Mastering OCR A Level Biology Section 5.2.1 (a-d) : Photosynthesis - Common Questions & Mark Scheme Insights

Mastering OCR A Level Biology Section 5.2.1: Photosynthesis - Common Questions & Mark Scheme Insights

After analysing extensive OCR past papers for specification section 5.2.1 a-d (Photosynthesis), I've identified the question patterns that consistently challenge students. Understanding how mark schemes assess pigment knowledge, chloroplast structure, chromatography calculations, and the relationship between photosynthesis and respiration is crucial for exam success. Let me guide you through five of the most frequently tested question types with real OCR examples.

Question Type 1: Naming Photosynthetic Pigments and Their Advantages

Why this question type is common: This tests fundamental knowledge of photosynthesis machinery and the ability to explain adaptive advantages - core concepts that underpin understanding of light harvesting.

How to answer each part:

Part i - Primary pigment (1 mark): "Chlorophyll, a/A"

Mark scheme insight: The mark scheme says "Mark the first answer" and warns that if you give an additional incorrect answer, you get 0 marks. It accepts "chlorophyll 680 and chlorophyll 700" but states "(Note that both are required for this option)." The mark scheme specifically says:

  • "IGNORE P680/P700"

  • "DO NOT CREDIT chlorophyll α" (alpha symbol not accepted)

Part ii - Accessory pigment (1 mark): Any one of: "Chlorophyll b / xanthophyll(s) / carotenoid(s) / (β/beta-)carotene"

Mark scheme insight: "DO NOT CREDIT karatine (as could be confused with keratin)" - spelling matters when it creates ambiguity. The examiner's comments note "those candidates who had given chlorophyll b as their answer to (i) frequently supplied an incorrect response to this section" - you can't use the same pigment twice.

Part iii - Advantage (1 mark): "Able to, absorb/use, a range of/different/more/other, (light) wavelengths/λ"

Mark scheme insight: The mark scheme provides a helpful example: "e.g. absorb wavelength(s) not absorbed by primary pigment" and specifies:

  • "IGNORE frequency"

  • "IGNORE absorb all wavelengths"

  • "IGNORE ref to chlorophyll b"

  • "DO NOT CREDIT ref to reflection where a pigment absorbs and reflects the same wavelength"

Examiner's comments: "The most common reason for not achieving the mark was to refer only to light frequencies or to simply state that more light could be absorbed."

Common mistakes:

  • Using "alpha" symbol instead of "a" or "A"

  • Mentioning P680/P700 (ignored, not credited)

  • Using the same pigment for both primary and accessory

  • Saying "absorbs more light" without mentioning different wavelengths

Question Type 2: Rf Value Calculation in Chromatography

Why this question type is common: This tests practical skills (PAG6) and mathematical competency (M1.1, M2.2) in a biological context. It's an excellent discriminator across ability levels.

How to calculate Rf value:

Formula: Rf = distance moved by pigment ÷ distance moved by solvent

Step-by-step approach:

  1. Measure distance from origin to centre of spot

  2. Measure distance from origin to solvent front

  3. Divide: spot distance ÷ solvent front distance

  4. Express to 2 significant figures

Example from mark scheme: Rf = 0.53 / 0.52 ✓✓ Pigment = chlorophyll a ✓

Mark scheme insight: "If incorrect: ALLOW for 1 mark for correct use of Rf e.g. Rf = [shows calculation] OR inappropriate use of sig. figs e.g. 0.533 / 0.5"

The mark scheme awards marks generously for showing method even if final answer is wrong. It also "ALLOW ECF if incorrect calculation" for identifying the pigment - if you calculate Rf wrong but correctly identify which pigment matches your calculated value, you still get the pigment mark.

For predicting pigment colours: "ALLOW ECF from calculated Rf value in part (ii) (for ECF looking for a pigment next highest in value than calculated as spot 4 has travelled further from origin than spot 3)"

Common mistakes:

  • Using inappropriate significant figures (e.g., 0.533 when 0.53 required)

  • Measuring from wrong starting point

  • Dividing solvent distance by pigment distance (wrong way round)

  • Not checking answer makes biological sense (Rf values are always between 0 and 1)

Question Type 3: Explaining Accessory Pigment Function in Different Wavelengths

Why this question type is common: This tests understanding of how photosystems work as a team, requiring application of pigment knowledge to interpret absorption spectra data.

How to structure your answer (3 marks maximum from 5 possible points):

  1. "(Alga has) accessory pigments"

  2. "(Other pigments) absorb, different/other, wavelengths (of light)"

  3. "Little/not all, light (wavelengths) is absorbed by, chlorophyll a/primary pigment"

  4. "(Light) energy is transferred to reaction centre"

  5. "For use in, light-dependent reaction/LDR"

Mark scheme insight: The mark scheme provides crucial allowances and restrictions:

  • Mark point 1: "IGNORE named pigments"

  • Mark point 2: "ALLOW longer/shorter/AW for different" and "ALLOW λ for wavelength" but "IGNORE more/wider range, wavelengths"

  • Mark point 4: "ALLOW chlorophyll a/primary pigment for reaction centre/photosystem" and "ALLOW AW e.g. accessory pigments harvest (light) energy for reaction centre"

Examiner's comments reveal common errors: "Some candidates did not recognise that this question was about photosynthetic pigments and described the features of hydrophytes. Marking points 1 and 2 were the most frequently given with only the higher attaining candidates going onto achieving a third mark. Many candidates knew that other pigments would be present, but some did not name them as 'accessory pigments' for marking point 1. Terms such as special chlorophyll, secondary pigments were used in place of accessory pigments."

Critical mistakes noted: "A common error was to write about different colours or percentages of light instead of wavelength. Credit was not given for stating that a 'wider' range of wavelengths would be absorbed or for failing to mention pigments anywhere in the account. Some candidates referred to the alga absorbing light rather than its pigments."

Question Type 4: Chloroplast Structure Identification and Adaptation

Why this question type is common: This links structure to function - a fundamental biological principle. It tests detailed knowledge of chloroplast ultrastructure and understanding of why features exist.

Part i - Naming structures (3 marks):

A. "Inner membrane (of, double membrane/envelope, surrounding organelle)" B. "Stroma" C. "Granum/grana/granal stack/thylakoid stack"

Mark scheme insight - Critical restrictions:

  • Part A: "DO NOT CREDIT inter membrane" or "inner envelope membrane" or "ref to cell/surface/plasma/membrane"

  • Part B: "Correct spelling only"

  • Part C: "IGNORE thylakoid unqualified/lamellae"

The mark scheme states "Mark the first answer on each prompt line. If the answer is correct and an additional answer is given that is incorrect or contradicts the correct answer then = 0 marks."

Part ii - Adaptations (2 marks maximum from 3 possible points):

  1. "Contain, (named) pigment (molecules)/photosystems"

  2. "Contain, (named) electron carriers/ETC/ATP synth(et)ase"

  3. "Idea that has a large surface area (in a small volume) for, light absorption/light dependent reaction(s)/light dependent stage/electron transport"

Mark scheme insight: The mark scheme gives helpful combinations:

  • "Note: 'the membranes containing the pigments have a large surface area for absorbing light' = 2 marks (mps 1 & 3)"

  • "Note: 'there is a large surface area for electron transport chain' = 2 marks (mps 2 & 3)"

It also specifies:

  • Mark point 1: "IGNORE 'accessory'"

  • Mark point 2: "IGNORE enzymes unqualified"

  • Mark point 3: "IGNORE ref to different wavelengths"

Examiner's comments: "The majority of candidates were able to describe at least one way in which the structure of the granum (part C) was adapted to its function. References to the presence of pigments, chlorophyll or photosystems on the granal membranes were very frequent and many candidates also went on to add that ATP synthase or the electron carriers would also be contained within the membranes."

Question Type 5: Relationship Between Photosynthesis and Respiration

Why this question type is common: This tests understanding of the interdependence of metabolic processes and ability to apply knowledge to explain real-world observations.

Part i - Identifying molecules (2 marks):

X = water / H₂O Y = carbon dioxide / CO₂ Z = oxygen / O₂

Mark scheme insight: "All three correct for TWO marks / One or two correct for ONE mark"

The examiner's comments note a "Misconception: It is a common misconception that candidates consider that ATP produced in respiration is used directly in photosynthesis." This is wrong - ATP cannot move between these processes.

Part ii - Explaining survival in sealed containers (3 marks maximum from 6 possible points):

  1. "Idea that light (energy) is the only requirement from outside the terrarium/AW"

  2. "Respiration provides carbon dioxide and water for photosynthesis OR photosynthesis provides glucose and oxygen for respiration"

  3. "Water used for photolysis OR oxygen used as final electron acceptor (in respiration)"

  4. "Carbon dioxide used for, light independent stage/Calvin cycle"

  5. "ATP (still) produced/energy provided, for (named) cell activities"

  6. "Decomposing plant material provides (named) mineral ions"

Mark scheme insight: The mark scheme allows chemical formulae throughout: "ALLOW O₂ for oxygen, H₂O for water, CO₂ for carbon dioxide and C₆H₁₂O₆ for glucose throughout"

Specific allowances:

  • Mark point 1: "ALLOW e.g. as light (energy) can pass through glass for photosynthesis" or "plants in glass containers will have access to light"

  • Mark point 2: "IGNORE equations unqualified"

  • Mark point 5: "ALLOW e.g. active transport/protein synthesis/active uptake of mineral ions" but "IGNORE produces energy"

  • Mark point 6: "IGNORE nutrients"

Examiner's comments: "Good responses showed good application of knowledge and understanding of photosynthesis and respiration and their interaction in plants. Higher attaining candidates set out their answers in a logical sequence and gave detailed accounts of the production and use of reactants for both processes."

Common mistakes:

  • Suggesting ATP moves directly between photosynthesis and respiration

  • Writing unqualified equations without explanation

  • Saying energy is "produced" rather than "provided" or "released"

  • Using vague terms like "nutrients" instead of specific mineral ions

General Tips for Section 5.2.1 Success

1. Know your pigments precisely

Primary pigment:

  • Chlorophyll a (or chlorophyll A)

  • NOT "chlorophyll α" (alpha symbol rejected)

  • NOT just "chlorophyll" (too vague)

Accessory pigments:

  • Chlorophyll b

  • Carotenoids / β-carotene

  • Xanthophylls

  • NOT "karatine" (confused with keratin)

Why they matter:

  • Primary pigment: receives energy, loses electrons to ETC

  • Accessory pigments: absorb different wavelengths, pass energy to reaction centre

2. Master Rf calculations

Formula: Rf = distance moved by substance ÷ distance moved by solvent

Key points:

  • Always between 0 and 1

  • More polar substances have lower Rf values

  • Less polar substances travel further (higher Rf)

  • Measure to appropriate significant figures

Common polarity order (least to most polar):

  1. Carotene (most nonpolar, highest Rf ~0.90)

  2. Pheophytin (~0.65)

  3. Chlorophyll a (~0.53)

  4. Chlorophyll b (~0.49)

  5. Xanthophylls (most polar, lowest Rf ~0.32-0.44)

3. Understand chloroplast structure terminology

Be precise with naming:

  • "Inner membrane" NOT "inter membrane" or "inner envelope membrane"

  • "Stroma" - correct spelling essential

  • "Granum/grana" NOT just "thylakoid" (too vague)

Locations of processes:

  • Light-dependent reactions: thylakoid membranes (in grana)

  • Light-independent reactions (Calvin cycle): stroma

  • DNA and ribosomes: stroma

4. Link structure to function effectively

For thylakoid membranes/grana:

  • Large surface area → more light absorption

  • Contains pigments → captures light energy

  • Contains photosystems → organises pigments

  • Contains electron carriers → electron transport

  • Contains ATP synthase → ATP production

Mark schemes reward combinations:

  • "Membranes containing pigments have large surface area for light absorption" = 2 marks

  • "Large surface area for electron transport chain" = 2 marks

5. Understand wavelength vs frequency vs colour

Mark schemes are strict:

  • Must say "wavelength" or "λ"

  • "Frequency" is IGNORED (not credited)

  • "Colour" alone is insufficient

Different wavelengths = different colours:

  • ~400-500 nm: blue/violet (short wavelength)

  • ~500-600 nm: green/yellow

  • ~600-700 nm: red/orange (long wavelength)

6. Know the photosynthesis-respiration cycle

What's recycled:

  • CO₂: produced by respiration → used in Calvin cycle

  • O₂: produced by photolysis → used as final electron acceptor in respiration

  • Water: produced by respiration → used in photolysis

  • Glucose: produced by Calvin cycle → used in respiration

What's NOT recycled:

  • ATP: made separately in each process, cannot move between them

  • NADP/NAD: different coenzymes, cannot substitute for each other

7. Apply knowledge to unfamiliar contexts

Common application scenarios:

  • Deep water algae: need pigments that absorb wavelengths that penetrate water

  • Carnivorous plants: obtain minerals from insects, not soil

  • Sealed terrariums: demonstrate the cycle of photosynthesis and respiration

  • Chromatography of different species: different pigment compositions

8. Read questions carefully for constraints

Mark schemes frequently note what to IGNORE:

  • "IGNORE chlorophyll b" (when discussing primary pigment advantage)

  • "IGNORE equations unqualified" (need explanation with equations)

  • "IGNORE cilia/flagella" (in cytoskeleton questions)

Watch for "DO NOT CREDIT" warnings:

  • These are wrong answers that seem plausible

  • Examples: "karatine" for carotene, "alpha" for a, "inter membrane"

Key Concepts to Master

Light harvesting:

  • Primary pigment (chlorophyll a) at reaction centre

  • Accessory pigments capture different wavelengths

  • Energy funnelled to reaction centre

  • Primary pigment becomes oxidised (loses electrons)

Chloroplast components:

  • Outer membrane: permeable to small molecules

  • Inner membrane: selectively permeable, transport proteins

  • Stroma: contains enzymes for Calvin cycle, DNA, ribosomes

  • Thylakoid membranes: site of light-dependent reactions

  • Grana (stacks of thylakoids): increase surface area

  • Inter-granal lamellae: connect grana

Light-dependent reactions (thylakoid membranes):

  • Photolysis of water → oxygen + protons + electrons

  • Electrons pass through ETC

  • Protons pumped into thylakoid space

  • Chemiosmosis: protons flow through ATP synthase

  • Products: ATP, reduced NADP, oxygen

Light-independent reactions (stroma):

  • Carbon fixation: CO₂ + RuBP → GP (catalysed by Rubisco)

  • Reduction: GP → TP (using ATP and reduced NADP)

  • Regeneration: TP → RuBP (using ATP)

  • Products: glucose, amino acids, lipids

Limiting factors:

  • At low light: light intensity limits rate

  • At high light: temperature or CO₂ concentration becomes limiting

  • Temperature affects enzyme-controlled Calvin cycle

  • CO₂ concentration affects carbon fixation

Remember that Section 5.2.1 connects photosynthesis to respiration, energy, and ecosystems. Master the details of chloroplast structure, pigment function, and the two stages of photosynthesis, and you'll find questions on limiting factors, adaptations, and practical investigations much more manageable.

The key to success with OCR mark schemes is precision in terminology, understanding what's accepted versus ignored versus rejected, and being able to apply knowledge to interpret data and explain observations. Mark schemes reward specific biological terms used correctly in appropriate contexts.

Good luck with your studies!