2. Biological Molecules

1. Place the sample solution in a test tube and add an excess of Benedict’s reagent.
2. Heat the mixture in a boiling water bath for a few minutes, keeping the heating conditions consistent when comparing samples.
3. Record the final colour and whether a precipitate forms: blue with no precipitate is a negative result; a shift away from blue with a coloured precipitate is positive.

1. Place the sample in a clean test tube or on a white spotting tile.
2. Add a few drops of iodine solution in potassium iodide to the sample.
3. Mix and observe against a light background: blue-black indicates starch; the iodine colour remains orange-brown when starch is not detected.

1. Add ethanol to the sample and shake to dissolve any lipid present.
2. Add the ethanol–sample mixture to water and mix.
3. Observe the final mixture: a cloudy or milky emulsion indicates lipid; a clear mixture is a negative result.

1. Add sodium hydroxide or potassium hydroxide solution to the sample to make it alkaline.
2. Add a few drops of blue copper(II) sulfate solution, or use Biuret reagent containing both components.
3. Mix and observe against a white background: a blue-to-lilac or purple change indicates protein; no change from blue is negative.

Prepare a range of known reducing-sugar concentrations, including a zero-sugar control, and test the unknown alongside them. Use the same sample volume, Benedict's reagent volume, tube size, water-bath temperature and mixing method. For colour comparison, heat every tube for the same fixed time. For timing, use the same predefined colour-change endpoint and start timing each tube at the same stage of heating.

1. Test the original sample with Benedict’s reagent first. Continue only if the initial result is negative for reducing sugar.
2. Add dilute hydrochloric acid to a fresh portion of sample and heat it in a boiling water bath.
3. Neutralise the acid with sodium hydrogencarbonate, checking with a suitable indicator; add a little extra so the mixture is slightly alkaline.
4. Repeat Benedict’s test: add Benedict’s reagent and heat in a boiling water bath.
5. A new orange-to-red precipitate supports the presence of a non-reducing sugar that was hydrolysed; without the initial negative control, the conclusion is inconclusive.

1. Draw the six-membered ring form with the ring oxygen and carbon positions in the same arrangement for both forms.
2. Identify the anomeric carbon next to the ring oxygen.
3. Draw the anomeric hydroxyl opposite to the CH₂OH group for α-glucose, and on the same side as the CH₂OH group for β-glucose.
4. Keep every other labelled group unchanged: the α/β distinction is the single anomeric hydroxyl orientation, not a different molecular formula.

• Monomer: a small subunit that can join to other subunits. A monosaccharide can act as the monomer of a carbohydrate polymer.
• Polymer: a large molecule made from many repeating monomer units. A polysaccharide is a carbohydrate polymer; a disaccharide is not a long polymer.
• Macromolecule: a very large biological molecule. Some macromolecules are polymers, but the terms are not interchangeable.
• Examples: glucose, fructose and galactose are monosaccharides; sucrose and maltose are disaccharides; starch, glycogen and cellulose are polysaccharides.

• Join: complementary reactive groups on subunits form a covalent link, creating a larger molecule from smaller units.
• Condensation: when the link forms, a water molecule is removed; repeating this process builds a polymer.
• Representative links: glycosidic bonds join monosaccharides in carbohydrate polymers, while ester bonds join glycerol to fatty acids in lipids such as triglycerides.
• Break: hydrolysis uses water to break a covalent link, allowing a polymer or larger molecule to become smaller subunits again.

1. Bring two monosaccharides together at suitable hydroxyl groups.
2. A condensation reaction forms the glycosidic bond and removes one molecule of water.
3. Repeating the joining process extends the carbohydrate chain; the bond type and position can differ, including α- or β-linked arrangements, but those details belong to the named molecule being formed.
4. Hydrolysis reverses the connection by adding water across the glycosidic bond, breaking a disaccharide into two monosaccharides or shortening a polysaccharide into smaller sugar units.

• Formation: three condensation reactions join the three fatty acids to glycerol by three ester bonds, releasing three water molecules.
• Fatty-acid variation: saturated tails have no carbon–carbon double bonds; unsaturated tails contain one or more double bonds, which can introduce bends and affect packing.
• Hydrophobic storage: a triglyceride does not dissolve in water, so many molecules can be stored together without the same osmotic effect as an equivalent store of free glucose.
• Energy: the molecule contains many energy-rich carbon–hydrogen bonds and little oxygen, so oxidation releases a large amount of energy per gram.

• Hydrophilic head: the phosphate-containing group interacts favourably with water.
• Hydrophobic tails: the two fatty-acid chains avoid water. Their saturation can affect how closely neighbouring tails pack.
• Amphipathic arrangement: in water, heads face the watery environments and tails point inward away from water.
• Bilayer consequence: two opposing layers create a hydrophobic core, forming the basic barrier of a cell-surface membrane; many water-soluble substances do not cross that core freely.

1. Position the carboxyl group of one amino acid beside the amine group of another.
2. Remove the hydroxyl group from the carboxyl group and a hydrogen from the amine group; together they form one water molecule.
3. Join the remaining carbonyl carbon to the nitrogen, creating a covalent peptide bond and a dipeptide.
4. Repeat the condensation reaction to join more amino acids and form a polypeptide chain.
5. In hydrolysis, add water across a peptide bond to break the chain back into smaller peptides or amino acids.

• Hydrophobic interactions: non-polar R groups tend to cluster away from water, helping form the protein’s interior.
• Hydrogen bonds: polar groups can form many relatively weak attractions that help stabilise particular folds.
• Ionic bonds: oppositely charged R groups attract and can support the folded arrangement.
• Disulfide bonds: two cysteine R groups can form a strong covalent bridge that stabilises the chain.
• Quaternary assembly: the same kinds of interactions can help hold multiple folded subunits together in a functional protein.

• Folding: R-group interactions fold the polypeptide into a compact tertiary structure rather than a long open chain.
• Solubility: hydrophobic R groups are mostly shielded inside, while hydrophilic R groups interact with water at the surface, helping the protein remain dispersed.
• Specific shape: the fold creates a particular three-dimensional surface, pocket or binding site.
• Physiological roles: solubility allows transport in body fluids and access to aqueous reactions; the specific shape allows roles such as enzyme catalysis, antibody recognition or transport.

• The four folded globin subunits are held together by non-covalent interactions that stabilise the quaternary assembly.
• Each globin subunit contains one non-protein prosthetic haem group, so one haemoglobin molecule has four haem groups.
• Each haem group contains an Fe2+ ion; the iron is part of haem rather than the polypeptide backbone.
• The compact assembly is water-compatible and positions the four haem groups within the globular protein.

• Binding site: Each haem group contains an Fe²⁺ ion that can reversibly combine with one oxygen molecule, forming oxyhaemoglobin.
• Loading and release: Reversible binding allows oxygen to be picked up where it is available and released where it is needed, rather than making oxygen permanently part of the pigment.
• Capacity: A haemoglobin molecule has four haem groups, so it can bind four oxygen molecules when all sites are occupied.
• Why the protein matters: Haemoglobin is soluble in blood, and its globin subunits position the haem groups so oxygen can be carried efficiently.

• Three-chain assembly: Each polypeptide chain has a helix shape, and three chains are held closely together by hydrogen bonds to form the triple helix.
• Repetitive sequence: Glycine occurs at about every third position. Its small R group allows the three chains to pack closely inside the helix.
• Within the helix: Many hydrogen bonds between the chains stabilise the triple-helix arrangement.
• Between molecules: Parallel triple helices form covalent cross-links between amino-acid R groups, holding collagen molecules together into fibrils.

• Molecular links: Covalent cross-links between neighbouring, parallel triple-helix molecules hold them together in fibrils.
• Fibre organisation: Many fibrils combine to form collagen fibres, with molecules arranged in a staggered pattern that adds strength.
• Force alignment: Collagen fibres can be lined up with the forces they must withstand, so the load is shared along the fibre direction.
• Functional result: The triple-helix stability, cross-links and parallel alignment together produce a strong supporting material suitable for tissues such as tendons and ligaments.

• Within one water molecule, oxygen and hydrogen are joined by polar covalent bonds.
• Between neighbouring molecules, a delta-plus hydrogen is attracted to a delta-minus oxygen.
• This intermolecular attraction is a hydrogen bond.
• Individual hydrogen bonds continually break and reform, while many together affect how water responds to solutes, heating and evaporation.