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Being one of the most important components of a cell, the plasma membrane, or cell membrane plays a huge role in regulating how things get in and out of cells. The plasma membranes unique structure enables it to let only certain molecules pass through the cell. This ability is called selective permeability. Most of the plasma membrane in most cells is made up of a phospholipid bilayer with proteins either embedded into it or sticking out from either side of the membrane. The heads of the phospholipids absorb water from the extracellular environment while the tails of the phospholipids repels it. This system helps the membrane regulate the flow of water in and out of the cell, and is also responsible for letting in hydrophobic or water repelling molecules into and out of the cell as well.
Molecules may enter the cell via two main kinds of transport: active and passive. Passive transport is the movement of molecules through the cell membrane that requires no energy. The molecules passing through the membrane move from the area of the higher concentration to the area with a lower concentration. In other words, the molecules move along the gradient and therefore do not need energy to help them move through the membrane. Diffusion is another word for passive transport and is categorized by two ways: facilitated and simple. In simple diffusion, the molecules are either hydrophilic or small enough to pass through the phospholipid bilayer without any assistance.
In facilitated diffusion, proteins embedded in the cell membrane, provide a doorway for particular solutes to go through. Even though the proteins move in order to make openings for solutes, this process does not use any energy. Water is crucial for the survival of cells. Water is able to diffuse through the membrane by a method of passive transport called osmosis.
Osmoregulation is the control of the water balance in cells. The amount of water in and out of cells has a great effect on the concentration gradient inside and outside cells. When there is more water inside the cell and more solutes outside the cell, the cell is hypotonic to the extracellular fluid. In other words, the cell is in a hypertonic solution. As a result of this, more water molecules leave the cell and the cell shrinks in a process called plasmolysis. If there is more solute inside the cell, the cell is hypertonic to the extracellular fluid.
Therefore, the cell would be in a hypotonic solution. More water molecules will enter the cell because there is more water outside the cell than inside. As a result, the cell swells and it may lyse, or rupture. If the concentrations of water and solutes are equal in and out of the cell, the cell and solution are isotonic.
In some cases, a solute must enter or leave the cell against the concentration gradient. This is called active transport. It requires the use of ATP energy to change the shape of the protein and to enable it to push out the solute. Active transport is mostly described as a pump. For example, some cells need to expel sodium for potassium.
This pump is called the sodium-potassium pump. ATP is used in this pump to change the shape of the protein and to pump the sodium ions out of the cell. After this, potassium ions bind to the protein to be pumped into the cell. For larger molecules, a different type of transport is used.
Instead of the use of only the membrane, the movement of the cytoplasm of the cell or vesicles contributes to the import or export of macromolecules across the plasma membrane. The process of moving large molecules outside the cell is called exocytosis, and it consists of vesicles surrounded by a membrane that contains large molecules, fusing with the plasma membrane and expelling the vesicles contents. Endocytosis describes the intake of large molecules by similar action. There are three types of endocytosis. One type, phagocytosis, is a process in which the plasma membrane wraps itself around and undissolved material and engulfs it, forming a vesicle. Pinocytosis, is when dissolved materials enter the cell by the plasma membrane folding over the dissolved material when it comes in contact with it and creates a vesicle.
Receptor-mediated endocytosis occurs when specialized receptors in the membrane are bonded by specific molecules. When they are bonded together, the membrane folds inward and a vesicle is created. The cell has a very unique way of transporting not only molecules, but signals. Signal-Transduction pathways, are systems in which proteins receive signals to be passed onto various cellular components until the signal reaches its target. The way the signal travels is like a relay. First, the protein receptors get the signal, then they pass their interpretation of their signals to other proteins in and out of the cell.
The relay of the signal is stopped once the signal reaches its target. An example of how these signal-transduction pathways work is in hormones and neurons.
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Research essay sample on Plasma Membrane Passive Transport
