1. 8. MEMBRANE TRANSPORT
1.4 – Particles move across
membranes by simple diffusion,
facilitated diffusion, osmosis
and active transport
1.4 – The fluidity of membranes
allows materials to be taken
into cells by endocytosis or
released by exocytosis. Vesicles
move materials within cells.

2. PASSIVE AND ACTIVE TRANSPORT
o There are two general types of cellular transport
o Passive transport
o Active transport
o Active transport requires energy (in the form of ATP), whereas
passive transport does not
o Passive transport occurs in situations where there are areas of
different concentrations of a particular substance. Movement of the
substance will occur from where it is in high concentration to where it
is in low concentration (along/down a concentration gradient. This
does not require cellular energy.
o When active transport occurs, the substance is moved against a
concentration gradient – movement occurs from a region where it is
in low concentration to a region where it is in high concentration.
This requires energy.

3. PASSIVE TRANSPORT: SIMPLE
DIFFUSION
o Movement of a solid, liquid or gas along a concentration gradient
o The rate of diffusion changes depending on the concentration
gradient
o The greater the difference in concentration between the two
regions the faster the diffusion
o Heat can also speed up the rate of diffusion. Why?
Examples
o Oxygen diffuses from the air into cells in the lungs and carbon
dioxide diffuses out
o Oxygen diffuses from the cells in the lungs into blood capillaries
and carbon dioxide diffuses out
o Anaesthetics & alcohol

4. PASSIVE TRANSPORT: FACILITATED
DIFFUSION
o Some larger molecules and electrically charged molecules move too slowly
by simple diffusion to satisfy the cells needs
o To overcome this problem, proteins in the cell membrane act as membrane
transporters moving them across the membrane
o If this movement is along a concentration gradient and no energy is
required & it is called facilitated diffusion
o Proteins involved in facilitated diffusion can either be channel proteins or
carrier proteins
o Channel proteins – span the cell membrane and open and close to let
substances pass through
oE.G. Potassium channels in axons (nerve cells)
o Carrier proteins – form a bond with the molecules and then change shape
to transport the molecule

6. PASSIVE TRANSPORT: OSMOSIS
o The movement of water through cell membranes is
passive, but because water is a charged molecule it
needs to move through membrane pores – called
aquaporins
o The rate of water movement through the membrane is
affected by two things:
1. Solute concentration – the concentration of dissolved
substances in the solution
2. Opposing physical pressure exerted on the water
i.e. water is only able to continue to move into a cell if the
cell is able to physically expand to allow space for the
incoming water

7. NB: If cells are placed in a solution of known osmolarity there are
three possibilities: the cells may gain mass, the cells may lose
mass or the cells may remain the same mass

8. SOLUTIONS
o A solution is a mixture made up of a solvent (e.g. water) in which a
solute (e.g. salt) is dissolved
o Hypertonic – used to describe the solution with the larger amount of
dissolved substances
o Hypotonic – used to describe the solution with the least amount of
dissolved substances
o Isotonic - If both solutions have the same concentration of solutes

9. NB: Tissues or organs to be
used in medical procedures
must be bathed in a solution
with the same osmolarity as
the cytoplasm to prevent
osmosis. WHY?

10. REVISION: SIZE AND CHARGE
o How easily a substance can move across a membrane passively
depends on two major factors: size and charge
o Substances that are small with no charge will move across easily.
E.g. oxygen, carbon dioxide, nitrogen
o Substances that are large, have a charge or both do not cross
membranes easily. E.g. glucose, potassium ions, chloride ions
Low Permeability High Permeability
- Anything that can dissolve in
water (water soluble)
- Anything with an electrical
charge
- Anything large in size
- Anything that can dissolve in
lipids (lipid soluble)
- Anything with a no charge (e.g.
CO2 and O2)
- Water (because of pores)

11. KIDNEY DIALYSIS
o A practical example of diffusion and osmosis is kidney dialysis
o Problems in regulating the solutes in many body spaces can arise as
the result of some sort of irregularity in the function of the kidneys.
This can be life threatening and so dialysis is carried out
o Blood is passed through a system of tubes composed of selectively
permeable membranes. These tubes are surrounded with a solution
that contains key solutes at levels close to the patient’s normal blood
levels. Wastes are kept at a low level in this solution. As blood moves
through the tubes the solution is replaced to keep waste levels low.

12. ACTIVE TRANSPORT
o Active transport requires energy (ATP) because it involves the
movement of substances against a concentration gradient
o This allows a cell to maintain interior concentrations of molecules
that are different from exterior concentrations
o E.G. Animal cells have a much higher concentration of potassium ions than their
exterior environment, where as sodium ions are more concentrated in the exterior
environment than in the cells. The cell maintains these conditions by actively
pumping sodium ions out and potassium ions in. The carrier protein responsible for
this is called The sodium-potassium pump.

13. ACTIVE TRANSPORT – SODIUM-
POTASSIUM PUMP
o The sodium potassium pump has five stages:
1. The specific protein in the phospholipid bilayer opens to the
intracellular side and attaches three sodium ions

14. ACTIVE TRANSPORT – SODIUM-
POTASSIUM PUMP
2. ATP attaches to the same protein. ATP has three phosphate groups
attached and during this process it looses one phosphate group –
called phosphorylation - this releases energy and is called
phosphorylation. The ATP now becomes ADP.

15. ACTIVE TRANSPORT – SODIUM-
POTASSIUM PUMP
3. The energy provided from the phosphorylation of ATP to ADP (step
2) causes the protein to change shape so that the protein is now open
to the exterior of the cell and closed to the interior of the cell. Sodium
is released to the exterior. ADP is released (but one phosphate group
remains attached to the protein).

16. ACTIVE TRANSPORT – SODIUM-
POTASSIUM PUMP
4. Two extracellular potassium ions bind to different regions of the
protein and this causes the release of the attached phosphate group
(likely through a conformational change)

17. ACTIVE TRANSPORT – SODIUM-
POTASSIUM PUMP
5. The loss of the phosphate group (step 4) restores the proteins
original shape where it is open to the interior. Potassium can then be
released into the interior
NB: Notice how
the pump is
exchanging
three positive
ions and only
receiving two
in return. Does
this have an
overall effect
on the charge
of a cell??

18. ACTIVE TRANSPORT – VESICULAR
TRANSPORT
o This type of transport involves vesicles (sac with a membrane made
up of phospholipids)
o This allows larger molecules to move across the plasma membrane
o There are two main types:
o Endocytosis – allows large molecules to enter the cell
o Exocytosis – allows large molecules to leave the cell
o Both processes depend on the fluidity of the plasma membrane

19. ACTIVE TRANSPORT – VESICULAR
TRANSPORT
Endocytosis
o Occurs when a portion of the plasma membrane
is pinched off to enclose a molecule.
o This pinching off involves a change in the shape
of the membrane which forms a vesicle that
enters the cytoplasm of the cell
o This could not occur if the plasma membrane
did not have a fluid nature
o e.g. Phagocytosis – by white blood cells to
consume foreign objects

20. ACTIVE TRANSPORT – VESICULAR
TRANSPORT
Exocytosis
o This is essentially the reverse of endocytosis.
Vesicles fuse with plasma membrane and eject
contents from the cell.
o Examples of exocytosis occur when: pancreas cells
produce insulin and secrete it into the blood stream to
regulate blood glucose levels; neurotransmitters are
released at synapses in the nervous system
o One example of cell exocytosis involves proteins
produced in the cytoplasm of a cell.

22. SUMMARY
In Da Club – Membranes & Transport

23. PRAC - OSMOLARITY
1.4 – Estimation of
osmolarity in tissues by
bathing samples in
hypotonic and hypertonic
solutions