) endocytosis.

a b

cytoplasm

plasma membrane

cytoplasm

R e v i e w

bioTERMSnucleusa membrane-bound compartment in eukaryotic cells that contains the chromosomal DNA

chromosomestructure composed of DNA and protein that contains along its length linear arrays of genes carrying genetic information; prokaryotes have one circular chromosome whereas eukaryotes have a number of linear chromosomes

chromatina complex of proteins and DNA in eukaryotic chromosomes

nucleoligranular structure within the nucleus where ribosomal RNA is transcribed and ribosome subunits are assembled

59


Figure 2.27 (a) The features of the nucleus. (b)Transmission electron micrograph of the nucleus of a pancreas acinar cell.

Organelles in actionThe plasma membrane holds the contents of the cell together. Within the cell various organelles work in collaboration to move substances from one part of the cell to another and prepare other substances for export from the cell (see Figure 2.2).

NucleusThe nucleus is one of the most prominent structures in the eukaryotic cell. It is the information centre of the cell and controls all activities of the cell because it controls the production of proteins. The code that directs all activities of the cell is found in the nucleic acid, DNA, on the chromosomes. The number of chromosomes within the nucleus is characteristic of the species. Chromosomes become highly visible in dividing cells but at other times are dispersed as DNA–protein fibres called chromatin.

The nucleus is contained within a double membrane called the nuclear envelope. The outer membrane is continuous with the endoplasmic reticulum, another membranous organelle that helps distribute materials throughout cells. The nuclear envelope contains numerous openings, called nuclear pores. These pores are channels by which water-soluble molecules can move between the nucleus and the cytoplasm.

Structures known as nucleoli are present in the nucleus. These are responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomes from rRNA and proteins. These ribosomes and the messenger RNA molecules that carry the coded instructions for assembling proteins move through the nuclear pores into the cytoplasm, where synthesis of proteins occurs.

18 Listfourtypesofsubstancesthataresecretedfromcells.

19 Describe,bymeansofsimplelabelleddiagrams,theprocessofcytosis.Distinguishbetweenendocytosis

andexocytosis.

20 Whyarecertainwhitebloodcellsknownasphagocytes?

a

chromatinnucleolus

ribosomesnuclear pore

outer membraneinner membrane {nuclear envelope

b

rough endoplasmic reticulum

R e v i e w

60 Unit 3

Figure 2.28 The structure of ricin. This protein has two chains linked together by a disulfide bond. The A chain is the toxic protein. The B chain is the glycoprotein that allows ricin to enter a cell.

Figure 2.29 Ribosomes are composed of strands of RNA and proteins. The two subunits of the ribosome lock around the messenger RNA (mRNA) molecule. The ribosome moves along the mRNA reading its nucleotides, three at a time, so that the correct amino acids are linked together to form a polypeptide chain.

bio B Y T EThe scene is London, 1978, during the Cold War. The Bulgarian dissident Georgi

Markov, a Communist defector, works for the BBC World Service. He leaves his office on 11 September and strolls across Waterloo Bridge. While waiting at the bus stop he feels a sharp pain in his thigh and notices a man picking up an umbrella. Four days later, after experiencing a severe fever, he lies dead on an autopsy table. The post-mortem reveals that his death is due to a tiny pellet lodged in his thigh that contains traces of the poison ricin.

protein pathwaysProteins are the workhorses of the cells. We have seen how important they are in structures and in moving molecules across the plasma membrane. Eukaryotic cells have elaborate mechanisms to assemble, package and transport proteins within the cell. Proteins can be tracked within the cell by labelling them with fluorescent dyes and radioactive atoms.

To understand the role played by various cell compartments in the protein pathway we will follow the journey of a particularly toxic protein molecule called ricin.

ribosomes produce proteinsRicin molecules, like all proteins, are synthesised on organelles called ribosomes. Ribosomes are extremely small organelles that exist free-floating in the cytosol of cells or attached to membranes of the endoplasmic reticulum, which is then described as rough endoplasmic reticulum. They are present in enormous numbers in cells, reflecting the work that they do in producing a constant supply of different kinds of proteins.

Ribosomes consist of two subunits that are composed of the nucleic acid RNA and various proteins (Figure 2.29). In eukaryote cells, ribosomal RNA (rRNA) molecules

are synthesised in the nucleolus. They pass through the nuclear pores into the cytosol. There they begin the process of synthesising polypeptide chains from amino acids according to the instructions given by mRNA copied from DNA within the nucleus.

Cells with high rates of protein synthesis have prominent nucleoli and many ribosomes. For example, the liver is the industrial centre of the body so it is not surprising that each liver cell has a few million ribosomes.

ricin blocks protein synthesis

Ricin is an enzymatic protein that is highly toxic to animals and insects. Found as a natural toxin in the endosperm cells that line the seed coats of the castor oil plant (Ricinus communis) (see Figure 1.31d), it is one of the most poisonous naturally occurring substances known. Less than 1 mg

21 Whatistheroleofthenucleusineukaryoticcells?

22 Whattypesofmoleculesaretransportedfromthenucleustothecytoplasm?

23 HowisDNApackagedwithinthenucleus?

A chainrandom

coilb-sheet

B chain α-helix

protein

small subunit large subunit

RNA

61


bio B Y T EThe brain works by sending messages in a controlled fashion. But sometimes extra messages

are sent which do not go down the correct routes. This can upset the normal function of the brain and can cause an epileptic seizure. Phenobarbitone is a barbiturate drug that is used to control these seizures and as a sedative to relieve anxiety. When large amounts of the drug phenobarbitone enter the circulation, unusually elevated amounts of detoxification enzymes are synthesised by hepatocytes (liver cells). The smooth ER in these cells doubles its surface area within a few days to accommodate these detoxifying proteins. Once the drug has been removed from the system, the excess smooth ER is digested by the action of lysosomes.

of ricin can kill an adult human. Ricin is just one of the many cytotoxic proteins produced by plants. It acts by denaturing the RNA in ribosomes, thus stopping protein synthesis. A single ricin molecule that enters the cytosol can inactivate over 1500 ribosomes per minute and kill the cell. This is why it is such a potent poison.

the endoplasmic reticulumThe endoplasmic reticulum (ER) is a labyrinth of interconnecting tubules and flattened sacs extending through the cytosol. Ribosomes can move freely in the cytosol but often they are associated with the surface of the ER.

When ribosomes are bound to the ER, they give it a ‘rough appearance’ so it is described as rough endoplasmic reticulum. Ribosomes bound to ER are involved in the production of membrane proteins and proteins to be secreted from the cell or stored in vesicles in the cytosol. Proteins travelling through the rough ER undergo modifications as enzymes act to ensure that they fold into their correct shape and prepare the protein for the later addition of carbohydrate groups. This is what happens to our ricin protein.

ER without ribosomes is called smooth endoplasmic reticulum. Its cavities are tubular rather than flattened sacs. Smooth ER is the site of phospholipid and cholesterol synthesis, which are required for particular purposes, such as making membranes.

Some carbohydrates are also produced on smooth ER and lipid-soluble drugs and poisons are detoxified. The smooth ER also stores calcium ions, which are necessary for muscle contraction and interactions between some membrane proteins.

the Golgi apparatusOnce protein and lipid molecules are produced they have to be packaged and moved on. This takes place in organelles called either Golgi bodies, the Golgi complex or the Golgi apparatus.

Figure 2.30 Protein pathways. Details of the production, transport and secretion of proteins in cells.

2. The protein chains (blue) are manufactured on ribosomes.

3. Some protein chains have signal peptides so they enter the lumen of the endoplasmic reticulum and are chemically modified.

4. Lipids are manufactured in the membrane of smooth endoplasmic reticulum.

5. Vesicles which bud from the endoplasmic reticulum membrane transport unfinished proteins and lipids to a Golgi apparatus.

6. Proteins and lipids take on final form in the space inside the Golgi apparatus.

7. Vesicles budding from the Golgi membrane transport finished products to the plasma membrane where they are released by exocytosis.

1. Protein building instructions move from the nucleus into the cytoplasm.

rough endoplasmic reticulum

assorted vesicles

smooth endoplasmic

reticulum

Golgi apparatus

62 Unit 3

Figure 2.31 Lysosomes contain enzymes that digest macromolecules, the products of which can be recycled

by the cell. The enzymes are processed by the ER and the Golgi apparatus

before becoming active.

These organelles are composed of an ordered series of flattened membrane-bound compartments that resemble tubes.

During their passage through the Golgi apparatus, protein and lipid molecules that were produced in the ER combine with carbohydrate groups, forming glycoproteins and glycolipids.

Membranes of the Golgi apparatus package the molecules in vesicles, which transport them to other parts of the cell or to the plasma membrane, where they are released from the cell by exocytosis. All eukaryote cells contain a Golgi apparatus but they are more numerous in cells that have a secretory function, such as pancreatic cells, which produce digestive enzymes and the hormone insulin.

the risk of ricin

When the ricin polypeptide enters the lumen of the ER it is folded by the formation of disulfide bonds. The molecule is then transported in a vesicle to the Golgi apparatus. Here, enzymes catalyse the addition of carbohydrate molecules to the ricin polypeptide to form a glycoprotein. Ricin then moves out of the Golgi apparatus and is stored in a protein body, where the final toxic form is achieved when enzymes cut part of the polypeptide chain to form two chains held together by the disulfide bond. Because it is toxic, ricin must remain inactive so that it does not poison its own ribosomes if the molecules accidentally ‘leak’ during transport.

powerful recyclersLysosomes are only present in eukaryotic cells. They are vesicles that contain various powerful digestive enzymes that are capable of breaking down all the major classes of biological macromolecules. It is no wonder that the rest of the cell is shielded from the enzymes found in lysosomes.

Lysosomes fuse with the membrane of food vacuoles and old organelles and empty their contents inside them. The enzymes break down the complex molecules into simpler, soluble and diffusible substances, such as amino acids and glucose. These pass through the membrane of the vacuole into the surrounding cytoplasm, where they are used in the synthesis of new biomolecules. Any material that is not digested is released by exocytosis into the extracellular fluid.

bio B Y T EWhen muscles are not used for prolonged

periods they tend to waste away. The body destroys the proteins that make up the muscle. This is a concern for bed-ridden patients and the elderly. The culprits are protein recycling units called proteosomes. These protein machines have been conserved throughout evolution, being found in the simplest bacteria through to the most complex organism – humans. Without them, organisms die. They are integral for ensuring that faulty or unused proteins are broken down for recycling. Proteins are tagged for destruction by the addition of a ubiquitin molecule, which is recognised by the proteosome complex. By stopping this tagging process, unused muscles could be saved.

PRA

CtiC

Al ACtivity 2.2

bioTERMlysosomea membrane-bound vesicle in eukaryotic cells containing enzymes which is involved in the breakdown and recycling of many types of molecules

Golgi apparatus

rough endoplasmic

reticulum

cell engulfs food particle

plasma membrane

lysosome engulfs damaged organelle

digestion

lysosome

vesicle contains inactive

enzymes

lysosome

digestion

R e v i e w

63


Lysosomes help to recycle cellular materials and offer protection. Not only do they digest worn-out organelles and macromolecules but in white blood cells they digest potentially dangerous bacteria.

Lysosomes play an important part in embryonic development. Some tissues, such as the webbing between fingers or the tail of tadpoles, are present only at certain stages. They are broken down by the activity of lysosomes and the products are used to build up new tissues.

Tay-Sachs disease is an inherited disorder in which fatty substance builds up in the nerve

cells of the brain. This build-up occurs because a particular enzyme that is responsible for

controlling the breakdown of fatty substance is missing from lysosomes. Accumulation of fatty

substance begins in the developing foetus and affects the nervous system to the extent that

death occurs by about 5 years of age.

A variety of enzymes are stored in lysosomes for the controlled breakdown of large

molecules. If one of these enzymes is unable to function, it will have adverse effects on the cell.

Molecular material will accumulate in a form that cannot be recycled or removed. Diseases

caused by this phenomenon are known as storage diseases.

Pompe’s disease is a glycogen storage disease caused by a defect in a single protein

(enzyme). It affects the liver, muscle and the heart. The enzyme works inside lysosomes to

break down the glucose storage molecule, glycogen. A deficiency of the specific enzyme causes

glycogen to accumulate in lysosomes, eventually rupturing them.

What is the problem common to all storage diseases?

Why do our cells store digestive enzymes within lysosomes rather than disperse them

throughout the cell?

As a research scientist trying to discover a treatment for Tay-Sachs disease, what would

you be working towards?

BioBox 2.6lysosomes laCking enzymes Cause disease

24 Whatisthestructureandfunctionofribosomes?Inwhatwaydotheydifferfromotherorganelles?

25 WhatdotheendoplasmicreticulumandtheGolgiapparatushaveincommonandhowdotheydiffer

functionally?

26 Whathappenstoproteinsproducedbyribosomesthatareattachedtotheendoplasmicreticulum?

Whatisthefinaldestinationforsuchproteins?

27 Howdoesacellprotectitselfduringtheproductionofatoxicmoleculesuchasricin?

28 Relatethestructureoflysosomestotheirfunction.

biolink

Cell organelles

64 Unit 3

Figure 2.33 Microtubules are involved in cell division. The arrangement of microtubules differs in (a) a cell in interphase and (b) when the chromosomes are about to separate on the spindle, which is made up of ropes of microtubules.

bio B Y T EKing Henry VII suffered from gout, an extremely painful form of arthritis caused

by the build-up of uric acid crystals in the joints. His doctors administered colchicine, a drug found in various plants, to reduce his painful symptoms. Later studies revealed that colchicine removes microtubules from the cytoskeleton of cells and inhibits cell division.

Figure 2.32 Actin microfilaments of the cytoskeleton control muscle contraction, maintain cell shape and carry out cellular movements. Fluorescence micrograph showing actin microfilaments in bundles in human fibroblasts, which form connective tissue. The green actin fibres provide a supporting network in the cell (magnification × 600).

Cell movements and connections

the cell cytoskeleton – a protein networkOnly cells of eukaryotes have a cytoskeleton. It consists of a network of protein fibres that give shape to the cell, hold and move organelles and coordinate cell movement. Three types of protein fibres criss-cross the cytosol of the cell:• microtubules: these are polymers of tubulin and are involved in the movement of

chromosomes, organelles, cilia and flagella• intermediate filaments: these provide tensile strength for the attachment of cells

to each other and their external environment to help maintain tissue shape and to support long nerve cell extensions

• microfilaments: these are composed of contractile fibres of actin that associate with myosin to control muscle contraction, maintain cell shape and carry out cellular movements, such as gliding, contraction and cell division.

The cytoskeleton is an internal skeleton of microtubules that extends throughout eukaryotic cells, giving them their shape, their ability to move and to arrange organelles. Extensions of the microtubules of the cytoskeleton, the centrioles, are involved in moving chromosomes apart in cell division.

The study of inherited disorders, such as Duchenne muscular dystrophy (MD), and the treatment of cells with chemicals has increased our knowledge of the structure and functioning of the cytoskeleton.

Dystrophin is a protein that forms a scaffold in cells that are essential for muscles to function. Patients with MD cannot replenish the protein dystrophin in their muscle cells. As a result, their muscle cells fall apart, causing progressive muscle weakness and wasting, and eventual death from respiratory failure.

The drug, taxol, from the Pacific yew tree, prevents cells from dividing by affecting the breakdown of microtubules. Because of this property it is now used as a localised cancer therapy to stop tumour cell growth.

a b

BioBox 2.7suppoRting tissues and oRgans

65


Figure 2.34 (a) Elastic and collagen fibres of the intercellular elastic cartilage of the ear are surrounded by spaces filled with extracellular matrix. (b) The connective tissue below epithelial cells contains a variety of cells and matrix components.

the extracellular matrix – glue that connectsSo far we have treated cells as if they end at the plasma membrane. However, most cells have some structures that are external to the plasma membrane but are still an integral part of the cell, both in structure and function. The cell wall in plants, fungi and bacteria is one such structure. Many kinds of cells, including most of our own body cells, are connected in some way to neighbouring cells. This is how body tissues form.

Cells in body tissues are surrounded by extracellular matrix (ECM). This consists of various macromolecules that have been produced and secreted by fibroblast cells found in the matrix. The ECM in animal cells usually consists of long flexible fibres that are embedded in a matrix made up of glycoproteins and glycolipids. Bone and cartilage are types of connective tissues that are largely composed of ECM materials. ECM plays an important role in determining the shape and mechanical properties of tissues and organs.

Our understanding of diseases that affect connective tissue has

highlighted the importance and function of the extracellular matrix

(ECM). Scurvy was the scourge of the seas. More sailors lost their lives

to scurvy between the 15th and 17th centuries than they did to war,

storms, shipwreck or other diseases. They were at sea for months

and many experienced rotting gums, aches and pains, blue and red

spots or stains on their skin and eventual death. In the 1740s James

Lind carried out a clinical trial on HMS Salisbury and found that

implementing citrus juice into the diet of sailors was an effective

treatment. We now know that diets lacking vitamin C lead to poor

wound healing, deficient growth and capillary weakness as a result

of the faulty processing of collagen fibres.

Collagen fibres are an important component of cartilage. A diet

lacking vitamin C results in changes to collagen proteins so that

they no longer form cross-links when secreted into the ECM. This

affects the linings of tissues such as the intestines and capillaries,

making them prone to rupture, and causes joint pain and an overall

breakdown of connective tissue.

Spectators were shocked when Olympian Flo Hyman died

suddenly during a volleyball game at age 31. She died of a tear in

the aorta artery wall, unaware that she had Marfan’s syndrome.

Marfan’s syndrome results from mutations in the extracellular protein, fibrillin, which

are inherited or occur spontaneously. Fibrillin is critical for strengthening connective

tissue. If it is defective, elastic fibres in the ECM arrange incorrectly and the weakened

connective tissue cannot provide adequate support or protection, leading to the many


a

b

collagen fibre

capillary

elastic fibre

mast cell

epithelium

macrophage

fibroblast

hyaluronan, proteoglycans and

glycoproteins

conn

ectiv

e tis

sue

50µm

R e v i e w

66 Unit 3


indicators of Marfan’s syndrome. Fibrillin and fibronectin proteins act like a strengthening

mesh to hold cells in place.

Proteoglycans are molecules found in the ECM that support cells and give strength to

tissues. By forming large groups, these molecules create a matrix that contributes to the

mechanical properties of tissues. Tissues that make up artery walls resist deformation caused

by blood pressure by increasing the proteoglycan content of the ECM to help prevent vessel

rupture. This can ultimately be dangerous, as an increase in the proteoglycan content of the

ECM traps more low-density lipoproteins in blood vessels, which accumulate and contribute

to the development of plaques. Plaque formations increase the risk of stroke and heart

attack.

Describe how the many symptoms of scurvy can be attributed to a lack of vitamin C in

the diet.

What is the role of fibrillin in the ECM?

What components of the ECM give strength to tissues?

How can elevated levels of proteoglycan molecules in artery walls contribute

to the risk of stroke and heart attack?

29 Listthreefunctionsofthecytoskeletonineukaryoticcells.

30 Howhavewegainedknowledgeabouttherolesplayedbythecytoskeleton?

31 DescribetherelationshipbetweenthestructureandfunctionoftheECM.

Figure 2.35 Coloured x-ray of the hands of a patient with Marfan’s Syndrome, showing elongation of the fingers.

67


Visual summary<

againstaconcentrationgradient

activetransport

LargeMolecules–SolidsandLiquids

endocytosis

vesicles

lysosomes

proteinchannels

freeincytosol

ribosomes

boundtoendoplasmicreticulum

membrane-boundorganelles

microfilaments

intermediatefilaments Cells

microtubules

FluidMosaicModel

glycoproteins

Structure

plasma Membranes

mitochondria

phospholipids SelectivelyPermeabletoSubstances

carrierproteins

carrierproteinmolecules

exocytosis

proteinsforexport

storageandsecretoryvesicles

packagedbyGolgiapparatus

proteins

facilitateddiffusion

osmosis passivetransport

diffusion alongaconcentrationgradient

68 Unit 3

Key termsactive transport endocytosis intermediate filament organelle ribosome

apoptosis endoplasmic reticulum isotonic osmosis selectively permeable

carrier proteins eukaryotic lysosome phagocytosis self

channel proteins exocytosis microfilament phospholipid solute

chromatin extracellular matrix microtubule pinocytosis solvent

chromosome facilitated diffusion non-self plasma membrane stomata

concentration gradient fluid mosaic model nuclear envelope plasmolysis turgid

cytoskeleton Golgi apparatus nucleolus prokaryotic turgor pressure

differentially permeable hypertonic nucleus protein pathway vacuole

diffusion hypotonic

apply understanding Radioactively labelled amino acids are supplied to

a pancreatic cell, which produces a protein called insulin that is secreted from the cell to control the blood sugar level. Which organelles would show the passage of this radioactivity and in what sequence, as the protein is produced and exported from the cell?

Explain why the alcohol in beer enters the body cells more rapidly than the sugar in a chocolate bar.

‘A particle that has been taken into a cell by phagocytosis is not truly inside the cell.’ Explain this statement.

Explain why red blood cells are suspended in saline (salt) solution but not in pure water.

If salad greens such as lettuce and celery are left for a period of time they become limp. To restore their

crispness they can be soaked in cold water. Explain the reason for this.

Plant cells produce many substances that could be toxic to the cell. Describe how a cell could protect itself from its own toxic products. Why would a plant produce a toxic product such as ricin?

A student made the comment that ‘The formation of vesicles by endocytosis should reduce the size of the plasma membrane’. Apply your knowledge of both endocytosis and exocytosis to critically examine this comment.

The internal structure of a eukaryotic cell is dominated by membrane-bound organelles. Explain the importance of these membrane- bound compartments for the functioning cell.

Investigate and inquire Using four different-sized cubes, construct a table to

show how the surface-area-to-volume ratio changes. Explain how increasing the size of a cell affects the cell’s ability to gain and lose substances by diffusion.

The amoeba, a single-celled organism, lives in fresh water. Identify a problem for this organism living in an aqueous environment and investigate how this organism overcomes this problem.

Investigate the advantages of the division of the cytoplasm of a eukaryotic cell into membrane-bound compartments. Explain why such organisation is needed in eukaryotic cells but not in prokaryotic cells.

Investigate the role of nanotechnology in medical research.

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