The Cell Cycle

Page
8

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

Stimuli for 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:

light,

CO2 depletion, and

an internal “clock” in guard cells.

All eukaryotic organisms have internal clocks; circadian rhyths are 24-hour cycles.

Slide 54

Effects of Transpiration on Wilting and Leaf Temperature

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

Adaptations That Reduce Evaporative Water Loss

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

Sugars are transported from leaves and other sources to sites of use or storage

The products of photosynthesis are transported through phloem by the process of translocation.

Slide 58

Movement from Sugar Sources to Sugar Sinks

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.