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An introduction to the microscope and magnification
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
unit
metre
decimetre
centimetre
millimetre
micrometre
nanometre
picometre
symbol
m
dm
cm
mm
μm
nm
pm
metres
1
0.1
0.01
0.001
0.000 001
0.000 000 001
0.000 000 000 01
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
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Advantages
The resolution is 0.1nm (2000x more than the light
microscope)
Can produce more detailed images of the structures
inside cells
The SEM produces a final 3D image not possible with
the light microscope
Limitations
Electron beams are deflected by air molecules, so the
sample has to be placed in a vacuum
Electron microscopes are extremely expensive
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.
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Cell ultrastructure and the importance of the cytoskeleton of cells
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
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The structure and function of the various organelles within animal and plant 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
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
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
The Golgi apparatus is a stack of membrane-bound sacs,
which looks very much like a pitta bread
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
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
A lysosome is a spherical sac surrounded by a single
membrane
Function
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
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 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
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
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
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 nucleus and endoplasmic
reticulum
►Golgi apparatus
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◄Mitochondrion
► Chloroplast
There are some organelles which are non membrane-bound…
Structure
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
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
Function
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
These are used in cell division, they form fibres known as
spindle which move the chromosomes during nuclear
division
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Structural and functional differences between the cells of prokaryotes and eukaryotes
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 antibioticresistant, 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.
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The structure and role of cell membranes
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.
hydrophilic
head
hydrophobic
tail
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.
hydrophobic tail
air
water
hydrophilic head
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.
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Components of the fluid mosaic model of the common membrane structure
Cell membranes are not just simple phospholipid bilayers. They contain many other key features which make it a fullyfunctional 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
glycoprotein
glycolipid
channel
protein
phospholipid
bilayer
cholesterol
extrinsic protein
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.
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Cell signalling and membrane-bound receptors
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.
Cell Signalling cells communicate with
one another by sending
and receiving signals
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.
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Diffusion, osmosis and active transport over cell membranes
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
Passive Processes processes which rely on the
kinetic energy in molecules
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
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outside cell
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.
glucose
plasma
membrane
glucose carrier
protein
inside cell
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.
Net movement of
water by osmosis
solute molecule
water molecule
partially-permeable
membrane
 Lower concentration of solute molecules
 Higher concentration of free water molecules
 Higher concentration of solute molecules
 Lower concentration of free water molecules
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
Lower water potential -10kPa
Very low water potential -500kPa
Pure water
Dilute solution
Concentrated solution
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No solute dissolved
Small amount of solute dissolved
Large amount of solute dissolved
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.
Active Transport the movement of molecules
across membranes using ATP to
drive the proteins used
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
molecule being actively transported
ATP (adenosine triphosphate)
produced during respiration,
almost all activities that
need energy in the cell are
driven by the energy
released from ATP
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
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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Processes involving moving large amounts of material into or out of the cell
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.
Endocytosis
plasma membrane
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
Exocytosis
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”
Diffusion
Passive processes (i.e. no
energy input from ATP
required)
Facilitated diffusion
Osmosis
Active processes (i.e.
energy input in the form
of ATP is required)
Active transport
Endocytosis and
exocytosis
Down a concentration gradient; lipid soluble or very small
molecules; through a lipid bilayer
Down a concentration gradient; charged or hydrophilic
molecules or ions; via a channel or carrier proteins
Down a water potential gradient; through bilayer or protein
pores
Against a concentration gradient via carrier proteins that use
energy from ATP to change shape
Bulk transport of materials via vesicles that fuse with or
break from the cell surface membrane
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Parent cells, daughter cells and the cell cycle
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 wornout 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
centromere
must be replicated. Two
replicas are produced. Each is
an exact copy of the original
chromatid
and they are held together at a
point called the centromere.
Each chromosome now
consists of a pair of sister
chromosome
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.
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Cell division for growth, repair and replacement
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:
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
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Production of cells which are genetically different
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).
Dave
46 chromosomes
(23 pairs)
Susan
46 chromosomes
(23 pairs)
Sperm cell
23 chromosomes
Egg
23 chromosomes
Jamie
46 chromosomes
(23 pairs)
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Stem cells and cell adaptations to particular functions
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
Erythrocyte (red blood cell)
Neutrophil (phagocyte)
Sperm cell
Palisade cell
Structure
Function
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
Granular cytoplasm due to many
lysosomes
Allows the breakdown of ingested pathogens
Lobed nucleus
Gives the cell greater flexibility to make
movement easier
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
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
Long hair-like projection
Increases the cell surface area, allowing for a
more rapid absorption rate of water
Root hair cell
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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.
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Tissues, organs and functioning organ systems within an organism
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:
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
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 O 2
4 – Lower epidermis: as upper epidermis, but also has
stomata for gas exchange, controlled by guard cells
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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.
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
Phloem tissue consists of sieve tubes and companion
cells. The meristem tissue produces cells which elongate and
Xylem
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.
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Questions on Units 1.1 – 1.4 on Cell Components
1
The figure below shows an electron micrograph of an animal cell.
A
B
centriole
C
nucleus
(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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(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)
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(d) The nucleus from the image is reproduced below. It has been magnified x 20,000
x 20,000
X
Y
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
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2
The figure below shows a bone marrow cell under the electron microscope.
A
B
C
D
nucleus
E
(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
nuclear envelope
D
Controls substances which enter or leave the cell
Contains digestive enzymes
Carries out aerobic respiration
Membrane surrounding the nucleus
Attaches to mRNA in protein synthesis
(4 marks)
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(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.
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(1 mark)
(ii) Explain the role of this structure.
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(3 marks)
Total: 8 marks
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Questions on Units 1.5 – 1.9 on Cell Membranes
1
The diagram below shows a cell surface (plasma) membrane.
A
B
C
D
X
E
(a) Name the structures A to E.
A
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B
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C
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D
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E
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(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)
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(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
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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.
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(1 mark)
(c) Explain why each of the molecules can or cannot cross the membrane directly:
(i)
water molecules
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(ii) glucose molecules
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(4 marks)
Total: 6 marks
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3
Below is a diagram showing two active processes of bulk transport.
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(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
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Questions on Units 1.10 – 1.14 on Cell Division
1
The below diagram shows a chromosome.
A
B
C
(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?
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(1 mark)
(c) The diagram below shows an animal cell in two sequential stages of division.
X
(i)
Y
Name the process of division this cell is going through.
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(1 mark)
(ii) Name the stage of cellular division the cell is at shown in Stage X.
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(1 mark)
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(iii) Name the stage of cellular division the cell is at shown in Stage Y.
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(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:
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(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.
.……………………………………………………………………………………………
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(1 mark)
(ii) Explain what happens after interphase to make a chromosome appear as it does above.
.……………………………………………………………………………………………
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(2 marks)
Total: 14 marks
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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.
.……………………………………………………………………………………………
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(2 marks)
(ii) State the number of chromosomes that would be found in a haploid cell from this animal.
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(1 mark)
(iii) Explain why haploid cells need to be produced for sexual reproduction.
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(3 marks)
Total: 7 marks
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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
A
interphase
What happens
adult cell complete with 46 replicated chromosomes
B
C
D
E
F
(10 marks)
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(b) The two cycles below represent the life cycles of two organisms, Organism A and Organism B.
adult
(2n)
adult
(2n)
gamete
(n)
(i)
young organism
(2n)
young organism
(2n)
Organism A
Organism B
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).
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(1 mark)
(iii) Describe the process which produces gametes.
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(4 marks)
Total: 16 marks
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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 .………………………………………………………………………………………………
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2 .………………………………………………………………………………………………
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3 .………………………………………………………………………………………………
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(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.
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(3 marks)
Total: 7 marks
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