Light-dependent reactions

Biology· 25 min read

1. Location and Core Overview★★☆☆☆⏱ 6 min

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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.

📘 Definition

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
1. The small thylakoid lumen volume allows a proton gradient to form rapidly after proton pumping.
3
1. ATP synthase enzymes embedded in the membrane enable protons to diffuse down their gradient to power ATP synthesis.

2. Non-Cyclic Photophosphorylation & Photolysis★★★☆☆⏱ 8 min

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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.

📘 Definition

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

3. Cyclic Photophosphorylation★★★☆☆⏱ 7 min

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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

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

4. Chemiosmosis★★★★☆⏱ 4 min

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

📘 Definition

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
1. This creates a higher concentration of $H^{+}$ in the lumen than stroma, forming an electrochemical gradient.
3
1. $H^{+}$ can only cross the membrane through ATP synthase channels, as the lipid bilayer is impermeable to protons.
4
1. The energy from the gradient drives ATP synthase to combine ADP and Pi into ATP.

5. Common Pitfalls

Wrong move:

Claiming oxygen is produced from splitting carbon dioxide in light-dependent reactions

Why:

Oxygen is always a product of photolysis of water, carbon dioxide is not used until the light-independent stage

Correct move:

State that oxygen is produced when water is split during photolysis at photosystem II

Wrong move:

Claiming cyclic photophosphorylation produces reduced NADP or oxygen

Why:

Electrons cycle back to PSI, so no photolysis is needed and no electrons are passed to NADP

Correct move:

State that cyclic photophosphorylation only produces ATP

Wrong move:

Locating light-dependent reactions in the chloroplast stroma

Why:

The stroma is the site of the light-independent Calvin cycle, light-dependent reactions are in thylakoid membranes

Correct move:

State that light-dependent reactions occur in thylakoid membranes / grana of chloroplasts

Wrong move:

Claiming chlorophyll absorbs mainly green light

Why:

Chlorophyll reflects green light (which is why leaves appear green), it absorbs red and blue light most effectively

Correct move:

State that chlorophyll absorbs primarily red and blue wavelengths, reflects green light

Wrong move:

Saying electrons from photolysis go directly to NADP

Why:

Electrons from photolysis first replace electrons lost by PSII, then travel through the ETC to PSI before reaching NADP

Correct move:

Explain the full sequence of electron flow from photolysis to PSII → ETC → PSI → NADP

6. Quick Reference 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

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 11
MCQ on site and products of photolysis
2023 · 22
Describe non-cyclic photophosphorylation
2021 · 13
Compare cyclic and non-cyclic pathways

Going deeper

syllabusCIE 9700 A-Level Biology SyllabusCheck for latest learning outcomes

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.