Mediated transport

In the process of mediated transport, carrier proteins embedded within the plasma membrane assist in the transport of larger polar molecules into or out of the cell. When a given substance attaches to a specific binding site on the carrier protein, the protein undergoes a conformational change such that this site with the bound substance moves from one side of the plasma membrane to the other. The substance is then released. Mediated transport displays three important characteristics influencing its function:




Carrier proteins display a high degree of specificity. In other words, each of these proteins may bind only with select substances that "fit" into its binding site. Another characteristic is competition; different substances with similar chemical structures may be able to bind to the same carrier protein and therefore compete for transport across the membrane. The third characteristic displayed by mediated transport is saturation. The greater the number of carrier proteins utilized at any given time, the greater the rate of transport is. Initially, as the concentration of a substance increases, the rate of transport increases; however, a finite number of carrier proteins exist in a given cell membrane. Once all these proteins are utilized in the transport process, any further increase in the concentration of the substance no longer increases the rate of transport because it has reached its maximum. At this point, the process is saturated.

Mediated transport has two forms:

• Facilitated diffusion

• Active transport

With facilitated diffusion, carrier proteins move across the membrane in either direction and will transport a substance down its concentration gradient. In other words, substances are moved from an area of high concentration to an area of low concentration — a passive process that requires no energy. An example of a substance transported by facilitated diffusion is glucose, which is a large polar molecule. Because cells are constantly utilizing glucose to form ATP, a concentration gradient is always available for diffusion into the cell.

With active transport, energy is expended to move a substance against its concentration gradient from an area of low concentration to an area of high concentration. This process is used to accumulate a substance on one side of the plasma membrane or the other. The most common example of active transport is the sodium-potassium pump that involves the activity of Na+-K+ ATPase, an intrinsic membrane protein. For each ATP molecule hydrolyzed by Na+-K+ ATPase, this pump moves three Na+ ions out of the cell and two K+ ions into it. As will be discussed further in the next chapter, the activity of this pump contributes to the difference in composition of the extracellular and intracellular fluids necessary for nerve and muscle cells to function.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook

Post a comment