Biology: Unit F211: Cells, Exchange and Transport
Module 1: Cells
1.1.2
– Cell Membranes
Membranes surround all cells, separating the cell contents from the outside world.
Eukaryotic cells also have organelles which are bound by membranes, such as the Golgi
body, the endoplasmic reticulum and the mitochondria.
Functions of Membranes
O2
Na+
+
1. Selectively Permeable - keeping certain things in and
out of the cell, for example, red blood cells must keep
haemoglobin inside the cell.
2. Identify the cell and communicate with others – identifying cells as your own, and
knowing which are invading cells.
3. Containing enzymes for chemical reactions – like respiration which is carried out
using enzymes in the mitochondria.
All membranes have the same basic structure – Phospholipids
Hydrophilic Phosphate Head –
Hydrophobic fatty acid tail –
hates water. Large molecule with no
charges and so moves away from
water.
Water outside cell
Hydrophobic Region
Water inside cell
loves water. It is made from a
molecule that is covered in small
charges, making it polar. This means
that it can interact with water.
These form a Phospholipid Bilayer:
The phospholipids naturally arrange themselves into a
Bilayer. The heads are hydrophilic, and so want to be
next to water. There is water both within the cell and
outside of the cell, and so this means that heads must
be on the outer edge of either side of the membrane.
Two layers of phospholipids line up, so that the heads
are on the outside, close to the water, and the tails,
which are hydrophobic, are away from the water.
Na+
Glucose
Protein
O2
CO2
H20
Small, uncharged
molecules like water,
carbon dioxide and
oxygen can pass through
the Phospholipid Bilayer
freely.
Large charged particles
cannot get through, like
glucose, sodium and
proteins.
However, slats, amino acids and glucose are all essential for our cells, and so there must be a
mechanism of getting these molecules into the cell, because they can’t travel through the
Phospholipid Bilayer. The diagram below shows the fluid mosaic model of the membrane.
Glycolipid
Cholesterol
Extrinsic
Protein
Intrinsic
Protein
Channel
Protein
Phospholipid Bilayer
Carrier
Protein
Glycoprotein
The Phospholipid Bilayer is 7nm thick and is selectively permeable. It prevents large,
charged molecules from passing through.
Channel Proteins form a hydrophilic channel through the Bilayer to allow specific large
and charged molecules to pass through.
Carrier Proteins – some of the intrinsic proteins act as carrier proteins are able to
actively move large and charged substances across the membrane. This process uses ATP
from respiration. The incoming molecule binds to a site causing a change in shape in the
carrier protein, which then deposits the molecule in the cell cytoplasm.
Glycoproteins are a combination of carbohydrate and protein which help in cell
signalling and cell interaction. They identify the cell as your own to your immune system.
They also are hormone receptors, responding to hormones in the blood stream, as well as
drugs.
Glycolipids are involved in cell adhesion. Glycolipids are a glucose joined to a
Phospholipid molecule. They stick cells together forming tissues and organs.
Intrinsic Proteins pass all the way through the Bilayer
Extrinsic Proteins are exposed on only one side of the membrane. They act as enzymes
in the membrane, carrying out reactions like respiration on the cristae of the mitochondria,
and photosynthesis on the thylakoids in the chloroplasts.
Cholesterol acts like a staple on the inside layer of the membrane to hold together the
phospholipids. It fits between the fatty acid tails, maintaining the membrane’s stability and
fluidity.
Communication and Cell Signalling
Hello! 
Cell Signalling is how cells communicate with each other.
In multicellular organisms, each cell has its individual role
to play, and so cells must be able to detect the various
internal and external signals used to coordinate and carry
out the processes involved in growth, development,
movement and excretion. Cells must then be able to carry
out reactions or functions in response to the signals. In
order to detect these signals, cells have receptors which
are capable of receiving signals. Receptors are often protein molecules or modified protein
molecules.
Hormone Receptors – Hormones are chemical messengers
produced in specific tissues and then released into the
organism. Any cell with a receptor for the hormone molecule is
a target cell. A hormone molecule will bind to the receptor on a
target cell plasma membrane because they have
complementary shapes, like a jigsaw. Binding of the hormone and receptor causes the
target cell to respond in a certain way.
Medicinal drugs have been
developed that are complementary
to the shape and type of a receptor
molecule. Beta-blockers prevent
heart muscle from increasing heart
rate. Drugs used to treat
schizophrenia mimic a natural
neurotransmitter that some
individuals cannot produce
themselves.
Insulin is a protein molecule hormone which is released
from the beta-islet cells of the pancreas in response to
increased blood sugar levels. It attaches to the insulin
receptors on the plasma membranes of muscle and liver
cells. This triggers internal responses in the cell which
allow more glucose to be taken up from the blood, so
reducing blood sugar levels.
Hijacking receptors – viruses enter cells by bonding to receptors
on the plasma membrane which would usually bind to signalling
molecules. The HIV virus fits onto the receptors of important
immune cells like T-lymhocytes. The virus may then reproduce
inside the cell and destroy it. Toxins can bind to receptors, like
the toxin used in BOTOX to paralyse small muscles in the face and
reduce the wrinkling in the skin.
Crossing Membranes
In order to survive, cells need nutrient molecules. They may need oxygen for aerobic
respiration. Waste products generated by the cell metabolism must be removed. Any
molecules that need to enter or leave a cell will usually have to cross a membrane to do so.
Passive Processes – A) Diffusion
Diffusion is the net movement of particles from an area of high concentration to low
concentration down a concentration
gradient. Particles tend to even out, so
that there are equal concentrations on
each side of the membrane. When
there are equal concentrations, the net
movement of particles has stopped.
Particles are still moving, but overall
there is no change. This is a state of
dynamic equilibrium.
The rate of diffusion is affected by:
1. Temperature – increasing the temperature gives the particles more kinetic energy, so the
rate of random movement increases, so rate of diffusion increases.
2. Concentration Gradient – Increased concentration gradient (a greater difference in
concentration gradient between the two sides) increases the rate of diffusion.
3. Movement – stirring a liquid increases the movement of molecules and so the rate of
diffusion.
4. Distance – if the membrane is thinner, rate of diffusion is quicker because there is a smaller
distance for the molecules to travel.
5. Size of molecule – smaller molecules diffuse more quickly than larger ones.
B) Facilitated Diffusion
Facilitated diffusion is when protein molecules are involved in the
movement of large or charged particles.
Facilitated Diffusion with Carrier Proteins moves larger molecules like
glucose and amino acids. They are a specific shape allowing one molecule
to fit, and the protein then changes shape to release the molecule.
Molecules can be carried in either direction according to the concentration gradient.
Facilitated Diffusion with Protein Channels moves charged molecules like sodium and calcium
ions. They are effectively pores in the membrane. Some are gated: at a synapse, a transmitter
substance binds to a receptor protein, which will then open the channel protein.
Active Processes – C) Active Transport
Active transport is the movement of molecules from an
area of low concentration to an area of a higher
concentration up a concentration gradient.
Sometimes, a cell cannot meet its needs by diffusion. A
plant cell may need more magnesium ions for
photosynthesis. It needs to be able to move these ions
into the cell against a concentration gradient.
Some of the carrier proteins found in plasma
membranes acts as pumps. Their shape is
complementary to the molecule they carry. These
protein pumps can only carry molecules one way across the plasma membrane, and they use
metabolic energy in the form of ATP to change their shape. By changing the shape using ATP, the
active transport protein ensures that the molecule can only go one way.
Calcium Ion movement in muscles – muscle fibres can only
contract if calcium ions are present. When a muscle is stimulated
to contract, calcium ions are released from specialised
endoplasmic reticulum, where they are in high concentration.
When the muscle needs to relax again, the calcium ions are
pumped back into the stores by the calcium ion pumps in the
plasma membranes of the specialised endoplasmic reticulum.
Bulk Transport – Endocytosis and Exocytosis
Endocytosis and Exocytosis are processes which bring large amounts of materials in (endo) or out
(exo) of the cell. This bulk transport is made possible because plasma membranes can easily fuse,
separate and pinch off vesicles. Lots of energy from ATP is required to form the vesicles that are
needed and to move the vesicles around the cell.
White blood Cells – engulf
invading microorganisms by
forming a vesicle around them.
This vesicle then fuses with the
lysosome so that the enzymes in
there can digest the
microorganism. These cells are
called Phagocytes.
Hormones – Pancreatic cells
make insulin in large
quantities. The insulin is
processed and packaged into
vesicle by the Golgi
apparatus, and these vesicles
then fuse with them
membrane to release insulin
into the blood.
D) Osmosis - Osmosis is the diffusion of water from an area of high water potential to an area
of low water potential.
Random movement of
molecules directly through
the Phospholipid Bilayer from
an area of high water
potential to an area of low
water potential.
Water potential is the desire of water to move out of a solution. It is measured in units of pressure –
Kpa (kilopascals). Its symbol is the Greek letter Psi, ψ. It could be thought of as the concentration of
free water molecules – the more free water, the higher the water potential.
In this solution, each water
molecule has a solute
molecule. The ratio between
water and solute is 1:1. This
means that the water does not
want to leave the solution, and
so water potential is low.
In this solution, each water molecule does
not have a solute molecule. The ratio
between water and solute is 3:1. This means
that the water does want to leave the
solution, and so water potential is high.
There are lots of water molecules which are
not attracted to solute molecules, and so can
leave the solution.
Osmosis takes place until there is no longer a water potential gradient. If both have equal water
potentials, there is no overall movement. Only water can move in osmosis, never the solute. The
movement of water is the thing which changes the volume of the cell. The cell can increase or
decrease in size according to the direction of water movement.
In pure water or a hypotonic solution with a very high water potential, water moves into the cell by
osmosis down a water potential gradient. Plant cell membrane will be pushed against the wall – the
cell will be turgid. An animal cell will burst open - it is haemolysed.
In a very concentrated solution with a very low water potential, water will move out of the cell by
osmosis down a water potential gradient. A plant cell’s plasma membrane will pull away from the
wall –the cell will be plasmolysed. An animal cell will shrink and wrinkle – it will be crenated.
In a hypotonic solution, there is less
solute, making it dilute (less
concentrated). This means there is more
free water, and so there is higher water
potential. Water moves from an area of
high water potential outside the cell to
the low water potential inside of the cell,
meaning that the cell volume increases.
An isotonic
solution will
have the same
water potential
as the cell, and
so no net
movement
occurs.
In a hypertonic solution, there is more
solute, making it more concentrated.
This means there is less free water, and
so there is lower water potential. Water
moves from an area of high water
potential inside the cell to the low water
potential outside of the cell, meaning
that the cell volume decreases.
Pure Water – no solute dissolved
Highest Water Potential – 0kPA
Dilute Solution – a little solute dissolved
Lower Water Potential – -10kPA
Concentrated Solution – lots of solute dissolved
Lower Water Potential – -500kPA
The plasma membrane forms a selectively permeable barrier around every cell. Signals must first
negotiate the plasma membrane before they can communicate with the nucleus. Chemicals which
change a cell’s behaviour are drugs and hormones. These signal molecules can either be made form
Lipids or Proteins.
Lipids are hydrophobic and so can
diffuse freely through the
Phospholipid Bilayer. Their target is
inside the cytoplasm, not on the
membrane.
Proteins are large molecules which
cannot pass through the Phospholipid
Bilayer, so their target is on the
outside of the membrane. Insulin is an
example.
There are 2 ways in which signals can be sent across membranes:
1. Receptors acting as ion channels:
Neurotransmitters such as acetylcholine attach to the receptors on the channel proteins. These
channel proteins only open when they are bonded to a
hormone signal molecule. When a signal molecule arrives at a
receptor, the channel opens, allowing things to pass into the
cell.
This is how nerve signals make muscles contract. Vesicles
containing a neurotransmitter are released from the nerve cell
by Exocytosis. These vesicles move to the plasma membrane of
the muscle cell. The neurotransmitter binds to a particular
channel protein in the muscle membrane, causing the channel
to open. Ca+ ions can then rush into the muscle, causing it to
become positive and contract.
2. Receptors acting as enzymes:
Intrinsic proteins have receptor sites for drugs and hormones,
with inactive enzymes attached to one side. When a protein
hormone or drug attaches to the receptor site, the proteins
are dragged closer together (dimerisation) and the enzymes
switch on.
This is how insulin reduces blood glucose: the insulin protein hormone binds to its glycoprotein
receptor. This causes the enzyme attached to the glycoprotein on the inside of the membrane to
activate. This starts other reactions inside the cell. It opens another channel protein which allows
glucose into the muscle, thereby reducing blood glucose, and converts glucose into fat and glycogen
for storage.
Temperature and the Permeability of Membranes
Any passive process (diffusion, facilitated diffusion, osmosis) is speeded up by an increased
temperature, because particles have a greater kinetic energy.
At lower temperatures, the membrane’s phospholipids do not
have very much kinetic energy. This means that there are no gaps
that large and charged molecules can fit through.
At higher temperatures, the membrane’s phospholipids have a
greater amount of kinetic energy. This means that larger
temporary gaps are created between them, increasing the chance
that large and charged molecules can pass through the
Phospholipid Bilayer.
If an organism lives in a warm environment, their membranes will have
more cholesterol between the phospholipids. This gives stability to the
membrane, and stops as many gaps being created.