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2 · Biological Molecules

Cambridge A Level Biology — Full F-drive source: 2 · Biological Molecules — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 24 of 25 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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2 · Biological Molecules — Lipids, Water and Membrane Fluidity — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe triglycerides and phospholipids, the properties of water, the emulsion test for lipids, and how fatty acids affect health and membranes.Structure …

  • Energy store
  • Other roles
  • High specific heat capacity
  • High latent heat of vaporisation
  • Saturated fatty acids
  • Trans fatty acids

Part 1 of 6Triglycerides

Triglycerides
Structure
  • Structure one glycerol joined to three fatty acids by three ester bonds, formed by condensation that releases three water molecules.
Properties
  • Properties non-polar and hydrophobic, so insoluble in water.
Energy store
  • Energy store many C–H bonds give about 37 kJ per gram, compared with 17 kJ for carbohydrates and proteins.
Other roles
  • Other roles insulation, buoyancy, protection of organs and metabolic water from oxidation.

Part 2 of 6Phospholipids

Structure
  • Structure glycerol, two fatty acid tails and a phosphate head.
Behaviour
  • Behaviour the hydrophilic head and hydrophobic tails make them amphipathic.
Bilayer
  • Bilayer in water the heads face the water and the tails face inwards, forming the basis of membranes.
Barrier
  • Barrier the hydrophobic core stops water-soluble molecules crossing freely.

Part 3 of 6Properties of water

Properties of water
Polarity
  • Polarity oxygen is δ− and hydrogen is δ+, so water is a dipole that forms hydrogen bonds.
Solvent
  • Solvent ionic and polar substances dissolve, allowing reactions and transport.
High specific heat capacity
  • High specific heat capacity temperature changes slowly, keeping conditions stable.
High latent heat of vaporisation
  • High latent heat of vaporisation evaporation of a little water cools organisms effectively.
Cohesion
  • Cohesion hydrogen bonding supports surface tension and movement in xylem.

Part 4 of 6Emulsion test for lipids

1
Add ethanol to the sample and shake to dissolve any lipid.
2
Pour the mixture into a test tube of water.
3
A milky white emulsion shows lipid is present.
4
A clear solution shows no lipid.
  • Reason
    • lipids dissolve in ethanol but not in water, so droplets form when the ethanol is diluted.

Part 5 of 6Fatty acids, health and membrane fluidity

Saturated fatty acids
  • Saturated fatty acids no C=C bonds, straight chains that pack closely and are solid at room temperature.
Unsaturated fatty acids
  • C=C bonds create kinks that prevent close packing, so oils are liquid.
Trans fatty acids
  • Trans fatty acids hydrogens on opposite sides of the double bond;
    • enzymes cannot metabolise them and they are linked to coronary heart disease.
Membrane fluidity
  • Membrane fluidity membranes with more unsaturated tails are more fluid, and those with more saturated tails are less fluid.

Part 6 of 6More from the specification

1Monosaccharides and Disaccharides
  • Monosaccharides like glucose are reducing sugars that join via covalent glycosidic bonds in condensation reactions to form maltose and larger carbohydrates.
2Polysaccharides
  • Starch and Glycogen — Insoluble storage polysaccharides include starch and glycogen (in animals),
    • (amylose and amylopectin in plants)
  • which are compact polymers of α-glucose.
3Structural Polysaccharide Cellulose
  • Cellulose is an unbranched structural polysaccharide formed from β-glucose monosaccharides, arranged into parallel microfibrils linked by hydrogen bonds.
4Glycosidic Bond Hydrolysis
  • Breaking down complex polysaccharides into free monosaccharides involves hydrolysis reactions,
  • which consume water molecules to split glycosidic covalent bonds.
5Monosaccharides and Disaccharides
  • Monosaccharides like glucose are reducing sugars that join via covalent glycosidic bonds in condensation reactions to form maltose and larger carbohydrates.
6Polysaccharides
  • Starch and Glycogen — Insoluble storage polysaccharides include starch and glycogen (in animals),
    • (amylose and amylopectin in plants)
  • which are compact polymers of α-glucose.
7Structural Polysaccharide Cellulose
  • Cellulose is an unbranched structural polysaccharide formed from β-glucose monosaccharides, arranged into parallel microfibrils linked by hydrogen bonds.
8Glycosidic Bond Hydrolysis
  • Breaking down complex polysaccharides into free monosaccharides involves hydrolysis reactions,
  • which consume water molecules to split glycosidic covalent bonds.
9Monosaccharides and Disaccharides
  • Monosaccharides like glucose are reducing sugars that join via covalent glycosidic bonds in condensation reactions to form maltose and larger carbohydrates.
10Polysaccharides
  • Starch and Glycogen — Insoluble storage polysaccharides include starch and glycogen (in animals),
    • (amylose and amylopectin in plants)
  • which are compact polymers of α-glucose.
11Structural Polysaccharide Cellulose
  • Cellulose is an unbranched structural polysaccharide formed from β-glucose monosaccharides, arranged into parallel microfibrils linked by hydrogen bonds.
12Glycosidic Bond Hydrolysis
  • Breaking down complex polysaccharides into free monosaccharides involves hydrolysis reactions,
  • which consume water molecules to split glycosidic covalent bonds.
13Sucrose
  • The sugar mainly transported in phloem.
14Covalent bond
  • A shared electron pair linking monomers.
15Glycosidic bond
  • The covalent link between two sugar units made by condensation.
16Non-reducing sugar
  • A sugar that cannot reduce Benedict's reagent until hydrolysed.
17Glucose
  • A simple sugar; the unit that starch, glycogen and cellulose are built from.
18Cellulose
  • A structural polysaccharide of β-glucose.
19Starch
  • An insoluble polysaccharide of glucose used by plants for energy storage.
20Glycogen
  • The polysaccharide in which glucose is stored in animals.
21Polysaccharide
  • Many sugars joined, such as starch, glycogen, cellulose or chitin.
22Polymer
  • A large molecule made of many repeating monomers.
23Monosaccharide
  • A single sugar unit such as glucose, fructose or galactose.
24Carbon
  • Element with four valence electrons that forms the backbone of organic molecules.

Key terms

TermMeaning
TriglycerideA lipid of glycerol and three fatty acids.
PhospholipidA lipid with glycerol, two fatty acids and a phosphate head.
AmphipathicHaving hydrophilic and hydrophobic regions.
Ester bondThe bond joining glycerol to a fatty acid.
EmulsionA suspension of tiny droplets of one liquid in another.
Trans fatty acidA fatty acid with hydrogens on opposite sides of the C=C bond.
Metabolic waterWater released when C–H bonds are oxidised.

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 different molecules.
3
Describing hydrophilic and hydrophobic parts the wrong way round — heads are hydrophilic and tails hydrophobic.
4
Writing that unsaturated fats have no double bonds — they have at least one C=C.

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