MECHANISMS OF TRANSPORT ACROSS THE CELL MEMBRANE

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

• 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 andc) 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.

• 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

HYPOTONIC, HYPERTONIC & ISOTONIC SOLUTION

• 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.

HYPOTONIC SOLUTION :• 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)

HYPERTONIC SOLUTION :• A solution that contains a HIGHER concentration of

solute molecules (LOWER concentration of water molecules) than the other solution (cell).

• Cell shrinks

ISOTONIC SOLUTION : A solution has the SAME solute concentration (EQUAL water concentration) than the other solution (cell).

Isotonic, Hypertonic, and Hypotonic Solutions

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

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

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

• 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.

• 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.

Factors affecting the rate of diffusion

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

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,