9.3 transpiration Flashcards
what is transpiration
loss of water vapour from leaves and stems of plants
What is a transpiration stream?
an uninterrupted stream of water and solutes taken up by the roots and transported via the xylem to the leaves where it evaporates into the air
How does the transpiration stream begin (leafs to xylem)
water molecules evaporate from surface of mesophyll cells into air spaces in leafs and move out of stomata into the air via diffusion by conc gradient
Loss of water lowers water potential of mesophyll cell, causing water to move into cell from an adjacent cell via osmosis
–> repeats across leaf to xylem
How does xylem use capillary action?
adhesion: water forming hydrogen bonds with carbohydrates in walls of xylem vessels
cohesion: water form hydrogen bonds with each other so tend to stick
combined effects= water exhibiting capillary action
How is water drawn up the xylem against gravity
due to capillary action, transpiration pull: water being drawn from xylem to replace water lost via evaporation
–> creates tension in xylem which helps move water across roots from soil
Evidence for cohesion-tension theory
diameter of trees changes depending on the time of day
–> highest point of transpiration= highest tension in xylem vessels= tree shrinks in diameter
–> lowest point of transpiration= low tension in xylem vessels= tree grows in diameter
When xylem is broken & air is pulled in, plant no longer moves water up stem due to cohesive forces being broken
What does transpiration deliver?
water and mineral ions dissolved in water
Stomata and their structure
rate of transpiration is controlled by the opening and closing of stomatal pores (turgor-driven process)
–> inner wall is thicker= less flexibility so when guard cells swell, the cell forms two bean like shapes, creating the pore
Factors affecting transpiraiton
light: more light= more stomata open= more water vapour lost= increased evaporation
Relative humidity: more humid= less water vapour evaporated due to lower conc gradient
