Photosynthesis

AQA spec ref: 3.5.2 - Photosynthesis

Photosynthesis is the process by which light energy is converted into chemical energy stored in organic molecules, primarily glucose. It takes place in the chloroplasts of plant cells and in some other photosynthetic organisms (algae, cyanobacteria). The overall equation is:

But this summary equation hides enormous complexity. Photosynthesis occurs in two linked stages: the light-dependent reactions (in the thylakoid membranes) and the light-independent reactions / Calvin cycle (in the stroma). Neither can function without the other - the light reactions supply the ATP and reduced NADP needed by the Calvin cycle, and the Calvin cycle regenerates NADP and ADP + Pᵢ needed by the light reactions.

Chloroplast Structure

The chloroplast is exquisitely adapted for photosynthesis. See Cell Structure for the full organelle detail; the key structural features relevant to photosynthesis are:

• Thylakoid membranes - highly folded internal membranes containing the photosynthetic pigments, electron carriers, and ATP synthase used in the light-dependent reactions. The folds are called grana (singular: granum), and grana are connected by lamellae.
• Stroma - the fluid-filled space surrounding the thylakoids. Contains the enzymes for the Calvin cycle (especially RuBisCO), as well as DNA, ribosomes, and starch grains.
• Large surface area of thylakoid membranes - maximises absorption of light and the number of sites for light-dependent reactions.
• Double membrane (envelope) - controls what enters and leaves the chloroplast (CO₂ and water in; glucose and O₂ out).

Photosynthetic Pigments

Chloroplasts contain several photosynthetic pigments that absorb different wavelengths of light. This broadens the range of light that can be used for photosynthesis.

• Chlorophyll a - the primary pigment. Absorbs red (around 680 nm) and blue-violet (around 430 nm) light. Reflects green light (which is why plants appear green).
• Chlorophyll b - an accessory pigment. Absorbs wavelengths slightly different from chlorophyll a, broadening the absorption spectrum.
• Carotenoids (including carotene and xanthophyll) - accessory pigments that absorb blue and green light. They pass absorbed energy to chlorophyll a. Also protect chlorophyll from photooxidative damage.

All these pigments are organised into photosystems in the thylakoid membrane:

• Photosystem II (PSII) - absorbs maximally at 680 nm. Contains P680 (the reaction centre chlorophyll a). Used first in the light-dependent reactions.
• Photosystem I (PSI) - absorbs maximally at 700 nm. Contains P700 (the reaction centre chlorophyll a). Used second.

The Light-Dependent Reactions

These occur in the thylakoid membranes. The overall purpose is to produce ATP (by photophosphorylation), reduced NADP (NADPH), and to release O₂ (as a by-product of water splitting).

Photolysis of Water

Light energy is absorbed by PSII and excites electrons to a higher energy level. To replace these lost electrons, water is split (photolysis):

The electrons replace those lost from PSII. The protons (H⁺) remain in the thylakoid lumen. The oxygen is released as a by-product - this is the source of all the O₂ in Earth's atmosphere.

The Electron Transport Chain and Chemiosmosis

The excited electrons from PSII pass along a series of electron carriers embedded in the thylakoid membrane, losing energy at each step. This energy is used to pump H⁺ ions from the stroma into the thylakoid lumen, creating a proton gradient.

The H⁺ ions then flow back from the lumen into the stroma through ATP synthase (CF₁CF₀-ATPase), and the energy released drives the synthesis of ATP from ADP + Pᵢ. This is exactly the same chemiosmotic mechanism as in Respiration, just in a different membrane.

This ATP synthesis driven by light is called photophosphorylation. It is non-cyclic because the electrons flow in one direction (from water through the ETC to NADP).

Reduction of NADP

After passing through the ETC, the electrons reach PSI. Here, additional light energy is absorbed, re-energising the electrons. These high-energy electrons are then donated to NADP (nicotinamide adenine dinucleotide phosphate - note this is NADP, not NAD as in respiration) along with a H⁺ ion:

Reduced NADP carries electrons (and energy) to the Calvin cycle, where it is used to reduce carbon dioxide to glucose.

Cyclic Photophosphorylation

Under some conditions, only PSI is involved. Electrons from PSI are re-cycled back to the electron carriers instead of going to NADP. This produces ATP but no reduced NADP and no O₂. It is less important than non-cyclic photophosphorylation but operates when the cell needs more ATP relative to reduced NADP (e.g. for active transport).

Summary of Light-Dependent Reaction Products

Per 2 H₂O split:

• 1 O₂ released
• 2 reduced NADP produced
• ATP produced (via chemiosmosis)

The Light-Independent Reactions (Calvin Cycle)

These occur in the stroma and do not directly require light - they require the products of the light reactions (ATP and reduced NADP). The Calvin cycle fixes CO₂ into organic molecules, ultimately producing glucose (and other carbon compounds). The key enzyme is RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase).

The Three Stages of the Calvin Cycle

Stage 1 - Carbon fixation:

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

Stage 2 - Reduction of GP:

GP is reduced to triose phosphate (TP), also a 3-carbon compound, using reduced NADP and ATP from the light reactions:

Triose phosphate is the first carbohydrate produced in photosynthesis and is the starting material for glucose synthesis. It can also be used to make amino acids, fatty acids, and glycerol.

Stage 3 - Regeneration of RuBP:

Most of the TP produced is used to regenerate RuBP, so the cycle can continue. This requires ATP. For every 5 molecules of TP, 3 molecules of RuBP are regenerated.

The glucose output: for every 6 turns of the Calvin cycle (fixing 6 CO₂), enough TP accumulates for 1 glucose to be produced (by condensing two 3C TP molecules into a 6C glucose-6-phosphate, and then to glucose).

Accounting for ATP and Reduced NADP Usage

Per CO₂ fixed (per turn of the cycle):

• 2 ATP used (1 to reduce GP→TP, 1 to regenerate RuBP)
• 2 reduced NADP used (to reduce GP → TP; NADP is regenerated)

This is why the Calvin cycle cannot run without a continuous supply of ATP and reduced NADP from the light reactions, and why it also regenerates NADP and ADP + Pᵢ for the light reactions.

Limiting Factors in Photosynthesis

The rate of photosynthesis is controlled by whichever factor is in the shortest supply - the limiting factor.

Light intensity: In low light, the light-dependent reactions are slow. Fewer photons mean less ATP and reduced NADP produced, limiting the rate of the Calvin cycle even if CO₂ and temperature are optimal. As light intensity increases, rate increases - but only up to the point where another factor becomes limiting.

CO₂ concentration: CO₂ is the substrate for RuBisCO in carbon fixation. Low CO₂ limits carbon fixation - RuBP cannot be carboxylated, so TP and glucose production slows. As CO₂ increases, rate increases until another factor limits.

Temperature: All enzymatic steps (especially RuBisCO catalysis in the Calvin cycle) are temperature dependent. Very low temperature = low enzyme activity = slow rate. But unlike simple chemical reactions, increasing temperature beyond the optimum denatures enzymes - rate falls sharply. The optimal temperature for photosynthesis is typically around 25 - 35°C.

Water: Water is a direct substrate in photolysis. Severe water stress closes stomata, reducing CO₂ entry. However, water shortage rarely limits photosynthesis directly unless it causes stomatal closure.

The concept of limiting factors is Blackman's Law: when a process is controlled by several factors, the rate is limited by the one present at the least favourable level.

Experimental Evidence: Investigating Photosynthesis

Chromatography: photosynthetic pigments can be separated by paper or thin-layer chromatography. Different pigments have different Rf values (Rf = distance pigment travels / distance solvent front travels). Chlorophyll a and b, carotene, and xanthophyll can all be identified.

DCPIP: a blue dye that is decolourised (reduced) when it accepts electrons from the light reactions. Can be used to show that isolated chloroplasts, in light, produce reducing power.

Measuring rate: by counting O₂ bubbles from aquatic plants (e.g. Elodea) or by measuring CO₂ uptake. Variables to control: light intensity (distance from lamp - intensity ∝ 1/d²), wavelength, temperature, CO₂ concentration.

Summary

• Light-dependent reactions (thylakoid membrane): light → excites electrons in PSII → photolysis of H₂O (O₂ released) → electron transport chain → H⁺ pumped into thylakoid lumen → chemiosmosis through ATP synthase → ATP made. Electrons re-energised in PSI → reduce NADP to NADPH.
• Calvin cycle (stroma): CO₂ + RuBP (5C) → 2GP (3C) [carbon fixation by RuBisCO] → GP reduced to TP using ATP + NADPH → TP used to make glucose or to regenerate RuBP (using ATP).
• Limiting factors: light intensity, CO₂ concentration, temperature
• PSII contains P680; PSI contains P700
• Photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂
• Products of light reactions: ATP, reduced NADP, O₂
• Products of Calvin cycle: TP (→ glucose, lipids, amino acids), NADP, ADP + Pᵢ

AQA Exam Tips

• Light-dependent vs light-independent: AQA often asks where each occurs. Light-dependent = thylakoid membranes. Calvin cycle = stroma.
• Why rate falls if light is removed: without light, no ATP/NADPH produced → GP cannot be reduced to TP → GP accumulates, TP and RuBP decrease. AQA will give you a graph and ask you to explain the fall in TP or RuBP.
• Calvin cycle changes when CO₂ removed: RuBisCO has no substrate → RuBP cannot be carboxylated → RuBP accumulates, GP and TP fall.
• Chemiosmosis in chloroplasts: H⁺ is pumped INTO the thylakoid lumen (not the intermembrane space as in mitochondria). Flow is from lumen back to stroma through ATP synthase.
• NADP not NAD: photosynthesis uses NADP as the hydrogen carrier; respiration uses NAD. This is a common error that loses marks.
• RuBisCO: always give its full role - it catalyses the fixation of CO₂ onto RuBP in the stroma.
• Rf values: in chromatography questions, Rf = distance spot moved / distance solvent moved. Can be used to identify pigments. State that a higher Rf means the pigment is less polar (more soluble in the organic solvent).
• Compensation point: the light intensity at which the rate of photosynthesis equals the rate of respiration, so there is no net gas exchange. AQA may ask you to identify this from a graph.