Physiology of the Cell Membrane

There are many functions to cell membranes. Cell membranes give cells their distinct shape and provides a barrier to the cell, which prevents the diffusion of certain substances in and out of the cell. Intracellular membranes, which are membranes within the cell, help organelles maintain their optimal environment by defining certain compartments within the cells.

There are also many components to the cell membrane. These components can be described by the fluid mosaic model. This model shows that proteins are integrated with the lipid bilayer. Different areas of the cell membrane have different functions. There are different proteins in these different areas of the cell membrane. This makes the membrane functionally asymmetrical. The main components of the cell membrane are phospholipids, proteins, and carbohydrates. These components are explained below.

A major component of membrane structure are phospholipids. Phospholipids are organized in a lipid bilayer and are amphipathic. Amphipathic means that the molecule has two or more different properties. The phospholipid has a hydrophilic head, which means that the phospholipid head is "water loving" or attracted to water. The phospholipid also has a hydrophobic tail, which means that the phospholipid tail is "water hating" or repelled by water. When phospholipid molecules are placed in an oil solution layered by water, the hydrophilic head is attracted to the water layer, while the hydrophobic tail becomes attracted to the oil. When phospholipid molecules are placed in regular aqueous solutions, they spontaneously form a bilayer. The hydrophobic tails are in the interior of the bilayer, while the hydrophilic heads are pointed to the exterior, where the aqueous solution is located.

Fluidity is the ease of movement of a phospholipid along their monolayer. Phospholipids can move freely along their monolayer but cannot flip-flop across to the opposite monolayer. One factor that effects membrane fluidity are the fatty acid tails of the phospholipids. Tails with zero double bonds are considered to be saturated. Tails with one or more double bonds are considered to be unsaturated. An increase in double bonds or an increase of unsaturation cause an increase in membrane fluidity. Another factor that affects fluidity of the cell membrane is cholesterol. Cholesterol makes bilayers less fluid by binding weakly to adjacent phospholipids. Finally, temperature affects the fluidity of the cell membrane. The cell membrane become stiffer at lower temperatures. Through evolution, animals have an alteration in the number of double bonds and saturation according to the climate of their habitat.

There are also proteins in the cell membrane. Integral proteins are proteins that penetrate the lipid bilayer. These types of proteins require detergents or organic solvents for removal and possess both hydrophobic and hydrophilic regions. Peripheral membrane proteins are held to the cell membrane by non-covalent bonds. These proteins can be removed easily without disruption to the membrane. There are many functions to these proteins. The five functional types are explained below according to Animal Physiology. Keep in mind that these functional types are not mutually exclusive.

Channel

Channels allows simple diffusion of solutes and osmosis of water in aqueous solutions into the cells.

Transporter

Transporters non-covalently bond to particles and move them across the cell membrane. If metabolic energy is involved, it is considered to be active transport. If no metabolic energy is consumed, it is considered to be passive transport.

Enzyme

Enzymes catalyze chemical reactions.

Receptor

Specific molecules bind noncovalently to these receptor proteins, which cause a change in membrane permeability or metabolism of the cell.

Structural Proteins

Structural proteins anchor intracellular elements and enables communication between adjacent cells by creating junctions between cells.

The final major component of the cell membrane are carbohydrates. These carbohydrate groups are usually covalently bonded to lipid or proteins, and are hydrophilic, which causes the carbohydrate group to be on the surface of the cell membrane. Carbohydrate groups always project from the extracellular face of the cell membrane. Proteins with a carbohydrate group attached are called glycoproteins. Lipids with a carbohydrate group attached are called a glycolipids. The carbohydrate groups have two functions, which are to serve as recognition sites for cell interactions and attachment sites for extracellular proteins.

Resources

Hill, Richard W., Gordon A. Wyse, and Margaret Anderson. Animal Physiology. Sunderland, MA: Sinauer Associates, 2008. Print.