In carrying out normal activities, cells use oxygen and produce carbon dioxide. the concentration of oxygen is higher in the blood than inside the cell, so oxygen moves into the cell. similarly, carbon dioxide moves out of the cell into the blood because the concentration of carbon dioxide inside the cell is greater than the concentration outside the cell. how do the small molecules of oxygen and carbon dioxide move through the cell membrane?
a. active transport by endocytosis
b. active transport by carrier proteins
c. passive transport by osmosis
d. passive transport by diffusion

One of the many functions performed by the plasma membrane is the control of substances entering and leaving the interior of the cell. In this sense, diffusion, also known as molecular diffusion, transports matter in order to maintain the balance between two solutions initially of different concentrations. An example of this can be seen in the question above, which shows the transport of oxygen and carbon dioxide between the inside and outside of the cell. This transport occurs because through diffusion, the cell seeks to balance the concentration of oxygen and carbon dioxide within the cell and in the blood.

Diffusion occurs due to thermal energy present in molecules that constantly move in fluids, whether they are liquid or gaseous. Thermal energy promotes the passage of solute to less concentrated regions. This transport is what allows the cell to absorb various nutrients and also allows any metabolic waste to be excreted.