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4.0Unit 4: Energy, Environment, Microbiology and Immunity

Overview: Energy, Environment, Microbiology and Immunity — IAL Edexcel Modular Revision Notes & Topical Questions

5 min read 6 parts Lesson 1 of 8 Unit quiz

📚 IAL Edexcel Modular Hub / Topical Revision
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Lesson summary

Unit 4: Energy, Environment, Microbiology and Immunity. Follow how photosynthesis supplies energy to ecosystems, how environmental change affects organisms, and how the body defends against pathogens.

  • The light-dependent reactions
  • Carbon fixation
  • Biomass and productivity
  • Measuring distribution and change
  • The carbon cycle and human activity
  • Effects on organisms

Part 1 of 6Photosynthesis and energy supply

1The light-dependent reactions.
The light-dependent reactions.
  • Chlorophyll absorbs light in thylakoid membranes.
    • Energised electrons pass through carriers, helping create a proton gradient used to make ATP.
  • Water is split to replace lost electrons. This photolysis releases oxygen and protons.
  • Electrons and protons reduce NADP to NADPH, which carries reducing power to the next stage.
2Carbon fixation.
Carbon fixation.
  • In the stroma, the Calvin cycle fixes carbon dioxide to RuBP using the enzyme rubisco.
  • ATP and reduced NADP from the light-dependent stage help convert the products into triose phosphate.
  • Some triose phosphate forms carbohydrates; the rest regenerates RuBP so the cycle can continue.

Part 2 of 6Energy flow through ecosystems

1
Biomass and productivity.
Biomass and productivity.
  • Producers convert light energy to chemical energy in organic molecules.
    • Gross primary productivity is the total fixed energy;
      • net primary productivity is what remains after producer respiration.
  • Net primary productivity supports growth and becomes available to consumers.
  • Only part of the energy at one trophic level reaches the next because material may be uneaten, undigested or used in respiration.
2
Measuring distribution and change.
Measuring distribution and change.
  • Quadrats estimate abundance or percentage cover in a defined area;
    • transects show change along an environmental gradient.
  • Random sampling reduces selection bias, while repeated samples improve the reliability of an estimate.
  • Ecological succession changes community composition over time as species alter conditions and other species colonise.

Part 3 of 6Climate and environmental evidence

1
The carbon cycle and human activity.
The carbon cycle and human activity.
  • Photosynthesis removes carbon dioxide from air; respiration, decomposition and combustion return it.
  • Burning fossil fuels and land-use change can increase atmospheric carbon dioxide, strengthening the greenhouse effect.
  • Carbon-cycle measurements and long-term temperature records are different lines of evidence that must be interpreted together.
2
Effects on organisms.
Effects on organisms.
  • Changes in temperature and rainfall may alter species ranges, breeding times and interactions within food webs.
  • Organisms with suitable heritable variation may be more likely to survive and reproduce;
    • isolated populations can diverge over many generations.
  • Climate models test possible futures, so state the scenario and uncertainty rather than treating one prediction as certain.

Part 4 of 6Microorganisms and growth

1Bacteria and viruses differ.
Bacteria and viruses differ.
  • Bacteria are cells with cytoplasm, a membrane and ribosomes;
    • they reproduce by binary fission when conditions allow.
  • Viruses are acellular particles containing genetic material in a protein coat and must use host cells to reproduce.
  • Antibiotics target bacterial processes; they do not directly stop viral replication.
2Interpreting a bacterial growth curve.
Interpreting a bacterial growth curve.
  • In the lag phase, cells adapt; exponential growth follows while resources are plentiful.
  • Growth slows and may become stationary as nutrients are depleted and waste builds up;
    • a decline phase can follow.
  • When comparing cultures, control temperature, nutrient supply, pH and sampling time, and use suitable aseptic technique.

Part 5 of 6Defence and immunity

1
Non-specific defences act first.
Non-specific defences act first.
  • Skin and mucous membranes limit pathogen entry.
    • Inflammation and phagocytes respond if a pathogen crosses these barriers.
  • Phagocytes engulf and digest foreign material; this response is not specific to one antigen.
  • Good experimental explanations distinguish these barriers from later antigen-specific responses.
2
Specific immunity and memory.
Specific immunity and memory.
  • B lymphocytes can produce antibodies that bind a matching antigen.
    • T lymphocytes help coordinate responses or destroy infected cells.
  • After an initial response, memory cells can produce a faster and larger response to the same antigen on later exposure.
  • Vaccination aims to establish this memory safely;
    • pathogen variation can reduce how well earlier immunity recognises a changed antigen.

Part 6 of 6Questions to test yourself

1
Explain why the Calvin cycle depends on products made in the light-dependent reactions.
2
Why does less energy reach each successive trophic level?
3
Compare a bacterial cell with a virus, and explain why antibiotics do not treat viral infection directly.
4
What makes a secondary immune response faster than a primary response?

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