TL;DR:
Photosynthesis builds sugar from carbon dioxide and water using light energy:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.The light reactions (thylakoid membranes) split water, release O₂ and make ATP and NADPH. The Calvin cycle (stroma) uses them to turn CO₂ into sugar.
The oxygen released comes from water, not from carbon dioxide.
A tree grows for decades, adding tonnes of wood, and most of that material came from carbon dioxide in the air and from water, not from the soil. The trick is photosynthesis: plants use sunlight to join carbon dioxide and water into sugar, and release oxygen on the way.
The overall equation
The summary reaction is 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. Glucose, C₆H₁₂O₆, is the sugar made. Most of the energy of sunlight ends up stored in its chemical bonds.
It is the reverse of the equation for burning glucose, which is why the next topic, cellular respiration and ATP, looks like a mirror image. Photosynthesis stores energy and respiration releases it.
Worked example: checking the equation
Does the equation balance? Count each element, as in balancing chemical equations by counting atoms.
Carbon: left
6 × 1 = 6. Right: 6 in glucose. Equal.Hydrogen: left
6 × 2 = 12. Right: 12 in glucose. Equal.Oxygen: left
6 × 2 + 6 × 1 = 18. Right:6in glucose plus6 × 2 = 12in O₂, which is 18. Equal.Mass check with molar masses CO₂ = 44.01, H₂O = 18.02, glucose = 180.16, O₂ = 32.00: left
6 × 44.01 + 6 × 18.02 = 264.06 + 108.12 = 372.18 g. Right:180.16 + 6 × 32.00 = 180.16 + 192.00 = 372.16 g.
The two totals agree to rounding. So one mole of glucose (180 g) needs 6 moles of CO₂ (about 264 g) and 6 moles of water (about 108 g). Plants therefore gain most of their dry mass from the carbon in carbon dioxide.
Where it happens: chloroplasts
In plant cells, photosynthesis takes place in chloroplasts. Inside each is a stack of flat sacs called thylakoids, whose membranes hold the green pigment chlorophyll, and a surrounding fluid called the stroma. Chlorophyll absorbs mainly red and blue light and reflects green, which is why leaves look green.
Stage 1: the light reactions
In the thylakoid membranes, chlorophyll absorbs light, and that energy excites electrons. Those electrons pass along a chain of carriers, and the energy released is used to make two energy carriers: ATP and NADPH.
To replace the electrons that chlorophyll has lost, the plant splits water: 2 H₂O → 4 H⁺ + 4 e⁻ + O₂. The oxygen gas is released as a by-product. That is where the oxygen in the air from plants comes from.
Stage 2: the Calvin cycle
In the stroma, the Calvin cycle uses the ATP and NADPH from stage 1 to build sugar from CO₂. An enzyme attaches CO₂ to a five-carbon molecule, and a series of steps rearranges the carbons into three-carbon sugars, with some used to rebuild the starting molecule so the cycle can continue. Glucose and other sugars are assembled from these three-carbon sugars.
The Calvin cycle does not use light directly, but it depends on the ATP and NADPH that light made, so it normally runs in the light.
Where does the oxygen come from?
A tempting guess is that the O₂ comes from carbon dioxide. It does not. Experiments using water labelled with a heavy form of oxygen showed that the oxygen released was the labelled kind. Water is split in the light reactions, and the oxygen in the CO₂ ends up in the sugar and in water made in the Calvin cycle.
A more detailed equation is 6 CO₂ + 12 H₂O → C₆H₁₂O₆ + 6 O₂ + 6 H₂O. Twelve water molecules are split or used and six new ones are made, so the net is 6 water molecules used, which is the summary equation.
Several things affect how fast a plant photosynthesises: how much light reaches the leaf, how much carbon dioxide is available, and the temperature. Each can become the limiting factor. If light is dim, adding more carbon dioxide will not help, and the reverse is also true. Too high a temperature can damage the enzymes of the Calvin cycle.
Leaves take in CO₂ through tiny pores called stomata. When the pores are open to let CO₂ in, water vapour escapes. Plants constantly balance these two needs, and it explains why plants in hot, dry places close their stomata during the hottest part of the day.
How to check your answer
Atoms. Each element must balance, 6 C, 12 H and 18 O.
Place. Light reactions in the thylakoid membranes, Calvin cycle in the stroma.
Inputs and outputs. The light reactions take light and water and give ATP, NADPH and O₂. The Calvin cycle takes CO₂, ATP and NADPH and gives sugar.
Direction. Photosynthesis is the reverse of respiration.
Mistakes that cost marks
Saying the oxygen comes from carbon dioxide.
Saying plants do photosynthesis instead of respiration. Plants do both.
Confusing the stroma and thylakoids.
Saying the Calvin cycle needs no light at all, when it depends on products of the light reactions.
Forgetting that chlorophyll reflects green light.
Where you will use it next
Photosynthesis feeds almost every food chain and sets the oxygen level of the air. It links to growth and cell division, as in mitosis and meiosis compared, and its energy accounting echoes endothermic and exothermic reactions.
Frequently asked questions
Do plants photosynthesise at night?
No, the light reactions need light. Plants still respire day and night, using some of the sugar they made.
Is photosynthesis only done by plants?
No. Algae and some bacteria also do it, and in water they provide a large part of the world's oxygen.
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