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Cotransport (Symport and Antiport)

How can a molecule be transported across a membrane against a concentration gradient? Cellular membranes function to keep the internal environment of the cell distinct from the external environment. Concentrations of many molecules differ across cellular membranes. Another function of membranes is to transport materials in and out of the cell. However, because of the difference in environment across the membrane, it is often necessary to transport a molecule from a region of low concentration to a region of higher concentration. Movement up a concentration gradient cannot happen passively, it requires a mechanism to drive the process. The mechanism can be powered directly by the chemical breakdown of ATP to ADP and phosphate or it can be powered by other molecules moving down a concentration gradient.

View the animation below, then complete the quiz to test your knowledge of the concept.






1Sugars can be transported into cells against their concentration gradient because of
A)osmosis.
B)simple diffusion.
C)facilitated diffusion.
D)antiport with Na+.
E)symport with Na +.



2Which of the following is an example of antiport?
A)simultaneous movement of an amino acid and protons into the cell
B)simultaneous movement of an amino acid and protons out of the cell
C)coupling of the inward movement of water with the outward movement of protons
D)coupling of the inward movement of Na+ with the outward movement of Ca ++
E)none of the above.



3Which of the following is the most direct source of energy for cotransport?
A)the movement of one of the transported substances up its concentration gradient
B)the movement of one of the transported substances down its concentration gradient
C)ATP hydrolysis
D)ATP formation
E)cotransport requires no energy



4The sodium-potassium pump maintains a high sodium ion concentration in the cell.
A)True
B)False



5Both symport and antiport require transport proteins.
A)True
B)False







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