2.22 · Biological Molecules
Carbohydrates & Lipids — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
2.2 Carbohydrates & Lipids — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure, bonding and functions of carbohydrates, triglycerides and phospholipids.Composition — carbohydrates contain C, H and O, with H and O in a 2:1 ratio, Cx(H2O)y.Monosacch…
- Monosaccharides
- Polymers formed
- Condensation
- Hydrolysis
- Storage properties
- Amylopectin
Part 1 of 8Carbohydrates and glucose isomers
Composition
- Composition carbohydrates contain C, H and O, with H and O in a 2:1 ratio, Cx(H2O)y.
Monosaccharides
- Monosaccharides trioses (3C), pentoses and hexoses (6C, glucose, fructose, galactose).
- (5C, ribose and deoxyribose)
- They are soluble, sweet and reducing.
Isomers
- Isomers alpha-glucose and beta-glucose differ only at carbon 1, with –OH below the ring in alpha and above in beta.
Polymers formed
- Polymers formed alpha-glucose gives starch and glycogen, and beta-glucose gives cellulose.
Part 2 of 8Glycosidic bonds
Glycosidic bonds
Condensation
- Condensation an –OH on one sugar and an –OH on another react, releasing water and forming a C–O–C glycosidic bond.
Hydrolysis
- Hydrolysis adding water breaks the glycosidic bond.
Types
- Types maltose has an alpha-1,4 bond, sucrose has an alpha-1,2 bond, and amylopectin and glycogen have alpha-1,6 branches.
Sucrose
- Sucrose a non-reducing sugar, so it must be hydrolysed with dilute HCl, neutralised, then tested with Benedict's.
Part 3 of 8Starch and glycogen
Storage properties
- Storage properties insoluble, compact, large and easily hydrolysed to glucose.
Amylose
- Amylose unbranched alpha-1,4 chain coiled into a compact helix.
Amylopectin
- Amylopectin has alpha-1,6 branches giving many ends for enzymes to act on.
Glycogen
- Glycogen more highly branched than amylopectin, so it is more compact and hydrolysed faster.
- (about every 8–12 residues)
- It is stored as granules in liver and muscle.
Part 4 of 8Cellulose
Cellulose
Structure
- Structure beta-glucose residues are each rotated 180° so chains are straight.
Strength
- Strength hydrogen bonds between parallel chains form microfibrils of high tensile strength.
Function
- Function cell walls resist bursting when turgid and stay freely permeable.
Digestion
- Digestion few organisms make cellulase, so cellulose is dietary fibre.
Part 5 of 8Triglycerides
Triglycerides
Structure
- Structure one glycerol joined to three fatty acids by three ester bonds, releasing three water molecules.
Fatty acids
- Fatty acids saturated chains are straight and pack closely, unsaturated chains are kinked (plant oils, liquid).
- (animal fats, solid)
Energy store
- Energy store many C–H bonds give high energy per gram, and they are insoluble so cause no osmotic effect.
Other roles
- Other roles thermal insulation, buoyancy, protection of organs and metabolic water.
Part 6 of 8Formation of a triglyceride
1
Start with one glycerol molecule, which has three –OH groups.
2
Line up three fatty acids, each with a –COOH group.
3
Each –COOH condenses with one –OH of glycerol, releasing water.
4
Three ester bonds form, releasing three water molecules.
5
The product is a non-polar, hydrophobic triglyceride.
Part 7 of 8Phospholipids
Phospholipids
Structure
- Structure glycerol, two fatty acid tails and a phosphate head.
- The phosphate is polar and hydrophilic; the tails are non-polar and hydrophobic.
Amphipathic
- Amphipathic in water they form a bilayer with heads facing water and tails facing inwards.
Role
- Role the basis of membranes, whose hydrophobic core is a barrier to water-soluble substances.
Fluidity
- Fluidity unsaturated tails make membranes more fluid, saturated tails make them less fluid.
Part 8 of 8More from the specification
1Monomer and polymer
- Monomer and polymer small repeating units such as monosaccharides join by covalent glycosidic bonds to make polysaccharides
2Condensation reaction
- Condensation reaction joining two sugars removes water and forms a glycosidic bond between carbon atoms;
- hydrolysis adds water to break it
3Reducing and non-reducing sugars
- Reducing and non-reducing sugars reducing sugars donate electrons to Benedict's reagent giving a brick-red precipitate;
- non-reducing sugars such as sucrose need acid hydrolysis first
4Polysaccharides
- Polysaccharides starch and glycogen store glucose, cellulose gives cell walls strength
5Triglycerides
- Triglycerides a glycerol joined to three fatty acids by ester bonds;
- saturated fats have no C=C double bonds and unsaturated fats have at least one
6Polysaccharide
- Many sugars joined, such as starch, glycogen, cellulose or chitin.
7Monomer
- A small molecule that links with others of its kind to build a polymer.
8Covalent bond
- A shared electron pair linking monomers.
Key terms
| Term | Meaning |
|---|---|
| Monosaccharide | A single sugar unit such as glucose. |
| Glycosidic bond | The covalent bond between two sugar units. |
| Triglyceride | A lipid of glycerol with three fatty acids joined by ester bonds. |
| Ester bond | The bond between a hydroxyl group and a carboxyl group. |
| Amphipathic | Having both hydrophilic and hydrophobic regions. |
| Condensation | A reaction joining molecules with loss of water. |
| Saturated fatty acid | A fatty acid with no C=C double bonds. |
Avoid these mistakes
1
Saying lipids are polymers — they are not made of repeating monomers.
2
Mixing up ester bonds (lipids) and glycosidic bonds (carbohydrates) — each links a different molecule type.
3
Describing hydrophilic and hydrophobic parts the wrong way round — heads are hydrophilic and tails hydrophobic.
4
Saying unsaturated fats have no double bonds — they have at least one C=C.
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