4.1 Structure of the cell surface membrane

All membranes in and around a cell, so this includes the membranes around organelles, have the same basic structure and are known as plasma membranes. The cell-surface membrane is the name given to the plasma membrane that surround the cell and is the boundary between the cytoplasm and the environment. This is what allows different conditions to occur inside the cell. It also controls the movement of substances into and out of the cell. There are a few different molecules that make up its structure and these are:

-          Phospholipids

-          Proteins

-          Cholesterol

-          Glycolipids

-          Glycoproteins

Phospholipids form a bilayer. They are important components of cell-surface membranes for the following reasons:

-          The hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane attracted by water on both sides

-          The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by water on both sides

Lipid soluble material moves through the membrane via the phospholipid portion. The functions of phospholipids in the membrane are to:

-          Allow lipid soluble substances to enter and leave the cell

-          Prevent water soluble substances entering and leaving the cell

-          Make the membrane flexible and self sealing

Proteins are interspersed throughout the cell surface membrane. They are embedded in the phospholipid bilayer in two main ways:

-          Some proteins occur in the surface of the bilayer and never extend completely across it. They act either to give mechanical support to the membrane or as cell receptors for molecules such as hormones

-          Other proteins completely span the phospholipid bilayer from one side to the other. Some are protein channels, which form water filled tubes to allow water soluble ions to diffuse across the membrane. Others are carrier proteins that bind to ions or molecules like glucose and amino acids, then change shape in order to move these molecules across the membrane

The functions of the proteins in the membrane are to:

Provide structural support, act as channels transporting water-soluble substances across the membrane, allow active transport across the membrane through carrier proteins, form cell surface receptors for identifying cells, help cells adhere together, act as receptors, for example for hormones.

Cholesterol molecules occur within the phospholipid bilayer of the cell surface membrane. They add strength to the membranes. Cholesterol molecules are very hydrophobic and therefore play an important role in preventing loss of water and dissolved ions from the cell. They also pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane as a whole too rigid.

The functions of cholesterol in the membrane are to:

- Reduce the lateral movement of other molecules including phospholipids

- Make the membrane less fluid at high temperatures

- Prevent leakage of water and dissolved ions from the cell

Glycolipids are made up of a carbohydrate covalently bonded with a lipid. The carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell surface receptor for specific chemicals, for example the human ABO blood system operates as a result of glycolipids on the cell-surface membrane.

The functions of glycolipids in the membrane are to:

- Act as recognition sites 

- Help maintain the stability of the membrane

- Help cells to attach to one another and so form tissues 

Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane. These glycoproteins also act as cell-surface receptors, more specifically for hormones and neurotransmitters.

The functions of glycoproteins in the membrane are to:

- Act as recognition sites

- Help cells attach to one another and so form tissues

- Allows cells to recognise one another, for example lymphocytes can recognise an organism's own cells  

The cell-surface membrane controls the movement of substances into and out of the cell. In general most molecules do not freely diffuse across it because many are:

- Not soluble in lipids and therefore cannot pass through the phospholipid layer

- Too large to pass through the channels in the membrane

- Of the same charge on the protein channels and so, even if they are small enough to pass through, they are repelled

- Electrically charged (polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.

The way in which all the various molecules are combined into the structure of the cell surface membrane. This arrangment is known as the fluid mosaic model for the following reasons:

- Fluid becuase the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing in shape.

- Mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic.