In which case would facilitated diffusion be needed? A. With large polar molecules moving against the concentration gradient.
B. With large non polar molecules moving down the concentration gradient.
C. With very small polar particles moving against the concentration gradient.
D. With very small non polar particles moving down the concentration gradient.

The voltage-gated potassium channels associated with an action potential provide an example of what type of membrane transport?

A. Simple diffusion.

B. Facilitated diffusion.

C. Coupled transport.

D. Active transport.

You are studying the entry of a small molecule into red blood cells. You determine the rate of movement across the membrane under a variety of conditions and make the following observations:

i. The molecules can move across the membrane in either direction.

ii. The molecules always move down their concentration gradient.

iii. No energy source is required for the molecules to move across the membrane.

iv. As the difference in concentration across the membrane increases, the rate of transport reaches a maximum.

The mechanism used to get this molecule across the membrane is most likely:

A. simple diffusion.

B. facilitated diffusion.

C. active transport.

D. There is not enough information to determine a mechanism.

Carrier proteins - exist in two conformations, altered by high affinity binding of the transported molecule. Moves material in either direction, down concentration gradient (facilitated diffusion). EXAMPLE: GluT1 erythrocyte glucose transporter.

Channel proteins - primarily for ion transport. Form an aqueous pore through the lipid bilayer. May be gated. Moves material in either direction, down concentration gradient (facilitated diffusion). EXAMPLES: Voltage-gated sodium channel, erytrhocyte bicarbonate exchange protein.

This might be helpful... because I don't know anything about facilitated diffusion.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.