Light-Dependent Reactions
Light energy splits water and builds ATP.
About this topic
The light-dependent reactions take place in the thylakoid membranes of chloroplasts. Chlorophyll absorbs light, exciting electrons that pass along an electron transport chain, generating ATP. Water is split (photolysis), releasing oxygen, protons and electrons, and NADP⁺ is reduced to NADPH.
The ATP and NADPH carry chemical energy to the Calvin cycle. Oxygen is a by-product. Light intensity and wavelength affect the rate.
A useful way to remember is that the light reactions charge the batteries. Light energy excites electrons in chlorophyll in photosystem II, which are passed down the chain to photosystem I; the energy released pumps hydrogen ions into the thylakoid space, and ATP synthase uses the resulting gradient to make ATP. The electrons end up on NADP, producing NADPH. Replacement electrons come from splitting water, which gives off oxygen.
Plants look green because chlorophyll absorbs red and blue light and reflects green. Chromatography of leaf pigments shows that other pigments such as carotenoids help to capture more of the spectrum. A common misconception is that the oxygen released comes from carbon dioxide; isotope experiments proved that it comes from water.
Key ideas
- In the thylakoid membranes
- Produces ATP, NADPH and O₂
- Photosystems capture photons
Quick check
Use cases: where Light-Dependent Reactions helps
- Homework: review the key idea behind Light-Dependent Reactions before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Light-Dependent Reactions without notes.
- Visual learning: play with the interactive lab to see how Light-Dependent Reactions changes when you adjust its inputs.
- Teaching a friend: explain Light-Dependent Reactions in your own words, then compare your version with the key ideas.
Biology underpins medicine, farming, conservation and biotechnology.
Frequently asked questions
What is Light-Dependent Reactions?
Light energy splits water and builds ATP.
What will I learn in Light-Dependent Reactions?
In the thylakoid membranes. Produces ATP, NADPH and O₂. Photosystems capture photons.
Is Light-Dependent Reactions part of Biology?
Yes. It is one of 150 Biology topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Light-Dependent Reactions lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Light-Dependent Reactions?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.