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Photosynthesis

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.

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

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