Question

There are two main ways in which molecules are transported into and out of cells - active transport and passive transport. Which of the following statements is true of passive transport?
A.) Energy, in the form of ATP, is essential for passive transport.
B.) Endocytosis and exocytosis are two forms of passive transport.
C.) Molecules move up a concentration gradient during passive transport.
D.) Carrier proteins are sometimes used during passive transport.

Answer 1

In the passive transport, the diffusion takes place down the concentration gradient of a solute, and do not require the wxpense of energy. But sometimes, carrier proteins are used for transport of the substances who can not cross the membrane, on their own, such as water and carbon dioxide. They are polar and have some charge, therefore, are not able to cross the hydrophobic membrane without carrier proteins. So, your answer is D.