3.6 Active Transport - Biological Concepts | OpenStaxName (2023)

electrochemical gradient

We have discussed simple concentration gradients (varying concentrations of a substance across a space or membrane), but in living systems gradients are more complex. Because cells contain proteins, most of which are negatively charged, and because ions move in and out of cells, there is an electrical gradient, a difference in charge, across the plasma membrane. The interior of living cells is electrically negative to the extracellular fluid in which they are bathed; At the same time, the cells have higher potassium concentrations (K⁺) and lower sodium concentrations (Na⁺) as extracellular fluid. Thus, in a living cell, the concentration gradient and the electrical gradient of Na⁺promotes the diffusion of the ion in the cell and the electrical gradient of Na⁺(a positive ion) tends to push it inwards when negatively charged. However, for other elements such as potassium, the situation is more complex. The electric gradient of K⁺promotes ion diffusionInof the cell, but the concentration gradient of K⁺promotes disseminationoutsidefrom cell (Figure 3.24). The combined gradient acting on an ion is called itselectrochemical gradient, and is particularly important for muscle and nerve cells.

Figure3.24 Electrochemical gradients arise from the combined action of concentration gradients and electrical gradients. N / A⁺Ions are in higher concentration outside the cell, and K⁺The ions are in a higher concentration inside the cell, and yet the interior of the cell has a net negative charge compared to the other side of the membrane. This is due to the presence of K⁺Binding proteins and other negatively charged molecules. The difference in electrical charges draws positively charged Na ions into the cell, the electrical gradient, while K ions tend to flow out of the cell through K channels due to the difference in electrical charges. (Source: modified work from "Synaptitude"/Wikimedia Commons)

movement against a gradient

In order to move substances against a concentration or an electrochemical gradient, the cell must expend energy. This energy is obtained from ATP, which is produced by cellular metabolism. Active transport mechanisms, collectively referred to as transport proteins or pumps, oppose electrochemical gradients. With the exception of ions, small substances constantly pass through plasma membranes. Active transport maintains the levels of ions and other substances needed by living cells in the face of these passive changes. A large part of a cell's metabolic energy supply can be used to maintain these processes. Since active transport mechanisms depend on cellular energy metabolism, they are sensitive to many metabolic toxins that disrupt ATP supply.

Two mechanisms exist for the transport of small molecular weight material and macromolecules. Primary active transport moves ions across a membrane and creates a charge difference across that membrane. The primary active transport system uses ATP to bring a substance, such as an ion, into the cell, and often a second substance is removed from the cell at the same time. The sodium-potassium pump, an important pump in animal cells, uses energy to move potassium ions into the cell and a varying amount of sodium ions out of the cell (Figure 3.25). The action of this pump results in a concentration and charge difference across the membrane.

Figure3.25 The sodium-potassium pump moves potassium and sodium ions across the plasma membrane. (Credit: Modification of work by Mariana Ruiz Villarreal)

Secondary active transport describes the movement of material using the energy of the electrochemical gradient established by primary active transport. Using the energy of the electrochemical gradient created by the primary active transport system, other substances such as amino acids and glucose can enter the cell through channels in the membrane. ATP itself is formed by secondary active transport using a hydrogen ion gradient in the mitochondria.

Endozytose

EndozytoseIt is a type of active transport that moves particles such as large molecules, cell parts and even whole cells within a cell. There are different variants of endocytosis, but they all share a common feature: the cell's plasma membrane invaginates and forms a pocket around the target particle. The pocket is pinched, trapping the particle in a newly created vacuole that forms from the plasma membrane.

Figure3.26 Three variants of endocytosis are shown. (a) In one form of endocytosis, phagocytosis, the cell membrane surrounds the particle and is compressed to form an intracellular vacuole. (b) In another type of endocytosis, pinocytosis, the cell membrane surrounds a small volume of fluid and sheds to form a vesicle. (c) In receptor-mediated endocytosis, the uptake of substances by the cell is directed to a single type of substance that binds to the receptor on the outer cell membrane. (Credit: Modification of work by Mariana Ruiz Villarreal)

PhagozytoseIt is the process by which large particles, such as B. cells, are absorbed by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil eliminates the invader through this process of encircling and engulfing the microorganism, which is then destroyed by the neutrophils (Figure 3.26).

A variant of the endocytosis is calledPinozitosis. This literally means "to drink from the cell" and was named at a time when the cell was intended to intentionally absorb extracellular fluid. This process actually takes solutes needed by the cell from the extracellular fluid (Figure 3.26).

A targeted variant of endocytosis utilizes binding proteins in the plasma membrane that are specific for certain substances (Figure 3.26). The particles bind to proteins and the plasma membrane invaginates, bringing the substance and proteins into the cell. If the passage through the membrane of the target ofreceptor-mediated endocytosisit is ineffective, it is not cleared from tissue fluids or blood. Instead, it stays in these liquids and increases its concentration. Some human diseases are caused by failure of receptor-mediated endocytosis. For example, the form of cholesterol called low-density lipoprotein, or LDL (also known as "bad" cholesterol) is cleared from the blood by receptor-mediated endocytosis. In familial hypercholesterolemia, a genetic human disease, LDL receptors are defective or absent altogether. People with this condition have life-threatening levels of cholesterol in their blood because their cells can't clear the chemical from their blood.

Exozytose

Contrasted with these methods of introducing material into a cell is the process of exocytosis.Exozytoseit is the opposite of the processes discussed above, since its purpose is to eject material from the cell into the extracellular fluid. A membrane-bound particle fuses with the interior of the plasma membrane. This fusion opens the membrane envelope to the outside of the cell and the particle is ejected into the extracellular space (Figure 3.27).

Figure3.27 During exocytosis, a vesicle migrates to the plasma membrane, binds and releases its contents outside the cell. (Credit: Modification of work by Mariana Ruiz Villarreal)