Biology-Lipids

What are lipids made from?

Lipids are different from proteins and carbohydrates because they are not polymers formed from long chains of monomers. Lipids are made from a variety of different components, but they all contain hydrocarbons (Molecules that contain only hydrogen and carbon atoms). The components they are made from relates to the lipids functions.

The true lipids you need to know about is triglycerides and phospholipids

Triglycerides

Triglycerides have one molecule of glycerol with three fatty acids attached to it. Fatty acid molecules have long ‘tails’ made of hydrocarbons. The tails are hydrophobic they repel water molecules. These tails make lipids insoluble in water.

Fatty acids

All fatty acids consist of the same basic structure, but the hydrocarbon tail varies. There are two kinds of fatty acids - saturated and unsaturated.

The difference is in their hydrocarbon tails.

Saturated fatty acids

Saturated fatty acids don’t have any double bonds between their carbon atoms.

The fatty acid is saturated with hydrogen

Unsaturated fatty acids

Unsaturated fatty acids do have double bonds between carbon atoms which causes the chain to link

Triglyceride formation

Triglycerides are formed by condensation reactions. The image below shows a fatty acid joining to a glycerol molecule. An ester bond forms between the two molecules releasing a molecule of water - this is a condensation reaction. The process happens twice more to form a triglyceride

Phospholipids

The lipids found in cell membranes aren’t triglycerides they’re phospholipids. Phospholipids are pretty similar to triglycerides except one of the fatty acid molecules is replaced by a phosphate group.

The phosphate Group is hydrophilic ( attracts water), The fatty acid tails are hydrophobic ( repel water). This is important in the cell membrane.

Properties of lipids- Triglycerides

Triglycerides Are mainly used as energy storage molecules. they’re good for this because the long hydrocarbon tails of the fatty acid contains lots of chemical energy - A lot of energy is released when they’re broken down. Because of these tails, lipids contain about twice as much energy per gram as carbohydrates.

Also, they’re insoluble in water so they don’t affect the water potential of the cell and cause water to enter the cells by osmosis ( which would make them swell).

The triglyceride bundle together as insoluble droplets in cells because the fatty acid tails are hydrophobic ( water repelling) -The tailed face inward shielding themselves from water with their glycerol heads.

Properties of lipids- Phospholipids

Phospholipids make up the bilayer of cell membranes. Cell membranes control what enters and leaves a cell.

Phospholipid heads are hydrophilic and their tails are hydrophobic so they form a double layer with their heads facing out towards the water on either sides. The centre of the Bay layer is hydrophobic. The water soluble substances can’t easily pass through the membrane act as a barrier to those substances.

The emulsion test for lipids

If you want to test for the presence of lipids in a sample, you’ll need to do the emulsion test:

-Shake the test substance with ethanol for about a minute, then pull the solution into water

-Any lipid will show up as a milky emulsion

-The more lipid there is the more noticeable the milky colour will be