AQA A-level Biology: Photosynthesis (7402) – Revision Notes

These notes condense section 3.5.1 (Photosynthesis) of the AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015, including required practicals 7 and 8. Section 3.5 is A-level only. For full explanations and worked examples, use the study guide. Then try the practice questions. See also the AQA A-level Biology hub, the printable checklist, the free diagnostics and the exam preparation page.

Key terms

Term	Meaning
Photoionisation	Light energy makes chlorophyll lose electrons, so it becomes ionised
Electron transfer chain	Carriers in a membrane that pass electrons on and release energy at each step
Chemiosmosis	ATP made by protons flowing down an electrochemical gradient through ATP synthase
Photolysis	Light-driven splitting of water into protons, electrons and oxygen
Reduced NADP	Carrier of hydrogen and electrons from the light-dependent reaction to the Calvin cycle
RuBP	Five-carbon compound that accepts carbon dioxide
Rubisco	Enzyme that catalyses the reaction of carbon dioxide with RuBP
GP	Glycerate 3-phosphate, three carbons, the first product
TP	Triose phosphate, three carbons, made by reducing GP
Limiting factor	The factor that, when changed, changes the rate while the others stay as they are

Where things happen

Stage	Site in the chloroplast	Inputs	Outputs
Light-dependent reaction	Thylakoid membranes	Light, water, ADP, phosphate, NADP	ATP, reduced NADP, oxygen
Light-independent reaction	Stroma	Carbon dioxide, ATP, reduced NADP, RuBP	TP, sugars, other organic substances, ADP, NADP

ADP, phosphate and NADP return to the thylakoids to be recycled.

Light-dependent reaction in steps

1. Chlorophyll absorbs light and electrons are released (photoionisation).
2. Electrons pass along the electron transfer chain, losing energy at each step.
3. Energy released moves protons from the stroma into the thylakoid space.
4. A proton gradient forms across the membrane.
5. Protons flow back through ATP synthase, which makes ATP from ADP and phosphate (chemiosmosis).
6. Electrons at the end of the chain, with a proton, reduce NADP.
7. Photolysis replaces the electrons lost from chlorophyll: H₂O → 2H⁺ + 2e⁻ + ½O₂.

Memory aid for photolysis products: electrons refill chlorophyll, protons build the gradient and reduce NADP, oxygen leaves.

Light-independent reaction in steps

1. Fixation: CO₂ + RuBP, catalysed by rubisco, gives two GP.
2. Reduction: GP to TP, using ATP and reduced NADP.
3. Regeneration: most TP is turned back into RuBP, using ATP.
4. Output: some TP is converted into useful substances such as glucose, other carbohydrates, lipids and amino acids.

Carbon accounting reminder

Nine CO₂ fixed with nine RuBP: 18 GP, then 18 TP. If three TP leave, the remaining fifteen TP (45 carbons) make nine RuBP (45 carbons). Carbon in equals carbon out.

What changes when conditions change

Change	ATP and reduced NADP	GP	RuBP
Light off	Fall	Rises	Falls
CO₂ removed	Build up	Falls	Rises
Electron chain blocked	Fall	Rises	Falls

The pattern: whatever is made by a step that has stopped falls, and whatever is used by that step builds up.

Limiting factors

Factor	Effect and reason
Light intensity	Rate rises with intensity until another factor limits; more photons mean more photoionisation
Carbon dioxide concentration	Rate rises as rubisco has more substrate, then levels off
Temperature	Rate rises as enzyme activity rises, peaks, then falls as enzymes denature
Wavelength	Red and blue light drive photosynthesis most strongly; green is reflected more

Reading a graph: on the rising part, the x-axis factor limits. On a plateau, another factor limits and the x-axis factor does not.

Agricultural practices

Practice	Limiting factor addressed	Cost to weigh
Carbon dioxide enrichment	CO₂ concentration	Gas, burners or cylinders
Artificial lighting	Light intensity, day length	Electricity, equipment
Heating	Temperature	Fuel

Method to evaluate data:

1. Calculate percentage change in yield: (new − old) ÷ old × 100.
2. Multiply extra yield by selling price to get extra income.
3. Subtract the extra cost to get net gain.
4. Comment on the quality of the evidence: size of trial, repeats, one crop, one site.

Required practical 7: chromatography

• Pencil origin line, concentrated pigment spot, solvent level below the line, lid on.
• Draw the solvent front line immediately after taking the paper out.
• Rf = distance moved by pigment ÷ distance moved by solvent front. No units, less than 1.
• Pigments separate because of different solubility in the solvent and different attraction to the paper.
• Compare plants by the number, position and size of spots. Identify pigments using reference Rf values for the same solvent.

Required practical 8: dehydrogenase

• Chloroplast extract made in cold, buffered, isotonic solution.
• DCPIP is an electron acceptor in place of NADP, and turns from blue to colourless.
• Measure time to decolourise, or absorbance with a colorimeter, while varying one named factor.
• Controls: boiled extract, no chloroplasts, dark (foil). Each shows what the colour change depends on.
• Rate = change in absorbance ÷ time.

Must-know distinctions

• Photoionisation (loss of electrons from chlorophyll in light) and photolysis (splitting of water).
• Electron transfer chain (moves protons) and ATP synthase (makes ATP as protons flow back).
• GP (first product, reduced) and TP (product of reduction, used to regenerate RuBP or make sugars).
• Light-dependent (thylakoid, makes ATP and reduced NADP) and light-independent (stroma, uses them).

Small worked reminders

Proton gradient size

Stroma pH 7.8, thylakoid space pH 4.8. The difference is 3.0 units, so the H⁺ concentration is 10³ = 1000 times higher in the thylakoid space. A bigger gradient means more energy available at ATP synthase.

Rate from a colorimeter

Absorbance falls from 0.80 to 0.52 in 8.0 minutes.

Rate = (0.80 − 0.52) ÷ 8.0 = 0.035 absorbance units per minute.

Always give the unit as “per minute”, and state the factor that was varied when you compare rates.

Percentage change

Yield rises from 45 to 54 kg per tray.

Change = (54 − 45) ÷ 45 × 100 = 20%.

Divide by the original value, not the new one.

Three-sentence graph answer

For a curve that rises then plateaus: (1) on the rising part the factor on the x-axis is limiting, because increasing it increases the rate; (2) name the reaction affected, for example more photoionisation or more substrate for rubisco; (3) on the plateau another factor, such as carbon dioxide, temperature or light, is limiting, because raising the x-axis factor makes no difference.

Other points worth a line

• The specification suggests investigating named environmental variables using aquatic plants, algae or immobilised algal beads. Beads hold the algae still, so they are easy to handle and can be reused or removed from the solution.
• Hydrolysis of ATP supplies the extra energy needed in the light-independent reaction. The ATP is made in the light-dependent reaction.
• Pigments other than chlorophyll a may be present in leaves, which is why a chromatogram shows several spots.

Quick self-test

1. What is lost from chlorophyll during photoionisation?
2. Where do the electrons that replace them come from?
3. State two roles of the electron transfer chain in the light-dependent reaction.
4. Stroma pH is 8.0 and thylakoid space pH is 6.0. How many times greater is the H⁺ concentration in the thylakoid space?
5. Name the enzyme that makes ATP and state what drives it.
6. Which compound is reduced by ATP and reduced NADP?
7. How many GP form when one CO₂ combines with one RuBP?
8. A pigment moved 4.5 cm and the solvent front 9.0 cm. Calculate Rf.
9. Why does GP level rise when light is switched off?
10. A trial raises yield from 800 kg to 1000 kg. Calculate the percentage increase.
11. Give one reason an isotonic solution is used to make a chloroplast extract.
12. State what the DCPIP boiled control shows.

Answers

1. Electrons.
2. Photolysis of water.
3. It passes electrons on, and releases energy used to move protons across the membrane. The final electrons also reduce NADP.
4. 100 times (10² for two pH units).
5. ATP synthase; the flow of protons down their electrochemical gradient.
6. GP, to give TP.
7. Two.
8. 4.5 ÷ 9.0 = 0.50.
9. ATP and reduced NADP are no longer made, so GP cannot be reduced while rubisco still makes it.
10. (1000 − 800) ÷ 800 × 100 = 25%.
11. It stops water entering or leaving by osmosis, which would burst or shrink the chloroplasts.
12. That the colour change needs active enzymes, because DCPIP stays blue when they are denatured.

Where marks are usually lost

• Answers that say energy is “made” or “produced”. Energy is transferred.
• Writing that oxygen comes from carbon dioxide.
• Leaving out the proton gradient when explaining how ATP is formed.
• Describing the light-independent reaction as happening only in the dark.
• Confusing NADP (oxidised) with reduced NADP, or saying reduced NADP is used to make ATP.
• Explaining graph changes without naming the reaction that stops.
• Calling a plateau limited by the factor on the x-axis.
• Evaluating agricultural data with only a percentage, and no mention of cost.
• Forgetting a boiled or dark control, or not saying what each control shows.
• Measuring Rf to the edge of a spot instead of its centre, or giving it units.

Official syllabus

AQA AS and A-level Biology (7401 and 7402), specification for first teaching 2015, published by AQA. Section 3.5.1 and required practicals 7 and 8.