Decay Detectives

Decay Detectives 🕵️

A fallen leaf, a dead animal — why doesn't the world pile up with them? Because an army of **decomposers** breaks the dead down, recycling their nutrients back into the soil. In this module: who the decomposers are, what speeds them up or slows them down, and how we harness decay in a **biogas generator**.

The clean-up crew 🍄

**Decomposition** is the breakdown of dead organisms and waste by **microorganisms** — mainly **bacteria** and **fungi**, the **decomposers**. They secrete enzymes onto the dead material to digest it, then absorb the nutrients — releasing carbon dioxide and mineral ions back into the ecosystem.

Who does the decomposing?

Which organisms are the main **decomposers**?

• Bacteria and fungi
• Green plants
• Large predators
• Only insects

What sets the pace 🌡️

How fast decay happens depends on **three factors**: • **Temperature** — warmth speeds up the decomposers' enzymes (up to a point). • **Water / moisture** — decomposers need water to survive and to respire. • **Oxygen availability** — most decomposers **respire aerobically**, so more oxygen means faster decay.

The three factors

Select the **three** factors that affect the **rate of decay**.

• Temperature
• Water / moisture
• Oxygen availability
• Light intensity

RP10: the milk test 🥛

Required practical 10 investigates how **temperature** affects the rate of decay of **fresh milk**. Microbes in the milk break it down, producing **acid** — so the **pH falls** over time. You start the milk **alkaline** with an indicator, incubate it at each temperature, and time how long it takes to turn **acidic** (the indicator changes colour). A shorter time means faster decay.

Run the experiment

An interactive activity.

Work out the rate

An interactive activity.

Reading the curve

On a graph of **rate of decay against temperature**, the rate rises, peaks around 30–40 °C, then **falls** at higher temperatures. Why does it fall?

• The decomposers' enzymes denature, so decay slows
• The reaction always gets faster as it heats up
• The microbes run out of milk to break down
• The milk freezes at high temperature

Harnessing decay ⚡

We can put decay to work. A **biogas generator** uses microorganisms to decompose waste (plant or animal) **without oxygen** — **anaerobic** decay — producing **biogas**, mainly **methane**, which is burned as a fuel. Generators work best kept **warm** and with a steady supply of waste — the same "faster when warmer" rule as the milk test.

In the exam 🎓

Case closed. Grade-9 habits for decay: • Decomposers are **bacteria and fungi**; rate depends on **temperature, water and oxygen**. • Rate rises with temperature to an **optimum**, then **falls as enzymes denature** — remember RP10 measures this by a falling **pH**. • A **biogas generator** uses **anaerobic** decay to make **methane** fuel — keep it warm, no oxygen.