Capital punishment and euthanasia utilize this method in their subjects. Figure Cells typically have a high concentration of potassium in the cytoplasm and are bathed in a high concentration of sodium. Injection of potassium dissipates this electrochemical gradient. Potassium injections are also used to stop the heart from beating during surgery. Figure If the pH outside the cell decreases, would you expect the amount of amino acids transported into the cell to increase or decrease?
The transport of amino acids into the cell will increase. How does the sodium-potassium pump make the interior of the cell negatively charged? What is the combination of an electrical gradient and a concentration gradient called? The cell harvests energy from ATP produced by its own metabolism to power active transport processes, such as the activity of pumps. How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell?
Glucose from digested food enters intestinal epithelial cells by active transport. Why would intestinal cells use active transport when most body cells use facilitated diffusion? Intestinal epithelial cells use active transport to fulfill their specific role as the cells that transfer glucose from the digested food to the bloodstream. Intestinal cells are exposed to an environment with fluctuating glucose levels.
Immediately after eating, glucose in the gut lumen will be high, and could accumulate in intestinal cells by diffusion. However, when the gut lumen is empty, glucose levels are higher in the intestinal cells. If glucose moved by facilitated diffusion, this would cause glucose to flow back out of the intestinal cells and into the gut. Active transport proteins ensure that glucose moves into the intestinal cells, and cannot move back into the gut.
It also ensures that glucose transport continues to occur even if high levels of glucose are already present in the intestinal cells. This maximizes the amount of energy the body can harvest from food. Describe why this transporter is classified as secondary active transport. The NCX moves sodium down its electrochemical gradient into the cell. Skip to content Structure and Function of Plasma Membranes.
Learning Objectives By the end of this section, you will be able to do the following: Understand how electrochemical gradients affect ions Distinguish between primary active transport and secondary active transport. Visual Connection. Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. Structures labeled A represent proteins.
Moving Against a Gradient To move substances against a concentration or electrochemical gradient, the cell must use energy. Carrier Proteins for 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 molecule or ion. A symporter carries two different molecules or ions, both in the same direction. 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.
Conversely, if their movements are in opposite directions it is called antiport. In primary active transport, membrane protein transporters include the ion pumps, ion channels, and ATPases. All of them are ATP-driven. In secondary active transport, the transporters are the antiporters and the symporters. An example of an antiporter is the sodium-calcium exchanger in the membranes of cardiac muscle cells.
As for symport mechanism, an example is the glucose symporter SGLT1 found in the internal lining of the small intestine, the heart, the brain, and the S3 segment of the proximal tubule in each nephron. Active transport is essential in multifarious biological processes. 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. 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 be brought into the cell through membrane channels.
ATP itself is formed through secondary active transport using a hydrogen ion gradient in the mitochondrion. 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. Phagocytosis is the process by which large particles, such as cells, are taken in by a cell. 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, which is then destroyed by the neutrophil Figure 3.
A variation of endocytosis is called pinocytosis. In reality, this process takes in solutes that the cell needs from the extracellular fluid Figure 3. Figure 3. Three variations of endocytosis are shown.
0コメント