9.1 Xylem Transport Flashcards
What is transpiration?
Transpiration is the loss of water vapour from the stems and leaves of plants
- What role does light energy play in transpiration?
Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata
- What does the evaporation of water cause? (transpiration)
New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
- What is the transpiration stream and what role does it play? transpiration
Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream)
What are stomata and what are their role?
Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis)
What requires stomata to be open and how does this affect transpiration?
As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis
What is transpiration a consequence of?
Hence, transpiration is an inevitable consequence of gas exchange in the leaf
How is water lost from the plant?
Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata
- Where is the first place that transpiration occurs and what happens?
Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
- Where does the water vapour go? What is created?
evaporation
This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
- What does this negative pressure create? What is its use?
evaporation
This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
- What causes the tension?
evaporation
The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls
How is water loss regulated?
The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata
What roles do guard cells play in the regulation of water loss? (specifically decreasing)
Guard cells flank the stomata and can obstruct the opening by becoming increasingly flaccid in response to cellular signals
What is released when plants wilt?
When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
What is the role of abscisic acid?
Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
What does loss of turgor do to the stomatal pore?
A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
When will transpiration rates be higher?
Transpiration rates will be higher when stomatal pores are open than when they are closed
What will affect levels of transpiration? (direct link, not just a factor)
Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration
What factors will affect transpiration?
humidity
temperature
light intensity
wind
What is the transpiration stream?
The flow of water through the xylem from the roots to the leaf, against gravity, is called the transpiration stream
What two key properties of water help it travel through the xylem?
Water rises through xylem vessels due to two key properties of water – cohesion and adhesion
What is cohesion?
Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)
How do water molecules exhibit cohesion?
Water molecules are polar and can form a type of intermolecular association called a hydrogen bond