Introduction


Plasma Membrane


Plasma Membrane, thin molecular layer that surrounds all living cells. The plasma membrane separates the cell from its surroundings, protects it from changes in the chemical and physical environment, and regulates the traffic of molecules into and out of the cell. Although flexible and exceedingly thinmore than 130,000 layers of plasma membrane placed one on top of the other would make a stack just 1 mm (0.04 in) highthe plasma membrane is very strong. In the cells of plants, bacteria, fungi, and most algae, the plasma membrane is surrounded by a cell wall, a rigid structure that helps support the cell and prevent it from drying out.

The plasma membrane is composed primarily


The plasma membrane is composed primarily of two types of moleculeslipids, which are fatty or oily molecules, and proteins. The basic structural framework of the plasma membrane is formed by two sheets of lipids, each sheet a single molecule thick. Within this double layer, or bilayer, of lipids, the protein molecules are embedded. Proteins are responsible for a host of functions, including transporting substances across the membrane, aiding communication between cells, and carrying out chemical reactions. In most cells, the plasma membrane is about 40 percent lipid and 60 percent protein, but these proportions vary greatly, from as little as 20 percent to as much as 75 percent protein depending on the type of cell.

Structure


Most of the lipids in the plasma membrane


Most of the lipids in the plasma membrane are of a specific type known as phospholipids. A phospholipid molecule has a head region at one end that is hydrophilicit can mix with water. At the other end are two long tails that are hydrophobicthey do not mix well with water. In the plasma membrane`s bilayer construction, phospholipid molecules are arranged so that their hydrophilic heads point outward on either side of the membrane, and their hydrophobic tails point toward each other in the middle of the membrane. This orientation keeps the hydrophobic tails away from the watery fluids that both fill and surround living cells. In fact, the plasma membrane stays intact precisely because the phospholipid molecules strongly resist any change in configuration that would expose their hydrophobic tails to the watery environment.

While the phospholipids are held in a bilayer

While the phospholipids are held in a bilayer, scientists believe the plasma membrane as a whole is a fluid structure because phospholipid molecules and some proteins can move sideways within the membrane. In one second, a single phospholipid molecule can travel the length of a large bacterial cell. Proteins drift more slowly through the membrane. With protein molecules scattered among the phospholipid molecules, the plasma membrane appears to be a mosaic of phospholipids and proteins. Some of the proteins are found on the inner or outer surface of the plasma membrane, while others span the membrane and protrude on either end. Scientists refer to this concept of the plasma membrane`s structure as the fluid mosaic model.

The movement of the phospholipid and protein


The movement of the phospholipid and protein components through the plasma membrane permits the membrane to change shape. This flexibility is crucial to many different types of cells. For example, a single-celled organism known as an amoeba moves by changing shape, stretching out one part of the cell in the direction of travel and dragging the rest along behind. Human red blood cells readily change shape as they squeeze through the body`s smallest blood vessels.

In animal cells

In animal cells, cholesterol also contributes to the fluidity of the plasma membrane. Cholesterol is a small lipid molecule that nestles among the hydrophobic tails of the phospholipids in the interior of the membrane. It prevents phospholipid molecules from packing together too tightly and making the membrane rigid. It also acts as an antifreeze for the plasma membrane, preventing the membrane from freezing to a jellylike consistency at low temperatures. Plants and fungi have similar molecules that increase the fluidity of their plasma membranes.

The lipid and protein molecules that make

The lipid and protein molecules that make up the plasma membrane are manufactured inside the cell and routed to the cell surface. The membrane is a dynamic structure, with molecules constantly being added to and removed from the plasma membrane as a cell moves and grows.

Function


The plasma membrane forms an extremely


The plasma membrane forms an extremely effective seal around the cell. Only a very few molecules can pass directly through the lipid bilayer to get from one side of the membrane to the other. Many substances that a cell needs in order to survive cannot cross the lipid bilayer on their own, including glucose (a sugar that cells burn for energy), amino acids (the building blocks of proteins), and ions, such as sodium and potassium. A cell uses two methods to move such substances from one side of the plasma membrane to another, known as passive transport and active transport. Both of these processes involve proteins in the plasma membrane.

Passive transport is accomplished by diffusion

Passive transport is accomplished by diffusion, the spontaneous movement of a substance from a region of greater concentration to a region of lesser concentration. The difference between the concentration of a substance in two different areas is known as a concentration gradient. Diffusion moves molecules down a concentration gradient in a manner that does not require the cell to expend energy. Water, oxygen, carbon dioxide, and a few other small molecules diffuse directly across the plasma membrane by passing between phospholipid molecules. Substances that cannot pass directly through the plasma membrane diffuse into or out of cells with the aid of hollow, channel-like proteins in a process known as facilitated diffusion. These channel proteins are shaped so that only one substance, or a small group of closely related substances, can pass through each type of protein. This specificity enables a cell to control precisely the molecules that travel in and out of the cell.

In order to move substances against a concentration

In order to move substances against a concentration gradientthat is, from the side of the plasma membrane where the concentration of a substance is lower to the side where it is already highera cell must expend energy in a process known as active transport. Active transport is achieved by membrane proteins called pumps, which have a docking site that is shaped to fit a specific substance. These pumps are open on either the inside or the outside of the cell. When the proper molecule or ion attaches to the docking site, the pump changes shape so that the docking site moves its opening to the other side of the plasma membrane, releasing the molecular cargo. Many pumps obtain the energy necessary to perform this work from adenosine triphosphate (ATP), a molecule that serves as the main energy currency of living cells.

Two additional transport mechanisms provide

Two additional transport mechanisms provide pathways for large molecules to pass in and out of cells. In endocytosis, the plasma membrane folds inward, forming a pouch that traps molecules. The pouch continues to press inward until it forms a closed sac that breaks loose from the plasma membrane and sinks into the cell. The second mechanism, exocytosis, is a reversal of endocytosis. A sac inside the cell containing proteins and other molecules moves toward the outer edge of the cell until it touches the plasma membrane. The membrane of the sac then joins with the plasma membrane, and the contents of the sac are released from the cell. Most of the proteins released by animal cells, such as hormones and antibodies, exit the cells where they are made through exocytosis.

In multicellular organisms

In multicellular organisms, the plasma membrane also plays a critical role in communication between cells. Proteins embedded in the plasma membrane act as receptors, binding to hormones and other molecules sent as signals from other cells. In animal cells, certain membrane proteins also act as markers that help the immune system distinguish the body`s own cells from foreign cells. These marker proteins help trigger the immune reaction that protects humans and other animals from disease-causing organisms such as bacteria, viruses, and fungi. These markers also play a role in the rejection of transplanted tissues and organs.

In certain types of cells

In certain types of cells, the plasma membrane has a wide variety of additional functions. Some membrane proteins are involved in holding neighboring cells together. In bacteria, plasma membrane proteins participate in photosynthesis and other reactions supplying the cell with energy.

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