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6.1.2Topic 6 · Immunity, Infection and Forensics

Micro-organisms- Decomposition and Nutrient Recycling — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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6.1.2 Micro-organisms- Decomposition and Nutrient Recycling — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how saprobiotic microorganisms decompose dead material, and describe the roles of microorganisms in the nitrogen and phosphorus cycles.Dead plants and animals, faece…

  • saprobionts
  • extracellular digestion
  • Temperature
  • Nature of the material
  • eutrophication
  • Exam-style question 1

Section 1 of 9Saprobionts and decomposition

Dead plants and animals, faeces and other waste contain organic compounds that would accumulate if they were not broken down.

Saprobionts
Bacteria and fungi that feed on dead matter are saprobionts (saprotrophs).
Extracellular digestion
They secrete digestive enzymes onto the material, a process known as extracellular digestion, and absorb the soluble products such as sugars, amino acids and fatty acids.
Fungi grow as networks of hyphae that penetrate the dead tissue, giving a large surface area for secretion and absorption.
They release carbohydrases, proteases and lipases, and some fungi produce enzymes that break down cellulose and lignin in wood.

The saprobionts use some of the absorbed nutrients for respiration and growth, and the rest is released when they die or are eaten. When they respire, carbon is returned to the atmosphere as CO2. Their action therefore recycles nutrients such as carbon, nitrogen, phosphorus and sulfur so that producers can take them up again.

Section 2 of 9Conditions that affect decomposition

Temperature
Temperature enzyme activity and growth increase to an optimum, so decay is faster in warm conditions and very slow in the cold.
Moisture
Moisture water is needed for enzyme action and microbial growth; dry material, as in a dried food, decays slowly.
Oxygen
Oxygen aerobic decomposers work faster; in waterlogged soils or peat anaerobic conditions slow decay and allow organic material to accumulate.
pH
pH acidic conditions, as in bogs, reduce microbial activity.
Nature of the material
Nature of the material woody tissues with lignin decay more slowly than soft leaves.

These principles explain methods of food preservation: freezing, drying, salting, pickling and canning slow or prevent microbial growth.

Section 3 of 9The nitrogen cycle

Organisms need nitrogen for proteins and nucleic acids. Although the atmosphere is about 78% nitrogen gas, most organisms cannot use it directly. Microorganisms carry out several stages.

ProcessOrganismsChange
Nitrogen fixationFree-living Azotobacter; Rhizobium in root nodules of legumes (mutualism)N2 to ammonia / ammonium ions
AmmonificationSaprobiontsProteins and urea in dead matter and waste to ammonia / ammonium
NitrificationNitrosomonas, then Nitrobacter (aerobic)Ammonium to nitrite (NO2−) to nitrate (NO3−)
DenitrificationDenitrifying bacteria, anaerobic conditionsNitrate to N2

Plants absorb nitrate (and some ammonium) through their roots and use it to make amino acids, proteins and nucleotides.

1
Animals obtain nitrogen by eating plants or other animals.
2
Waterlogging of soil encourages denitrification and so reduces the fertility of the soil; ploughing and drainage restore oxygen and promote nitrification.
3
Legumes enrich soil, so crop rotation with clover or beans reduces the need for fertiliser.
4
Lightning also fixes small amounts of nitrogen, and industrial processes make fertiliser.

Section 4 of 9The phosphorus cycle

Phosphorus is needed for ATP, nucleic acids and phospholipids.

It occurs in rocks as phosphate.
Weathering and erosion release phosphate ions into the soil and water, where plants and algae absorb them.
Phosphate passes along food chains.
When organisms die, saprobionts decompose them and release phosphate to the soil, so that it can be taken up again.
Mycorrhizal fungi form a mutualism with the roots of most plants: the fungus extends the area for absorption of phosphate and water, and receives sugars from the plant.
Unlike the carbon and nitrogen cycles, the phosphorus cycle has no major gaseous stage and is slower.
Eutrophication
Excess phosphate and nitrate from fertilisers cause eutrophication.

Section 5 of 9Importance and links

Without decomposition, nutrients would remain locked in dead material and growth would stop. The actions of microorganisms underpin soil fertility, composting, sewage treatment and biogas production. These ideas link to the carbon cycle (climate topic), to enzymes, and to forensic uses: decomposition and microbial succession are used as evidence in estimating time of death.

Section 6 of 9Applications and data skills

Composting uses these principlesa heap with a mix of green and woody waste, adequate air, moisture and warmth supports aerobic saprobionts, producing humus that improves soil structure and water retention.

Applications and data skills
Biogas digesters work in the absence of oxygen, using anaerobic microorganisms that make methane from organic waste.
Sewage treatment relies on aerobic bacteria to break down organic matter in activated sludge.

Data questions may give the mass of leaf litter remaining over time at different temperatures or in different soils.

Applications and data skills
Describe the rate of loss using figures (for example, percentage mass lost per month), compare conditions, and explain differences using enzyme activity, oxygen and moisture.
The rate can be found from the gradient of a graph of mass against time.
In the investigation, litter bags with different mesh sizes can show the contribution of large invertebrates such as earthworms, which fragment the material and increase the surface area for microbial action.

Section 7 of 9Exam-style questions

  1. Explain how saprobionts break down dead leaves. [4]4 marks

  2. Describe the roles of bacteria in the conversion of ammonium ions to nitrate in soil, and state why this is important for plants. [4]4 marks

  3. Suggest why waterlogged soil is often low in nitrate. [3]3 marks

Section 8 of 9Key terms

TermMeaning
SaprobiontAn organism that feeds on dead organic matter by extracellular digestion.
Extracellular digestionDigestion by enzymes secreted outside the cell.
Nitrogen fixationConversion of nitrogen gas into ammonia or ammonium.
NitrificationOxidation of ammonium to nitrite and then nitrate.
DenitrificationConversion of nitrate to nitrogen gas by anaerobic bacteria.
AmmonificationProduction of ammonia from nitrogenous waste and dead matter.
MycorrhizaA mutualistic association between a fungus and plant roots.
EutrophicationEnrichment of water with nutrients, leading to algal blooms and oxygen depletion.

Section 9 of 9Exam technique

1
Saying decomposers 'eat' the material. Saprobionts digest it externally using enzymes.
2
Confusing nitrification with nitrogen fixation: fixation turns N2 into ammonia; nitrification oxidises ammonium to nitrate.
3
Forgetting that nitrification needs oxygen and denitrification occurs without it.
4
Saying plants take in nitrogen gas. They absorb nitrate or ammonium ions.

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