Is sunlight or water keeping this plant alive?
After you watchIs sunlight or water keeping this plant alive?
The short answer
A plant dies in a dark closet even when you water it because plants don't drink their food — they build it. A leaf is a tiny food factory that uses light energy to turn carbon dioxide from the air and water from the roots into sugar, the food the plant lives on. With the light off the factory makes no sugar, so the plant burns through its stored food and starves, even though the soil is soaking wet.
Try this next
- What if you gave the dark plant a lamp partway through, after its sugar tank was almost empty? In the race, predict first: does a late lamp save it or is it too late? Then run it and watch whether the sugar tank can climb back up once you switch the light on.
- What if a plant had bright light but you never watered it at all? Push the water down to nothing and predict what the sugar tank does. Water is an ingredient too, so watch whether a sunny, bone-dry plant can still build any sugar.
- What if the closet weren't fully dark, just dim? Try a low light setting instead of zero and predict: does the tank still drain, just slower? Watch whether dim light is enough to keep the factory ahead of the plant's hunger.
The whole story
How it works
Plants make their own food through photosynthesis. In their leaves, light energy powers a reaction that combines carbon dioxide gas pulled from the air with water drawn up from the roots, building sugar and releasing oxygen. That sugar is the plant's food and fuel. Like every living thing, a plant constantly burns some of its food just to stay alive, day and night. In the dark there is no light to run the factory, so no new sugar is made; the plant keeps spending its stored sugar until it runs out and starves. Water still climbs the plant and keeps its cells plump, but water carries no energy, so watering alone can never feed a plant in the dark.
What people get wrong
Many people think a plant gets its food by drinking water and soaking up soil through its roots, so watering it should keep it fed. In fact the roots only supply water and a few dissolved minerals. The plant's actual food is sugar that it builds in its leaves using light, air, and water together. That is why a perfectly watered plant still starves in the dark: its food factory has no power, and water is just one ingredient, never the energy.
The catch
Water is a genuine must-have: it is one of the ingredients the leaf builds sugar from, and it keeps cells plump so the plant doesn't wilt — without it the plant dies too. But water alone can't feed a plant because it carries no energy. Light is the energy that runs the whole factory, but light alone isn't enough either: with no water or no air the leaf has nothing to build sugar from. A plant needs light and water and air together, and light is simply the part people most often forget.
Questions kids ask
If a plant can't make food in the dark, how does it stay alive at night?
During the day a plant builds extra sugar and stores some of it as starch. At night it lives off that stored food. A single night is fine, but in constant darkness it never refills its store, so it eventually runs out and starves.
Do plants really get their food from the air and not the soil?
Mostly, yes. The bulk of a plant's body is built from carbon that comes out of carbon dioxide gas in the air, joined with water. The soil mainly provides water and a few minerals, not the plant's main food. That's why a big tree's wood comes mostly from the air, not from the dirt it grew in.
Will any light work, like a lamp instead of the sun?
Yes. A plant's food factory runs on light energy, and bright indoor or grow lights can power it just like the sun. The plant doesn't care where the light comes from, only that there is enough of it to keep building sugar.
So can a plant survive on just light with no water?
No. Light is the energy, but water is one of the ingredients the leaf builds sugar from, and it keeps the plant's cells plump. A plant needs light, water, and air from the air together — take away any one of them and it dies.
Talk about it
- We watered this plant every day and it still died in the corner — guess what it was actually missing, and why water couldn't fix it.
- Where do you think most of a big tree's weight comes from — the soil, the water, or somewhere you can't see?
- If a plant makes its own food, what do you think it's doing with that food all night long while it's dark?
For grown-ups
This is photosynthesis. Inside the chloroplasts of leaf cells, light energy drives the reaction of carbon dioxide (taken from the air through pores called stomata) with water into glucose, releasing oxygen: roughly 6CO2 + 6H2O + light -> C6H12O6 + 6O2. The plant then respires that glucose for energy day and night. With no light the plant cannot fix any carbon, so it lives off stored starch and eventually starves even with ample water. The roots supply water and a few dissolved minerals, never the bulk food — which is also why a tree's wood is built mostly from carbon taken out of the air, not soaked up from the soil.
Keep going
What else makes you wonder?
- If a plant builds its body mostly from air, where does all the wood in a giant tree actually come from?
- Plants make food from light — could an animal ever do that too, or is there a reason none of them grow leaves?
- Plants give off oxygen while they build sugar in the light, but what are they doing to the air at night when the factory is off?