Cell membrane and Transport

CELL MEMBRANES
AND TRANSPORT

LO:

• describe and explain the fluid mosaic model of
membrane structure, including an outline of
the roles of phospholipids, cholesterol,
glycolipids, proteins and glycoproteins ;

• All living cells – surrounded by the cell surface
membrane

• Controls the exchange of materials
• Regulates the transport across the

membranes
• Receive messages

Phospholipids

- Made of two fatty acid tails and
glycerol head that contains a
phosphate group

- Hydrophilic head and
hydrophobic tails

- Can form little bags in which
chemicals are isolated from the
external environment = cells
and organelles

• When in water:

– Spread as a single layer of molecules on the
surface of water

– Forms micelles surrounded by water
– Forms bilayer
– Bilayers form membrane-bound compartments





Structure of membranes

• The bilayer is seen under the electron
microscope at very high magnification

• 1972 – fluid mosaic model

– Membrane made of phospholipids and proteins
that can move about by diffusion

– Phospholipids move sideways in their own layers
– Some proteins move as well
– Mosaic – the pattern made by the scattered

protein molecules when viewed from above



• Phospholipid bilayer

– Molecules move

– Tails inwards – form non-polar hydrophobic
interior; the more unsaturated they are, the more
fluid the membrane (because the tails are bent
and therefore fit more loosely); the longer the tail
the less fluid the membrane

– The lower the temperature the less fluid the
membranes are (some organisms respond by
increasing the number of unsaturated fatty acids)

– Heads – face the aqueous medium that surrounds
the membranes

• Proteins

• Integral proteins (intrinsic)

– In the inner layer, outer layer or spanning the
whole membrane (transmembrane proteins)

– Have hydrophilic and hydrophobic regions
– Mostly float like icebergs in the phospholipid

layer; some are fixed

• Peripheral proteins (extrinsic)

– Found on the inner or outer surface of the
membrane

• Cell markers = antigens – allow cell
recognition

• Enzymes – e.g. digestive enzymes in the
alimentary canal

• Transport proteins – form hydrophilic
channels or passageways for ions and polar
molecules to pass through the membrane

– Channel proteins
– Carrier proteins

• Carbohydrates

– Short branching chains of carbohydrates are
attached to the proteins and lipids

– On the side of the molecule which faces the
outside of the membrane

– Glycoproteins
– Glycolipid

• Glycoproteins and glycolipids

– Molecules on the outer surface of the membrane
that have short carbohydrate chains attached to
them

– The chains project like antennae into water fluids
where they form hydrogen bonds and so help
stabilise the membrane structure

– Can form a sugary coating = glycocalyx

– Glycoproteins and glycolipids act as receptor
molecules

• Signalling receptors – part of signalling system that
coordinates the activities of animal cells, this receptor
recognises messenger molecules like hormones and
neurotransmitters

• Receptors involved in endocytosis

• Receptors involved in binding cells to other cells in
tissues and organs

https://www.wisc-online.com/learn/natural-
science/life-science/ap1101/construction-of-
the-cell-membrane

TRANSPORT ACROSS THE CELL
SURFACE MEMBRANE

• Diffusion
• Facilitated diffusion
• Osmosis
• Active transport
• Bulk transport

LO:

• describe and explain the processes of
diffusion, facilitated diffusion, osmosis, active
transport, endocytosis and exocytosis

• investigate the effects on plant cells of
immersion in solutions of different water
potential;

DIFFUSION

• Net movement of a substance from a region of
its higher concentration to a region of its
lower concentration

• This movement is a result of random motion
of its molecules or ions; caused by the natural
kinetic energy or the molecules

• The molecules or ions move down the
concentration gradient

• Some molecules and ions are able to pass
through cell membranes by diffusion

Factors affecting the rate of diffusion

• The steepness of the concentration gradient

– Difference in concentration of the substance on the
two sides of the surface

– The greater the difference in concentration, the
greater the number of molecules passing in the two
directions – the faster the net rate of diffusion

• Temperature

– at high temperature, molecules have higher kinetic
energy than at low temperatures; they move around
faster = rate of diffusion higher

At a higher temperature the particles have more kinetic energy and are
moving around faster. Therefore in a given time more diffusion will occur.

• The surface area

– The greater the surface area the more ions or
molecules can cross it at any moment = faster
diffusion (microvilli, cristae)

• The nature of the molecules or ions

– Large molecules require more energy to get them
moving than small ones do = small molecules
diffuse faster

– Non-polar molecule diffuse much more easily
through cell membranes as they are soluble in the
non-polar phospholipid tails

• Three cubes of agar are prepared which contain
the indicator phenolphthalein.

• These1cmare placed in2cm hydrochloric3cmacid which will
diffuse into the cubes.

• As it diffuses in it will turn the indicator
colourless.
1cm
2cm
3cm

• As the size of the cube increases the surface
area to volume ratio decreases.

1cm
2cm
3cm
1cm 2cm
3cm

Width of cube Surface area Volume Surface area: volume
(cm) (cm2) (cm3)
1 6 1 6
3
2 24 8 2

3 54 27

• The cubes look like this after a few minutes.

1cm
2cm
3cm
1cm
• If real c2ecmlls then the b3icgmger cell would
these were

not have received what it needs to all parts of the

cell.

• Therefore it would need a bigger surface area in

order to rely on diffusion.

• Watch this video to see the experiment in
action.

• As the rate of diffusion relies on the surface
area.The parts of organisms that rely on
diffusion therefore tend to have a large
surface area.

FACILITATED DIFFUSION

• Diffusion with the help of certain protein
molecules

• Large molecules as glucose and amino acids,
sodium ions, chloride ions

• Channel proteins

– Water filled pores; allow charged substances to
diffuse; have fixed shape

– Most of them are gated = part of the protein
molecule on the inside surface of the membrane
can move to close or open the pore = control of
ion exchange

– Example: nerve cell surface membranes – one
type allows sodium ions – production of an action
potential; the other allows exit of potassium
during the recovery phase (Na-K pump)

• Carrier proteins

– Constantly flip between two shapes

– The binding site is alternately open to one side of
the membrane, than the other

– The rate of diffusion is affected by the number of
opened channel proteins

– Example: cystic fibrosis is caused by a defect in a
channel protein which should be present in the
membranes of the cells lining the lungs; this
protein allows chloride ions to move out of the
cells



OSMOSIS

• Special type of diffusion involving water
molecules only

• Solute + solvent = solution
• Sugar + water = sugar solution
• Partially permeable membrane is present

• Water moves from a dilute solution to a more
concentrated one across the partially
permeable cell membrane.

Rlawson at en.wikibooks

Water potential and solute potential

• Water potential = tendency of water to move
from one place to another

• Symbol for water potential (psi)
• Water always moves from a region of higher

water potential to a region of lower water
potential (down a water potential gradient)
• Equilibrium is reached when the water
potentials are equal

• OSMOSIS

– Net movement of water molecules from a region
of higher water potential to a region of lower
water potential through a partially permeable
membrane

– Pure water has the highest possible water
potential;

– Ψpure water= 0; solute potential is always negative

• OSMOSIS IN ANIMAL CELLS

• It is important to maintain a constant water
potential inside the bodies of animals

• In animal cells water potential = solute
potential

PRESSURE POTENTIAL

– The greater the pressure applied, the greater the
tendency for water molecules to be forced back
from solution B to solution A

– Increasing the pressure increases the water
potential of solution B

– Ψp = pressure potential
– Pressure potential makes the water potential less

negative, and is therefore positive

ACTIVE TRANSPORT

• Against the concentration gradient
• Achieved by carrier proteins – they are specific

for a particular type of molecule or ion
• Energy required – ATP (adenosinetriphosphate);

produced by respiration inside the cell
• Energy is used to change the shape of carrier protein

in the process
• The energy consuming transport of molecules or

ions across a membrane against a concentration
gradient

• SODIUM POTASSIUM PUMP - Na+ – K+ pump





• IMPORTANCE OF ACTIVE TRANSPORT

– In kidneys – re-absorption into the blood

– Absorption of some products of digestion in guts
– In plants – to load sugar from the

photosynthesising cells of leaves into the phloem
tissue; to load inorganic ions from the soil into
root hairs

BULK TRANSPORT

• Transport of large quantities of materials into
cells and out of cell

• Endocytosis – transport into the cells
• Exocytosis – transport out of the cells

• ENDOCYTOSIS

– Involves the engulf of the material by the cell
surface membrane to form a small sack

– PHAGOCYTOSIS – cell eating; specialized cells =
phagocytes; engulfing of bacteria by certain white
blood cells

– PINOCYTOSIS – cell drinking; uptake of liquid;
human egg cell takes up nutrients from cells that
surround it

Phagocytosis of a bacterium by a white blood cell

• EXOCYTOSIS

– Reverse of endocytosis – materials are removed
from cell

– E.g. secretion of digestive enzymes from cells of
the pancreas

– Secretory vesicles from the Golgi apparatus carry
the enzymes to the cell surface and release their
contents

– Plant cells use exocytosis to get their cell wall
building materials to the outside of the cell
surface membrane