Sec 3 Combined Science (Biology) Biological Molecules notes

Carbohydrates are the body's quick-charge battery: starch (in bread, rice, potatoes) is digested to glucose, which cells burn in respiration for immediate energy. Fats are the savings account and the winter coat: they store more than twice as much energy per gram as carbohydrate, ideal for long-term storage, and the fat layer under the skin insulates the body against heat loss. Proteins are the building crew: their amino-acid units are assembled into new cells, enzymes and muscle fibre, so protein in the diet is needed for growth and repair — the role of a growing teenager's meals and of a wound healing.

Each food class betrays itself with a colour change. For starch, add a few drops of iodine solution: blue-black means starch is present; staying brown means absent. For reducing sugars, add Benedict's solution and heat the tube: no sugar keeps the blue colour, a little turns green or yellow, more turns orange, and plenty gives a brick-red precipitate. For protein, add biuret solution: a purple colour means protein; staying blue means absent. For fats, mix the food with ethanol, then pour the ethanol layer into water: a cloudy white emulsion means fat is present; staying clear means absent.

The giant molecules of food are all assembled from a handful of small units, like Lego from standard bricks. Glucose units join into the three big carbohydrates: starch (the plant's store), glycogen (the animal's store, in liver and muscle) and cellulose (the plant's structural wall material). Amino acids join in long chains to make proteins — twenty-odd amino-acid types can be ordered into endless different proteins. Glycerol joined to fatty acids makes fats. Digestion simply reverses the process, cutting the big molecules back into their small units small enough to absorb.

Exam phrasing to memorise exactly: starch, glycogen and cellulose are all "synthesised from glucose"; proteins "from amino acids"; fats "from glycerol and fatty acids". Swapping any unit — starch from amino acids is the classic wrong answer — costs the mark.

An enzyme is a biological catalyst — a protein that speeds up a reaction without being used up itself. Every enzyme carries a dent called the active site, whose shape exactly matches one substrate, the way one key fits one lock. The substrate slots into the active site, forming an enzyme–substrate complex; held in exactly the right position, the substrate reacts — split or joined — and the products drift away, leaving the enzyme free to accept the next substrate.

That shape-matching explains specificity: amylase's active site fits starch but not protein, so amylase only ever catalyses starch digestion. It also explains denaturation ahead: anything that changes the active site's shape — excessive heat, the wrong pH — stops the substrate fitting, and the enzyme stops working.

Warm an enzyme reaction and it speeds up — molecules move and collide faster — up to an optimum temperature, about 37 °C for human enzymes. Push further and the rate falls off a cliff: heat denatures the enzyme, permanently changing the active site's shape so the substrate no longer fits. pH works the same way on a different dial: each enzyme has an optimum pH — around 2 for the protein-digesting pepsin in the acidic stomach, around 7 for amylase in the mouth — and on either side of its optimum the rate drops, dropping for good if the shape is wrecked.