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Transport of substances into and out of the leaf
The leaf is designed to transport water, sugars, carbon dioxide and oxygen across its surface. Each of these involves separate processes and cells which we will discuss below.
Movement of oxygen and carbon dioxide Stomata are the site of gaseous exchange in the leaf. There are two major metabolic processes that take place in plants that involve the exchange of oxygen and carbon dioxide:
- Photosynthesis:takes place during the day when the chloroplasts can absorb radiant energy from the sun. Photosynthesis requires carbon dioxide and releases oxygen as a by-product. Therefore, during daylight hours, the concentration of carbon dioxide is low in the leaf and the concentration of oxygen is high. As a result, during the day, carbon dioxide enters the leaf and oxygen is released.
- Cellular Respiration:occurs continuously throughout the day and night. Cellular respiration requires oxygen and releases carbon dioxide as a waste product. During the day, the plant can use some of the oxygen from photosynthesis for cellular respiration. During the night, when photosynthesis stops, the concentration of oxygen in the plant drops and the concentration gradient switches: the concentration of carbon dioxide is high and the concentration of oxygen is low. Therefore at night time, oxygen enters the leaves, and carbon dioxide is released.
Movement of water into leaf
Water is constantly being lost by the leaf through transpiration. This results in the cells in the mesophyll having a lower water concentration than the vascular bundles. Water thus moves down a concentration gradient from the xylem vessel into the living cells of the mesophyll layer and to the surface of the mesophyll cell walls. This causes water to move up from the stem by means of transpirational pull. The movement of water is maintained because water molecules constantly evaporate into leaf inter-cellular air space out of the stomatal pore and into the atmosphere.
Movement of sugars
Chloroplasts found in the palisade layer capture radiant energy from the sun to make glucose via photosynthesis. This glucose is used to make the simple sugar sucrose. Sucrose is transported to the rest of the plant through the phloem vessels present in the vascular tissue in the leaf. Plants convert sugars to starch for long-term storage.
Opening and closing of stomata: The opening and closing of the stomata is important for gaseous exchange, transpiration and the movement of sugars. Stomata open when it is bright and when there is high humidity. When water concentration in the soil is low, indicating that the plant is dry, chemical changes in the plant result in the closing of the stomata.
Figure 4.41: Confocal microscope image of guard cells and stoma. The red-staining region is chlorophyll.