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.
In phagocytosis, a portion of the membrane invaginates and flows around the particle, eventually pinching off and leaving the particle wholly enclosed by an envelope of plasma membrane. When ATP loses a phosphate group, energy is released. The carrier protein changes shape, and as it does, it pumps the three sodium ions out of the cell.
At that point, two potassium ions bind to the carrier protein. The process is reversed, and the potassium ions are pumped into the cell. Sodium is the principal ion in the fluid outside of cells. Normal sodium concentrations are about 10 times higher outside than inside of cells. Potassium is the principal ion in the fluid inside of cells. Normal potassium concentrations are about 30 times higher inside than outside of cells. Vesicle Transport Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell.
Endocytosis Endocytosis is a type of vesicle transport that moves a substance into the cell. The membrane then pinches off to form a phagosome food vacuole.
Middle Pinocytosis is when the membrane folds to form a vesicle that carries substances dissolved in the extracellular fluid. On the left Receptor-mediated endocytosis occurs when the receptors on the plasma membrane bind to specific particles. The coated pit region of the membrane forms a coated vesicle containing the receptors with their bound particles.
Exocytosis Exocytosis is a type of vesicle transport that moves a substance out of the cell. Material destined for export is packaged into a vesicle inside the cell.
The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space. Homeostasis and Cell Function For a cell to function normally, a stable state must be maintained inside the cell. Feature:Feature: My Human Body Maintaining the proper balance of sodium and potassium in body fluids by active transport is necessary for life itself, so it's no surprise that getting the right balance of sodium and potassium in the diet is important for good health.
Most salt in the diet is found in processed foods or added with a salt shaker. Stop adding salt and start checking food labels for sodium content. It's easy to add potassium to the diet by choosing the right foods, and there are plenty of choices. Most fruits and vegetables are high in potassium, but especially potatoes, bananas, oranges, apricots, plums, leafy greens, tomatoes, lima beans, and avocado. Other foods with substantial amounts of potassium are fish, meat, poultry, and whole grains.
Review Define active transport. What is the sodium-potassium pump? Why is it so important? Name two types of vesicle transport. Which type moves substances out of the cell? What are the similarities and differences between phagocytosis and pinocytosis?
The sodium-potassium pump is a: Phospholipid Protein Carbohydrate Ion What is the functional significance of the shape change of the carrier protein in the sodium-potassium pump after the sodium ions bind?
A potentially deadly poison derived from plants called ouabain blocks the sodium-potassium pump and prevents it from working. What do you think this does to the sodium and potassium balance in cells? Explain your answer. Active transport requires energy for the process by transporting molecules against a concentration or electrochemical gradient.
Active transport is an energy-driven process where membrane proteins transport molecules across cells, mainly classified as either primary or secondary, based on how energy is coupled to fuel these mechanisms. The former constitutes means by which a chemical reaction, e. The latter employs those established gradients to transport other molecules.
These gradients support the roles of other membrane proteins and other workings of the cell and are crucial to the maintenance of cellular and bodily homeostasis.
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