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Photosynthesis

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Photosynthesis

Oxygen — the oxygen that we breath — is just a waste product of what reaction?

Every split second that sunlight hits that leaf, photosynthesis is initiated, bringing energy into the ecosystem. It could be said that this is one of the most important - if not the absolutely most important - biochemical reactions. And it all starts with the leaf.

Factories for Photosynthesis

Photosynthesis is the process that uses energy from the sun, together with carbon dioxide and water, to make glucose and oxygen. The primary role of photosynthesis is to make the carbohydrate, suggesting that oxygen, which is released back into the atmosphere, is just a waste product.

You can think of a single leaf as a photosynthesis factory. A factory has specialized machines to produce a product. It’s also connected to a transportation system that supplies it with raw materials and carries away the finished product. In all these ways, a leaf resembles a factory. The cross section of a leaf in Figure below lets you look inside a leaf “factory.”

There’s more to a leaf than meets the eye. Can you identify the functions of each of the labeled structures in the diagram?

A leaf consists of several different kinds of specialized tissues that work together to make food by photosynthesis. The major tissues are mesophyll, veins, and epidermis.

  • Mesophyll makes up most of the leaf’s interior. This is where photosynthesis occurs. Mesophyll consists mainly of parenchymal cells with chloroplasts.
  • Veins are made primarily of xylem and phloem. They transport water and minerals to the cells of leaves and carry away dissolved sugar.
  • The epidermis of the leaf consists of a single layer of tightly-packed dermal cells. They secrete waxy cuticle to prevent evaporation of water from the leaf. The epidermis has tiny pores called stomata (singular, stoma) that control transpiration and gas exchange with the air. For photosynthesis, stomata must control the transpiration of water vapor and the exchange of carbon dioxide and oxygen. Stomata are flanked by guard cells that swell or shrink by taking in or losing water through osmosis. When they do, they open or close the stomata (see Figure below).

For photosynthesis, stomata must control the transpiration of water vapor and the exchange of carbon dioxide and oxygen. Stomata are flanked by guard cells that swell or shrink by taking in or losing water through osmosis. When they do, they open or close the stomata.

Summary

  • Specialized cells and tissues in leaves work together to perform photosynthesis.

Practice

Use this resource to answer the questions that follow.

  1. Describe the role of the leaf in photosynthesis.
  2. Why do leaves have veins?
  3. What is the purpose of stomata?
  4. Why is the size and shape of the leaf important?
  5. What is the main role of the chloroplast?

Review

1. Explain how a leaf is like a factory.

2. Explain the role of stomata during photosynthesis.

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