Decomposition depends on microbial activity
Definition
Decomposition
The breakdown of dead organisms and waste by microorganisms, which releases nutrients back into the environment.
- Bacteria and fungi secrete digestive enzymes onto biological material and absorb the soluble products.
- Temperature changes enzyme activity and the rates of microbial respiration, growth and reproduction.
- Water is needed for enzyme-controlled reactions and for soluble substances to move into microbial cells.
- Oxygen allows aerobic decomposers to respire and release enough energy for rapid growth and active transport.
Food preservation removes favourable conditions
- Refrigeration lowers temperature, reducing enzyme activity and slowing microbial growth without killing every microorganism.
- Freezing makes liquid water unavailable and lowers enzyme activity, so decay becomes very slow until the food thaws.
- Drying removes water, while high concentrations of salt or sugar draw water out of microbial cells by osmosis.
- Vacuum packaging and sealed containers reduce oxygen availability, slowing aerobic decomposers.
- Canning combines heating to kill microorganisms with sealing to prevent re-entry and restrict oxygen.
- Pasteurisation uses controlled heating to kill many microorganisms and is followed by cooling to slow survivors.
Example
- Fresh fruit kept warm, moist and exposed to air provides favourable conditions for decomposers, so it decays rapidly.
- The same fruit lasts longer when chilled because lower temperature reduces enzyme activity and microbial reproduction.
- Drying or sealing it removes another requirement, so the explanation should name the factor and its effect on microorganisms.
Composting supplies favourable conditions
- A compost heap is kept moist so decomposers have water, but excess water fills air spaces and restricts oxygen.
- Turning the heap introduces oxygen for aerobic respiration and mixes undecomposed material with microorganisms.
- Microbial respiration releases heat, so a large, insulated heap can remain warm and support rapid enzyme activity.
- Very high temperature slows some decomposers because enzymes denature and microorganisms may die.
- Shredding material increases its surface area, giving decomposer enzymes more contact with the material.
- A mixture of soft green material and tougher dry material helps retain moisture without removing all the air spaces.
Exam technique
- For each factor, write a causal chain from the condition to microbial enzyme activity or respiration and then to the rate of decomposition.
- Avoid saying cold kills bacteria because refrigeration mainly slows their reactions and growth.
Preservation and composting use opposite aims
- Preservation keeps food cold, dry or low in oxygen to reduce the rate at which decomposers break it down.
- Composting keeps material warm, moist and aerated to increase decomposition and produce nutrient-rich compost.
- Compost returns mineral ions to soil, where plants can absorb them for new growth.
- Conditions must be balanced because a waterlogged heap has little oxygen, while a dry heap limits enzyme-controlled reactions.
Self review
- How does low temperature slow food decomposition?
- Why do drying, salt and sugar preserve food?
- Why does turning a compost heap increase decomposition?
- Why can both too little and too much water slow composting?
