Transpiration. 1) Discuss the role of stomata in transpiration (7) 2) What is meant by Transpiration Stream? (3) 3) Describe the factors affecting the rate of transpiration (10)

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Transpiration

) Discuss the role of stomata in transpiration (7)

2) What is meant by Transpiration Stream? (3)

3) Describe the factors affecting the rate of transpiration (10)

) Leaves are the primary photosynthetic organs of most plants. Leaf surfaces are equipped with small openings or pores called stomata, which allow carbon dioxide to enter the leaf and oxygen to escape to facilitate photosynthesis. In addition, water is lost through stomata during a process called transpiration. It is estimated that approximately 99% of the water absorbed by the roots of the plant is lost by the leaves in transpiration.

Plants must exchange gasses through their leaves in order to conduct photosynthesis and respiration; they also must permit evaporation / transpiration in order to assist in the movement of water from the ground to leaves, where it is needed to build carbohydrates. Yet, if transpiration is uncontrolled, a plant may become dehydrated and die.

The stoma is a pore formed by a pair of guard cells. The guard cells are located in both epidermal layers of the leaf, with a higher concentration on the underside (this is a strategy to reduce water loss). Plants contain a range of 10,000 - 100,000 stomata per cm2. Stomatal opening is regulated by turgor in the guard cells. The inner walls of the stomata are fused at both ends, and, as the cells fill with water, the fused walls stay the same length, while the outer walls stretch, causing the cells to curve and opening the pore between them wider. As to differ from this process, when guard cells lose turgidity, they loose their curvy structure, closing the pore. This stretching is limited by the strength of lignin fibres surrounding the cells. Water enters and leaves guard cells in response to changes in solute concentrations. Potassium is actively pumped into the guard cells from surrounding cells. This in turn increases solute concentration; as a result, water potential decreases and water enters the guard cell against a pressure gradient.
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2) Water is absorbed from the root and is transported to all areas of the plant; this passage of water is called the transpiration stream. Water is absorbed by the roots of a plant, which possess many root hairs with large surface areas for high

absorption of water. Osmosis occurs across a concentration gradient, and this is the case here, when water is required and the concentration of water in the roots is low, water is absorbed from the higher concentration of water found in the ground. This same process happens for the concentration difference between ...

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