Cell-surface membrane structure

• The membranes around and within cell organelles have the same basic structure. They are called plasma membranes.
• The cell-surface membrane is the name specifically given to the plasma membrane that surrounds cells. The overall function of this membrane is to regulate transport of materials entering and exiting the cell.
• The molecules that make up the cell surface membrane include:

  Phospholipids

  Proteins

  Cholesterol

  Glycolipids

  Glycoproteins
• Phospholipids

  • Made up of phosphate, glycerol and two fatty acids.
  • The hydrophilic heads point to the outside of the cell surface membrane.
  • Lipid soluble substances move through the membrane through the phospholipid portion.
• Proteins

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

  1. Some proteins exist in the surface of the bilayer, and they never extend completely across it. They either act to give mechanical support or together with glycolipids act as receptors for proteins such as hormones.

  Other proteins completely span the phospholipid bilayer. Some are protein channels. Others are carrier proteins that bind to ions, or molecules like glucose. They then change shape in order to move the molecules across the membranes.
• The function of proteins are to:

  Provide structural support

  Act as channels

  Allow active transport across the membrane through carrier proteins

  Form cell-surface receptors for identifying cells

  Help cells adhere (join) together

  Act as receptors for hormones
• Cholesterol

  Reduces the movement of other molecules including phospholipids.

  Makes the membrane less fluid at high temperatures.

  Helps to prevent the leakage of water and dissolved ions.
• Glycolipids

  Made up of a carbohydrate bonded with a lipid.

  The functions of glycolipids are to:

  Act as recognition sites

  Help maintain the stability of the membrane

  Help cells attach to each other
• Glycoproteins

  Made up of carbohydrates attached to proteins.

  The functions of glycoproteins are to:

  Act as recognition sites

  Help cells attach to each other

  Allow cells to recognise each other
• Permeability

  The cell surface membrane controls the movement of substances in to and out of the cell. Most molecules do not move freely across it because many are:

  Not soluble in lipids

  Too large to pass through channels

  Electrically charged
• The way in which molecules combine to make up the surface of the cell is described as the Fluid Mosaic Model

  -          Fluid because the phospholipid molecules can move in relation to each other.

  -          Mosaic because the proteins that are embedded in the membrane form a pattern that looks like a mosaic.