#11. Lipids - Triglycerides and Phospholipids.

Lipids:
- Include triglycerides + phospholipids.
- Molecules contain C, H, O atoms
- Very small proportion of O.
- Insoluble in water.



1.Triglycerides

- Made of glycerol 'backbone', attached to 3 fatty acids by ester bonds.

Fatty acids

• Have long chains of C and H atoms. 
• Each C atom has 4 bonds: 2 to C atoms, 2 to H atoms. 
• Sometime, only 1 H atom attached --> C atom has a 'spare' bond, attached to the next-door C atom (also has 1 H bonded) ---> double bond. 
• Unsaturated fatty acids has ≥ 1 C-C double bonds (do not contain quite as much H).
• Saturated fatty acids has no double bonds. 

• Lipids containing unsaturated fatty acids ---> unsaturated lipids (in plant) 
• Lipids containing completely saturated fatty acids ---> saturated lipids (in animal)  
• Unsaturated lipids tend to have lower melting points than saturated lipids.

Triglycerides are insoluble in water ---> energy storage in plants, animals and fungi. They contain more energy per gram than polysaccharides. 

     a. In mammals 

Triglyceride are build up beneath the skin in the form of adipose tissue:

• It cells contain oil droplets of triglycerides;
• Helps to insulate body against heat loss. 
• Relatively low density ---> ↑ buoyancy ---> useful for aquatic mammals living in cold water (whales, seals).
• Forms a protective layer around organs (e.g. kidneys).

    b.In plants
Triglycerides = major part of energy stores in seeds:

• Cotyledons (sunflower seeds...) 
• Endosperm (castor beans...).

2. Phospholipids

• Made of glycerol 'backbone', 2 fatty acids and 1 phosphate group.

• Fatty acid chains are hydrophobic: no electrical charge ---> not attracted to H₂O molecules.
• Phosphate group is hydrophilic: has electrical charge ---> attracted to H₂O molecules.
• In H₂O, phospholipid molecules arranges into a bilayer: hydrophilic heads facing outwards into the water + hydrophobic tails facing inwards, avoiding water.
• This is the basic structure of a cell membrane. 

3. Emulsion (Ethanol) test for lipids

• Dissolve the substance by mixing it with absolute ethanol. 
• Decant the ethanol into water.
• If lipids are present in the mixture, it will precipitates and forms an milky emulsion.

A milky emulsion indicates the presence of lipid. 

Syllabus 2015  (e) describe the molecular structure of a triglyceride and a phospholipid and relate these structures to their functions in living organisms;

Syllabus 2016  - 2018 f) describe the molecular structure of a triglyceride  with reference to the formation of ester bonds and relate  the structure of triglycerides to their functions in living organisms g)   describe the structure of a phospholipid and relate  the structure of phospholipids to their functions in living organisms