Transpiration, translocation and the stomata

Why would a tree pull water all the way up from its roots, only to let it go into the air? A large oak can give off around a hundred and fifty thousand litres of water that way in a year. The easy guess is that the tree has water to spare. It doesn't: in a dry spell, plants run short of water and wilt. The real reason sits in the tiny pores on the underside of its leaves.

That water leaves through the stomata, and the loss of water vapour from a plant's leaves has a name: transpiration. Inside the leaf, water evaporates from the surfaces of the mesophyll cells into the air spaces, as water vapour. There's more water vapour inside the leaf than in the air outside. Which way will it move? Out. It diffuses out through the open stomata, from more water vapour inside to less outside. The rest of the leaf surface lets very little water through, so almost all of it leaves by the stomata. Each stoma is a pore with a guard cell on each side of it. Which is right: stomata control guard cells, or guard cells control stomata? Guard cells control the stomata. The stoma is only the hole, and the two cells around it open and close it. When the guard cells take in water, they swell and curve apart, and the pore opens. When they lose water, they go limp, and the pore closes. The osmosis behind that is in the video on osmosis and the change in mass of plant tissue. One examiner's report on a Foundation paper notes that "the two terms were commonly confused by a significant proportion of students, with many thinking that the pores are called guard cells, which are controlled by the stomata." One sentence to copy is the fix: the guard cells open and close the stoma.

Open stomata cost the plant water. They also let in something it cannot do without. What does an open stoma let in that the leaf needs for photosynthesis? Carbon dioxide. It diffuses in through the open pore, while oxygen diffuses out. That's the trade-off, and it answers the oak. Open, a stoma lets carbon dioxide in but lets water vapour out. Closed, it saves water but shuts the carbon dioxide out. So stomata are usually open in the light, when photosynthesis can happen, and closed in the dark. They also close when the plant is short of water. Now one plant at two times. A geranium's stomata are open at ten in the morning, on a cool, bright day. At three in the afternoon on the hottest day of summer, with the soil dry, they are closed. For each time, what does the plant gain, and what does it give up? At ten, open, the plant's gain is carbon dioxide for photosynthesis, and what it gives up is water, lost as vapour through the pores. At three, closed: it gains the water it saves, which matters with the soil dry, and it gives up carbon dioxide, so photosynthesis slows. Every arrow through a stoma is a gas: carbon dioxide in, oxygen out, water vapour out. Guard cells do have a few chloroplasts, but the stoma is a door for gases. The photosynthesis happens in the mesophyll behind it. One examiner's report on a Higher paper says some students "linked stomata to uptake of water which is a common misconception". It adds that "it is true that the guard cells of stomata have some chloroplasts this is not the role of the stomata." Instead, draw every arrow at a stoma as a gas, and keep the water-in arrow at the root hair.

The water lost at the top does more than leave: it pulls more water up behind it. Take a tomato plant in a greenhouse on a hot afternoon, with six things happening to its water, jumbled up. Which of the six sets all the others off? Water evaporating from the cells inside the leaf. The cause is at the top, even though the water travels up from the bottom. So water vapour builds up in the air spaces, and it diffuses out through the stomata, from more inside to less outside. The leaf cells have lost water, so water moves out of the xylem in the leaf to replace it. Water has just left the top of the xylem. What happens to the water below it in the stem? It is pulled up to fill the gap. Water molecules stick together, so pulling at the top moves the column as one. In turn, water moves up out of the root, and more water enters the root hair cells from the soil, by osmosis. That continuous flow, from the roots, up through the xylem and out of the leaves, is called the transpiration stream. If that chain is landing, you are in good company. One examiner's report on a Higher paper, on how water moves up a stem, says "Many candidates scored maximum marks, demonstrating an excellent understanding." The stream is pulled from the top, not pumped from the bottom. The xylem's cells are dead, and the stream uses no energy from the plant. Mineral ions came in at the root hairs. How do you think they reach the leaves? Dissolved in the water, carried up the xylem in the transpiration stream. The stream does three jobs. It brings water for photosynthesis, it keeps the cells firm, and it delivers mineral ions to the leaves. And that's the oak's other answer. The water it loses at the top is what pulls the next water up from its roots.

The second flow runs through the phloem, and it carries sugar instead of water. Translocation is the movement of dissolved sugars, mainly sucrose, through the phloem. They travel from the leaves, where the sugars are made, to the parts that use or store them. Those parts include growing tips, roots, fruits and storage organs such as potatoes. A leaf sits halfway up a stem. Can its sugar travel up, down, or both? Both. Sugar travels from where it is made to wherever it is needed, so it can head up the stem or down it. Unlike the xylem, the phloem is living, and translocation uses energy. The details of how the sap moves are beyond GCSE. Here's one student's answer to a question asking for a difference between the two flows: the xylem carries water and mineral ions. What's missing from that answer, if it's meant to be a difference? The other flow. It describes the xylem only, so there is nothing to compare it with. One examiner's report on a Higher paper records that "Over forty per cent of students failed to gain a mark on this question, mostly due to a lack of comparative detail." It says that "many students just described one or the other, which was not creditworthy." The habit to build is one sentence per difference, with both flows in it, joined by whereas. A potato plant moves water from its roots to its leaves, and sugar from its leaves down to the potatoes underground. Write three differences between its water flow and sugar flow, both in each sentence. One: the water moves in the xylem, whereas the sugar moves in the phloem. Two: the xylem carries water and mineral ions, whereas the phloem carries sucrose dissolved in water. Three: the water only moves up the plant, whereas the sugar can move up or down, to wherever it is needed. A fourth works too: the xylem is dead and uses no energy, whereas the phloem is living and uses energy. In the second one, the phloem's sugar is sucrose. The same report notes that "A large proportion of pupils stated that glucose and mineral ions were transported, which was ignored, unless a correct reference to sugars or sucrose was given." Write sucrose instead, never glucose.

Let's run back over the two flows, one question at a time. Which end of the plant drives the transpiration stream: the roots or the leaves? The leaves. Water lost from them pulls the column up behind it. Now a tree you have not met. A strip of bark is cut away all round its trunk, taking the phloem with it but leaving the xylem. Which part of the tree can its sugar no longer reach? The roots, below the cut. Sugar made in the leaves cannot get past the gap, so everything beneath it misses out. One more: a plant's guard cells cannot take up water. What happens to its stomata, and to its growth? Its guard cells stay limp, so the stomata stay closed. Water is saved, but carbon dioxide cannot get in, so photosynthesis and growth slow down. So the oak's water is not wasted. Losing it is the price of letting carbon dioxide in, and the pull that lifts the next water up.

Up to this video, that's the chapter. If you own this one, a thumbs up is your reminder that it's done, so you never sit through it twice. If the two flows are still tangled, save the playlist, give it a day, and look again.

Next in the chapter: what changes the rate of transpiration, and measuring it. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh five, oh four.

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