2.2.8 Lipids: Strucure & Functions

Question 1

Triglycerides

Answer 1

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
- protection

Question 2

Energy Storage

Answer 2

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 ore can be stored
- plants store triglycerides, in the form of oil, 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 adds kinks to the fatty acids chains, altering their properties
- mammals store triglycerides as oil droplets in adipose tissue to help them survive when food is scare (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 left to drink
Bird and reptile embryos in their shells also use this water

Question 3

Insulation

Answer 3

Triglycerides are part of the composition of the myelin sheath that surrounds nerve fibres
- the myelin sheath provides insulation which increase 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 (e.g. blubber of whales)

Question 4

Buoyancy

Answer 4

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

Question 5

Protection

Answer 5

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

Question 6

Phospholipids

Answer 6

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Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule in a phospholipid as one had been replaced by a phosphate ion (PO4 3-)
As the phosphate is polar, it is soluble in water (hydrophilic) whereas the fatty acid ‘tails’ are non-polar and therefore insoluble in water (hydrophobic)
This means that phospholipids are amphipathic
- i.e. they have both hydrophobic and hydrophilic parts
As a result of of being amphipathic, phospholipids molecules form monolayers or bilayers in water
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 from H-bonds with water, allowing the cell membrane to be used to compartmentalise
- compartmentalisation enable 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 acids tails then the membrane will be more fluid
Phospholipids control membrane protein organisation
- weak hydrophobic interaction between phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer

Question 7

Cholesterol

Answer 7

Another another important lipid molecule found in the cell membrane of eukaryotic cells is cholesterol
Just like phospholipid molecules, the 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 distrusts the close-packing of phospholipids, increasing the flexibility of the membrane
- 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