Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. We have discussed simple concentration gradients—differential concentrations of a substance across a space or a membrane—but in living systems, gradients are more complex.
Because cells contain proteins, most of which are negatively charged, and because ions move into and out of cells, there is an electrical gradient, a difference of charge, across the plasma membrane. The situation is more complex, however, for other elements such as potassium. The combined gradient that affects an ion is called its electrochemical gradient, and it is especially important to muscle and nerve cells.
To move substances against a concentration or an electrochemical gradient, the cell must use energy. This energy is harvested from ATP that is generated through cellular metabolism. Active transport mechanisms, collectively called pumps or carrier proteins, work against electrochemical gradients. With the exception of ions, small substances constantly pass through plasma membranes.
Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive changes. Because active transport mechanisms depend on cellular metabolism for energy, they are sensitive to many metabolic poisons that interfere with the supply of ATP.
Two mechanisms exist for the transport of small-molecular weight material and macromolecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane.
The primary active transport system uses ATP to move a substance, such as an ion, into the cell, and often at the same time, a second substance is moved out of the cell.
The sodium-potassium pump, an important pump in animal cells, expends energy to move potassium ions into the cell and a different number of sodium ions out of the cell Figure 2.
The action of this pump results in a concentration and charge difference across the membrane. Figure 2. The sodium-potassium pump move potassium and sodium ions across the plasma membrane.
However, the situation is more complex for other elements such as potassium. We call the combined concentration gradient and electrical charge that affects an ion its electrochemical gradient. This is how capital punishment and euthanasia subjects die. Why do you think a potassium solution injection is lethal? To move substances against a concentration or electrochemical gradient, the cell must use energy. Active transport mechanisms, or pumps , work against electrochemical gradients.
Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances that living cells require in the face of these passive movements. A cell may spend much of its metabolic energy supply maintaining these processes. A red blood cell uses most of its metabolic energy to maintain the imbalance between exterior and interior sodium and potassium levels that the cell requires.
Two mechanisms exist for transporting small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport does not directly require ATP: instead, it is the movement of material due to the electrochemical gradient established by primary active transport.
An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three protein types or transporters Figure. A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction.
An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also in facilitated diffusion, but they do not require ATP to work in that process. Both of these are antiporter carrier proteins.
Both are pumps. The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur.
The second transport method is still active because it depends on using energy as does primary transport Figure. Because of this, cellular energy e. ATP is used in active transport in contrast to passive transport that utilizes kinetic and natural energy. ATP can be generated through cellular respiration. Active transport may be primary or secondary. A primary active transport is one that uses chemical energy in the form of ATP whereas a secondary active transport uses potential energy often from an electrochemical potential difference.
In primary active transport, there is a direct coupling of energy such as ATP. An example is the active transport involving the sodium-potassium pump. Another example is the active transport driven by the redox energy of NADH when it moves protons across the inner mitochondrial membrane against concentration gradient. Photon energy can also drive primary active transport such as when the protons are moved across the thylakoid membrane. This leads to the generation of proton gradient such as during photosynthesis.
In secondary active transport, there is no direct ATP coupling. Rather, the transport is powered by the energy from electrochemical potential difference as the ions are pumped into and out of the cell. In secondary active transport, one ion is allowed to move down its electrochemical gradient. This results in increased entropy that can be used as a source of energy. Thus, secondary active transport is also called coupled transport or cotransport.
Coupled transport is defined as the simultaneous transport of two substances across a biological membrane. It may be a symport or antiport depending on the direction of movement of the two substances. If both move in the same direction it is a symport type of coupled transport. When the glucose concentration in the intestine is lower than in the intestinal cells, movement of glucose involves active transport. The process requires energy produced by respiration. Active transport Substances are transported passively down concentration gradients.
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