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Biology · Photosynthesis · Class 11

Light Reactions of Photosynthesis — Step by Step

Light reactions are the first stage of photosynthesis. They occur on the thylakoid membranes, use light energy to split water, and produce ATP, NADPH, and oxygen. These products power the Calvin cycle.

Explanation

Think of light reactions as the power plant inside a chloroplast. Sunlight hits chlorophyll molecules arranged in two clusters — Photosystem II (PS II) and Photosystem I (PS I). PS II absorbs light first, energizing electrons that then travel through a chain of proteins embedded in the thylakoid membrane.

As electrons move through this chain, they lose energy — and that energy is used to pump hydrogen ions (H+) into the thylakoid space, building a concentration gradient. When these ions flow back out through ATP synthase (like water through a dam), ATP is generated. Meanwhile, PS I re-energizes the electrons with another photon of light. These high-energy electrons combine with NADP+ and H+ to form NADPH.

The electrons that PS II lost must be replaced — and they come from splitting water. This photolysis releases O2 as a byproduct.

Step-by-Step Breakdown

  1. Photosystem II Absorbs Light

    Chlorophyll molecules in PS II absorb photons. Electrons in the reaction centre (P680) become excited and are passed to the primary electron acceptor.

  2. Water Splitting (Photolysis)

    To replace the lost electrons, water is split: 2H2O → 4H+ + 4e- + O2. The oxygen is released. The H+ ions stay inside the thylakoid lumen.

  3. Electron Transport Chain

    Excited electrons pass through plastoquinone (PQ), cytochrome b6f complex, and plastocyanin (PC). Energy released pumps H+ into the thylakoid lumen.

  4. ATP Synthesis (Chemiosmosis)

    The H+ concentration gradient drives H+ back through ATP synthase, generating ATP from ADP + Pi. This is called photophosphorylation.

  5. Photosystem I Re-energizes Electrons

    Electrons arriving at PS I (P700) absorb another photon and are re-energized. They pass through ferredoxin to the enzyme NADP+ reductase.

  6. NADPH Formation

    NADP+ reductase combines the high-energy electrons with H+ and NADP+ to produce NADPH — an essential electron carrier for the Calvin cycle.

Key Definitions

TermDefinition
Photosystem II (PS II)The first protein complex in light reactions. Contains chlorophyll P680. Splits water.
Photosystem I (PS I)The second protein complex. Contains chlorophyll P700. Produces NADPH.
PhotophosphorylationATP production using light energy during the light reactions of photosynthesis.
ChemiosmosisThe process where H+ ions flow down their gradient through ATP synthase to generate ATP.
Plastoquinone (PQ)A mobile electron carrier in the thylakoid membrane that shuttles electrons from PS II to cytochrome b6f.

Common Mistakes Students Make

1

Thinking Photosystem I comes first because of its name. PS II actually acts first in the Z-scheme. The numbering reflects discovery order, not function order.

2

Confusing cyclic and non-cyclic photophosphorylation. Non-cyclic involves both PS I and PS II and produces ATP + NADPH + O2. Cyclic involves only PS I and produces only ATP.

3

Forgetting that the H+ gradient is across the thylakoid membrane — H+ accumulates inside the thylakoid lumen, not in the stroma.

4

Writing that light reactions produce glucose. They do not. Light reactions produce ATP and NADPH. Glucose is made in the Calvin cycle.

Exam Focus — How This Is Asked

NEET frequently asks: 'What is the role of PS II?' and 'Differentiate between cyclic and non-cyclic photophosphorylation.' CBSE 5-mark questions often ask you to draw the Z-scheme or describe the entire electron flow. Always mention: PS II → ETC → PS I → NADPH, and that O2 comes from water splitting at PS II.

Practice Questions

Q1.Why is Photosystem II named 'II' even though it acts first?2m
The naming reflects the order of discovery, not the order of function. PS II was discovered after PS I, but in the light reaction pathway, PS II acts before PS I.
Q2.What are the products of the light reactions?2m
ATP, NADPH, and O2. ATP and NADPH are used in the Calvin cycle. O2 is released as a byproduct of water splitting.
Q3.Differentiate between cyclic and non-cyclic photophosphorylation.3m
Non-cyclic: involves PS II and PS I, produces ATP + NADPH + O2, electrons flow linearly from water to NADP+. Cyclic: involves only PS I, produces only ATP, electrons cycle back to PS I, no O2 released, no NADPH produced.

Photosynthesis — 30-Second Revision

  • •Light reactions occur on thylakoid membranes
  • •PS II acts first (despite the name), PS I acts second
  • •Water is split at PS II → O2 released
  • •Electron flow: H2O → PS II → ETC → PS I → NADP+ reductase → NADPH
  • •H+ gradient drives ATP synthase → ATP (chemiosmosis)
  • •Products: ATP + NADPH + O2
  • •Non-cyclic = both PS, cyclic = only PS I (ATP only)

Related Topics

Frequently Asked Questions

What is the Z-scheme in photosynthesis?
The Z-scheme describes the pathway of electron flow through PS II and PS I. When plotted on a graph of redox potential vs. electron flow, the path looks like the letter Z — electrons drop in energy through the ETC, then get re-energized at PS I.
Why is water needed in the light reactions?
Water is the electron donor. When PS II loses electrons (to the ETC), those electrons are replaced by splitting water. Without water, PS II cannot function and the entire light reaction chain stops.

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