A mixed solution of water and small molecules is exposed to a cell membrane and allowed to interact for an hour. Which molecule would be most likely to cross the phospholipid bilayer without the aid of transport proteins and diffuse across the membrane the fastest?

Question

A.  Water
B.  NH4+
C.  NH3
D.  CH3COO-
E.  Na+

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?

Gather Content
· What do you know about diffusion? What other information is related to the question?

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  NH3

Clarify Question
· What is the key concept addressed by the question?
        o The question asks about diffusion.
· What type of thinking is required?
        o You are being asked to apply your knowledge about diffusion.

Gather Content
· What do you know about diffusion? What other information is related to the question?
        o Diffusion occurs when a molecule can move down its concentration gradient. If the protein must cross a membrane, polar or charged molecules require the aid of transport proteins. Gases and small non-polar molecules can diffuse directly across the membrane. Water moves by osmosis, but typically only in response to differences in salt concentrations across the membrane.

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o NH3 is ammonia gas and can diffuse most rapidly across a membrane. Water can diffuse, but would do so mostly in response to differences in ion concentration. The other three choices are all ions and would not diffuse rapidly without the aid of transport proteins.

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked you to apply your knowledge about diffusion. If you got the correct answer, great job! If you got an incorrect answer, where did the process break down? Did you think that ions could diffuse across the membrane without the aid of transport proteins? Did you think that water would diffuse more rapidly than a gas?