# Light-dependent reactions

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u14-light-dependent-reactions/

This module covers the first stage of photosynthesis, occurring in the thylakoid membranes of chloroplasts. You will learn how light energy is converted to chemical energy in ATP and reduced NADP, which power the subsequent light-independent stage.

**Prerequisites:** [Chloroplast structure and function](https://www.owlsprep.com/study/cie-9700-u14-chloroplast-structure/); [ATP as an energy currency](https://www.owlsprep.com/study/cie-9700-u06-atp-energy/)

## Learning objectives

- Outline the location and key steps of the light-dependent reactions of photosynthesis
- Explain the role of chlorophyll, electron carriers and photolysis in ATP and reduced NADP production
- Distinguish between cyclic and non-cyclic photophosphorylation
- Relate chloroplast structure to the function of light-dependent reactions

## Location and Core Overview

All light-dependent reactions occur across the thylakoid membranes of chloroplast. These membranes contain photosynthetic pigments arranged into two photosystems (PSI and PSII) and an electron transport chain, specialised for converting light energy to chemical energy.

**Photoionisation of Chlorophyll** — Process where light energy excites electrons in chlorophyll, raising their energy enough to leave the chlorophyll, ionising it in the process

*Example:* Chlorophyll $a$ in PSII loses 2 excited electrons when it absorbs 680nm wavelength light.

**Worked example:** Explain how thylakoid membrane structure is adapted for light-dependent reactions

1. 1. Folded into thin discs creates a large surface area for embedding photosystems and electron transport proteins.
2. 2. The small thylakoid lumen volume allows a proton gradient to form rapidly after proton pumping.
3. 3. ATP synthase enzymes embedded in the membrane enable protons to diffuse down their gradient to power ATP synthesis.

> **tip**
>
> Grana are just stacks of thylakoids, so you can refer to the location as either thylakoid membranes or grana, both are accepted in exams.

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## Non-Cyclic Photophosphorylation & Photolysis

Non-cyclic photophosphorylation is the main pathway that produces both ATP and reduced NADP, and requires both PSI and PSII. When light hits PSII, photoionisation occurs, and electrons must be replaced.

**Photolysis of Water** — Enzyme-catalysed splitting of water at PSII to replace lost electrons, producing the products below

*Example:* The overall reaction for photolysis is:

$$2H_2O \rightarrow 4H^+ + 4e^- + O_2$$

**Worked example:** Describe the sequence of events in non-cyclic photophosphorylation starting from light hitting PSII

1. 1. Light absorption causes photoionisation of chlorophyll in PSII, excited electrons leave chlorophyll.
2. 2. Electrons pass along the electron transport chain, losing energy at each carrier.
3. 3. Photolysis splits water to replace the electrons lost from PSII, producing $H^+$, $e^-$ and $O_2$.
4. 4. Energy from electrons is used to pump $H^+$ from stroma into the thylakoid lumen, creating a proton gradient.
5. 5. Protons diffuse through ATP synthase back to stroma, driving ATP synthesis via chemiosmosis.
6. 6. Electrons move from PSII to PSI to replace electrons lost from PSI after photoionisation.
7. 7. Excited electrons from PSI are transferred to NADP along with $H^+$ from stroma to form reduced NADP.

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## Cyclic Photophosphorylation

Cyclic photophosphorylation is a secondary pathway that only uses PSI, and produces only ATP (no reduced NADP or oxygen). Electrons that leave PSI are passed back to PSI instead of being passed to NADP.

**Worked example:** Compare key features of cyclic and non-cyclic photophosphorylation

1. | Feature | Cyclic | Non-cyclic |
| --- | --- | --- |
| Photosystems used | Only PSI | PSI + PSII |
| Products | Only ATP | ATP, reduced NADP, oxygen |
| Electron fate | Return to PSI | End in reduced NADP, do not return |
| Requires photolysis | No | Yes |

> **info**
>
> Cyclic photophosphorylation produces extra ATP to meet the higher energy demand of the Calvin cycle, which requires more ATP than reduced NADP.

*Calculator:* forbidden

## Chemiosmosis

Chemiosmosis is the process that generates ATP using the potential energy stored in a proton electrochemical gradient across the thylakoid membrane.

**Chemiosmosis** — Diffusion of protons down their electrochemical gradient through membrane-bound ATP synthase, which provides energy to catalyse ATP formation from ADP and inorganic phosphate (Pi)

**Worked example:** Explain how a proton gradient is generated and used to make ATP

1. 1. As electrons move down the electron transport chain, they release energy used to pump $H^+$ from the stroma into the thylakoid lumen.
2. 2. This creates a higher concentration of $H^+$ in the lumen than stroma, forming an electrochemical gradient.
3. 3. $H^+$ can only cross the membrane through ATP synthase channels, as the lipid bilayer is impermeable to protons.
4. 4. The energy from the gradient drives ATP synthase to combine ADP and Pi into ATP.

*Calculator:* forbidden

## Common pitfalls

- **Wrong:** Claiming oxygen is produced from splitting carbon dioxide in light-dependent reactions
  - Why it fails: Oxygen is always a product of photolysis of water, carbon dioxide is not used until the light-independent stage
  - Correct: State that oxygen is produced when water is split during photolysis at photosystem II
- **Wrong:** Claiming cyclic photophosphorylation produces reduced NADP or oxygen
  - Why it fails: Electrons cycle back to PSI, so no photolysis is needed and no electrons are passed to NADP
  - Correct: State that cyclic photophosphorylation only produces ATP
- **Wrong:** Locating light-dependent reactions in the chloroplast stroma
  - Why it fails: The stroma is the site of the light-independent Calvin cycle, light-dependent reactions are in thylakoid membranes
  - Correct: State that light-dependent reactions occur in thylakoid membranes / grana of chloroplasts
- **Wrong:** Claiming chlorophyll absorbs mainly green light
  - Why it fails: Chlorophyll reflects green light (which is why leaves appear green), it absorbs red and blue light most effectively
  - Correct: State that chlorophyll absorbs primarily red and blue wavelengths, reflects green light
- **Wrong:** Saying electrons from photolysis go directly to NADP
  - Why it fails: Electrons from photolysis first replace electrons lost by PSII, then travel through the ETC to PSI before reaching NADP
  - Correct: Explain the full sequence of electron flow from photolysis to PSII → ETC → PSI → NADP

## Cheatsheet

| Process | Location | Key Products | Key Notes |
| --- | --- | --- | --- |
| Light-dependent reactions | Thylakoid membranes / grana | ATP, reduced NADP, $O_2$ | Requires light |
| Non-cyclic photophosphorylation | Thylakoid membrane | ATP, reduced NADP, $O_2$ | Uses PSI + PSII |
| Cyclic photophosphorylation | Thylakoid membrane | ATP only | Uses only PSI |
| Photolysis | PSII (thylakoid lumen) | $H^+$, $e^-$, $O_2$ | Replaces electrons lost from PSII |
| Chemiosmosis | Thylakoid membrane | ATP | Uses proton gradient through ATP synthase |

## What's next

The ATP and reduced NADP produced during the light-dependent reactions are the energy source and reducing power needed for the next stage of photosynthesis, the light-independent reactions (Calvin cycle) in the stroma of the chloroplast. Understanding the details of this stage is critical for explaining how carbon dioxide is fixed into organic molecules, and for analysing how limiting factors affect overall photosynthesis rate. This topic also shares core mechanisms with oxidative phosphorylation in aerobic respiration, so mastering it will help you compare and contrast these two key energy transfer processes in exam answers. You can now build on this foundation to learn about the Calvin cycle and how photosynthesis is regulated in plants.

- [Light-independent reactions](https://www.owlsprep.com/study/cie-9700-u14-light-independent-reactions/)
- [Limiting factors of photosynthesis](https://www.owlsprep.com/study/cie-9700-u14-limiting-factors-of-photosynthesis/)
- [Chloroplast structure](https://www.owlsprep.com/study/cie-9700-u14-chloroplast-structure/)

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