Start here: how does a plant build itself?#
Seventy-two seconds, start to finish. Watch it before you read anything below, because everything after this is the same story told slowly.
How Does a Plant Build Itself?
- 0:00How does a plant build itself?
- 0:05Where in a plant does photosynthesis happen?
- 0:07What is chlorophyll, and where is it found inside a leaf?
- 0:11What does chlorophyll do when light hits it?
- 0:15What happens to the energy a photon delivers?
- 0:24Where does the oxygen a plant releases come from?
- 0:29What does a plant do with the energy it captures?
- 0:36Which sugar does a plant build?
- 0:39What are the two things a plant can do with glucose?
- 0:48If not from the soil, where does a plant's mass come from?
- 0:53What supplies the energy for photosynthesis?
- 0:55Where does a plant get the carbon it is built from?
- 0:58What does photosynthesis turn light, water and air into?
Video made by A Wandering Mind, a Kestrel Exams LLC channel. The visuals are animated and the narration is a synthetic voice. Embedded through youtube-nocookie.com, so YouTube does not set a tracking cookie unless you press play.
What photosynthesis actually is#
Think of a leaf as a workshop that takes deliveries. Three things arrive: light from above, water pulled up from the roots, and carbon dioxide gas drifting in through pores on the underside of the leaf. One thing goes out as waste, oxygen, and one thing gets built and kept, sugar.
Photosynthesis is that whole conversion. The word itself is two plain words stuck together: photo means light, and synthesis means putting together. Putting something together using light. That is the entire name, and it describes the entire job.
It happens in a specific place. Inside a leaf cell are small green compartments called chloroplasts, and packed inside those is a green pigment called chlorophyll. Chlorophyll is what actually catches the light. Everything else in the process follows from that one catch.
Light is the power supply, water is the electron supply, and carbon dioxide is the building material. Almost every exam question about photosynthesis is asking which of those three a given step is using.
The two halves, and why they get mixed up#
Photosynthesis is usually taught as one process, which is why it feels like a wall of names. It is easier as two jobs done by two different crews in two different parts of the chloroplast.
| Light-dependent reactions | Calvin cycle | |
|---|---|---|
| Job | Capture energy | Spend it on building |
| Needs | Light and water | Carbon dioxide, ATP, NADPH |
| Makes | ATP, NADPH, and oxygen as waste | Sugar |
| Where | The stacked membranes inside the chloroplast | The fluid around those stacks |
The link between them is two rechargeable carriers, ATP and NADPH. A carrier is exactly what it sounds like: a molecule that picks something up in one place and puts it down in another. The first half charges them, the second half drains them, and they go back for more. Nothing in the second half touches light directly.
The Calvin cycle is sometimes called the dark reactions, and the name has misled students for decades. It does not need darkness and it does not mainly happen at night. It needs ATP and NADPH, and the light-dependent reactions are what supply those, so in practice it runs while the sun is up. Dark means it does not use light directly, not that it happens after dark.
Where the oxygen comes from#
Here is the part almost everyone gets backwards. A plant takes in carbon dioxide, which is full of oxygen atoms, and it releases oxygen. The obvious conclusion is that the plant strips the oxygen off the carbon dioxide and lets it go. That is not what happens.
The oxygen comes from water. Chlorophyll, having caught a photon, loses an electron, and the plant has to replace it. It gets the replacement by pulling water apart. What is left over once the electrons are taken is oxygen, and the plant has no use for it, so it goes out through the same pores the carbon dioxide came in through.
The oxygen you are breathing right now is the discarded half of a water molecule.
This is not a story, it is a measured result. In 1941 Ruben, Randall, Kamen and Hyde fed plants water made with a heavy oxygen isotope, O-18, and the released oxygen carried the heavy label. When they put the heavy label in the carbon dioxide instead, the released oxygen did not carry it. The label follows the water.
Where a plant's mass comes from#
A tree weighs tonnes. The soil around it does not go down by tonnes. People have noticed this for four hundred years and mostly guessed wrong about it.
Most of a plant's dry mass is carbon, and that carbon arrives as a gas. Carbon dioxide comes in through the pores, the Calvin cycle assembles it into glucose, and glucose becomes either fuel the plant burns or the material it builds cell walls, stems, roots and leaves out of. Water contributes too, and the soil supplies mineral nutrients the plant genuinely cannot live without. But the bulk of the body came out of the air.
That is the sentence worth keeping: a tree is mostly air, assembled by light.
The rest of this module#
Module 07 covers photosynthesis one question at a time. Each short answers exactly one of them, and each lands on this page as it publishes. The opener above is the whole story at speed; these are the slow versions.
- Live07.0 · How does a plant build itself? The whole module in 72 seconds.
- Next07.1 · How does carbon dioxide get into a leaf? Stomata, and the trade a plant makes every time it opens one.
- Planned07.2 · What is a chloroplast?
- Planned07.3 · What does chlorophyll do?
- Planned07.4 · Why do plants look green?
- Planned07.5 · How does water get from the roots to the leaves?
- Planned07.6 · What are the light-dependent reactions?
- Planned07.7 · Why does a plant split water?
- Planned07.8 · Where does the oxygen you breathe come from?
- Planned07.9 · What do ATP and NADPH carry?
- Planned07.10 · What is the Calvin cycle?
- Planned07.11 · How does a plant build glucose?
- Planned07.12 · What slows photosynthesis down?
Key terms#
- Photosynthesis
- Building sugar out of carbon dioxide and water, using light as the power supply.
- Chloroplast
- The small green compartment inside a plant cell where photosynthesis happens.
- Chlorophyll
- The green pigment inside a chloroplast that absorbs the light.
- Stoma (plural stomata)
- An adjustable pore, mostly on the underside of a leaf, that lets carbon dioxide in and water vapour out.
- Light-dependent reactions
- The first half: light is caught, water is split, and two carriers get charged.
- ATP and NADPH
- The two rechargeable carriers that move energy and electrons from the first half to the second.
- Calvin cycle
- The second half: carbon dioxide is assembled into sugar using the charged carriers.
- Glucose
- The sugar a plant builds, spent either as fuel or as building material.
Check yourself#
Answer each one out loud before you open it. Reading the answer without trying first feels like learning and is not.
1. A plant releases oxygen. Which molecule did that oxygen come from, and how do we know?
Water. Ruben and colleagues labelled the oxygen in water with the heavy isotope O-18 in 1941 and found the label in the released oxygen. Labelling the carbon dioxide instead did not put the label in the released oxygen.
2. Name the two carriers that connect the two halves, and say which half charges them.
ATP and NADPH. The light-dependent reactions charge them; the Calvin cycle spends them.
3. A tree gains 500 kg over ten years. Where did most of that mass come from?
The air, as carbon dioxide. Soil supplies water and mineral nutrients, but not the bulk of the dry mass.
4. Why is "dark reactions" a bad name for the Calvin cycle?
Because it suggests the cycle runs at night. It does not use light directly, but it depends on ATP and NADPH from the light-dependent reactions, so it runs while the light is on.
Frequently asked questions#
Where does a plant's mass come from? Mostly from the air, as carbon dioxide. Soil supplies water and mineral nutrients, which matter, but not the bulk of the dry mass.
Does the oxygen a plant releases come from carbon dioxide? No, from water, established by isotope labelling in 1941.
Why are plants green? Chlorophyll absorbs red and blue light strongly and green light poorly, so green is what gets reflected. The colour you see is the light the plant did not want.
What is the difference between the light-dependent reactions and the Calvin cycle? The first captures energy from light and splits water. The second spends that energy building sugar from carbon dioxide.
Is the Calvin cycle the same as the dark reactions? Same reactions, misleading name. See the callout above.
Do plants photosynthesise at night? No. They do keep respiring, which is why a plant is a net oxygen producer across a whole day rather than in every hour of it.
Sources#
Ruben, S., Randall, M., Kamen, M. and Hyde, J. L. (1941). Heavy oxygen (O18) as a tracer in the study of photosynthesis. Journal of the American Chemical Society, 63(3), 877–879. The isotope experiment establishing that photosynthetic oxygen comes from water rather than from carbon dioxide.
Clark, M. A., Douglas, M. and Choi, J. (2018). Biology 2e, Chapter 8: Photosynthesis. OpenStax, Rice University. Open-access textbook treatment of the light-dependent reactions and carbon fixation.