Biological Molecules - Lipids Flashcards
What is a triglyceride made up of?
One molecule of glycerol
with three fatty acids attached to it.
Fatty acid molecules have long ‘tails’
made up of hydrocarbons.
The tails are ‘hydrophobic’ (they repel water molecules).
These tails make lipids insoluble in water.
All fatty acids have the same basic structure,
but the hydrocarbon tail varies.
How are triglycerides formed?
Triglycerides are formed by condensation reactions.
OH in glycerol + OH in fatty acid –> triglyceride
Two more fatty acids are attached to the glycerol
in the same way (with ester bonds).
When the ester bond is formed a molecule of water is released.
What is the basic structure of a fatty acid?
O=C-R
|
R
What are the two kinds of fatty acids?
What is the difference between the two?
Saturated and unsaturated.
The difference between the two is their hydrocarbon tails (R group):
Saturated fatty acids don’t have any double bonds between their carbon atoms.
The fatty acid is ‘saturated’ with hydrogen.
Unsaturated fatty acids have at least one double bond between carbon atoms,
which cause the chain to kink.
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Monounsaturated means one C=C bond
Polyunsaturated means more than one C=C bond
If fatty acids are unsaturated, their shape is altered from a saturated molecule so the molecules in the lipid push apart, thus making it more fluid and oily.
Animals tend to have more saturated, and consequently solid at room temperature lipids whereas plants tend to have more unsaturated and so fluid at room temperature lipids.
What lipids are found in cell membranes?
Phospholipids.
What is the difference between a phospholipid and a triglyceride?
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.
What are triglycerides mainly used as?
Triglycerides are mainly used as energy storage molecules.
Why are triglycerides good for energy storage?
The long hydrocarbon tails of the fatty acids
contain lots of chemical energy
– a load of energy is released when they’re broken down.
Because of these tails, lipids contain about twice as much energy per gram as carbohydrates.
They’re insoluble,
so they don’t affect the water potential of the cell
and don’t cause water to enter cells by osmosis
(which would make them swell).
The triglycerides clump together as insoluble droplets in cells
because the fatty acid tails are hydrophobic (water-repelling)
– the tails face inwards, shielding themselves from water with their glycerol heads.
Why are phospholipids good for cell membranes?
Phospholipids make up the bilayer of cell membranes.
Cell membranes control what enters and leaves the cell.
- Their heads are hydrophilic and their tails are hydrophobic,
so they form a double layer
with their heads facing out towards the water on either side. - The centre of the bilayer is hydrophobic, so water-soluble substances can’t easily pass through it
- the membrane acts as a barrier to those substances.
What test do you use for lipids?
The emulsion test for lipids.
- Shake the test substance with ethanol for about a minute so that it dissolves, then pour 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.
Properties of lipids in comparison to carbohydrates?
In reference to energy, mass and solubility.
Lipids:
- lower proportion of O:C
- double energy:mass ratio than carbohydrates
- insoluble in water
- soluble in organic substances
- high H:O ratio so releases water when oxidised
Functions of lipids?
- energy storage
- making biological membranes
- insulation
- protection (e.g. against plant leaves from drying up)
- boyancy
- acting as hormones
- waterproofing
