The image represents a human cell in an isotonic solution. Some common cellular molecules that can freely move across the plasma membrane are shown5

Question

What do you think will happen if no energy is used?

a
Some H2O will leave the cell.
b
Some O2 will enter the cell.
c
Some CO2 will leave the cell.
d
Some H2O will enter the cell.

Answer 1

In an isotonic solution, the concentration of solutes outside the cell is equal to the concentration of solutes inside the cell. This means that there is no net movement of water into or out of the cell, as the osmotic pressure is balanced.

Given the options you provided:

a. Some H2O will leave the cell.
b. Some O2 will enter the cell.
c. Some CO2 will leave the cell.
d. Some H2O will enter the cell.

The correct answer is b. Some O2 will enter the cell and c. Some CO2 will leave the cell.

• O2 can freely diffuse into the cell due to a concentration gradient (higher concentration of O2 outside than inside, assuming cellular respiration is happening).
• CO2 is produced as a waste product in the cell and will diffuse out of the cell where it is in higher concentration.

However, regarding the movement of water, since the solution is isotonic, there would be no net movement of water (thus neither option a nor option d would necessarily occur).

So, the two net continua would be b and c due to the concentration gradients of O2 and CO2, respectively.