What it does
In the light reactions of photosynthesis, light energy is turned into two useful molecules: ATP and NADPH. It happens through a chain of electron carriers in the thylakoid membrane of the chloroplast. Drawn by energy, the chain looks like a sideways letter Z, so it is called the Z-scheme.
This page draws the Z and moves one electron along it, one step at a time, with a short explanation of what happens at each carrier: where light lifts the electron, where water is split and oxygen is released, where H⁺ is pumped into the thylakoid lumen, and where NADP⁺ is turned into NADPH. Switch to the cyclic path to see how photosystem I alone can make extra ATP. Below, a tally counts the photons, water, oxygen, NADPH, H⁺ and ATP for any number of electrons.
It fits Class 11 Biology (photosynthesis in higher plants: the light reaction, the electron transport chain, splitting of water, cyclic and non-cyclic photophosphorylation and chemiosmosis). First see which colours of light the pigments catch in pigment spectra; see what limits the overall rate in photosynthesis lab.
How to use it
- Leave Non-cyclic chosen and press Next step. The red dot is the electron; the big line names the carrier it is on and the text explains the step. Back goes one step back; Play steps by itself.
- Look at the dashed orange lines marked light: the two places where a photon lifts the electron (at P680 in photosystem II and at P700 in photosystem I).
- Switch to Cyclic and step through again. The electron leaves P700, reaches ferredoxin, then goes back to plastoquinone and cytochrome b6f and returns to P700.
- In the tally, slide Electrons sent through and read what is made.
Worked example
Send 12 electrons through the non-cyclic chain. Each electron is lifted twice, so 24 photons are absorbed (12 at each photosystem). Water gives 4 electrons for every 2 molecules split, so 6 H₂O are split and 3 O₂ given off. Each NADPH takes 2 electrons, so 6 NADPH are made. The lumen gains 12 H⁺ from water and 24 from cytochrome b6f, 36 in all, and with about 14 H⁺ per 3 ATP that makes 36 × 3 / 14 ≈ 7.71 ATP.
Switch to Cyclic with the same 12 electrons: 12 photons, no water split, no O₂, no NADPH, 24 H⁺ and about 5.14 ATP.
Tips
- The electron only gains energy at the two photosystems. Everywhere else it goes downhill, and some of that energy is used to pump H⁺.
- ATP is not made by the electrons directly. They build up H⁺ in the lumen, and H⁺ flowing back through ATP synthase makes ATP (chemiosmosis).
- The Calvin cycle needs 3 ATP for every 2 NADPH. Non-cyclic flow alone gives a little less ATP than that, which is one reason plants also run cyclic flow.
FAQ
Why is it called P680 and P700? They are special pairs of chlorophyll a molecules that absorb best at about 680 nm and 700 nm.
Why does the diagram go “up” for negative potentials? A more negative redox potential means the electron is held less tightly and has more energy to give. Putting those at the top makes the energy picture easy to read.
Is 14 H⁺ per 3 ATP exact? No. It is a rounded figure often used for the chloroplast ATP synthase; textbooks also use simpler numbers. The idea that matters is that more H⁺ means more ATP.
Where the numbers come from
- The order of carriers in the light reactions (P680, pheophytin, plastoquinone, cytochrome b6f, plastocyanin, P700, ferredoxin, NADP⁺ reductase), the 2 H₂O → 4 H⁺ + O₂ + 4 e⁻ and NADP⁺ + 2 e⁻ + H⁺ → NADPH equations, and the cyclic path: general plant biology. Redox potentials are rounded textbook values, drawn only to show the shape of the Z.
- H⁺ counts (1 per electron from water, 2 per electron through cytochrome b6f) and about 14 H⁺ per 3 ATP are rounded, commonly used figures; real values vary and are still studied.
- Checked in the tests against hand counts: 12 electrons give 24 photons, 6 H₂O, 3 O₂, 6 NADPH, 36 H⁺ and 36 × 3/14 = 7.71 ATP. Teaching model only. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.