Quick questions on Food chemistry: biomolecules, enzymes and energy content: VCE Chemistry Unit 4

Monosaccharides (simple sugars) are the building blocks. The most important are glucose (C6H12O6, an aldehyde sugar) and fructose (also C6H12O6, a ketone sugar). They differ in functional group but share the same molecular formula (functional-group isomers).

Amino acids are the monomers. Each has an alpha-carbon bonded to an NH2 (amine), a COOH (carboxylic acid), an H, and an R side chain. The 20 standard amino acids differ only in R. R groups can be non-polar (hydrophobic), polar (hydrophilic), acidic, or basic.

Triglycerides (or triacylglycerols) are esters of glycerol (a triol) with three fatty acid chains. Formation by condensation:

Vitamins are small organic molecules required in trace amounts. Two classes:

Enzymes are protein catalysts. Two models for substrate binding:

A bomb calorimeter burns a known mass of food sample in pure oxygen inside a sealed steel "bomb" surrounded by water. The temperature rise of the calorimeter is measured. The calorimeter is calibrated with a known electrical input (heater of known V, I, t) to give a calibration factor (CF) in J deg C^-1 or kJ deg C^-1.

Two models for substrate binding:

rate rises with T up to an optimum (typically around 37 deg C for human enzymes), then falls sharply as the enzyme denatures: the weak bonds holding the tertiary structure together break, the active site distorts, and the enzyme loses activity.

each enzyme has a pH optimum. Pepsin (stomach) optimum ~2; trypsin (small intestine) optimum ~8; salivary amylase optimum ~7. Outside the optimum range, the charge state of the active site residues changes, weakening substrate binding or catalysis.

rate increases with [S] until the enzyme is saturated, at which point the rate plateaus at V_max.

q = 5.40 x 2.50 = 13.5 kJ for 0.500 g, so 27.0 kJ g^-1.

the calorimetry value (27.0) exceeds the macronutrient estimate (22.4) by about 20%, in line with the bomb measuring full combustion (including some of the fibre) while the metabolic estimate counts only digestible macronutrients. This is the expected direction of any discrepancy.

Bomb gives total combustion energy; metabolic energy is what the body actually uses. Bomb values are usually higher.

GI measures the speed of blood glucose rise from a fixed serving of carbohydrate; energy content is total kJ per gram. They are independent.

Humans lack cellulase. Cellulose is fibre, undigestible, contributes 0 kJ g^-1 metabolically.