The Light-Dependent Reaction — AQA A-level Biology

SPEC 3.5.1 - ENERGY TRANSFERS

Photosynthesis: Light-Dependent Reaction

Photoionisation, the electron transfer chain, chemiosmosis, ATP synthesis, reduced NADP and photolysis of water

Photosynthesis: The Big Picture

Photosynthesis converts light energy into chemical energy stored in organic molecules. It occurs in two linked stages inside the chloroplast.

Light-Dependent Reaction

Location: Thylakoid membranes (grana)

Inputs: Light, water, ADP+Pᵢ, NADP

Outputs: ATP, reduced NADP, O₂

Light-Independent Reaction (Calvin Cycle)

Location: Stroma

Inputs: CO₂, ATP, reduced NADP

Outputs: Glucose (via triose phosphate)

The Link

The light-dependent reaction produces the ATP and reduced NADP that power the Calvin cycle. Without these, the cycle stops. Remove the light and both stages ultimately halt.

Synoptic Link

ATP structure (Lesson 1.07): adenosine + 3 phosphate groups. In photosynthesis, ATP is synthesised by photophosphorylation. In respiration (Lesson 5.03–5.04), it is synthesised by oxidative phosphorylation. The mechanism — chemiosmosis through ATP synthase — is the same in both.

Chloroplast Structure

Understanding where reactions occur requires knowing the chloroplast ultrastructure.

Key structures

Thylakoid membranes — folded into stacks (grana); contain photosystems, electron transfer chain proteins and ATP synthase

Thylakoid lumen — the space inside the thylakoid; protons accumulate here during the light-dependent reaction

Stroma — fluid surrounding the grana; site of the Calvin cycle; contains rubisco, circular DNA and ribosomes

Photoionisation

Definition — Photoionisation

The process in which light energy is absorbed by chlorophyll, causing electrons to be emitted from the chlorophyll molecule.

Chlorophyll molecules are organised into photosystems (PS I and PS II) embedded in the thylakoid membrane. Each photosystem contains a reaction centre chlorophyll and antenna pigments that funnel light energy to it.

1

Light energy is absorbed by chlorophyll (and accessory pigments) in Photosystem II (PSII)

2

Electrons in the chlorophyll molecule gain energy and are excited to a higher energy level — they are emitted from the chlorophyll

3

The excited electrons enter the electron transfer chain in the thylakoid membrane

4

Photolysis of water provides replacement electrons for the PSII chlorophyll (covered on the next slide)

Photosynthetic Pigments — Required Practical 7

Different pigments absorb different wavelengths: chlorophyll a absorbs red and blue-violet; chlorophyll b absorbs blue-violet; carotenoids absorb blue and blue-green wavelengths. Together they broaden the range of wavelengths used in photosynthesis. Chromatography separates these pigments; Rf values identify each one. Green light is mostly reflected by all chloroplast pigments — this is why plants appear green.

Photolysis of Water

Definition — Photolysis

The splitting of water molecules using light energy, releasing electrons, protons (H⁺ ions) and oxygen.

2H₂O → 4H⁺ + 4e⁻ + O₂

What each product does

Electrons (e⁻) — replace those lost from PSII chlorophyll during photoionisation

Protons (H⁺) — contribute to the proton gradient and are used to reduce NADP

Oxygen (O₂) — released as a by-product into the atmosphere

Where it occurs

Photolysis occurs at Photosystem II, on the lumenal side of the thylakoid membrane.

The light energy absorbed by PSII chlorophyll provides the energy to split water molecules.

Examiner Alert — O₂ Source

Oxygen is produced by photolysis of water in the light-dependent reaction — NOT in the Calvin cycle and NOT from CO₂. Many students incorrectly state that O₂ comes from CO₂. The oxygen atoms in CO₂ end up in GP (glycerate 3-phosphate) in the Calvin cycle — they are never released as O₂.

Electron Transfer Chain & ATP Synthesis

As electrons pass along the electron transfer chain (ETC), they lose energy. This energy drives active transport of protons, creating the gradient used to make ATP.

1

Excited electrons from PSII enter the electron transfer chain — a series of carrier proteins embedded in the thylakoid membrane

2

As electrons pass along the chain, energy is released. This energy is used to actively pump protons (H⁺) from the stroma into the thylakoid lumen

3

A proton gradient (electrochemical gradient) builds up across the thylakoid membrane — high [H⁺] in lumen, low in stroma

4

Protons diffuse down the gradient through ATP synthase (embedded in the thylakoid membrane) by chemiosmosis → ATP is synthesised from ADP + Pᵢ (photophosphorylation)

Definition — Chemiosmosis

The movement of protons (H⁺) down an electrochemical gradient through ATP synthase, driving the synthesis of ATP.

Thylakoid Membrane: Proton Gradient

Green arrows = H⁺ pumped from stroma INTO lumen by ETC (energy from electron transfer). Red arrow = H⁺ flowing from lumen back through ATP synthase into stroma (chemiosmosis → ATP). Black arrows = electron flow.

Photosystem I & Reduced NADP

After passing through the electron transfer chain, electrons arrive at Photosystem I (PSI) with reduced energy. A second absorption of light re-energises them.

1

Electrons arrive at PSI having lost energy through the ETC

2

PSI absorbs more light energy — electrons are re-energised to a higher energy level

3

The high-energy electrons, together with H⁺ from photolysis, reduce NADP:

Reaction at PSI

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

Definition — Reduced NADP

A hydrogen carrier produced in the light-dependent reaction that carries H⁺ and electrons to the light-independent reaction (Calvin cycle), where it is used to reduce GP to triose phosphate.

Products of the Light-Dependent Reaction

The two key products that drive the Calvin cycle are: ATPReduced NADP

Oxygen is released as a by-product of photolysis and exits through stomata.

Full Sequence: Light-Dependent Reaction

1

Photoionisation at PSII: Light absorbed by chlorophyll → electrons excited and emitted → enter ETC

2

Photolysis at PSII: Water split → 4H⁺ + 4e⁻ + O₂ (full equation: 2H₂O → 4H⁺ + 4e⁻ + O₂). Electrons replace those lost from chlorophyll. O₂ released as by-product.

3

Electron transfer chain: Electrons pass down ETC → energy released → H⁺ pumped into thylakoid lumen → proton gradient established

4

Chemiosmosis / ATP synthesis: H⁺ diffuse back through ATP synthase → ADP + Pᵢ → ATP (photophosphorylation)

5

PSI: Electrons re-energised by light → combine with H⁺ (from photolysis) and NADP → reduced NADP formed

6

Products passed to Calvin cycle: ATP and reduced NADP enter stroma to power the light-independent reaction

Examiner Alert — Reduced NAD vs Reduced NADP

In the light-dependent reaction, the carrier is reduced NADP (NADPH) — NOT reduced NAD. Reduced NAD appears in glycolysis and the Krebs cycle (respiration). Using the wrong carrier loses marks — examiners report this error repeatedly.

Required Practicals 7 & 8

Required Practical 7 — Chromatography of Photosynthetic Pigments

Method: Spot plant extract onto chromatography paper → run solvent up the paper → pigments separate by solubility.

Rf value: Rf = distance moved by pigment ÷ distance moved by solvent front

Expected result: 4 bands from top to bottom: carotene (most soluble, highest Rf), xanthophyll, chlorophyll a, chlorophyll b (least soluble)

Why multiple pigments? Different pigments absorb different wavelengths of light — together they absorb a wider spectrum and maximise light absorption.

Required Practical 8 — Effect of a Named Variable on the Rate of Dehydrogenase Activity in Chloroplast Extracts

Principle: DCPIP (a blue dye) acts as an alternative hydrogen acceptor. Dehydrogenases in the light-dependent reaction remove hydrogen from water (via the ETC pathway); when this hydrogen is passed to DCPIP instead of NADP, DCPIP is reduced and turns from blue → colourless. Rate of decolourisation reflects the rate of the light-dependent reaction.

Typical independent variables: Light intensity, wavelength of light, temperature.

Control variables: Same volume/concentration of DCPIP and chloroplast suspension; same temperature (use water bath); maintain constant wavelength if varying intensity.

Factors Affecting the Light-Dependent Reaction

Any shortage of inputs limits the rate of the light-dependent reaction — and therefore the Calvin cycle.

FactorEffect when limitingWhy

Light intensityRate decreasesLess photoionisation → less ATP and reduced NADP produced → Calvin cycle slows

Water availabilityRate decreasesLess substrate for photolysis → fewer replacement electrons → chlorophyll cannot cycle → ETC slows

TemperatureModest effect on this stageLight reactions are largely non-enzymatic; temperature matters more in the Calvin cycle (rubisco)

What happens in darkness?

When light is removed: no photoionisation → ATP and reduced NADP production stops → GP cannot be reduced to TP in the Calvin cycle → GP accumulates. Without TP, RuBP cannot be regenerated (regeneration requires ATP) → TP falls, RuBP falls.

(This is a classic exam question: trace all three molecules — GP, TP and RuBP.)

Agricultural Application

Glasshouse growers can raise CO₂ concentration, increase light intensity with artificial lighting, and maintain optimal temperature to overcome limiting factors and increase photosynthetic rate — and therefore crop yield.

DEEP Framework: Extended Answer

12-mark question style: "Describe the role of light energy in the light-dependent reaction of photosynthesis."

DEEP Answer Framework

DDefine: The light-dependent reaction uses light energy to produce ATP and reduced NADP, which are used to drive the Calvin cycle.

EExplain mechanism: Light is absorbed by chlorophyll in PSII → photoionisation → electrons emitted → pass down ETC → energy released pumps H⁺ into thylakoid lumen → H⁺ diffuse through ATP synthase by chemiosmosis → ATP synthesised. Second absorption at PSI → electrons re-energised → reduce NADP to reduced NADP using H⁺ from photolysis. Water is split by photolysis (2H₂O → 4H⁺ + 4e⁻ + O₂) — providing electrons for PSII and H⁺ for NADP reduction. O₂ released as by-product.

EEvidence/example: DCPIP experiments show that the light-dependent reaction is directly proportional to light intensity up to a saturation point, confirming light drives photoionisation and electron flow.

PPractical/quantitative: As light intensity doubles from 1000 to 2000 lux, DCPIP decolourisation rate approximately doubles — consistent with a linear relationship before saturation.

Synoptic Links

Link to Respiration (5.03–5.04)

Both photosynthesis and respiration use chemiosmosis through ATP synthase to produce ATP. In photosynthesis this is in the thylakoid membrane (photophosphorylation); in respiration it is in the inner mitochondrial membrane (oxidative phosphorylation). Same enzyme, same mechanism, different locations.

Link to Cell Membranes (2.05)

The proton gradient that drives ATP synthase is an electrochemical gradient — the same type of gradient used in co-transport across the ileum epithelium. Facilitated diffusion, active transport and chemiosmosis all rely on concentration gradients and membrane proteins.

Link to Protein Structure (1.04)

ATP synthase is a large multi-subunit protein — a rotary molecular motor. Its quaternary structure is essential for function. Denaturation of ATP synthase (e.g. extreme pH or temperature) would halt ATP production from chemiosmosis.

Link to Ecosystems (5.05)

The rate of photosynthesis determines gross primary production (GPP). Understanding limiting factors in the light-dependent reaction directly explains why GPP varies with season, latitude and canopy coverage.

Practice Question

A student investigated the rate of the light-dependent reaction using isolated chloroplasts and DCPIP, a blue dye that turns colourless when reduced.

The student illuminated the chloroplasts at different light intensities and recorded the time taken for DCPIP to become colourless. They found that at 500 lux the decolourisation time was 8 minutes, at 1000 lux it was 4 minutes, and at 4000 lux it was also 4 minutes.

(a) Explain why DCPIP becomes colourless during the light-dependent reaction. [3 marks]

(b) Explain why the decolourisation rate did not increase between 1000 and 4000 lux. [3 marks]

(c) Suggest one way the student could verify that the decolourisation was caused by the light-dependent reaction and not a chemical reaction with DCPIP. [1 mark]

(a) Light is absorbed by chlorophyll → photoionisation → electrons emitted from chlorophyll → pass to electron transfer chain / DCPIP acts as electron acceptor → DCPIP gains electrons → reduced DCPIP is colourless. (3 marks: electrons from photoionisation; DCPIP accepts electrons; reduced DCPIP colourless)

(b) At 1000 lux the reaction rate has reached its maximum — another factor is now limiting the rate. At these light intensities, a factor other than light intensity (e.g. concentration of ADP/Pᵢ, amount of NADP, or number of chlorophyll molecules / photosystems available) limits the rate, so increasing light intensity further has no effect. (3 marks: plateau reached; named limiting factor; correct explanation)

(c) Include a control tube containing DCPIP and chloroplasts but kept in the dark (covered with foil) — should show no decolourisation, confirming light (and therefore the light-dependent reaction) is required. (1 mark)

Lesson Summary

Light-Dependent Reaction — Key Points

Location: Thylakoid membranes (grana) of chloroplasts

Photoionisation: Light absorbed by chlorophyll → electrons emitted from PSII

Photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂ (at PSII); O₂ released as by-product; electrons replace those lost from chlorophyll

ETC: Electrons pass along carrier proteins → energy used to pump H⁺ into thylakoid lumen → proton gradient

ATP synthesis: H⁺ diffuse through ATP synthase by chemiosmosis → ATP (photophosphorylation)

Reduced NADP: At PSI, electrons re-energised by light → combine with H⁺ and NADP → reduced NADP

Products: ATP + reduced NADP → used in Calvin cycle; O₂ released

Required Practicals: RP7 — chromatography (Rf values); RP8 — DCPIP as electron acceptor

Never Write

O₂ comes from CO₂ → O₂ comes from photolysis of water

Reduced NAD is produced → it is reduced NADP in photosynthesis

ATP synthase is in the inner mitochondrial membrane → here it is in the thylakoid membrane