Ecology, human impact and biotechnology
Food chains and energy flow, nutrient cycles, populations, how humans affect ecosystems, and how microorganisms and genetic modification are used.
📌 Key points
- The Sun is the principal source of energy for biological systems. Energy flows through food chains from producers to primary, secondary and tertiary consumers; decomposers break down dead material.
- Energy is lost at each trophic level (e.g. as heat from respiration, in egestion, and in uneaten parts), so food chains usually have fewer than five trophic levels. Pyramids of biomass are usually a true pyramid shape; pyramids of numbers can be inverted.
- Carbon cycle: photosynthesis removes carbon dioxide from the air; respiration, decomposition and combustion return it; carbon can be stored in fossil fuels.
- Nitrogen cycle: nitrogen-fixing bacteria turn nitrogen gas into nitrogen compounds; decomposers release ammonium ions; nitrifying bacteria make nitrates; denitrifying bacteria return nitrogen gas to the air.
- A population growth curve has lag, exponential (log), stationary and death phases. Population size is affected by food supply, predation and disease.
- Deforestation causes loss of habitats, extinction, soil erosion, flooding and more carbon dioxide in the air.
- Eutrophication: fertiliser or sewage adds nutrients to water; algae grow quickly and block light; plants die; decomposers break them down using oxygen in aerobic respiration; oxygen falls and fish die.
- Conservation methods include protected areas, captive breeding, seed banks, fishing quotas, and replanting forests (sustainable resources).
- Bacteria are useful in biotechnology because they reproduce rapidly and can make complex molecules. Yeast makes ethanol for biofuels and carbon dioxide for bread; pectinase extracts fruit juice; biological washing powders contain enzymes; penicillin and insulin are made in fermenters.
- Genetic modification is changing the genetic material of an organism by removing, changing or inserting individual genes, e.g. inserting the human insulin gene into bacteria (using restriction enzymes, ligase and plasmids), or making pest-resistant or herbicide-resistant crops.
📖 Key terms
- Producer
- An organism that makes its own organic nutrients, usually by photosynthesis.
- Consumer
- An organism that gets its energy by feeding on other organisms.
- Decomposer
- An organism that gets its energy from dead or waste organic material.
- Trophic level
- The position of an organism in a food chain or food web.
- Ecosystem
- A unit containing the community of organisms and their environment, interacting together.
- Eutrophication
- Enrichment of water with nutrients, leading to algae growth and loss of oxygen.
- Genetic modification
- Changing the genetic material of an organism by removing, changing or inserting genes.
✏️ Worked example
In a food chain the producers contain 20 000 kJ, the primary consumers 2000 kJ and the secondary consumers 180 kJ. Calculate the efficiency of energy transfer from primary to secondary consumers.
- Efficiency = energy in next level ÷ energy in previous level × 100.
- Efficiency = 180 ÷ 2000 × 100.
- 180 ÷ 2000 = 0.09.
- Efficiency = 9%: the rest is lost, e.g. as heat from respiration and in faeces.
In eutrophication, the oxygen is used up by the aerobic respiration of decomposing bacteria, not by the algae themselves. Write each step of the sequence in order.
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