Xylem and phloem

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How does water move up the stem in the xylem?
Water evaporates from mesophyll cells, water molecules form H bonds so stick together (cohesion), W forms a continuous/unbroken column across mesophyll cells+down the xylem, as W evaps more w mols are drawn up bc cohension, transpiration pull
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What evidence is there to support the cohesion-tension theory?
Change in diameter of tree trunks according to rate of transpiration (day=faster rate, more tension, pulls xylem vessels inwards, trunk shrinks). If a xylem vessel is broken and air enters the tree can't take up water bc the column is broken.
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Describe potometer experiment
Leafy shoot cut under water (no water on leaves), potometer filled with water, fit leafy shoot to potometer with rubber tube, seal joints with waterproof jelly, air bubble intro to capillary tube, distance moved by ab in given time measured, cont
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calc mean, calc vol of water lost in tiume, plot on graph, when ab nears junction of reservoir and cap tube tap open and syringe push down so ab pushed back to start
.
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Name the 4 factors that affect the rate of transpiration
Light, temperature, humidity and air movement
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How does light affect the rate of transpiration?
Stomata open in light and close in dark. Higher the light intensity, faster the rate if transpiration
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How does temperature affect the rate of transpiration?
Alters kinetic energy of water molecules and humidity. Higher temp= faster rate
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How does humidity affect the rate of transpiration?
Affects water potential gradient between air-spaces in leaf and the atmosphere. Higher humidity=slower rate
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How does air movement affect the rate of transpiration?
Changes water potential gradient by altering rate that moist air is removed. More air movement= faster rate
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Define translocation
The process by which organic molecules and some mineral ions are transported in a plant
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Define sources
Site of sugar production
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Define sinks
Where sugars are used directly or stored for future use
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What are the 3 stages of the mass flow theory?
1. Transfer of sucrose into sieve elements from photosynthesising tissue 2. mass flow of sucrose through sieve tube elements 3. transfer of sucrose from the sieve tube elements into storage or other sink cells
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Explain the 1st stage
Sucrose diffuses down conc gradient via FD from photos cells to companion cells, hydrogen ions AT from comp cells to spaces within cell walls, h ions then diffuse down conc grad through carrier proteins into sieve elements, sucrose molecules cont
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transported with the hydrogen ions (co-transport)
.
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Explain the 2nd stage
Sieve tubes have a lower water potential bc sucrose, xylem has higher WP so water moves into sieve tubes creating high hydrostatic pressure within them, hydrostatic gradient created so mass flow down it in the sieve tubes
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What can mass flow be affected by?
Metabolic poisons
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Explain the 3rd stage
Sucrose is transported by companion cells, out of sieve tubes and into sink cells
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Describe the ringing experiment
Section of outer layer (protective layer and phloem) are removed, region above missing ring begins to swell, samples rich in sugars, non-photosynthetic tissues below missing ring begin to die but those above don't
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What does this prove?
Shows that the phloem is responsible for transloaction
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What are tracer experiments?
Radioactive isotopes useful for tracing movement of substances in plants. Incorporated into sugars traced using autoradiography.
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Other cards in this set

Card 2

Front

What evidence is there to support the cohesion-tension theory?

Back

Change in diameter of tree trunks according to rate of transpiration (day=faster rate, more tension, pulls xylem vessels inwards, trunk shrinks). If a xylem vessel is broken and air enters the tree can't take up water bc the column is broken.

Card 3

Front

Describe potometer experiment

Back

Preview of the front of card 3

Card 4

Front

calc mean, calc vol of water lost in tiume, plot on graph, when ab nears junction of reservoir and cap tube tap open and syringe push down so ab pushed back to start

Back

Preview of the front of card 4

Card 5

Front

Name the 4 factors that affect the rate of transpiration

Back

Preview of the front of card 5
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