Hi Tara. Here is the shape of Topic 8 before you start. Sub-topic 8.1 is two tissues, two cargoes and three pictures — you must be able to point at the xylem and the phloem in a root, a stem and a leaf, and there is a single rule that gets all three right at once. Sub-topic 8.2 is the journey from soil water into the xylem, and it is really Topic 3 wearing new clothes: osmosis for the water, active transport for the mineral ions. Sub-topic 8.3 is the biggest, and it contains the two sentences that earn the most marks in the whole topic — the two-step exit of water from a leaf, and the transpiration pull. Sub-topic 8.4 is short and is almost entirely about two words, source and sink, plus the idea that the same organ can be each of them at different times of year. Then 8.5 is a checklist for the night before a paper.
One connection worth holding onto the whole way through: a plant transpires because it must open its stomata to let carbon dioxide in. Water loss is the price of photosynthesis, not a purpose. Almost every “explain why” question in this topic becomes easier once you see it that way.
Start With the Cargo
A plant runs two separate transport systems, and they never mix. Learn what each one carries first, because almost every question in this sub-topic is answerable the moment you are certain of that.
| Tissue | What it transports | Direction | Other function | Living or dead? |
|---|---|---|---|---|
| Xylem | Water and mineral ions | Upwards only, roots to leaves | Support — it holds the plant up | Dead, and empty |
| Phloem | Sucrose and amino acids | Either way — up or down | None required by the syllabus | Alive |
Cambridge pays for mineral ions, not “minerals” and not “nutrients”. It pays for sucrose, not “sugar” and not “glucose”. The leaf makes glucose in photosynthesis and then converts it to sucrose before loading it into the phloem, so glucose is a genuinely wrong answer rather than a loose one. And when phloem is meant, the process is called translocation, not simply “transport”.
The direction row is worth a second look. Xylem is a one-way street: water enters at the roots and leaves at the leaves, and nothing ever comes back down. Phloem is two-way, because it delivers to whichever part of the plant currently needs sugar — and in early spring that part is at the top. “Translocation is always downwards” is one of the most common wrong sentences in this topic, and 8.4 will show you exactly when it is upwards.
Where They Sit: One Rule for Three Pictures
You have to be able to identify xylem and phloem in a transverse section of a root, a stem and a leaf of a non-woody dicotyledonous plant. Three diagrams, and most people learn them as three unrelated facts and then mix them up under pressure. There is a better way.
Xylem is always nearer the centre of the plant; phloem is always nearer the outside.
In the root, the centre of the organ is the centre of the plant, so the xylem is a star right in the middle with the phloem tucked between its arms. In the stem, the bundles are arranged in a ring, and within each bundle the xylem is on the inner side, the phloem on the outer. In the leaf, a vein is a stem bundle that has been folded out sideways — so the side that was facing the middle of the plant is now facing up. Xylem on top, phloem underneath.
If a paper gives you a section you have never seen, do not start by hunting for xylem. Start by asking which organ is this? A near-circular section with a star in the middle is a root. A near-circular section with a ring of separate patches is a stem. A flat slab with an upper and a lower surface is a leaf. Once the organ is identified the rule above tells you the answer, even if the drawing is unlike any you have practised on.
Supplement: How a Xylem Vessel Is Built
You need to relate the structure of a xylem vessel to its function, limited to exactly three points.
This is a structure-and-function question, which means every feature you name has to be followed by what it achieves. Naming all three features and explaining none of them typically scores half.
| Feature | What it achieves |
|---|---|
| Thick walls containing lignin | Lignin is strong and waterproof, so the vessel does not collapse when water is pulled up under tension — and the same rigidity gives the plant support |
| No cell contents | No cytoplasm, no nucleus, nothing in the lumen, so water flows through an open pipe with almost no resistance. The cells died to make this possible |
| No cross walls — the cells are joined end to end | The end walls broke down, so many cells became one long continuous tube running from root to leaf with nothing to cross |
The syllabus says you do not need to know how a wall becomes lignified. It does expect the word lignin itself. “Thick, strong walls” on its own is usually one mark short of “thick walls containing lignin”.
And the Other Tube: Why Phloem Has to Be Alive
You are not required to describe phloem structure in the same detail, but the contrast is worth holding because it explains several exam answers. Phloem sieve tubes keep their cell membranes and cytoplasm and are helped by neighbouring companion cells. Loading sucrose into a sieve tube costs energy, which is why translocation stops when phloem tissue is poisoned or starved of oxygen, and why phloem — unlike xylem — cannot work in a dead plant.
Xylem is dead and empty; phloem is alive and busy. A dead pipe can still carry water, because the sun is doing the pulling. Nothing dead can carry sucrose to a root tip in the dark, because that takes work.
Separate patches arranged in a ring means a stem. A root would show one central star; a leaf would not be circular. Doing this first means you never have to remember which tissue is which in the abstract.
The blue-stained region is on the inner side of each bundle, so it is the xylem. The justification is the mark: “because in a dicotyledonous stem the xylem lies on the inside of each vascular bundle, nearer the centre”. Lignin also takes up many stains strongly, so a heavily stained thick-walled tissue is another clue.
The outer region is phloem, which translocates sucrose and amino acids from sources to sinks. “Carries food” would not be credited — food is not a chemical.
The phloem, which lies just inside the bark — on the outer side of the vascular tissue. The xylem is deeper in.
Water is carried by the xylem, which is untouched, so the leaves are still supplied and photosynthesis continues.
Sucrose travelling down from the leaves cannot pass the damaged phloem, so it accumulates above the block. The swelling is on the source side — which also tells you the direction of travel at that time of year.
The Root Hair Cell
A root hair is not a separate structure and it is not a small root. It is a single epidermal cell of the root that has grown a long, thin extension out between the soil particles. There are millions of them, they last only a few days, and together they are responsible for essentially all the water and mineral ions a plant ever takes in.
| Adaptation | Why it helps |
|---|---|
| A long, thin projection | Gives a very large surface area, so more water and mineral ions are absorbed per second. This is the point the syllabus states explicitly |
| A thin cell wall | A short distance for water to cross, so uptake is faster |
| Many mitochondria | Release energy in respiration for the active transport of mineral ions |
| No waxy cuticle | A cuticle is waterproof. An absorbing surface is the one place a plant cannot afford one |
| No chloroplasts | It is underground, so there is no light and photosynthesis is impossible. Do not draw them |
Large surface area for what? Finish the sentence: a large surface area for the absorption of water and mineral ions. The same half-answer costs marks in every topic that has ever mentioned surface area — villi, alveoli, root hairs — and it is the easiest habit in Biology to fix.
Two Different Journeys Into the Same Cell
Water and mineral ions arrive at the root hair together and get in by completely different means. Keeping them apart is worth several marks a paper.
The soil solution is very dilute, so it has a higher water potential than the cell sap inside the root hair. Water therefore moves down the water potential gradient, through the partially permeable cell membrane, into the cell. That is osmosis, and it costs the plant nothing.
The soil solution is dilute in ions too — often more dilute than the inside of the cell. So ions must be moved against the concentration gradient, from a lower to a higher concentration, by protein carriers in the cell membrane, using energy released in respiration. That is why a root hair cell is full of mitochondria, and why a waterlogged plant — whose roots cannot get oxygen — goes short of minerals long before it goes short of water.
Waterlogged soil has its air spaces filled with water, so the root cells cannot get enough oxygen for aerobic respiration. Less energy is released, so less active transport, so mineral ion uptake falls. Water uptake is unaffected, because osmosis needs no energy. If you can write that chain you have also answered every question about respiratory poisons and about temperature effects on ion uptake.
The Pathway, in the Order Cambridge Lists It
The syllabus gives the route as a single line, and questions quote it back at you in scrambled order. Learn the sequence, not just the parts.
Two things about the middle of that journey are worth understanding rather than memorising. First, water crosses the cortex before it reaches the xylem — it has to, because in a root the xylem is right in the centre and the cortex is in the way. Second, it crosses the cortex by the same process it used to get in: each cell nearer the centre has a slightly lower water potential than the one outside it, so water moves from cell to cell by osmosis all the way to the xylem. No pump, no energy, no exceptions.
If you kill a stem by heating it and then stand it in dye, the dye still rises. That single fact rules out any answer that has living cells pumping water upwards, and it is the reason the mechanism in 8.3 had to be a pull from above rather than a push from below.
The Practical: Following Water With a Stain
The syllabus asks you to investigate the pathway of water through the above-ground parts of a plant using a suitable stain. The classic version uses a stick of celery or a white carnation standing in water coloured with eosin or methylene blue.
1. Cut the stem under water, so no air bubble is drawn into the xylem to break the column.
2. Stand the shoot in dilute stain and leave it in a warm, bright, moving-air place for one to two hours — conditions that keep transpiration fast, so the dye is drawn up quickly.
3. Take it out, rinse it, and cut thin transverse sections at several heights up the stem.
4. Examine the sections and record where the stain is.
The result: the stain appears in the xylem only — in a stem, as a ring of coloured dots on the inner side of each bundle. Stained veins run out into the leaves as well.
A very common wrong answer is to measure how much coloured water disappeared from the beaker. That tells you how much water moved and nothing at all about which tissue carried it — and the question asked for the pathway. Any experiment about a route has to end with looking at a section.
2 × π × 0.01 × 0.6 = 0.0377 mm² (3 significant figures). Working shown earns a mark even if the arithmetic slips, so always write the substitution line out.
0.0377 ÷ 0.0003 = 126 times greater. Sanity check: the hair must be the larger of the two, so the answer has to be well above 1. If you had got 0.008 you would know instantly that the division was upside down.
The huge increase in surface area means far more water and mineral ions can be absorbed per second than the same patch of epidermis could manage. Multiply that by millions of root hairs and you have the entire water supply of the plant.
Water enters by osmosis, which is passive: it is driven by the water potential gradient and needs no energy from the plant. Removing oxygen therefore makes almost no difference to it.
Ions are taken up largely by active transport, against the concentration gradient, using energy from aerobic respiration. Bubbling nitrogen displaces the oxygen, aerobic respiration slows, less energy is available, and the protein carriers cannot move as many ions.
Both solutions were bubbled, so the stirring and the disturbance were the same in each. The only variable changed was the gas. If plant B had simply been left still, you could not tell whether the difference was caused by the lack of oxygen or by the lack of mixing.
The Definition, and the Word That Carries It
Transpiration is the loss of water vapour from leaves.
Not “the loss of water”. Not “the movement of water up the plant” — that is the transpiration stream, which is a consequence of transpiration and a different phrase. Water leaves a leaf as a vapour, through a hole, and the whole mechanism depends on that being true.
The Two-Step Exit — the Highest-Value Sentence in Topic 8
Ask most students how water gets out of a leaf and they will say “through the stomata”. That is the second half of the answer. There are two distinct stages, with two different verbs, and mark schemes routinely award a mark for each.
1. Water evaporates from the wet cell walls of the mesophyll cells into the air spaces of the leaf.
2. The water vapour then diffuses out through the stomata, down a concentration gradient from the humid air spaces to the drier air outside.
Evaporation is a change of state. Diffusion is a movement of the vapour that results. Using either verb for both stages loses a mark, and swapping in “osmosis” loses the question — there is no membrane in a stoma, only a hole.
Why Leaves Lose So Much: Surface Area and Stomata
Two features control how much water a leaf can lose, and the syllabus names both.
The large internal surface area. The spongy mesophyll is a loose network of cells with big air spaces between them, and every one of those cells has a wet wall facing an air space. Add it up and the internal surface of a leaf is many times its external surface. More wet surface exposed to air means more evaporation — the same reason spilt water dries faster than water in a glass.
The size and number of the stomata. Every molecule of water vapour has to leave through a pore. More pores, and wider pores, mean a faster rate; fewer and smaller, and the rate falls. That is why plants adapted to dry places have fewer, smaller, sometimes sunken stomata, and why closing the stomata is the fastest emergency measure a plant has.
That huge internal surface area is not there for transpiration. It is there so that carbon dioxide can diffuse from the air spaces into every mesophyll cell for photosynthesis, and the stomata are open for the same reason. A wet surface exposed to air must evaporate. So transpiration is best described as an unavoidable consequence of the leaf being built for gas exchange — not as something the plant does on purpose. Examiners will accept that transpiration has useful side effects; they will not accept that it has a purpose.
Transpiration Pull — How Water Gets Up a Tree
There is no pump in a plant. Root pressure exists but is far too weak to lift water more than a metre or two, and in any case a plant killed by heat still conducts water. So the force has to come from the top, and it does.
1. Water evaporates from the mesophyll cells and diffuses out of the stomata as vapour.
2. Those mesophyll cells now have a lower water potential, so water moves into them from the xylem beside them.
3. This creates a tension — a pull — on the water in the xylem.
4. Because there are forces of attraction between water molecules, the column does not break; it behaves like a rope, so pulling the top drags the whole thing up.
5. Water is therefore drawn all the way from the roots. This is the transpiration pull, and the water in the xylem is a continuous column.
The phrase is transpiration pull. Not “suction”, not “capillary action”, and not “the plant sucks water up”. And the reason the column holds together is the attraction between water molecules — that exact idea is a mark of its own, and it is also why an air bubble in the xylem is such a disaster: it breaks the rope.
Three Factors That Change the Rate
The syllabus names temperature, wind speed and humidity. Every one of them works by changing either how fast water evaporates inside the leaf or how steep the water vapour gradient is at the stoma. If you explain them that way you never have to memorise a table.
| Factor | Effect on rate | Explanation the mark scheme wants |
|---|---|---|
| Temperature increases | Increases | Water molecules gain more kinetic energy, so evaporation from the mesophyll walls is faster and diffusion out of the stomata is faster. Warm air also holds more water vapour, keeping the gradient steep |
| Wind speed increases | Increases | Moving air blows away the humid layer just outside the stomata, so the water vapour concentration gradient between the air spaces and the outside stays steep and diffusion is faster |
| Humidity increases | Decreases | The air outside already contains a lot of water vapour, so the gradient is shallower and diffusion out of the leaf is slower |
Bright light makes stomata open, which increases transpiration, and you may meet it as data. But the three factors the syllabus asks you to explain are temperature, wind speed and humidity. If a question gives you light intensity, the explanation runs through stomatal opening, not through kinetic energy.
Measuring It: The Potometer
Cut the shoot under water so that no air enters the xylem. Assemble the whole apparatus under water so there are no bubbles anywhere except the one you introduce deliberately. Seal the joint with petroleum jelly so it is airtight. Then let it settle for a few minutes before the first reading.
What ruins a potometer: an air leak at the seal; an air bubble drawn into the xylem when the stem was cut in air; and taking readings immediately, before the shoot has equilibrated.
A potometer measures the rate of water uptake, not the rate of water loss. A small proportion of the water taken up is used in photosynthesis and retained to keep cells turgid, so uptake slightly overstates the volume actually transpired. Over a short period the two are close enough for uptake to be a good estimate — and saying exactly that, rather than pretending they are identical, is what earns the mark.
Planning the investigation
Choose one factor to change. Keep everything else the same.
- Wind speed. Put an electric fan in front of the shoot. Change the wind speed by moving the fan to set distances (for example 100 cm, 75 cm, 50 cm and 25 cm) or by using its speed settings. Run one set of readings with the fan off as the comparison.
- Temperature. Put the whole potometer in rooms or a growth cabinet at different air temperatures (for example 15, 20, 25 and 30 °C). Measure the air temperature next to the leaves with a thermometer. Do not use a lamp to heat the shoot: a lamp also changes the light intensity, so you would be changing two factors at once.
- Keep these the same: the same leafy shoot (same number and size of leaves), the light intensity, the humidity, and the time each reading lasts.
Method. After each change, wait about 5 minutes so the shoot settles to the new conditions. Time how far the bubble moves in a set time (for example 5 minutes). Use the reservoir tap to push the bubble back to the start. Take three readings at each setting and calculate the mean. Rate = distance moved ÷ time, in mm per minute.
What you find. As the temperature rises, the rate of transpiration rises. As the wind speed rises, the rate rises, then levels off at high wind speeds. Always describe results with figures from your table.
Wilting
Wilting is what happens when the sums stop working: the rate of water loss becomes greater than the rate of water uptake.
Loss exceeds uptake → cells lose water by osmosis → the vacuoles shrink and the cells become flaccid → there is no longer enough turgor pressure pushing outwards on the cell walls → the soft, non-woody tissues can no longer support themselves → the leaves and young stems droop.
Wilting is damaging, because a drooping leaf intercepts less light and the plant closes its stomata, which shuts off the carbon dioxide supply and stops photosynthesis. But it is also partly protective: drooping reduces the surface area exposed to the sun, and closed stomata cut water loss dramatically. A plant that wilts in the middle of a hot afternoon and recovers by evening has done exactly what it should.
Every mark in the wilting chain comes from osmosis vocabulary you already have: turgid, turgor pressure, flaccid. A soft green plant is held up by water pressure inside its cells pressing outwards on the cell walls. Take the water away and there is nothing left holding it up — which, incidentally, is why the trunk and branches of a tree never droop: lignin holds them up. Its leaves and young shoots have no lignin to rely on, so in a drought they wilt just like a soft plant.
22 mm ÷ 10 min = 2.2 mm per minute. Write the unit. A bare “2.2” is often not credited, because a rate without a unit is not a rate.
The increase is 51 − 22 = 29 mm. As a percentage of the original: 29 ÷ 22 × 100 = 132 %. A very common error is to calculate 51 ÷ 22 × 100 = 232 %, which is what the new value is as a percentage of the old, not the increase. Read which one was asked for.
Water vapour lost from the leaves is trapped by the bag, so the air around the shoot becomes very humid. The water vapour concentration gradient between the air spaces inside the leaf and the air outside becomes much shallower. So less water vapour diffuses out through the stomata, and both transpiration and therefore uptake fall — here to 0.6 mm per minute, roughly a quarter of the control.
Compare W and X. Blocking the upper surface (W) barely changed anything — 0.68 against the 0.79 of the untreated leaf — while blocking the lower surface (X) cut the loss to 0.15 g. So most of the water is being lost through the lower surface, which is where most of the stomata are.
Y has both surfaces blocked and still lost 0.04 g. That is the water lost by routes other than the stomata — through the cuticle, and through the sealed stalk if the seal was imperfect. It is the control, and without it you could not know whether X’s 0.15 g was real stomatal loss or background.
Stomatal loss through the lower surface is roughly 0.68 − 0.04 = 0.64 g; through the upper surface roughly 0.15 − 0.04 = 0.11 g. So the lower surface loses about six times as much. Quoting figures like this is usually worth a mark on its own.
It is shaded and cooler, so the rate of evaporation from an open stoma there is lower than it would be on the upper surface facing the sun. The plant still gets its carbon dioxide, and pays less water for it.
The Definition
Translocation is the movement of sucrose and amino acids in the phloem, from sources to sinks.
Four things in one sentence, and each is examinable: the substances, the tissue, and the two ends of the journey.
Notice how little room that leaves for vague answers. “Phloem carries food around the plant” contains no chemical, no tissue detail and no direction, and would earn nothing at all in a two-mark definition. Naming sucrose and amino acids is the easiest mark in the sub-topic.
Sources and Sinks
| Term | Definition | Typical examples |
|---|---|---|
| Source | A part of the plant that releases sucrose or amino acids | A photosynthesising leaf in summer; a storage organ in spring, emptying its store |
| Sink | A part of the plant that uses or stores sucrose or amino acids | A growing root tip; a developing fruit or seed; a flower; a storage organ in summer, filling up |
Source and sink are defined by what the tissue is doing right now, not by what organ it is. A leaf that is still growing and cannot yet make more sugar than it needs is a sink. A potato tuber sprouting in spring is a source. Any definition that names an organ instead of an action will fail on exactly the question examiners like to set.
The Same Organ, Both Things, at Different Times
This is the syllabus point that separates a full-mark answer from a half one, and one worked example teaches it better than any definition.
In midsummer the leaves are photosynthesising hard. They make far more sugar than they need, convert it to sucrose, and load it into the phloem. The tuber underground receives that sucrose and stores it, mostly as starch. Leaves = source, tuber = sink, and the movement in the phloem is downwards.
In early spring there are no leaves at all — just a tuber and a bud. The stored starch is broken down to sucrose, loaded into the phloem, and sent up to the growing shoot, which uses it for respiration and growth. Tuber = source, shoot = sink, and the movement in the phloem is now upwards.
Xylem has one job with one geometry: water enters at the bottom and leaves at the top, so it only ever needs to flow one way. Phloem delivers to whatever needs feeding, and what needs feeding is somewhere different in every season — sometimes above the source, sometimes below it, often both at once in different sieve tubes. That is why “translocation is the movement of sugar downwards” is marked wrong.
The Two Classic Experiments
Ringing (girdling). A complete ring of bark, which contains the phloem, is cut away from a tree trunk. The xylem, which lies deeper in the wood, is left intact. Over the following weeks a swelling develops above the ring, where sucrose coming down from the leaves has accumulated and cannot pass. The leaves stay green and healthy for a long time, because the xylem is still delivering water. Eventually the roots, cut off from their sugar supply, starve and the tree dies.
Two things at once, and both are worth marks. It proves that sucrose is transported in the bark (phloem), because removing the bark stops it. And it proves that water is transported somewhere else, because removing the bark did not stop the leaves being supplied. One experiment, two tissues.
Radioactive tracing. A single leaf is enclosed in a small chamber and supplied with carbon dioxide containing a radioactive isotope of carbon. The leaf photosynthesises, so the label ends up first in glucose, then in sucrose, then in the phloem. A few hours later the labelled sucrose is found in the roots, in developing fruits and in growing buds — the sinks — but not in neighbouring mature leaves, which are sources of their own and need nothing delivered.
Everything Side by Side
| Transpiration stream (xylem) | Translocation (phloem) | |
|---|---|---|
| Substances | Water and mineral ions | Sucrose and amino acids |
| Direction | Upwards only | Source to sink — up, down, or both at once |
| Driving force | Transpiration pull, from evaporation at the leaves | Loading and unloading by living cells |
| Energy from the plant? | None — the sun supplies it | Yes — from respiration |
| Cells | Dead, no contents, no cross walls | Alive, with cytoplasm and companion cells |
| Stops if the tissue is killed? | No | Yes |
The roots (24 %) and the developing pods (29 %). Neither can photosynthesise enough for itself, and both are growing, so both use or store the sucrose delivered to them. The stem’s 5 % is mostly label in transit through the phloem rather than a true sink.
Other mature leaves are sources, not sinks — they are making their own sucrose and exporting it, so nothing is delivered to them. The 1 % is a trace, not a transport route. Answers that treat a small number as “experimental error” miss the point that the number is telling you something.
Some of the labelled carbon was built into the leaf’s own structures or respired there, some is still stored as starch waiting to be converted to sucrose, and six hours is simply not long enough to export all of it. Translocation is continuous, not instant.
The Eight Sentences That Cost the Most Marks
Every one of these is written by thousands of candidates a year, sounds perfectly sensible, and earns nothing. Fix these eight and you are several marks up before you have learned any new biology.
| Never write | Write instead | Why |
|---|---|---|
| “Xylem carries water and minerals” | “Xylem carries water and mineral ions” | Minerals are rocks. The syllabus wording is mineral ions |
| “Phloem carries food” | “Phloem carries sucrose and amino acids” | Food is not a chemical. Name the substances |
| “Phloem carries glucose” | “Phloem carries sucrose” | The leaf converts glucose to sucrose before loading it. Glucose is wrong, not just imprecise |
| “Transpiration is the loss of water” | “Transpiration is the loss of water vapour from leaves” | The state matters — it leaves as a vapour, by diffusion through a pore |
| “Water is sucked up the xylem” | “Water is drawn up by transpiration pull” | Suck is not a mechanism. The named term carries the mark |
| “Water diffuses out of the mesophyll cells” | “Water evaporates from the mesophyll cells into the air spaces, then diffuses out of the stomata” | Two stages, two verbs, usually two marks |
| “Water vapour leaves by osmosis” | “Water vapour leaves by diffusion” | Osmosis needs a partially permeable membrane. A stoma is a hole |
| “The plant transpires in order to cool itself” | “Transpiration is an unavoidable consequence of having stomata open for carbon dioxide” | Cooling is a side effect, not a purpose |
Reading the Command Word
State / Name — one word or one short phrase, no explanation. Adding one wastes time you will want later.
Identify — usually pointing at a diagram. Give the name, and if asked, the evidence you used.
Describe — what happens, or what something looks like. For a xylem vessel that means the three features, not the advantages.
Explain — say why. Every mark needs a because. This is where kinetic energy, water potential and concentration gradients live.
Suggest — apply what you know to something unfamiliar. There is often more than one acceptable answer and the mark is for the reasoning.
Calculate — show the working and give the unit. Working earns a mark even when the final number is wrong.
How to Attack a Topic 8 Data Question
Almost every data question here is a potometer table, a mass-loss table or a tracer experiment. Work through them in the same order every time.
1. Read the headings and units first. Distance moved is not a rate. Mass lost is not a percentage. Uptake is not loss.
2. Find the control. There is almost always one condition that everything else is compared against. Identify it before you start explaining differences.
3. Say which way each number moves — up, down, unchanged — before you attempt any explanation.
4. Quote figures. “The rate more than doubled, from 2.2 to 5.1 mm per minute” is routinely worth a mark that “the rate increased” is not.
5. Explain through the gradient. Nearly every effect in this topic works by changing either the rate of evaporation inside the leaf or the steepness of the water vapour gradient at the stoma. Say which.
Three Scenarios to Test Yourself On
Brighter light opens the stomata wider and the windowsill is warmer, so evaporation from the mesophyll is faster; the breeze blows away the humid air just outside the stomata, keeping the water vapour gradient steep. Both raise the rate of transpiration sharply.
The rate of water loss now exceeds the rate of uptake. Cells lose water, the vacuoles shrink, the cells become flaccid, turgor pressure falls, and the soft tissues can no longer support themselves. The roots have not changed — the demand has.
Water more often (raise the supply), or reduce the demand: move it out of the direct draught, shade it during the hottest part of the day, or raise the humidity around it — grouping plants together or standing the pot on a wet gravel tray both work by making the outside air more humid and so shallowing the gradient.
(1) Water is absorbed by osmosis, not active transport — it is the mineral ions that need energy. (2) It travels in the xylem, not the phloem. (3) It is pulled, not pushed — transpiration pull, driven by evaporation at the leaves. (4) It leaves as water vapour, by diffusion, not as water by osmosis.
“Water is absorbed by the root hairs by osmosis, crosses the cortex by osmosis, and is drawn up the xylem by the transpiration pull. In the leaf it evaporates from the mesophyll cells into the air spaces and diffuses out through the stomata as water vapour.”
It contains six separate mark points and covers 8.2 and 8.3 between them. If you can produce it without hesitating, most of the long-answer questions in this topic are already half written.
The rate of transpiration depends on the size and number of the stomata. In a windy, dry place the gradient at the stoma is very steep and water would be lost extremely fast, so fewer and smaller pores reduce the loss to something the roots can keep up with.
A pit traps a pocket of humid air immediately outside the pore, which the wind cannot easily strip away. That keeps the water vapour gradient shallower and slows diffusion out — the same physics as the plastic bag in Worked Example 3, built into the leaf.
Stomata are the way carbon dioxide gets in. Fewer, smaller, sunken pores mean less carbon dioxide reaching the mesophyll, so the maximum rate of photosynthesis is lower and Q will grow more slowly. Every water-saving adaptation in a plant is paid for in sugar — and saying so is usually the last mark.
The Night-Before Checklist
1. Xylem: water and mineral ions, upwards, plus support. Phloem: sucrose and amino acids, either way.
2. Xylem is nearer the centre: root = central star, stem = inner side of each bundle, leaf = upper side of the vein.
3. Xylem vessel: lignified walls, no cell contents, no cross walls — and what each achieves.
4. Root hair: large surface area for the absorption of water and mineral ions.
5. Water in by osmosis; mineral ions by active transport, using energy from respiration.
6. Pathway: root hair → root cortex → xylem → mesophyll.
7. Transpiration = loss of water vapour from leaves. It evaporates from the mesophyll into the air spaces, then diffuses out of the stomata.
8. Transpiration pull, and a continuous column held together by forces of attraction between water molecules.
9. Rate: up with temperature and wind speed, down with humidity — all explained through evaporation or the steepness of the gradient.
10. Wilting: loss exceeds uptake → flaccid cells → no turgor pressure → droop. Source releases; sink uses or stores.