2.2.8 Lipids: Structure & Function

Triglycerides

• Triglycerides are fats and oils
• Fatty acid and glycerol molecules are the components that make up triglycerides
• Fats and oils have a number of important functions in organisms: energy storage, insulation, buoyancy, and protection

Energy storage

• The long hydrocarbon chains in triglycerides contain many carbon-hydrogen bonds with little oxygen (triglycerides are highly reduced)
  • So when triglycerides are oxidised during cellular respiration this causes these bonds to break releasing energy used to produce ATP

• Triglycerides, therefore, store more energy per gram than carbohydrates and proteins (37kJ compared to 17kJ)
• As triglycerides are hydrophobic they do not cause osmotic water uptake in cells so more can be stored
  • Plants store triglycerides, in the form of oils, in their seeds and fruits. If extracted from seeds and fruits these are generally liquid at room temperature due to the presence of double bonds which add kinks to the fatty acid chains altering their properties
  • Mammals store triglycerides as oil droplets in adipose tissue to help them survive when food is scarce (e.g. hibernating bears)

• The oxidation of the carbon-hydrogen bonds releases large numbers of water molecules (metabolic water) during cellular respiration
  • Desert animals retain this water if there is no liquid water to drink
  • Bird and reptile embryos in their shells also use this water

Insulation

• Triglycerides are part of the composition of the myelin sheath that surrounds nerve fibres
  • The myelin sheath provides insulation which increases the speed of transmission of nerve impulses

• Triglycerides compose part of the adipose tissue layer below the skin which acts as insulation against heat loss (eg. blubber of whales)

Buoyancy

• The low density of fat tissue increases the ability of animals to float more easily

Protection

• The adipose tissue in mammals contains stored triglycerides and this tissue helps protect organs from the risk of damage

Phospholipids

• Phospholipids are a type of lipid, therefore they are formed from the monomer glycerol and fatty acids
• Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule in a phospholipid as one has been replaced by a phosphate ion (PO₄^3-)
• As the phosphate is polar it is soluble in water (hydrophilic)
• The fatty acid ‘tails’ are non-polar and therefore insoluble in water (hydrophobic)

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.

• Phospholipids are amphipathic (they have both hydrophobic and hydrophilic parts)
• As a result of having hydrophobic and hydrophilic parts phospholipid molecules form monolayers or bilayers in water

In the presence of water due to the hydrophobic and hydrophilic parts phospholipids will form monolayers or bilayers

• Phospholipids are the main component (building block) of cell membranes
• Due to the presence of hydrophobic fatty acid tails, a hydrophobic core is created when a phospholipid bilayer forms
  • The core acts as a barrier to water-soluble molecules

• The hydrophilic phosphate heads form H-bonds with water allowing the cell membrane to be used to compartmentalise
  • Compartmentalisation enables cells to organise specific roles into organelles, helping with efficiency

• The composition of phospholipids contributes to the fluidity of the cell membrane
  • If there are mainly saturated fatty acid tails then the membrane will be less fluid
  • If there are mainly unsaturated fatty acid tails then the membrane will be more fluid

• Phospholipids control membrane protein orientation
  • Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer

Cholesterol

• Another important lipid molecule found in the cell membrane of eukaryotic cells is cholesterol
• Just like phospholipid molecules, cholesterol molecules have hydrophobic and hydrophilic regions
  • Their chemical structure allows them to exist in the bilayer of the membrane

• Molecules of cholesterol are synthesised in the liver and transported via the blood
• Cholesterol affects the fluidity and permeability of the cell membrane
  • It disrupts the close-packing of phospholipids, increasing the rigidity of the membrane (makes the membrane less flexible)
  • It acts as a barrier, fitting in the spaces between phospholipids. This prevents water-soluble substances from diffusing across the membrane

• Molecules of cholesterol are used to produce steroid-based hormones such as oestrogen, testosterone and progesterone

The structure of a cholesterol molecule possesses both a hydrophobic region and a hydrophilic region