Decomposition (biology only)

Drag from cold to hot: what are the enzymes doing?

Temperature (°C): 40. Rate of decay (relative): 100

Drag the point from cold to hot and stop at each part of the curve. The shape is drawn for the milk-decay practical later in this lesson, where the optimum is about 35–45 °C. The heights show the pattern only: they are not measurements.

AQA mark schemes credit a warmer-is-faster explanation of decay in terms of greater kinetic energy and more collisions, higher enzyme activity, or bacteria reproducing faster — but give no credit for simply saying the enzymes work better.

Predict, then check

Temperature is only part of the story. The microorganisms need other things too.

A damp heap of garden waste is sealed under plastic so no air can get in. What happens to the decay?

The milk-decay practical

Milk decays slowly on its own, so the method adds an enzyme, lipase, to speed the decay up. That makes the practical a model of decay you can time.

1. 1Set up two tubesPut 5 cm³ of lipase solution in a boiling tube labelled 'lipase'. In a tube labelled 'milk', add five drops of cresol red indicator, 5 cm³ of milk and 7 cm³ of sodium carbonate solution. The milk tube should be purple.+
2. 2Warm both tubesStand both tubes in a water bath at your chosen temperature. Wait until the contents reach the water-bath temperature.+
3. 3Add the lipase and start timingTransfer 1 cm³ of lipase into the milk tube and start the stop clock immediately. Stir the contents.
4. 4Time the colour changeKeep stirring until the solution turns yellow. Record the time taken, in seconds.+
5. 5Repeat across a rangeRepeat the investigation at different water-bath temperatures. A suitable range is 20–60 °C, ideally with at least five different temperatures.+
6. 6Record and graphRecord the temperature against the time taken to turn yellow. Plot the results as a graph, choosing appropriate scales for both axes.+

This is AQA GCSE Biology Required practical activity 10: investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change. It covers AT skills 1, 3, 4 and 5.

What you need to know

• Microorganisms use enzymes to decompose biological material.
• Temperature, water and the availability of oxygen all affect the rate of decay.
• Decay gets faster as the temperature rises to an optimum, then slows as enzymes are denatured.
• Compost is used as a natural fertiliser; anaerobic decay in a biogas generator produces methane for fuel.
• In the milk-decay practical, the time for cresol red to turn from purple to yellow shows how fast decay is at each temperature.

The big picture

Microorganisms decompose biological material using enzymes, so temperature affects decay by affecting the rate of enzyme-controlled reactions: decay speeds up towards an optimum, then slows as enzymes are denatured. Water matters because microorganisms need it to survive, and oxygen availability affects aerobic and anaerobic microorganisms. People use decay to make compost, a natural fertiliser, and use anaerobic decay in biogas generators to produce methane as a fuel.

⚠ Watch out

Mixing up the two slow ends of the curve. In the cold, decay is slow because successful collisions are rare. When it is extremely hot, decay is slow because the active site has changed shape and the enzyme is denatured. Denaturing belongs to the hot end.

Memory hook

Cold: rare collisions. Optimum: enzymes at their busiest. Too hot: wrong shape — denatured for good.

Check yourself

Without looking back, explain why decay is slow in the cold, fastest at the optimum and very slow when extremely hot. Use the words collisions, active site and denatured.