From Media Hex
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.
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 bilayerWhile 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 cellsIn 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 makeThe 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.
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.