Home›Biology›Plants

How photosynthesis works

In the 1600s Jan Baptist van Helmont grew a willow in 200 pounds of dried soil; after five years the tree had gained 164 pounds and the soil had lost about 2 ounces.

Where does a tree get its mass?

If you had to guess where the wood of a tree comes from, the soil seems like the obvious answer. Van Helmont tested that idea. He dried 200 pounds of earth in an oven, planted a willow shoot that weighed five pounds, and watered it only with rain or distilled water for five years. The tree then weighed about 169 pounds and 3 ounces, yet the dried soil came back to 200 pounds less about 2 ounces. He concluded that the wood, bark and roots had come from water alone.

He was half right. Water does go into the plant, but most of the dry mass of a tree is carbon, and that carbon comes from the air. Plants, algae and cyanobacteria take carbon dioxide gas, which is low in energy, and use the energy of sunlight to build it into sugars, which are high in energy. OpenStax Biology calls them photoautotrophs, self-feeders that use light. Animals, fungi and most bacteria are heterotrophs: they live on sugars that photosynthesis made first.

The overall reaction is usually written as 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ with light as the energy source. Six molecules of carbon dioxide and six of water become one molecule of glucose and six of oxygen. The equation looks simple, but the real process runs through many steps with short-lived intermediate molecules.

BackgroundPlants that repair air. In 1771 the English clergyman Joseph Priestley burned a candle in a closed container until the air would no longer let it burn. He put a sprig of mint inside, and after several days a candle could burn in that air again. In 1779 the Dutch physician Jan Ingenhousz published Experiments upon Vegetables, showing that the plant restored the air only in light, and only with its green parts. In 1782 it was shown that the plant makes this gas at the expense of another one, fixed air, which had been identified the year before as carbon dioxide.

1600s: Van Helmont weighs a willow and its soil over five years.
1771: Priestley: a mint sprig restores air spoiled by a candle.
1779: Ingenhousz: only in light, only the green parts.
1782: Oxygen is shown to form at the expense of fixed air (carbon dioxide).
1905: Blackman: limiting factors.

Inside the chloroplast

In a leaf, photosynthesis happens mainly in the middle layer, the mesophyll, inside organelles called chloroplasts. A chloroplast has an outer and an inner membrane. Inside are flat discs called thylakoids, stacked into piles called grana, and around them is a fluid called the stroma. Chlorophyll sits in the thylakoid membranes. Chlorophyll a absorbs blue and red light from the two ends of the visible range (400 to 700 nanometres) and reflects green, which is why leaves look green.

A chloroplast (not to scale). A: a granum, a stack of thylakoids where the light-dependent reactions run. B: the stroma, where the Calvin cycle builds sugar. C: the double membrane.

Two stages

The light-dependent reactions happen in the thylakoid membranes. Light energy is passed from pigment to pigment, 300 to 400 chlorophyll molecules in each antenna, to a reaction centre. There the energy knocks an electron loose, and the missing electron is replaced by splitting water. That split is where the oxygen comes from. The energy is stored for a moment in two carrier molecules, ATP and NADPH. About 10 percent of the oxygen is used by the leaf’s own mitochondria; the rest goes into the air.

The second stage, the Calvin cycle, happens in the stroma. An enzyme called RuBisCO attaches one carbon dioxide molecule to a five-carbon sugar, RuBP, and the result splits into two three-carbon molecules. ATP and NADPH then turn these into a sugar called G3P. Each turn of the cycle fixes only one carbon, so it takes three turns to export one G3P and six turns to make enough carbon for one glucose. The old name “dark reaction” is misleading: it suggests that the cycle runs only at night or without light, yet the cycle needs the ATP and NADPH made by the light reactions.

Drag to turn. Scroll or pinch to zoom.

Move the light slider and watch the curve. In dim light the rate climbs quickly; in bright light it levels off because something else runs short.

The lab uses a made-up teaching model, with the rate given as a percentage of the best possible. It shows the shape of real light curves, not the numbers of any particular plant.

Key points

Plants, algae and cyanobacteria build sugar from CO₂ and water using light.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Light-dependent reactions: thylakoid membranes, water split, O₂ released, ATP and NADPH made.
Calvin cycle: stroma, RuBisCO fixes CO₂, three turns per G3P, six per glucose.

Photoautotroph	An organism that makes its own food using light.
Chloroplast	The organelle where photosynthesis happens.
Thylakoid	Flat membrane disc holding chlorophyll; stacks are grana.
Stroma	The fluid around the grana, site of the Calvin cycle.
RuBisCO	The enzyme that fixes CO₂ onto RuBP.
G3P	Three-carbon sugar made by the Calvin cycle.

Where does most of the dry mass of a tree come from?
- Carbon dioxide from the air
- Minerals from the soil
- Sunlight itself
- Oxygen from the air
Van Helmont’s willow went from 5 pounds to about 169 pounds. How many pounds did it gain (ignore the 3 ounces)?
The soil lost about 2 ounces (1 pound = 16 ounces). How many times larger was the tree’s gain of 164 pounds than the soil’s loss?
Which gas did Priestley’s mint sprig restore so a candle could burn again?
- Oxygen
- Carbon dioxide
- Nitrogen
- Water vapour
What did Ingenhousz add in 1779?
- Plants restore air only in light and only with green parts
- Plants take in carbon dioxide
- Water is split in photosynthesis
- The Calvin cycle
Where do the light-dependent reactions take place?
- Thylakoid membranes
- Stroma
- Cell wall
- Nucleus
The oxygen released by a plant comes from which molecule?
- Water
- Carbon dioxide
- Glucose
- ATP
How many turns of the Calvin cycle supply the carbon for one glucose molecule?
Why do leaves look green?
- Chlorophyll absorbs blue and red light and reflects green
- Chlorophyll absorbs green light
- Leaves give off green light
- Carotenoids absorb red light
Why is “dark reaction” now considered a poor name for the Calvin cycle?
- It needs ATP and NADPH from the light reactions, so it does not run independently of light
- It only happens at night
- It produces light
- It happens outside the chloroplast

Sources for this course (16)

-fix5: gravity calculation corrected using rho*g*h = 1000*9.8*h Pa
the cited textbook has a tenfold coefficient error. Sources (checked 2026-10-17): OpenStax Biology 2e 8.1 Overview of Photosynthesis (photoautotrophs, mesophyll, stomata on the underside, guard cells, chloroplast, thylakoid, granum, stroma, two stages; openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis)
8.2 The Light-Dependent Reactions (400 to 700 nm, chlorophyll a absorbs blue and red, carotenoids, 300 to 400 chlorophylls per antenna, water split at PSII, about 10 percent of the oxygen used by the leaf's mitochondria; .../8-2-the-light-dependent-reactions-of-photosynthesis)
8.3 Using Light Energy to Make Organic Molecules (RuBisCO, 3-PGA, G3P, three turns per G3P and six per glucose, "dark reaction" outdated, cacti fix CO2 at night; .../8-3-using-light-energy-to-make-organic-molecules)
30.3 Roots (tap and fibrous roots, root cap, three zones in the first centimetre, root hairs, endodermis and Casparian strip; .../30-3-roots)
30.4 Leaves (lamina, petiole, midrib, parallel and reticulate venation, Ginkgo, cuticle, trichomes, palisade in one to three layers, spongy mesophyll, guard cells the only epidermal cells with chloroplasts, xylem and phloem, needles and spines; .../30-4-leaves)
30.5 Transport of Water and Solutes (redwoods near 116 m, turgor 0.6 to 0.8 MPa and up to 1.5 MPa = 210 psi vs tyres 30 to 34 psi, gravity -0.1 MPa per metre, -2 MPa at the leaf surface, cohesion-tension, up to 90 percent of the water lost, sources and sinks, phloem sap up to 30 percent sugar; .../30-5-transport-of-water-and-solutes-in-plants)
31.3 Nutritional Adaptations (rhizobia, nodules, nitrogenase, biological fixation 65 percent of nitrogen used in agriculture, mycorrhizae in more than 80 percent of land plants, up to 20 percent of the carbon; .../31-3-nutritional-adaptations-of-plants)
32.2 Pollination and Fertilization (self and cross pollination, bees cannot see red, nectar guides, bats, birds, wind, water, at least 25,000 orchid species, double fertilization 2n zygote and 3n endosperm, seed parts, scutellum, fruit types, dispersal, dormancy; .../32-2-pollination-and-fertilization)
Britannica, Photosynthesis: history (Priestley 1771, Ingenhousz 1779, 1782: oxygen formed at the expense of fixed air, identified the year before as carbon dioxide) (britannica.com/science/photosynthesis/History-of-the-study-of-photosynthesis)
Smithsonian Libraries, Ingenhousz, Experiments upon Vegetables, 1779 (library.si.edu/digital-library/book/experimentsuponv00inge)
van Helmont's account (200 pounds of earth, 5 pound willow, 169 pounds and about 3 ounces after five years, soil short by about 2 ounces)
quoted from Howe 1965 by S. G. Saupe, College of St. Benedict and St. John's University (employees.csbsju.edu/ssaupe/biol115/van_helmont.htm)
F. F. Blackman, Optima and Limiting Factors, Annals of Botany 19:281-296, April 1905 (doi.org/10.1093/oxfordjournals.aob.a089000)
NOAA Global Monitoring Laboratory, Trends in CO2 (gml.noaa.gov/ccgg/trends/: Mauna Loa September 2026 monthly mean 426.07 ppm)
NOAA Climate.gov, Climate change: atmospheric carbon dioxide (280 ppm or less before the Industrial Revolution of the mid-1700s, from ice cores; climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide).

Checked: 2026-10-17