Cell Membrane

Cell Membrane
This image shows a section of permeable cell membrane, including proteins, carbohydrate chains, and cholesterol. Credit: Science Picture Co/Getty Images

The cell membrane is a thin semi-permeable membrane that surrounds the cytoplasm of a cell, enclosing its contents. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell, while keeping other substances out.

The cell membrane also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus, it helps to support the cell and maintain its shape.

In addition, several cell organelles are also enclosed by a single outer membrane covering. Mitochondria and chloroplasts are surrounded by a double membrane.

Cell Membrane Components

The cell membrane is primarily composed of a mix of proteins and lipids. While lipids help to give membranes their flexibility, proteins monitor and maintain the cell's chemical climate and assist in the transfer of molecules across the membrane.

Phospholipids are a major component of cell membranes. They form a lipid bilayer in which their hydrophilic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane.

Cell membranes also include cholesterol and glycolipids.

Glycolipids are lipids that have a carbohydrate attached to them. Glycolipids provide energy for the cell and also aid in communication and recognition between cells.

Cell Membranes and Molecule Transport

One of the main functions of the cell membrane is to control what enters or exits a cell. Needed substances that can not diffuse across the membrane are helped across by facilitated diffusion or active transport.

In facilitated diffusion, molecules and ions such as glucose, sodium, and chloride, are transported into the cell by ion channel proteins and carrier proteins. In active transport, energy in the form of ATP (adenosine triphosphate) is needed to pump the molecules against their concentration gradients.