1
Van Helmont’s willow
Scrub through five years. The tree gains about 74 kg. How much does the soil lose?
Photosynthesis in one sentence
Photosynthesis is the process plants, algae and cyanobacteria use to turn light energy into chemical energy: they take in carbon dioxide and water, capture energy from light, and make sugar while releasing oxygen.1
The photosynthesis equation
Written as a balanced chemical equation:
6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
| Reactants (inputs) | Products (outputs) |
|---|---|
| Carbon dioxide (from the air, through the stomata) | Glucose and other sugars (the plant's food and building material) |
| Water (from the soil, through the roots) | Oxygen (released into the air) |
| Light energy (absorbed by chlorophyll) |
The equation is a summary. The real process takes dozens of steps, split into two stages.1
Where does photosynthesis take place?
In plants, it happens mainly in the leaves, in a middle layer of cells called the mesophyll. Carbon dioxide enters, and oxygen leaves, through tiny pores called stomata, which sit mostly on the underside of the leaf. Each stoma is flanked by two guard cells that open and close it.1
Inside the mesophyll cells are chloroplasts. Each chloroplast holds stacks of disc-shaped thylakoids surrounded by a fluid called the stroma. Stage one happens in the thylakoid membranes; stage two happens in the stroma.1
Why are leaves green?
The thylakoid membranes contain chlorophyll, a pigment that absorbs mostly blue and red light and reflects green light. The green light bounces back to our eyes, so leaves look green.3
The steps of photosynthesis
Stage 1: The light-dependent reactions (thylakoids)
- Chlorophyll absorbs light energy.
- That energy is used to split water molecules. This releases oxygen, so the oxygen plants give off comes from water.34
- The captured energy is stored in two short-lived carrier molecules, ATP and NADPH, which last only millionths of a second.2
Stage 2: The Calvin cycle, or light-independent reactions (stroma)
- Fixation: the enzyme RuBisCO attaches CO₂ to a five-carbon molecule called RuBP.
- Reduction: ATP and NADPH from stage 1 are spent to turn the result into a three-carbon sugar, G3P.
- Regeneration: most G3P is recycled to rebuild RuBP. Every three turns of the cycle, one G3P leaves to become glucose, sucrose, starch and cellulose.2
The Calvin cycle doesn't use light directly, but it depends on the ATP and NADPH made in the light. Without light, it soon stops.1
What limits the speed of photosynthesis?
The scarcest input sets the pace. More light speeds photosynthesis up only until the plant can't use it any faster. On hot, dry days, guard cells close the stomata to save water, which also cuts off carbon dioxide and slows photosynthesis.16
Common misconceptions
“Plants get their food from the soil.” Mostly not. In the 1600s, Jan Baptist van Helmont grew a willow for five years. It gained about 74 kg while the soil lost only 57 g.7 Most of a plant's mass is carbon taken from CO₂ in the air, plus water.2 Soil supplies water and small amounts of minerals.
“The oxygen comes from carbon dioxide.” No. The oxygen released comes from splitting water.3
“Plants use green light best.” The opposite. Green is the color chlorophyll absorbs least.3
Photosynthesis and cellular respiration
They are two halves of one cycle. Photosynthesis stores the Sun's energy in sugar and releases oxygen. Cellular respiration, which plants do too, uses oxygen to break sugar down and release that energy, giving off CO₂ and water.2
Why is photosynthesis important?
- It powers about 99% of Earth's ecosystems. Nearly every food chain starts with it.1
- Ocean phytoplankton and cyanobacteria are thought to produce between a third and a half of atmospheric oxygen.4
- Coal, oil and natural gas are the remains of organisms that captured sunlight hundreds of millions of years ago.15
- Oxygen-producing cyanobacteria transformed Earth's atmosphere, starting with the Great Oxidation Event about 2.4 billion years ago.5
FAQ
How does photosynthesis work in simple words?
A plant uses the energy in light to turn carbon dioxide from the air and water from the soil into sugar, its food. Oxygen is released as a by-product.
What are the reactants and products of photosynthesis?
The reactants are carbon dioxide and water, powered by light energy. The products are glucose (and other sugars) and oxygen.
Where does photosynthesis take place?
In chloroplasts, mostly in the mesophyll cells of leaves. The light-dependent reactions happen in the thylakoid membranes and the Calvin cycle happens in the stroma.
What are the two stages of photosynthesis?
The light-dependent reactions, which capture light energy, split water and make ATP and NADPH; and the Calvin cycle, which uses that ATP and NADPH to build sugar from carbon dioxide.
Do humans do photosynthesis?
No. Humans and other animals are heterotrophs: we get our energy by eating plants, or animals that ate plants. Only plants, algae and some bacteria photosynthesize.
Does photosynthesis happen at night?
The light-dependent reactions need light, so they stop in the dark. The Calvin cycle doesn't use light directly, but it quickly runs out of the ATP and NADPH made in the light.
Where does the oxygen from photosynthesis come from?
From water. Light energy is used to split water molecules, and the oxygen atoms are released as oxygen gas.
Did this help it click?
Sources
- OpenStax, Biology 2e, 8.1 Overview of Photosynthesis
- OpenStax, Biology 2e, 8.3 Using Light Energy to Make Organic Molecules
- National Geographic Education, Photosynthesis
- Nature Education (Scitable), Photosynthetic Cells
- Encyclopaedia Britannica, Photosynthesis
- Wikipedia, Photosynthetic efficiency (light saturation)
- Wikipedia, Jan Baptist van Helmont (willow experiment)