Water is chemically simple but biologically exceptional
Oxygen attracts shared electrons more strongly than hydrogen, making water polar. The partially negative oxygen of one molecule attracts the partially positive hydrogen of another, forming hydrogen bonds. Each bond is weak, but large networks produce important collective properties.
Water is an effective solvent for ions and polar molecules, supports transport and reaction chemistry, and has a high specific heat capacity that resists rapid temperature change. Cohesion contributes to continuous water columns in plants, while the high latent heat of vaporisation makes sweating and transpiration effective cooling processes.
Unequal charge distribution.
Water molecules attract one another.
Cohesion, heat capacity and solvent action.
Transport, temperature stability and metabolism.
Carbohydrates combine energy and architecture
Monosaccharides such as glucose are small units that can be joined by glycosidic bonds through condensation reactions. Hydrolysis breaks those bonds by adding water. Disaccharides contain two monosaccharides, while polysaccharides contain long chains.
Starch and glycogen are compact energy stores built from alpha-glucose. Branching creates many ends for rapid enzyme action, especially in glycogen. Cellulose is built from beta-glucose in straight chains that hydrogen-bond into strong microfibrils, making it suitable for plant cell walls rather than energy storage in humans.
| Molecule | Building unit and shape | Main role |
|---|---|---|
| Starch | Alpha-glucose; amylose and branched amylopectin | Plant energy storage |
| Glycogen | Highly branched alpha-glucose polymer | Animal and fungal energy storage |
| Cellulose | Straight beta-glucose chains in microfibrils | Plant cell-wall strength |
Lipids store energy without attracting much water
A triglyceride forms when glycerol joins three fatty acids by ester bonds. Long hydrocarbon regions make the molecule largely hydrophobic. Lipids therefore store energy compactly without binding large amounts of water and yield substantial energy when oxidised.
Saturated fatty acids lack carbon–carbon double bonds; unsaturated fatty acids contain one or more. Cis double bonds introduce bends that reduce packing and often lower melting point. Phospholipids replace one fatty acid with a phosphate-containing polar head, creating the amphipathic molecules that self-assemble into membranes.
Protein function depends on three-dimensional shape
Amino acids join through peptide bonds to form polypeptides. The primary structure is the amino-acid sequence. Local hydrogen bonding creates alpha helices and beta sheets; interactions among side chains create tertiary structure; and some proteins assemble multiple polypeptide subunits into quaternary structure.
A change in primary sequence can alter folding, stability or an active site. Heat, extreme pH or chemicals can disrupt weak interactions and denature a protein. Denaturation changes higher-level structure and function but does not usually hydrolyse every peptide bond.
Amino-acid sequence.
Local helices and sheets.
Overall three-dimensional folding.
Assembly of multiple polypeptide subunits.
Food tests reveal chemical groups
Benedict's reagent tests for reducing sugars after heating, iodine solution tests for starch, the Biuret test detects peptide bonds, and the ethanol-emulsion test reveals lipids through a cloudy suspension. Each test requires a control and a clear statement of the positive result.
A colour change is evidence for a chemical feature, not proof that every molecule in a broad category is present. For example, a positive Biuret test detects peptide bonds but does not identify a particular protein.
The ideas to carry forward
- Water's polarity and hydrogen bonding produce its life-supporting properties.
- Small differences in glucose bonding distinguish storage polysaccharides from cellulose.
- Hydrophobicity makes triglycerides compact stores and phospholipids membrane-formers.
- Protein sequence guides folding, and folding enables function.
Answer first. Then reveal the marking logic.
01Why is glycogen well suited to rapid glucose release?2 marks · show the biological link
Answer: Its extensive branching creates many terminal points where enzymes can add or remove glucose units simultaneously.
02Why does denaturation not necessarily destroy the primary structure?2 marks · show the biological link
Answer: It usually disrupts weak interactions responsible for secondary, tertiary or quaternary folding without hydrolysing the covalent peptide backbone.
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