Why is active transport necessary

Active transport then occurs across the root so that the plant takes in the ions it needs from the soil around it. In animals, glucose molecules have to be moved across the gut wall into the blood.

The glucose molecules in the intestine might be in a higher concentration than in the intestinal cells and blood — for instance, after a sugary meal — but there will be times when glucose concentration in the intestine might be lower. All the glucose in the gut needs to be absorbed. 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.

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. This fusion opens the membranous envelope to the exterior of the cell, and the particle is expelled into the extracellular space Figure 4. The combined gradient that affects an ion includes its concentration gradient and its electrical gradient. Living cells need certain substances in concentrations greater than they exist in the extracellular space.

Moving substances up their electrochemical gradients requires energy from the cell. Active transport uses energy stored in ATP to fuel the transport. Active transport of small molecular-size material uses integral proteins in the cell membrane to move the material—these proteins are analogous to pumps. Some pumps, which carry out primary active transport, couple directly with ATP to drive their action. In secondary transport, energy from primary transport can be used to move another substance into the cell and up its concentration gradient.

Endocytosis methods require the direct use of ATP to fuel the transport of large particles such as macromolecules; parts of cells or whole cells can be engulfed by other cells in a process called phagocytosis.