Xylem Flashcards
What is transpiration?
the loss of water vapour from the stomata by evaporation →moves out of leaf down water potential gradient
What are the factors affecting transpiration rate?
Light intensity
Temperature
Humidity
Wind
How does light intensity affect transpiration rate?
positive correlation →more light causes more stomata to open - larger surface area for evaporation
- when its dark stomata usually close so there is little transpiration
How does temperature affect transpiration rate?
positive correlation →more heat means more kinetic energy (faster moving molecules) - evaporate from ells inside leaf faster which increases water potential gradient between the inside and outside of the leaf making water diffuse out of the leaf faster
How does humidity affect transpiration rate?
negative correlation →the lower the humidity the faster the transpiration - more water vapour in air will make water potential more positive on the outside of the leaf which reduces water potential gradient
How does wind affect transpiration rate?
positive correlation →more wind will blow away humid air containing water vapour, maintaining the water potential gradient.
What equipment is used to measure transpiration rate?
a potometer measures the rat eof uptake of water from a plant which is assumed to be directly related to water loss by the leaves (transpiration)
What is the process of measuring transpiration rate?
1.) cut a shoot underwater to prevent air entering the xylem and breaking the water column
2.) potometer is filled with water and all air bubbles removed
3.) the cut leafy plant is then attached to the potometer using rubber seals and petroleum jelly to make the equipment air tight- is waterproof which prevents water leaking out to make sure it can only leave by evaporation
4.) one air bubble introduced into equpment → distance travelled is recorded
5.) distance and volume of cylinder used to find volume of water loss to work out transpiration rate = volume/time
How can we test transpiration rates in different conditions?
reservoir used to move the bubble to reset the experiment
What is the function of the xylem?
transports water and mineral ions in solution
What is the structure of the xylem?
- long, tube like structures formed from dead cells joined end to end (no cytoplasm)
- cell walls thickened with lignin which is in rings and allows cells to stretch and grown longer (waterproof substance) and spaces between the rings allows water to enter easily
- no end wall on cells making an uninterrupted tube so water can pass through middle easily
What is cohesion tension theory?
1.) water molecules are hydrogen bonded to each other (between hydrogen and oxygen molecules) and stick together (creates cohesion) → water travels in continuous water column →as water evaporates from mesophyll cells into the air spaces more water is drawn up by cohesion
- a column of water is pulled up the xylem due to transpiration pull which puts the xylem under tension
2.) adhesion of water → water sticks to other molecules and water adheres to xylem walls → narrower the xylem , the bugger the impact of capillarity.
3.) as water moves into roots by osmosis it increases the volume of liquid in root so pressure in the root increases →forces water above it upwards
What is the overall process of movement of water up the xylem?
1.) water evaporates out of stomata(transpiration)so more water is pulled up by xylem to replace it
2.) due to hydrogen bonds between H2 O molecules they were cohesive which creates column of water within the xylem
3.) H2O molecules adhere to walls of xylem which helps pull water column upwards
4.) as column of water is pulled up the xylem it creates tension →xylem narrows/ pulls xylem in
