When a cell is in equilibrium with its environment, which of the following processes occurs for substances that can diffuse through the plasma membrane?
a) there is random movement of substances into and out of the cell.
b) all movement of molecules across the plasma membrane occurs by active transport.
c) there is directed movement of substances into and out of the cell.
d) there is no movement of substances into or out of the cell.

The correct answer is A.

Explanation:

Diffusion is the net movement of a substance from a region of higher concentration to a region of lower concentration. The difference in concentration between two environments creates a concentration gradient which is what drives diffusion. Therefore, this movement does not require aid such as extra energy molecules for active transport or integral membrane proteins for directed movement, hence the answer is not B nor C. If the cell is in equilibrium with its environment, it means that the concentration of substances inside the cell is equal to the concentration outside the cell, and any movement that takes place will be random movement of substances across the plasma memberane. Movement in the cell will not cease as the cell is still active. Therefore, the answer cannot be D either.