Water concentration in plants 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Definitions Stomata – are microscopic pores found on the under side of leaves Guard Cells - control the stomatal openings in the leaf transpiration – is the evaporation of water from plants. It occurs chiefly at the leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis. Adhesion – when water sticks to something other then its self Cohesion – when water molecules stick together Capillary action - is a physical effect caused by the interactions of a liquid with the walls of a thin tube

Guard Cells Three environmental factors regulate the guard cells.  These factors are light, CO2 concentration and water availability Opening and closing stomata - water enters the guard cells by osmosis.  This causes the guard cells to swell.  As the guard cells swell up they bend and open. Closed guard cells lack water. When the stoma is open CO2 can diffuse into the leaf and enter the Calvin Cycle Water vapor also escapes from the stoma by the process of  Transpiration.  As water transpires, other water molecules are pulled up through the plant behind it.

Significance of transpiration Loss of water from plants' surface is called transpiration Water normally leaves the plant as water vapor Transpiration occurs from the stomata About 90% of the water is lost through transpiration 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Transpiration – when plants lose water to the atmosphere as they obtain CO2

Closing the stomata can control water loss The gradient is maintained by solar energy and evaporation of water from the surface of the plant (transpiration) Closing the stomata can control water loss However, the opening of stomata at times helps CO2 to enter A plant must respond to both the need to conserve water and the requirement of CO2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Plants must take up water at their roots to make up for transpiration Plants use osmotic potential to draw water into their roots (root cells are hyperosmotic) Water is pulled up through the xylem into the leaves. Water leaves the plant through the stomata (transpiration).

Water loss from stomata increases with temperature. Adaptations to prevent water loss in dry environments: hairs and spines thick, waxy covering on leaves stomata located deep in pits shedding leaves in dry season wilting or closing leaves