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.