Summary AQA AS-level Biology

Biological Molecules

Monomers and Polymers

• Monomer: smaller, repeating unit from which larger molecules/polymers are made
• Polymer: chains of repeating monomer units, formed during polymerisation


Condensa(on reac(on Hydrolysis reac(on

• Joins monomers to form polymers • Breaks down polymers into monomers
• Involves crea3on of bond • Breaks chemical bond
• Involves removal of water • Involves addi(on of water


• Hydrogen bonding
- In polar molecules, electrons not evenly distributed
- Some regions more negatively charged
- Negative region of a polarised molecule attracted to positively charged region of
another
- Electrostatic bond forms
- Polar molecules = soluble in water



Carbohydrates

Monosaccharides
- Reducing sugars
• Galactose
• Fructose
• Glucose
(C6H12O6)




Benedict’s Test for reducing sugars
1. Add Benedict’s reagent
2. Heat in boiling water bath
3. If present, insoluble red precipitate of copper oxide forms



Disaccharides
- Polymers
- Condensation reaction between 2
monosaccharides
- Glycosidic bond:
• Maltose (reducing) = glucose + glucose
• Sucrose (non-reducing) = glucose + fructose
• Lactose (reducing) = glucose + galactose

,Polysaccharides
- Polymers
- Non-reducing sugars
- Condensation reaction between many monosaccharides
- Glycosidic bonds
• Glycogen = α-glucose
• Starch = α-glucose
• Cellulose = β-glucose




Benedict’s test for non-reducing sugars
1. Carry out reducing sugars test - after negative result
2. Add hydrochloric acid
3. Boiling water bath
4. Hydrolyses non-reducing sugar into monosaccharides
5. Allow tube to cool
6. Neutralise acid with sodium hydrogen carbonate
7. Test for reducing sugars

,Polysaccharides

Starch
Glycogen Cellulose
Amylose Amylopec(n

Source Animal Plant Plant

Purpose Storage compound of Storage compound of glucose Strong cellulose fibres
glucose Resist turgor pressure
Prevents burs3ng

Glucose α-glucose α-glucose β-glucose

Glycosidic
bonds 1,4 1,4
1,4 1,4
(Condensa(on 1,6 1,6
reac(on)

Branched Branched Unbranched Branched Unbranched

• Insoluble; doesn’t affect • Insoluble; doesn’t affect water poten3al • Molecules inverted 180O
water poten3al from previous molecule
• Helical: compact • Prevents coiling
• Helical: compact; • Straight, unbranched chains
suitable for storage • Large molecule so doesn’t leave cell run parallel joined by H-
bonds
• Large molecule so • Amylose: slowly hydrolysed (only 2 ends - Forms cross-linkages
doesn’t leave cell accessible) between adjacent chains
• Amylopec3n = branched, quickly - Collec3vely strong
• Highly branched: hydrolysed • Chains grouped to form
- Many ends acted on fibrils
simultaneously by
enzymes
- Rapidly hydrolysed
Structure




Benedict’s test for non- Iodine test for starch: Benedict’s test for non-reducing
reducing sugars • Add iodine solu(on sugars
Food test • Presence of starch indicated by colour
change (blue/black)
• Reac3on between I3- & amylose helix

, Lipids

Emulsion test for lipids
1. Add ethanol (lipids = soluble in organic solvents)
2. Shake tube thoroughly to dissolve any lipid in sample
3. Add water
4. shake gently
5. If present, white emulsion forms on surface
6. If not solution remains clear



Structure of fatty acids
• Hydrophobic
• Insoluble in water




Hydrocarbon chain (R group):
Saturated Mono-unsaturated Poly-unsaturated




No double C-bonds 1 double C-bond > 1 double C-bond

• Molecules can be closely packed • Molecules can’t be compacted
• Stronger intermolecular forces • Weaker intermolecular forces
• Solid at room temp • Liquid at room temp


Triglycerides
- Non-polar (insoluble in water)
- Ester bonds; between carboxyl group of fatty acid & hydroxyl group of glycerol