How Does Water Climb Trees?

A short video explaining how very tall trees are able to get water despite needing to fight off so much gravity. I hope that you guys all learned something new from this video :)

Timestamps:
00:00 Intro
00:27 Transport systems in vascular plants
01:32 Transpiration
01:54 Water pressure
02:20 Cohesion force
02:59 Conclusion

Sources:

Transcipt:
Water is an essential molecule that almost all organisms need to survive. For us humans, water is transported by our blood and then distributed to the rest of our body, but what about trees? They don’t have any blood to transport water, and they also have to somehow fight against gravity and transport water from their roots all the way to their tallest branches. Some of them have to transport water over 100 meters (328 ft)! We will go over how trees manage to achieve it in this video.

There are two main components of the transport system inside a tree, or any vascular plant, which are the phloem and xylem. The phloem is responsible for transporting sugar (sap), while the xylem is responsible for transporting water, so we will be mainly focusing on the xylem. The transportation of water is mostly a passive process, which means that it doesn’t require energy from the plant to actually transport the water, unlike blood, which is an active transport process that requires energy to transport stuff around our body. The xylem is made of mostly dead cells, as they provide more strength and structure to the xylem, and also don’t accidentally take up the water needed for leaves. In a tree, the “wood” part inside the tree bark is actually considered a part of the xylem, as wood tissue is mostly dead cells. So when you look at tree rings, you are actually looking at layers of xylem cells.

Recall that earlier, I mentioned that the water is passively transported through the xylem to the top of the tree. If there is nothing pumping the water to the top, then how does water fight against gravity to reach into the leaves? The answer actually lies within the leaves themselves. Water is needed as part of the process of photosynthesis, where the plant converts carbon dioxide into sugar and oxygen. The water that gets used in this reaction will evaporate through the openings in leaves called stomatas, and this process of evaporation during photosynthesis is called transpiration. As water gets evaporated at the leaves, the water potential, or in other words, the pressure, also decreases at the leaves, as there is less water occupying the same space. Fluids such as water will always flow from high pressure to low pressure, which means that as the pressure of the leaves drops below the pressure of the roots, water from the roots will flow towards the leaves, as there is a lower pressure there. Another factor that helps water climb up a 100 meter xylem is the cohesion force of water. Water molecules are made of one oxygen atom, being slightly negatively charged, and two hydrogen atoms, being slightly positively charged. Since opposite charges attract, water molecules tend to attract towards each other, or towards other things like the xylem walls by forming hydrogen bonds. That is also why when you pour yourself a drink, sometimes the water will flow down the side of the cup and spill everywhere. So when some water molecules are slowly flowing up towards the leaves due to the lower pressure, they are dragging more of their buddies with them because of the hydrogen bonds that they formed. This is also why plants need more water in the summer, since the temperature is higher and there is more sunlight, more transpiration and photosynthesis occurs, which leads to more water being drawn from the roots as the leaves are constantly having lower pressure.

And there we have it! Extremely tall trees are able to get water to the very top through their xylem thanks to the change in water pressure. It’s pretty much like drinking through a straw, where the roots of a tree is where the water is, and by sucking on the straw, or through transpiration, pressure at the leaves will drop enough to the point where the water will get sucked up into the leaves. I hope that all of you learned something new today and thank you for watching!

#teamtrees #transpiration #zeleonscience