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    Hi, I am not really sure about how to answer this question :

    The scientists also measured the diameter of the trunk of the tree on which the twig had been growing. The diameter was less at 12.00 than it was at 03.00 hours.Explain why the diameter was less at 12.00 hours

    On the graph the rate of transpiration is the highest at 12 and lower at 3.

    The answer is
    1. Water pulled up trunk/moves up at fast rate;2. (Water column under) tension;3. Sticking/adhesion (between water and)cells/walls/xylem;4. Pulls xylem in;

    So if the rate of transpiration is higher at 12, the adhesive forces between xylem vessel and water molecules will be greater this makes the trunk smaller because the adhesion is greater. The bit about pulling xylem in, is that the result of a greater adhesion??

    Why isn't this have to do with sucrose being used up? This is Q8 in this paper http://cdn.biology-innovation.co.uk/...W-QP-JAN11.pdf
    Thanks !
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    (Original post by coconut64)
    Hi, I am not really sure about how to answer this question :

    The scientists also measured the diameter of the trunk of the tree on which the twig had been growing. The diameter was less at 12.00 than it was at 03.00 hours.Explain why the diameter was less at 12.00 hours

    On the graph the rate of transpiration is the highest at 12 and lower at 3.

    The answer is
    1. Water pulled up trunk/moves up at fast rate;2. (Water column under) tension;3. Sticking/adhesion (between water and)cells/walls/xylem;4. Pulls xylem in;

    So if the rate of transpiration is higher at 12, the adhesive forces between xylem vessel and water molecules will be greater this makes the trunk smaller because the adhesion is greater. The bit about pulling xylem in, is that the result of a greater adhesion??

    Why isn't this have to do with sucrose being used up? This is Q8 in this paper http://cdn.biology-innovation.co.uk/...W-QP-JAN11.pdf
    Thanks !

    It's not so much as a result of adhesion but rather the tension of the water effectively 'pulling' the stream of water upwards. The pressure change causes the xylem to cave in (it doesn't implode though, I can't remember the exact structure of xylem). Tension in this context is similar to suction (I remember some textbooks using these interchangeably)

    Since transpiration is greater, more water is leaving the xylem causing a more lower hydrostatic pressure. This increases tension.

    Adhesion is more of a momentary phenomenon. The water molecule adheres to the xylem by attractive forces but that's for an instant (this ensuring that it doesn't fall down by gravity). Since there are more water molecules, more adhesion is taking place but only at any instant. By the time the water molecules moves up the xylem, the adhesion force is broken.
    Spoiler:
    Show
    A **** analogy but if it works, hey, it works.


    It's like if I run, I pull my shoe up with my foot. There's friction between the shoe and ground which I can overcome and continue running. But if I put crazy glue on my shoes, to keep running I have to pull my shoes up more which stretches the shoe. The water molecule isn't pulling in the xylem purely by it's adhesion force (even on a macromolecular scale, however, if research has proven otherwise then wow)


    Spoiler:
    Show
    Just realised this is a tangent so don't read if you don't want to bother with them.




    It's similar to the reason for having moist alveolar membranes (thanks to water vapour diffusing). When we breathe out, it's because pulmonary pressure is greater than atmospheric pressure so air rushes out of the alveoli. The pressure change 'sucks' in the alveoli which is why is our lungs deflate. The water vapour equalises surface tension across the alveoli preventing the alveolar walls from collapsing in on itself (which is called atelectasis).
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    (Original post by RMNDK)
    It's not so much as a result of adhesion but rather the tension of the water effectively 'pulling' the stream of water upwards. The pressure change causes the xylem to cave in (it doesn't implode though, I can't remember the exact structure of xylem). Tension in this context is similar to suction (I remember some textbooks using these interchangeably)

    Since transpiration is greater, more water is leaving the xylem causing a more lower hydrostatic pressure. This increases tension.

    Adhesion is more of a momentary phenomenon. The water molecule adheres to the xylem by attractive forces but that's for an instant (this ensuring that it doesn't fall down by gravity). Since there are more water molecules, more adhesion is taking place but only at any instant. By the time the water molecules moves up the xylem, the adhesion force is broken.
    Spoiler:
    Show
    A **** analogy but if it works, hey, it works.


    It's like if I run, I pull my shoe up with my foot. There's friction between the shoe and ground which I can overcome and continue running. But if I put crazy glue on my shoes, to keep running I have to pull my shoes up more which stretches the shoe. The water molecule isn't pulling in the xylem purely by it's adhesion force (even on a macromolecular scale, however, if research has proven otherwise then wow)
    Spoiler:
    Show
    Just realised this is a tangent so don't read if you don't want to bother with them.




    It's similar to the reason for having moist alveolar membranes (thanks to water vapour diffusing). When we breathe out, it's because pulmonary pressure is greater than atmospheric pressure so air rushes out of the alveoli. The pressure change 'sucks' in the alveoli which is why is our lungs deflate. The water vapour equalises surface tension across the alveoli preventing the alveolar walls from collapsing in on itself (which is called atelectasis).

    So the greater the tension, the smaller the diameter of the trunk? Why is adhesion mentioned in the mark scheme then if it has nothing to do with it... Thanks
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    (Original post by coconut64)
    So the greater the tension, the smaller the diameter of the trunk? Why is adhesion mentioned in the mark scheme then if it has nothing to do with it... Thanks
    Yes in principle, as the tension force at the top of the trunk increases, the diameter of the trunk decreases.
    I guess adhesion is mentioned because its a factor that increases tension. Because more adhesion is taking place, it prevents water from falling back down the xylem. This means at any moment there is a higher volume of water being "pulled up" to top of the trunk, which means a greater pressure gradient, which means a greater tension force.
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    (Original post by RMNDK)
    Yes in principle, as the tension force at the top of the trunk increases, the diameter of the trunk decreases.
    I guess adhesion is mentioned because its a factor that increases tension. Because more adhesion is taking place, it prevents water from falling back down the xylem. This means at any moment there is a higher volume of water being "pulled up" to top of the trunk, which means a greater pressure gradient, which means a greater tension force.
    But how can a greater tension lead to a smaller trunk. If more water passes through, surely it should be bigger right?
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    (Original post by coconut64)
    But how can a greater tension lead to a smaller trunk. If more water passes through, surely it should be bigger right?
    It's not that it's allowing a higher volume of water at any instant, it's that because it's transpiring at a faster rate that the volume of water transported is greater.

    Say you have two trunks, A and B, and they are both transpiring at the same rate.
    However, trunk B is carrying more water.
    Thus, B will be fatter because at any moment in time, if you were to measure the volume of water at a portion of the xylem, there's more water in B which causes the xylem to bulge outwards and increase the diameter of the trunk.

    Now in a different case, B is transpiring at a greater rate than A. The water in both A and B is the same, but the water in B is moving upwards quicker. Thus, in the same amount of time, B transports more water.

    Water is leaving B and diffuses out of the stomata much quicker than A. Thus there is a much lower pressure at the top of the xylem than there is in A. Because of this, tension is greater because the pressure wants to equalise. There's two ways to equalise the pressure, increase the volume of the substance or decrease the volume of the container (the xylem). This is partly achieved because a bit more water can actually be transported (however, it's small as it offsets the rate of transpiration), but the main reason is because the xylem caves in. This decreases the volume so pressure increases so that the pressure gradient is reduced.
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    (Original post by RMNDK)
    It's not that it's allowing a higher volume of water at any instant, it's that because it's transpiring at a faster rate that the volume of water transported is greater.

    Say you have two trunks, A and B, and they are both transpiring at the same rate.
    However, trunk B is carrying more water.
    Thus, B will be fatter because at any moment in time, if you were to measure the volume of water at a portion of the xylem, there's more water in B which causes the xylem to bulge outwards and increase the diameter of the trunk.

    Now in a different case, B is transpiring at a greater rate than A. The water in both A and B is the same, but the water in B is moving upwards quicker. Thus, in the same amount of time, B transports more water.

    Water is leaving B and diffuses out of the stomata much quicker than A. Thus there is a much lower pressure at the top of the xylem than there is in A. Because of this, tension is greater because the pressure wants to equalise. There's two ways to equalise the pressure, increase the volume of the substance or decrease the volume of the container (the xylem). This is partly achieved because a bit more water can actually be transported (however, it's small as it offsets the rate of transpiration), but the main reason is because the xylem caves in. This decreases the volume so pressure increases so that the pressure gradient is reduced.
    Oh okay thank you.
 
 
 
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