3.1.3 Lipids Flashcards
2 groups of lipid
- Triglycerides (fats and oils)
- Phospholipids
Characteristics of lipids
- Water insoluble
- Soluble in organic solvents (e.g. alcohols)
- Contain C, H and O. Contain less O compared to carbohydrates
How are triglycerides formed?
Condensation of 3 fatty acid molecules and 1 glycerol molecule
* Condensation reaction between glycerol and fatty acid (RCOOH) = ester bond
Roles of lipids
- Cell-surface membranes
- Source of energy
- Insulation
- Protection
Roles of lipids: Cell-surface membranes
- Phospholipids contribute to the flexibility of membranes and the transfer of lipid-soluble substances across them
Roles of lipids: Source of energy
When lipids are oxidised:
* lipids produce more than twice the amount of energy that the same amount of carbohydrate produces
* releases valuable water
Roles of lipids: Insulation
- Slow conductors of heat
- so when beneath the body surface, help to retain heat
- electric insulator in the myelin sheath around nerve cells
Roles of lipids: Protection
- Stored around delicate organs (e.g. kidney)
Triglycerides
- Made up of 3 fatty acids and 1 glycerol
What causes the differences in the properties of different triglycerides (fats and oils)?
- Variations in the fatty acids (many different ones)
Structure of fatty acid
- Carboxyl (-COOH) group attached to hydrocarbon
Different types of fatty acid
- Saturated
- Mono-unsaturated
- Polyunsaturated
Fatty acids: Saturated
No double bonds between adjacent C atoms in a hydrocarbon chain
Fatty acids: mono-unsaturated
1 double bond between adjacent C atoms in a hydrocarbon chain
Fatty acids: polyunsaturated
more than 1 double bond between adjacent C atoms in a hydrocarbon chain
Adaptations of triglycerides
- High energy ratio stored in C-H bonds: C atoms = good source of energy
- Low mass: energy ratio = good storage molecules (beneficial to animals as less mass to carry as they move around)
- Large and non-polar, so water insoluble = storage of lipids doesn’t affect osmosis or water potential
- High H: O ratio = triglycerides release water when oxidised
Phospholipids
- 1 fatty acid in lipids is replaced by a phosphate molecule
Polarity of triglycerides
Non-polar
Polarity of phospholipids
polar
Parts of a phospholipid
- Hydrophilic phosphate head
- Hydrophobic fatty acids tail
- The fatty acids face away from water, and the phosphate faces towards water
Hydrophilic
Attracted to water
Hydrophobic
Repels water
Adaptations of phospholipids
- Polar = in aqueous environments form a bilayer in cell-membranes (forms hydrophobic barrier between inside and outside of cell)
- Hydrophilic phosphate heads = help to hold
- Structure allows them to form glycolipids (by combining with carbohydrates within the cell-surface membrane)
Glycolipids
- Phospholipids + carbohydrates
- Important for cell recognition
Test for lipids
- To a dry and grease-free test tube, add the sample and ethanol
- Shake thoroughly (to dissolve the lipid)
- Add water
- Shake gently
- If lipid is present, a cloudy white emulsion will form
- As a control, repeat with water instead of sample (should remain clear)
Why does a cloudy-white emulsion form?
Dispersion of lipid in the water
How to test a solid sample for lipids?
- Dissolve in alcohol
- Add water
