Lipids
Functions
-Energy source
-Energy storage (adipose cells)
-Cell surface membranes
-Insulation (whale blubber)
-Protection (waxy cuticles)
-Some hormones

All energy storage fats/oils contain glycerol and different fatty acids (some of which - essential fatty acids- must be taken in by the body as they cannot be made). All lipids have the same glycerol molecule but fatty acids can vary, they:

-Consist of an acid group and hydrocarbon chain (2-20 Cs long)
-Can have an unsat or sat C-H chain; unsat fatty acids will be more fluid as molecules push apart (oils contain unsat acids whilst solid fats are sat)

Types of lipid:
1) Triglycerides
1 glycerol and 3 fatty acids
Condensation reaction forms ester bond
Insoluble in water

2) Phospholipids
Structure very similar to triglycerides but only 2 fatty acids; the 3rd OH group joins to a hydrophilic phosphate 'head' instead
When surrounded by water, forms a bilayer that is the basis of plasma membranes in cells
Since phospholipids contain fatty acid tails they can be saturated or unsaturated- this controls the fluidity of the membranes and allows animals to adapt (animals in cold climates will have more unsaturated fatty acids in their membrane to keep it moving)

3) Cholesterol
Made from 4 carbon based rings
Fits in between phospholipids in membranes and provides mechanical stability

4)Cholesterol based hormones
Testosterone, oestrogen and vitamin D are all steroid hormones made from cholesterol
They can pass straight through the bilayer as well as the nuclear envelope and bind to their target receptor site (often in the nucleus)

Note: Cholesterol can form gallstones in bile and cause atherosclerosis in blood vessels. FHC is a cholesterol related genetic disease that causes strokes and mycardial infarctions; people do not have the receptor that binds to the protein that controls cholesterol in the blood so keep making more.

The respiration of lipids- Ester bond is broken in hydrolysis reaction (CO2, water and 'energy' released).
Long term, lipids can be stored in cells without affecting water potential for respiration at another time (e.g. camels store fat to release water).

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