4. Large polar molecules such as
glucose and amino acids, cannot
diffuse across the phospholipid
bilayer. Also ions such as Na+ or
Cl- cannot pass.
These molecules pass through
protein channels instead.
Diffusion through these
channels is called
FACILITATED DIFFUSION.
Movement of molecules is still
PASSIVE just like ordinary
diffusion, the only difference is,
the molecules go through a
protein channel instead of
passing between the
phospholipids.
AS Biology, Cell membranes and Transport 4
5. Gas exchange at the alveoli — oxygen from air to
blood, carbon dioxide from blood to air.
Gas exchange for photosynthesis — carbon
dioxide from air to leaf, oxygen from leaf to air.
Gas exchange for respiration — oxygen from
blood to tissue cells, carbon dioxide in opposite
direction.
Transfer of transmitter substance —
acetylcholine from presynaptic to postsynaptic
membrane at a synapse.
Osmosis — diffusion of water through a
semipermeable membrane.
12. Two major groups of integral membrane
proteins are involved in facilitated
diffusion;
Carrier proteins
Ion Channels
Ion channels allow currents to be carried
across the membrane and are thus of
particular importance in the physiology of
excitable cells like neurons
13. Bind a specific type of solute
Discharges the solute
Certain of the hexose transporters, which transport
glucose and similar monosaccharides into and out of
cells
14. Not really bind the solute
Faster than by carrier proteins
Contain a "gate" which is functions to control
the channel's permiability
