2.8+Cells-photosynthesis

Photosynthesis
In order to carry on [|photosynthesis], green plants need a supply of **carbon dioxide** and a means of disposing of **oxygen**. In order to carry on [|cellular respiration], plant cells need **oxygen** and a means of disposing of **carbon dioxide** (just as animal cells do). Unlike animals, plants have no specialized organs for gas exchange (with the few inevitable exceptions!). The are several reasons they can get along without them: 
 * Each part of the plant takes care of its own gas exchange needs. Although plants have an elaborate liquid transport system, it does not participate in gas transport.
 * Roots, stems, and leaves respire at rates much lower than are characteristic of animals. Only during photosynthesis are large volumes of gases exchanged and each leaf is well adapted to take care of its own needs.
 * The distance that gases must diffuse in even a large plant is not great. Each living cell in the plant is located close to the surface. While obvious for leaves, it is also true for stems. The only **living** cells in the stem are organized in thin layers just beneath the bark. The cells in the interior are dead and serve only to provide mechanical support. [[image:http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/stoma.gif width="297" height="432" align="right"]]
 * Most of the living cells in a plant have at least part of their surface exposed to air. The loose packing of [|parenchyma] cells in leaves, stems, and roots provides an interconnecting system of air spaces. Gases diffuse through air several thousand times faster than through water. Once oxygen and carbon dioxide reach the network of intercellular air spaces (arrows), they diffuse rapidly through them.
 * Oxygen and carbon dioxide also pass through the cell wall and plasma membrane of the cell by diffusion.


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