Biological compounds: the roles of water and inorganic ions; the structure of carbohydrates, lipids and proteins; condensation and hydrolysis; and the biochemical tests for these molecules.

What this dot point is asking

Eduqas wants you to know the biological roles of water and inorganic ions, the structures of carbohydrates, lipids and proteins, how monomers join by condensation and split by hydrolysis, and how to carry out and interpret the biochemical food tests. This is a Core Concepts statement, so it can appear on any of the three papers.

Water and inorganic ions

Water is a polar molecule: oxygen pulls the shared electrons more strongly than the hydrogens, so the oxygen is slightly negative ($\delta^-$) and the hydrogens slightly positive ($\delta^+$). These partial charges let water molecules form hydrogen bonds with one another, which underlies almost every useful property Eduqas asks about.

Inorganic ions have specific roles: calcium ions ($\text{Ca}^{2+}$) in bones, teeth and blood clotting; iron ions ($\text{Fe}^{2+}$) at the centre of each haem group in haemoglobin, where oxygen binds; phosphate ions ($\text{PO}_4^{3-}$) in ATP, phospholipids and the backbone of nucleic acids; and hydrogen ions ($\text{H}^+$) in setting pH and in chemiosmosis during respiration and photosynthesis.

Carbohydrates

Carbohydrates contain only carbon, hydrogen and oxygen. Monosaccharides (glucose, in $\alpha$ and $\beta$ forms, plus fructose and galactose) are the monomers. Two monosaccharides join by condensation to form a disaccharide and a glycosidic bond, releasing one water molecule (for example glucose plus fructose gives sucrose).

Polysaccharides are condensation polymers of many monosaccharides. Starch (amylose and amylopectin) is the plant energy store, compact and insoluble; glycogen is the more highly branched animal store, allowing fast hydrolysis when energy is needed; cellulose is structural, made of $\beta$-glucose units that form straight chains held side by side by hydrogen bonds into strong microfibrils that give plant cell walls their tensile strength.

Lipids

Phospholipids have one fatty acid replaced by a phosphate group, giving a hydrophilic head and two hydrophobic tails. This is what makes them form a bilayer in water, the structural basis of every cell membrane. Triglycerides are excellent energy stores because they are compact, insoluble and release roughly twice the energy per gram of carbohydrate when respired.

Proteins

Proteins are polymers of amino acids joined by peptide bonds formed by condensation. There are four levels of structure: primary (the sequence of amino acids), secondary (the $\alpha$-helix and $\beta$-pleated sheet held by hydrogen bonds along the backbone), tertiary (the overall 3D fold held by hydrogen, ionic and disulfide bonds between R groups), and quaternary (two or more polypeptide chains together, as in haemoglobin with its four chains and iron-containing haem groups). The precise tertiary shape is what gives enzymes and antibodies their specificity.

The biochemical tests

• Reducing sugars: add Benedict's reagent and heat in a water bath; a positive result turns from blue to brick-red.
• Non-reducing sugars: first hydrolyse with dilute acid, neutralise, then test with Benedict's; brick-red is positive.
• Starch: add iodine in potassium iodide solution; a positive result turns from orange-brown to blue-black.
• Proteins: add biuret reagent; a positive result turns from blue to lilac or purple.
• Lipids: the emulsion test, mix the sample with ethanol then add water; a positive result gives a cloudy white emulsion.

Examples in context

Example 1. Cellulose versus starch. Both are glucose polymers, yet cellulose is tough and structural while starch is a soft energy store. The difference is the isomer: $\beta$-glucose in cellulose forces alternate units to flip, giving straight chains that hydrogen-bond into rigid microfibrils, whereas $\alpha$-glucose in starch coils into a compact, easily hydrolysed store. This is a classic Eduqas structure-and-function comparison.

Example 2. Haemoglobin and quaternary structure. Haemoglobin's four polypeptide chains, each holding an iron-containing haem group, let one molecule carry four oxygen molecules and show cooperative binding. This shows why quaternary structure matters: the assembled protein does something none of its single chains could, linking biological compounds directly to gas transport.

Try this

Q1. Name the bond formed when two monosaccharides join, and the reaction involved. [2 marks]

• Cue. Glycosidic bond, formed by condensation.

Q2. Explain why water is described as a good transport medium. [2 marks]

• Cue. It is polar, so it dissolves ions and polar solutes; it is a liquid that flows, carrying dissolved substances around the body.

Q3. Explain how the structure of cellulose makes it suited to its role in plant cell walls. [3 marks]

• Cue. Made of $\beta$-glucose forming straight chains; chains held side by side by many hydrogen bonds into microfibrils; this gives high tensile strength to support the cell against turgor pressure.