Cell Membranes Health Article

Definition

A cell membrane (also known as a plasma membrane) is a thin semifluid structure that separates the contents of a cell or organelle from its surroundings.

Description

The environment inside of a cell is drastically different from that of its surroundings. Outside the cell, water- soluble ions and molecules create a harsh and toxic environment. From this, the cell must selectively absorb nutrients that are essential to its growth and function. It must also excrete toxic byproducts of its metabolism. The cell membrane subsequently has two major functions:

• It acts as a barrier, enclosing and protecting the components of a cell.
• It acts as a gate, controlling the flow of molecules in and out of the cell.

Higher-level cells known as eukaryotes contain specialized components, called organelles, that play dedicated roles in its growth and development. Each organelle is surrounded by a separate membrane whose function is similar to that of plasma membranes, but with a slightly different composition that enables the organelle to perform specific tasks.

Structure

Plasma and organelle membranes are composed primarily of lipids (fatty acids, sterols, or other water-insoluble molecules) and proteins (chains of amino acids). They differ in their proportion of lipids to proteins. For example, cell membranes of structures predominantly involved in energy production (e.g., the mitochondria) have a higher percentage of proteins, while membranes acting as insulators (e.g., the Schwann cell, which insulates some nerve fibers) have a higher proportion of lipids.

LIPIDS. A membrane is actually two layers of lipids that form a shell around the cell. This lipid bilayer is composed primarily of phosopholipids (lipids containing one or more phosphate groups), each with a hydrophilic (water-soluble) "head" and a hydrophobic (water-insoluble) "tail." The bilayer is the most stable configuration for phospholipids in a water environment, with the water- repelling tails sequestered in the middle of two layers of water-soluble heads. Thus the membrane forms a stable yet flexible configuration with a certain amount of fluidity: individual phospholipids can move rapidly across the surface of the membrane, and part to allow molecules soluble in organic media (e.g., other lipids, dissolved gases, etc.) to enter the cell.

PROTEINS. One type of protein can be loosely associated with the outside of the membrane; these are called extrinsic (or peripheral) proteins. Other proteins are tightly embedded in the membrane, and may extend from one side of the membrane to the other; these intrinsic or integral proteins are difficult to remove without destroying the membrane itself.

There are two general types of membrane proteins: transporters and receptors. Although some lipid-soluble molecules can permeate the cell membrane, many of the nutrients that a cell needs to function are too large to readily enter the cell. Transporters allow the cell to be

selective in which molecules it allows into its cytoplasm. Examples of transporters are channels, which facilitate free movement of molecules across the membranes, and pumps, which require a certain amount of energy in order to transport molecules. Transport proteins also exist in organelle membranes; transport channels have been shown to exist in the organelles of yeast cells and are essential to cell viability.

A cell must be able to communicate with its surroundings if it is going to adapt to changing conditions. Receptors are transmembrane proteins that detect signals from the extracellular environment and translate those signals into a cellular response. An example of a signal is the compound epinephrine (also known as adrenaline). Receptors specific to epinephrine detect its presence in the environment and bind to the molecule. This binding induces a cascade of events in the cell, resulting in increased production of glucose used as energy.