B1.1 Carbohydrates and lipids

Carbohydrates and lipids are the molecules cells reach for when they need energy and structure. Both are built mainly from carbon, hydrogen and oxygen, yet small differences in how their atoms are arranged give them very different jobs — from the instant fuel of glucose to the long-term energy store of fat and the membranes that enclose every cell. B1.1 is a topic where structure explains function at every turn, so the strategy for the exam is to connect each molecule’s shape and bonding to the role it plays.

Carbohydrates from simple to complex

Carbohydrates are built from sugar units. Monosaccharides are the single-unit building blocks, such as glucose (C₆H₁₂O₆), fructose and galactose. They are soluble, sweet and the immediate source of energy in respiration.

Two monosaccharides join to form a disaccharide — for example glucose + fructose gives sucrose, and glucose + galactose gives lactose. Many monosaccharides join to form a polysaccharide. The syllabus focuses on three made from glucose:

• Starch (in plants) and glycogen (in animals) are energy stores. They are compact, branched or coiled, and insoluble, so they store glucose without affecting the cell’s water balance and can be broken down quickly when needed.
• Cellulose is a structural polysaccharide that forms the tough fibres of plant cell walls.

Strikingly, starch and cellulose are both polymers of glucose, yet they behave completely differently — the explanation lies in how the glucose units are linked, covered next.

Condensation, hydrolysis and the alpha/beta distinction

Sugar units are joined by condensation reactions, in which a bond forms between two monomers and a molecule of water is released. The bond joining two sugars is a glycosidic bond. The reverse reaction, hydrolysis, uses water to break the bond and split the polymer back into monomers — this is how stored polysaccharides and dietary carbohydrates are digested.

Glucose exists in two forms, alpha-glucose and beta-glucose, which differ only in the orientation of one hydroxyl group. This tiny difference has large consequences:

• Starch and glycogen are made of alpha-glucose, giving chains that coil or branch into compact stores that enzymes can readily digest.
• Cellulose is made of beta-glucose, which forces alternate units to flip over. This produces straight chains that line up and are cross-linked by hydrogen bonds into strong, rigid fibres — ideal for support but indigestible to most animals.

Lipids: triglycerides and phospholipids

Lipids are a varied group united by being largely non-polar and insoluble in water (hydrophobic). The two on the syllabus are triglycerides and phospholipids.

A triglyceride is formed by condensation of one glycerol molecule with three fatty acids, joined by ester bonds. Fatty acids may be saturated (no carbon–carbon double bonds, straight chains that pack closely, usually solid at room temperature, as in animal fats) or unsaturated (one or more double bonds creating kinks, so they pack loosely and are usually liquid oils, as in plants).

A phospholipid is similar but has one fatty acid replaced by a phosphate group. This makes the molecule amphipathic: the phosphate head is hydrophilic and the two fatty-acid tails are hydrophobic. In water, phospholipids therefore arrange themselves into a bilayer, heads facing out towards the water and tails tucked inside — the basis of every cell membrane.

Comparing carbohydrates and lipids as energy stores

Both carbohydrates and lipids store energy, but they suit different needs — a common comparison question:

• Energy density: lipids store roughly twice as much energy per gram as carbohydrates, so fat is the more efficient long-term store and is lighter to carry.
• Accessibility: carbohydrates such as glycogen can be mobilised and respired more quickly, making them better for short-term, readily available energy.
• Solubility: both stores are insoluble, so neither disturbs the water potential of the cell.
• Additional roles: stored fat under the skin also provides thermal insulation and physical protection of organs — functions carbohydrates do not perform.