• 19 parts
  • Checked October 2026

Light-Dependent Reactions Diagram

Follow an electron from water to NADPH across the thylakoid membrane, then open up one photosystem.

Light-Dependent Reactions Diagram

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
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Write the name of each numbered part.Answer key

  1. Photosystem II
  2. Plastoquinone
  3. Cytochrome complex (b6f)
  4. Plastocyanin
  5. Photosystem I
  6. Ferredoxin
  7. ATP synthase
  8. Water
  9. Oxygen
  10. NADPH
  11. ATP
  12. Thylakoid membrane
  13. Stroma
  14. Thylakoid lumen (thylakoid space)
  15. Light-harvesting complex
  16. Reaction center
  17. Special pair (chlorophyll)
  18. Electron acceptor
  19. Pigment molecule

Drawn for this site on 2026-10-01 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Next: the Calvin cycle, where the ATP and NADPH go

All 19 parts on the diagram

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The path of the electrons 6

  1. Photosystem II The first photosystem in the chain. It takes electrons from water, and oxygen is released.
  2. Plastoquinone A small carrier that moves within the membrane. It takes electrons from photosystem II to the cytochrome complex.
  3. Cytochrome complex (b6f) It pumps hydrogen ions into the thylakoid lumen as electrons pass. The pumping builds the gradient.
  4. Plastocyanin A small protein that contains copper. It carries electrons from the cytochrome complex to photosystem I.
  5. Photosystem I Light raises the electrons to a high energy a second time. They leave on the stroma side.
  6. Ferredoxin A small protein with an iron-sulfur center. It passes the electrons on toward NADP+.

What goes in and comes out 5

  1. ATP synthase Hydrogen ions flow back to the stroma through it. That flow drives the making of ATP.
  2. Water The source of the electrons. Photosystem II removes them from water.
  3. Oxygen Released when water gives up its electrons. It is a by-product.
  4. NADPH Made on the stroma side. Electrons and hydrogen ions join NADP+ to form it.
  5. ATP Made in the stroma by ATP synthase. The hydrogen ion gradient supplies the energy.

The membrane and its two sides 3

  1. Thylakoid membrane It holds both photosystems, the carriers between them and ATP synthase.
  2. Stroma The fluid of the chloroplast outside the thylakoid. NADPH and ATP are released into it.
  3. Thylakoid lumen (thylakoid space) The space inside the thylakoid. Hydrogen ions are pumped into it and build up there.

Inside one photosystem 5

  1. Light-harvesting complex Together these form the antenna complex. It collects light energy and passes it to the reaction center.
  2. Reaction center A complex of proteins and chlorophyll at the heart of the photosystem. Here light energy becomes chemical energy.
  3. Special pair (chlorophyll) Two reaction-center chlorophyll molecules. The collected energy reaches them, and one gives up an electron.
  4. Electron acceptor It takes the excited electron from the special pair. The electron then moves on along a chain of acceptors.
  5. Pigment molecule A light-harvesting complex holds many. Each can capture light energy and hand it to a neighbor.

How this list was compiled: Every name, place and step checked on 2026-10-01 against Alberts and others, Molecular Biology of the Cell, 4th edition, chapter 14, Chloroplasts and Photosynthesis (NCBI Bookshelf NBK26819, archived copy) and against Wikipedia: Photosystem and Photosynthetic reaction centre, text only. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

The light-dependent reactions happen in the thylakoid membrane of a chloroplast. The first chart follows the electrons along the membrane from water to NADPH; the second shows how one photosystem catches light.

Read the full guide

Read the first chart from left to right

Photosystem II takes electrons from water, and oxygen is released. Plastoquinone carries the electrons to the cytochrome complex, which pumps hydrogen ions into the thylakoid lumen. Plastocyanin carries them on to photosystem I, where light raises their energy again. Ferredoxin passes them toward NADP+, which becomes NADPH. The hydrogen ions flow back to the stroma through ATP synthase, and that flow makes ATP. This path from water to NADPH is the noncyclic pathway. In the cyclic pathway electrons from photosystem I return to the cytochrome complex, so ATP is made without NADPH or oxygen.

  • The stroma is above the membrane on this chart and the lumen below.
  • Small round shapes are carriers that move; blocks stay in the membrane.

Questions and answers

What are the parts of the noncyclic pathway?

In the order the electrons meet them: water, photosystem II, plastoquinone, the cytochrome complex, plastocyanin, photosystem I, ferredoxin and NADP+, which becomes NADPH. Oxygen is released at the start, and ATP synthase uses the hydrogen ion gradient to make ATP.

What are the major components of a photosystem?

A reaction center surrounded by light-harvesting complexes. The complexes hold pigment molecules that collect light energy and pass it inward to a special pair of chlorophyll molecules, which hands an electron to an electron acceptor.

Is this the "label the major components of the photosynthetic reaction complex" figure?

No. That sentence is the title of a publisher's exercise. This drawing is our own and shows the same structures, so it helps you learn the names, but it is not an answer key.

Where do the light-dependent reactions take place?

In the thylakoid membranes inside a chloroplast. The photosystems, the carriers and ATP synthase sit in the membrane, hydrogen ions build up in the thylakoid lumen, and NADPH and ATP are released into the stroma.

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