(a) Changes in guard cell shape and stomatal opening and closing
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
(b) Role of potassium ions in stomatal opening and closing
Generally, stomata open during the day and close at night to minimize water loss.
Stomatal opening at dawn is triggered by:
CO2 depletion, and
an internal “clock” in guard cells.
All eukaryotic organisms have internal clocks; circadian rhyths are 24-hour cycles.
Plants lose a large amount of water by transpiration.
If the lost water is not replaced by sufficient transport of water, the plant will lose water and wilt.
Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes.
Xerophytes are plants adapted to arid climates.
They have leaf modifications that reduce the rate of transpiration.
Some plants use a specialized form of photosynthesis called crassulacean acid metabolism CAM where stomatal gas exchange occurs at night.
Xerophytic - Desert Plants Adaptations
Oleander leaf cross section and flowers
Upper epidermal tissue
Ocotillo leaves after a heavy rain
Lower epidermal tissue
Ocotillo after heavy rain
Old man cactus
Ocotillo - leafless
The products of photosynthesis are transported through phloem by the process of translocation.
Phloem sap is an aqueous solution that is high in sucrose = disaccharide.
It travels from a sugar source to a sugar sink: Source to sink
A sugar source is an organ that is a net producer of sugar, such as mature leaves.
A sugar sink is an organ that is a net consumer or storer of sugar, such as a tuber or bulb.
A storage organ can be both a sugar sink in summer and sugar source in winter.