Photosynthesis¶

Part of Module 5: Communication, homeostasis and energy.

Photosynthesis is the process by which light energy is used to drive the endothermic synthesis of organic molecules from inorganic precursors — carbon dioxide and water. It underpins virtually all food chains on Earth and is responsible for atmospheric oxygen. At A level, the focus shifts from the overall equation to the two-stage mechanism: a light-dependent series of reactions in the thylakoid membranes, and a light-independent cycle in the stroma.

What You Need to Learn¶

Further detail: AS Biology A (H020) and A Level Biology A (H420).

The structure of the chloroplast and the pigments used in photosynthesis, how the light-dependent reactions produce ATP, reduced NADP and oxygen, how the Calvin cycle uses them to fix carbon dioxide into carbohydrate, and how light, carbon dioxide and temperature limit the rate.

Overview¶

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light energy)

This summary equation conceals a complex sequence of reactions. The energy input is light; the light energy is converted first into ATP and reduced NADP, then used to fix CO₂ into organic molecules. Oxygen is a waste product of water splitting.

The process takes place in the chloroplast, an organelle present in the mesophyll cells of leaves.

Chloroplast Structure and Adaptation¶

Thylakoid membranes contain photosynthetic pigments arranged into photosystems (PS I and PS II). Photosystems are antenna complexes: accessory pigments (carotenoids, xanthophylls) absorb light of various wavelengths and funnel the energy to a central reaction centre chlorophyll (P700 in PS I, P680 in PS II) where it is used to excite electrons.

Photosynthetic Pigments and Chromatography¶

The main photosynthetic pigments are:

• Chlorophyll a: primary pigment; absorbs red and blue-violet light, reflects green (hence the colour of most leaves); present in all photosynthetic organisms
• Chlorophyll b: accessory pigment; broadens the range of absorbed wavelengths; found alongside chlorophyll a in the light-harvesting complexes
• Xanthophylls and carotenoids: absorb wavelengths not absorbed by chlorophyll, further broadening the usable spectrum

Chromatography can separate these pigments. Each pigment travels a different distance up chromatography paper. The Rf value identifies each pigment:

Rf = distance travelled by pigment from origin
     ─────────────────────────────────────────
     distance travelled by solvent from origin

Each pigment has a characteristic Rf value (carotene travels farthest; chlorophyll b travels least far).

Stage 1: The Light-Dependent Reactions¶

The light-dependent reactions occur on the thylakoid membranes. Their products are ATP, reduced NADP (NADPH) and oxygen.

Photosystem II (P680)¶

1. Light energy is absorbed by PS II antenna pigments; energy is transferred to the P680 reaction centre
2. Energy excites electrons to a higher energy level; the excited electrons leave P680
3. These electrons are accepted by the primary electron acceptor and passed to the electron transport chain in the thylakoid membrane
4. P680 is left with a deficit — it has a very positive redox potential; it obtains replacement electrons from the photolysis of water:

2H₂O → 4H⁺ + 4e⁻ + O₂

1. Oxygen is released as a waste product; protons (H⁺) accumulate in the thylakoid lumen

Electron Transport Chain and ATP Synthesis (Photophosphorylation)¶

1. Electrons pass through a series of electron carriers in the thylakoid membrane (plastoquinone, cytochrome b₆f complex, plastocyanin)
2. As electrons move to lower energy levels, energy is released and used to pump H⁺ ions from the stroma into the thylakoid lumen (chemiosmosis)
3. H⁺ ions accumulate in the thylakoid lumen, creating a proton gradient (high H⁺ inside thylakoid)
4. H⁺ ions flow back down the gradient through ATP synthase (located in the thylakoid membrane)
5. This drives the synthesis of ATP from ADP and Pᵢ — called photophosphorylation

Photosystem I (P700)¶

1. Electrons arriving at the end of the transport chain are re-energised by PS I (absorbs light at 700 nm)
2. The re-excited electrons are passed to the final electron acceptor ferredoxin
3. Ferredoxin reduces NADP using the electrons and the H⁺ from the lumen:

NADP + 2e⁻ + H⁺ → NADPH (reduced NADP)

Cyclic and Non-Cyclic Photophosphorylation¶

In cyclic photophosphorylation, electrons from PS I return to the electron transport chain via ferredoxin → cytochrome b₆f complex → plastocyanin → PS I. No NADPH is produced, but extra ATP is generated to meet the demand of the Calvin cycle.

Summary of Light-Dependent Reaction Products¶

Stage 2: The Light-Independent Reactions (Calvin Cycle)¶

The Calvin cycle occurs in the stroma of the chloroplast. It uses ATP and NADPH from the light-dependent stage to fix CO₂ into organic molecules. It does not directly require light, but it is dependent on the continuous supply of ATP and NADPH.

The Calvin Cycle Steps¶

Step 1 — Carbon fixation:

• CO₂ combines with the 5-carbon acceptor molecule ribulose bisphosphate (RuBP) in a reaction catalysed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase)
• This produces an unstable 6-carbon intermediate that immediately splits into two molecules of glycerate-3-phosphate (GP) — a 3-carbon compound

CO₂ + RuBP (C5) → 2 × GP (C3)

Step 2 — Reduction:

• GP is reduced using ATP and NADPH from the light-dependent reactions
• GP is converted to triose phosphate (TP) — a 3-carbon sugar phosphate (specifically glyceraldehyde-3-phosphate, G3P)

GP + ATP + NADPH → TP + ADP + NADP + Pᵢ

Step 3 — Regeneration of RuBP:

• Most TP molecules (5 out of every 6) are used to regenerate RuBP using ATP
• This step maintains the cycle by ensuring the CO₂ acceptor is continuously available

Step 4 — Production of organic molecules:

• The remaining TP (1 in 6 molecules) is used to synthesise glucose and other organic compounds
• Glucose is then converted to starch, sucrose, lipids, amino acids, nucleotides or cellulose

Fate of Triose Phosphate¶

Worked Example: Effect of Changing Light on Calvin Cycle Intermediates¶

If light is suddenly removed:

• Light-dependent reactions stop; no more ATP or NADPH produced
• GP can no longer be reduced to TP → GP accumulates
• TP is no longer available to regenerate RuBP → TP and RuBP levels fall

If CO₂ concentration is suddenly reduced:

• Less CO₂ fixation occurs; less GP is produced → GP levels fall
• TP continues to be used to regenerate RuBP for a while → RuBP accumulates initially

Factors Affecting the Rate of Photosynthesis¶

The rate of photosynthesis is determined by whichever factor is in shortest supply — this is the limiting factor.

Light Intensity¶

• Higher light intensity → more photons available → more PS II reaction centre activation → more photophosphorylation → more ATP and NADPH → faster Calvin cycle
• If light is limiting: GP rises (CO₂ still being fixed but less ATP/NADPH to reduce it); TP and RuBP fall

CO₂ Concentration¶

• Higher CO₂ → more carbon fixation by RuBisCO → more GP → more TP → more glucose
• If CO₂ is limiting: RuBP accumulates (not being used for fixation); GP falls

Temperature¶

• Temperature affects enzyme activity: RuBisCO and Calvin cycle enzymes have an optimal temperature
• At low temperatures, enzyme kinetic energy is low, reactions are slow, rate of photosynthesis limited
• At very high temperatures, enzymes denature
• The light-dependent reactions are less temperature-sensitive (photochemical processes, not enzyme-controlled), so temperature mainly limits the Calvin cycle

Saturation Point¶

The saturation point is the value of a limiting factor at which it is no longer limiting the rate of photosynthesis — another factor has become limiting instead. At and beyond the saturation point, increasing the original factor produces no further increase in the rate of photosynthesis.

Combined Effect: Compensation Point¶

The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration — there is no net gas exchange. Below the compensation point, the plant consumes more O₂ (respiration) than it produces. Shade-adapted plants have lower compensation points.

Common Confusions¶

• Light-dependent and light-independent: the light-independent reactions still depend on the products of the light-dependent stage, so they stop soon after dark.
• Source of oxygen: oxygen released in photosynthesis comes from the photolysis of water and not from carbon dioxide.
• Net and gross photosynthesis: the net rate is what is measured after subtracting the oxygen used in respiration.
• Limiting factor: it is the factor in shortest supply at that moment. When a graph plateaus, a different factor has become limiting.
• GP, TP and RuBP: under low light, GP accumulates and TP and RuBP fall, because there is too little ATP and reduced NADP to convert GP.

Check Yourself¶

1. Describe where in the chloroplast the light-dependent and light-independent reactions take place.
2. Explain the role of photolysis of water in the light-dependent reactions.
3. Describe how the Calvin cycle fixes carbon dioxide, naming the enzyme and the first stable product.
4. Predict and explain what happens to the concentrations of GP and RuBP if the carbon dioxide supply to a plant is suddenly reduced.
5. A graph of rate against light intensity levels off. Suggest two reasons.
6. Explain what is meant by the compensation point.

Key Terms¶

• Chloroplast: organelle in plant and algal cells where photosynthesis takes place.
• Photosystem II: pigment-protein complex that absorbs light and drives photolysis and electron excitation.
• Photolysis: splitting of water by light.
• Photophosphorylation: synthesis of ATP using light energy during the light-dependent stage.
• Chemiosmosis: ATP production driven by proton flow down an electrochemical gradient through ATP synthase.
• Reduced NADP: electron-carrying molecule formed in the light-dependent stage and used in the Calvin cycle.
• Photosystem I: pigment-protein complex that re-excites electrons so NADP can be reduced.
• Cyclic photophosphorylation: pathway in which electrons cycle back to PSI and generate ATP but not reduced NADP.
• RuBisCO: enzyme that catalyses fixation of CO2 to RuBP in the Calvin cycle.
• GP: glycerate 3-phosphate, the first stable product of carbon fixation in the Calvin cycle.
• TP: triose phosphate, the reduced three-carbon product used to make carbohydrates and regenerate RuBP.
• RuBP: ribulose bisphosphate, the five-carbon carbon acceptor in the Calvin cycle.
• Rf value: the ratio of pigment travel distance to solvent travel distance.
• Saturation point: the value of a limiting factor beyond which it no longer limits the rate of photosynthesis.
• Compensation point: light intensity at which photosynthesis and respiration occur at equal rates.

Connected Pages¶

• 5.2.2 Respiration (complementary energy processes; ATP)
• 2.1.3 Nucleotides and nucleic acids (ATP structure)
• 2.1.2 Biological molecules (products of Calvin cycle — glucose, lipids)
• 6.3.1 Ecosystems (gross and net primary productivity)
• Photosynthesis vs respiration
• Module 5: Communication, homeostasis and energy