Cell Membrane Structure and Function

The Fluid Mosaic Model

This is the currently accepted model of the cell membrane structure.

Fluid - the molecules which make up the cell membrane are constantly moving.

Mosaic - It is composed of many different molecules.

Model - as the cell membrane is far too small to be seen, even with an electron microscope, we use a model to describe its behaviour and composition.

Structure

Phospholipid Molecules:

The main structure consists of phospholipid molecules.

Phospholipids are molecules made up of lipids with a phosphate group attached.

In a cell membrane, they arrange in two layers, called a bilayer.

Each phospholipid molecule has two distinct regions:

1. A head that is hydrophilic or polar (water loving)

2. A tail that is hydrophobic or non-polar (water hating)

The heads are always facing outside as they are attracted to the intra/extracellular fluid. The tails face inwards, as they are repelled by the same fluids.

Cholesterol:

Cholesterol molecules are wedged between the phospholipid molecules.

They provide flexibility and improve the structural integrity of the membrane.

Proteins:

Receptor proteins - peripheral proteins that allow cell membrane to communicate with other cells.

Channel proteins - integral proteins that transport small, polar molecules such as water.

Carrier proteins - integral proteins that are responsible for facilitated diffusion and active transport.

Functions of the Cell Membrane

Acts as a physical barrier - separates the intracellular fluid (cytoplasm) from the extracellular fluid.

Regulates the passage of materials - controls the movement of materials in and out of the cell.

Able to sense changes - contains receptors that can respond to changes in the extracellular fluid very quickly.

Helps to support the cell - contains structural molecules that give the cell shape and allow it to join with other cells.