Secondary Active Transport involves two steps.
The first requires the active transport of a substance (ex. Na+) across a
membrane to establish a concentration gradient, and the second is allowing that
substance to move back into the cell via a carrier molecule that also carries
another substance (ex. Glucose). Since the first substance (Na+) is moving back
inside the membrane from a high concentration to a low one, the concentration
gradient provides enough energy for the carrier molecule to transport the
second substance (glucose) against its concentration gradient by co-transport. Facilitated
Diffusion uses a carrier molecule to transport substances down their
concentration gradients and because of this ATP is not required. When a
substance like glucose binds with a carrier molecule on the outside of a
membrane, it changes shape and releases the glucose on the inside of the
membrane. Secondary active transport is used to absorb glucose into the cells
of the small intestine in order to maintain an adequate glucose concentration
in the cells. Since the glucose levels in the digestive tract are always
changing, if the cells relied on facilitated diffusion for their glucose
absorption then whenever the body went a few hours without eating and glucose
levels became low in the gut, facilitated diffusion would allow for all of the
glucose to be transported down the concentration gradient from the blood and
tissues into the intestines, a situation which could be very harmful and
potentially fatal. Secondary Active Transport does not allow for any reverse
flow of glucose away from the cells of the body, no matter what the
concentration is in our intestines. Facilitated Diffusion is used to transport
glucose out of the cell to be taken up by the circulation and vice versa.
Because blood glucose levels are monitored very closely by the body and the
environment in the circulation is much more stable than the intestines,
secondary active transport is not needed. Since diffusion transports substances
along the concentration gradient, the glucose levels within cells can never
exceed the concentrations within the circulation and they will always be