The Plasma MembraneExamples of different membrane proteins

include

Ion channels

Carriers

Receptors

The Plasma MembraneExamples of different membrane proteins

include

Enzymes

Linkers

Cell identity markers

• 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, lipid-

soluble substances can freely pass. Water is a special case - it is highly polar, yet still freely

permeable.

The Plasma Membrane

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 ProcessesIn 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 ProcessesSolutes 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.