Well ascent of sap is due to suction force which is due to transpiration. transpiration is an active process, because cells need to constantly pump ions (actively) into stomata guard cells to keep stomata open. Thus, if you really want to stretch the term, ascent of sap is active
Xylem vessels consist of dead cells and use purely evapotranspiration(and meniscus to an extent) as a suction force to carry water only(with dissolved ions) and as a result can only move water towards the leaves.
Pheolem cells on the other hand are alive and can pump sap in either direction, they use active transport to achieve this, carbohydrates have a lot of trouble diffusing because of their size.
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Only the fittest chickens cross the road.
the process of transpiration is an active transport. Plants need to constantly keep pumping H+ ions from the stomata cells in order to keep the stomata open. If plant cells ran out of ATP (hypothetically) then all stomata would close and the water column would stop. Therefore, it is an active mechanism
The movement of water up a plant is probably caused (primarily) by transpirational pull. This is not an active process for the plant, the energy that evaporates/transpires water from leaves is not provided by the plant - it comes from the environment, the sun and wind.
Plants can stop transpiration, to some extent, by closing guard cells but this is not a direct use of energy to cause transpiration. Think about the cold tap in your bath - if you open the tap the water flows out because of gravitational energy, not because of the energy you expended in opening the tap.
Yes, but think about a different situation: your tap is always closed, and the only way to keep it open is if you exert mechanical force on it. If you stop exerting force, the tap closes by itself again. Sure you are not providing the energy of the water falling, but you do need to expend energy to keep the tap open to allow the water to fall. Sure, that's not how taps work, but it is how stomata work on the molecular level. Try to think of the process as a molecular biology process, not a physical one. Plants are not pipes.
You make a good point, and I agree that plants do need to extend energy to keep stomata open. My own view however is that this is not a direct 'cause' of transpiration. A plant will expend the same amount of energy to keep it's stomata open irrespective of the rate of transpiration, the stomata act like a switch turning transpiration on and off, this is not the same as 'active transport' in it's biological meaning.
For it to be 'active transport' the rate of transpiration would have to be correlated to the amount of energy the plant puts in, and this is not the case.
n.b. (Looking back at the original question, I should make it clear that I've been talking about the flow of water up through the Xylem - not the movement of liquid and solutes in the phloem).
I think I my last answer was confused - I'll try to clarify it a little....
Transpiration, the evaporation of water, mainly, through stomata is a passive process. Although plants expend energy to keep stomata open the actual energy which causes transpiration is environmental - so this would not be considered to be 'active transport'.
Transpiration may pull a limited amount of water up the xylem vessels, however it is unlikely that transpiration by itself is sufficient to explain the volume of water that moves within plants or the heights that the water has to be rasied through. The 'Mass flow hypothesis' is an attempt to do this.
The idea behind mass flow is that at sugar sources, eg. photosynthesising leaves, plants actively pump sugar into the phloem, while at sugar sumps, e.g. non-photosyhthesing roots, the plants actively pump sugar out of the phloem. Consequently in the sink(leaf) water moves by osmosis from the xylem into the phloem while in the sump(root) water moves by osmosis from the phloem into the xylem. This produces regions of high and low hydrostatic pressure in the xylem and phloem and a circular flow of liquid is established. This definitely is active transport.
There is evidence to support the theory from ringing experiments, measurements of hydrostatic pressure and radioactive tracer experiments.
well we're just toying with words here. The point i was making is that you need energy to carry out transpiration. If you don't want to consider it active transport, fine, I don't care. If we agree on the mechanism, that's good enough
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