Cell membranes and transport
• You should be able to:
• describe the structure of cell membranes
• explain the functions of the molecules that
• make up cell membranes
• explain how substances enter and leave cells
• find the water potential of plant tissues
• through experiment
• A liposome is a tiny bubble (vesicle), made out
of the same material as a cell membrane.
Liposomes can be filled with drugs, and used
to deliver drugs for cancer and other diseases.
• a microscopic spherical particle formed by a
lipid bilayer enclosing an aqueous
• The name liposome is derived from two Greek
words: 'Lipos' meaning fat and 'Soma'
• Liposomes were first described by British
haematologist Dr Alec D Bangham FRS in 1961
(published 1964), at the Babraham Institute,
Cell membrane It is composed of four different
types of molecules:
• Phospholipids form the bilayer, which is the basic
structure of the membrane.
• Because the tails of phospholipids are non-polar,
it is difficult for polar molecules, or ions, to pass
through membranes, so they act as a barrier to
most water-soluble substances.
• For example, water-soluble molecules such as
sugars, amino acids and proteins cannot leak out
of the cell, and unwanted water-soluble
molecules cannot enter the cell.
• Some phospholipids can be modified chemically to act
as signaling molecules.
• They may move about in the phospholipid bilayer,
activating other molecules such as enzymes.
• Alternatively, they may be hydrolyzed to release small,
water-soluble, glycerol-related molecules.
• These diffuse through the cytoplasm and bind to
• One such system results in the release of calcium ions
from storage in the ER, which in turn brings about
exocytosis of digestive enzymes from pancreatic cells.
• If the phospholipids are shaken up with water,
they can form stable ball-like structures in the
water called micelles.
• Here all the hydrophilic heads face outwards
into the water, shielding the hydrophobic tails,
which point in towards each other.
• The phospholipid bilayer is visible using the
electron microscope at very high
magnifications of at least × 100 000.
• Cholesterol molecules are made up of four rings of
hydrogen and carbon atoms. They are hydrophobic and
are found among the hydrophobic tails in the lipid
• Cholesterol molecules are important for maintaining
the consistency of the cell membrane.
• They strengthen the membrane by preventing some
small molecules from crossing it.
• Cholesterol molecules also keep the phospholipid tails
from coming into contact and solidifying. This ensures
that the cell membrane stays fluid and flexible.
• A channel protein is a protein that allows the
transport of specific substances across a cell
• They are intrinsic proteins, so span across the
whole membrane They basically make a
channel/pathway/hole for stuff to go through
• The channel that the channel proteins make,
is full of water. This means only water soluble
substances can pass through.
• carrier proteins bind solute on one side of a
membrane, undergo conformational changes,
and release them on the other side of the
• These proteins can mediate both active and
• carrier proteins are glycoproteins.
Fluid mosaic model.
• In 1972, two scientists, Singer and Nicolson,
used all the available evidence to put forward
a hypothesis for membrane structure. They
called their model the fluid mosaic model.
• It is described as ‘fluid’ because both the
phospholipids and the proteins can move
about by diffusion
• The phospholipid bilayer has the sort off fluidity.
• The phospholipids move sideways, mainly in their own
• Some of the protein molecules also move about within
the phospholipid bilayer, like icebergs in the sea.
• Others remain fixed to structures inside or outside the
• The word ‘mosaic’ describes the pattern produced by
the scattered protein molecules when the surface of the
membrane is viewed from above.
• The individual phospholipid molecules move
about by diffusion within their own
• The phospholipid tails point inwards, facing
each other and forming a non-polar
• The phospholipid heads face the aqueous
medium that surrounds the membranes.
• If the lipids are more unsaturated the more
fluid the membrane.
• This is because the unsaturated fatty acid tails
are bent and therefore fit together more
• Fluidity is also affected by tail length: the
longer the tail, the less fluid the membrane.
• As temperature decreases, membranes
become less fluid,
Some organisms which cannot regulate their
own temperature, such as bacteria and yeasts,
respond by increasing the proportion of
unsaturated fatty acids in their membranes.
• Extrinsic membrane proteins are entirely
outside of the membrane.
• Intrinsic membrane proteins are embedded in
the membrane. Many of them extend from
one side of the membrane to the other and
are referred to as transmembrane proteins.
• Most of the intrinsic protein molecules float
like mobile icebergs in the phospholipid layers,
although some are fixed like islands to
structures inside or outside the cell and do not
• second type of protein molecule is the
extrinsic protein (or peripheral protein). These
are found on the inner or outer surface of the
Glycoproteins and Glycolipids
• Lipid and proteins on the cell membrane
surface often have short carbohydrate chains
protruding out from the cell surface, known as
glycolipids and glycoproteins.
• The carbohydrate chains form a sugary coating
to the cell, known as the glycocalyx.
• It provides cushioning and protection for the
plasma membrane, and it is also important in
cell recognition. Based on the structure and
types of carbohydrates in the glycocalyx, your
body can recognize cells and determine if they
should be there or not. The glycocalyx can also
act as a glue to attach cells together.
• Some glycolipids and glycoproteins act as cell
markers or antigens, allowing cell–cell
• Each type of cell has its own type of antigen,
rather like countries with different flags.
Cells communicate by sending and receiving
chemical and electrical signals.
This molecular conversation allows the cells in
your body to coordinate their activities.
• Cell-cell signaling involves the transmission of
a signal from a sending cell to a receiving cell.
However, not all sending and receiving cells
are next-door neighbors.
• Cells communicate by means of extracellular signaling
molecules that are produced and released by signaling
• These molecules recognize and bind to receptors on
the surface of target cells where they cause a cellular
response by means of a signal transduction pathway.
• Cells have proteins called receptors that bind to
signaling molecules and initiate a physiological
• Different receptors are specific for different molecules.
• Typical signaling pathway starts with the signal
arriving at a protein receptor in a cell surface
• The receptor is a specific shape which recognizes
the signal. Only cells with this receptor can
recognize the signal.
• The signal brings about a change in the shape of
• Changing the shape of the receptor allows it to
interact with the next component of the pathway,
so the message gets transmitted.
• G proteins, also known as guanine nucleotide-
binding proteins, are a family of proteins that
act as molecular switches inside cells, and are
involved in transmitting signals from a variety
of stimuli outside a cell to its interior.
• G proteins acts as a switch to bring about the
release of a ‘second messenger’,
• Apart from the secretion of chemical signals,
direct cell-cell contact is another mechanism
• This occurs, for example, during embryonic
development and when lymphocytes detect
foreign antigens on other cells.
Movement of substances into
and out of cells
• A phospholipid bilayer around cells makes a
very effective barrier, particularly against the
movement of water-soluble molecules and
• The aqueous contents of the cell are therefore
prevented from escaping.
• There are five basic mechanisms by which this
exchange is achieved:
• Facilitated diffusion
• Active transport and bulk transport.
• The process by which molecules spread from
areas of high concentration, to areas of low
concentration in order to reach equilibrium.
• The rate at which a substance diffuses across a
membrane depends on a number of factors.
• The ‘steepness’ of the concentration gradient
- The greater the difference in concentration,
the greater the difference in the number of
molecules passing in the two directions, and
hence the faster the rate of diffusion.
• Temperature. At high temperatures, molecules
and ions have much more kinetic energy than
at low temperatures.
• They move around faster, and thus diffusion
takes place faster.
• The surface area across which diffusion is
• The greater the surface area, the more
molecules or ions can cross it at any one
moment, and therefore the faster diffusion
• The nature of the molecules or ions - Large
molecules require more energy to get them
moving than small ones do, so large molecules
tend to diffuse more slowly than small molecules.
• Non-polar molecules, such as glycerol, alcohol
and steroid hormones, diffuse much more easily
through cell membranes than polar ones,
because they are soluble in the non-polar
• The respiratory gases – oxygen and carbon
dioxide – cross membranes by diffusion.
• They are uncharged and non-polar, and so can
cross through the phospholipid bilayer directly
between the phospholipid molecules.
• Water molecules, despite being very polar,
can diffuse rapidly across the phospholipid
bilayer because they are small enough.
• Facilitated diffusion is the diffusion of a
substance through transport proteins in a cell
membrane; the proteins provide hydrophilic
areas that allow the molecules or ions to pass
through the membrane which would
otherwise be less permeable to them.
• There are two types of protein involved,
namely channel proteins and carrier proteins.
Each is highly specific, allowing only one type
of molecule or ion to pass through it.
• Channel proteins are water-filled pores. They
allow charged substances, usually ions, to
diffuse through the membrane.
• Most channel proteins are ‘gated’. This means
that part of the protein molecule on the inside
surface of the membrane can move to close or
open the pore, like a gate.
• This allows control of ion exchange.
• channel proteins have a fixed shape, carrier
proteins can flip between two shapes.
• As a result, the binding site is alternately
open to one side of the membrane, then the
• The rate at which this diffusion takes place is
affected by how many channel or carrier
protein molecules there are in the membrane,
and, in the case of channel proteins, on
whether they are open or not.
• is caused by a defect in a channel
protein that should be present in
the cell surface membranes of
cells lining the lungs.
• This protein normally allows
chloride ions to move out of the
• If the channel protein is not
• correctly positioned in the
membrane, or if it does not
• open the chloride channel as and
when it should, then the
• chloride ions cannot move out.
• Osmosis is a special type of diffusion involving
water molecules only.
• This movement of water molecules from a
dilute solution to a concentrated solution,
through a partially permeable membrane, is
• The tendency of water to move from one area
to another due to osmosis, gravity, mechanical
pressure, or matrix effects such as capillary
• Water potential is the 'measure of the ability of water
molecules to move freely in solution'.
• All this means is that is a solution of pure water where
there is no solute, all of the water molecules are free to
move, so the water potential is high.
• If a solute is added to the solution, it is attracted to the
water molecules, so those water molecules can no
longer move freely and the water potential is lower.
• In osmosis, water molecules move down the water
potential gradient, from a high water potential to a
lower water potential.
Factors influencing the water potential
• The major factors influencing the water
• Water always moves from a region of high
water potential to a region of low water
• This will happen until the water potential is
the same throughout the system, at which
point we can say that equilibrium has been
• A solution containing a lot of water (a dilute
solution) has a higher water potential than a
solution containing only a little water (a
• Osmosis is the net movement of water
molecules from a region of higher water
potential to a region of lower water potential,
through a partially permeable membrane.
Solute potential Ѱs
• The presence of solute molecules in a solution
lowers its water potential.
• The greater the concentration of solutes the
lower is the water potential.
• This change in water potential as a
consequence of the presence of solute
molecules is called Solute potential.
Pressure potential Ѱp
• Pressure potential is based on mechanical pressure.
• Pressure potential increases as water enters a cell.
• As water passes through the cell wall and cell
membrane, it increases the total amount of water
present inside the cell, which exerts an outward
pressure that is opposed by the structural rigidity of
the cell wall.
• By creating this pressure, the plant can maintain turgor,
which allows the plant to keep its rigidity.
• Without turgor, plants will lose structure and wilt.
• If the water potential of the solution
surrounding the cell is too high, the cell swells
• If it is too low, the cell shrinks, This shows one
reason why it is important to maintain a
constant water potential inside the bodies of
Osmosis in Plant cells
• Plant cells are surrounded by rigid cellulose walls,
(unlike animal cells), but plant cells still take in
water by osmosis when placed in pure water.
• However, plant cells do not burst because their
cellulose cell walls limit how much water can
move in. The cell walls exert pressure, called
turgor pressure, as the cells take up water. This is
useful as plants do not have a skeleton.
• For plant cells water potential is a
combination of solute potential and pressure
• This can be expressed in the following
• ψ = ψs + ψp
• The living part of the cell in plant cells I.e.,
excluding cell wall.
• The contraction of the protoplasm of cells
within plants due to the loss of water through
osmosis. It is when the cell membrane peels
off of the cell wall and the vacuole collapses
when placed in a hypertonic environment.
• The stage of plasmolysis at which the first sign
of shrinkage of cell contents from cell wall
becomes detectable is called incipient
plasmolysis. At incipient plasmolysis the
protoplast has just ceased to exert any
pressure against the cell wall, so the cell is
• Active transport describes what happens
when a cell uses energy to transport
• Active transport is against concentration
gradient, means they are pumping something
from areas of lower to higher concentration.
• The energy required for active transport is
supplied by the molecule ATP.
• The energy is used to make the carrier protein
change its shape, transferring the molecules
or ions across the membrane in the process.
sodium–potassium (Na+ – K+) pump
• The Sodium-Potassium Pump uses energy to
transport Sodium and Potassium ions in and
out of the cell. It plays a vital role in
maintaining a cell's homeostasis.
• The pump is powered by a molecule of ATP.
The ATP allows the shape of the pump to
change, emptying its contents either into or
out of the cell.
• Three sodium ions form inside the cell bind to the
• The Phosphate group form a molecule of ATP
binds to the pump.
• The pump changes shape and the sodium ions
are released outside the cell.
• Two potassium ions bind to the pump.
• The phosphate group is released form the pump
and the pump again changes shape and releases
the ions into the inside of the cell.
• Active transport is important in reabsorption in
the kidneys, where certain useful molecules and
ions have to be reabsorbed into the blood after
filtration into the kidney tubules.
• It is also involved in the absorption of some
products of digestion from the gut.
• In plants, active transport is used to load sugar
from the photosynthesizing cells of leaves into
the phloem tissue for transport around the plant
and to load inorganic ions from the soil into root
• Exocytosis is the reverse of endocytosis and is
• by which materials are removed from cells