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Botany · Process

Photosynthesis Diagram

How a chloroplast turns light, water and carbon dioxide into sugar and oxygen: the light-dependent reactions and the Calvin cycle.

Plants, algae and cyanobacteria use the energy of light to make sugar from carbon dioxide and water, giving off oxygen. Above, a chloroplast in section; below, one thylakoid enlarged, with the light-dependent reactions on its membrane and the Calvin cycle in the stroma around it. Step through it, or press any part. Not to scale.

Photosynthesis in a chloroplast. Above, a chloroplast in section shows its outer and inner membranes, grana of stacked thylakoids and the stroma. Below, a thylakoid membrane is enlarged: light reaches photosystem II, which splits water into oxygen and hydrogen ions; electrons pass through plastoquinone, the cytochrome b6f complex, which pumps hydrogen ions into the lumen, and plastocyanin to photosystem I, which reduces NADP+ to NADPH; hydrogen ions flow back through ATP synthase to make ATP. In the stroma, the Calvin cycle fixes 3 CO2 onto 3 RuBP with RuBisCO, making 6 3-PGA, which 6 ATP and 6 NADPH reduce to 6 G3P; one G3P leaves to make sugars and five remake RuBP with 3 more ATP. ADP, phosphate and NADP+ return to the membrane.A chloroplast, in sectionOuter membraneInner membraneStromaGranum(a stack of thylakoids)ThylakoidStromaLight-dependent reactionson the thylakoid membraneThylakoid lumenLightinPSIIb₆fPSIPQPCe⁻H₂OinO₂outH⁺H⁺H⁺H⁺H⁺H⁺NADP⁺NADPHNADP⁺ reductaseH⁺ADP + PiATPATPsynthaseCalvin cyclein the stromaRuBisCO6 ATP6 NADPH5 G3P3 ATP3 RuBP5 carbons6 3-PGA3 carbons6 G3P3 carbons3 CO₂inGlucose andother sugarsout1 G3POverall: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
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Step 1 of 10

Photosynthesis at a glance

Photosynthesis stores the energy of light in sugar. It runs in two linked stages in the chloroplast: the light-dependent reactions, on the thylakoid membranes, make ATP and NADPH and give off oxygen; the Calvin cycle, in the stroma, spends them to build sugar from carbon dioxide. Summed up: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The Calvin cycle is sometimes called the dark reactions, but several of its enzymes are switched off in the dark, so it runs in the light.

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Step 1 of 10: Photosynthesis at a glance

Download the diagramChecked against its sources on 29 October 2026.

Step by step

The steps of the Photosynthesis

  1. 01Photosynthesis at a glance

    Photosynthesis stores the energy of light in sugar. It runs in two linked stages in the chloroplast: the light-dependent reactions, on the thylakoid membranes, make ATP and NADPH and give off oxygen; the Calvin cycle, in the stroma, spends them to build sugar from carbon dioxide. Summed up: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The Calvin cycle is sometimes called the dark reactions, but several of its enzymes are switched off in the dark, so it runs in the light.

  2. 02Where it happens: the chloroplast

    A chloroplast has an outer and an inner membrane round a fluid, the stroma. Inside lies a third membrane, folded into flattened sacs called thylakoids, many of them stacked into grana; the space inside them is the lumen. The light-dependent reactions run on the thylakoid membrane, and the Calvin cycle in the stroma.

  3. 03Light reaches photosystem II

    Each photosystem holds hundreds of chlorophyll and other pigment molecules, which pass the energy of absorbed light to a special pair of chlorophylls at its reaction centre. In photosystem II this energy lifts an electron to a high energy level, and the electron is handed to an acceptor, starting a flow of electrons along the membrane.

  4. 04Water is split and oxygen released

    Photosystem II replaces the electrons it loses by taking them from water. A cluster of manganese atoms splits two water molecules into four hydrogen ions, four electrons and one molecule of oxygen: 2H₂O → 4H⁺ + 4e⁻ + O₂. The hydrogen ions stay in the lumen; the oxygen leaves the leaf through the stomata.

  5. 05Electrons pass along and pump hydrogen ions

    Plastoquinone carries the electrons to the cytochrome b₆f complex, and plastocyanin carries them on to photosystem I. As they pass through the cytochrome b₆f complex, it pumps hydrogen ions from the stroma into the lumen, which becomes far more acidic than the stroma: about pH 5.5 to 6 against pH 8.

  6. 06Photosystem I makes NADPH

    Photosystem I absorbs more light and lifts the electrons to a higher energy level again. They pass to ferredoxin, a small protein on the stroma side, and the enzyme NADP⁺ reductase uses them to reduce NADP⁺ to NADPH, the reducing power the Calvin cycle needs.

  7. 07ATP synthase makes ATP

    The hydrogen ions crowded in the lumen can flow back to the stroma only through ATP synthase. Their flow drives the enzyme to join ADP and phosphate (Pi) into ATP, on the stroma side of the membrane. Making ATP this way, with energy from light, is called photophosphorylation.

  8. 08RuBisCO fixes carbon dioxide

    In the stroma, the enzyme RuBisCO adds carbon dioxide to ribulose bisphosphate (RuBP), a five-carbon sugar. The six-carbon product splits at once into two molecules of 3-phosphoglycerate (3-PGA), so 3 CO₂ and 3 RuBP give 6 3-PGA. RuBisCO is slow, so leaves hold a great deal of it; it is probably the most abundant protein on Earth.

  9. 093-PGA is reduced to G3P

    Each 3-PGA takes a phosphate from ATP and is then reduced by NADPH to glyceraldehyde 3-phosphate (G3P), a three-carbon sugar. For 6 3-PGA this uses 6 ATP and 6 NADPH. The ADP, phosphate and NADP⁺ left over go back to the thylakoid membrane to be recharged.

  10. 10One G3P leaves, five remake RuBP

    Of the 6 G3P, one leaves the cycle: the net gain from fixing 3 CO₂. Two G3P make one glucose, and the cell builds sucrose, starch and its other molecules from G3P. The other 5 G3P are rearranged, using 3 more ATP, into 3 RuBP, so the cycle turns again. In all, 3 CO₂ make 1 G3P for 9 ATP and 6 NADPH.

Parts

What each part of the photosynthesis does

Outer membrane
The outer of the chloroplast's two envelope membranes. Pore proteins in it let small molecules pass freely.
Inner membrane
The inner envelope membrane. It lets ions and other molecules through only by its transport proteins, so it controls what enters and leaves the stroma. G3P leaves through it in exchange for phosphate.
Thylakoids and grana
Flattened sacs of membrane inside the chloroplast, many of them stacked into piles called grana and joined by unstacked lamellae. Their membrane holds the chlorophyll, the photosystems, the electron carriers and ATP synthase.
Stroma
The fluid inside the inner membrane, around the thylakoids. It holds the enzymes of the Calvin cycle, including RuBisCO, and the chloroplast's own DNA and ribosomes. Starch is stored here as grains.
Thylakoid lumen
The space inside the thylakoids. In the light, hydrogen ions released and pumped into it build up until it is about pH 6, or nearer pH 5.5 in bright light, against about pH 8 in the stroma.
Light
Sunlight, absorbed by chlorophyll, the green pigment of the thylakoid membrane, and by accessory pigments such as carotenoids, which collect light of other wavelengths and protect the chlorophyll.
Photosystem II
An antenna of pigments that funnels light energy to a reaction centre, joined to an enzyme that splits water. It comes first in the chain, though it was named second for historical reasons.
Water
The source of the electrons. Photosystem II splits it, and its oxygen atoms become the oxygen the plant gives off. Water reaches leaf cells from the roots.
Oxygen
A by-product of splitting water. It leaves the leaf through the stomata. Almost all the oxygen in the air was made this way, and the plant's own mitochondria use some of it.
Electron carriers
Plastoquinone, which moves within the membrane, carries electrons from photosystem II to the cytochrome b₆f complex; plastocyanin, a small copper protein in the lumen, carries them on to photosystem I.
Cytochrome b₆f complex
Passes electrons from plastoquinone to plastocyanin and uses their energy to pump hydrogen ions from the stroma into the lumen. It resembles the cytochrome b-c₁ complex of mitochondria.
Hydrogen ions
Hydrogen ions (H⁺), released from water and pumped in by the cytochrome b₆f complex, crowd into the lumen. Their push to flow back out to the stroma stores the energy that ATP synthase uses.
Photosystem I
A second antenna and reaction centre. Light absorbed here lifts each electron high enough to reduce NADP⁺, by way of the small protein ferredoxin and the enzyme NADP⁺ reductase.
NADPH
The reduced form of the coenzyme NADP⁺, made on the stroma side of the membrane by NADP⁺ reductase. It carries high-energy electrons to the Calvin cycle and comes back as NADP⁺ to be reduced again.
ATP synthase
An enzyme in the thylakoid membrane with its head in the stroma. Hydrogen ions flowing through it from the lumen to the stroma drive it to make ATP from ADP and phosphate, as the same kind of enzyme does in mitochondria.
ATP
The cell's energy carrier, made from ADP and phosphate (Pi) by ATP synthase. The Calvin cycle spends 9 ATP for every 3 CO₂ it fixes, and the ADP and phosphate return to the membrane to be made into ATP again.
Carbon dioxide
Enters the leaf through the stomata and reaches the stroma. The Calvin cycle builds it into sugar: this is how carbon from the air enters living things.
RuBisCO
Ribulose bisphosphate carboxylase/oxygenase, the enzyme that fixes carbon dioxide onto RuBP. It is slow, handling about three molecules a second, so leaves make a great deal of it. It sometimes adds oxygen instead, which wastes carbon (photorespiration).
RuBP
Ribulose 1,5-bisphosphate, a five-carbon sugar carrying two phosphate groups. It takes up the carbon dioxide, and the cycle makes it again from G3P.
3-PGA
3-Phosphoglycerate, a three-carbon acid: the first stable product of carbon fixation. Each CO₂ fixed onto a RuBP gives two of it.
G3P
Glyceraldehyde 3-phosphate, a three-carbon sugar and the cycle's product. Of every six made, one is the net gain and five go to remake RuBP.
Glucose and other sugars
G3P that leaves the cycle is built into glucose, into sucrose that is carried round the plant, into starch for storage, and into the cell's other organic molecules.

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