OxfordAQA International AS and A-level Biology: Photosynthesis and limiting factors (9610) – Revision Notes

These revision notes condense OxfordAQA International AS and A-level Biology (9610), Version 5.1, section 3.3.2: 3.3.2.1 the light-dependent reaction, 3.3.2.2 the light-independent reaction, 3.3.2.3 limiting factors, and Required practical 7. For full explanations read the study guide. Then try the practice questions.

Use the course page, the checklist and the free diagnostics to find your weak spots.

Key terms

Term	Meaning
Chlorophyll	Pigment that absorbs light energy in the chloroplast
Excited electron	An electron raised to a higher energy level by light energy
Electron transfer chain	Carriers in the chloroplast membrane that pass electrons on, releasing energy at each step
ATP synthase	Membrane enzyme; protons flow through it and it makes ATP from ADP and inorganic phosphate
Photolysis	Light-driven breakdown of water; it yields protons, electrons and oxygen
Reduced NADP	Hydrogen-carrying coenzyme made in the first stage; it supplies reducing power
RuBP	Five-carbon acceptor of CO₂
Rubisco	Enzyme that catalyses the reaction of CO₂ with RuBP
GP	Glycerate 3-phosphate, a three-carbon compound
Triose phosphate	Three-carbon sugar phosphate, the product of reducing GP
Limiting factor	The factor in shortest supply that sets the rate

3.3.2.1 Light-dependent reaction: method in steps

1. Chlorophyll absorbs light.
2. Light energy excites electrons in chlorophyll, and they leave it.
3. The electrons pass along the electron transfer chain in the chloroplast membranes, losing energy at each step.
4. The released energy pumps protons across the membrane, building a proton gradient.
5. The protons return through ATP synthase, and the enzyme makes ATP (chemiosmosis).
6. Photolysis of water replaces the electrons: 2H₂O → 4H⁺ + 4e⁻ + O₂.
7. Protons, electrons and NADP combine to form reduced NADP.

Photolysis summary

Product	Where it goes
Electrons	Back into chlorophyll
Protons	Into the proton pool; combine with NADP to form reduced NADP
Oxygen	Released as a waste gas, or used in respiration

Worked reminder. 8 mmol of oxygen released means 8 × 2 = 16 mmol of water split and 8 × 4 = 32 mmol of electrons.

3.3.2.2 Light-independent reaction: method in steps

1. CO₂ + RuBP (5C) → 2 GP (3C), catalysed by rubisco.
2. GP is reduced to triose phosphate, using ATP and reduced NADP.
3. Most triose phosphate is used to regenerate RuBP (the Calvin cycle).
4. The remainder is converted to useful organic substances.

Where each product goes

Substance	Made from
Hexose, then starch and cellulose	Two triose phosphates join; hexoses are then polymerised
Lipids	Triose phosphate gives glycerol and fatty acids
Amino acids	Carbon from the cycle plus nitrogen from nitrate

Worked reminder: three CO₂ fixed. 3 CO₂ + 3 RuBP gives 6 GP, then 6 triose phosphate, using 6 ATP and 6 reduced NADP. Carbon in is 3 + 15 = 18, matching 6 × 3 = 18. Regenerating 3 RuBP needs 15 carbons, which is 5 triose phosphate. One triose phosphate (3 carbons) leaves the cycle.

Quick numerical rules

For n CO₂ fixed	Amount
GP formed	2n
Triose phosphate formed	2n
ATP and reduced NADP used in reduction	2n of each
Triose phosphate for RuBP regeneration	5n ÷ 3
Triose phosphate that leaves the cycle	n ÷ 3

The last two rows give whole numbers only when n is a multiple of 3. They keep the carbon balanced: 5n ÷ 3 triose phosphate has 5n carbons, enough for n RuBP.

Must-know distinctions

	Light-dependent	Light-independent
Needs light directly	Yes	No
Main inputs	Light, water, ADP, phosphate, NADP	CO₂, RuBP, ATP, reduced NADP
Main outputs	ATP, reduced NADP, oxygen	Triose phosphate, then organic substances
Key enzyme	ATP synthase	Rubisco

• GP versus RuBP. GP is the product of fixation. RuBP is the acceptor that is regenerated.
• Oxygen versus carbon. The oxygen released comes from water, not from CO₂.
• Reduction. GP is reduced to triose phosphate. Reduced NADP is the reducing agent.

3.3.2.3 Limiting factors

The factor nearest its minimum, or in shortest supply, limits the rate. Increasing a different factor has no effect until the limiting factor changes.

Factor	Why it limits	Typical graph shape
Light	Too little ATP and reduced NADP for the light-independent reaction	Rate rises with intensity, then levels off
CO₂	Too little substrate for rubisco	Rate rises with concentration, then levels off
Temperature	Enzymes in the Calvin cycle are slow when cold and denature when hot	Rises to an optimum, then falls steeply

Reading a graph in four steps

1. Where the line climbs, the factor plotted on the horizontal axis is the limit.
2. On the plateau, a different factor is limiting. Name the likely candidates.
3. If a curve with a higher CO₂ or temperature continues to rise where the other has levelled off, that factor was limiting on the first curve.
4. Quote figures and calculate a percentage change where you can.

Interpreting two curves

If one curve plateaus at a low CO₂ concentration and a second curve at a higher concentration keeps rising, then at the plateau of the first curve CO₂ was limiting. If both curves plateau at the same value, neither CO₂ nor light is limiting there. The rate is then limited by something else, most likely temperature, or by the amount of enzyme or chlorophyll.

Worked reminder. A rate rises from 9 to 14 units when CO₂ is increased at fixed light and temperature. The percentage increase is (14 − 9) ÷ 9 × 100 = 55.6%, so CO₂ was limiting.

Answer frameworks

“Explain why the rate of photosynthesis falls when the light is switched off.”

1. No light, so no excited electrons from chlorophyll.
2. Less ATP and reduced NADP are made.
3. GP is not reduced to triose phosphate, so GP builds up.
4. Less triose phosphate means less RuBP is regenerated, so the amount of RuBP falls and less CO₂ is fixed.

“Explain why a higher CO₂ concentration increases the rate.”

1. More CO₂ is available to react with RuBP.
2. Rubisco forms more GP, so more triose phosphate is made.
3. The effect stops when another factor, such as light, becomes limiting.

“Explain the effect of high temperature.”

1. Above the optimum, hydrogen bonds in the enzyme break.
2. The tertiary structure and active site change shape, so the enzyme denatures.
3. Fewer enzyme-substrate complexes form, so the rate falls.

Required practical 7: summary

• Aim: investigate the effect of a limiting factor, such as light intensity, on the rate of photosynthesis.
• Independent variable: distance of the lamp (light intensity is proportional to 1 ÷ distance²).
• Dependent variable: bubbles per minute, or volume of gas collected per unit time.
• Control: temperature (water bath or heat shield), CO₂ supply (hydrogencarbonate solution), the same plant, time allowed to adjust.
• Method: allow a few minutes at each distance before counting, and use at least three repeats.
• Evaluation: bubble size varies; some oxygen dissolves or is used in respiration; counting is subjective.

Quick self-test

1. What is the role of chlorophyll in the light-dependent reaction?
2. State the three products of photolysis.
3. What is the role of ATP synthase?
4. Which enzyme catalyses the reaction between CO₂ and RuBP, and what is formed?
5. What do ATP and reduced NADP do in the light-independent reaction?
6. Six CO₂ molecules are fixed. How many GP are formed?
7. In the same cycle, how many triose phosphate leave the cycle as product?
8. A leaf releases 12 mmol of oxygen. How many mmol of water were split?
9. On a light intensity graph the rate has levelled off. Name two factors that could now be limiting.
10. Why does the rate fall steeply above the optimum temperature?
11. Lamp distance changes from 10 cm to 20 cm. How does light intensity change?

Answers

1. It absorbs light energy, which excites its electrons.
2. Protons, electrons and oxygen.
3. Protons flow through it, and it catalyses the production of ATP from ADP and inorganic phosphate.
4. Rubisco; two molecules of GP.
5. They reduce GP to triose phosphate (ATP supplies energy, reduced NADP supplies hydrogen).
6. 12 (6 × 2).
7. 2, since 12 triose phosphate are made and 10 regenerate 6 RuBP.
8. 24 mmol (12 × 2).
9. Carbon dioxide concentration and temperature.
10. Enzymes, including rubisco, denature, so the active site no longer fits the substrate.
11. It falls to (10 ÷ 20)² = 0.25, or one quarter.

Where marks are usually lost

• Saying chlorophyll “makes” energy rather than absorbing light.
• Missing that the electrons lost from chlorophyll are replaced from water.
• Giving the products of photolysis as “hydrogen and oxygen” without separating protons and electrons.
• Reversing RuBP and GP, or the numbers of carbons in each.
• Forgetting that the Calvin cycle needs ATP and reduced NADP, so it stops soon after the light is switched off.
• Writing “temperature” as limiting on a plateau without evidence that points to it.
• Describing a graph without figures.
• Describing enzyme “death” instead of denaturation.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, published by OxfordAQA. Section 3.3.2 and Required practical 7.