structures, properties and reactions (condensation and hydrolysis) of the major biomacromolecules in food (carbohydrates, proteins and lipids) and the role of vitamins, enzymes (active site, lock-and-key/induced-fit models, effects of temperature and pH) and the determination of the energy content of food using bomb calorimetry, including the influence of macronutrient composition and glycaemic index

What this dot point is asking

VCAA wants you to describe the structures and reactions of the three major biomacromolecules (carbohydrates, proteins, lipids), to know the role of vitamins and enzymes (including the lock-and-key and induced-fit models and the effect of temperature and pH), and to determine and interpret the energy content of food using bomb calorimetry, with reference to macronutrient composition and the glycaemic index.

The answer

Carbohydrates

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).

Disaccharides form by condensation between two monosaccharides, losing one water molecule and forming a glycosidic bond:

• Glucose + glucose -> maltose + H2O
• Glucose + fructose -> sucrose + H2O
• Glucose + galactose -> lactose + H2O

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds:

The reverse reaction, hydrolysis, adds water across the glycosidic bond to break the carbohydrate back into its monomers. Hydrolysis is catalysed by acid + heat (chemistry lab) or by specific enzymes (amylase in saliva and pancreas; sucrase, lactase, maltase in the small intestine).

Proteins

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.

Peptide bond formation (condensation) joins the COOH of one amino acid to the NH2 of another, eliminating water and producing an amide (-CO-NH-) link:

H2N-CHR1-COOH + H2N-CHR2-COOH -> H2N-CHR1-CO-NH-CHR2-COOH + H2O

The result is a dipeptide; further condensations build a polypeptide, then a protein.

Four levels of protein structure:

1. Primary: the sequence of amino acids, held by covalent peptide bonds.
2. Secondary: local folding into alpha-helix and beta-pleated sheet, stabilised by hydrogen bonds between backbone C=O and N-H groups.
3. Tertiary: overall 3D shape of one polypeptide, stabilised by hydrogen bonds, hydrophobic interactions, ionic bridges, and disulfide bonds between R groups.
4. Quaternary: assembly of multiple polypeptide subunits into a functional complex (e.g. haemoglobin = 4 subunits).

Hydrolysis (acid + heat, or proteases such as pepsin and trypsin) cleaves the peptide bond back to free amino acids.

Lipids

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

Glycerol + 3 fatty acids -> triglyceride + 3 H2O

Each glycerol-to-fatty-acid bond is an ester linkage. Hydrolysis (acid or base catalysis, or lipase enzymes in the digestive tract) reverses the reaction.

Fatty acids vary in:

• Chain length (commonly 12 to 24 carbons).
• Saturation: saturated (no C=C), monounsaturated (one C=C), polyunsaturated (more than one C=C).
• Geometry: cis (the two chain segments on the same side of the C=C; produces a kink; lowers melting point) or trans (chain segments on opposite sides; straighter molecule; higher melting point).

Saturated and trans fatty acids pack closely (high melting point, solid at room temperature: butter, lard). Cis-unsaturated fatty acids have kinks that prevent close packing (lower melting point, liquid at room temperature: olive oil, fish oil).

Diet implications:

• Saturated and trans fats raise LDL cholesterol and cardiovascular risk.
• Cis-unsaturated fats are neutral or beneficial.
• Omega-3 polyunsaturated fatty acids (found in fish oils, flaxseed) are essential.

The iodine number measures the degree of unsaturation: iodine adds across C=C bonds, so more I2 absorbed per gram of lipid means more double bonds. Useful for ranking oils by unsaturation.

Vitamins

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

• Water-soluble (B-complex, C): not stored long-term; excess excreted in urine; needed regularly. Most B vitamins act as coenzymes in metabolic enzyme reactions.
• Fat-soluble (A, D, E, K): stored in adipose (fat) tissue and liver; excess can accumulate to toxic levels; required less frequently in the diet.

Vitamin C is famous in chemistry for being a reducing agent (you can titrate it with iodine) and for its role in collagen synthesis (and the prevention of scurvy).

Enzymes

Enzymes are protein catalysts. Two models for substrate binding:

• Lock-and-key: the substrate fits the rigid active site exactly, like a key in a lock. Strong on specificity, weak on flexibility.
• Induced fit: the active site is somewhat flexible and adjusts its shape on binding the substrate (like a glove fitting a hand). This is the currently accepted refined model.

Enzymes are highly specific (each catalyses one reaction or a narrow class) and highly efficient (rate enhancements of 10^6 to 10^17 over the uncatalysed reaction).

Effect of temperature: 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.

Effect of pH: 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. Extreme pH also denatures the protein.

Effect of substrate concentration: rate increases with [S] until the enzyme is saturated, at which point the rate plateaus at V_max.

Energy content of food: bomb calorimetry

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.

Energy released:

q = CF x deltaT

Energy per gram of food = q / mass of sample.

Approximate metabolic energy contents (per gram, as the body uses them):

Two important nuances:

1. Bomb calorimetry gives gross combustion energy, not metabolic energy. Bomb energy is generally slightly higher than metabolic energy because the bomb fully combusts substances (including fibre) that the body does not use.
2. Glycaemic index (GI) is not energy content. GI measures how fast a carbohydrate is digested and raises blood glucose, compared to a reference glucose (GI 100). Low-GI foods (oats, lentils, GI 30 to 55) digest slowly; high-GI foods (white bread, mashed potato, GI 70+) digest fast and spike blood glucose. Two foods can have the same energy content but very different GIs.

Worked example

A 0.500 g sample of cashew is combusted in a bomb calorimeter with CF = 5.40 kJ deg C^-1. The temperature rises by 2.50 deg C. The cashew is approximately 18% carbohydrate, 18% protein and 44% fat (the rest is water and fibre). Calculate the energy content from the calorimetry and compare to the macronutrient prediction.

Calorimetry

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

Macronutrient prediction (using metabolic energy):
Carb: 0.18 x 17 = 3.06 kJ g^-1
Protein: 0.18 x 17 = 3.06 kJ g^-1
Fat: 0.44 x 37 = 16.3 kJ g^-1
Total: ~22.4 kJ g^-1 (with the rest of the mass contributing little or nothing).

Discussion: 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.

Common traps

Confusing bomb-calorimetry energy with metabolic energy. Bomb gives total combustion energy; metabolic energy is what the body actually uses. Bomb values are usually higher.

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

Calling cellulose an energy source for humans. Humans lack cellulase. Cellulose is fibre, undigestible, contributes 0 kJ g^-1 metabolically.

Listing peptide bonds as anything but amide (condensation) bonds. The peptide bond is a -CO-NH- amide, formed by condensation (loss of water).

Forgetting that enzymes are proteins. A few non-protein catalysts (ribozymes, RNA-based) exist, but in VCE all enzymes are proteins, and pH/temperature effects act through the protein tertiary structure.

Saying trans fats are healthy because they are unsaturated. Trans configuration packs almost as tightly as saturated fats and behaves more like saturated fat metabolically. Cis-unsaturated is the beneficial form.

In one sentence

Carbohydrates, proteins and lipids are made by condensation of their respective monomers and broken by hydrolysis (catalysed by acid + heat or specific enzymes), enzymes are protein catalysts whose tertiary structure (and so their active site) is sensitive to temperature and pH, and the energy content of food is measured by bomb calorimetry (q = CF x deltaT) with macronutrient composition (17 kJ g^-1 carb/protein, 37 kJ g^-1 fat) and glycaemic index providing complementary nutritional information.