ABSTRACT
Throughout every cell the movement of molecules from one region to another is important to meet the demands of the processes that sustain life. A small molecule, such as ATP (the energy currency of cells), is synthesized by enzymes at the membranes of mitochondria, but is then needed in remote parts of the cell, where it is used for reactions such as powering chaperones for protein folding. Proteins themselves are synthesized by ribosomes in the cytoplasm, but may need to function elsewhere in the cell, such as by binding to DNA in the nucleus.
Much of the movement of molecules in cells is passive, requiring no energy source, and occurs in random directions. e random motions of molecules arise from constant collisions with water and other molecules. Such collisions cause transient accelerations in random directions and also cause the molecules to rotate. e resulting motions are maintained for only a very short time before further collisions change both the direction and the speed of the motion. Over time, these random motions cause molecules to move signi cant distances in a process referred to as diff usion. e same e ects occur with larger particles, resulting in Brownian motion (in Brownian motion, the di using particles are large enough to see with a microscope). Passive transport, which occurs through di usion alone, can be too slow to meet all of the needs of a cell. As a consequence, active transport, which requires an energy source, is also important. Some of the concepts that we shall develop in this chapter to describe di usion are also useful for understanding aspects of active transport.
