International A Level Biology / Decomposition and forensics
Microorganisms, decomposition and the carbon cycle
Edexcel International A Level Biology (YBI11), chapter 19: Decomposition and forensics, specification point 6.16. Free lesson.
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Decomposers use extracellular digestion to break down dead matter, then absorb and respire the products. This returns carbon to the atmosphere and helps explain why decomposition speeds up in some conditions but almost stops in waterlogged soil.
How decomposers digest dead matter
Decomposers are bacteria and fungi that feed on dead organic matter, such as leaves, dead animals and faeces. They release enzymes onto the material, so digestion happens outside their cells. This is called extracellular digestion. Fungi release enzymes from hyphae, the fine threads that grow through dead material. The enzymes hydrolyse large, insoluble molecules into small, soluble molecules. Hydrolysis means a bond is broken by adding water. The small soluble products are absorbed into the decomposer's cells. The decomposer can then use them to grow or respire.
Which enzymes break down which molecules
Different enzymes act on different molecules in dead matter. Their products are small and soluble, so they can be absorbed. These reactions are all forms of hydrolysis. Proteases digest proteins into amino acids. Muscle and skin contain a lot of protein. Cellulases digest cellulose into glucose. Cellulose is a main part of plant cell walls. Many bacteria and fungi can make cellulase, but most animals cannot. Amylase digests starch into maltose, then maltase digests maltose into glucose. Lipases digest lipids, including triglycerides, into fatty acids and glycerol.
How decomposers return carbon to the air
Carbon dioxide in the atmosphere is fixed by producers during photosynthesis and made into organic molecules. Feeding passes this carbon along food chains. When organisms die or produce faeces, some of the carbon becomes dead organic matter. Decomposers close the loop by digesting this material and respiring its products. The absorbed glucose can be used as a respiratory substrate. Respiration releases energy for making ATP and produces carbon dioxide. The carbon dioxide returns to the soil and air, where plants can use it in photosynthesis. Some carbon becomes part of the decomposer's…
What makes decomposition faster or slower
Four conditions affect decomposers' enzymes or their respiration. Decomposition is fastest when material is warm, moist, well supplied with oxygen and broken into small pieces. Temperature: warmth increases collisions between enzymes and substrates. If it gets too hot, enzymes denature. Compost heaps often rot faster in summer. Water: hydrolysis uses water, and microorganisms need water to grow. Dried food keeps for months because it has little available water. Oxygen: most decomposers respire aerobically, which releases more ATP per glucose than anaerobic respiration. Turning a…
Why waterlogged peat slows decay
Waterlogged soil contains very little oxygen. Most decomposers cannot respire aerobically there, so decomposition almost stops and carbon remains locked in dead matter. Decomposition is carried out by microorganisms' enzymes, not by the dead organism's own enzymes. Describe the conditions as waterlogged and low in oxygen, then explain that microorganisms cannot respire aerobically and decomposition is limited.