When Blood Supplies Oxygen and Food to a Cell Surface

Multiple Choice

When blood supplies oxygen and food to a cell surface and these molecules diffuse across,we might expect some "resting time." However,if we inspected most cell membranes in a living human body,we would find a nearly continuous flow of substances back-and-forth across the plasma membrane.Which of the following is not a reason why?

A) Cell metabolism is constantly using up oxygen and raw materials; this lowers their concentration so more will diffuse into the cell.
B) Cell metabolism is constantly producing waste molecules; this increases their concentration so more will diffuse out of the cell.
C) Cells produce useful secretions; this increases their concentration and more will diffuse out of the cell.
D) Cell metabolism constantly produces carbon dioxide in respiration; this increases its concentration so more will diffuse out of the cell.
E) Cells are living entities and in order to live can never "rest."