Skip to content
Study Helper
Chapter 8 · Part 7 of 10
  1. Chapter overview
  2. 1What photosynthesis does
  3. 2Chloroplasts and the two stages
  4. 3Light and how pigments absorb it
  5. 4The pigments plants use
  6. 5How photosystem II captures light
  7. 6Linear electron flow from water to NADPH
  8. 7How chloroplasts make ATP
  9. 8The Calvin cycle
  10. 9Photorespiration
  11. 10How C₄ and CAM plants avoid photorespiration

How chloroplasts make ATP

0:00 / 9:10
1.0×

Linear electron flow makes NADPH and O₂ directly, but ATP is made another way. Hydrogen ions (H⁺) pile up inside the thylakoids, and as they flow back out, their movement is harnessed to build ATP.

An H⁺ gradient drives ATP synthase

Chloroplasts make ATP by adding a phosphate to ADP, a step called phosphorylation, using energy that originally came from light. The process is photophosphorylation, and it works by chemiosmosis, the same method mitochondria use in oxidative phosphorylation. In chemiosmosis, a membrane keeps H⁺ concentrated on one side, and the only way back across is through an enzyme called ATP synthase, which uses that flow to make ATP.

In the chloroplast, H⁺ piles up in the thylakoid lumen and is scarce in the stroma. Because each H⁺ carries a positive charge, the difference across the membrane is electrical as well as a difference in concentration, so it’s called an H⁺ electrochemical gradient. The light reactions build it in three ways:

  1. Splitting water in PSII releases H⁺ into the lumen.
  2. The cytochrome complex pumps H⁺ into the lumen as electrons pass along the electron transport chain.
  3. Making NADPH uses up H⁺ in the stroma.

H⁺ then flows from the lumen back into the stroma through ATP synthase, which spans the thylakoid membrane. That flow drives the enzyme to join ADP and phosphate into ATP on the stroma side.

Where each product is made

The three products of the light reactions end up in two different places:

Product Where it’s made How
O₂ Thylakoid lumen PSII splits water, and removing two electrons from one H₂O gives 2 H⁺ and ½ O₂. The O₂ then diffuses out of the chloroplast.
NADPH Stroma NADP⁺ reductase adds two high-energy electrons (boosted by light in both PSII and PSI) and one H⁺ to NADP⁺.
ATP Stroma ATP synthase makes it, powered by H⁺ flowing out of the lumen.

Cyclic electron flow makes extra ATP

Linear electron flow turns out ATP and NADPH in roughly equal amounts, but the Calvin cycle needs more ATP than NADPH: 18 ATP for every 12 NADPH, or 3 ATP for every 2 NADPH. Plants make up the shortfall with a second pattern of electron flow that produces only ATP.

In cyclic photophosphorylation, also called cyclic electron flow, electrons travel in a loop powered by PSI alone:

  1. Light excites P700 in PSI, which sends a high-energy electron to its primary electron acceptor and then to ferredoxin.
  2. Instead of going on to NADP⁺ reductase, the electron passes from ferredoxin back to plastoquinone.
  3. It moves through the cytochrome complex, which pumps H⁺ into the lumen, and then to plastocyanin.
  4. Plastocyanin returns the electron to PSI, closing the loop.

The pumped H⁺ adds to the same gradient, so ATP synthase makes more ATP. No NADPH is made, and because PSII isn’t involved, no water is split and no O₂ is released. The name sums it up: the electron path is cyclic, light energizes the electrons (photo), and ADP gains a phosphate (phosphorylation).

H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ ADP Pᵢ ATP e⁻ e⁻ Plastoquinone Stroma Light ADP + Pᵢ ATP Ferredoxin Plastocyanin Photosystem I Cytochrome complex ATP synthase Thylakoid lumen
Figure 8.6 Electrons leave photosystem I and come back to it, and their energy pumps H⁺ into the lumen. ATP synthase uses the H⁺ gradient to make ATP, but no NADPH or O₂ is produced.

Which pattern dominates depends on what the chloroplast is short of:

Linear electron flow Cyclic photophosphorylation
Photosystems used PSII and PSI PSI only
Source of electrons Water The same electrons, recycled back to PSI
Where the electrons end up In NADPH Back in PSI
Products ATP, NADPH, and O₂ ATP only
Favored when NADP⁺ is high and NADPH is low NADP⁺ is low and NADPH is high, or ATP is low

The cytochrome complex in the thylakoid membrane (called cytochrome b₆-f) resembles a complex in the electron transport chain of mitochondria (cytochrome b-c₁). Each contains a similar protein: cytochrome b₆ in chloroplasts and cytochrome b in mitochondria. Both accept electrons from a quinone (a small mobile carrier, such as plastoquinone) and pump H⁺ across their membrane.

These two proteins are homologous: their genes descend from the same ancestral gene. Similar genes code for similar amino acid sequences, and because a protein’s sequence determines its shape, the two proteins have very similar three-dimensional structures.

That shared ancestry is older than chloroplasts and mitochondria themselves. Both organelles arose by endosymbiosis, in which once free-living bacteria came to live inside ancestral cells: chloroplasts from cyanobacteria and mitochondria from proteobacteria. The original cytochrome gene belonged to a bacterium that was an ancestor of both groups, and it later diverged into the two versions.

Check yourselfAre ATP, NADPH, and O₂ made in the stroma or in the thylakoid lumen?

ATP and NADPH are made in the stroma, where the Calvin cycle uses them. O₂ is made in the thylakoid lumen, where water is split.

Check yourselfWhy is cyclic photophosphorylation useful to a plant?

Linear electron flow makes ATP and NADPH in roughly equal amounts, but the Calvin cycle uses more ATP than NADPH. Cyclic flow makes ATP without making NADPH, which raises the supply of ATP relative to NADPH.

Practice questions

  1. 1 of 2

    How does cyclic photophosphorylation help a plant keep the Calvin cycle supplied?

  2. 2 of 2

    Chloroplasts make ATP by chemiosmosis, using an H⁺ gradient built with energy from light. What is this process called?