Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient, that is, if the concentration of the substance inside the cell must be greater than its concentration in the extracellular fluid, the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight material, such as ions, through the membrane.

In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles. Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that the uptake and release of large particles by the cell requires energy. A large particle, however, cannot pass through the membrane, even with energy supplied by the cell.

Primary Active Transport

There are several types of active transport. The principle one that will be discussed is primary active transport, which uses a combination of ATP energy and a transport protein to move substances across the membrane against the concentration gradient. ATP is hydrolyzed, via an enzyme- catalyzed reaction, to ADP and the lost phosphate group attaches to the protein. This joining causes a conformational change in the shape of the transport protein and the particular substance is moved across the membrane against the concentration gradient. An example of primary active transport is the sodium-potassium pump, which is involved in nerve impulses and is discussed in a later chapter.

Endocytosis

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created vacuole that is formed from the plasma membrane (Figure 3.19).

Phagocytosis is the process by which large particles, such as cells, are taken in by a cell (Figure 3.19 a). For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil removes the invader, through this process, surrounding and engulfing the microorganism, whis is then destroyed.

Pinocytosis is the process by which small volumes of liquids are taken in by a cell. This process is also known as “cell drinking” (Figure 3.19 b).

In receptor-mediated endocytosis substances bind to a receptor on the external face of the cell membrane. The bound substance is then taken in by the cell (Figure 3.19 c). Many nutrients and other metabolic substances, proteins, and hormons, as well as many viruses are taken in by cells through this process.

Exocytosis

Exocytosis is the opposite of the processes discussed above in that its purpose is to expel material from the cell into the extracellular fluid. A particle enveloped in membrane fuses with the interior of the plasma membrane. This fusion opens the membranous envelope to the exterior of the cell, and the particle is expelled into the extracellular space (Figure 3.20).

Section Summary

Primary active transport uses energy stored in ATP to fuel the transport. Active transport of small molecular-size material uses integral proteins in the cell membrane to move the material — these proteins are analogous to pumps. Some pumps, which carry out primary active transport, couple directly with ATP to drive their action.

Endocytosis methods require the direct use of ATP to fuel the transport of large particles such as macromolecules; parts of cells or whole cells can be engulfed by other cells in a process called phagocytosis. In phagocytosis, a portion of the membrane invaginates and flows around the particle, eventually pinching off and leaving the particle wholly enclosed by an envelope of plasma membrane. The cell expels waste and other particles through the reverse process, exocytosis. Wastes are moved outside the cell, pushing a membranous vesicle to the plasma membrane, allowing the vesicle to fuse with the membrane and incorporating itself into the membrane structure, releasing its contents to the exterior of the cell.