The sodium-potassium pump is, therefore, an electrogenic pump a pump that creates a charge imbalance , creating an electrical imbalance across the membrane and contributing to the membrane potential. In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient. Unlike in primary active transport, in secondary active transport, ATP is not directly coupled to the molecule of interest.
Instead, another molecule is moved up its concentration gradient , which generates an electrochemical gradient. The molecule of interest is then transported down the electrochemical gradient. While this process still consumes ATP to generate that gradient, the energy is not directly used to move the molecule across the membrane, hence it is known as secondary active transport.
Both antiporters and symporters are used in secondary active transport. Co-transporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions across the cell membrane. Secondary active transport brings sodium ions, and possibly other compounds, into the cell.
As sodium ion concentrations build outside the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane.
Many amino acids , as well as glucose , enter a cell this way. This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP.
The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP.
Secondary Active Transport : An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport. Privacy Policy. Skip to main content. Module 4: The Cellular Level of Organization.
Search for:. Learning Objectives Define an electrochemical gradient and describe how a cell moves substances against this gradient. Key Takeaways Key Points The electrical and concentration gradients of a membrane tend to drive sodium into and potassium out of the cell , and active transport works against these gradients. To move substances against a concentration or electrochemical gradient , the cell must utilize energy in the form of ATP during active transport.
Primary active transport, which is directly dependent on ATP, moves ions across a membrane and creates a difference in charge across that membrane.
Secondary active transport , created by primary active transport, is the transport of a solute in the direction of its electrochemical gradient and does not directly require ATP. Primary Active Transport The sodium-potassium pump maintains the electrochemical gradient of living cells by moving sodium in and potassium out of the cell.
Learning Objectives Describe how a cell moves sodium and potassium out of and into the cell against its electrochemical gradient. When the sodium-potassium- ATPase enzyme points into the cell, it has a high affinity for sodium ions and binds three of them, hydrolyzing ATP and changing shape.
As the enzyme changes shape, it reorients itself towards the outside of the cell, and the three sodium ions are released. The enzyme changes shape again, releasing the potassium ions into the cell.
After potassium is released into the cell, the enzyme binds three sodium ions, which starts the process over again. Key Terms electrogenic pump :An ion pump that generates a net charge flow as a result of its activity. Secondary Active Transport In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient. Learning Objectives Differentiate between primary and secondary active transport. Key Takeaways Key Points While secondary active transport consumes ATP to generate the gradient down which a molecule is moved, the energy is not directly used to move the molecule across the membrane.
Secondary active transport brings sodium ions into the cell , and as sodium ion concentrations build outside the plasma membrane , an electrochemical gradient is created. If a channel protein is open via primary active transport, the ions will be pulled through the membrane along with other substances that can attach themselves to the transport protein through the membrane.
Secondary active transport is used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy in the hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase , and that energy is used to convert ADP into ATP.
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.
A targeted variation of endocytosis employs binding proteins in the plasma membrane that are specific for certain substances Figure 3. The particles bind to the proteins and the plasma membrane invaginates, bringing the substance and the proteins into the cell. If passage across the membrane of the target of receptor-mediated endocytosis is ineffective, it will not be removed from the tissue fluids or blood.
Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by a failure of receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely.
People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear the chemical from their blood. Figure 4. In exocytosis, a vesicle migrates to the plasma membrane, binds, and releases its contents to the outside of the cell. In contrast to these methods of moving material into a cell is the process of 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.
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