OCR A Level Biology: Mastering Biodiversity and Simpson's Index (Section 4.2.1) (a-d)

OCR A Level Biology: Mastering Biodiversity and Simpson's Index (Section 4.2.1 a-d)

Prior Knowledge to Recap

Before diving into biodiversity questions, ensure you understand these foundational concepts:

• The three levels of biodiversity: genetic diversity (variation within species), species diversity (number and abundance of species), and habitat/ecosystem diversity (range of different habitats)

• Species richness vs species evenness: richness is the total number of different species present, whilst evenness refers to how similar the abundance of each species is

• Simpson's Index formula: D = 1 - Σ(n/N)² where n is the number of individuals of each species and N is the total number of all individuals

• Sampling techniques: random sampling (using coordinates), systematic sampling (transects), and stratified sampling (proportional sampling across different areas)

• The purpose of sampling: it's often impractical to count every organism, so representative samples provide estimates of biodiversity

Links to GCSE Content

• Ecosystems and communities: understanding how organisms interact within habitats (GCSE Biology)

• Variation and classification: recognising that organisms show variation both within and between species (GCSE Biology)

• Mathematical skills: calculating percentages, means, and working with formulas (GCSE Maths)

Common Question Types and How to Answer Them

Let me walk you through five frequently asked questions from past OCR papers, showing you exactly what examiners are looking for.

Question 1: Defining Biodiversity

How to Answer:

The correct answer is C: the variety of genes, species and habitats.

This is a straightforward recall question testing whether you know the complete definition. Many students incorrectly choose A because they only think of species diversity, but biodiversity encompasses all three levels: genetic, species, and habitat diversity.

Mark scheme guidance: Award 1 mark for C only.

Common mistake: Option D refers only to genetic diversity within a single species, whilst option A refers only to species diversity. The complete definition of biodiversity must include all three levels.

Question 2: Calculating Simpson's Index

How to Answer:

This question tests your mathematical skills. Follow these steps systematically:

Step 1: Calculate N (total) N = 6 + 7 + 3 + 8 = 24

Step 2: Create a working table

Species n n/N (n/N)² Meadow buttercup 6 0.250 0.063 Common daisy 7 0.292 0.085 Red clover 3 0.125 0.016 Ribwort plantain 8 0.333 0.111

Step 3: Sum the (n/N)² column Σ(n/N)² = 0.063 + 0.085 + 0.016 + 0.111 = 0.275

Step 4: Subtract from 1 D = 1 - 0.275 = 0.725

Step 5: Round to 2 s.f. D = 0.73

Mark scheme guidance:

• Correct answer of 0.73 = 3 marks (even without working)

• Σ(n/N)² = 0.275 and 1 - Σ = 0.725 = 2 marks

• Some correct values for n/N and (n/N)² = 1 mark

• Error carried forward allowed if method correct

Top tip: Always add extra columns to tables for your working. This makes your calculations clearer and helps you spot errors. Keep at least 3 decimal places in your working, only round at the final answer.

Question 3: Species Richness vs Species Evenness

Find this question in the PDF: Question 4(b)(i) and (ii) (pages 4-5)

Copy and paste Question 4(b) parts (i) and (ii) from your PDF to see Table 2.1 with the butterfly data.

How to Answer:

(i) Species richness:

Answer: Area 2

Justification: Area 2 has 6 species present (including silver-studded blue), whereas Area 1 has only 5 species (silver-studded blue is absent).

Mark scheme guidance: Award 1 mark for identifying Area 2 with correct justification (more/6 species).

(ii) Species evenness:

Answer: Area 2

Justification: The range of individual numbers is smaller in Area 2 (2-11, range = 9) compared to Area 1 (0-16, range = 16), showing more even distribution of individuals across species.

Mark scheme guidance: Award 1 mark for identifying Area 2 with justification that the range of n is smaller.

Key definitions to remember:

• Species richness = the number of different species present

• Species evenness = how similar the population sizes are across all species (the relative abundance of each species)

Common mistake: Students often state "Area 2 has more species" without being specific. Always give the actual numbers (6 species vs 5 species) for a strong justification.

Question 4: Sampling Strategy

How to Answer:

Your answer should include three key elements:

1. Sampling strategy (1 mark): Use stratified AND random sampling

2. Explanation of proportional sampling (1 mark): The number of samples within each area should be proportional to their size

3. Specific calculation (1 mark):

• Total area = 800 + 2400 + 3200 = 6400 m²

• Conifer: (800 ÷ 6400) × 100 = 12.5% → 8 samples (if taking 64 total)

• Marshy: (2400 ÷ 6400) × 100 = 37.5% → 24 samples

• Grazed: (3200 ÷ 6400) × 100 = 50% → 32 samples

Example full answer: "The scientists should use stratified random sampling. They should divide the ecosystem into the three distinct habitat areas and take samples randomly within each one to avoid bias. The number of samples in each area should be proportional to its size. For example, if taking 64 samples in total: 8 samples in the conifer area (12.5%), 24 in the marshy area (37.5%), and 32 in the heavily grazed area (50%)."

Mark scheme guidance:

• Stratified AND random (within each area) = 1 mark

• Idea that number of samples should be proportional to area size = 1 mark

• Correct suggestion for number of samples (e.g., 8, 24, 32) = 1 mark

Why stratified sampling? When a habitat has distinct different zones or types, stratified sampling ensures all areas are represented fairly in proportion to their size.

Question 5: Interpreting Simpson's Index Values

Copy and paste Question 35 from your PDF to see the multiple choice question about ancient woodland.

How to Answer:

The correct answer is A: Biodiversity is high.

Understanding Simpson's Index values:

• The index ranges from 0 to 1

• Values close to 1 = high biodiversity (many species, evenly distributed)

• Values close to 0 = low biodiversity (few species or one dominant species)

• A value of 0.85 is close to 1, indicating high biodiversity

Why the other options are wrong:

Option B is incorrect because 0.85 is high, not low. Values below 0.3 would typically indicate low biodiversity.

Option C is incorrect because Simpson's Index doesn't measure interspecific variation (differences between species as groups). It measures species diversity (richness and evenness).

Option D is incorrect because Simpson's Index doesn't measure intraspecific variation (genetic differences within a single species). That would be genetic diversity, measured differently.

Mark scheme guidance: Award 1 mark for A only.

Critical concept: Don't confuse the types of variation and biodiversity:

• Genetic biodiversity = variety of alleles within and between populations (intraspecific)

• Species biodiversity = variety and abundance of species (what Simpson's Index measures)

• Habitat biodiversity = variety of different habitats in an area

Simpson's Index specifically measures species biodiversity by combining species richness (how many species) and species evenness (how evenly distributed).

Additional Exam Technique Tips

1. For multiple choice questions: Eliminate obviously wrong answers first. Often you can narrow it down to two options, then think carefully about the precise definitions.

2. For calculations:

   • Always show your working in a clear table format

   • Don't round intermediate values too early (keep 3+ decimal places)

   • Only round your final answer to the specified number of significant figures

   • Even if you get the wrong answer, clear working can earn method marks

3. For "justify" questions: Simply restating the question isn't enough. You must provide specific evidence from the data (e.g., actual numbers, ranges, or calculations).

4. For sampling questions: Always specify:

   • The type of sampling (random/systematic/stratified)

   • How you'd implement it (coordinates, transects, etc.)

   • The number of samples and why

5. Time management: These questions appear throughout the papers. Don't spend too long on 1-mark multiple choice questions. If unsure, make an educated guess and move on.

By practising with actual past paper questions in their original format, you'll become familiar with the exam style and what examiners expect

OCR A Level Biology: Mastering Photosynthesis - The Calvin Cycle and Limiting Factors (Section 5.2.1 e, f, g)

OCR A Level Biology: Mastering Photosynthesis - The Calvin Cycle and Limiting Factors (Section 5.2.1 e, f, g)

Prior Knowledge to Recap

Before diving into these photosynthesis questions, ensure you understand these foundational concepts:

• The two stages of photosynthesis: the light-dependent stage (in thylakoid membranes) produces ATP and reduced NADP, whilst the light-independent stage (Calvin cycle in the stroma) uses these products to fix CO₂

• The Calvin cycle pathway: CO₂ combines with RuBP (catalysed by RuBisCO) to form GP, which is reduced to TP using ATP and reduced NADP, with most TP recycled to regenerate RuBP

• Limiting factors concept: when a factor is in short supply, it prevents the rate of a process from increasing, even if other factors are optimal

• Enzyme properties: enzymes are affected by temperature (kinetic energy, denaturation) but not directly by light

• Products of photosynthesis: oxygen is released from photolysis, glucose/carbohydrates are synthesised from TP

Links to GCSE Content

• Photosynthesis equation: understanding that plants use CO₂ and water to produce glucose and oxygen using light energy (GCSE Biology)

• Factors affecting photosynthesis: light intensity, CO₂ concentration, temperature, and chlorophyll (GCSE Biology)

• Enzymes and temperature: recognising that enzymes work faster at higher temperatures until they denature (GCSE Biology)

Common Question Types and How to Answer Them

Let me walk you through five frequently asked questions from past OCR papers on the Calvin cycle and limiting factors.

Question 1: Interpreting Calvin's Experiment Data

How to Answer:

(i) Explaining Calvin's conclusions:

You need to use specific times from the graph:

Point 1: GP was the only compound seen after 1 second Point 2: TP appears after 5 seconds

This shows that GP must be formed first (as it appears earliest and alone), and then GP is converted into TP (which appears later).

Mark scheme guidance:

• Award 1 mark for stating GP was the only compound seen after 1 second

• Award 1 mark for stating TP appears after 5 seconds

• Allow 'glycerate 3-phosphate' for GP and 'triose phosphate' for TP

(ii) What happens to TP:

Answer: TP is converted into/used to synthesise sugar phosphates, amino acids (e.g. glutamic acid, serine, glycine), or RuBP.

Mark scheme guidance: Award 1 mark. Must be the idea of synthesis/conversion into something, not breaking down.

Common mistake: Don't say TP is "broken down" - it's used as a building block to synthesise other molecules. Also, don't confuse TP with GP.

Question 2: Effects of Light Intensity on Calvin Cycle Intermediates

How to Answer:

The correct answer is B: Only statements 1 and 2 are correct.

Why each statement is correct or incorrect:

Statement 1 is CORRECT: At low light intensity, less GP is converted into TP because there is less ATP and reduced NADP available (products of the light-dependent stage).

Statement 2 is CORRECT: At high light intensity, RuBP concentration is high because more TP is regenerated into RuBP (due to more ATP and reduced NADP being available to convert GP to TP).

Statement 3 is INCORRECT: At high light intensity, RuBP does not accumulate because it cannot be converted to GP. In fact, at high light intensity, RuBP is rapidly converted to GP because there's plenty of CO₂ available (assuming CO₂ isn't limiting).

Mark scheme guidance: Award 1 mark for B only.

Key concept: Light intensity affects the light-dependent stage directly (producing ATP and reduced NADP), which then affects the concentrations of Calvin cycle intermediates. Low light = less ATP/reduced NADP = GP accumulates (can't be reduced to TP). High light = more ATP/reduced NADP = RuBP accumulates (TP is rapidly regenerated to RuBP).

Question 3: Completing a Passage about the Calvin Cycle

How to Answer:

This tests your precise knowledge of the Calvin cycle terminology:

1. RuBP / ribulose bisphosphate (CO₂ combines with this 5-carbon molecule)

2. GP / glycerate 3-phosphate (the unstable 6-carbon molecule breaks into two of these)

3. ATP (used to reduce GP to TP)

4. NADPH / reduced NADP (also used to reduce GP to TP)

5. sucrose (hexose phosphates converted to this for transport)

Mark scheme guidance: Award 1 mark for each correct answer (5 marks total). ATP and NADPH can be in either order. Allow abbreviations like GP and RuBP.

Common mistakes:

• Writing "ribulose biphosphate" instead of "bisphosphate"

• Confusing GP with "glycerol-phosphate"

• Writing "NADH" instead of "NADPH" (confusing with respiration)

• Writing "glucose" instead of "sucrose" for transport (the question specifically mentions transport elsewhere in the plant)

• Confusing RuBP (the substrate) with RuBisCO (the enzyme)

Top tip: If you find the full chemical names difficult to spell correctly, use the abbreviations RuBP and GP - there's much less opportunity for error!

Question 4: Effects of Reducing CO₂ Concentration on Calvin Cycle

How to Answer:

You need to describe and explain changes in both RuBP and GP:

For GP (glycerate 3-phosphate):

• Description: GP concentration decreases

• Explanation: Because less CO₂ is available to react with RuBP to produce GP / less carbon fixation taking place

For RuBP (ribulose bisphosphate):

• Description: RuBP concentration increases AND then decreases

• Explanations:

  • RuBP increases because it is not being converted to GP (no CO₂ to react with)

  • RuBP increases because it is still being produced/regenerated from TP

  • RuBP then decreases because less GP is available to regenerate RuBP

Mark scheme guidance: Award up to 3 marks. Maximum 2 marks for RuBP explanations (from MPs 3, 4, 5, and 6).

Understanding the logic: When CO₂ is reduced:

1. Less CO₂ + RuBP → less GP formed (GP decreases)

2. RuBP not being used up → RuBP initially increases

3. But TP is still being used to regenerate RuBP → RuBP continues to increase temporarily

4. Eventually less GP means less TP, means less regeneration → RuBP then decreases

Common mistake: Students often only describe one molecule (usually GP) and forget to discuss RuBP, or they don't explain the biphasic nature of the RuBP graph (increases then decreases).

Question 5: Why Temperature Affects Light-Independent Stage More

How to Answer:

This requires you to link enzyme action to the Calvin cycle:

Point 1: The light-independent stage is controlled by enzymes (e.g. RuBisCO, and others)

Point 2: Higher temperature increases kinetic energy of enzyme molecules / number of successful collisions / enzyme-substrate complexes formed

Alternative Point 2: High temperatures may denature enzymes (describing denaturation: active site changes shape, substrate no longer complementary/fits)

Mark scheme guidance: Award up to 2 marks maximum.

Full answer example: "The light-independent stage is controlled by enzymes such as RuBisCO, which catalyses the fixation of CO₂ to RuBP. Higher temperatures increase the kinetic energy of enzyme and substrate molecules, leading to more successful collisions and more enzyme-substrate complexes formed per unit time. This increases the rate of reactions in the Calvin cycle. However, at very high temperatures, these enzymes may denature, causing the active site to change shape so substrates can no longer bind, dramatically reducing the rate."

Why the light-dependent stage is affected less: The light-dependent reactions are mainly driven by light energy exciting electrons in photosystems, not by enzyme-catalysed reactions. Whilst some enzymes are involved (e.g. ATP synthase), the rate-limiting steps are physical processes (light absorption, electron transport) rather than enzyme catalysis.

Mark scheme guidance notes:

• Ignore "no enzymes in light-dependent stage" (this is incorrect but was ignored)

• Allow "fewer enzymes in light-dependent stage"

• Don't confuse NADP with NAD (from respiration)

Common mistakes:

• Stating vaguely that "temperature affects enzymes" without explaining how (kinetic energy, collisions, ESCs)

• Not mentioning that enzymes control the light-independent stage

• Confusing the light-independent stage with requiring photons/light energy

• Not relating high temperatures to denaturation

Additional Exam Technique Tips

1. For Calvin cycle questions: Always think about the sequence: CO₂ + RuBP → GP → TP → (mostly back to RuBP, some to make other molecules). If one intermediate increases, trace through what must happen to the others.

2. For limiting factors: Remember that a limiting factor doesn't reduce the rate - it prevents it from increasing further. The rate plateaus when a factor becomes limiting.

3. For practical investigations: Always consider:

   • Independent variable (what you change)

   • Dependent variable (what you measure)

   • Control variables (what you keep constant)

   • How to improve precision and reduce anomalies

4. For enzyme-related questions: Link temperature to:

   • Kinetic energy → more successful collisions

   • Enzyme-substrate complex formation

   • But also potential denaturation at high temperatures

5. Terminology precision: Use the correct names:

   • GP not "glycerol phosphate"

   • RuBP not "RuBisCO" (enzyme vs substrate)

   • NADPH not "NADH" (photosynthesis vs respiration)

   • Sucrose for transport, not glucose

By practising with actual past paper questions and understanding what examiners are looking for, you'll be well-prepared for photosynthesis questions in your OCR A Level Biology exam.