Cell biology

Cell membranes are vital to the way cells function. In animal cells, they form the outer layer of the cell – the ultimate barrier between the inside of the cell and its surroundings. In plant cells, the cell surface membrane is inside a relatively rigid cellulose cell wall but the properties of the membrane still control much of what moves into and out of the cell. Most of the organelles inside a eukaryotic cell are also membrane-bound. Understanding the properties of cell membranes is key to understanding how cells work.

The structure of the cell membrane

The cell membrane, however, is more than a simple unit membrane. Our current model is of a fluid phospholipid bilayer with many other molecules associated with it, floating or embedded in the lipid sea. These other molecules include cholesterol, glycolipids, proteins and glycoproteins and they all have different functions in the membrane. This is the fluid mosaic model of membrane structure and it explains many of the properties of membranes that we can observe experimentally.

The fluid mosaic model of the cell membrane.

• A Phospholipids: lipid molecules with a hydrophilic ‘head’ region around the ionic phosphate group and a long hydrophobic hydrocarbon tail that form a bilayer in aqueous solutions.
• B Cholesterol: a lipid with a steroid ring structure, and hydrophilic and hydrophobic regions. It makes up part of the membrane structure - there may be up to one cholesterol molecule for every two phospholipids. Cholesterol makes the membrane stiffer and more rigid – so the amount of cholesterol in the structure affects the rigidity of the membrane.
• C Glycolipids: lipids that have a carbohydrate chain attached to them. The carbohydrate chain is attached to the outside of the cell and is part of the cell recognition system.
• D Proteins: a wide variety of molecules that carry out many of the very specific functions of the cell membrane. There are integral proteins and peripheral proteins. They can form temporary and permanent channels in the membrane, allowing different molecules to pass in and out of the cell. They may be enzymes involved in active transport systems or enzymes linked to biochemical pathways such as photosynthesis or respiration. Proteins also act as receptor molecules for other molecules such as hormones and neurotransmitters.
• E Glycoproteins: proteins that have a carbohydrate chain attached to them. The carbohydrate chain sticks out of the outside of the cell and is part of the cell recognition system.