Lipids (College Board AP® Biology) : Study Guide
Structure & function in lipids
Lipids are macromolecules which contain carbon, hydrogen and oxygen atoms
There are two groups of lipids:
Triglycerides
Phospholipids
Triglycerides
Triglycerides are nonpolar, hydrophobic molecules
The monomers are glycerol and fatty acids
Glycerol is an alcohol
Fatty acids contain a methyl group at one end of a hydrocarbon chain (known as the R group) and at the other is a carboxyl group
The shorthand chemical formula for a fatty acid is RCOOH
A triglyceride is formed through a dehydration synthesis reaction
Covalent bonds form between glycerol and the fatty acid chains
For each bond formed a water molecule is released
Three fatty acids join to one glycerol molecule to form a triglyceride
Formation of a triglyceride from a glycerol molecule and three fatty acid molecules
Saturation
Fatty acids can vary in:
length of the hydrocarbon chain (R group)
saturation of the fatty acid chain (R group), which may be saturated or unsaturated
Differences in saturation determine the structure and function of lipids
Saturated fatty acids contain no double bonds between the carbon atoms
These chains are straight
Being straight means that the fatty acids can pack together tightly and so these fats are solid at room temperature
E.g. butte
Unsaturated fatty acids contain at least one carbon-carbon double bond
These chains have a bend wherever a double bond is present
The bends in the chain prevent the fatty acids from packing together tightly, meaning that these fats are liquid at room temperature
E.g. olive oil


Functions of lipids
Lipids are energy-dense
They are insoluble so do not affect osmosis; therefore, they do not risk upsetting the water balance of the organism
When lipids are respired, a lot of water is produced compared to the respiration of carbohydrates
This is called metabolic water and can be used as a dietary water source when drinking water is unavailable
A camel's hump is not a water sac, it is a lipid rich storage organ that yields metabolic water for the camel in its dry desert habitat
All these features make lipids ideal for long-term energy storage
Other roles of lipids include:
Physical protection of soft organs, e.g., visceral fat around the heart
Thermal insulation from subcutaneous fat, e.g. whale blubber
Subcutaneous fat as a buoyancy aid (fat is less dense than water so assists flotation)
Waterproofing secretions, e.g. birds' preening glands or waxy cuticles on leaf surfaces
Electrical insulation, e.g. the myelin sheath around certain nerve axons
Certain photosynthetic pigments, e.g. carotenoids
Glycolipids, typically as cell-surface recognition molecules/receptors
Examiner Tips and Tricks
Ensure that you are familiar with the structure of a triglyceride and that you can recognize whether the fatty acids are saturated or unsaturated. A saturated lipid is saturated with hydrogen atoms; it cannot contain more hydrogen atoms.
Phospholipids
Phospholipids are a type of lipid; they are formed from the monomer glycerol and fatty acids
Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule as one has been replaced by a phosphate ion (PO43-)
As phosphate is polar it is soluble in water (hydrophilic)
The fatty acid "tails" are nonpolar and, therefore, insoluble in water (hydrophobic)

The bipolar nature of phospholipids
Phospholipids contain both hydrophobic and hydrophilic parts so they can be regarded as bipolar
As a result of having hydrophobic and hydrophilic parts, phospholipid molecules form monolayers or bilayers in water

Functions of phospholipids
Phospholipids are the main component of cell membranes
The presence of hydrophobic fatty acid tails acts as a barrier to water-soluble molecules
The hydrophilic phosphate heads form H-bonds with water allowing the cell membrane to be used to compartmentalize
This enables the cells to organize specific roles into organelles helping with efficiency
The composition of phospholipids contributes to the fluidity of the cell membrane
If there are mainly saturated fatty acid tails, the membrane will be less fluid
If there are mainly unsaturated fatty acid tails, the membrane will be more fluid
Phospholipids control membrane protein orientation
Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer
Examiner Tips and Tricks
Ensure you know the difference between phospholipids and triglycerides.
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