Lecture 20 – Uptake RequiringATP
How things get into Cells
Proteins and small molecules
◦ Free Diffusion, Simple Diffusion – These are lipid soluble compound, can pass directly
through the plasma membrane, thus aliphatic compounds like steroids.
◦ Facilitated Diffusion – Occurs through a channel, is like Free diffusion in that it does not
require extra energy, flow through these channels follows the diffusion gradient. Ions,
certain sugars, water (aquaporin in plasma membrane and porin in mitochondria) flow
through these channels. Without these channels, flow through the membrane of these
substances would be greatly slowed.
◦ For channels, thus Facilitated Diffusion, you will reach a saturation point. Not so with Free
◦ There are also Gated Channels, in which the opening is controlled in various ways.
◦ Answering the question of how a Potassium channel blocks Na ions; there are carbonyl
groups all around the entrance of the channel, Na is small so it is drawn particularly to one
random carbonyl, and this results in it being stopped from continuing downward. K is
much larger, and because of its size it is equally drawn to all the carbonyl sites, so remains
in the relative center of the channel, and so passes through.
◦ Gated Channels, K , Na , Ca , etc.
◦ There a