Transport In Plants Flashcards
(21 cards)
Structure of xylem
F: transport water and ions in plants
1) long tube like structures formed from dead cells joined end to end
2) no end walls = create an uninterrupted tube allow water to pass up in middle easily = increase sA for water to transport
3) thick cell wall made of lignin(a sugar) = keeps vessels rigid and strengthen
Cohesion tension theory in transpiration
1) water evaporates from leaves at the top
2) creates tension = causes a stronger upwards pull of water = creates continuous water column
3) as water pass along= tension pulls in xylem walls making vessel narrower= increase action of capillarity
4) water have hydrogen bonds that causes cohesion as polar water molecules can stick together in the vessel tube = allow the continuous column of water to move up the continuous tube
What is the effect on diameter of xylem vessel causes it to change ?
- tension of water = in day more transpiration = more tension= narrow diameter = allow continuous flow along the column
- night= less transpiration/ tension = diameter increase = limit water loss
What is the effect of root pressure on transpiration ?
- more water moves into xylem by osmosis from soil = increase volume of vessel =PRESSURE increase TOO inside the root = higher pressure forces water moves upwards= faster transpiration rate
4 factors affect transpiration Rate
- light intensity
- temperature
- humidity
- wind /air flow
How does light intensity affect transpiration ?
- the more light = faster rate
- stomata opens to allow CO2 in for photosynthesis, when it’s dark, stomata closes to limit water loss= so less transpiration when less light
Effect of temperature on rate of transpiration ?
- higher temperature = faster rate
- warmer causes molecules gain more KE to vibrate around = causes quicker evaporation from inside leaf
= cause an increase in w.p gradient between inside and outside of leaf, causes more water to evaporate diffuse out of the leaf down the conc.grad
How does humidity affect rate of transpiration ?
- lower the humidity = drier = faster rate
- negative correlation = if air is dry= causes increase in w.p grad between leaf and air = water easier to diffuse out by osmosis high to low conc = increase transpiration
How does wind affect transpiration ?
- more wind= more transpiration= increase rate
- lots of air movement causes water molecules to vibrate more around the stomata = increases w.p gradient= faster rate
Explain why diameter of branch decreased when rate of water flow increase ? (2)
1) increase transpiration = produce a greater tension
2) reduce adhesive forces between xylem and water
Describe and explain the changes in rate of water flow in branch from day to night hours (3)
1) rate of flow increases to max at mid day then decreases
2) because increase transpiration
3) transpiration creates tension= increases transpirational pull = allow water to pull up of continuous column of xylem
Explain how water enters a plant roots from the soil and travels through the xylem (5)
1) water enters root hair cells of the plants roots
2) by osmosis
3) down water potential gradient, high w.p in soil into low w.p in root cells
4) via apoplastic cell wall pathway
5) and symplastic cytoplasm pathway
Use knowledge of cohesion tension theory, explain the changes in circumference of a tree from day to night (5)
1) circumference of tree decrease because of increase of light in day causes stomata to open
2) water evaporates, more water loss by transpiration = causes a stronger tensional pull in water columns
3) hydrogen bonding of water molecules cause cohesion that holds water together = stronger pull cause diameter of column to decrease
4) diameter increase at night as decrease in sunlight= water evaporation slows
5) causes tension falls and so water column collapse
Explain why phloem pressure is reduced at the hottest time of the day with understanding of transpiration (3)
1) hotter = higher rate of evaporation
2) more water lost through stomata/ greater tensional pull in xylem
3) causes less water movement from xylem to phloem as insufficient water potential in phloem to draw water
Explain how pressure is generated inside the phloem (3)
1) sucrose actively transported into phloem from C.C
2) lower w.p in phloem
3) causes water to move from xylem to phloem down w.p grad by osmosis
Describe the mass flow hypothesis for the mechanisms of translocation in plants (5)
1) sucrose actively transported from source of leaf into phloem
2) by companion cells
3) reduces w.p of phloem causes water to move across from xylem to phloem by osmosis
4) causes increase pressure, forces mass movement of sucrose towards the sink
5) sucrose used in roots for respiration or storage as starch
Describe the cohesion tension theory of water transport in xylem (6)
1) water loss from leaf by evaporation and transpiration of water by stomata
2) lowers w.p at the leaves
3) creates a greater tensional pull of water in xylem
4) water molecules stick together by hydrogen bonding
5) forms a continuous water column
6) adhesion of water cause them to stick to walls of xylem
Describe the ringing experiment
Remove a ring of phloem of a plant= cause a bulge to appear around top part of the chopped off area
= extract the sucrose out from that area = found sucrose accumulate in the bulge and the bottom part of the ring did not have any sucrose = prove sucrose is moving downwards
Describe tracer experiment
Exposed 14C radioactive carbon to use by plants for photosynthesis for few days = use auto radiographs to see where the sucrose made from photosynthesis is moving in the plant
Tracing the movement of radioactive sucrose along the phloem = prove phloem is moving from source to sink
Explain why values for pressure in xylem are negative (1)
Inside xylem pressure is lower than atmospheric pressure
Explain importance of xylem being kept open as a continuous tube (3)
1) allow unbroken water column
2) cohesion of polar water molecules form H bonds between molecules
3) allow evaporation to create more tension to pull water up
