Xylem and Transpiration Flashcards
What part of the stem does water travel up?
Xylem
Name the 6 adaptations of the xylem and explain them
-have no cytoplasm/hollow: reduces resistance to flow of water to ensure a continuous column
-xylem vessels are stacked on top of each other: ensure a continuous column of water
-lignin in cell walls: makes walls waterproof and allows them to withstand tension
-xylem is dead: no water potential gradient is created by a living cell
-narrow tubes: increases SA for adhesion
-xylem cells have pores in side walls: enables the sideways flow of water
What is transpiration?
the evaporation of water from the leaf through stomata. It drives water transport up the xylem.
explain the process of the cohesion-tension theory
-water is lost from leaf because of transpiration
-this lowers the water potential of mesophyll cells
-water moves from xylem into mesophyll cells by osmosis
-as water molecules move out of the xylem into the leaf, water is pulled up the xylem which creates a ‘negative pressure’ called tension
-water molecules cohere together by hydrogen bonds to form a continuous water column
-hydrogen bonding causes adhesion of water molecules to walls of xylem. This causes the xylem to be pulled inwards.
what is cohesion?
attraction of water molecules to each other
what is adhesion?
attraction of water molecules to other substances
How does light intensity affect transpiration?
Increase in light intensity= increase in rate of transpiration. More stomata open in high light intensity for gas exchange, so more water evaporates out.
How does humidity affect transpiration?
Increase in humidity= decrease in rate of transpiration. There is a reduced water potential gradient at high humidity so less water evaporates out.
How does temperature affect transpiration?
increase in temperature= increase in rate of transpiration. high temp means water molecules have more kinetic energy, reducing the humidity and increasing evaporation.
how does air movement affect transpiration?
increase in air movement= increase in rate of transpiration. High air movement reduces water vapour concentration at the stomata, so increased water potential gradient.
How do you make a potometer more reliable when measuring rate of transpiration?
-seal joints to ensure airtight
-cut the shoot under water
-dry off leaves
-insert plant into apparatus under water
-ensure no air bubbles are present
-shut the tap
-note where bubble is at start position
Why does a potometer actually measure water uptake, not water loss by transpiration?
Not all water is lost via transpiration:
-water is used for support/ turgidity
-water used in photosynthesis
-water produced in respiration
Using a potometer, a student wanted to calculate the rate of water uptake by the shoot in cm3 per minute. what measurement should they take?
-distance moved by bubble
-time taken
-radius/ diameter/ area of capillary tube.
