3. Facilitated Diffusion (Assisted diffusion)

Facilitated diffusion: diffusion of specific particles through
______transport proteins_________ found in the membrane from
_high____ to __low____ concentration

Involves transport proteins called
___carrier proteins ____________ which are
_____specific_____________ for one type of molecule
How do carrier proteins work?
1. carrier protein binds to molecule to be transported
2.
protein changes shape – shields molecule from interior of
cell membrane
3.
molecule moves through protein and is released
4.
protein returns to original shape and is reusable
4. Diffusion Through Ion Channels

Diffusion through ion channels: diffusion of __ions____ from
_high____ to __low_____ concentration

Involves transport proteins called __ion channels_________ or
__channel proteins________ which are usually __specific _____
for one type of ion
ions are _charged__________ so…
o they’re __insoluble_____ in lipids
o they can’t get through the
__nonpolar interior ______________ of membrane
some ion channels are always open; others open and close
(gated channels)
o opening/closing may be controlled by
1. membrane stretching
2. electrical signals
3. chemical signals
examples of some common ions:
Examples of molecules that move through carrier proteins:
Na+, K+, Ca+2, Mg+, Cl-, PO4-, HCO3******* GLUCOSE !!!! – it’s needed quickly in the cell but
is too large to diffuse quickly on its own
Why are these ions important?
other small polar molecules (like some amino acids)
heartbeat, nerve conduction, fluid balance, O2 delivery,
acid-base balance
Facilitated diffusion is similar to another process that we talked about earlier this year. What process is that? How are the two processes
similar?