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A close-up view of a leaf, showing the veins. Each vein is made up of a bundle of tubes, the transport vessels. One type is the...Read More >>A close-up view of a leaf, showing the veins. Each vein is made up of a bundle of tubes, the transport vessels. One type is the xylem, which brings water and minerals from the roots up into the leaf. The other is the phloem, through which sap, water with dissolved sugars produced by photosynthesis, passes from the leaf to the rest of the plant. Surrounding the tubes is a layer of woody tissue called lignin, which helps keep the leaf rigid.A plant obtains its energy by a process known as photosynthesis—a word meaning “making with light”. It takes in sunlight and carbon dioxide from the air. Water and nutrients taken up by the roots are carried up through the plant's stem, which contains tiny tubes. In a chemical reaction, the carbon combines with water to make sugars, which are food for the plant. Most plants have broad, flat surfaces, such as leaves or fronds, where this happens. The leaves contain tiny tubes, some bringing water from the roots up into the leaf (xylem), others taking the sap (water with the dissolved sugars produced by photosynthesis in the leaf) to the rest of the plant (phloem).

A diagram showing how photosynthesis works
Palisade cells containing chloroplasts and stomata.

Inside a leaf

A plant’s leaves are “light-powered food factories”. They are broad and flat so that as much light as possible falls on them. Inside the tall palisade cells in the leaf’s upper surface are a large number of tiny blobs or discs called chloroplasts. These contain chlorophyll, a green substance that absorbs the energy in light. The plant uses this energy to make a chemical reaction, called photosynthesis. Water, taken up from the soil, and carbon dioxide, taken in from the air, join together to form glucose, a type of sugar, which contains lots of energy in chemical form. The plant then uses this sugar to power its life activities.

A stoma in a leaf, see though a microscope
A cross-section through a leaf, greatly magnified so we can see the palisade cells and transport vessels.
The carbon dioxide for photosynthesis comes from the air. It seeps into the leaf through tiny holes in its lower surface, known as stomata (the plural of stoma). In addition to sugar, photosynthesis also produces oxygen and water vapour, which seeps out into the air. Living things including ourselves need oxygen to survive. Plants help to top up its level in the air.
A bust of Jan Ingenhousz


Photosynthesis was discovered by Dutch biologist Jan Ingenhousz (1730–1799). He found that plants in sunlight absorbed carbon dioxide and produced fresh oxygen that animals could breathe. Plants in the dark, however, did the opposite: they absorbed oxygen and gave off carbon dioxide. More oxygen was given off in the light than carbon dioxide given off in the dark, meaning that plants produced far more oxygen than they absorbed.
Ingenhousz showed that plants did not produce new mass (for example, leaves or stems) by absorbing matter from the ground, since the soil did not lose mass as a plant grew. Instead, new plant growth must have come from sunlight. In the presence of sunlight, plants captured carbon from carbon dioxide in the air and converted it into new plant matter.

 Chris Jarvis

The sequence of chemical changes in photosynthesis takes place in a millionth of a second.

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