Decomposition and carbon recyclingEdexcel International A Level Biology: Revision notes
Section 1
Decomposition and saprobiotic nutrition
Decomposition is the breakdown of dead organic matter (dead plants, animals and their waste). It is carried out mainly by saprobiotic (saprotrophic) bacteria and fungi, which are the decomposers.
Saprobionts feed by extracellular digestion: they secrete enzymes (such as cellulases, proteases and lipases) onto the dead material. These hydrolyse large insoluble molecules into small soluble ones, which the organism absorbs.
Fungi and bacteria do not engulf dead material. They secrete enzymes and absorb the soluble products.
Section 2
Respiration of decomposers
Decomposers use some of the absorbed products for respiration, releasing carbon dioxide and heat. This is why compost heaps become warm. The rest is used for growth, making new microbial biomass, or remains in the soil as organic matter. Decomposers also release mineral ions such as nitrates and phosphates, which plants absorb.
Section 3
The carbon cycle
Carbon is recycled between the atmosphere and living things:
- Photosynthesis fixes carbon dioxide into organic compounds in plants
- Feeding passes carbon along food chains
- Respiration by plants, animals and decomposers releases carbon dioxide
- Decomposition of dead matter and waste returns carbon dioxide to the atmosphere
- Combustion of wood and fossil fuels releases carbon dioxide quickly
Without decomposers, carbon would stay locked in dead material and the supply of carbon dioxide and minerals for plants would run out.
Section 4
Factors affecting the rate of decomposition
The rate depends on how active the decomposers are:
- Temperature – a warmer temperature increases enzyme activity and respiration, up to the optimum, above which enzymes denature
- Water – decomposers need moisture; very dry conditions stop activity
- Oxygen – aerobic decomposers need oxygen. In waterlogged soil and peat bogs, decomposition is very slow, so carbon builds up as peat
- Nutrients and pH – very acidic conditions slow decomposers
Link any factor to enzyme activity or respiration, e.g. 'low temperature means slower enzyme-controlled reactions, so slower decomposition'.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Decomposition and carbon recycling
- A gardener makes a compost heap from grass cuttings, vegetable peelings and fallen leaves. After eight weeks the material has become a dark, crumbly substance, and the heap has been noticeably warm throughout.Explain how fungi in the heap digest the dead plant material.2 marks
- A student buries identical masses of dry leaves in mesh bags in soil kept equally moist at four different temperatures. After four weeks she measures the percentage loss in mass of the leaves: 4% at 5 degrees Celsius, 13% at 15 degrees Celsius, 32% at 25 degrees Celsius and 21% at 35 degrees Celsius.Explain why the leaves lost mass during the investigation.2 marks
- In a woodland, carbon dioxide from the atmosphere is incorporated into the leaves of trees. When the leaves fall, they are broken down in the soil by bacteria and fungi. In a waterlogged peat bog nearby, dead plant material has built up over thousands of years to form deep layers of peat.Describe how carbon in atmospheric carbon dioxide is incorporated into a leaf and later returned to the atmosphere after the leaf has fallen.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).