AQA A Level Biology Lipids

what type of molecule are lipids?

large molecules, not polymers

what are the two main types of lipid?

• triglycerides

• phospholipids

• (cholesterol)

what are the functions of lipids?

• energy storage

• insulation

• protection of internal organs

what type of molecule do all lipids contain?

hydrocarbons

describe the structure of a triglyceride

• made up of 1 glycerol molecule

• and 3 fatty acid molecules

• joined by ester bonds

• non polar molecule

describe the structure of a fatty acid

• carboxyl group bonded to a hydrocarbon tail

• hydrocarbon tail is hydrophobic

what is the formula of a fatty acid?

• RCOOH (C=O)

• R is the variable hydrocarbon tail

what are the two types of fatty acid?

• saturated

• unsaturated

what is the difference between saturated and unsaturated fatty acids?

• saturated fatty acids contain no double bonds

• unsaturated fatty acids contain C=C double bonds, which cause the chain to kink

• saturated fatty acids are normally solid at rtp

• unsaturated fatty acids are normally oils at rtp

describe the structure of a glycerol molecule

• propanol, but with 3 OH groups

• CH2OHCHOHCH2OH

what type of reaction forms a triglyceride?

• condensation

• 3 water molecules produced

draw the condensation reaction used to produce glycerol

which molecules are removed from glycerol and fatty acids in a condensation reaction?

• H from glycerol

• OH from fatty acid

describe the structure and function of triglycerides

• hydrophobic tails make the molecule insoluble, also non polar - have no osmotic effect on cells so good for storing energy - as tails are hydrophobic, they form droplets in cells (why they emulsify)

• large - cannot cross cell membranes so good energy stores

• high proportion of carbon and hydrogen atoms - contains lots of energy, more per gram tahn carbohydrates

• good heat insulators - stored as blubber in marine animals

• low density - buoyancy aid in animals

describe the structure of phospholipids

• similar to triglycerides, one fatty acid is replaced by a phosphate group

• so glycerol, 2 fatty acids, phosphate group

describe the structure and function of phospholipids

• heads are hydrophilic and tails hydrophobic, so they form a double layer with heads facing out towards the water on both sides of the cell membrane

• water soluble substances can’t easily pass through the membrane - acts as a barrier

• in water single or double layered spheres called micelles can form, or monolayers on the surface

what test is carried out to test for the presence of lipids?

emulsion test

describe how the emulsion test would be carried out

• 2cm³ of sample added to 2cm³ of ethanol in a test tube

• shake for 30s

• decant into test tube of cold water

• if a white emulsion forms, sample contains lipids

how does the emulsion work chemically?

• ethanol dissolves lipids,

• so if the food sample contains lipids when added to water the insoluble dissolved lipids form droplets

• these prevent light passing through