Module 2: Biomolecules - Water, Carbohydrates, Lipids, Proteins and Enzymes
Biochemistry relates water, carbohydrates, lipids and proteins to biological functions. Interpret structure, chemical bonds and supplied observations. This is shared conceptual practice rather than a claim about an official module or paper.
Water
Water is dipolar, so it forms hydrogen bonds: high specific heat capacity and high latent heat of vaporisation (thermal stability, cooling by sweating), cohesion and surface tension (xylem transport), a good solvent for ions and polar molecules, and ice floating because it is less dense than liquid water.
Carbohydrates and lipids
Monosaccharides (trioses to hexoses; α- and β-glucose differ only in the position of the –OH on carbon 1) join by condensation, forming glycosidic bonds, to give disaccharides (maltose, sucrose, lactose) and polysaccharides. Starch (amylose helix plus branched amylopectin) stores energy in plants, glycogen is more branched for animals, cellulose is unbranched β-1,4 chains held by hydrogen bonds into microfibrils for strength. Reducing sugars (all monosaccharides, maltose, lactose) reduce Benedict's solution; sucrose is non-reducing. Triglycerides form by condensation of glycerol with three fatty acids into ester bonds; saturated chains pack tightly, cis-unsaturated chains commonly kink and reduce close packing; trans configurations behave differently. Phospholipids are amphipathic and build bilayers.
Proteins and enzymes
Amino acids join through peptide bonds. Protein primary structure is sequence; secondary alpha-helices and beta-sheets involve backbone hydrogen bonding; tertiary structure depends on several interactions and quaternary structure joins subunits. Fibrous and globular describe broad structural patterns rather than universally separate functions. Most enzymes are proteins, while some RNA molecules also catalyze reactions. Catalysts lower an activation barrier without changing the reaction equilibrium. Rate depends on temperature, substrate, enzyme amount and assay conditions. High temperature can destabilize a protein; pH can change protonation and binding without necessarily causing unfolding. Denaturation usually disrupts the folded state rather than cleaving every peptide bond. In a simple reversible competitive model apparent Km increases while Vmax is unchanged. In a pure noncompetitive model Vmax decreases while Km is unchanged. Those kinetic patterns do not uniquely identify an atomic binding pocket. Cofactors can support catalytic activity.
Worked example
A supplied food-test dataset reports no reducing-sugar signal from sucrose before hydrolysis and a positive signal afterward, with suitable positive and negative controls. Hydrolysis can release glucose and fructose, explaining the changed observation. Interpret the recorded chemistry rather than treating this paragraph as an operating procedure. Appropriate assay conditions and controls matter: a negative result can reflect a failed assay as well as absence of the target compound.
Traps that cost marks
Avoid calling every polysaccharide reducing, confusing alpha- and beta-glucose linkages, or treating enzymes as organisms that are killed. A declining measured rate can reflect substrate depletion, pH effects, inhibition or instability; inspect the supplied conditions before assigning a cause. A typical glycerophospholipid has two fatty-acid tails and a phosphate-containing head, unlike a triacylglycerol with three fatty acids.