B1.1 Carbohydrates and lipids

Why carbon

Carbon atoms form four covalent bonds and can bond to each other in chains, branches and rings, single or double bonded. This versatility allows an almost unlimited range of stable molecules, which is why biological macromolecules are built on carbon skeletons.

Condensation and hydrolysis

Large molecules (polymers) are made from smaller subunits (monomers).

• Condensation: two monomers join, a covalent bond forms and a molecule of water is released. Energy (from ATP, via enzymes) is needed.
• Hydrolysis: water is used to split a bond, breaking polymers back into monomers, for example during digestion.

flowchart LR
  A["Monomer + Monomer"] -- "condensation: water released" --> B["Dimer or polymer"]
  B -- "hydrolysis: water added" --> A

Carbohydrates

Monosaccharides such as glucose, fructose and ribose are soluble, easily transported and quickly respired. Glucose exists as alpha and beta forms that differ in the position of the hydroxyl group on carbon 1. This small difference has large consequences:

Starch and glycogen are good stores because they are compact and insoluble, so they do not draw water into cells by osmosis, and branching lets enzymes add or remove glucose rapidly.

Glycoproteins and cell recognition

Short carbohydrate chains attached to membrane proteins form glycoproteins that face outward from the cell. Their pattern acts like an identity tag, allowing cells to recognise each other and the immune system to tell "self" from "non-self". The ABO blood groups depend on differences in these carbohydrate chains on red blood cells.

Lipids

Lipids are a varied group that are hydrophobic or only partly hydrophilic.

• Triglycerides form by condensation of one glycerol and three fatty acids, producing three ester bonds and three waters. Saturated fatty acids have no carbon double bonds and are straight, so they pack tightly and are usually solid at room temperature (fats). Unsaturated fatty acids have one or more double bonds; cis double bonds create a kink, so they pack loosely and are usually liquid (oils).
• Phospholipids have glycerol bonded to two fatty acids and a phosphate group. The phosphate head is hydrophilic and the tails hydrophobic, so the molecule is amphipathic and forms bilayers in water.
• Steroids have a structure of four fused carbon rings; examples include cholesterol and sex hormones such as testosterone and oestradiol.

Energy storage and insulation

Lipids release roughly twice as much energy per gram as carbohydrates when respired and are stored without water, making them a light, compact long-term store. Adipose tissue under the skin also insulates mammals, and blubber helps marine mammals keep warm and stay buoyant. Carbohydrates are better for short-term storage because they can be mobilised and respired faster.

Common mistakes

• Saying hydrolysis "releases" water. It uses water; condensation releases it.
• Stating cellulose is made of alpha-glucose, or that starch can make strong fibres.
• Calling a triglyceride a polymer; it is not built of repeating monomers.
• Forgetting that phospholipids have two fatty acids, not three.

Exam tips

• Outline condensation of a disaccharide: name the monomers, the bond formed (glycosidic), and water released.
• Compare and contrast triglycerides and phospholipids: both have glycerol and ester bonds; they differ in number of fatty acids and in the phosphate head.
• Calculate questions on energy content: divide carefully and keep units such as kJ g⁻¹.
• Link structure to function every time: branching to fast release, straight chains to strength.