[asker94]

I have been arguing with my friends that wether ascent of sap is diffusion or active transport?
please help me coz nothing is on the internet about it?

[MrMistery]

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

[futurezoologist]

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.

[freshbiology]

Movement water upwards in the xylem is caused by transpirational pull. Water is being pulled from above. Water is therefore, under tension as it moves upwards. It is by no means an active process.

[MrMistery]

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

[Dougalbod]

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.

Dougal

[robsabba]

I agree with Dougal. Transpiration is primarily driven by pressure gradients. Therefore, it is basically not an "active" process.

[slindsey]

😆 think its diffusion

[MrMistery]

@dougalbod

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.

[Dougalbod]

MrMistery

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.

Dougal

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).

[Dougalbod]

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.

Dougal

[MrMistery]

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

[jwalin]

another question
a transpirational pull tends to pull the water up. its just like sucking a fine straw. but if there are 50m long trees what then. i come to know and think of tat the pull is just not enough. try taking a longer straw. harder its to bring the water to the tap.
the column tend to break.
in the larger plants there would be higher probabilities of air spaces. would there be?? i do think so. what then.
i also do understand that cohesion and adhesion play a great role.

[JackBean]

Where will you take the air bubbles inside of a tree? I guess the length doesn’t really matter.

[jwalin]

i do not think so. the straw example.
if you think keeping a little physics in mind what about the external resistance???
air, by xylem???
someone please help.

[JackBean]

Yeah, it will be harder to bring water to 2m instead of 1, but it does not matter, whether you move it for 1m in 2m or 10m long tube 😉
In my opinion

[jwalin]

??
unclear
??

[JackBean]

Imagine, that you have some luggage and you are supposed to take it by stairs to 1st or 2nd floor. When will you have to use more energy? I guess in the second case, right?
But what if you have to take it from basement to 1st floor OR from 5th to 6th? I guess here you will need the same energy in both cases, don’t you think so?

[jwalin]

that is right.
but taking from ground to 50 th is different from ground to first.

[JackBean]

sure, that is the first case 😉 I’m talking about that, that if plant once start (as a very little seedling), than it just continues and doesn’t care, how high

[abarnett]

active transport is when the particles go into the cell using a special pathway because they are too big 😛

[addisongrace22]

Does active transport occur in plant cells as well as animal cells? Or is it specific to certain types of cells?

[Vsalazar]

well I think its diffusion because it spreads widely. So The sap would spread. That’s just my opinion. 😀

[whawth]

The example of the straw example works but when the tree uses water it has different compartments in it that each use the water so it would be like using a straw with tiny holes in it all the way up the straw and instead of pulling in air the pull the water out of the inside of the straw so if there is any air bubbles they would be move to above the water the come out the pores in the tree bark at the top. After all the tree breaths through the bark.

[Michaelguan]

it is very interesting to see your discussion. i learned much about the spirit. \
Cheers!!!

[Darwin420]

Yea….Mistery brings a good point. The act of the water evaporating is not active but in order for transpiration to occur, stomata must be open and for stomata to be open ATP is required.

[jwalin]

ohhh that slipped my mind
very true

mass flow moves the water but the evaporation pull (suction) is due to transpiration that needs energy

thank you soo much

[jwalin]

still have one question
there are 50m long trees what then. i come to know and think of tat the pull is just not enough. try taking a longer straw. harder its to bring the water to the tap.
the column tend to break.

[jwalin]

oh found the answer
its just that more energy would required

[daddyjames]

Transpiration is a passive process.

What has not being taken into account is the primary reason why the stomata open and close. Stomata open and close to allow gas exchange for photosynthesis to occur. It lets carbon dioxide into the plant and oxygen out of the plant.

Carbon dioxide and oxygen, as well as water, does not passively diffuse into the plant.

Obviously, carbon dioxide is necessary for photosynthesis.

If the amount of oxygen increases within the plant (due to the splitting of water during photosynthesis) the photosynthetic rate decreases until it completely ceases through a process called photorespiration. This is due, in part, to the fact that RuBisCo (the enzyme responsible for the first step in carbon fixation) also fixes oxygen.

The opening and closing of the stomata to regulate carbon dioxide and oxygen levels within the plant allows for the "inadvertant" escape of water through diffusion. Under stressful conditions, drought, the plant will close stomata as an attempt to regulate water loss. But eventually, the plant has to open the stomata to regulate carbon dioxide/oxygen levels and additional water is lost. That, in part, is why plants have evolved different mechanisms (C3 versus C4 and CAM plants) to minimize water loss from the need to photosynthesize.

Under extremely high levels of humidity, the rate of transpiration decreases even though the stomata are open.

Although the opening and closing of the stomata may be an active process. Tranpiration itself is a passive process. The water is not being actively pumped out of the plant through the stomata.

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