What Are the Products of the Light-Dependent Reactions?

Short answer

• The products of the light-dependent reactions are ATP, NADPH and oxygen.
• ATP is the cell's short term energy carrier. It is made by ATP synthase as hydrogen ions flow through it, and it is spent in the Calvin cycle to build sugars.
• NADPH is the reducing power of photosynthesis. It carries high energy electrons and hydrogen to the Calvin cycle, where it helps turn carbon dioxide into sugar.
• Oxygen comes from splitting water. It is released as a byproduct, leaves the leaf through the stomata, and is not used to build sugar.
• Sugar is not a product of the light-dependent reactions. Sugar is built later, in the light-independent reactions (the Calvin cycle), using the ATP and NADPH made here.

Introduction: A Short Question With a Precise Answer

“What are the products of the light-dependent reactions?” is one of the most asked questions in introductory biology, and it has a short answer: ATP, NADPH and oxygen. The question is popular for a good reason. It sits at the exact point where light energy stops being light and starts being chemistry. Everything that follows in photosynthesis, including the building of sugar, depends on getting this step right.

The confusion usually starts because textbooks describe photosynthesis in two stages, and the stages have similar names. The light-dependent reactions need light directly. The light-independent reactions, also called the Calvin cycle, do not use light directly, but they depend completely on the products of the first stage. If you mix up which stage makes what, the whole process stops making sense. This article keeps the two stages strictly separate, explains each product in turn, shows how each one is made, and follows each one to where it goes next.

For the full photosynthesis overview, including the overall equation and how the two stages fit together, see Cells, Mitosis, Meiosis and Photosynthesis Explained Simply. This article is the focused piece: the light-dependent reactions only, their three products, and nothing else claimed for them.

The Three Products at a Glance

The table below is the whole answer in one place. The rest of the article explains each row: where the product comes from, what it does, and where it goes.

Two of the three products are used inside the chloroplast. One leaves it. That split is the key to remembering the list: ATP and NADPH are the useful products that power the next stage, and oxygen is the byproduct that escapes. Both groups matter, but for different reasons. ATP and NADPH matter because without them the Calvin cycle cannot run. Oxygen matters because almost all the oxygen in the air comes from this reaction.

Where the Light-Dependent Reactions Happen

The light-dependent reactions take place in the thylakoid membrane of the chloroplast. Thylakoids are flattened, disk shaped sacs stacked inside the chloroplast, and their membranes hold the photosystems, the electron transport chain and ATP synthase. The fluid inside a thylakoid is called the lumen. The fluid surrounding the thylakoids, still inside the chloroplast, is called the stroma. The Calvin cycle runs in the stroma, which is why ATP and NADPH only have a short journey to make: they are made at the thylakoid membrane and used just outside it.

This location matters for understanding the products. The thylakoid membrane is not just a surface where reactions happen. It is a barrier that lets the chloroplast build up hydrogen ions on one side, in the lumen, and keep them scarce on the other side, in the stroma. That difference in concentration is stored energy, and it is what drives the production of ATP. Without the membrane, there would be no gradient, and without the gradient, there would be no ATP from this stage.

Chlorophyll sits in the thylakoid membrane, held in protein complexes called photosystems. Chlorophyll absorbs light in the blue and red parts of the visible spectrum and reflects green, which is why leaves look green. Carotenoids, the other major class of photosynthetic pigment, absorb mainly blue light and help dispose of excess energy as heat, protecting the system from damage in full sun. When a photon of light is absorbed by a pigment molecule, the energy is passed from chlorophyll to chlorophyll until it reaches the reaction center of a photosystem, where the real chemistry begins. For a labeled view of where chloroplasts sit inside a plant cell, see Plant Cell Diagram Labeled.

Product 1: ATP, the Energy Carrier

ATP stands for adenosine triphosphate. It is the cell’s standard way of carrying usable energy from a reaction that releases energy to a reaction that needs it. ATP is not a fuel store for the long term. It is a short term carrier, made where it is needed and spent almost at once. OpenStax Biology 2e states this directly: the products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of a second, while the carbohydrate products of the light-independent reactions can survive almost indefinitely. That contrast is worth remembering. ATP and NADPH are made to be spent, not stored.

How ATP is made: chemiosmosis

ATP in the light-dependent reactions is made by a process called photophosphorylation, which means making ATP using light energy. The light itself does not attach the phosphate to ADP. The light drives a chain of events that builds a hydrogen ion gradient, and the gradient drives ATP synthase. The steps are:

1. Light excites electrons in photosystem II, and those electrons travel along the electron transport chain toward photosystem I.
2. As the electrons move through the chain, their energy is used to pump hydrogen ions (protons) from the stroma into the thylakoid lumen.
3. Splitting water at photosystem II adds still more hydrogen ions to the lumen.
4. Making NADPH at the end of the chain removes hydrogen ions from the stroma.
5. The result is a high concentration of hydrogen ions inside the lumen and a low concentration outside it, in the stroma. The lumen becomes more acidic than the stroma.

That gradient is stored energy, much like water held behind a dam. ATP synthase is the opening in the dam. It is a protein channel that spans the thylakoid membrane. Hydrogen ions rush through it from the lumen back into the stroma, flowing from high concentration to low concentration, and the energy released by that flow allows ATP synthase to attach a third phosphate group to ADP (adenosine diphosphate), forming ATP. This flow of ions through ATP synthase is called chemiosmosis. The same basic mechanism, a proton gradient driving ATP synthase, is used in cellular respiration in mitochondria, which is why the two processes are often taught side by side. For the respiration side of that comparison, see Chemical Equation for Cellular Respiration.

What ATP does next

ATP made in the light-dependent reactions goes into the stroma, where the Calvin cycle uses it. The Calvin cycle needs ATP in two of its three stages: in the reduction stage, where it helps convert 3-PGA into G3P, and in the regeneration stage, where it helps rebuild RuBP, the molecule that accepts the next carbon dioxide. For every three molecules of carbon dioxide fixed, the Calvin cycle consumes nine molecules of ATP and six molecules of NADPH to produce one net molecule of G3P. That figure, nine ATP and six NADPH per G3P, is the standard textbook stoichiometry, and it shows why the light-dependent reactions must keep producing ATP at a steady rate. When the light reactions stop, the ATP supply stops, and the Calvin cycle stops with it.

After ATP gives up its energy in the Calvin cycle, it becomes ADP and inorganic phosphate again. Those empty carriers return to the light-dependent reactions, where ATP synthase recharges them into ATP. The cycle of ATP to ADP and back is continuous while light is available. Nothing in this loop is consumed permanently. What is consumed is light energy and water, and what is produced is ATP, NADPH and oxygen.

Cyclic photophosphorylation: ATP without NADPH or oxygen

There is one variation worth knowing, because it appears in exam questions. In cyclic photophosphorylation, electrons flow in a loop involving only photosystem I. They return to photosystem I instead of going on to make NADPH, and no water is split, so no oxygen is released. The loop still pumps hydrogen ions and still drives ATP synthase, so it produces ATP only. The cell uses this route to top up ATP when the Calvin cycle needs more ATP relative to NADPH than the main noncyclic route supplies. The main route, noncyclic photophosphorylation, uses both photosystems and produces all three products. The cyclic route is a supplement, not a replacement.

Product 2: NADPH, the Reducing Power

NADPH is the reduced form of NADP+, nicotinamide adenine dinucleotide phosphate. Its job is to carry high energy electrons, plus hydrogen, from the light-dependent reactions to the Calvin cycle. Where ATP supplies energy, NADPH supplies reducing power: the electrons that actually reduce carbon dioxide into sugar. The two carriers work as a pair, and the Calvin cycle needs both. ATP alone cannot build sugar, because building sugar is a reduction, and reduction needs electrons. NADPH alone cannot build sugar either, because the cycle also needs energy to drive its steps. The light-dependent reactions exist to supply both together, in roughly the ratio the Calvin cycle needs.

How NADPH is made

NADPH is made at the end of the electron transport chain. The sequence runs through photosystem I:

1. Electrons arrive at photosystem I from photosystem II, having traveled along the electron transport chain and lost some energy along the way.
2. A second photon, absorbed by the photosystem I antenna, re-energizes the electrons at the photosystem I reaction center, called P700.
3. The high energy electrons are passed to a small iron sulfur protein called ferredoxin.
4. Ferredoxin delivers the electrons to an enzyme called ferredoxin-NADP+ reductase.
5. That enzyme uses the electrons, plus a hydrogen ion from the stroma, to reduce NADP+ into NADPH.

In plain terms, NADP+ is the empty carrier and NADPH is the loaded one. It picks up a high energy electron and a proton and becomes NADPH. When it later gives up its electron in the Calvin cycle, it is converted back to NADP+, the empty form, and returns to the light-dependent reactions to be loaded again. Biology LibreTexts makes the same point with a useful comparison: photosynthesis uses NADPH in a way comparable to how cellular respiration uses NADH and FADH2, but NADPH is a different carrier, used for building molecules up rather than breaking them down.

What NADPH does next

NADPH goes into the stroma and into the reduction stage of the Calvin cycle. There it donates its electrons to convert 3-PGA, the product of carbon fixation, into G3P (glyceraldehyde 3-phosphate), the three carbon sugar that is the net output of the cycle. Six molecules of NADPH are used for every three molecules of carbon dioxide fixed, matching the six molecules of ATP used in the same reduction stage. The remaining three ATP of the nine total are used in the regeneration stage, which rebuilds RuBP. Once NADPH has delivered its electrons, it returns to NADP+ and cycles back to the light-dependent reactions, just as ADP returns to be recharged into ATP.

Do not confuse NADPH with NADH. The names differ by one letter, and the molecules are related, but their roles are different. NADH is mainly an electron carrier in cellular respiration, where it delivers electrons to the mitochondrial electron transport chain to make ATP. NADPH is mainly an electron carrier in photosynthesis, where it delivers electrons to build sugar. Mixing them up is one of the most common errors in this topic, and examiners test it precisely because the names are so similar.

Product 3: Oxygen, the Byproduct That Changed the Air

Oxygen is the third product, and it is different from the other two in an important way: it is not used by the plant to build sugar. It is released. It comes from water, not from carbon dioxide, which is another common error. The carbon dioxide taken in by the leaf supplies the carbon for sugar. The oxygen released by the leaf comes from the water split at photosystem II.

How oxygen is made: photolysis

The process is called photolysis, the splitting of water by light energy. It happens at photosystem II, at a structure called the oxygen evolving complex. When light excites the reaction center of photosystem II, called P680, its chlorophyll molecules give up an electron. That electron must be replaced, or the photosystem cannot respond to the next photon. The replacement electron comes from water. The overall reaction for water splitting is:

2H2O gives 4H+ + 4e- + O2

In words, splitting two molecules of water releases four electrons, four hydrogen ions and one molecule of oxygen gas. Splitting a single water molecule releases two electrons, two hydrogen ions and one atom of oxygen, and two such splits are needed to form one molecule of diatomic oxygen gas. Each product of the split has a job:

• The electrons replace the electrons lost by photosystem II, keeping the electron transport chain supplied.
• The hydrogen ions add to the proton gradient in the thylakoid lumen, helping to drive ATP production.
• The oxygen is released as a byproduct.

The oxygen molecules produced exit the leaf through the stomata, the small pores on the leaf surface, and find their way into the surrounding air. OpenStax notes that about 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation, and the remainder escapes to the atmosphere, where it is used by aerobic organisms to support respiration. That 10 percent figure is an approximate textbook value, not a precise measurement for every leaf in every condition, but it makes the point clearly: most of the oxygen leaves, and only a small share is used locally.

Why oxygen matters beyond the leaf

The oxygen released by photolysis is the source of nearly all the oxygen in the air. Every breath taken by an aerobic organism depends, ultimately, on water being split in a thylakoid membrane somewhere. That is a large claim for a small reaction, and it is why the light-dependent reactions are often described as one of the most important chemical processes on Earth. They do not just feed the plant. They maintain the oxygen supply for nearly all aerobic life, and they do it as a byproduct, while the plant’s real purpose in the reaction is to make ATP and NADPH for itself.

The Full Sequence, Step by Step

The three products do not appear all at once. They are made at different points in one connected sequence. Read in order, the sequence is:

1. A photon strikes the antenna pigments of photosystem II. Energy passes to the reaction center, P680.
2. P680 gives up a high energy electron to the primary electron acceptor. Light energy has now become chemical energy in the form of an excited electron.
3. Water is split at photosystem II to replace the lost electron. Oxygen and hydrogen ions are released, and the electrons enter the chain.
4. The electron travels through the electron transport chain, from plastoquinone to the cytochrome complex to plastocyanin, on its way to photosystem I. As it travels, it loses energy, and that energy pumps hydrogen ions into the thylakoid lumen.
5. A second photon is absorbed at photosystem I. The electron is re-energized at the reaction center, P700.
6. The electron passes through ferredoxin to ferredoxin-NADP+ reductase, which reduces NADP+ to NADPH.
7. Meanwhile, hydrogen ions flow back through ATP synthase from the lumen to the stroma, and ATP is made from ADP and inorganic phosphate.

The numbering of the photosystems is a historical accident worth knowing about, because it confuses many learners. Photosystem II was discovered before photosystem I, so it carries the higher number even though it acts first in the sequence. Electrons flow from photosystem II to photosystem I, not the other way around. OpenStax states this directly: the numbering of the photosystems is derived from the order in which they were discovered, not in the order of the transfer of electrons.

The Net Equation

The light-dependent reactions are often summarized in a single net equation. The version given in standard sources is:

2H2O + 2NADP+ + 3ADP + 3Pi gives O2 + 2NADPH + 3ATP

In words: two molecules of water, two molecules of NADP+, three molecules of ADP and three inorganic phosphates, plus light energy, produce one molecule of oxygen, two molecules of NADPH and three molecules of ATP. Hydrogen ions appear on the product side in some versions of the equation, and the exact numbers of ATP per NADPH can vary with conditions and with how much cyclic photophosphorylation is running. Treat the equation as a summary of the inputs and outputs, not as a fixed recipe that is identical in every cell at every moment. What does not vary is the list of products: ATP, NADPH and oxygen, made from water, NADP+, ADP, inorganic phosphate and light.

Where Each Product Goes Next: The Handoff to the Calvin Cycle

The light-dependent reactions and the Calvin cycle are two halves of one process, and the handoff between them is exact. ATP and NADPH cross from the thylakoid membrane into the stroma, where the Calvin cycle waits. Oxygen leaves the leaf. The Calvin cycle then runs in three stages:

1. Fixation. The enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. RuBisCO is short for ribulose-1,5-bisphosphate carboxylase/oxygenase, and it acts on RuBP, a five carbon molecule.
2. Reduction. ATP and NADPH convert 3-PGA into G3P. This is the stage that consumes the products of the light-dependent reactions.
3. Regeneration. Some G3P is used to rebuild RuBP so the cycle can continue, using more ATP. The rest leaves the cycle toward making glucose and other organic molecules.

Only one carbon dioxide molecule is incorporated at a time, so the cycle must turn three times to produce a single three carbon G3P molecule, and six times to produce a six carbon glucose molecule. That is why the stoichiometry matters: three turns consume nine ATP and six NADPH for one net G3P. Two G3P molecules combine to make one glucose. The light-dependent reactions must therefore run continuously in daylight to keep the Calvin cycle supplied, and the Calvin cycle must keep consuming ATP and NADPH, returning ADP and NADP+ to be recharged. If either half stops, the other stops too.

Note what the Calvin cycle does not do: it does not use light directly, and it does not split water or release oxygen. Those are light-dependent jobs. The common nickname “dark reactions” for the Calvin cycle is misleading, because the cycle normally runs in daylight, powered by products that only exist while light is available. It is called light-independent because light is not a direct reactant, not because it happens in the dark. Several of its enzymes are in fact switched on by light dependent signals.

What the Products Are Not

Several wrong answers appear again and again, in search results and in exam scripts. Clearing them up is part of answering the question properly.

Sugar is not a product of the light-dependent reactions. Glucose and other sugars are built in the Calvin cycle, from carbon dioxide, using ATP and NADPH. The light-dependent reactions make the energy carriers, not the sugar itself. If an answer lists glucose among the products of the light reactions, it has confused the two stages.

Carbon dioxide is not a product. Carbon dioxide is a reactant of photosynthesis as a whole, taken in for the Calvin cycle, where its carbon is fixed into organic molecules. It is not produced by the light-dependent reactions.

Water is not a product of this stage. Water is a reactant here: it is split to supply electrons, hydrogen ions and oxygen. Water is produced elsewhere in the overall chemistry of photosynthesis and respiration, but in the light-dependent reactions it is consumed, not made.

NADH is not a product. The carrier made here is NADPH, not NADH. The two are related but distinct, with different roles, as explained above.

ADP and NADP+ are not products. They are the empty carriers left behind after ATP and NADPH are spent in the Calvin cycle, and they are the starting materials the light-dependent reactions recharge. Listing them as products reverses the direction of the process.

Factors That Affect How Much Product Is Made

The rate at which the light-dependent reactions produce ATP, NADPH and oxygen depends on the conditions around the leaf. The main factors are:

• Light intensity. More light, up to a point, means more photons absorbed, more electrons excited, and more ATP and NADPH made. Beyond the point where the photosystems are saturated, extra light does not increase the rate, and excess energy must be dissipated safely, partly by carotenoids, to avoid damage.
• Light wavelength. Chlorophyll absorbs mainly blue and red light, and reflects green. Light in the wavelengths chlorophyll cannot absorb contributes little to the reactions, however bright it looks to the human eye.
• Water availability. Water is the electron donor. Without water to split, photosystem II cannot replace its lost electrons, and the whole chain stalls. A plant short of water closes its stomata to conserve water, which also limits carbon dioxide intake for the Calvin cycle.
• Temperature. The reactions involve enzymes, including ATP synthase and ferredoxin-NADP+ reductase, and enzymes work fastest within a range. Very low temperatures slow them, and very high temperatures can damage them.
• Carbon dioxide supply. Carbon dioxide is not a reactant of the light-dependent reactions, so it does not affect them directly. It affects them indirectly: if the Calvin cycle slows for lack of carbon dioxide, ATP and NADPH are not consumed, ADP and NADP+ are not returned, and the light reactions back up. The two stages are linked through the carrier cycle.

These factors explain why photosynthesis is fastest on a bright, warm day with plentiful water and open stomata, and why it slows in shade, drought or cold, even though the chemistry itself has not changed.

Common Exam Traps and How to Avoid Them

This topic is heavily tested, and the same traps appear year after year. The most frequent are:

1. Naming only two products. Many answers list ATP and NADPH and forget oxygen. Oxygen is a product, even though it is a byproduct and leaves the leaf. The complete answer has three items.
2. Saying the oxygen comes from carbon dioxide. It comes from water. This was established by classic experiments using heavy oxygen isotopes, and it is one of the most tested facts in the topic. The oxygen atoms in the released O2 are the oxygen atoms from split water molecules.
3. Putting sugar in the light-dependent reactions. Sugar is made in the Calvin cycle. The light reactions make ATP, NADPH and oxygen only.
4. Placing the reactions in the wrong part of the chloroplast. The light-dependent reactions happen at the thylakoid membrane. The Calvin cycle happens in the stroma. Swapping them is a common error.
5. Running the photosystems in the wrong order. Electrons flow from photosystem II to photosystem I. The numbers reflect discovery order, not function order.
6. Calling the Calvin cycle the dark reactions and assuming it happens at night. It is light-independent, not dark. It runs in daylight, powered by ATP and NADPH made in the light.
7. Confusing NADPH with NADH. Different carrier, different process, different purpose.
8. Forgetting cyclic photophosphorylation. When a question asks what cyclic photophosphorylation produces, the answer is ATP only: no NADPH, no oxygen.

Why the Answer Matters

The products of the light-dependent reactions matter for three reasons that go beyond passing a test. First, they are the energy link between sunlight and food. Nearly every food chain on Earth starts with ATP and NADPH made in a thylakoid membrane, because nearly every organism eats plants, algae or something that ate them. Second, the oxygen byproduct maintains the air that aerobic organisms breathe. Third, the division of labor between the two stages, one capturing energy, the other building molecules, is a pattern repeated across biology: capture energy first, spend it carefully second. Understanding that pattern in photosynthesis makes cellular respiration, which runs the same chemiosmosis machinery in reverse, much easier to understand. The comparison is worth making explicit: both processes use an electron transport chain, a proton gradient and ATP synthase to make ATP, but one stores light energy in chemical carriers and the other releases chemical energy from food. For the energy concepts behind both, see Definition of Energy for Science.

Frequently Asked Questions

What are the products of the light-dependent reactions?

The products are ATP, NADPH and oxygen. ATP and NADPH go to the Calvin cycle to build sugar, and oxygen is released as a byproduct when water is split.

What are the products of the light-dependent reactions and where do they go?

ATP and NADPH go into the stroma of the chloroplast, where the Calvin cycle uses them to turn carbon dioxide into sugar. Oxygen leaves the leaf through the stomata and enters the air. ADP and NADP+, the empty carriers left after the Calvin cycle spends ATP and NADPH, return to the light-dependent reactions to be recharged.

Is glucose a product of the light-dependent reactions?

No. Glucose is not made in the light-dependent reactions. It is built later, in the Calvin cycle, from carbon dioxide, using the ATP and NADPH made in the light reactions. The direct products of the light reactions are ATP, NADPH and oxygen only.

What is the equation for the light-dependent reactions?

A commonly given net equation is 2H2O + 2NADP+ + 3ADP + 3Pi gives O2 + 2NADPH + 3ATP, with light energy as the input. The exact ATP count can vary with conditions, because cyclic photophosphorylation can add extra ATP without making NADPH or oxygen.

Where do the light-dependent reactions take place?

They take place in the thylakoid membrane of the chloroplast. The photosystems, the electron transport chain and ATP synthase are all embedded in that membrane. The Calvin cycle, by contrast, takes place in the stroma, the fluid surrounding the thylakoids.

What is photolysis?

Photolysis is the splitting of water by light energy at photosystem II. It releases electrons, hydrogen ions and oxygen. The electrons replace those lost by chlorophyll, the hydrogen ions help build the gradient that drives ATP production, and the oxygen is released as a byproduct.

What is the difference between cyclic and noncyclic photophosphorylation?

Noncyclic photophosphorylation uses both photosystem II and photosystem I, splits water, and produces ATP, NADPH and oxygen. Cyclic photophosphorylation uses only photosystem I, in a loop, and produces ATP only, with no NADPH and no oxygen. The cell uses the cyclic route to adjust the ratio of ATP to NADPH.

Does the Calvin cycle use the products of the light-dependent reactions?

Yes. The Calvin cycle uses ATP and NADPH from the light-dependent reactions to fix carbon dioxide into sugar. For every three molecules of carbon dioxide fixed, it consumes nine ATP and six NADPH to produce one net molecule of G3P. Oxygen is not used by the Calvin cycle; it is released.

Why is oxygen released during photosynthesis?

Oxygen is released because water must be split to replace the electrons that photosystem II loses when light excites it. The oxygen atoms from the water molecules have nowhere else to go, so they combine into O2 and leave the leaf. The oxygen comes from water, not from carbon dioxide.

Sources & references

• OpenStax, Biology 2e, Section 8.2, "The Light-Dependent Reactions of Photosynthesis." Accessed 5 November 2026. https://openstax.org/books/biology-2e/pages/8-2-the-light-dependent-reactions-of-photosynthesis
• OpenStax, Biology 2e, Section 8.3, "Using Light Energy to Make Organic Molecules" (the Calvin cycle). Accessed 5 November 2026. https://openstax.org/books/biology-2e/pages/8-3-using-light-energy-to-make-organic-molecules
• Biology LibreTexts, "14.5: The Light-Dependent Reactions." Accessed 5 November 2026. https://bio.libretexts.org/Workbench/Bio_11A_-_Introduction_to_Biology_I/14%3A_Photosynthesis_-_Light_Reactions/14.05%3A_The_Light-Dependent_Reactions
• Wikipedia, "Light reactions." Accessed 5 November 2026 via search results. https://en.wikipedia.org/wiki/Light_reactions