Pearson Edexcel International A Level Biology: Photosynthesis (YBI11) – Revision Notes

These are condensed notes for the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, statements 5.1 to 5.8 of Topic 5, including Core Practical 10. Topics 5 to 8 are International A2 (IA2), so this is A2 content, with no tiers. For full explanations use the study guide, then test yourself with the practice questions. See also the course hub, the checklist and the free diagnostics.

5.1 and 5.2 Overall reaction and ATP

Item	Recall
Equation	6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
Role of light	Energy to split the strong bonds in water
Hydrogen	Stored in glucose by combining it with CO₂
Oxygen	From water, released to the atmosphere
Photophosphorylation	ADP + Pi → ATP, energy from light
ATP hydrolysis	ATP + H₂O → ADP + Pi, immediate energy for processes

Remember: ATP is the immediate energy currency. Glucose is the stored fuel.

5.3 Light-dependent reactions

Site: thylakoid membranes. Needs: light, water, ADP + Pi, NADP.

Method in steps (non-cyclic):

1. Photon absorbed, electron in chlorophyll excited and leaves PSII.
2. Electrons pass down the electron transport chain, losing energy.
3. Energy pumps H⁺ into the thylakoid space; H⁺ returns through ATP synthase, making ATP (chemiosmosis).
4. PSI excited; electrons reduce NADP.
5. PSII refilled from photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂.

	Non-cyclic	Cyclic
Photosystem	PSII + PSI	PSI
Photolysis	Yes	No
ATP	Yes	Yes
Reduced NADP	Yes	No
O₂	Yes	No

Why cyclic exists: extra ATP without extra reduced NADP, so the supply can be balanced with demand.

5.4 Light-independent reactions

Site: stroma. Needs: CO₂, ATP, reduced NADP, enzymes.

Compound	Carbons	Role
RuBP	5	CO₂ acceptor
RUBISCO	enzyme	Fixes CO₂ to RuBP
GP	3	First stable product; reduced to GALP
GALP	3	Used to regenerate RuBP or to make carbohydrate and other molecules

Method in steps:

1. CO₂ + RuBP → unstable 6C → 2 GP (RUBISCO).
2. GP → GALP (uses ATP and reduced NADP).
3. Most GALP → RuBP (uses ATP).
4. A fraction of GALP leaves the cycle.

Totals for one glucose: 6 turns, 6 CO₂, 18 ATP, 12 reduced NADP. Per CO₂: 3 ATP and 2 reduced NADP.

Worked reminder: 7.5 µmol CO₂ fixed needs 7.5 × 3 = 22.5 µmol ATP and 7.5 × 2 = 15 µmol reduced NADP.

Light off: GP up, RuBP down. CO₂ off: RuBP up, GP down.

5.4 (ii) Uses of the products

Product of photosynthesis	Made into
Simple sugars	Respiration (all organisms that obtain them)
Glucose units	Starch, cellulose and other polysaccharides
Carbon skeletons plus nitrate	Amino acids, then proteins
GALP-derived compounds	Lipids
Sugars and phosphate with nitrogenous bases	Nucleic acids

5.5 Chloroplast structure

Structure	Function
Envelope (two membranes)	Selective barrier
Grana and thylakoids	Large surface area for photosystems, carriers, ATP synthase
Intergranal lamellae	Connect grana
Thylakoid space	Small, so H⁺ gradient forms quickly
Stroma	Calvin cycle enzymes, starch grains
Own DNA and ribosomes	Make some chloroplast proteins

Tip: link every structure to a function with a “so that” statement.

5.6 Spectra

Term	Axes	Shows
Absorption spectrum	Absorbance against wavelength	Which wavelengths a pigment absorbs
Action spectrum	Rate of photosynthesis against wavelength	Which wavelengths drive photosynthesis

Chlorophylls absorb mainly blue and red and reflect green. Accessory pigments widen the range of wavelengths used. The two spectra have similar shapes. A difference means that not all absorbed light is used effectively.

5.7 Chromatography

Rf = origin-to-spot distance ÷ origin-to-front distance.

Method in steps:

1. Pencil line (origin) on the strip.
2. Add concentrated pigment extract as a small spot and let it dry.
3. Stand the strip in solvent below the origin and let it rise.
4. Mark the solvent front at once.
5. Measure to the middle of each spot, calculate Rf, then compare with reference values from the same solvent.

Worked reminder: front 7.5 cm, spot at 6.0 cm gives Rf = 6.0 ÷ 7.5 = 0.80.

A pigment with a higher Rf is more soluble in the solvent and is held less firmly by the paper.

5.8 Core Practical 10

Variable	Independent	Method	Controls
Light intensity	Lamp distance	Ruler	Temperature, CO₂
Wavelength	Filter colour	Filters over lamp	Intensity, temperature
Temperature	Water bath	Thermometer	Light, CO₂
CO₂	Hydrogencarbonate concentration	Dilution series	Light, temperature

Dependent variable: bubbles per minute, or gas volume per minute.

Reminders:

• Intensity ∝ 1 ÷ d². Lamp from 40 cm to 20 cm: (40 ÷ 20)² = 4 times.
• Gas in a capillary: V = πr²l.
• Use a heat shield or LED lamp and allow time for the plant to adjust.
• Repeat and calculate a mean.
• Rate becomes steady when another factor is limiting. Identify it from conditions.

Worked reminders for calculations

Rate from counts. Bubbles in 3 minutes: 33, 30, 36. Mean = 33. Rate = 33 ÷ 3 = 11 bubbles per minute.

Percentage change. A rate rises from 8.0 to 10.4 bubbles per minute. Change = 2.4. Percentage increase = 2.4 ÷ 8.0 × 100 = 30%.

Gas volume. A bubble 8.0 mm long in a capillary of radius 0.5 mm: V = π × 0.5² × 8.0 = 6.3 mm³. If it forms in 4 minutes, the rate is 6.3 ÷ 4 = 1.6 mm³ per minute.

Dilution. To make 40 cm³ of 0.02 mol dm⁻³ hydrogencarbonate from 0.10 mol dm⁻³ stock: volume of stock = 0.02 × 40 ÷ 0.10 = 8 cm³, plus 32 cm³ of water.

Cost of fixing carbon. A turn of the cycle that fixes 1 CO₂ uses 3 ATP. For 9 CO₂: 27 ATP and 18 reduced NADP.

Answer frameworks

Explain how ATP is made in the light-dependent reactions: photon absorbed, electron excited, electron passes along the chain, energy released, H⁺ pumped into the thylakoid space, gradient forms, H⁺ flows through ATP synthase, ADP + Pi joined.

Explain a change in GP or RuBP: say what has changed in the supply of ATP, reduced NADP or CO₂, say which step is affected, and then say which compound builds up and which runs out.

Explain a limiting factor graph: where the curve climbs, the factor being varied is the one in short supply, so more of it raises the rate. In the flat part, something else is limiting; name the likeliest factor from the conditions given in the question. A fall at high temperature is usually enzyme denaturation, including RUBISCO.

Link structure to function: name the structure, state the feature, then say what that makes possible. For example, thylakoid membranes have a large surface area, so many photosystems and ATP synthase molecules fit in.

Compare cyclic and non-cyclic: always give a similarity and differences, with both columns of the table complete.

Must-know distinctions

• Absorption spectrum against action spectrum.
• Photophosphorylation (making ATP using light) against ATP hydrolysis (releasing energy).
• Light-dependent (thylakoids, makes ATP and reduced NADP) against light-independent (stroma, uses them).
• GP (first product) against GALP (reduced product).

Quick self-test

1. State the source of the oxygen released in photosynthesis.
2. Name the process by which water is split in the light-dependent reactions.
3. Which photosystem is used in cyclic photophosphorylation?
4. Name the enzyme that combines CO₂ with RuBP.
5. A leaf disc fixes 12 µmol CO₂. How much ATP is needed?
6. Define Rf and work it out for a spot 3.6 cm from the origin with a solvent front at 9.0 cm.
7. A lamp is moved from 50 cm to 25 cm. How many times greater is the light intensity?
8. What happens to the amount of GP when the light is switched off, and why?
9. Why is the thylakoid space small?
10. Define action spectrum.

Answers

1. Water (photolysis).
2. Photolysis.
3. Photosystem I (PSI).
4. RUBISCO.
5. 12 × 3 = 36 µmol ATP.
6. Rf is the origin-to-spot distance divided by the origin-to-front distance; 3.6 ÷ 9.0 = 0.40.
7. (50 ÷ 25)² = 4 times.
8. GP rises. ATP and reduced NADP fall, so GP cannot be reduced to GALP, but CO₂ is still being fixed.
9. So that the H⁺ concentration inside rises quickly, making a steeper gradient for ATP synthase.
10. A graph of rate of photosynthesis against wavelength of light.

Where marks are usually lost

• Writing that CO₂ is split to give oxygen.
• Saying “light energy is used to make ATP” without mentioning excited electrons and the electron transport chain.
• Leaving out reduced NADP as a product of the non-cyclic route, or including it for the cyclic route.
• Writing “light-independent reactions need no light” with no mention that they need the products of the light-dependent stage.
• Giving only a name for a chloroplast structure and no function.
• Describing an action spectrum as the amount of light absorbed.
• Rf given with units, or as a value above 1, or measured to the edge of the spot.
• In Core Practical 10, changing more than one variable, or naming the control variables without saying how they are kept constant.
• Describing a plateau in a graph without naming the limiting factor.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology (YBI11) specification, Issue 2, published by Pearson Education Limited.