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Intracellular and extracellular fluid of neurons contain various kinds of charged ions. Additionally, these fluids contain many negatively charged protein molecules called anions A-. The movement of such ions across neural membranes creates electrical activity. As ions move in and out of the cell, they bring their respective charges either positive or negative with them, thus influencing the voltage of cellular membranes.
The resting potential refers to an inactive axon's difference in electrical charge across its membrane, as measured by a voltmeter. The inside of the membrane is millivolts mV relative to the extracellular side.
This charge across the membrane is a store of potential energy and can be used at a future time. The charged particles mentioned above are unequally distributed across the membrane.
The cell membrane forms the border of a neuron and acts to control the movement of substances into and out of the cell. This membrane is composed of two layers of lipid or fat molecules phospholipids, in particular. The intracellular fluid is the clear viscous fluid within a neuron.
It composes the bulk of cellular material, and provides a suspension medium for organelles and free-floating molecules. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions - but diffusion does not occur in solids.
Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example. 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. Vacuoles are broken down by the cell, with the particles used as food or dispatched in some other way. Pinocytosis is a similar process on a smaller scale.
The cell expels waste and other particles through the reverse process, exocytosis. Wastes are moved outside the cell, pushing a membranous vesicle to the plasma membrane, allowing the vesicle to fuse with the membrane and incorporating itself into the membrane structure, releasing its contents to the exterior of the cell.
Skip to main content. Module 6: Structure and Function of Plasma Membrane. Search for:. Active Transport Learning Objectives By the end of this section, you will be able to: Understand how electrochemical gradients affect ions Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis Understand the process of exocytosis.
Electrochemical Gradient Figure 1.
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