Transpiration and Translocation A-Level Biology AQA Watch

Imofisher
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Please can someone explain this to me in simpler terms than the textbook. I do not understand it. Could you give me mark-scheme points as well?
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Psychology109
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(Original post by Imofisher)
Please can someone explain this to me in simpler terms than the textbook. I do not understand it. Could you give me mark-scheme points as well?
Transpiration is the passive process where water evaporates out of the leaf, through the stomata, causing more water to be drawn from the soil.
The "Tension-Cohesion" theory is used to explain the transpiration pull;

1) Heat from the Sun causes water to evaporate from mesophyll cells and into the air spaces of the leaf.
2) This decreases the water potential of the cells; as a result, water is absorbed from neighbouring cells by osmosis, creating a chain of events until mesophyll cells right next to the xylem absorb water directly from the xylem vessels, creating tension.
3) Water molecules are very cohesive due to the strong hydrogen bonds between water molecules.
4) As a result, a continuous, unbroken column of water is created across the mesophyll cells and down the xylem vessels.
5) As more water evaporates out of the leaf through the stomata, e.g. due to an increase in temperature, more water is absorbed from the soil by root hair cells and transported up the xylem.

Translocation is the process where sugars and dissolved solutes are transported throughout the plant to wherever they are needed. Phloem vessels consist of sieve tube elements - narrow, tube-like structures that consist of living cells, arranged end-to-end. The end walls of the sieve tube elements are perforated to form sieve tube plates. Associated with the sieve tube elements are companion cells.
The "Mass Flow" hypothesis is used to explain translocation:

1) Solutes, e.g. sucrose produced by the source diffuses into the companion cells by facilitated diffusion. In here, sucrose is actively transported into the sieve tube elements using ATP.
2) This decreases the water potential of the sieve tubes, causing water from the companion cells and the xylem to diffuse into the phloem by osmosis, increasing the hydrostatic pressure.
3) Near the sink, sucrose is either used up or converted to starch for storage. Either way, this decreases the water potential of the sink.
4) As a result, water diffuses from the sieve tubes into the sink by osmosis, decreasing the hydrostatic pressure in the phloem.
5) As a result, there is a high hydrostatic pressure gradient as there's a high hydrostatic pressure near the companion cells and a low hydrostatic pressure near the sink. Therefore, we have a mass flow of solutes throughout the plant.
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mini.me
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Um, hi, I don't know what exam board you are doing but for the edexcel biology B spec, it says that we need to know strengths and weaknesses of the mass-flow hypothesis... could anyone help?
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Psychology109
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(Original post by mini.me)
Um, hi, I don't know what exam board you are doing but for the edexcel biology B spec, it says that we need to know strengths and weaknesses of the mass-flow hypothesis... could anyone help?
Heya, i'm doing the AQA specification and we also have to know some evidence for the theories I mentioned above.

Evidence for the Mass flow Hypothesis:
If you cut the stem of a plant, more specifically its xylem vessel(s), the plant can no longer absorb water and dissolved mineral ions from the soil. This is because the column of water in the xylem is interrupted by air particles so hydrogen bonds are unable to form between water molecules.

That's the only evidence I need to know they don't ask much about proof.
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mini.me
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(Original post by Psychology109)
Heya, i'm doing the AQA specification and we also have to know some evidence for the theories I mentioned above.

Evidence for the Mass flow Hypothesis:
If you cut the stem of a plant, more specifically its xylem vessel(s), the plant can no longer absorb water and dissolved mineral ions from the soil. This is because the column of water in the xylem is interrupted by air particles so hydrogen bonds are unable to form between water molecules.

That's the only evidence I need to know they don't ask much about proof.
Ah ok thank you so much!
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