Lipids

• Triglycerides and phospholipids are two groups of lipids 
• Lipids include fats and oils (triglycerides, steroids and sterols and waxes).
• Lipids Contain Carbon, hydrogen and oxygen, however the proportion of Oxygen is very small compared with carbohydrates. 
• Lipids are not polymers.
• A triglyceride molecule has one molecule of glycerol joined by three covalent ester bonds formed in condensation reactions to three fatty acids. Three molecules of water are removed.
• There are over 70 fatty acids each with a COOH group with a hydrocarbon chain attached. 
• Functions of triglycerides include:

  • Source of energy
  • Act as insulators
  • Waterproofing
• Triglycerides- Source of energy – lipids have a higher proportion of Hydrogen to oxygen atoms and when oxidised release twice the energy as the same mass as carbohydrates and proteins 
• Triglycerides- Act as insulators – fat is a slow conductor of heat and helps retain body heat (thermal insulator). It also acts as an electrical insulator as the myelin sheath in nerve cells 
• Triglycerides- Waterproofing - Are large non-polar molecules which are insoluble in water, therefore useful for waterproofing eg waxy cuticle in plants.
• Triglycerides- Protection- fat is stored around delicate organs and acts as a shock absorber.
• Fatty Acids:

  A) Saturated

  B) Unsaturated

  
• Saturated triglycerides – Fatty acid has no C=C bonds, all the carbon atoms are linked to the maximum number of hydrogen atoms.   
• Mono-unsaturated triglycerides – one C=C bond in fatty acid 
• Polyunsaturated triglycerides – have more than one C=C bonds  
• A Phospholipid:

  A) Fatty Acid

  B) Glycerol

  C) Phosphate

  
• Phospholipids are found in cell membranes. They have a similar structure to triglycerides.
• Phospholipids contain a glycerol molecule and two fatty acids. The third fatty acid is replaced by a phosphate group.
• The phosphate group of phospholipids is electrically charged (polar) which makes it attracted to water – hydrophilic.
• The Hydrocarbon chain/tail of phospholipids is repelled by water and is described as hydrophobic (non-polar).
• In water phospholipids form a bilayer – the hydrophobic tails point towards the centre of the bilayer, the hydrophilic heads point outwards.  
• The phospholipid bilayer forms a barrier which controls the movement of molecules through it. The hydrophobic tails prevent water soluble molecules passing directly through the bilayer, however lipid soluble molecules can diffuse through directly. 
• Small, non-polar, lipid soluble molecules can pass through the phospholipid bilayer.
• Large, charged, polar, water soluble molecules cannot pass through the phospholipid bilayer.