Long Term Storage
Lipids are excellent storage compounds
- Lipid macromolecules, like carbohydrates, contain carbon, hydrogen and oxygen atoms
- However, unlike carbohydrates, lipids contain a low proportion of oxygen
- More of the oxygen required for their respiration has to come from the air
- This allows lipids to be energy-dense, maximising the energy content per gram versus carbohydrates
- They contain 2× more energy per gram than most carbohydrates
- Less body mass is required to store a given amount of energy
- Lipids are insoluble so do not affect osmosis, so do not risk upsetting the water balance of the organism
- 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 a water sac, it 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
- This is called metabolic water and can be used as a dietary water source when drinking water is unavailable
- All these features make lipids ideal for long term energy storage
Forms of lipid storage
- In animals, lipids are stored in various areas
- Subcutaneous fats are stored below the skin
- Visceral fats are stored around the major internal organs
- There are genetic and gender differences between how individuals store fat
- 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
- 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
Other roles of lipids
- As well as energy storage molecules, lipids have a number of other roles
- Physical protection of soft organs eg. visceral fat around the heart
- Thermal insulation from subcutaneous fat eg. whale blubber
- Subcutaneous fat as a buoyancy aid eg. in seals (fat is less dense than water so assists flotation)
- Waterproofing secretions eg. birds' preening glands or waxy cuticles on leaf surfaces
- Electrical insulation eg. the myelin sheath around certain nerve axons
- Certain photosynthetic pigments eg. carotenoids
- Glycolipids, typically as cell-surface recognition molecules/receptors
Examiner Tip
Ensure that you are familiar with the structure of a triglyceride and that you can recognise whether the fatty acids are saturated or unsaturated.