Question

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

i. The molecules can only move across the membrane to the inside of the cell
ii. The molecules always move up their concentration gradient
iii. When ATP synthesis in the cell is inhibited, transport of the molecule stops

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

Answer 1

The molecule's transport across the cell membrane is most likely achieved through active transport, as it moves against its concentration gradient and requires ATP, distinguishing it from passive transport mechanisms.

Step-by-step explanation:

The mechanism most likely used for the transport of a small molecule across a cell membrane, given the observations that (i) the molecule moves inside the cell only, (ii) it moves up its concentration gradient, and (iii) transport stops when ATP synthesis is inhibited, is active transport. This process is characterized by the energy-dependent movement of molecules against their concentration gradient, typically facilitated by membrane-bound proteins that function as pumps.

Passive transport, in contrast, does not use energy and moves molecules down their concentration gradient. Facilitated diffusion is a type of passive transport that utilizes carrier proteins or channels to move molecules down their concentration gradient, but it does not require energy. Because the transport of the molecule in question stops when ATP synthesis is inhibited, passive transport, including simple and facilitated diffusion, is ruled out.