1 Cells

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MAGNIFICATION AND RESOLUTION

Because cells are too small to be seen with the naked eye, the light microscope was developed to produce enlarged and

more detailed images of cells. The magnification of an image is how much bigger it appears under the microscope than

it is in real life, and is worked out using the following formula:

magnification = image size ÷ actual size

However, magnification on its own does not increase the level of

detail seen, it just increases the size. The term resolution refers

to the ability to see two distinct points separately. For example, if

the resolution of a light microscope is 200nm (0.2μm), this means

it can see any two different points as separate objects if they are

200nm apart or more; but if they are any closer than this amount,

they appear as one object.

THE LIGHT MICROSCOPE

Light microscopes use a number of lenses to produce an image that can be viewed directly at the eyepiece. Light passes

from a bulb under the stage, through a condenser lens and then through the specimen. This beam of light is passed

through an objective lens and then the eyepiece lens. The light microscope usually has a number of objective lenses

which can be rotated into position, these are x4, x10, x40 and x100 lenses. The eyepiece lens then magnifies the image

again by x10. So the final magnifications the microscope is capable of producing are x40, x100, x400 and x1000.

overall magnification = objective lens magnification x eyepiece lens magnification

You can view some specimens directly using the light microscope. Others have to be prepared to get around the issues

involving the fact that biological material may not be coloured and so detail cannot be seen; also that some materials

distort when you cut them into small sections:

1 staining – coloured stains are chemicals that bind to chemicals on or in the specimen, this allows the specimen to be

seen

2 sectioning – specimens are embedded in wax – thin sections are then cut out without distorting the specimen – this

is especially useful for making sections of soft tissue, such as brain

THE ELECTRON MICROSCOPE

Light microscopes have low resolution, of about 200nm (0.2μm), so structures closer together than this appear as one

object. A higher resolution can be achieved with an electron microscope. Electron microscopes generate a beam of

electrons, which have a wavelength of 0.004nm. They can distinguish objects 0.2nm apart. There are two types:

A – Transmission Electron Microscope (TEM)

The electron beam passes through a very thin prepared sample, and the electrons pass through denser parts less easily,

giving some contrast in the final 2D image produced. Maximum possible magnification of x500,000

B – Scanning Electron Microscope (SEM)

The electrom beam is directed onto a sample. The electrons don’t pass through the specimen, they bounce off,

producing a final 3D image view of the surface of the sample. Maximum possible magnification of x100,000

unit symbol metres

metre m 1 decimetre dm 0.1 centimetre cm 0.01 millimetre mm 0.001

micrometre μm 0.000 001 nanometre nm 0.000 000 001 picometre pm 0.000 000 000 01

An introduction to the microscope and magnification


Advantages Limitations

The resolution is 0.1nm (2000x more than the light microscope)

Electron beams are deflected by air molecules, so the sample has to be placed in a vacuum

Can produce more detailed images of the structures inside cells

Electron microscopes are extremely expensive

The SEM produces a final 3D image not possible with the light microscope

Preparing samples and using the electron microscope both require a high degree of skill and training

Electron micrographs are

sometimes shown in

colour. The final image

produced from an

electron microscope is

always in greyscale; the

colours are added

afterwards using

specialised computer

software. Such images will

be labelled as false-colour

electron micrographs.


When you look at animal or plant cells under the electron microscope, you can see a lot more detail. You are able to see

the inside structures – organelles – of the cells, which together make a cell’s ultrastructure. Most organelles are

common to both animal and plant cells. They have the same function in teach type of cell. Each organelle has its own

specific role within the cell, all working together and each contributing towards the survival of the cell. This process is

called division of labour.

CYTOSKELETON

Cells contain a network of fibres made of protein. These fibres keep the cell’s shape stable by providing an internal

framework called the cytoskeleton:

Some of the fibres, called actin filaments are able to move against each other – these cause the movement seen in

some white blood cells, and they move some organelles around inside cells

There are other fibres, called microtubules. These are cylinders about 25nm in diameter made of a protein called

tubulin, and may be used to move a microorganism through a liquid or to waft a liquid past a cell. Other proteins

present on the microtubules move organelles and other cell contents along the fibres – these proteins are called

microtubule motors

UNDULIPODIA & CILIA

Structurally, flagella of eukaryotes (correctly named undulipodia) and cilia

are the same. Each one is made up of a cylinder than contains nine

microtubules arranged in a circle and another two microtubules in a central

bundle. Undulipodia are longer than cilia.

The undulipodium that forms the tail of a sperm cell can move the entire

cell. Undulipodia and cilia can move because the microtubules can use

energy produced by ATP (adenosine triphosphate).

Some bacteria have flagella. These look like the same as eukaryotic

undulipodia, but their internal structure is different. These are true motors;

they are made of a spiral of protein, called flagellin, attached by a hook to a

protein disc at the base. Using energy from ATP, the disc rotates, spinning

the flagellum

Cell ultrastructure and the importance of the cytoskeleton of cells


Many of the organelles found within cells are membrane-bound, this means that they have their own surrounding

membranes to separate them from the rest of the contents of the cell. They have the same structure as the main cell

membrane. The organelles form separate compartments within the cell, a process called compartmentalisation.

Structure Function

The nucleus is the largest organelle in the cell. When stained, it shows darkened patches known as chromatin. It is surrounded by a nuclear envelope. This is a structure made of two membranes with fluid between them. A lot of holes, called nuclear pores, go right through the envelope. These holes are large enough for relatively large molecules to pass through. There is a dense, spherical structure, called the nucleolus, inside the nucleus

The nucleus stores the majority of the cell’s genetic material. The chromatin consists of DNA and proteins. It contains the instructions for making proteins. Some of these proteins regulate the cell’s activities. When a cell divides, chromatin condenses into visible chromosomes. The nucleolus makes RNA and ribosomes. These pass into the cytoplasm and proteins are assembled at them

Endoplasmic reticulum (ER) consists of a series of flattened membrane-bound sacs called cisternae. They are continuous with the outer nuclear membrane. Rough ER is studded with ribosomes, smooth ER does not have ribosomes

Rough ER transports proteins that were made on the attached ribosomes. Some of these proteins may be secreted from the cell. Some will be placed on the cell surface membrane. Smooth ER is involved in making the lipids that the cell needs

The Golgi apparatus is a stack of membrane-bound sacs, which looks very much like a pitta bread

The Golgi apparatus is responsible for receiving proteins and modifying them. It receives proteins from the ER and may add sugar molecules to them. It then packages the modified proteins into vesicles that can be transported. Some modified proteins go to the cell surface so they can be secreted

A single mitochondrion is spherical or sausage-shaped. It has two membranes separated by a fluid-filled space. The inner membrane is highly-folded to form cristae. The central part of the mitochondrion is the matrix

Mitochondria are the site where ATP is produced during respiration. ATP is sometimes called the universal carrier energy as it drives most of the cellular processes

Chloroplasts are only found in plant cells, and have two membranes separated by a fluid-filled space. The inner membrane is continuous, with an elaborate network of flattened membrane sacs called thylakoids. A stack of thylakoids is a granum (plural: grana). Chlorophyll molecules are present on the thylakoids membranes and in the intergranal membranes

These are the site of photosynthesis in plant cells. Light energy is used to drive the reactions, in which carbohydrate molecules are made from carbon dioxide and water

A lysosome is a spherical sac surrounded by a single membrane

These contain powerful digestive enzymes which are there to break down materials. For example, white blood cell lysosomes help to break down invading microorganisms; and the specialised lysosome in the head of a sperm cell helps penetrate the female egg cell

The structure and function of the various organelles within animal and plant cells

◄The nucleus and endoplasmic

reticulum

►Golgi apparatus


There are some organelles which are non membrane-bound…

Structure Function

A ribosome is a tiny organelle that consists of two subunits. They can be found in the cytoplasm or attached to the ER making rough ER

Ribosomes are the site of protein synthesis in the cell (where new proteins are made). They act as an assembly line where coded information (mRNA) from the nucleus is used to assemble proteins from amino acids

Centrioles are small tubes of protein fibres (microtubules) which are present only in animal cells and cells of some protoctists. They are found in a pair next to the nucleus

These are used in cell division, they form fibres known as spindle which move the chromosomes during nuclear division

◄Mitochondrion

► Chloroplast


Any cell which is eukaryotic (literally meaning “having a true nucleus”) has a complicated internal structure containing

many organelles, a lot of which will be membrane-bound and performing their own specific roles. The breakdown of cell

components into individual tasks performed by separate organelles is referred to as division of labour.

Cells which are prokaryotic (bacteria) are much smaller than eukaryotic cells. Features of prokaryotes include:

they have only one membrane, the cell surface (plasma) membrane, and do not contain any membrane-bound

organelles such as chloroplasts of mitochondria

they are surrounded by a cell wall, although it is made from a different substance to eukaryotic cell walls

many prokaryotes are contained within a capsule which provides protection

they contain ribosomes, but these are far smaller than eukaryotic ribosomes

ATP production happens in specially infolded regions of the plasma membrane called mesosomes

their DNA is found loose within the cytoplasm and is in the form of a single loop – this loop of DNA is often

referred to as a circular chromosome or bacterial chromosome – many prokaryotic cells also contain many smaller

loops of DNA called plasmids

there is no membrane surrounding the DNA (unlike the nuclear envelope of eukaryotic cells), but the general area

containing the DNA is called the nucleoid

many prokaryotes have flagella (these are functionally the same as eukaryotic undulipodia, but are internally

different)

There are many bacteria which are well known

because of the diseases which they cause.

Some strains of bacteria are antibiotic-

resistant, such as MRSA. These resistant strains

cause problems because the resistance is

coded on plasmid DNA. Bacteria can share

plasmids among one another, so resistance is

easily passes on between prokaryotes.

However, there are some useful bacteria, for

example, those used in food production, and

skin being covered with a type of bacteria

which prevents harmful pathogens getting into

the body.

Structural and functional differences between the cells of prokaryotes and eukaryotes


All cells have a surrounding membrane, and in eukaryotic cells many of the organelles inside the cell have their own

membranes to separate them from each other. Other than separating cell components from each other, cell

membranes have a number of other purposes:

they separate the cell contents from each other and the cell’s outside environment

they are involved in cell recognition and signalling

they control the transport of certain materials going into or coming out of the cell

The basic structure of all cell surface membranes is the same. They consist of a number of arranged phospholipids.

A phospholipid consists of a phosphate head which is very hydrophilic (water-loving),

attached to two fatty acid tails which are hydrophobic (water-hating). When the

phospholipids are mixed with water, they arrange themselves in a layer at the surface of

the water with the hydrophobic tails sticking out, as shown by below.

If phospholipids become completely surrounded by water, a phospholipid bilayer can form. Phosphate heads on each

side of the bilayer stick into the water, while the hydrophobic fatty acid tails point towards each other in the centre.

This means the hydrophobic tails are held away from the water molecules. In this state, the phospholipid molecules can

move freely, just as fluid molecules do. This phospholipid bilayer is the basic structure of all biological membranes.

The bilayer creates a barrier to many molecules and separates the cell

contents from the outside world. This thin layer of oil is ideal as a boundary

in living systems, where most metabolic reactions take place in a water

environment.

A simple phospholipid bilayer would be incapable of performing all of the

functions of biological membranes. It would also be too fragile to function as

a barrier within or around cells. Other components are needed to make it a

fully-functional biological membrane.

All membranes are permeable to water because water molecules can diffuse through the lipid bilayer. Some

membranes are up to 1000 times more permeable to water because they contain aquaporins (protein channels that

allow water molecules through them). Cell membranes that are permeable only to water and some solutes are

described as partially permeable membranes.

The structure and role of cell membranes

hydrophilic

head

hydrophobic

tail

hydrophobic tail

hydrophilic head water

air


Cell membranes are not just simple phospholipid bilayers. They contain many other key features which make it a fully-

functional biological membrane. The fluid mosaic model shows the components found in a membrane. It is now widely

excepted as the model which explains how membranes form and function. Its main features are:

a phospholipid bilayer giving its basic structure

various protein molecules floating around in the bilayer, some completely free, others bound to other components

some proteins (extrinsic) partially embedded in the bilayer on the inside or the outside face, other proteins

(intrinsic) completely spanning the bilayer

Some of the phospholipid molecules which make the bilayer, and some of the proteins that are part of the membrane

have carbohydrate chains attached to them. When a phospholipid has a carbohydrate part attached to it, it is called a

glycolipid. When a protein has a carbohydrate part attached to it, it is called a glycoprotein.

The cholesterol gives the membranes of many eukaryotic cells some mechanical stability. This steroid fits nicely

between the fatty acid tails and makes the barrier more complete, so that water molecules and other substances

cannot pass through the membrane so easily. Channel proteins allow the movement of some substances across the

membrane. Molecules of sugars, such as glucose, are too large and too hydrophilic to pass directly through the

membrane and so they use these channel proteins instead. Carrier proteins actively move substances around the

membrane.

Other features found on membranes might include receptor sites. These can allow hormones to bind with the cell so

that a cell response can be carried out. These are also important in allowing drugs to bind, and so affect metabolism.

Enzymes and coenzymes are also present, which are used in some stages of respiration (in the membranes of the

mitochondrion) and in photosynthesis (in the membranes of the chloroplasts).

MEMBRANES AND TEMPERATURE

Increasing the temperature gives molecules more kinetic energy, so they move faster. This increased movement of

phospholipids and other components makes membranes leaky, which allows substances that would normally not do so

to enter or leave the cell.

Organisms that live in very hot or very cols environments need differently adapted molecular components of their

membranes, for example the cholesterol content, so that their membranes can perform the functions needed to

maintain life.

Components of the fluid mosaic model of the common membrane structure

phospholipid

bilayer

channel

protein

glycolipid glycoprotein

cholesterol extrinsic protein


One of the most important attributes of living organisms is their ability to respond efficiently to changes in their

environment. On a simple level, amoeba must be able to detect nutrient molecules in the water around it so it can

move towards the nutrient and take the molecules into the cell. If it cannot detect them, it cannot take them, it cannot

survive. In a multicellular organism, each cell must play its part. It must detect signals, both internal and external, and

base chemical reactions on the signals it receives.

In order to detect signals, cells must have on their surface sensors capable of receiving

these signals – these sensors are known as receptors. They are usually (modified) protein

molecules.

Multicellular organisms use hormones more often than not to communicate between cells. These are chemical

messengers packaged into tissues and released into the organism. Any cell with a receptor for the hormone is a target

cell. A specific hormone will bind to a receptor on a target cell because their shapes fit perfectly, like enzymes.

An example of a hormone receptor is the insulin receptor. Insulin is released from the pancreas by special beta cells in

response to increased blood sugar levels. The insulin is a molecule which can attach to the plasma membranes of many

cells, including muscle and liver cells. When insulin attaches to its receptor, it triggers internal responses win the cell

that lead to more glucose channels being present in the plasma membrane. This allows the cell to take up more glucose

from the blood, and ultimately reduce blood glucose level.

Viruses enter cells by binding with receptors on the surface of the plasma membrane

that normally bind to the signalling molecules. HIV, which causes AIDS, can infect

humans because it can enter the cells of the immune system – it has a shape that fits

into one of the receptors on the cell surface of some important types of immune

cells, such as T-blood cells. Once it enters a cell, it may reproduce inside the cell and

eventually destroy it. The diagram shows a HIV particle with its receptors.

Some poisons also bind with receptors. The toxin extracted from the bacterium

Clostridium botulinum binds with receptors on muscle fibres and prevents them from

working properly, causing paralysis.

Whilst this toxin is deadly, it is used in small amounts in cosmetic surgery, under the name Botox, to paralyse small

muscle cells in the face and reduce wrinkling of the skin.

Cell signalling and membrane-bound receptors

CCeellll SSiiggnnaalllliinngg --

cceellllss ccoommmmuunniiccaattee wwiitthh

oonnee aannootthheerr bbyy sseennddiinngg

aanndd rreecceeiivviinngg ssiiggnnaallss


In order to survive, cells need a supply of nutrient molecules; and most cells also need an oxygen supply for aerobic

respiration. Also, the reactions within living cells (collectively known as metabolism) generate waste products which

need to be removed from the cell. Any molecule needs to cross a membrane to move in or out of a cell.

DIFFUSION

In a fluid (gas or liquid), the molecules or ions move around freely, even if it is not

mixed or stirred, this is because the molecules are not held together like in a solid,

they possess kinetic energy that keeps them moving. Processes such as diffusion

that depend only on this energy are termed as passive processes.

The process of diffusion is the evening out of molecules across an area. It is the net movement of particles from an area

of high concentration to an area of lower concentration; which continues until the concentration of the particles is

consistent throughout.

The diagram shows a phospholipid bilayer (cell

membrane) with carbon dioxide molecules on one side of

it in the first stage. In the second stage, the carbon

dioxide molecules are travelling through the bilayer

because they are small enough to pass between the

phospholipids. The molecules move through to the other

side of the membrane. Diffusion stops when the amount

of molecules on both sides is even. This is an example of

diffusion as would happen with photosynthesis with the

gas exchange.

Even when the molecules have been distributed evenly via diffusion, movement doesn’t stop completely. The molecules

still move around, but not in any one particular direction – because they still have this kinetic energy. We refer to this

state as equilibrium, when there is no net movement.

Factors affecting the rate of diffusion are: temperature – an increase in temperature means an increase in kinetic energy, so the rate of random movement of

the molecules increases as does the rate of diffusion

concentration gradient – having more molecules on one side of the membrane increases the concentration gradient

and so increases the rate of diffusion

size of molecules – smaller molecules diffuse more quickly than larger ones

thickness of membrane –diffusion is slowed down by thick membranes as molecules have to cross large distances

surface area – diffusion occurs more quickly when there is a larger surface area to diffuse across

There is a second type of diffusion, called facilitated diffusion, which is the movement of a specific molecule down a

concentration gradient, passing through the membrane via a specific protein carrier. The two types of protein are:

1 the channel protein forms pores in the membrane, which are usually shaped only to allow the one type of molecule

or ion through – and many are also gated, meaning they can be opened and closed

2 the carrier protein is shaped so that a specific molecule (e.g. glucose in the diagram on the following page) can fit

into the protein at the membrane surface, and when the molecule fits, the protein changes shape to allow the

molecule to pass through to the other side

Diffusion, osmosis and active transport over cell membranes

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pprroocceesssseess wwhhiicchh rreellyy oonn tthhee

kkiinneettiicc eenneerrggyy iinn mmoolleeccuulleess


The diagram shows a glucose

molecule entering a carrier

protein which is shaped to

specifically hold that molecule.

When it is securely in there, the

carrier protein changes shape to

allow the glucose molecule to

travel through the protein and

out through the other end.

Glucose molecules are too big to

diffuse through the

phospholipids, so they have to

use these proteins instead.

OSMOSIS

A special type of diffusion is osmosis, which is specifically concerned with water molecules across a partially-permeable

membrane. Water molecules are also free to move from areas of a high water concentration, to areas of low water

concentration. Having a substance dissolved in the water will affect the number of free water molecules, and this

decreases the water concentration.

The measure of the tendency of water molecules to move from one place to another is called water potential (ψ).

Water always moves from an area of high water potential to an area of low water potential, i.e. from areas with lots of

these “free” water molecules to areas with fewer water molecules.

As with diffusion, net movement

of molecules occurs until the

concentrations are evened out, so

osmosis will occur until the water

potential is the same on both

sides of the membrane.

solute molecule

water molecule

partially-permeable

membrane

The water potential of cells is lower than that of pure water, because of all the sugars, salts and other substances

dissolved in the cytoplasm. The water potential of pure water is zero, which is in fact the highest water potential.

Everything else has a water potential lower than that of pure water, which is measured using negative numbers. The

unit of measurement for water potential is kilopascals (kPa). The larger the negative figure, the more solute dissolved

and the lower the water potential (e.g. -14kPa has a lower water potential than -3kPa).

Highest water potential 0kPa Pure water No solute dissolved Lower water potential -10kPa Dilute solution Small amount of solute dissolved

Very low water potential -500kPa Concentrated solution Large amount of solute dissolved

glucose

plasma

membrane

inside cell

outside cell

glucose carrier

protein

Net movement of

water by osmosis

Lower concentration of solute molecules

Higher concentration of free water molecules

Higher concentration of solute molecules

Lower concentration of free water molecules


The cell membrane is a partially permeable membrane. Placing plant or animal cells in pure water, or in any solution

with a water potential higher than the cell contents, means there is a water potential gradient from outside to inside

the cells. Water molecules will move down the water potential gradient into the cells by osmosis. The cells will swell. In

the case of animal cells, the cell will eventually burst open – it is haemolysed. In a plant cell, the swelling vacuole and

cytoplasm will push the membrane against the cell wall. It will not burst because the wall will eventually stop the cell

getting any larger. Osmosis will then stop at this point, even if the concentration gradient remains. The cell is turgid.

Placing animal or plant cells in a salt or sugar solution (with a water potential lower than the cell contents) means there

is a water potential from insider to outside the cells, so water molecules move out of the cells by osmosis. The cells will

shrink, and in the case of animal cells, the cell contents will shrink and the membrane will wrinkle up – the cell has

crenated. With plant cells, the cytoplasm and vacuole will shrink as they lose water, the cell surface membrane will pull

away from the cell wall – this is called plasmolysis.

ACTIVE TRANSPORT

A cell cannot get everything it needs via diffusion and osmosis. Sometimes a

cell will need more of a particular substance than there is outside of the cell; or

in other cases, it may just be that the cell needs to get a particular substance

inside the cell quicker than simple diffusion allows. This would obviously

require energy to drive the process.

Some of the carrier proteins found in membranes act as “pumps.” These proteins are similar to the carrier proteins used

for facilitated diffusion. They are shaped in a way that is complementary to the molecules they carry. They carry larger

or charged ions through membranes. These are the molecules that cannot pass through the lipid bilayer using diffusion.

These protein pumps differ significantly from the proteins used in facilitated diffusion:

they carry specific molecules one way across the membrane

in carrying molecules across the membrane, they use energy in the form of ATP

they can carry molecules against the concentration gradient (from low to high)

they can carry molecules at a much faster and more efficient rate than diffusion

The energy which is used in the active transport process is used to change the shape of the transport protein. The shape

change means that specific molecules to be transported fit into the protein on one side of the membrane only. As the

molecule is carried through, the carrier uses the energy from ATP to chane shape so that the molecule being carried

across now leaves the carrier protein. The molecule cannot enter the transport protein, because the protein is now a

different shape and so it will not fit.

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tthhee mmoovveemmeenntt ooff mmoolleeccuulleess

aaccrroossss mmeemmbbrraanneess uussiinngg AATTPP ttoo

ddrriivvee tthhee pprrootteeiinnss uusseedd

AATTPP --

((aaddeennoossiinnee ttrriipphhoosspphhaattee))

pprroodduucceedd dduurriinngg rreessppiirraattiioonn,,

aallmmoosstt aallll aaccttiivviittiieess tthhaatt

nneeeedd eenneerrggyy iinn tthhee cceellll aarree

ddrriivveenn bbyy tthhee eenneerrggyy

rreelleeaasseedd ffrroomm AATTPP molecule being actively transported

shape change of active transport protein

requires ATP – the shape change does not

allow the molecule to go the “wrong way”

active transport protein is shaped so that

the molecule it transports can only fit on

one side of the protein


Some cells need to move large quantities of material either in or out. The process of bringing materials into the cell is

called endocytosis and the process of moving materials out of the cell is exocytosis. This bulk transport is possible

because membranes can easily fuse, separate and “pinch off.” Like active transport, bulk transport requires energy in

the form of ATP. In this case, the energy is used to move the membranes around to form the vesicles that are needed,

and to move the vesicles around the cell.

Some examples of bulk transport are:

hormones – pancreatic cells make insulin in

large quantities, the insulin is processed and

packaged into vesicles by the Golgi body, and

these vesicles fuse with the outer membrane

to release insulin into the blood

in plant cells, materials required to build the

cell wall are carried outside in vesicles

some white blood cells engulf invading

microorganisms by forming a vesicle around

them – this vesicle then fuses with

lysosomes so that the enzymes from the

lysosomes can digest the microorganisms –

such cells are called phagocytes

MOVEMENT ACROSS MEMBRANES – A SUMMARY

Different names are given to the movements of materials in bulk transport:

endo = inwards exo = outwards phago = solid material pino = liquid material

So the bulk movement of liquid material out of a cell is described as “exopinocytosis”

Passive processes (i.e. no energy input from ATP required)

Diffusion Down a concentration gradient; lipid soluble or very small molecules; through a lipid bilayer

Facilitated diffusion Down a concentration gradient; charged or hydrophilic molecules or ions; via a channel or carrier proteins

Osmosis Down a water potential gradient; through bilayer or protein pores

Active processes (i.e. energy input in the form of ATP is required)

Active transport Against a concentration gradient via carrier proteins that use energy from ATP to change shape

Endocytosis and exocytosis

Bulk transport of materials via vesicles that fuse with or break from the cell surface membrane

Processes involving moving large amounts of material into or out of the cell

plasma membrane Endocytosis

Exocytosis


Many organisms consist of millions of living cells. New daughter cells are constantly being formed from parent cells in a

series of events which together make the cell cycle. A daughter cell must be able to carry out exactly the same functions

as the parent cell, whether the daughter cell is a new cell in a growing organism, or a new cell simply replacing a worn-

out cell, such as a dead skin cell.

All eukaryotic cells have chromosomes which contain one molecule of DNA each. These contain specific lengths of DNA

called genes. The chromosomes hold the instructions, often called the “blueprint” for making new cells. Daughter cells

produced during the cell cycle must contain a copy of all these instructions, so they must each carry a full set of

chromosomes, copied exactly from the single set of the parent.

The diagram to the left shows the cell cycle:

G1 indicates the first growth stage, this includes making new proteins

and organelles

S indicates “synthesis” where each chromosome is duplicated so that

each has two chromatids

The cell ‘checks itself’ after this stage to ensure it has two copies of

each chromosome, if not, the cell cycle stops

G2 indicates the second growth stage, the enlargement of the

developing cell

There is another ‘checkpoint’ next where the cell checks its progress

M is nuclear division (mitosis) where the cell eventually divides

The line at the end of mitosis marks cytokenesis (cleavage of cytoplasm)

The outside letters M and I represent mitosis (nuclear division) and

interphase

Before a cell can divide, the

DNA of each chromosome

must be replicated. Two

replicas are produced. Each is

an exact copy of the original

and they are held together at a

point called the centromere.

Each chromosome now

consists of a pair of sister

chromatids.

The DNA double helix, as shown in the right diagram, is

coiled and wrapped in loops around cores of proteins.

These are called histones.

Parent cells, daughter cells and the cell cycle

chromosome

chromatid

centromere


Mitosis refers to the type of cell division where two genetically-identical nuclei are formed from one parent cell

nucleus. The process happens in a number of stages:

Cell division for growth, repair and replacement

Interphase refers to the state a complete parent cell is in when it has

all 46 chromosomes that have been replicated. There are two centrioles

situated at opposite ends of the cell

Early prophase occurs when the chromosomes supercoil (shorten

and thicken). At this point they consist of a pair of sister chromatids. The

two daughter centrioles begin to move around the cell

Late prophase involves the centrioles moving completely round to

opposite ends of the cell (opposite poles). Each centriole begins to make

the spindle, a structure made of protein threads. The nuclear envelope has

broken down at this point

Metaphase happens next. The individual chromosomes move to the

central region of the spindle (the equator) and align themselves. Each

chromosome becomes attached to the spindle thread as the spindle locks

onto the centromere of each chromosome

Anaphase happens when the centromeres split and each individual

chromatid (now effectively its own chromosome). The spindle fibres

shorten, which pulls the chromatids further apart to opposite poles of the

cell. They have a V-shaped appearance because they are being pulled by

the centromere, which leads

Telophase is the final stage of nuclear division where a new nuclear

envelope reforms around each individual set of chromatids to create

two new nuclei. The spindle breaks down and disappears and the

chromosomes uncoil again. The cell then splits in two, so that the two

daughter cells each have a nucleus and are genetically identical. This

splitting action is called cytokenesis


Sexual reproduction involved the fusion of two cell nuclei from two different individuals in order to produce offspring.

Each cell contributes half of the total genetic information (genome) required by the offspring. This means that special

cells containing half the adult number of chromosomes must be produced. Such cells are called gametes. The fusion of a

male and female gamete produces a zygote which can divide via mitosis to grow into a new individual organism.

The process which produces gametes is not mitosis, but a different process called meiosis which happens at specific

regions of the adult organism – the gonads (sex organs). Most adult cells of eukaryotes contain two sets of

chromosomes (for example, humans have 46) – they are said to be diploid. The chromosomes are homologous, this is

all contain the same genes but different alleles (versions of a gene). During meiosis, only one chromosome from each

homologous pair goes into the daughter cell.

The daughter cell will therefore be haploid (only contain one set of chromosomes, for example, humans have 23). The

haploid cells are not all genetically identical because they contain different alleles of the genes they were allocated from

the adult cells.

Meiosis is different to mitosis in two important ways:

Meiosis produces cells containing half the number of chromosomes

Meiosis produces cells that are genetically different to each other and to the adult cell

These features, together with the fusion of gametes from different individuals, means that the offspring of sexually

reproducing organisms are always different from each other (apart from identical twins, which are natural clones).

Production of cells which are genetically different

Susan

46 chromosomes

(23 pairs)

Dave

46 chromosomes

(23 pairs)

Sperm cell

23 chromosomes Egg

23 chromosomes

Jamie

46 chromosomes

(23 pairs)


CELL DIFFERENTIATION

A stem cell is a cell which is potentially capable of becoming any one cell which is found in the organism it belongs to.

These cells are described as being omnipotent (all types), totipotent (any type), pluripotent (every type) and

multipotent (many types) – all of these words mean the same thing basically.

Stem cells only occur in small numbers in adult animals. In humans, they can be found in bone marrow. The stem cells

here can become any type of blood cell or bone cell needed. They can differentiate into different specialised cells by

switching on or off certain genes. Cells can differ in size, shape and the number (or presence) of certain organelles. Cell

differentiation is an irreversible process.

SPECIALISED CELLS

Cell Structure Function

Erythrocyte (red blood cell) Packed with haemoglobin (Hb)

Hb bind reversibly with oxygen to carry it around the body

Biconcave disc (concave on both sides of the cell)

Provides an increased surface area for exchange; and makes it more flexible to pass through narrow capillaries

No nucleus Allows for more space for haemoglobin

Neutrophil (phagocyte) Granular cytoplasm due to many lysosomes

Allows the breakdown of ingested pathogens

Lobed nucleus Gives the cell greater flexibility to make movement easier

Sperm cell Undulipodium

Rapid undulation gives the cell propulsion for movement

Acrosome (with hydrolytic enzymes)

Breaks down the outer coating of the egg cell

Haploid cell (only has half the chromosomes of an adult)

Means that the full complement is restored after fusion with the egg

Many mitochondria Produce ATP for movement

Palisade cell Large numbers of chloroplasts Capture a lot of sunlight for photosynthesis

Chloroplasts circulate around cell Minimalises the heat damage to organelles

Tall, thin and long in shape Means there are fewer cell walls for the sunlight to pass through

Root hair cell

Long hair-like projection Increases the cell surface area, allowing for a more rapid absorption rate of water

Stem cells and cell adaptations to particular functions


Erythrocytes (red blood cells) and neutrophils (phagocytes – a certain type of white blood cell) play very different roles,

both are human cells and each began with the same set of chromosomes, so each is potentially capable of carrying out

exactly the same function. All blood cells are produced from undifferentiated stem cells in bone marrow.

The cells destined to become erythrocytes lose their nucleus, mitochondria, Golgi body and rough ER. They become

packed full of the protein haemoglobin (Hb) and their shapes change dramatically into biconcave discs.

Cells destined to become neutrophils keep their nucleus, and it becomes lobed (the picture to the

right shows what a lobed nucleus looks like). Their cytoplasm appears granular due to the enormous

numbers of lysosomes which are produced. These potent enzymes have a role in the blood, which is

to kill invading pathogens, so these are specialised to attack all invading microorganisms.


In order to ensure an organism made from many millions of cells survives, the cells’ activities and functions needs to be

organised. The level of organisation within multicellular organisms can be described under three main headings:

EPITHELIAL TISSUES IN ANIMALS

Animal tissues in general are grouped under four main categories:

epithelial tissue – layers and linings

connective tissue – hold structures together and provide support

muscle tissue – cells specialised to contract and move certain body parts

nervous tissue – cells that convert certain stimuli into electrical impulses and conduct those impulses

Epithelial tissues form sheets which cover surfaces. Almost all organs in the body have some kind of epithelial tissue

involved. Simple epithelia are one cell thick. Cells rest on a basement membrane (a network of collagen and

glycoprotein, secreted by the underlying cells and that holds the epithelial cells in position).

Squamous (pavement) epithelia cover many surfaces in the body including the cheeks, blood vessels and alveoli. The

individual cells are smooth, flat and very thin. They fit closely together to provide a low-friction surface. Their thinness

allows for rapid diffusion.

Ciliated epithelia have cilia. Cells can be cubodial, as in the bronchioles, of columnar, as in the oviduct. The cells with

cilia waft rhythmically, moving material over the surface, for example to move the egg along the oviduct. There are also

mucus-secreting goblet cells present. In breathing tubes, the mucus traps dirt and microbes whilst cilia move it

upwards.

TISSUES IN A LEAF

1 – Upper epidermis: secretes a waxy cuticle to protect

against pathogens and prevent dramatic water loss.

Transparent to allow the light to reach palisade layer

2 – Palisade mesophyll: many chloroplasts circulating the

cells for photosynthesis – long and thin cells mean fewer

cell walls for light to pass through

3 – Spongy mesophyll: spread out to provide air spaces

for uptake of CO2 for palisade cells and excretion of O2

4 – Lower epidermis: as upper epidermis, but also has

stomata for gas exchange, controlled by guard cells

Tissues, organs and functioning organ systems within an organism

Tissues

A collection of cells that are similar

to each other and perform a

common function. These may be

attached to each other but may

not be. Examples include phloem

and xylem in plants, and epithelial

and nervous tissues in animals

Organs

A collection of tissues working

together to perform a particular

function. Examples include the

leaves of plants and the liver in

animals

Organ systems

Made up of several organs

working together to perform an

overall life function. Examples

include the excretory system and

the reproductive system


TRANSPORT TISSUES IN PLANTS

Plants need to move water and minerals from the soil through their roots and stems and up into

their leaves. They also need to be able to move the products of photosynthesis around the plant to

be used for growth or stored in other places for later use.

In a plant, meristems are points at which meristem cells are produced. These are the only

undifferentiated cells in a plant which can specialise into any other form of cell needed. Meristem

regions are the root, shoot tips and a ring around the stem or trunk. Meristem cells can become

cells which can become part of the transport tissues needed for the above functions.

Phloem

Xylem

Some meristem cells produce small cells which elongate to become xylem cells. The xylem

vessel walls are reinforced and made waterproof by deposits of lignin.

The lignin kills the cell contents and breaks down the ends

of the cells so that the stack of cells becomes one long

hollow tube, with a wide lumen. Xylem tissue is used

to transport water and minerals up the plant.

Phloem tissue consists of sieve tubes and companion

cells. The meristem tissue produces cells which elongate and

line up end-to-end to form a long tube. Their ends do not

completely break down, but form sieve plates between the cells.

The plates allow the movement of materials up or down the tubes. Next

to each sieve tube is a companion cell. These are very metabolically active. They move the products of photosynthesis up

and down the plant.


1 The figure below shows an electron micrograph of an animal cell.

(a) Name and state the function of the following structures.

(i) Structure A

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(ii) Structure B

……………………………………………………………………………………………

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(iii) Structure C

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(6 marks)

(b) Describe the roles of centrioles in cellular division.

…………………………………………………………………………………………………

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Questions on Units 1.1 – 1.4 on Cell Components

A

B

centriole

C

nucleus


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(8 marks)

(c) The nucleus is labelled on the diagram. Complete the table by putting a tick () or a cross () in

each box to identify which properties belong to eukaryotic and prokaryotic cells.

Structure or function Eukaryotic cells Prokaryotic cells

Nucleolus

Nuclear envelope

Nucleoid

Presence of DNA

(4 marks)


(d) The nucleus from the image is reproduced below. It has been magnified x 20,000

Use the line XY as the length of the nucleus to work out its actual size.

Show your working.

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(3 marks)

Total: 21 marks

X Y

x 20,000


2 The figure below shows a bone marrow cell under the electron microscope.

(a) Complete the table below to show the functions of the structures labelled A to D.

One has been done for you.

Function Structure Label

Controls substances which enter or leave the cell

Contains digestive enzymes

Carries out aerobic respiration

Membrane surrounding the nucleus nuclear envelope D

Attaches to mRNA in protein synthesis

(4 marks)

A B

C

D

nucleus

E


(b) The nucleus has been labelled in the diagram. There are several darkened stain patches present.

(i) State the name of these darkened patches which appear when stained.

……………………………………………………………………………………………

(1 mark)

(ii) Explain the role of this structure.

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(3 marks)

Total: 8 marks


1 The diagram below shows a cell surface (plasma) membrane.

(a) Name the structures A to E.

A ……………………………………………………………………………………………

B ……………………………………………………………………………………………

C ……………………………………………………………………………………………

D ……………………………………………………………………………………………

E ……………………………………………………………………………………………

(5 mark)

(b) Label X shows a protein. Which of the following words correctly describes it?

Put a ring around your answer

extrinsic intrinsic

(1 mark)

(c) State the width of a cell surface (plasma) membrane.

There is one mark available for providing the correct unit of measurement.

…………………………………………………………………………………………………

(2 marks)

Questions on Units 1.5 – 1.9 on Cell Membranes

A B C

E X

D


(d) Write the letter Z on the side of the membrane which the cytoplasm of the cell is on. (1 mark)

(e) Give a reason for your answer to part (d) using evidence from the diagram on the previous page.

…………………………………………………………………………………………………

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(2 marks)

Total: 10 marks


2 The diagram below is showing the water molecules and glucose molecules over a membrane.

The water molecules are the smaller circles. The glucose molecules are the larger circles.

(a) Show the net movement of water by drawing in an arrow across the membrane. (1 mark)

(b) Name the process of transport shown in the diagram.

…………………………………………………………………………………………………

(1 mark)

(c) Explain why each of the molecules can or cannot cross the membrane directly:

(i) water molecules

……………………………………………………………………………………………

……………………………………………………………………………………………

(ii) glucose molecules

……………………………………………………………………………………………

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(4 marks)

Total: 6 marks


3 Below is a diagram showing two active processes of bulk transport.

(a) Label the two processes of endocytosis and exocytosis in the spaces provided in the diagram.

(1 mark)

(b) Explain the process of bulk transport.

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(6 marks)

(c) Another active method of transport is active transport.

(i) Explain how this method of transport is an active process.

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(1 mark)

(ii) Describe the processes of active transport and osmosis, highlighting differences between them

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(7 marks)

Total: 15 marks


1 The below diagram shows a chromosome.

(a) Name the structure labelled A in the diagram.

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(1 mark)

(b) Look at structures B and C in the diagram.

(i) What can you say about the structures?

.……………………………………………………………………………………………

.……………………………………………………………………………………………

(1 mark)

(ii) What is the name given to the structures A and B?

.……………………………………………………………………………………………

(1 mark)

(c) The diagram below shows an animal cell in two sequential stages of division.

X Y

(i) Name the process of division this cell is going through.

.……………………………………………………………………………………………

(1 mark)

(ii) Name the stage of cellular division the cell is at shown in Stage X.

.……………………………………………………………………………………………

(1 mark)

Questions on Units 1.10 – 1.14 on Cell Division

A B

C


(iii) Name the stage of cellular division the cell is at shown in Stage Y.

.……………………………………………………………………………………………

(1 mark)

(iv) In the space below, draw an annotated diagram which shows the stage of cellular division

which takes place after Y.

Name of stage:

.……………………………………………………………………………………………

(5 marks)

(d) The diagram below shows the same cell at interphase and a chromosome during division.

(i) Suggest one difference between a chromosome at interphase and during division.

.……………………………………………………………………………………………

.……………………………………………………………………………………………

(1 mark)

(ii) Explain what happens after interphase to make a chromosome appear as it does above.

.……………………………………………………………………………………………

.……………………………………………………………………………………………

(2 marks)

Total: 14 marks


2 Below is a diagram showing a nucleus of an animal cell at early prophase.

(a) On the diagram, shade one pair of homologous chromosomes. (1 mark)

(b) The cell is described as 2n=6.

(i) Explain what 2n=6 means in terms of cell nuclei.

.……………………………………………………………………………………………

.……………………………………………………………………………………………

.……………………………………………………………………………………………

(2 marks)

(ii) State the number of chromosomes that would be found in a haploid cell from this animal.

.……………………………………………………………………………………………

(1 mark)

(iii) Explain why haploid cells need to be produced for sexual reproduction.

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.……………………………………………………………………………………………

.……………………………………………………………………………………………

.……………………………………………………………………………………………

.……………………………………………………………………………………………

(3 marks)

Total: 7 marks


3 The diagrams below represent an animal cell at various stages of mitosis.

A B

C D

E F

(a) Complete the table below to show what happens at each stage of mitosis.

One has been done for you.

Stage What happens

A interphase adult cell complete with 46 replicated chromosomes

B

C

D

E

F

(10 marks)


(b) The two cycles below represent the life cycles of two organisms, Organism A and Organism B.

Organism A Organism B

(i) Name the types of reproduction taking place in both organisms:

Organism A: …………………..……………………………………………………………

Organism B: …………………..……………………………………………………………

(1 mark)

(ii) Explain why a gamete is described as (n).

.……………………………………………………………………………………………

.……………………………………………………………………………………………

(1 mark)

(iii) Describe the process which produces gametes.

.……………………………………………………………………………………………

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.……………………………………………………………………………………………

(4 marks)

Total: 16 marks

adult

(2n)

young organism

(2n)

adult

(2n)

young organism

(2n)

gamete

(n)


4 Study the diagrams below which show an erythrocyte (red blood cell) at two different views.

surface view cross section

(a) Explain three adaptations to an erythrocyte to suit its function.

1 .………………………………………………………………………………………………

.………………………………………………………………………………………………....

2 .………………………………………………………………………………………………

.………………………………………………………………………………………………....

3 .………………………………………………………………………………………………

.………………………………………………………………………………………………....

(3 marks)

(b) Erythrocytes and neutrophils (phagocytes – a type of white blood cell) both start out with the same

number of chromosomes and are potentially capable of performing the same roles.

(i) State the name of the process which turns undifferentiated cells into differentiated cells.

.……………………………………………………………………………………………

(1 mark)

(ii) Explain what happens to erythrocytes and neutrophils to differentiate them.

.……………………………………………………………………………………………

.……………………………………………………………………………………………

.……………………………………………………………………………………………

.……………………………………………………………………………………………

.……………………………………………………………………………………………

(3 marks)

Total: 7 marks

1 Cells

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