Decomposers and decay revision notes

Edexcel International GCSE Biology (4BI1), chapter 14: Ecosystems, specification point 4.6. Free lesson.

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Decomposers break down dead material and return its nutrients to the soil. Decay is fastest in warm, moist, oxygen-rich conditions, so understanding these conditions explains both nutrient cycling and food preservation.

How decomposers feed

A decomposer is a fungus or bacterium that feeds on dead material. It does not take food into its body first. Instead, it releases enzymes onto the material, so digestion happens outside its body. This is called saprotrophic nutrition. Enzymes break large insoluble molecules into small soluble molecules. The decomposer absorbs the soluble products through its cell surface. Unused nutrients are released into the soil, where plants can take them up.

How to measure the rate of decay

The amount of carbon dioxide produced gives a way to compare decay rates. Decomposers release carbon dioxide as they respire, so more carbon dioxide in the same length of time means faster respiration and faster decay. To test temperature fairly, change the temperature but keep the other conditions the same.

Conditions that affect decay

Decay is fastest when conditions allow decomposers and their enzymes to work well. Temperature has an optimum, usually around 30 to 40 °C. Above this, enzymes become denatured, meaning their shape changes so they no longer work properly. Cold, dry or airless conditions slow decay. Warmth: raises the rate up to the optimum. Too much heat denatures enzymes, so the rate falls. Moisture: is needed for decomposers to grow and for substances to dissolve. Oxygen: allows aerobic respiration, which releases carbon dioxide as decomposers break down material.

Why preservation slows or stops decay

Food preservation works by removing conditions decomposers need, or by removing the microorganisms themselves. A freezer makes enzyme activity extremely slow. Canning heats the food to kill microorganisms, then seals it so more cannot get in. In a freezer, the temperature is far below the optimum, so decay almost stops. In a sealed tin, heating kills the microorganisms and the seal prevents others from entering.

Why decomposers matter

Decomposers return nutrients from dead organisms to the soil. Plants can absorb these nutrients and use them to grow, so decomposers are essential to nutrient cycles.