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Nature and farming · How do plants live?

Photosynthesis and light

Does a huge tree get its mass from the soil or from the air? Explore photosynthesis, from van Helmont's willow experiment to why leaves are green, how light limits growth and why not every plant wants full sun.

  • 16 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

A walnut tree starts out as a single walnut that fits in the palm of your hand. Fifty years later, with its trunk, branches and roots, it weighs several tonnes. Where did all that mass come from? Most of us would first answer "from the soil". But in that case, shouldn't the soil around the tree shrink over the years, leaving a great pit around the trunk? We never see a pit like that in any garden.

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No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Plant growth

The best conditions for a bean

Move the light, water and temperature sliders. Look at each factor's score and at which factor is limiting growth. When growth reaches its highest level, press Check.

Prediction

Predict first

You give a bean plant full sun, but the soil is bone dry. What do you think decides its growth now?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Plant growth

How much light does a fern want?

Move the light, water and temperature sliders. As you raise the light step by step, watch how the light score changes. When growth reaches its highest level, press Check.

Prediction

Predict first

What do you think would happen if you moved a fern from a dim corner into full sun?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Lab

The leaf factory

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Frequently asked

5 questions

Doesn't a plant get its food from the soil?

It takes water and minerals from the soil, but it makes its food, sugar, itself in its leaves. The raw materials of that sugar are carbon dioxide from the air and water. Van Helmont's willow gained more than 70 kilograms while the soil lost only a few tens of grams.

Isn't fertiliser the plant's food?

No. Fertiliser is a supplement that makes up for missing minerals such as nitrogen, phosphorus and potassium. The plant uses these minerals to build its proteins and chlorophyll, but it gets its energy and most of its mass from the sugar it makes by photosynthesis.

Do plants respire at night too?

Yes, plants respire day and night. In daylight photosynthesis runs faster than respiration, so overall the plant gives off oxygen. In the dark photosynthesis stops and the plant gives off carbon dioxide overall.

Why do leaves look green?

Chlorophyll absorbs blue-violet and red light the most. It uses some green light too, but it reflects and transmits green far more than the other colours. The green light that bounces back is what reaches your eyes.

Does more light always mean faster growth?

No. Growth is limited by whichever factor is scarcest at the moment: if water, temperature or carbon dioxide falls short, adding light does not help. For shade plants such as ferns, full sun is even harmful; their leaves turn yellow and their edges scorch.