Root hair cells, xylem and phloem

Stand a stick of celery in red food colouring overnight, then slice across it. Where has the red gone? Not all through the stalk. It sits in a row of tiny red dots near the outer edge, and the rest of the slice is clean and green. The water climbed the stalk through just a few tubes, and those tubes are made of dead cells. Why a living plant pipes water through dead cells starts at the root.

Water gets into a plant from the soil, through its roots. A root hair cell sits on the surface of the root. Part of it grows out into a long, thin extension that pokes between the soil particles. What does that long, thin extension give the cell, and why does that help? A very large surface area, so the cell can take in water and mineral ions much faster. Mineral ions are the dissolved minerals a plant needs. Water moves in by osmosis. The soil water is more dilute than the cell's contents, so water moves from the soil into the cell, down its concentration gradient. Osmosis itself is in the video on osmosis and the change in mass of plant tissue. Mineral ions are a different story. Take nitrate ions: there are usually fewer of them in the soil water than inside the cell. Which process moves nitrate ions into the cell: diffusion, osmosis or active transport? Active transport. The ions must move from a lower concentration to a higher one, against their gradient, and that takes energy from respiration. Diffusion only moves particles from where there are more to where there are fewer, so it would carry nitrate ions out, not in. And osmosis is for water only. What would a root hair cell need plenty of, to fuel active transport? Mitochondria, where respiration releases the energy. Root hair cells have lots of them. So at the root hair, two things come in by two routes. Water comes in down its gradient, by osmosis, with no energy. Mineral ions come in against theirs, by active transport, using energy. Questions also test which level you answer at. Here is one bean plant, asked about twice. Which organ absorbs a bean plant's water, and which cells on it do the absorbing? The organ is the root, and the cells are root hair cells. Read the question for organ or cell before you answer. One examiner's report on a Higher paper notes that "Most students gained credit for stating that water enters the root hair cells from the soil." It goes on: "However, answers that referred only to roots or incorrectly described the process as active transport were not credited." So the fix is to name the cells, root hair cells, and to give water its own process, osmosis. Active transport belongs to the ions.

From the root, water and mineral ions head up the plant in the xylem, through the stem and into the leaves. Xylem must carry water up a tall stem without collapsing. What would its walls be like? Thick and strong. Xylem walls are thickened with a tough substance called lignin, and walls like that are called lignified. Lignin stops the tube collapsing, and it also helps to hold the plant up. Its cells are joined end to end. As they mature, the cells die, and the end walls between them break down. Why is it useful for a water pipe to be dead and to have no end walls? Nothing gets in the way. With no cell contents and no end walls, the xylem is one hollow tube, so water flows straight through it. That is the celery. Its red dots are xylem, hollow and dead, and that is why the dyed water could climb the stalk so easily. Any feature you give for xylem has to pass one test. It must let the tube do something an ordinary plant cell could not. Which is a real xylem adaptation: a cell wall, a cell membrane, or lignin? Lignin. Every plant cell has a wall and a membrane, so they adapt nothing. Lignin keeps the xylem open and strong. One examiner's report on a Foundation paper notes that "Some candidates did not score due to just referencing normal plant cell features such as a rigid cell wall or cell membrane rather than adaptations shown by xylem." The fix is that test. A feature counts only if an ordinary plant cell does not have it.

The other tube in each bundle is the phloem. It carries sugar made in the leaves to the rest of the plant. Phloem is made of living cells joined end to end into tubes. The tube cells have very little inside them, and no nucleus. Their end walls are still there, but they have holes in them, called sieve plates, so the sap can pass from one cell to the next. Moving the sugar takes energy, and the living cells supply it. How the sap is pushed along is not needed at GCSE. Which does phloem carry: sucrose only, sucrose in water, or water and mineral ions? Sucrose dissolved in water. The sap is a liquid with the sugar dissolved in it, so the phloem carries water too. One examiner's report on a Foundation paper records that "Approximately eighty per cent of candidates did not identify correctly that water and sucrose are transported in the phloem." It adds that "The presence of water in the phloem is a common omission when teaching plant transport." So whenever you write phloem, write water beside the sucrose. And call the sugar sucrose, or dissolved sugars, never glucose.

Now sort it all out. Eight features, and each one belongs to a root hair cell, the xylem or the phloem. Sort each one, and for every feature say what it lets that structure do that an ordinary plant cell could not. Have a go at sorting all eight, with a reason for each, before I show you. Root hair cell: the long, thin extension, for a huge surface to take in water and ions, and many mitochondria, for the energy active transport needs. Xylem: no end walls and dead cells, so nothing blocks the water, and lignin, so the tube does not collapse. Phloem: end walls with holes, so sap passes from cell to cell, and living cells that use energy to move the sugar. The feature about carrying sucrose belongs to the phloem too. Line the two tubes up in columns, and the differences read straight across: dead or living, lignin or none, open ends or sieve plates, no energy or energy. One word sits in the carries row of both columns. Which word is it? Water. Xylem carries water and mineral ions, and phloem carries sucrose dissolved in water. Last, a student's sentence to mark: water moves into the root hair cell by active transport, then travels up to the leaves in the phloem, along with the mineral ions. It reads smoothly, and yet it holds two mistakes. Which two mistakes does it make, and what should each one be? The first mistake is the process: water enters by osmosis, not active transport. It moves down its own concentration gradient, so it needs no energy. Second, water and mineral ions travel up in the xylem, not the phloem. The phloem carries sucrose dissolved in water.

Time to check what stuck, one question at a time. Which process brings water into a root hair cell, and which way does it move? Osmosis. Water moves from the dilute soil water into the more concentrated cell, down its gradient. Now one on tubes. Picture xylem with no lignin in its walls. What would go wrong? The weak walls could collapse and block the water, and the stem could no longer hold the plant up. Try this one: a greenfly pierces a stem to drink sugary sap. Which tube does it tap? The phloem is the tube it taps. Sugar dissolved in water travels there, while the xylem carries water and mineral ions. And the celery's red dots were xylem: dead, hollow tubes with no end walls, which is why the water could flow up through them.

Here are the chapter's videos so far, this one at the end. Once it's in the bag, press thumbs up on it, so you can see at a glance which ones you never need again. If it still feels wobbly, save the playlist and try a few exam questions on it; by the third, it starts to feel normal.

Next in the chapter: transpiration, translocation and the stomata. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh five, oh three.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.