The Plasma Membrane Examples of different membrane proteins

The Plasma Membrane Examples of different membrane proteins include § Ion channels § Carriers § Receptors

The Plasma Membrane Examples of different membrane proteins include § Enzymes § Linkers § Cell identity markers

The Plasma Membrane • Because of the distribution of lipids and the proteins embedded in it, the membrane allows some substances across but not others; this is called Selective permeability – Rule of thumb: small, neutrally-charged, lipidsoluble substances can freely pass. Water is a special case - it is highly polar, yet still freely permeable. courtesy of Dr. Jim Hutchins

Membrane Permeability • For those substances that are needed by the cell but for which the membrane is impenetrable (impermeable), transmembrane proteins act as channels and transporters. – They assist the entrance of certain substances that either can’t pass at all (glucose) or for which the cell needs to hasten passage (ions).

Transport Processes • Passive processes involve substances moving across the cell membranes without the input of any energy - they are said to move “with” or “down” their concentration gradient ([gradient] , where [ ] indicates “concentration”). • Active processes involve the use of energy, primarily from the breakdown of ATP, to move a substance against its [gradient].

Transport Processes • Passive processes – Diffusion of solutes – Diffusion of water (called osmosis) – Facilitated diffusion (requires a specific channel or a carrier molecule, but no energy is used) • Active processes – Various types of transporters are used, and energy is required.

Passive Transport Processes • Diffusion is the passive spread of particles through random motion, from areas of high concentration to areas of low concentration. – It is affected by the amount of substance and the steepness of the concentration gradient. – Temperature – Surface area – Diffusion distance

Passive Transport Processes • Channel-Mediated Facilitated Diffusion • Carrier-Mediated Facilitated Diffusion

Passive Transport Processes • An example of Channel-Mediated Facilitated Diffusion is the passage of potassium ions through a gated K+ Channel • An example of Carrier-Mediated Facilitated Diffusion is the passage of glucose across the cell membrane.

• Passive Transport Processes Osmosis is the net movement of water through a selectively permeable membrane from an area of high water concentration to one of lower water concentration. • Water can pass through plasma membrane in 2 ways: – through lipid bilayer by simple diffusion – through aquaporins (integral membrane proteins)

Passive Transport Processes In the third tube, the force generated by the movement of water from the left to the right side is called osmotic pressure.

Passive Transport Processes In the body, tonicity refers to the concentration of salt solutions in the blood and elsewhere. Since semipermeable membranes separate these fluid compartments, osmosis of water is free to occur between any fluid space and another. The effect of changing tonicity is demonstrated in this graphic, as water moves in and out of red blood cells.

Active Transport Processes Solutes can also be actively transported across a plasma membrane against their concentration gradient ([low] to [high]) by using energy (usually in the form of ATP). The sodium-potassium pump is found in all cells.

Secondary Active Transport Mechanisms • Antiporters carry two substances across the membrane in opposite directions. • Symporters carry two substances across the membrane in the same direction.