Lipids (HL IB Biology)

• Examples of lipids in living organisms are
  • Fats
  • Oils
  • Waxes
  • Steroids
• Lipid macromolecules contain carbon, hydrogen and oxygen atoms

Basic lipid structure diagram

Lipid molecules are composed of a glycerol molecule and fatty acid hydrocarbon chains

Lipid solubility

• The structure of lipids affects their solubility
• Lipids contain hydrocarbon molecules which contain many non-polar covalent bonds
• The non-polar nature of lipid molecules means that lipids are insoluble in water or other polar solvents
• In living organisms, lipid solubility can be improved by combining lipid molecules with other molecules, e.g.
  • Glycolipids
  • Lipoproteins

Formation of triglycerides

• Some lipids are categorised as triglycerides
• Three fatty acids join to one glycerol molecule to form a triglyceride
  • Fatty acids contain hydrocarbon chains that can be either saturated or unsaturated
    • Saturated fatty acids contain only single carbon-carbon bonds
    • Unsaturated fatty acids contain one or more double bonds
• Triglycerides are formed by a process known as esterification
  • An ester bond forms when the hydroxyl (-OH) group of a glycerol molecule bonds with the carboxyl group (-COOH) of a fatty acid
  • The formation of an ester bond is a condensation reaction
    • For each ester bond formed a water molecule is released
    • Therefore for one triglyceride to form, three water molecules are released

Formation of a triglyceride diagram

A triglyceride forms from a glycerol molecule and three fatty acid molecules by the process of esterification

Formation of phospholipids

• Phospholipids are also formed from glycerol and fatty acids
• Unlike triglycerides, phospholipids contain only two fatty acids bonded to a glycerol molecule, as the third has been replaced by a phosphate ion (PO₄^3-)
• As the phosphate is polar, it is soluble in water, or hydrophilic
• The fatty acid ‘tails’ are non-polar and therefore insoluble in water, or hydrophobic
• Phospholipids are said to be amphipathic, meaning that they have both hydrophobic and hydrophilic regions
  • As a result of having hydrophobic and hydrophilic parts, phospholipid molecules can form monolayers or bilayers when placed in water

Structure of a phospholipid diagram

Phospholipids are the major components of cell surface membranes. They have fatty acid tails that are hydrophobic and a phosphate head that is hydrophilic, attached to a glycerol molecule.

Lipids as an energy store

• The hydrolysis of triglycerides releases glycerol and fatty acids, which can form useful respiratory substrates
• Lipids are energy-dense in comparison to carbohydrates due to their high number of C-H bonds
  • They contain 2× more energy per gram than most carbohydrates
• Lipids are insoluble so are not transported around the body easily and remain in their storage cells
• When lipids are respired a lot of water is produced compared to the respiration of carbohydrates
  • This is called metabolic water and can be used as a dietary water source when drinking water is unavailable
    • A camel's hump is not filled with water, but is a lipid-rich storage organ that yields metabolic water for the camel in its dry desert habitat
    • A bird's egg also makes use of lipid-rich yolk to provide energy and metabolic water to the growing chick
• All these features make lipids ideal for long term energy storage

Storage of lipids

• In animals, lipids are stored in adipose tissue
  • Subcutaneous fats are stored below the skin
  • Visceral fats are stored around the major internal organs
• Fat is stored in adipose cells, which are specialised to contain large globules of fat
  • Adipose cells shrink when the fat is respired to generate metabolic energy
• Adipose tissue can be used as a thermal insulator in animals that live in particularly cold environments
  • Seals and walruses are endotherms and have thick adipose tissue called blubber which helps trap heat generated by respiration

• In many plants, seeds have evolved to store fats to provide energy for a growing seedling plant
  • Olives, sunflowers, nuts, coconuts and oilseed rape are good examples of crops whose oils are harvested for edible oil production by humans