The document summarizes the main mechanisms of transport across the cell membrane: passive transport and active transport. Passive transport includes osmosis, simple diffusion, and facilitated diffusion, and does not require energy. Active transport requires the cell to use energy to move molecules against a concentration gradient. The document also describes types of solutions (hypotonic, hypertonic, isotonic) and how they affect cell volume. Diffusion is defined as the net movement of substances from an area of high concentration to low concentration down a concentration gradient.

2. MECHANISMS OF TRANSPORTACROSS THE CELL
MEMBRANE

3. Solute move across the plasma membrane
by two main process:
a) Passive transport- which does not require
a cell to use energy.
b) Active transport- which requires a cell to
use energy to move molecules through its
cell membrane.
Types of transport across the plasma membrane

4. • Passive transport consists of:
a) Osmosis is the special name given to simple
diffusion of water molecules through a semi-
permeable membrane.
b) Simple diffusion and
c) Facilitated diffusion (aided by carrier protein or
channel protein of the plasma protein).
Passive Transport
• Passive transport is a means of moving biochemicals,
and other atomic or molecular substances, across
membranes.
• This process does not involve chemical energy.
• Passive transport is dependent on the permeability
of the cell membrane, which, in turn, is dependent
on the organization and characteristics of the
membrane lipids and proteins.

5. • Osmosis is the movement of water molecules from region of high
water concentration to low water concentration through a semi-
permeable membrane.
• Osmosis is the diffusion of water only and not of the substances
that dissolved in water.
OSMOSIS

6. HYPOTONIC, HYPERTONIC& ISOTONICSOLUTION
• The direction of movement of substances
across the plasma membrane in the cell
depends on the concentration of the
solution around it.
• There are 3 types of solution; hypotonic,
hypertonic & isotonic solution.

7. HYPOTONICSOLUTION:
• A solution that contains a LOWER concentration of
solute molecules (HIGHER concentration of water
molecules) than the other solution (cell).
• Cell expands (or may burst)

8. HYPERTONICSOLUTION:
• A solution that contains a HIGHER concentration of
solute molecules (LOWER concentration of water
molecules) than the other solution (cell).
• Cell shrinks

9.  ISOTONICSOLUTION:
A solution has the SAME solute concentration (EQUAL
water concentration) than the other solution (cell).

10. Isotonic, Hypertonic, and Hypotonic Solutions

11. Cell membranes are permeable to water,
therefore, the environment the cell is exposed
to can have a dramatic effect on the cell.

12. Diffusionis the net movement of material from an area of high
concentration of that substance to an area with lower
concentration of that substance.

13. Simple diffusion
• Simple diffusion is the random movement of
ions or molecules from a region of their high
concentration to a region of their low
concentration down a concentration gradient
until an equilibrium is achieved.
• Molecules have kinetic energy, move randomly,
and collide with each other.
• There are more collision in a region of high
concentration than in a region of low
concentration.
• Random collision of molecules spread the
molecules out, down the concentration
gradient.
Concentration gradient = difference in
concentration of a particular substance in
one region compared to another region

14. • Simple diffusion of lipid-soluble substances can take place
through the lipid bilayer, its rate dependent on how highly
lipid soluble it is (e.g. oxygen, carbon dioxide, nitrogen,
alcohol).
• Water & lipid-insoluble substances simply diffuse through
protein channels, the number and size of openings available
determining its rate.
• The protein channels involved in simple diffusion are
distinguished by 2 important characteristics:
1. They are often selectively permeable to certain substances.
2. Many of the channels can be opened or closed by gates.

15. • Fick’s Law of Diffusion:
The net diffusion rate of a gas across a
fluid membrane is proportional to the difference
in concentration, to the surface area of the
membrane, to the permeability of the membrane
to the substance and inversely proportional to
the thickness of the membrane and molecular
weight of the molecule.

16. Factors affecting the rateof diffusion

17. c) Eventually the
molecules spread
throughout the water
uniformly.
Purple
solution
A sample experiment to illustrate the physical process of diffusion
b) The molecules
diffuse out along the
concentration gradient
in all direction.
a) The crystals starts to
dissolve, forming a region of
high concentration of solute
molecules.
Distilled water
Potassium
manganate (vii)
crystals

18. References:
• Nelson, Philip (December 16, 2013). Biological Physics.
W.H. Freeman.
• Diffusion Processes, Thomas Graham Symposium, ed.
J.N. Sherwood, A.V. Chadwick, W.M.Muir, F.L. Swinton,
Gordon and Breach, London, 1971.
• Jump up^ L.W. Barr (1997), In: Diffusion in Materials,
DIMAT 96, ed. H.Mehrer, Chr. Herzig, N.A. Stolwijk, H.
Bracht, Scitec Publications, Vol.1,