Lipids
Lipids
- Lipids are the fats and oils — the body's long-term energy store.
- They do not mix with water, and that single fact explains what they do.
- One special lipid builds every cell membrane.
Triglycerides
- A triglyceride is one glycerol joined to three fatty acids by ester bonds (each bond forms by condensation, so three waters are removed).
- It is non-polar and hydrophobic — it will not mix with water.
- Fatty acids are saturated (no C=C double bonds, straight, usually solid fats) or unsaturated (one or more double bonds, kinked, usually liquid oils).
A triglyceride is made from:
A triglyceride = 1 glycerol + 3 fatty acids, joined by ester bonds (each forms by condensation).
An unsaturated fatty acid differs from a saturated one because it:
Unsaturated fatty acids contain C=C double bonds, which kink the chain; saturated ones have none and are usually solid.
Why fat is a great energy store
- It releases about twice the energy per gram as carbohydrate.
- It is insoluble, so it stays put and has no osmotic effect.
- It stores no extra water, so it adds little mass.
- Under the skin it also gives insulation and protects organs.
One reason triglycerides are good long-term energy stores is that they:
Lipids release ~2× the energy per gram of carbohydrate, are insoluble, and store no water.
Phospholipids and the bilayer
- A phospholipid is a triglyceride with one fatty acid replaced by a phosphate group.
- This gives it two ends: a hydrophilic ("water-loving") polar phosphate head and two hydrophobic fatty-acid tails.
- In water the heads face out and the tails hide inside — forming the bilayer that makes every cell membrane.
A phospholipid forms membranes because it has:
The polar head is attracted to water and the non-polar tails avoid it, so phospholipids arrange into a bilayer.
In a phospholipid bilayer, the hydrophobic tails point inward and the hydrophilic heads face the water.
Correct — heads face the watery surroundings on both sides; tails meet in the middle, away from water.
You've got it
- triglyceride = 1 glycerol + 3 fatty acids joined by ester bonds (condensation)
- saturated = no C=C (solid fats); unsaturated = has C=C double bonds (liquid oils)
- great energy store: ~2× energy/gram, insoluble, no water, insulation
- phospholipid = hydrophilic head + hydrophobic tails → forms the membrane bilayer