Triglycerides & Phospholipids

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Triglycerides

Triglycerides are a type of lipid that are mainly used as energy storage molecules.

Formation of triglycerides

Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid.
- Ester bonds form between the glycerol and the fatty acid chains.
- One water molecule is released per ester bond.
  - So, three molecules of water are released per triglyceride formed.

Structure of fatty acids

Fatty acids have long 'tails' made of a chain of hydrocarbons with 4-36 carbon atoms.
The hydrocarbon tail is variable but most fatty acids contain 12-18 carbons.
Glycerol links to the central carbon atom on fatty acids.

Types of fatty acids

The three fatty acids in the triglyceride may be similar or dissimilar.
Fatty acids may be saturated or unsaturated.
Some fatty acids have names that come from their origin.
- E.g. Palmitic acid is a saturated fatty acid that comes from the palm tree.

Functions of Triglycerides

The structures of triglycerides are related to their functions as molecules responsible for storing energy.

Triglycerides in energy release

Chemical energy is stored in the fatty acid hydrocarbon tails.
So, lots of energy is released when triglycerides are broken down.

Lipids contain lots of energy

Carbohydrates contain half the amount of energy per gram as lipids do.

Triglycerides repel water

Triglycerides are insoluble in water because the fatty acid tails are hydrophobic.
This means that the cell's water potential is NOT affected by triglycerides.
- This is important because if triglycerides didn't repel water, the water would enter the cell through the process of osmosis. This would make the cells swell up.

Lipid droplets

In cells, the insoluble triglycerides crowd together as droplets because the hydrophobic fatty acid tails face inwards.

Phospholipids

Phospholipids are a type of lipid that forms a bilayer. They are the main component of cell membranes and are responsible for controlling what goes into and out of cells.

Structure of phospholipids

Phospholipids and triglycerides are quite similar. They are both composed of fatty acid chains attached to glycerol.
The main difference is that one of the three hydrophobic fatty acid tails is replaced by a hydrophilic phosphate group in phospholipids.
This means that a phospholipid molecule has a hydrophobic and hydrophilic part.
- This kind of molecule is called an amphipathic molecule.

Hydrophobic tail

Phospholipids have two fatty acid chains.
The two fatty acid chains in phospholipids are hydrophobic and cannot interact with water.
In membranes, the hydrophobic fatty acid tails face inwards in the bilayer.
- This means that water-soluble substances can't easily pass through the membrane.

Hydrophilic head

The modified phosphate group in phospholipids is hydrophilic and can interact with water.
In membranes, the hydrophilic phosphate group faces the outside.