Carbohydrates and lipids

Monosaccharides — simplest carbohydrates with formula (CH₂O)ₙ. Common examples (all hexoses, C₆H₁₂O₆):

• Glucose (energy source).
• Fructose (in fruits).
• Galactose (in milk).

Disaccharides — two monosaccharides joined by glycosidic bond (formed by condensation):

• Maltose = glucose + glucose.
• Sucrose = glucose + fructose.
• Lactose = glucose + galactose.

Polysaccharides — many monosaccharides joined.

Polysaccharide	Monomer	Structure	Function	Found in
Starch (amylose + amylopectin)	α-glucose	Coiled (amylose), some branching (amylopectin)	Energy storage	Plants
Glycogen	α-glucose	Highly branched, compact	Energy storage	Animals (liver, muscle)
Cellulose	β-glucose	Long straight chains, H-bonded into fibres	Structural (cell walls)	Plants

α vs β glucose. The orientation of the OH group at C1 determines whether the polysaccharide can coil (α) or forms straight chains (β). β-glucose's alternating orientation enables strong H-bonded cellulose microfibrils — giving plant cell walls tensile strength.

Branching matters. Highly branched glycogen has many ends — enzymes can release glucose rapidly when energy is needed. Less branched starch is broken down more slowly.

Triglyceride = 1 glycerol + 3 fatty acids, joined by ester bonds (condensation reactions; 3 H₂O released).

Fatty acids vary in:

• Length (number of carbons).
• Saturation:
  • Saturated — no C=C double bonds. Tails pack tightly → solid at room T (e.g. butter, lard).
  • Monounsaturated — one C=C → kink → looser packing → liquid (e.g. olive oil).
  • Polyunsaturated — multiple C=C → very loose packing → liquid (e.g. omega-3 fish oils).

Functions of triglycerides:

• Long-term energy storage — 38 kJ/g vs 17 kJ/g for carbohydrates.
• Insulation — subcutaneous fat reduces heat loss in mammals.
• Buoyancy in aquatic mammals.
• Protection of internal organs.

Phospholipid = glycerol + 2 fatty acids + phosphate group (replacing the third fatty acid). The phosphate-containing head is hydrophilic; the fatty acid tails are hydrophobic — making phospholipids amphipathic. In water they self-assemble into bilayers (basis of all membranes — see Membrane Structure).

Saturated fat and health. Diets high in saturated fat raise LDL cholesterol → atherosclerosis risk. Trans fats (industrial partial hydrogenation) are particularly harmful.