Biology › Plant transport and mineral nutrition › Xylem and the transpiration stream: the pull comes from the top

Xylem and the transpiration stream: the pull comes from the top

A redwood lifts water a hundred metres without a single moving part. The energy comes from the sun evaporating water off a leaf, the rope is a column of water held together by hydrogen bonds, and the whole thing runs in tension rather than under pressure.

Before this Water potential and osmosis in plant cells · Active transport and carrier proteins

What you should be able to do

• Describe how a root hair cell is built for absorbing water and mineral ions.
• Trace water across the root cortex by the apoplast and symplast routes, and say where each one has to cross a membrane.
• Explain what the Casparian strip does and why it hands the plant control over what reaches the xylem.
• Account for every feature of a xylem vessel in terms of the job the tissue does.
• State the cohesion-tension theory and name the property of water each step depends on.
• Set up a potometer, and say exactly which quantity the reading is.
• Name the ions a plant takes up and what a shortage of each one looks like.

Getting water out of the soil

A root hair is one epidermal cell drawn out into a projection perhaps a millimetre long. It is not a separate cell type and it does not last a week. What it gives the plant is surface: thousands of hairs on a centimetre of root turn a smooth cylinder into something with an enormous area against the film of water round the soil particles, and they push between those particles into water a plain root surface would never reach.

Water enters by osmosis, so the cell must sit at a lower water potential than the soil solution. In a moist soil that solution is dilute, only a few tens of kPa below zero, while the cytoplasm and vacuole of the root hair cell sit several hundred kPa negative. Water moves in down the gradient without the plant spending anything on it.

Keeping the inside that concentrated does cost something, and the mineral ions are how it is paid for. A root hair cell is packed with mitochondria, and carrier proteins in its membrane move nitrate, phosphate, potassium and the rest inwards against their concentration gradients. That active transport does two jobs at once: the plant gets the ions it needs, and the solute potential of the cell contents drops, which is what keeps water flowing in.

Root hair cell

An epidermal cell of the root extended into a long narrow projection, giving a large surface area for the absorption of water and mineral ions.

Apoplast pathway

Movement of water through the cellulose cell walls and the spaces between them, without entering the cytoplasm of any cell.

Symplast pathway

Movement of water through the cytoplasm of the cells, passing from one to the next through plasmodesmata.

Casparian strip

A band of suberin in the walls of the endodermal cells that is impermeable to water, so the apoplast route is blocked and water must cross a plasma membrane.

Which ions, and what each one is for, is examined in its own right, and the deficiency symptoms are more logical than they look. Every one of them follows from the molecule the ion is part of.

Notice which leaves go first. Nitrogen, magnesium, phosphorus and potassium all travel in phloem, so a plant running short strips them out of its old leaves for the new growth and the damage shows on the old ones. Calcium cannot move in phloem once delivered, so the growing point is what suffers. Handed a photograph of a deficient plant, look at which end of it is dying.

Two routes across the cortex, and a wall that stops one

Once through the root hair, water has to cross the cortex to reach the xylem in the middle. There are two ways across and they behave quite differently.

The apoplast route runs through the cellulose walls themselves. Cell walls are an open mesh of fibres soaked in water, and they are continuous from cell to cell, so water can travel through them without ever entering a cytoplasm. Nothing is selected, nothing is excluded, and because there is no membrane to cross the resistance is low. Most of the water crossing a root cortex goes this way.

The symplast route goes through the cells. Water crosses a plasma membrane once, at the root hair, then passes from cytoplasm to cytoplasm through the plasmodesmata that thread the pores in adjacent walls, so the living contents of a root are one connected compartment. A third description, the vacuolar route, has water crossing the tonoplast as well; OCR A and CAIE name it explicitly, and on AQA the two that earn marks are the apoplast and the symplast.

At the inner edge of the cortex sits a single ring of cells, the endodermis, and their walls carry a band of suberin — a waxy, waterproof material — running right round each cell. That is the Casparian strip, and it is the reason the endodermis is worth a name. Water travelling in the walls arrives at the strip and cannot go on. It has to enter the cytoplasm of an endodermal cell, which means crossing a plasma membrane, which means passing whatever that membrane will let through.

That single constraint is what makes a root selective: everything reaching the xylem has passed a membrane, so toxic ions can be excluded and useful ones concentrated. It also lets the endodermis pump ions into the xylem. The xylem sap's water potential falls, water follows osmotically, and the positive hydrostatic pressure that builds is root pressure — which the strip stops from leaking straight back out along the walls.

Root pressure is where guttation comes from — the beads of liquid water squeezed out along the leaf margins of grass on a still, humid morning, which is not dew, because dew condenses onto a surface. It is also easy to switch off. Cool the roots, flood them so they run short of oxygen, or poison them with cyanide, and the exudation stops, because the active transport driving it has stopped. That dependence on respiration is the evidence that root pressure is real; its small size is the reason it cannot be the main mechanism.

The pipe, and why it is dead

Xylem is a tissue, not a cell type, and the part that carries water is the vessel: a file of cells that grew end to end, laid down lignin in their walls, then died and lost their contents. What is left is a hollow tube, sometimes metres long, with nothing inside to obstruct flow.

The lignin matters more than it looks. A tube carrying water under tension is being squeezed inwards by the atmosphere, and an unreinforced cellulose tube of that diameter would flatten. Laid down in rings, spirals or a pitted mesh, lignin stiffens the wall while leaving a stem flexible enough to bend in wind, waterproofs it so water does not seep out along the whole length, and is strong enough that xylem doubles as the plant's skeleton. Wood is mostly old xylem.

Pits are the gaps where lignin was not laid down, so water can pass sideways to the living cells that need it and from one vessel into its neighbour. That second function is a repair route. If air gets in, the column in that vessel breaks — an embolism — and the vessel is finished for the season, but water detours through the pits into the vessel alongside. Many narrow tubes survive damage that would cripple one wide one.

Conifers manage without vessels at all. They have only tracheids, narrower cells that keep their tapered end walls and pass water through pits, and a pine still gets water sixty metres up. The vessel's advantage is lower resistance; the tracheid's is that one embolism spreads less easily.

Cohesion-tension: the pull starts at a leaf

Now the mechanism, and the reason the lie at the top of this lesson is a lie. Suction cannot lift water more than about ten metres, whatever pump you attach: atmospheric pressure is 101 kPa, and 101 kPa supports a column of water 10.3 m high. Yet water reaches the top of a redwood at nearly a hundred metres. It follows that the water up there is not being held up by pressure from below. It is hanging, under tension, from the leaves.

Take the chain link by link. Water evaporates from the wet cellulose walls of the mesophyll cells into the air spaces of the leaf, and diffuses out through the stomata. That cell has now lost water, so its water potential falls, so it draws water osmotically from the cell next to it, and so on back to the xylem in the vein. Water is pulled out of the vessel.

Two properties of water carry the rest. Cohesion — hydrogen bonding between water molecules — means the column does not pull apart when you tug on the top of it; the whole thread moves as one, so removing a molecule at the leaf draws one in at the root. Adhesion — hydrogen bonding between water and the wall — holds the column against the lignified surface and helps it up the narrow tube. Neither is a detail you can leave out of an answer: the mark scheme wants hydrogen bonding named.

The tension is measurable and it is why the theory has the name it does. Xylem sap in a transpiring tree at midday sits at perhaps −1000 to −2000 kPa. Three observations follow, and all three are used as evidence:

Every step of that chain runs downhill in water potential: soil at about −30 kPa, root and stem below it, leaf mesophyll near −1500 kPa, and beyond the stoma an atmosphere lower still. Nothing in the plant has to work at it. The sun does the work, at the leaf surface, by evaporating water.

The potometer, and what the reading actually is

The standard apparatus for this topic is the potometer, and the standard mark lost is the one for saying what it measures.

Setting it up is a sequence of precautions, each with a reason a question can ask for. Cut the shoot under water and assemble it submerged, so no air enters the xylem. Cut the stem at a slant, for a larger surface in contact with the water. Dry the leaves, so you are not measuring a film left on the surface. Seal every joint with petroleum jelly. Then leave it ten minutes to settle, and take repeats, resetting the bubble from the reservoir between them.

What moves is the air bubble, and it moves because the shoot is drawing water in. That is water uptake. Transpiration is the water leaving through the stomata, and the two are not the same quantity: a small fraction of what the plant takes up is used as a raw material in photosynthesis or held in the cells to keep them turgid as they grow. The fraction is genuinely small, usually quoted at around one per cent, so uptake is a good estimate of transpiration — but it is an estimate of it, and an answer that calls the reading a transpiration rate is asking to lose the mark.

In the exam

• Water moves down a water potential gradient, and 'higher' means less negative. Pure water is 0 kPa and there is no such thing as a positive value for a solution, so −200 kPa is higher than −900 kPa.
• Name the bonding. Cohesion-tension answers that say the water 'sticks together' will not score; 'hydrogen bonds between water molecules' will.
• The apoplast route crosses no membrane until the endodermis. That is the whole reason the Casparian strip is examinable, so say what the strip forces water to do, not just where it is.
• Root pressure is not the mechanism. If a question offers it, the marks are for saying why it is too small and why chilling the roots stops it.
• A potometer measures uptake. Write 'rate of water uptake' every time.
• For a deficiency, ask which molecule the ion belongs to, then whether it moves in phloem — that decides whether old or new leaves suffer first.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

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