A membrane surrounds every living cell, keeping the cell's interior separated and protected from the outside world. Many factors affect how this membrane behaves and temperature is one of the most important. Temperature helps determine what can enter or leave the cell and how well molecules found within the membrane can function. Temperatures that are too high or too low can seriously damage and, in the extreme temperature ranges, kill the cell through their effect on the cell's membrane.
What Makes a Cell Membrane?
A cell membrane is called a bilayer because it's made of two layers that face each other and surround the cell. Chemically, each layer is formed by fatty molecules called phospholipids. Each molecule has an end that repels water, called its head, and another end called the tail that repels water. The nature of the phospholipids in the membrane helps keep it fluid and semi-permeable, so that some molecules like oxygen, carbon dioxide and small hydrocarbons can move through it and enter the cell, while other molecules that might be harmful or unneeded by the cell are kept out.
A cell membrane also contains proteins, either on its inner or outer surface – called peripheral proteins – or embedded in the membrane and called integral proteins. Because the membrane is fluid and not rigid, these proteins can move within the membrane to serve the cell's needs and help keep it healthy. Also, as cells grow and enlarge, the membrane also increases in size and maintains its fluidity to allow this growth to proceed smoothly.
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High Temperature Increases Fluidity
Cells function best at normal physiological temperature, which is 98.6 degrees Fahrenheit in warm-blooded animals like humans. If body temperature increases, for example during a high fever, the cell membrane can become more fluid. This happens when the fatty acid tails of the phospholipids become less rigid and allow more movement of proteins and other molecules in and through the membrane. This can change the permeability of the cell, possibly allowing some potentially harmful molecules to enter. Both integral and peripheral proteins in the membrane can also be damaged by high temperatures and, if extremely high, heat might cause these proteins to break down, or denature.
Low Temperature Stiffens the Membrane
A decrease in temperature can also have a negative effect on cell membranes and cells. At low temperature, the fatty acid tails of the phospholipids move less and become more rigid. This decreases the overall fluidity of the membrane, also decreasing its permeability and potentially restricting entry of important molecules such as oxygen and glucose into the cell. Low temperature can also slow cell growth by prevent the cell's increase in size. In extreme situations, such as prolonged exposure to sub-freezing temperatures, liquid in the cell can begin to freeze, forming crystals that pierce the membrane and might ultimately kill the cell.