B1.1 Carbohydrates and lipidsIB Biology HL: Revision notes

A covalent bond is a shared pair of electrons between two atoms. A carbon atom can form up to four covalent bonds, as single bonds or a combination of single and double bonds, with other carbon atoms or with other non-metals (H, O, N, S, P).

So carbon compounds can be unbranched chains, branched chains, single rings or multiple rings, which gives the huge diversity of molecules life is based on.

NOS: scientific conventions such as the SI prefixes kilo (10³), centi (10⁻²), milli (10⁻³), micro (10⁻⁶) and nano (10⁻⁹) are based on international agreement.

Condensation reactions link monomers into polymers, releasing water: e.g. monosaccharides → polysaccharides, amino acids → polypeptides, nucleotides → nucleic acids.

Hydrolysis digests polymers into monomers. A water molecule is split to provide the —H and —OH groups added to the monomers, hence the name.

Monosaccharides are single sugar units. Pentoses have 5 carbons (ribose, deoxyribose) and hexoses have 6 (glucose, galactose, fructose). In ring form, a hexose such as glucose has a six-membered ring (five C and one O) with one carbon outside the ring; a pentose such as ribose has a five-membered ring (four C and one O).

Glucose properties and uses:

• Soluble (polar) — dissolves in cytoplasm and plasma.
• Transportable — carried in blood.
• Chemically stable — does not react spontaneously.
• Yields energy when oxidised in respiration.

Starch (plants) and glycogen (animals) are energy stores made of α-glucose:

• Compact: coiling and branching during polymerisation.
• Relatively insoluble because of their large size, so no osmotic effect.
• Easy to build or mobilise: α-glucose units are added or removed by condensation and hydrolysis at chain ends; branching gives many ends.

Cellulose is a structural polysaccharide of β-glucose. Alternate monomers are inverted, giving straight chains that lie side by side in bundles, cross-linked by hydrogen bonds into strong fibres for plant cell walls.

Glycoproteins are proteins with carbohydrate chains attached, projecting from the outer surface of the plasma membrane. They act in cell–cell recognition.

ABO antigens on red blood cells are an example: group O has the basic chain, group A adds N-acetylgalactosamine, group B adds galactose, group AB has both. Antibodies bind non-self antigens and cause agglutination after a mismatched transfusion.

Lipids dissolve in non-polar solvents but are only sparingly soluble in water: fats, oils, waxes and steroids.

By condensation, one glycerol links three fatty acids (a triglyceride) or two fatty acids and a phosphate (a phospholipid).

Saturated fatty acids have no C=C; monounsaturated one; polyunsaturated two or more. Cis double bonds kink chains, so they pack less closely and melting point falls. Endotherms store fats rich in saturated fatty acids (solid at room temperature); plants store unsaturated oils (liquid).

Triglycerides in adipose tissue are ideal for long-term energy storage (about twice the energy per gram of carbohydrate, stored without water, stable) and thermal insulation, which matters most for endotherms in cold habitats.

Phospholipids are amphipathic: a hydrophilic phosphate head and two hydrophobic fatty acid tails. In water they form a bilayer, heads facing the water on each side and tails inside, the basis of membranes.

Steroids have four fused carbon rings (three six-membered and one five-membered). Oestradiol and testosterone are non-polar, so they pass through the phospholipid bilayer to reach receptors inside cells.