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I am a 5th grader learning about the plant transport mechanics and I learnt that the xylem is made up of dead cells.So if water travels up the xylem and water travels only 1 way, is it like how water travel up a tissue paper?

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I'm in year 5 too; and (with a bit of help of my dad) I have some bits of answers (my challenge of the day).

A paper tissue is principally made of cellulose. And cellulose is a polar molecule. And so is water. So, the paper will easily absorb water.
See these short clips [1] for water polarity and here for cellulose [2].

In plants, xylem vessels are also made of cellulose and that helps the small molecules of water to stick to the vessels walls.
Think of it as a lot of tiny drinking straws made of wood.

There are at least two reasons why water manages to 'climb up' from the roots to the leaves through the xylem.

During the day, when the sunlight is available, water will be consumed by the leaves during photosynthesis [3]. For each molecule of glucose, 6 molecules of water are needed. So the water already present from the xylem in the leaves needs to be replenished by water (sap) from the twigs, which is replenished by the water from the xylem in the branches,... And so on, till the water is pulled from the roots.[4]
Remember little molecules of water cling to each other because they are polar. This is why they tend to pull each other up, as they are consumed in the leaves [1'].

And this is why it is important that both the water molecule and the cellulose molecule are polar. If H2O would be like CO2, there would be no vascular plants as we know them.

But there is no sun, so no photosynthesis, during the night.
Then the stomata, which are holes used by vascular plants to "breath" ( transpiration), are usually closed.
There is also another reason why water moves up the xylem. Here it is. Roots not only 'pump' water, their role is also to capture nutrients. These nutrients are like salts, not table salt, more like ions (potassium, phosphorus, zinc, mangnese, etc) which are needed by the plant cells. So the water is a bit more 'salty' inside the roots. Water outside the roots tends to try to diffuse into the roots in order to compensate the relative saltiness [5].
This in turn pushes the water from the root below the ground towards the branches.[4]

What is remarkable is that the whole cycle of water is passive, the plant does not spend energy to move the water 'upwards' to the leaves.

Here are a few sources which I can recommend and from which I learned most of these things. Crash course biology. They talk fast but it's quite funny.

[1] water polarity: 'Water - Liquid Awesome: Crash Course Biology #2'
https://youtu.be/HVT3Y3_gHGg?list=PL3EED4C1D684D3ADF&t=171
[1'] To picture molecules of water pulling one another up the xylem... well: https://youtu.be/IlmgFYmbAUg?t=154 (Monkey chain in Madagascar 3)

[2] cellulose: ' Plant Cells: Crash Course Biology #6'
https://youtu.be/9UvlqAVCoqY?list=PL3EED4C1D684D3ADF&t=255

[3] Photosynthesis ' Photosynthesis: Crash Course Biology #8'
https://youtu.be/sQK3Yr4Sc_k?list=PL3EED4C1D684D3ADF&t=75

[4] For a simple presentation of xylem, I use Simple English in https://simple.wikipedia.org/wiki/Xylem_vessel and 'Vascular Plants = Winning! - Crash Course Biology #37' https://youtu.be/h9oDTMXM7M8?t=373

[5] Osmosis (water compensating solutes) "In Da Club - Membranes & Transport: Crash Course Biology #5" https://youtu.be/dPKvHrD1eS4?list=PL3EED4C1D684D3ADF&t=148

Ian (and dad <= all errors and approximations are his :) ).

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    $\begingroup$ Ah, and yes, this is me in the avatar. When I was one. $\endgroup$ – Alain Pannetier Jan 16 '18 at 20:44
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    $\begingroup$ Thanks for your highly informative answer. But at night, if the temperature is still higher than 0° so won't some water evaporate through the stomata? $\endgroup$ – Joshua Jan 17 '18 at 11:17
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    $\begingroup$ @Joshua, under normal circumstances, stomata are closed during the night. youtu.be/IlmgFYmbAUg?t=154 but you are also right to add that photosynthesis is not the main reason why leaves consume water. Water evaporation is also the way plants regulate their temperature in warmer conditions. $\endgroup$ – Alain Pannetier Jan 17 '18 at 13:42
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Yes, the analogy is quite correct with respect to the mechanisms involved. In xylem as well as in tissue paper there are there are tiny pores throught out them through which water rises up. This mechanism is called cappilary action of water which you shall study in high school physics course. In fact tissue paper is made up of the same substance that xylem is made up of, i.e Cellulose.When you dip tissue paper in a bucket it spreads in all directions but imagine looping it to form a pencil like shape. Now thats like a real xylem. Hope it helps.....

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