Lipids, Phopholipids and the Plasma Membrane

Summary of Cell membrane structure and function from AQA biology unit one. Hope they're helpful!

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Fatty acids

Fatty acids have a long hydrocarbon (carbon and hydrogen) chain with a carboxyl (acid) group. The chains usually contain 16 to 18 carbons.

Glycerol contains 3 carbons and 3 hydroxyl groups. It reacts with 3 fatty acids to form a triglyceride or fat


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Emulsion test for lipids

1. Mix ethanol and the test solution in equal volumes. Shake to help dissolve any lipids present

2.  Add an equal volume of water and shake again. Milky white precipitate means lipids! 

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Phospholipids are a special type of trygliceride (hydrocarbon chain and 3 fatty acid groups) which contain a phosphate group instead of one of the fatty acid components. 

  • The phosphate group is polar and therefore capable of interacting with water molecules.
  • Hydrocarbon chains of the two fatty acid components don't dissolve in water and are hydrophobic
  • structure and function of cell membranes depends on hydrophilic and hydrophobic properties of phopholipid molecules
  • Phospholipids spontaneously form a bilayer in a watery environment. They arrange themselves so that the polar heads are oriented toward the water and the fatty acid tails are oriented toward the inside of the bilayer (see the diagram below). 


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Phospholipids (2)

  • These have hydrophilic heads (phosphate group) and hydrophobic tails (hydrocarbon chains) 
  • Cells are aqueous within and bathed in water without.
  • As a result, water phospholipid molecules spontaneously form a stable two layer framework. This is a phospholipid bilayer. 
  • Hydrophobic tails point inwards, shielded from the water, while hydrophilic tails point outwards to form H bonds with the water
  • The hydrocarbon tails of many of the phospholipid molecules and kinked because of the double bonds present. As a result, the phospholipids are loosely packed; the plasma membrane is fluid and so proteins and other molecules can move sideways within the membrane.
  • Cholesterol is lipid. It's molecules (wedged into the bilayer) help keep the plasma membrane fluid at low temperatures.
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The plasma membrane

  • Cells are surrounded by a plasma membrane, which looks like this:  (
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Proteins within the phospholipid bilayer

Different proteins are embedded in the phospholipid bilayer. Some extend through the membrane, others are local to one side of the membrane or the other. The different proteins carry out many of the different functions of the plasma membrane. Functions include:

  • action as enzymes- e.g. enzymes on the surfaces of cells lining the intestine catalyse reactions which digest food
  • Transport of substances across the membrane
  • Maintenance of cell shape
  • Formation of structures which stick cells together
  • binding of messenger molecules. Proteins that messenger molecules bind to are called receptors. They trigger particular activities in the cell. 
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Glycolipids and Glycoproteins

  • sugars bond to protein and lipids embedded in the outside surface of the plasma membrane
  • A combination of a carbohydrate and protein forms a glycoprotein
  • A combination of a carbohydrate and lipid forms a glycolipid
  • Glycolipids are receptors for chemicals that act as signals between cells. This is important because it means that cells can recognise each other. Glycoproteins work similarly
  • Glycoproteins also allow cells of the immune system to recognise foreign cells which may be pathogens
  • Glycoproteins and  Glycolipids vary form species to species and from one individual to another of the same species. They act as little tiny identification tags on cells.
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