In short
Photosynthesis is a process where living things use light energy, usually from the sun, to make sugars they can use as fuel.
In most cases, plants split water and release oxygen, and then turn carbon dioxide into sugar.
Why it matters
Photosynthesis supplies most of the energy for complex life and helps keep oxygen in the air we breathe.
Understanding it helps explain how food chains and the air around us depend on sunlight.
All 13 key points from the article are covered
- Photosynthesis is a set of processes in which plants, algae and cyanobacteria use light energy, usually sunlight, to make chemical energy for their metabolism.core
- The energy is stored in carbohydrates such as sugars, starches and cellulose. Cells later break these down through cellular respiration to release energy.core
- In the common form, oxygenic photosynthesis, carbon dioxide and water are turned into sugar and oxygen using light. Water is split, and oxygen is released as a byproduct.core
- Photosynthesis has two stages. Light-dependent reactions capture light energy and make ATP and NADPH. Light-independent reactions use these products to fix carbon dioxide into sugar.core
- In the light-dependent reactions, chlorophyll absorbs light and loses electrons. Water is split to replace them, which releases oxygen. The energy flow makes ATP and NADPH.core
- In the Calvin cycle, the enzyme RuBisCO captures carbon dioxide. The cycle uses ATP and NADPH to make three-carbon sugars, which later form glucose, sucrose and starch.core
- Photosynthesis and cellular respiration are roughly opposite processes. They use different chemical reactions and take place in different parts of the cell.
- Some bacteria and archaea use anoxygenic photosynthesis. It does not release oxygen, and some bacteria use hydrogen sulfide instead of water.
- Photosynthesis made Earth's oxygen-rich atmosphere possible. Cyanobacteria produced the oxygen that changed the planet.core
- In plants and algae, photosynthesis takes place in chloroplasts. Light-dependent reactions happen in the thylakoid membranes inside them.
- Hot, dry conditions can cause photorespiration, which wastes energy. C4 and CAM plants have evolved ways to reduce this loss.
- Plants usually convert about 3 to 6 percent of light energy into chemical energy. Efficiency can range from 0.1 to 8 percent depending on conditions.
- Main factors that limit photosynthesis are light, water, carbon dioxide and temperature.
How photosynthesis fits together
This diagram shows how light energy becomes stored sugar that cells can later use for energy.
Light absorbed by chlorophyll
Pigments in chloroplasts absorb light energy.
Light-dependent reactions
Water is split, releasing oxygen, and ATP and NADPH are made.
ATP and NADPH
Energy carriers that power sugar-making.
Calvin cycle
RuBisCO fixes carbon dioxide into three-carbon sugars.
Sugars such as glucose and starch
Carbohydrates that store chemical energy.
Cellular respiration uses the sugars
Cells break down stored sugars to release energy.
Based on
Section by section: a short excerpt from the article, then what it means in plain words.
What photosynthesis is
From the article
“Photosynthesis[note 1] is a system of biological processes by which photopigment-bearing autotrophic organisms, such as most plants, algae and cyanobacteria, convert light energy—typically from sunlight—into the chemical energy necessary to fuel their metabolism. The term photosynthesis usually refers to oxygenic photosynthesis, a process that releases oxygen as a byproduct of water splitting. Photosynthetic organisms store the converted chemical energy within the bonds of intracellular organic compounds (complex compounds containing carbon),…” ¶3
“In plants, algae, and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle. In this process, atmospheric carbon dioxide is incorporated into already existing organic compounds, such as ribulose bisphosphate (RuBP).[6] Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose. In other bacteria, different mechanisms like the reverse…” ¶6
In plain words
- Photosynthesis is how plants, algae and some bacteria (called autotrophs, meaning they make their own food) turn light energy, usually sunlight, into chemical energy they can use.
- Cells store this energy in sugars and starches. When they need energy, they break these sugars down in cellular respiration.
- Chlorophyll, a green pigment, catches the light. In plants it sits inside chloroplasts, which are small parts of leaf cells. Light splits water, which releases oxygen, and the freed hydrogen helps make two energy carriers, NADPH and ATP (the cell's main energy currency).interpretation
- In the Calvin cycle, the plant takes carbon dioxide from the air and uses the energy carriers to build it into sugars such as glucose.
- Photosynthesis is not one single method. Some bacteria use other substances instead of water and give off no oxygen. Early life probably used hydrogen or hydrogen sulfide before cyanobacteria appeared.
- The map shows where photosynthesis happens around the world, both in ocean plankton and on land plants.
From sunlight to sugar
This flowchart shows the main steps of photosynthesis, from light to stored sugar.
Sunlight is absorbed by chlorophyll
Chlorophyll sits inside chloroplasts in plant leaf cells.
Water is split and oxygen is released
Some of the light energy strips electrons from water.
Energy carriers NADPH and ATP are made
The hydrogen from water helps make these molecules.
Calvin cycle turns carbon dioxide into sugar
The plant uses NADPH and ATP to build glucose from air.
Cells use or store the sugar
Cells break sugar down in cellular respiration when they need energy.
Based on
What this shows: A sunflower with a leaf and roots shows the basic inputs and outputs of photosynthesis. Light, carbon dioxide taken in by the leaf, and water from the soil go in, and oxygen comes out while carbohydrates are made and stored in the plant.
What this shows: A world map colour-coded to show where photosynthesis happens, both in ocean phytoplankton (chlorophyll concentration) and on land (vegetation index). Blue and green areas mark high activity in the ocean and on land, while yellow and tan areas, such as deserts, show low activity. A colour scale at the bottom gives the measurement ranges.
Think of it like this: Think of sugar as a rechargeable battery. Sunlight charges it inside the plant, and the cells drain it later when they need energy.
Overview and the chemical equation
From the article
“Most photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms use carbon dioxide as a source of carbon atoms to carry out photosynthesis; photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon.[2]” ¶10
“Carbon dioxide is converted into sugars in a process called carbon fixation; photosynthesis captures energy from sunlight to convert carbon dioxide into carbohydrates. Carbon fixation is an endothermic redox reaction. In general outline, photosynthesis is the opposite of cellular respiration: while photosynthesis is a process of reduction of carbon dioxide to carbohydrates, cellular respiration is the oxidation of carbohydrates or other nutrients to carbon dioxide. Nutrients used in cellular respiration…” ¶13
In plain words
- Most photosynthetic organisms are photoautotrophs, which means they make their own food. They use light energy to build food from carbon dioxide and water.
- Plants, algae and cyanobacteria release oxygen while they make food. This oxygen-releasing type, called oxygenic photosynthesis, is by far the most common. Some bacteria use anoxygenic photosynthesis, which releases no oxygen.
- Carbon fixation means turning carbon dioxide into sugar (a carbohydrate). Cellular respiration does the opposite: it breaks sugar down to release energy. Photosynthesis takes in light energy, while respiration gives energy out.
- The simple equation is carbon dioxide plus water plus light gives a carbohydrate and oxygen. Water appears on both sides, so it cancels out. The net version is carbon dioxide plus water plus light gives sugar and oxygen.
- The figure sums it up: photosynthesis turns sunlight into chemical energy, splits water to release oxygen, and fixes carbon dioxide into sugar.
- Water is not the only option. Some microbes use arsenite instead of water to supply electrons, so the equation changes for them.
Photosynthesis vs. cellular respiration
Photosynthesis builds sugar and releases oxygen, while cellular respiration breaks sugar down to release energy.
| Photosynthesis | Cellular respiration | |
|---|---|---|
| Energy | Captures light energy and stores it in sugar | Releases chemical energy from nutrients |
| Starting materials | Carbon dioxide and water (plus light) | Nutrients such as carbohydrates, amino acids and fatty acids |
| End products | Carbohydrate (sugar) and oxygen | Carbon dioxide and water |
| Where it happens | Not named in this part | Mitochondria |
What this shows: A flow diagram showing photosynthesis in two stages. Light reactions take in light, water (H2O) and produce oxygen (O2), plus energy carriers ATP and NADPH. These feed the Calvin Cycle, which takes in carbon dioxide (CO2) and produces sugar.
Think of it like this: Think of photosynthesis as a solar-powered kitchen. Light is the stove, carbon dioxide and water are the ingredients, sugar is the finished food, and oxygen is the byproduct released into the air.
Two stages: light and sugar-making
From the article
“Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the hydrogen carrier NADPH and the energy-storage molecule ATP. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.” ¶23
“In the light-independent (or "dark") reactions, the enzyme RuBisCO captures CO2 from the atmosphere and, in a process called the Calvin cycle, uses the newly formed NADPH and releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is[30]: 128” ¶48
In plain words
- Photosynthesis works in two stages. The light-dependent reactions catch light energy and store it in ATP and NADPH. The light-independent reactions then use that stored energy to turn carbon dioxide into sugar.
- ATP and NADPH are energy-carrying molecules. Think of them as charged batteries that the cell can spend later. In the light stage, light knocks electrons loose from chlorophyll, and the electrons pass along a chain that builds up these batteries. Splitting water also releases oxygen.
- The light stage happens in the thylakoid membranes inside chloroplasts. In the usual route (non-cyclic flow), electrons make both ATP and NADPH. In the cyclic route, electrons loop back and make only ATP.
- In the sugar stage, the enzyme RuBisCO grabs carbon dioxide from the air. The Calvin cycle then uses NADPH and ATP to build three-carbon sugars, which later combine into sucrose and starch.
- Making sugar from three carbon dioxide molecules takes 9 ATP and 6 NADPH. The sugar stage is often called the dark reactions, but it does not need light directly. It only needs the energy made in the light stage.
From light to sugar
Light energy is stored in ATP and NADPH, and the Calvin cycle uses them to build sugar.
Light hits chlorophyll in thylakoid membranes
Chlorophyll absorbs a photon and loses an electron.
Light-dependent reactions
Make ATP and NADPH, and split water to release oxygen.
ATP and NADPH energy carriers
Store energy for the next stage.
Calvin cycle (light-independent reactions)
RuBisCO takes in CO2, and ATP and NADPH power sugar-making.
Three-carbon sugars
Later combined into sucrose and starch.
Based on
What this shows: A diagram of the light-dependent reactions on the thylakoid membrane. Light hits photosystems PSII and PSI, electrons move along a chain through plastoquinone, cytochrome b6f and plastocyanin, and protons build up inside the thylakoid lumen. ATP synthase uses that proton flow to make ATP, and ferredoxin-NADP reductase makes NADPH.
What this shows: A Z-scheme energy diagram of the light-dependent reactions. Electrons rise in energy when light strikes photosystem II (P680) and photosystem I (P700), then fall down an electron transport chain that releases energy for ATP synthesis. At the end, electrons reduce NADP+ to NADPH, and water is split to release oxygen.
What this shows: A cycle diagram of the Calvin Cycle with three phases: carbon fixation, reduction, and regeneration of ribulose. Carbon dioxide joins RuBisCO and the cycle uses ATP and NADPH to make sugar, while part of the product regenerates ribulose 5-phosphate so the cycle can continue.
Think of it like this: Think of the light stage as charging rechargeable batteries in the sun. The sugar stage is like using those charged batteries to build something, here sugar from carbon dioxide.
How plants cope with heat, dryness and energy limits
From the article
“In hot and dry conditions, plants close their stomata to prevent water loss. Under these conditions, CO2 will decrease and oxygen gas, produced by the light reactions of photosynthesis, will increase, causing an increase of photorespiration by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and decrease in carbon fixation. Some plants have evolved mechanisms to increase the CO2 concentration in the leaves under these conditions.[32]” ¶56
“Plants usually convert light into chemical energy with a photosynthetic efficiency of 3–6%.[42][43] Absorbed light that is unconverted is dissipated primarily as heat, with a small fraction (1–2%) reemitted as chlorophyll fluorescence at longer (redder) wavelengths. This fact allows measurement of the light reaction of photosynthesis by using chlorophyll fluorometers.[44]” ¶66
In plain words
- On hot, dry days, plants close their stomata, the tiny pores on leaves. This saves water, but less carbon dioxide gets in. Oxygen from the light reactions then builds up, and the plant wastes energy on photorespiration, a process that makes no sugar.
- C4 plants solve this by first grabbing carbon dioxide in mesophyll cells. They pass it to inner bundle sheath cells, where RuBisCO does its work. This keeps RuBisCO away from oxygen, so less sugar is lost. Maize, sorghum, sugarcane and millet are C4 plants.
- CAM plants, such as cacti, open their stomata at night to take in carbon dioxide. They store it and use it during the day, when their stomata stay closed to save water.interpretation
- Plants turn only a small part of sunlight into usable chemical energy, about 3 to 6 percent. Solar panels do better, at roughly 6 to 20 percent for common panels.
- Scientists measure photosynthesis by checking the glow of chlorophyll and by tracking gas flows in and out of leaves. This shows how well each stage is working.
- Light energy in cells moves from one light-catching molecule to the next, like a hopping path, until it reaches the reaction centre where it is stored.
Three ways plants capture carbon dioxide
This table compares how C3, C4 and CAM plants first take in carbon dioxide and where they are found.
| C3 | C4 | CAM | |
|---|---|---|---|
| How carbon dioxide is first captured | Directly by RuBisCO in the Calvin cycle | First added to PEP in mesophyll cells, then moved to bundle sheath cells | Taken in at night when stomata are open, stored as malic acid |
| Example plants | Most plants, such as cotton and sunflower | Maize, sorghum, sugarcane, millet | Cacti and most succulents |
| How common | Over 90% of plants | About 3% of plants | About 16,000 species |
| Stage | Event | Site | Time scale |
|---|---|---|---|
| 1 | Energy transfer in antenna chlorophyll | Thylakoid membranes in the chloroplasts | Femtosecond to picosecond |
| 2 | Transfer of electrons in photochemical reactions | Picosecond to nanosecond | |
| 3 | Electron transport chain and ATP synthesis | Microsecond to millisecond | |
| 4 | Carbon fixation and export of stable products | Stroma of the chloroplasts and the cell cytosol | Millisecond to second |
Think of it like this: Closing stomata on a hot day is like shutting the windows of a busy kitchen to save air-conditioning. The room stays cool, but the cooks soon run short of fresh air.
How photosynthesis evolved and partnered with other life
From the article
“Oxygenic photosynthesis is the main source of oxygen in the Earth's atmosphere, and its earliest appearance is sometimes referred to as the oxygen catastrophe. Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around two billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic, using water as an electron donor, which is oxidized to molecular oxygen in the…” ¶77
“An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with photosynthetic bacteria, including a circular chromosome, prokaryotic-type ribosome, and similar proteins in the photosynthetic reaction center.[70][71] The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis) by early eukaryotic cells to form the first plant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like mitochondria, chloroplasts…” ¶81
In plain words
- The first photosynthetic life probably used hydrogen or hydrogen sulfide, not water, as its source of electrons. Cyanobacteria came later and were the first to split water and release oxygen.interpretation
- The oxygen that cyanobacteria released changed Earth's air. This made complex life possible. Cyanobacteria stayed the main oxygen producers for a very long time.
- Chloroplasts, the green parts that do photosynthesis in plant cells, may have started as bacteria taken in by early cells. This is the endosymbiotic theory: one living thing lives inside another. Chloroplasts still have their own DNA, which looks like cyanobacterial DNA.
- Symbiosis means two kinds of living things living closely together. Corals, sponges and sea anemones host photosynthetic algae. Some sea slugs keep stolen chloroplasts and can live on sunlight alone for months.
- Photosynthesis is spread across many life groups. Algae and some other single-celled and many-celled life can do it. In bacteria, eight groups are known to photosynthesize, but archaea almost never do.
Key dates in the evolution of photosynthesis
This timeline shows the main ages when photosynthesis and oxygen-making life appeared in Earth's history.
About 3.4 billion years ago
Likely early photosynthetic-style life
Fossils of filament-like photosynthetic organisms are dated to about 3.4 billion years ago.
About 2,450 to 2,320 million …
Cyanobacteria gain the ability to use water
The ability to use water for electrons evolved once, in a common ancestor of cyanobacteria.
Around 2 billion years ago
Oxygen-making photosynthesis becomes important
Geological evidence shows oxygenic photosynthesis became important in the Paleoproterozoic era.
About 1.75 billion years ago
First direct evidence of photosynthesis
Thylakoid membranes preserved in ancient rock give direct evidence of photosynthesis.
251 to 66 million years ago
Modern-type oceans and shelves
Dinoflagellates, coccolithophorids and diatoms make marine oxygen production take its modern form.
Based on
What this shows: A microscope photo of moss cells, with many small green chloroplasts visible inside each cell. It shows what photosynthetic cells look like in living tissue.
Think of it like this: Chloroplasts are like a guest who moved into a house long ago, stayed, and now helps run the household by making food for everyone.
Discovery and the factors that limit photosynthesis
From the article
“In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterward, Nicolas-Théodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO2 but also to the incorporation of water. Thus, the basic reaction by which organisms use photosynthesis to produce food…” ¶113
“Total photosynthesis is limited by a range of environmental factors. These include the amount of light available, the amount of leaf area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), the rate at which carbon dioxide can be supplied to the chloroplasts to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis.[114]” ¶138
In plain words
- Jan van Helmont weighed a potted plant and its soil. The soil barely changed, so he guessed the plant's growth came from water. He was partly right: much of the gain came from carbon dioxide and water.
- Later scientists built on this. Joseph Priestley showed that plants can restore air that candles and mice had used up. Jan Ingenhousz found that sunlight is needed for this. Jean Senebier showed that plants take in carbon dioxide and give off oxygen in light.
- Cornelis van Niel showed that photosynthesis depends on light. Samuel Ruben and Martin Kamen used radioactive versions of atoms, called isotopes, as tracers. They found that the oxygen released came from water.
- Melvin Calvin and his team mapped the steps that turn carbon dioxide into sugar. This series of steps is called the Calvin cycle. Calvin later won a Nobel Prize for this work.
- Several things limit how much photosynthesis happens: light, leaf area, the supply of carbon dioxide, water, and temperature. The factor that is in shortest supply is likely the one that sets the pace.interpretation
- When carbon dioxide is low, the enzyme RuBisCO (the protein that captures carbon dioxide) can grab oxygen instead. This wastes energy and makes no sugar. The wasteful process is called photorespiration.
Key steps in understanding photosynthesis
Major discoveries about photosynthesis, shown in the order they were made.
Mid-17th century
Jan van Helmont weighs soil and plant
Suspects plant growth comes from water.
1779
Jan Ingenhousz finds sunlight is needed
1796
Jean Senebier links light to carbon dioxide and oxygen
Plants consume carbon dioxide and release oxygen under light.
1937 and 1939
Robert Hill shows oxygen comes from water
1950 and 1954
Photophosphorylation found in living cells and in isolated …
Light-driven ATP formation is observed.
1961
Melvin Calvin wins Nobel Prize
For mapping the carbon steps of photosynthesis.
Based on
What this shows: A black-and-white photo of Melvin Calvin, a scientist in a suit, seated in his laboratory with glassware on shelves behind him. The caption says he works in his photosynthesis lab, linking to the article's note that Calvin, Benson and Bassham worked out the carbon reduction cycle now called the Calvin cycle.
What this shows: A close-up photo of a green leaf lit from behind, showing its veins clearly. The caption says the leaf is the main site of photosynthesis in plants.
Think of it like this: Like a lemonade stand: the number of cups you can sell is set by whichever ingredient runs out first.
Key takeaways
- Photosynthesis is the main way energy from sunlight enters living things. Cells store that energy in sugars, then break the sugars down through respiration when they need energy.
- The oxygen in our air is a leftover from photosynthesis. Cyanobacteria released it long ago, which made complex life possible.
- Photosynthesis works in two linked stages. Light reactions capture light and make energy carriers called ATP and NADPH. The Calvin cycle then uses those carriers to turn carbon dioxide into sugar.
- Photosynthesis is not very efficient. Plants usually turn only about 3 to 6 percent of light into chemical energy, which is less than typical solar panels.
- Plants in hot, dry places have workarounds. Maize uses the C4 method, and cacti use CAM, to raise carbon dioxide levels inside the leaf.
From the original article
More figures and tables from the article.
What this shows: A labelled cross-section of a chloroplast, the organelle where photosynthesis happens. Numbered labels point to the outer and inner membranes, the stroma fluid, stacked thylakoid discs (grana), starch, ribosomes, DNA, and lipid droplets.
What this shows: A diagram of C4 carbon fixation across two plant cell types. In mesophyll cells, carbon dioxide is captured into PEP and then malate, which moves into bundle sheath cells, where carbon dioxide is released to enter the Calvin Cycle. The diagram also shows the light-dependent reactions supplying ATP and NADPH.
What this shows: A line chart of light absorbance against wavelength in nanometres, from about 400 to 700 nm. The blue line for chlorophyll a peaks sharply near 660 nm, and the red line for chlorophyll b peaks near 460 nm and again near 640 nm. The point is that chlorophylls absorb mainly blue and red light, which powers photosynthesis.
What this shows: A diagram of the photorespiration pathway across three cell compartments, shown with labelled molecules and enzymes. It starts with RuBisCO in the Calvin cycle on the left, which takes in oxygen and produces a two-carbon compound, and the compound moves through enzymes such as GOX, GGT, SHMT and GDC to recycle carbon. The point is that when oxygen is used by RuBisCO, some carbon is lost and must be processed back.
What the article doesn't say
- Efficiency is given only as a range, from 0.1 to 8 percent. It changes with light type, light strength, temperature and carbon dioxide levels.
- The article says some steps of photosynthesis are still not fully understood. It also says when oxygen-making photosynthesis first evolved is still unanswered.
- The article focuses on how photosynthesis works in living things. It mentions only briefly that scientists hope to grow plants with higher yields, so practical uses are not covered in depth.interpretation