Vacuole
Guard cell
(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
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
K+
Slide 53
Generally, stomata open during the day and close at night to minimize water loss.
Stomatal opening at dawn is triggered by:
light,
CO2 depletion, and
an internal “clock” in guard cells.
All eukaryotic organisms have internal clocks; circadian rhyths are 24-hour cycles.
Slide 54
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.
Slide 55
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.
Slide 56
Xerophytic - Desert Plants Adaptations
Oleander leaf cross section and flowers
Cuticle
Upper epidermal tissue
Ocotillo leaves after a heavy rain
Trichomes (“hairs”)
Crypt
Stomata recessed
Lower epidermal tissue
100 µm
Ocotillo after heavy rain
Old man cactus
Ocotillo - leafless
Slide 57
The products of photosynthesis are transported through phloem by the process of translocation.
Slide 58
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.