Phospholipids are similar to triglycerides but instead of three fatty acids they have two and a phosphate group.

As well as proteins the membrane is composed of phospholipids. A phospholipid is made up of two parts:

• the phosphate head which is hydrophilic
• two fatty acid tails which are hydrophobic

In the cell membrane they arrange themselves into a double layer or bilayer. Their phosphate head, which is polar and hydrophilic (likes water), faces out into the water on either side. Their fatty acid tails, which are non-polar and hydrophobic (hates water), face each other in the centre of the bilayer.

The lipid bilayer is semi permeable and the passage of substances in and out of the cell is regulated. H₂O and particular little uncharged molecules, like O₂ and CO₂ can pass through.

The strength of a membrane depends on the fatty acids it contains. Animal cell membranes consist of cholesterol which stabilises the membrane by linking the fatty acids together.

——————————————————

Proteins

Proteins make up about 50% of the membrane mass and carry out a number of functions:

• carriers for water-soluble molecules like glucose
• channels for ions like sodium and chloride
• pumps to move water-soluble molecules and ions against the diffusion gradient
• receptors which allow nerve transmitters and hormones to bind to cells
• recognition sites so that the cell can be recognised
• adhesion sites which helps cells stick together
• enzymes to speed up chemical reactions occurring at the membrane edge
• adhesion molecules which allow cells to hold to the extracellular matrix

Like phospholipids they’re composed of hydrophilic amino acids, which are in contact with the water, and hydrophobic amino acids, which keep contact with the fatty acids in the centre of the membrane.

There are a number of different types of protein to carry out their specific roles:

• Transport proteins: these are integral proteins which allow small molecules to enter and leave the cell via facilitated diffusion or active transport.
• Receptor proteins: these are peripheral proteins. They have a specific binding site to which chemicals like hormones can bind, creating a hormone-receptor complex. Once this happens other processes, inside or outside the cell, are triggered.
• Recognition proteins: these are usually glycoproteins and are involved in cell recognition.
• Structural proteins: these peripheral proteins and can be located on the inner or outer membrane surface. Those located on the inside surface of the cell membrane and assist in either maintaining the structure of the cell or modifying it when necessary. Those located on the outside layer are involved with cell adhesion, where cells stick together either temporarily or permanently.
• Enzymes: these catalyse reactions either inside or outside the cell.