Histology

1. D E PA RT M E N T O F P H Y S I O L O G Y,
C O L L E G E O F M E D I C I N E ,
E K I T I S TAT E U N I V E R S I T Y.
PHS 201
CELL MEMBRANE TRANSPORT
MECHANISM.
Dr. (Mrs) M.A. Adefisayo

2. Cell Membrane Transport Mech.
 Cell transport is the movement of materials across the cell
membrane.
 Cell transport includes active transport and passive transport.
Passive transport does not require energy whereas active
transport requires energy to proceed.
 Passive transport proceeds through diffusion, facilitated diffusion
and Osmosis.

3. Diffusion
 Regarding cells, diffusion is usually defined in terms of
concentration of ions and molecules in the interior (Intracellular)
and exterior (extracellular) compartments of the cell.
 The extracellular and intracellular fluid is made up of solutes,
ions and molecules which differs in concentration.
 This differences cause a movement of ions/molecules from the
compartments with high concentration gradients to the
compartments with low concentration gradients.

4. Diffusion Cont’d
 Diffusion is of two types: Simple diffusion and facilitated
diffusion.
 The concentration differences across the membrane is
responsible for the simple diffusion and the required energy is
not in excess of the kinetic energy of motion.
 Simple diffusion through the cell membrane can occur through
either of two pathways;
a) Watery channels in transport proteins for water soluble ions
and molecules.
b) Interstices of the lipid bilayer for ions and molecules which
are insoluble in water but soluble in lipids.

5. Diffusion Cont’d
 In facilitated diffusion, the diffusing particles are unable to cross
the cell membrane due to their large size and insolubility in lipids
e.g glucose and amino acids.
 Special proteins called ‘carrier proteins’ aid the particles to cross
the membrane by combining with them to release them to the
other side.
 The rate of facilitated diffusion can be limited by the number of
carrier proteins present on the membrane.

6. Diffusion Cont’d
 The net rate of diffusion is important in this mechanism, this is
because particles that move from one side of a membrane to the
other can still move back in the counter direction. This is because
the concentration gradient is what determine the direction of
movement. The factors that influence the net rate of diffusion are
listed;
 Concentration difference: The magnitude of the concentration
difference determines the net rate of diffusion. Net diffusion as
regards to conc. Difference can be calculated as; Net diffusion =
diffusion coefficient x (concentration outside - concentration
inside) or Net diffusion = D.(C1 – C2).

7. Diffusion Cont’d
 Difusion Coefficient: It is determined by multiplying the rate of
diffusion in a single unit of the membrane (P) by the total area of
the membrane (A). D = P x A.
 Membrane Permeability: It is the rate at which an ion or
molecule diffuses across a single point of the membrane. It can be
affected by thickness of membrane, temperature, availability of
membrane channels, molecular weight of particle and lipid
solubility of the particle.

8. Diffussion Cont’d
 Pressure Difference: Pressure in this process is the sum of all the
forces exerted by the different ion/molecules at a unit surface area
of the membrane at a given time. The difference in the pressure
exerted on both sides may influence the net rate of diffusion.
 Electrical Potential Difference: Ions move from areas of greater
electrical potential to areas of less electrical potential, regardless
of concentration differences

9. OSMOSIS
 Osmosis involves the solvent instead of solute and the
membranes in this case are semi-permeable i.e the membranes
only allow selected particles to pass through.
 Osmosis is therefore defined as the movement of water
moleculed from a region of higher concentration of water to a
region of lower concentration of water.

10. Osmotic Pressure
 Osmotic pressure is the pressure exerted by the non-diffusible
particles; which impede osmosis from the side where water is
more concentrated to the area with less concentration of water.
 The terms isotonic, hypotonic and hypertonic are used to define
the level of concentration of body fluids compared to water.
 Solutions of the same osmotic pressure with body fluids are said
to be isotonic. A solution with less osmotic pressure than body
fluids are said to be hypotonic while solutions with higher
osmotic pressure than body fluids are called hypertonic.

11. ACTIVE TRANSPORT
 Active transport is the movement of ions and molecules from a
region of lower concentration to a region of higher concentration
across the cell membrane with the use of energy as well as carrier
proteins.
 The energy used is supplied from cellular metabolism and a
cell may use up to 40% of it’s generated energy to serve active
trasport processes.
 The carrier proteins transport particles from either side of the
membrane and continue to do so until the need for transport
ceases.

12. Endocytosis
 Endocytosis is a process by which a large cell membrane can
move large particles from the intracellular fluids outwards
(exocytosis) or from the extracellular fluid inwards (pinocytosis).
Both processes are energy driven.
 In pinocytosis, cells take in tiny droplets of fluid in company of
large particles from the extracellular fluid inward. When this
happens, a small part of the cell membrane invaginates and forms
a sealed small vesicle of about 0.1 micrometre. The vesicle then
detached from, the surface and indent into the cytoplasm
breaking in the process.
 This is called ‘cell drinking.’

13. Endocytosis
 Phagocytosis: It’s a form of endocytosis in which solids are taken
into the cell. This process concerns the type of white blood cells
called phagocytes which takes in solid particles e.g bacteria. The
phagocyte attaches itself to the particle and then surround the
particles engulfing it. The particle then detach from the surface
into the cytoplasm and digestive enzymes decomposes and
destroys the particle.
 Exocytosis: Particles formed within the cell are packaged within
a vesicle which fuses with the cell membane and is released to
the exterior of the cell.