Edexcel A Level Biology A Topic 5: On the Wild Side Exam Questions

About Edexcel A Level Biology A Topic 5: On the Wild Side

Topic 5 of Edexcel A Level Biology A (Salters-Nuffield) studies ecosystems and how energy flows through them. You learn about biotic and abiotic factors, niches, succession to a climax community and ecological sampling techniques. Photosynthesis is covered in detail, from chloroplast structure and photophosphorylation to the Calvin cycle and the products made from GALP. You then calculate productivity and the efficiency of energy transfer between trophic levels. The topic covers evidence for climate change, its causes and effects on organisms, the use of models, and how scientists debate the evidence. Natural selection and speciation link back to Topic 4. Topic 5 is assessed in Paper 1 and Paper 3.

What you need to know

• Ecosystems Populations are affected by biotic factors (competition, predation) and abiotic factors (temperature, light). Succession proceeds from pioneer species through to a climax community, and quadrats and transects are used to sample.
• Light-dependent reactions On the thylakoid membranes, light energy excites electrons; photolysis of water releases oxygen; ATP is made by photophosphorylation and NADP is reduced.
• Calvin cycle In the stroma, carbon dioxide combines with RuBP, catalysed by RUBISCO, to form GP. GP is reduced to GALP using ATP and reduced NADP; GALP is used to make glucose, amino acids and lipids and to regenerate RuBP.
• Productivity NPP = GPP - R. Efficiency of energy transfer = energy in next trophic level / energy in previous level x 100. Energy is lost through respiration, heat, egestion and excretion.
• Climate change Evidence includes temperature records, dendrochronology, pollen in peat bogs and carbon dioxide from ice cores. Rising temperature affects enzyme activity, development rates, distribution and timing of life cycles.
• Evolution and speciation Natural selection can lead to new species when populations are reproductively isolated. Molecular evidence and peer review support scientific claims about evolution.

Common mistakes in Topic 5: On the Wild Side

• Writing that GPP is the energy available to the next trophic level; NPP is, because energy used in respiration is lost.
• Saying oxygen in photosynthesis comes from carbon dioxide; it comes from photolysis of water.
• Confusing GP and GALP; GP is reduced to GALP using ATP and reduced NADP.
• Stating that all climate change data proves human causes; you should evaluate the evidence and discuss correlation and uncertainty.
• Describing succession without mentioning that each community changes the abiotic conditions, allowing new species to colonise.

Try three Topic 5: On the Wild Side exam-style questions