Carbohydrates and Lipids

Carbohydrates contain the elements C, H and O only, generally in the ratio $C_n(H_2O)_n$.

Monosaccharides are the simplest sugars and act as monomers:

• Trioses (3C) — glyceraldehyde, an intermediate in respiration.
• Pentoses (5C) — ribose (RNA, ATP) and deoxyribose (DNA).
• Hexoses (6C) — glucose, fructose, galactose. The body's main respiratory substrate is glucose.

α-glucose and β-glucose. Both are hexoses with the molecular formula $C_6H_{12}O_6$. In water they exist mostly as ring forms, and the position of the OH on carbon 1 distinguishes them:

• α-glucose: OH on C1 is below the ring (same side as C6 / CH₂OH).
• β-glucose: OH on C1 is above the ring (opposite side from C6).

This tiny geometric difference is biology's great divider: α-glucose builds storage polysaccharides (starch, glycogen), β-glucose builds the structural polysaccharide cellulose.

All hexose monosaccharides reduce Benedict's reagent because of their open-chain aldehyde or ketone group, so they are reducing sugars.

A glycosidic bond is the covalent —O— bond between two monosaccharides. It is formed by condensation (one H₂O removed) and broken by hydrolysis (one H₂O added).

The three Cambridge-named disaccharides are:

Disaccharide	Monomers	Bond	Reducing?
Maltose	α-glucose + α-glucose	α-1,4	Yes
Sucrose	α-glucose + fructose	α-1,β-2	No
Lactose	β-glucose + galactose	β-1,4	Yes

Sucrose is the only common non-reducing sugar — its two reducing groups are buried in the bond. This is why the non-reducing sugar test exists: hydrolyse with acid and heat → neutralise → repeat Benedict's → brick-red precipitate appears.

Hydrolysis is the opposite process. A water molecule splits the —O— bond. Different enzymes hydrolyse different disaccharides (e.g. maltase hydrolyses maltose, sucrase hydrolyses sucrose, lactase hydrolyses lactose).

Starch is the plant energy store, found as starch grains in chloroplasts and amyloplasts. It is a mixture of two polymers:

• Amylose — chains of α-glucose joined exclusively by α-1,4-glycosidic bonds. Each successive glucose is rotated relative to the previous one, which curls the whole chain into a helix with about six glucose units per turn. The helix is compact and slightly easier for triiodide ions to slot into, giving starch its blue-black colour with iodine.

• Amylopectin — α-1,4 backbone with α-1,6 branch points every 24-30 residues. The branches stick out, giving multiple free ends.

Glycogen is the animal energy store, found in liver and muscle cells as glycogen granules. It has the same α-1,4 + α-1,6 structure as amylopectin but with much more frequent branching (every 8-12 residues). The branches are also shorter.

Why store as a polysaccharide?

• Insoluble in water — no effect on water potential of the cell.
• Compact — high glucose density per unit volume.
• Easily hydrolysed — α-1,4 and α-1,6 bonds are broken by phosphorylase and debranching enzymes for rapid glucose release.
• Branching = many free ends = many enzyme attack points = fast glucose release. Animals need this for sudden energy demand; very-branched glycogen wins out over moderately-branched amylopectin.

Monomer. Cellulose is built from β-glucose. Joining two β-glucose molecules by a β-1,4 bond requires every successive monomer to be rotated 180° so that the —OH on C1 and the —OH on C4 can line up. The result is a perfectly straight, unbranched chain — no helix is possible.

Hydrogen bonding. The —OH groups projecting from each chain form many hydrogen bonds with —OH groups on neighbouring parallel chains. Individually each hydrogen bond is weak, but a cellulose microfibril contains ~60-70 parallel chains and thousands of hydrogen bonds.

Hierarchy. Glucose → cellulose chain → microfibril (~10 nm) → macrofibril → cellulose layer of the plant cell wall.

Function.

• High tensile strength — the plant cell wall withstands the outward pressure exerted by the central vacuole (turgor).
• Insoluble and chemically inert — does not interfere with cytoplasm chemistry.
• Freely permeable to water and small solutes — the cell wall is no barrier to transport.
• Permanent — most plant cells never replace their cellulose, in contrast with the constant turnover of glycogen in animals.

Few organisms can hydrolyse the β-1,4 bond (it needs cellulase). Herbivores rely on symbiotic gut microbes for this — explaining why ruminants have specialised foregut chambers full of cellulose-digesting bacteria.

Lipids contain C, H and O but with much less oxygen relative to carbon than carbohydrates. They are insoluble in water but dissolve in organic solvents (the basis of the emulsion test).

Triglyceride. One glycerol + three fatty acids joined by three ester bonds, with three water molecules removed (condensation).

• Glycerol — a three-carbon alcohol with three —OH groups.
• Fatty acid — long hydrocarbon tail (typically 14-22 C) with a carboxyl —COOH group at one end.
• Ester bond — formed by condensation between an —OH and a —COOH (—O—CO— linkage).

Saturated vs unsaturated.

• Saturated — no C=C double bonds in the tail. Straight chain. Tight packing → many van der Waals forces → high melting point → solid at room temperature (animal fats: butter, lard, suet).
• Unsaturated — one (mono-) or more (poly-) C=C double bonds. Each introduces a kink. Loose packing → fewer van der Waals forces → low melting point → liquid at room temperature (plant and fish oils).

Triglyceride functions.

• Long-term energy store — yields more than twice the ATP per gram compared with carbohydrate, because the hydrocarbon tails are highly reduced.
• Insulation — adipose tissue is a poor conductor of heat.
• Buoyancy — less dense than water (whales, sharks).
• Protection — adipose around vital organs.
• Metabolic water — oxidation produces water, vital for desert organisms (e.g. camel hump).

Phospholipid. A modified triglyceride where one fatty acid is replaced by a polar phosphate group (often via a glycerol C3 oxygen). The molecule is amphipathic:

• The phosphate head is hydrophilic — interacts with water.
• The two fatty acid tails are hydrophobic — repelled by water.

In an aqueous environment, phospholipids spontaneously self-assemble into a bilayer — heads facing the watery cytoplasm and extracellular fluid, tails sandwiched in the middle. This bilayer is the structural basis of every cell membrane: a partially permeable barrier that retains the contents of the cell while allowing selective transport.