For each of the following scenarios described where a molecule or ion is moving from one side of a membrane to the other, select the method by which the molecule or ion is moving. Each answer can be used more than once, or not at all.

- Simple Diffusion

- Facilitated diffusion by a channel protein

- Facilitated diffusion by a carrier/transport protein

- Active transport by a pump

- Could be facilitated diffusion by a channel or a carrier; not enough information is given

a. While water can freely diffuse across the membrane, it does not do so fast enough for living organisms to function properly. Therefore, membrane proteins known as aquaporins can increase the rate at which water moves across the membrane. The movement of water across the membrane via aquaporins (which do not change shape) is an example of which type of transport?

b. Many snake venoms induce paralysis by acting on acetylcholine receptors. Nicotinic acetylcholine receptors are transmembrane proteins that allow Na⁺, K⁺ and Ca²⁺ to flow through an open pore in the protein in order to cross the membrane, but only in response to acetylcholine binding to that protein. What kind of transport is this?

c. 3,4-Methylenedioxymethamphetamine (MDMA, Ecstasy) is a compound that is structurally similar to the endogenous neurotransmitter serotonin. MDMA is so similar to serotonin that it binds the same protein that is used to move serotonin across the membrane. The protein changes shape in response to serotonin (or MDMA) binding it. What kind of transport occurs here?

d. Carbon monoxide (CO) is a poisonous, lipid-soluble gas that can freely cross the cell membrane. What type of transport is this? (P.S. please invest in a carbon monoxide detector in your home if you have not done so already!).

e. Na⁺/K⁺-ATPase is an enzyme (a type of protein) found in the animal cell membrane that helps move Na⁺ and K⁺ against their concentration gradients in order for the organism to function properly. What type of transport is this?