Active Transport
Definition
Active transport is the movement of ions or molecules across a cell membrane in the direction opposite that of diffusion, that is, from an area of lower concentration to one of higher concentration. Active transport requires the assistance of a type of protein called a carrier protein, using energy supplied by ATP. One type of active transport channel will bind to something it is supposed to transport – for example, a sodium ion and hold onto it until a molecule of ATP comes along and binds to the protein.
Another type of active transport is “secondary” active transport. In this type of active transport, the protein pump does not use ATP itself, but the cell must expend ATP in order to keep it functioning.
In 1848, the German physiologist Emil Heinrich du Bois-Reymond suggested the possibility of active transport of substances across membranes. Rosenberg (1948) formulated the concept of active transport based on energetic considerations, but later it would be redefined.
Unlike passive transport, which uses the kinetic energy and natural entropy of molecules moving down a gradient, active transport uses cellular energy to move them against a gradient, polar repulsion, or other resistance. Active transport is usually associated with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino acids.
Classifications and Functions of Active Transport
Active transport mechanisms can be divided into two categories. Primary active transport directly uses a source of chemical energy (e.g., ATP) to move molecules across a membrane against their gradient. Secondary active transport (cotransport), on the other hand, uses an electrochemical gradient – generated by active transport – as an energy source to move molecules gainst their gradient, and thus does not directly require a chemical source of energy such as ATP.
Primary Active Transport: Most of the enzymes that perform this type of transport are transmembrane ATPases. A primary ATPase universal to all animal life is the sodium-potassium pump, which helps to maintain the cell potential. One of the most important pumps in animal cells is the sodium-potassium pump, which moves Na+ out of cells and K+ into them. Because the transport process uses ATP as an energy source, it is considered an example of primary active transport.
Secondary Active Transport: In secondary active transport, the movement of the sodium ions down their gradient is coupled to the uphill transport of other substances by a shared carrier protein (a cotransporter). For instance, in the figure below, a carrier protein lets sodium ions move down their gradient, but simultaneously brings a glucose molecule up its gradient and into the cell. The carrier protein uses the energy of the sodium gradient to drive the transport of glucose molecules.
In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption. Crane’s discovery of cotransport was the first ever proposal of flux coupling in biology.
Types of Active Transport
- Antiport Pumps – Anti-port pumps are pumps that transport one substance in one direction, while transporting another substance the other way. These pumps are extremely efficient because many of them can use one ATP molecule to do these two different tasks.
- Symport Pumps – Symport pumps take advantage of diffusion gradients – differences in concentration that cause substances to naturally move from areas of high to low concentration to move substances. In the case of a symport pump, a substance that “wants” to move from an area of high concentration to low concentration is used to “carry” another substance against its concentration gradient.
- Endocytosis – In endocytosis, the cell uses proteins in its membrane to fold the membrane into the shape of a pocket. This pocket grows until it is pinched off, re-forming the cell membrane around it and trapping the pocket and its contents inside the cell. These membrane pockets, which are used to carry materials inside of or between cells, are called “vesicles.”
- Exocytosis – Exocytosis is the opposite of endocytosis. In exocytosis, the cell creates a vesicle to enclose something that is inside itself, for the purpose of moving it outside. In eukaryotic cells, protein products are made in the endoplasmic reticulum. They are often packaged by the endoplasmic reticulum into vesicles, and sent to the Golgi apparatus.
Plants need to absorb mineral salts from the soil or other sources, but these salts exist in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the direction of the concentration gradient.
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