Lipids are primarily composed of carbon, hydrogen and oxygen, although some forms of lipids may include additional elements (e.g. N and P)

Unlike the other three types of organic molecules, lipids do not form polymers (although they may be constructed from identifiable subunits)

Lipids are synthesised in the smooth endoplasmic reticulum and come in a wide variety of structural and chemical forms

Types of Lipids

  • Triglycerides:  Function as a long-term energy source in animals (fats) and plants (oils)
  • Phospholipids:  Structural component of cell membranes
  • Steroids:  Act as hormones in plants and animals, and is a structural component of animal cell membranes (cholesterol)
  • Waxes:  Act as a protective layer against water loss in plant leaves and animal skin
  • Carotenoids:  Light-absorbing accessory pigment in plants (involved in photosynthesis)
  • Glycolipids:  Complex of carbohydrate and lipid that acts a cell receptor molecule

Function of Lipids

Lipids can serve many functions within the cell, including:

  • Storage of energy for long-term use (e.g. triglycerides)
  • Hormonal roles (e.g. steroids such as estrogen and testosterone)
  • Insulation (retention of thermal energy)
  • Protection of internal organs (e.g. triglycerides and waxes)
  • Structural components (e.g. phospholipids, cholesterol)

Biosynthesis of Lipids

  • Triglycerides are formed when glycerol is joined to three fatty acid chains via condensation reactions (producing three molecules of water)
  • Animals tend to store triglycerides as fats (solid form) while plants tend to store triglycerides as oils (liquid form)
  • Triglycerides can be either saturated (fatty acids have no double bonds) or unsaturated (fatty acids have double bonds)
  • Phospholipids are synthesised when a phosphate group is bonded to glycerol instead of a third fatty acid
  • Because the phosphate is polar and the fatty acids are non-polar, phospholipids contain both hydrophilic and hydrophobic regions

Triglycerides and Phospholipids