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

This study guide is aligned to the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, section 3.3.2 (photosynthesis), covering 3.3.2.1 (the light-dependent reaction), 3.3.2.2 (the light-independent reaction) and 3.3.2.3 (limiting factors). It sits in Unit 3, Populations and genes, which the specification lists as International A-level only. It also covers Required practical 7.

Test yourself afterwards with the revision notes and the practice questions. The course page, the checklist and the free diagnostics help you plan your revision.

What this unit covers

Section	What you must be able to do
3.3.2.1	Show that chlorophyll absorbs light and that light energy excites electrons in chlorophyll
3.3.2.1	Explain how energy from excited electrons generates ATP and reduced NADP
3.3.2.1	Describe the electron transfer chain, the passage of protons across chloroplast membranes and the role of ATP synthase
3.3.2.1	Describe the photolysis of water and its three products
3.3.2.2	Describe the reaction of carbon dioxide with ribulose bisphosphate (RuBP), catalysed by rubisco, to form glycerate 3-phosphate (GP)
3.3.2.2	Explain how ATP and reduced NADP reduce GP to triose phosphate
3.3.2.2	Explain how some triose phosphate regenerates RuBP in the Calvin cycle, and how the rest is converted to useful organic substances
3.3.2.3	Apply the principle of limiting factors to carbon dioxide concentration, light and temperature
Required practical 7	Investigate the effect of a limiting factor, such as light intensity, on the rate of photosynthesis

The specification says the written papers test the procedures in a required practical, evaluation of the techniques, and interpreting specimen results. Treat Required practical 7 as examinable content.

The big picture

Photosynthesis is the main route by which energy enters an ecosystem. In a chloroplast it happens in two linked stages.

• The light-dependent reaction captures light energy as ATP and reduced NADP and gives off oxygen.
• The light-independent reaction builds organic substances from carbon dioxide, powered by the ATP and reduced NADP made in the first stage.

The two stages depend on each other. The second cannot run without the products of the first. The first would run out of NADP and ADP if the second stopped. Many questions test this link.

3.3.2.1 The light-dependent reaction

Chlorophyll and excited electrons

Chlorophyll is a photosynthetic pigment. It absorbs light, mainly in the red and blue-violet parts of the spectrum, and reflects green. When a chlorophyll molecule absorbs light, the light energy excites electrons. They rise to a higher energy level and leave the chlorophyll. The electrons are now an energy source.

Making ATP: the electron transfer chain

The excited electrons are passed along a series of carrier molecules, the electron transfer chain, embedded in the chloroplast membranes. The electrons give up energy as they pass from carrier to carrier. Follow the order:

1. The energy released is used to pump protons (H⁺) across the chloroplast membrane. This builds up a high proton concentration on one side.
2. The protons then flow back down their concentration gradient through a channel in the enzyme ATP synthase, which is embedded in the same membranes.
3. The flow of protons provides the energy for ATP synthase to join ADP and inorganic phosphate, forming ATP.

This coupling of electron transfer, a proton gradient and ATP synthesis is chemiosmosis. Protons cannot cross the lipid bilayer on their own, so ATP synthase is the route back. The specification asks for the passage of protons across the membranes and the catalytic role of ATP synthase.

Making reduced NADP and photolysis

Electrons leave chlorophyll, so they must be replaced. They come from water. Light also drives photolysis, the splitting of water:

2H₂O → 4H⁺ + 4e⁻ + O₂

Photolysis of water produces three things:

Product	Fate
Electrons	Replace those lost from chlorophyll
Protons	Join the proton pool; at the end of the chain they combine with NADP and electrons to form reduced NADP
Oxygen	A waste product; it leaves the leaf or is used in respiration

The end results are ATP and reduced NADP, which carry energy and reducing power to the next stage.

Worked example 1. A suspension of chloroplasts releases 5.0 mmol of oxygen. How much water was split and how many electrons were released?

• The equation shows 2H₂O gives 1 O₂, so water split = 5.0 × 2 = 10.0 mmol.
• It also shows 4e⁻ per O₂, so electrons = 5.0 × 4 = 20.0 mmol.

3.3.2.2 The light-independent reaction

This stage takes place in the stroma of the chloroplast and does not use light directly. It is a cycle, the Calvin cycle.

1. Carbon fixation. Carbon dioxide reacts with the five-carbon sugar ribulose bisphosphate (RuBP). The reaction is catalysed by the enzyme rubisco. The unstable six-carbon product breaks down into two molecules of glycerate 3-phosphate (GP), three carbons apiece.
2. Reduction. ATP and reduced NADP from the light-dependent reaction reduce GP to triose phosphate. The ATP gives energy. The reduced NADP donates hydrogen. ADP and NADP return to the light-dependent reaction.
3. Regeneration. Some of the triose phosphate is used to regenerate RuBP, so the cycle can continue.
4. Products. The rest of the triose phosphate is converted to useful organic substances.

Useful organic substances

Two triose phosphate molecules can join to make a hexose sugar such as glucose. From there the plant makes:

• starch for storage and cellulose for cell walls (carbohydrates);
• lipids, using glycerol and fatty acids made from triose phosphate;
• amino acids and then proteins; these also need nitrogen from nitrate ions taken up by the roots.

Worked example 2: carbon accounting. Six molecules of CO₂ are fixed. Show how many molecules of each substance are involved and that the carbon balances.

• 6 CO₂ react with 6 RuBP, giving 12 GP (6 × 2).
• Reduction gives 12 triose phosphate. This uses 12 ATP and 12 reduced NADP, one of each per GP.
• Carbon in: 6 (CO₂) + 6 × 5 (RuBP) = 36. Carbon out: 12 × 3 (GP) = 36. It balances.
• To keep the cycle going, 6 RuBP must be regenerated. That is 6 × 5 = 30 carbons, which needs 10 triose phosphate (10 × 3 = 30).
• The remaining 2 triose phosphate (6 carbons) leave the cycle. Their carbon is the six carbons of one hexose, from the six CO₂ fixed.

3.3.2.3 Limiting factors

The principle of limiting factors is that the rate of a process that depends on several factors is limited by the factor that is nearest to its minimum value, the one in shortest supply. Raising that factor increases the rate. Raising any other factor makes no difference until the limiting factor changes. When the rate stops rising, a different factor has become limiting.

Light

Light drives the light-dependent reaction. At low intensity there is little ATP and reduced NADP, so GP is not reduced fast enough and the whole process is slow. The rate rises in proportion to light intensity until another factor takes over.

Carbon dioxide concentration

CO₂ is the substrate that combines with RuBP. At low concentrations there is not enough CO₂ to keep rubisco working, so the rate is low even in bright light. The atmosphere contains about 0.04% CO₂, and this is often limiting.

Temperature

The light-independent reaction is controlled by enzymes, including rubisco. Warmer conditions give particles more kinetic energy, so enzymes meet substrate molecules more frequently and the rate climbs towards the optimum. Above the optimum the enzymes begin to denature and the rate falls steeply. At low temperature, enzymes work slowly, so temperature is limiting even in bright light with plenty of CO₂.

Worked example 3: reading a light curve. The rate of oxygen release from a water plant (mm³ per minute) at different light intensities, with temperature and CO₂ constant:

Light intensity (arbitrary units)	0	20	40	60	80	100
Rate	0	4.0	7.8	11.5	12.3	12.4

• From 0 to 60 the rate rises almost steadily, about 4 mm³ per minute for every 20 units. Light is limiting.
• From 20 to 40 the rate rises by 3.8, a percentage increase of 3.8 ÷ 4.0 × 100 = 95%.
• From 80 to 100 it rises by only 0.1, or 0.1 ÷ 12.3 × 100 = 0.8%. Light is no longer limiting.
• At 80 to 100 the limiting factor must be CO₂ concentration or temperature (or the amount of rubisco). The data cannot say which. You would repeat with more CO₂, then with a higher temperature.

Required practical 7: a limiting factor

A common version investigates light intensity using a water plant such as Elodea or Cabomba.

1. Place a length of pondweed in a boiling tube of sodium hydrogencarbonate solution, which supplies CO₂. Stand the tube in a water bath to keep the temperature constant.
2. Place a lamp at a measured distance. Allow the plant to adjust for a few minutes.
3. Count the bubbles of oxygen released per minute, or collect the gas in a capillary tube and measure its length.
4. Repeat at several distances, with at least three repeats at each, and calculate a mean.

Light intensity and distance. Intensity is proportional to 1 ÷ distance². A lamp at 10 cm gives intensity 1 (relative); at 20 cm it gives (10 ÷ 20)² = 0.25; at 40 cm it gives 0.0625. Moving from 10 cm to 25 cm cuts the intensity 6.25 times. Plotting rate against 1 ÷ distance² gives a more useful graph than rate against distance.

Controls and limitations.

• Keep temperature constant (a water bath or heat shield stops the lamp warming the tube), keep CO₂ constant and use the same plant.
• Bubble size varies, so counting bubbles is a poor measure of gas volume. Collecting and measuring the gas is better.
• Oxygen is also used in respiration and dissolves in the water, so the volume collected underestimates the rate of photosynthesis.

Common errors

• Writing that light energy “makes” ATP directly. The energy of excited electrons drives the electron transfer chain and ATP synthase makes the ATP.
• Saying photolysis occurs in the light-independent reaction.
• Describing RuBP as the product of fixation. GP is the product; RuBP is the acceptor.
• Saying the Calvin cycle “uses light”. It uses ATP and reduced NADP.
• Saying a rate is limited by “all factors”. Name one factor and justify it from the data.
• Stating that “enzymes die” at high temperature. They denature.

Next steps

Work through the revision notes, then attempt the practice questions. Use the checklist to record progress and the course page to find related topics.

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.