⚡ Challenge Paper Preparation

Challenge Prep: Transport in Plants

IGCSE Biology 0610 — Topic 8 — Extended

Topic 8 has no equations and only four sub-topics, which makes it look like an easy topic and makes it examined like a precision one. Water leaves a leaf as water vapour, not water. Xylem carries mineral ions, not minerals. Phloem carries sucrose, not sugar and not glucose. Water evaporates from the mesophyll and then diffuses out of the stomata — two verbs, two marks. Water is pulled up, never pushed or sucked. And translocation is not always downwards. Twelve traps, six walkthroughs, six lookalike pairs, a concept map and ten full practice questions below — every one aimed at a place where a sensible-sounding sentence earns nothing.

⚠️ Common Traps & Misconceptions

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Twelve traps that cost marks on Topic 8 challenge papers. Every one is an answer that sounds right and that mark schemes refuse.

⚠️ TRAP

Trap 1: Defining transpiration as the loss of water

The Trap“Transpiration is the loss of water from the leaves.” One word short, and it is the word being tested. It survives because the sentence sounds complete and because we all talk about plants “losing water” in ordinary speech.

The TruthTranspiration is the loss of water vapour from leaves. The state matters: the water changes into a gas inside the leaf and leaves through a pore by diffusion. If it left as a liquid there would be no gradient, no diffusion and no mechanism.

Why It MattersThe definition is worth a mark on its own and appears on nearly every Topic 8 paper. It also sets up the whole of the rest of the sub-topic: because it is a vapour, humidity and wind matter, and because it leaves through a hole, the process is diffusion rather than osmosis.

Example Question“Define transpiration. [1]”

⚠️ TRAP

Trap 2: Putting the phloem on the inside of the bundle

The TrapLabelling a stem section with the xylem outside and the phloem inside — or expecting a root to show a ring of bundles. Under time pressure people reach for whichever diagram they revised last.

The TruthOne rule fixes all three organs: the xylem is always nearer the centre of the plant. Root: a central star of xylem with phloem between the arms. Stem: bundles in a ring, xylem on the inner side of each. Leaf: xylem on the upper side of the vein, because the upper side faces the middle of the plant.

Why It Matters“Identify in diagrams and images” is a syllabus statement, so this is examined directly every year, often on a section you have not seen before. Identify the organ first — circular with a star, circular with a ring, or a flat slab — and the tissue follows automatically.

Example Question“The diagram shows a transverse section of a root. Name the tissue labelled P and state what it transports. [2]”

⚠️ TRAP

Trap 3: Writing that phloem carries food

The Trap“Phloem carries food around the plant.” It is what most textbooks said at Key Stage 3, and it feels like an answer. It contains no chemical, so it earns nothing.

The TruthPhloem transports sucrose and amino acids. Both are named in the syllabus and both are usually needed for the mark. “Food” is not a substance, in the same way that “stuff” is not.

Why It MattersEvery marker is looking for the two named substances. It costs nothing to write them, and the same habit — name the chemical — is worth marks in digestion, photosynthesis and respiration too.

Example Question“State the substances transported by xylem and by phloem. [3]”

⚠️ TRAP

Trap 4: Saying glucose instead of sucrose

The Trap“The glucose made in photosynthesis is transported in the phloem to the roots.” Everything about that sentence is nearly right, which is what makes it dangerous.

The TruthGlucose is what photosynthesis makes, but the leaf converts it to sucrose before loading it into the phloem. Sucrose is relatively unreactive, so it travels without being used up on the way. In this topic, glucose is a wrong answer, not a vague one.

Why It MattersIt is one word, it appears in the mark scheme of every translocation question, and it is the difference between a full-mark definition and a zero-mark one. If you are asked for an advantage, the answer is that sucrose is unreactive and is not consumed in transit.

Example Question“Name the sugar transported in the phloem and suggest one advantage of using it rather than glucose. [2]”

⚠️ TRAP

Trap 5: Saying water leaves the stomata by osmosis

The Trap“Water passes out of the stomata by osmosis.” Osmosis is the word this whole area of the syllabus has trained you to use for water, so the hand writes it automatically.

The TruthOsmosis requires a partially permeable membrane. A stoma is a hole. Water vapour leaves by diffusion, down a concentration gradient from the humid air spaces to the drier air outside.

Why It MattersGetting this wrong destroys the explanation of humidity and wind, because both work by changing the steepness of a diffusion gradient. It also flags to the examiner that osmosis has been learned as a word for water rather than as a defined process.

Example Question“Name the process by which water vapour leaves a leaf and explain how wind increases its rate. [3]”

⚠️ TRAP

Trap 6: Using one verb for the two-stage exit

The Trap“Water evaporates out of the stomata”, or “water diffuses out of the mesophyll cells and out of the leaf.” Both compress two marks into one.

The TruthTwo stages, two verbs. Water evaporates from the wet cell walls of the mesophyll cells into the air spaces; the water vapour then diffuses out through the stomata. Evaporation is a change of state; diffusion is the movement that follows.

Why It MattersThis is the highest-frequency sentence in Topic 8 and routinely carries two of the marks in a longer answer. It is also the sentence that makes the “large internal surface area” point make sense — the area is the surface the water evaporates from.

Example Question“Describe how water is lost from a leaf. [2]”

⚠️ TRAP

Trap 7: Saying the plant sucks the water up, or that root pressure does it

The Trap“The roots push the water up the stem”, or “the leaves suck the water up.” Both describe something happening; neither names a mechanism.

The TruthThe named term is transpiration pull. Evaporation from the mesophyll lowers the water potential there, water leaves the xylem, the column comes under tension, and the tension is transmitted all the way down because of the forces of attraction between water molecules. Root pressure exists but is far too weak to reach the top of a tree.

Why It MattersTwo separate marks live here: the named term, and the reason the column does not break. It also explains a set of otherwise unrelated facts — why a stem must be cut under water, why an air bubble is fatal, and why lignin has to stop the vessel collapsing.

Example Question“Explain how water moves up the xylem of a tree 30 m tall. [5]”

⚠️ TRAP

Trap 8: Treating transpiration as something the plant does on purpose

The Trap“The plant transpires in order to cool itself and to pull water and mineral ions up from the roots.” Both consequences are real, which is why this is such a persuasive wrong answer.

The TruthThe stomata are open so that carbon dioxide can diffuse in for photosynthesis, and the leaf has a huge internal surface area for the same reason. A wet surface exposed to air must evaporate. Transpiration is therefore an unavoidable consequence, not a purpose. Examiners will credit the useful side effects; they will not credit purpose.

Why It MattersEvery “suggest why” question about desert plants, sunken stomata, thick cuticles and wilting turns on this idea, because it supplies the cost: anything that reduces water loss also reduces carbon dioxide entry. That trade-off is usually the last mark in the question.

Example Question“A cactus has very few stomata. Suggest one disadvantage of this. [2]”

⚠️ TRAP

Trap 9: Swapping osmosis and active transport at the root

The Trap“Water is absorbed by active transport and mineral ions enter by osmosis.” Confident, fluent, and exactly the wrong way round.

The TruthWater enters by osmosis, down a water potential gradient, through the partially permeable cell membrane, using no energy. Mineral ions are taken up largely by active transport, against their concentration gradient, by protein carriers, using energy from respiration. That is why a root hair cell is full of mitochondria. And write mineral ions, not minerals.

Why It MattersThe swap makes three favourite exam contexts unanswerable: waterlogged soil, cold soil, and roots in a solution bubbled with nitrogen. In each of them ion uptake collapses and water uptake barely changes, and only the correct pairing explains why.

Example Question“A field is flooded for four days. Explain why the crop shows mineral deficiency although the soil is rich in mineral ions. [4]”

⚠️ TRAP

Trap 10: Saying translocation is the movement of sugar downwards

The Trap“Sucrose is translocated down from the leaves to the rest of the plant.” True in July, false in March, and the mark scheme knows it.

The TruthTranslocation is movement from sources to sinks, and a sink can be above or below the source. A potato tuber in spring is a source sending sucrose up to a growing shoot; a developing fruit high on a plant is a sink fed from below. Different sieve tubes can even be carrying material in opposite directions at once.

Why It Matters“Explain why some parts of a plant may act as a source and a sink at different times” is a syllabus statement in its own right, so this is examined directly. It is also the cleanest way to show you understand that source and sink describe an action, not an organ.

Example Question“A potato tuber sprouts in spring. State whether it is a source or a sink and explain the direction of movement. [3]”

⚠️ TRAP

Trap 11: Claiming a potometer measures transpiration exactly

The Trap“The potometer shows that the rate of transpiration was 3.2 mm per minute.” The number is fine; the noun is not.

The TruthA potometer measures the rate of water uptake. A small proportion of the water taken up is used in photosynthesis and retained in the cells to keep them turgid, so uptake slightly exceeds loss. Over a short period the two are close enough for uptake to be a good estimate of transpiration — and saying exactly that is what earns the evaluation mark.

Why It MattersEvaluation marks are handed out for accurate qualification, not for complaining that an experiment is unreliable. Notice too that the weighed-plant method fixes this weakness by measuring loss directly, which is why comparing the two methods is such a common exam question.

Example Question“A student says the potometer has measured the rate of transpiration. Evaluate this. [2]”

⚠️ TRAP

Trap 12: Assuming xylem must be alive because it does a job

The Trap“The xylem cells actively transport the water upwards”, or drawing a xylem vessel with a nucleus in it. Living tissue feels like the default for anything that works.

The TruthA mature xylem vessel is dead: thick lignified walls, no cell contents, and no cross walls, so it is one long open pipe. That is not a defect — it is what makes the flow fast. Phloem, by contrast, is alive, keeps its cytoplasm and needs energy from respiration.

Why It MattersIt explains the classic pair of experiments: a stem killed by steam still conducts water, but translocation stops within minutes of a respiratory poison. Three structural features and three consequences are also a Supplement statement, so they can be asked for directly.

Example Question“Relate the structure of a xylem vessel to its function. [3]”

🔍 Step-by-Step Walkthroughs

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Six challenge-level questions worked through in the order you should actually think about them. Try each part before revealing the next step.

Walkthrough 1 — Four Leaves and a Layer of Petroleum JellyFour leaves of equal area from one plant had their stalks sealed with wax and petroleum jelly applied as shown. After 24 hours in the same room the mass lost was: W upper surface only, 0.62 g. X lower surface only, 0.14 g. Y both surfaces, 0.03 g. Z neither surface, 0.71 g. (a) What do the results show about the stomata? [3] (b) Why was leaf Y included? [2] (c) Calculate the loss through the lower surface alone. [2]

Z is the comparison, Y is the control

Z has nothing blocked, so it shows what a normal leaf loses: 0.71 g. Y has everything blocked, so it shows what is lost by routes other than the stomata: 0.03 g. Identifying these two before touching W and X is what turns four numbers into an argument.

Blocking the lower surface is what makes the difference

Blocking the upper surface (W) reduced the loss only from 0.71 to 0.62 g. Blocking the lower surface (X) reduced it from 0.71 to 0.14 g. So most of the water was leaving through the lower surface, and therefore most of the stomata are on the lower surface. Notice the last step: the data tell you where the water went, and one further sentence converts that into a conclusion about stomata.

It measures the background, and it disproves a myth

Y measures water lost through the cuticle and through any imperfection in the wax seal. Its small value (0.03 g) means the differences between the other leaves can safely be attributed to the stomata. It also shows the cuticle is not perfectly waterproof — small, but not zero.

Subtract the background, then compare

Leaf W has only its lower surface working, so its stomatal loss through the lower surface is 0.62 − 0.03 = 0.59 g. Leaf X has only its upper surface working: 0.14 − 0.03 = 0.11 g. The lower surface loses more than five times as much. Quoting a ratio like that is very often worth a mark on its own.

Full-mark answerMost stomata are on the lower surface: blocking it cut the loss from 0.71 to 0.14 g, while blocking the upper surface cut it only to 0.62 g. Leaf Y is the control, measuring the 0.03 g lost through the cuticle. Correcting for that, the lower surface accounts for 0.59 g and the upper for 0.11 g.

Walkthrough 2 — A Potometer Under Four ConditionsThe same leafy shoot was tested for 12 minutes under each condition. Still air 20 °C: 84 mm. Moving air 20 °C: 158 mm. Still air 30 °C: 121 mm. Still air 20 °C, shoot in a clear plastic bag: 19 mm. (a) Calculate the rate in still air at 20 °C. [2] (b) Calculate the percentage increase caused by moving the air. [2] (c) Explain the plastic bag result. [3] (d) State what the potometer actually measures. [1]

Divide, and write the unit

84 ÷ 12 = 7.0 mm per minute. The working line earns a mark of its own, so write the division out even if you can do it in your head, and never leave the unit off — a number without a unit is not a rate.

The difference goes on the top

The increase is 158 − 84 = 74 mm. As a percentage of the original: 74 ÷ 84 × 100 = 88 %. The tempting alternative, 158 ÷ 84 × 100 = 188 %, is the new value as a percentage of the old one, which is a different quantity. Read the question and decide which it wants before you touch the calculator.

Trapped vapour, shallow gradient, slow diffusion

The bag traps the water vapour lost from the leaves, so the air around the shoot becomes very humid. The water vapour concentration gradient between the air spaces and the outside air becomes much shallower, so less vapour diffuses out through the stomata and uptake falls from 84 to 19 mm. Three linked ideas, three marks.

Uptake, and why that is not quite loss

A potometer measures the rate of water uptake. Some of the water taken in is used in photosynthesis and kept in the cells, so uptake is slightly greater than the volume actually transpired. Over 12 minutes the difference is small, so uptake is a good estimate — but the question asked what was measured, and the honest answer is uptake.

Full-mark answer7.0 mm per minute; an 88 % increase with moving air; the bag raises humidity and flattens the water vapour gradient so less diffuses out; and the apparatus measures uptake, not loss.

Walkthrough 3 — Two Stems, Two Different Ways of Being DamagedStem A is held in steam for one minute so that every cell is killed, then stood in red dye. Stem B is living but is treated with a respiratory poison, then stood in the same dye. In both stems the dye rises into the leaf veins. In stem B, however, no sucrose leaves the leaves. Explain what each result shows. [5]

One removes life; the other removes energy

Steam kills every cell, so nothing living remains anywhere. The poison leaves the cells structurally intact but stops them releasing energy in respiration. Two different removals, and the difference between the results is what tells you something.

Water movement needs no living cells at all

So the force cannot come from anything the plant is doing. It comes from evaporation at the leaves — the transpiration pull — and the energy is supplied by the sun. That also disposes of root pressure, since the stem has no roots.

Translocation is work, and work needs energy

The phloem cells are still there but cannot respire, and translocation stops. So loading sucrose into the phloem requires energy from respiration and therefore requires living cells. Xylem is dead, phloem is alive, and this pair of experiments proves both halves at once.

Each experiment is the other one’s control

Stem A alone would tell you nothing about the phloem, and stem B alone would leave open the possibility that the poison had blocked the vessels physically. Because the dye still rose in B, you know the xylem is unaffected, so the failure must be in the phloem. Designing experiments in matched pairs is what makes an inference safe.

Full-mark answerWater moves in dead xylem, so the transpiration stream needs no living cells and is driven by evaporation at the leaves. Sucrose does not move without respiration, so translocation needs energy and living phloem. Together the two results separate the tissues cleanly.

Walkthrough 4 — Two Curves Through One Hot DayA graph shows the rate of water loss and the rate of water uptake for the same plant from midnight to midnight. Loss stays near 2 units until 04:00, peaks at 92 units at about 10:00 and is back to 2 by 20:00. Uptake rises more slowly, peaks at about 70 units at 12:00 and stays above the loss curve from about 14:00 until the evening. (a) When is the plant most likely to wilt? [1] (b) Explain your answer. [3] (c) Explain the afternoon crossover. [2]

Look for the biggest vertical distance between the curves

The instinctive answer is “when loss is highest”, and it is wrong. Wilting begins when loss exceeds uptake by the largest margin, which here is at about 10:00 — 92 against roughly 55. Reading two curves means reading the difference between them.

The cells run down a water deficit

Because more water is leaving than arriving, the cells lose water and become flaccid. With too little turgor pressure pressing outwards on the cell walls, the soft, non-woody tissues cannot support themselves, and the leaves droop.

The debt is repaid in the afternoon

As the temperature falls and the stomata begin to close, the rate of loss drops quickly. Uptake continues, so the deficit built up in the morning is repaid and the cells become turgid again. That is why a plant that wilts at midday can look perfectly healthy by evening.

A small overnight loss is a real result

The loss curve settles near 2 units rather than at zero, because a little water still escapes through the cuticle and not every stoma closes completely. When a graph disagrees with a remembered rule, the graph wins — and here the non-zero value is telling you the cuticle is a minor but genuine route.

Full-mark answerWilting is most likely around 10:00, when loss exceeds uptake by the widest margin; the cells become flaccid and lose the turgor pressure that supports them. In the afternoon loss falls faster than uptake, so the deficit is repaid and the plant recovers.

Walkthrough 5 — Barley Seedlings, Oxygen and ColdIdentical seedlings were grown in dilute nutrient solution for a week. Air bubbled, 20 °C: water 42 cm³, potassium 100 units. Air bubbled, 5 °C: 31 cm³, 24 units. Nitrogen bubbled, 20 °C: 39 cm³, 18 units. Nitrogen bubbled, 5 °C: 28 cm³, 6 units. Explain these results as fully as you can. [6]

One column barely moves; the other collapses

Water uptake falls from 42 to 28 cm³ — a drop of about a third. Potassium uptake falls from 100 to 6 units — a drop of 94 %. Two very different responses to the same treatments means two different mechanisms, and saying so is the first mark.

Passive versus active

Water enters by osmosis, which is passive and needs no energy from the plant, so removing oxygen hardly touches it. Potassium ions are taken up largely by active transport against their concentration gradient, which does need energy, so anything that reduces the energy supply reduces it sharply.

Two ways of reducing the same thing

Nitrogen displaces oxygen, so aerobic respiration falls and potassium uptake drops from 100 to 18. Cold slows the enzyme-controlled reactions of respiration, so uptake drops from 100 to 24. Doing both gives the lowest value of all, 6 units, because both routes to releasing energy have been restricted at once.

Passive does not mean unaffected

Water uptake still falls at 5 °C because the molecules have less kinetic energy, so osmosis and diffusion are slower, and because a cooler plant transpires less so the pull is weaker. This is the mark most often missed, and it is there for candidates who explain every feature of the data rather than the obvious one.

Full-mark answerWater uptake is passive (osmosis) so it changes little; potassium uptake is active so it collapses when either oxygen or temperature restricts respiration; both together give the lowest value; and the modest fall in water uptake at 5 °C reflects lower kinetic energy and reduced transpiration.

Walkthrough 6 — Following Labelled Carbon Around a Bean PlantOne mature leaf was supplied with radioactively labelled carbon dioxide for 30 minutes. Six hours later the label recovered was: labelled leaf 38 %, other mature leaves 2 %, stem 6 %, roots 22 %, developing pods 32 %. (a) Explain how the label got into the phloem. [2] (b) Identify the sinks and justify your choice. [2] (c) Explain the 2 % and the 38 %. [3]

Carbon dioxide, then glucose, then sucrose

The labelled carbon entered the leaf as carbon dioxide and was fixed in photosynthesis, first into glucose. The leaf then converted the glucose to sucrose and loaded it into the phloem. Note the conversion: it is the reason “glucose is translocated” is marked wrong.

Roots and pods, and the justification matters

The roots (22 %) and the developing pods (32 %) received the label. Both are growing and neither can photosynthesise enough for itself, so both use or store what is delivered — which is the definition of a sink. The stem’s 6 % is mostly label in transit rather than a true sink.

2 % is not experimental error

Other mature leaves are sources in their own right: they make more sucrose than they need and export it, so almost nothing is delivered to them. That near-zero value is the evidence, not the noise. Treating small numbers as error is one of the most expensive habits in data questions.

Export takes time — and it goes both ways

The 38 % still in the labelled leaf reflects carbon respired or built into the leaf itself, carbon stored temporarily as starch, and the simple fact that translocation is continuous rather than instant. Notice too that the label reached the roots (below) and the pods (above): translocation is not restricted to one direction.

Full-mark answerThe label was fixed in photosynthesis, converted to sucrose and loaded into the phloem. The roots and pods are sinks, receiving 22 and 32 %. Other mature leaves are sources, so they receive almost none. The 38 % remaining reflects respiration, storage as starch, and the time export takes.

🔍 Spot the Difference

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Six pairs that look almost identical and have different answers. The distinction is where the marks live.

Question A

Name the process by which water gets from a mesophyll cell wall into an air space.

Evaporation. Liquid water on the wet cell wall changes into water vapour. It is a change of state, and it needs heat energy — which is why temperature matters so much.

Question B

Name the process by which the water vapour then leaves through a stoma.

Diffusion. The vapour moves down its own concentration gradient, from the humid air spaces to the drier air outside. No change of state, and no membrane involved.

Key DifferenceTwo stages, two verbs, and usually two marks. Evaporation happens inside the leaf, diffusion happens out of it. Compressing them into one verb is the commonest way to score one mark instead of two on the highest-frequency question in the topic.

Question A

Where is the xylem in a transverse section of a root?

In the centre, as a star. The phloem sits in the gaps between the arms of the star. There is no ring of bundles in a root.

Question B

Where is the xylem in a transverse section of a stem?

On the inner side of each bundle, and the bundles are arranged in a ring near the outside of the stem. The phloem is on the outer side of each bundle.

Key DifferenceBoth follow the same rule — xylem nearer the centre — but the arrangement differs: one central mass in the root, separate bundles in a ring in the stem. Identify the organ first from the shape of the whole section, and the tissue follows without having to remember a picture.

Question A

A root is deprived of oxygen. What happens to water uptake?

Almost unchanged. Water enters by osmosis, which is passive: it is driven by the water potential gradient and costs the plant nothing.

Question B

The same root is deprived of oxygen. What happens to mineral ion uptake?

It falls sharply. Ions are moved against their concentration gradient by active transport, which needs energy from aerobic respiration.

Key DifferencePassive versus active is the whole of this pair, and it is why waterlogged soil produces mineral deficiency rather than drought. Any answer that has water entering by active transport, or ions entering by osmosis, will get both halves wrong at once.

Question A

In July, a potato tuber is swelling while the leaves are fully grown. Source or sink?

Sink. It is receiving sucrose from the photosynthesising leaves and storing it, so material moves down the phloem.

Question B

In March, the same tuber is sprouting a shoot and there are no leaves. Source or sink?

Source. The store is converted to sucrose and exported to the growing shoot, so material moves up the phloem.

Key DifferenceNothing about the organ has changed — only what it is doing. Source and sink describe an action, never an organ, and this pair is the reason “translocation is the movement of sugar downwards” is marked wrong.

Question A

Define transpiration.

The loss of water vapour from leaves. It is an event at the leaf surface, and the word “vapour” is doing the work.

Question B

What is meant by the transpiration stream?

The movement of water up the xylem, from roots to leaves, drawn by the transpiration pull. It is the consequence, not the definition.

Key DifferenceOne is the loss at the top; the other is the flow that results. Offering “the movement of water up the plant” when asked to define transpiration answers the wrong one of these two, and it is the single most common way of losing the definition mark.

Question A

What does a potometer measure?

The rate of water uptake. Uptake is slightly greater than loss, because some water is used in photosynthesis and retained in the cells — so it is a good estimate, not an identity.

Question B

What does weighing a potted plant, with the pot sealed in a bag, measure?

Water loss, directly. Sealing the pot stops evaporation from the soil being counted, so the mass lost is water lost from the leaves.

Key DifferenceThe second method fixes exactly the weakness of the first. If a question asks you to compare two methods, that is the comparison it wants — and being able to say what one method does better is worth more than listing faults in both.

🔗 Transport in Plants Concept Map

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Click each node to see how the sub-topics connect. Three frameworks carry the whole topic.

⭐ CORE FRAMEWORK 1

Two pipes → two cargoes → one rule about position

What Each Tube Carries, and Which Way ▶

Xylem: water and mineral ions, upwards only. It also provides support.

Phloem: sucrose and amino acids, from sources to sinks — so up, down, or both at once.

Four words decide most of the marks in 8.1: mineral ions rather than minerals, and sucrose rather than sugar or glucose.

One Position Rule for Three Organs ▶

Xylem is always nearer the centre of the plant. Root: central star. Stem: inner side of each bundle in a ring. Leaf: upper side of the vein.

A leaf vein is a stem bundle folded outwards, so the side that faced the middle of the plant now faces up. That is why the rule holds in all three.

In an unfamiliar section, identify the organ first: flat slab = leaf, circular with a star = root, circular with a ring of bundles = stem.

Dead and Empty, or Alive and Busy ▶

Xylem vessel: lignified walls (rigid, so it does not collapse under tension, and it supports the plant), no cell contents (an open pipe), no cross walls (one continuous tube).

Phloem: living, with cytoplasm and companion cells. Loading sucrose is work, so it needs energy from respiration.

Test: a stem killed by steam still conducts water; translocation stops within minutes of a respiratory poison. One pair of experiments, both conclusions.

⭐ CORE FRAMEWORK 2

Soil → root hair → cortex → xylem → mesophyll → air

Getting In: Two Substances, Two Mechanisms ▶

Water enters by osmosis: the dilute soil solution has the higher water potential, and the cell membrane is partially permeable. No energy required.

Mineral ions enter largely by active transport, against the gradient, through protein carriers, using energy from respiration. Hence the many mitochondria.

The root hair is one epidermal cell drawn into a long thin projection: a huge surface area for absorption, and a thin wall for a short distance.

Going Up: The Pull From the Top ▶

Water crosses the cortex cell to cell by osmosis, down a falling water potential gradient, and enters the xylem in the centre of the root.

Evaporation in the leaf lowers the mesophyll water potential, water leaves the xylem, and the column comes under tension: the transpiration pull.

Forces of attraction between water molecules keep the column continuous, so the pull reaches all the way to the root. Break the column with an air bubble and the flow stops.

Getting Out, and the Three Factors ▶

Water evaporates from the mesophyll into the air spaces, then diffuses out through the stomata as water vapour.

Rate rises with temperature (more kinetic energy) and wind speed (the humid layer is stripped away, keeping the gradient steep); it falls with humidity (shallower gradient).

Loss greater than uptake → cells flaccid → no turgor pressure → wilting. Damaging, and partly protective.

⭐ CORE FRAMEWORK 3

Source → phloem → sink, in whichever direction the plant needs today

Definitions That Name an Action, Not an Organ ▶

Source: a part that releases sucrose or amino acids. Sink: a part that uses or stores them.

A newly unfolded leaf is a sink, because it is still growing. A tuber in spring is a source, because it is emptying. The organ tells you nothing on its own.

That is exactly why the syllabus asks you to explain how a part can be both, at different times.

The Evidence: Ringing, Tracers and Aphids ▶

Ringing: remove a complete ring of bark and a swelling forms above it, while the leaves stay green — sucrose moves in the bark, water does not.

Radioactive tracer supplied to one leaf as carbon dioxide appears in roots and fruits but not in other mature leaves — sources export, sinks import, and the movement goes both ways.

Aphids tapping a single sieve tube get sap under pressure, rich in sucrose — a direct sample rather than an inference.

Where This Connects to Everything Else ▶

Osmosis (3.2) explains water uptake, water movement across the cortex, guard cells opening, and wilting. Active transport (3.3) explains mineral ion uptake.

Photosynthesis (6.1) supplies the glucose that becomes the sucrose in the phloem, and it is the reason the stomata are open at all.

Leaf structure (6.2) supplies the air spaces, the mesophyll and the stomata — so most of 8.3 is a Topic 6 diagram being read for a different purpose.

❌ “Why Is This Wrong?” Exercises

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Six real student answers. Find the fault before you reveal it.

Exercise 1: “Describe how water is lost from a leaf. [2]”

Student’s Answer“Water passes out of the stomata by osmosis, which is called transpiration.”

The FlawTwo errors and one omission. Osmosis needs a partially permeable membrane and a stoma is a hole. The water leaves as a vapour, not as liquid water. And the first stage — evaporation inside the leaf — is missing altogether.

Correct Answer“Water evaporates from the wet cell walls of the mesophyll cells into the air spaces [1], and the water vapour then diffuses out through the stomata, down a concentration gradient [1].”

Key RuleTwo stages, two verbs. If a mark scheme offers two marks for “how water is lost”, it is almost always one for each stage.

Exercise 2: “Explain how water moves from the roots to the top of a tall tree. [4]”

Student’s Answer“The roots absorb water by active transport and push it up the xylem. The leaves also suck it up, and it travels up the phloem to the top.”

The FlawFour errors in two sentences. Water is absorbed by osmosis, not active transport. It is pulled, not pushed. “Suck” is not a mechanism. And it travels in the xylem, not the phloem — the answer even contradicts itself.

Correct Answer“Water evaporates from the mesophyll cells and diffuses out of the stomata [1]. Those cells then have a lower water potential, so water moves into them from the xylem [1]. This puts the column under tension — the transpiration pull [1]. Because there are forces of attraction between water molecules the column stays continuous, so the pull is transmitted all the way to the roots [1].”

Key RuleTwo named ideas carry this whole question: transpiration pull, and the attraction between water molecules. Neither is optional, and neither can be replaced with a verb like suck or push.

Exercise 3: “A field is flooded for four days and the crop shows mineral deficiency although the soil is rich in mineral ions. Explain. [3]”

Student’s Answer“The flood water washed all the mineral ions out of the soil, so there were none left for the plant to absorb.”

The FlawIt contradicts the question, which says the soil is still rich in mineral ions. Whenever an answer has to deny the information you were given, it is the wrong answer — the stem is not decoration.

Correct Answer“Flooding fills the air spaces in the soil, so the roots cannot obtain enough oxygen [1]. The rate of aerobic respiration falls, so less energy is released [1]. Less energy means less active transport, so fewer mineral ions are absorbed even though plenty are present [1].”

Key RuleRead the stem for what it rules out. The phrase “although the soil is rich in mineral ions” is there precisely to eliminate the answer this student gave.

Exercise 4: “Explain why a plant wilts on a hot afternoon even though it has been watered. [3]”

Student’s Answer“The heat dries out the plant and the xylem vessels collapse, so no more water can get to the leaves.”

The Flaw“Dries out” is not a mechanism, and lignified xylem does not collapse — preventing exactly that is what the lignin is for. The real explanation is a mismatch between two rates, and the answer never mentions either.

Correct Answer“On a hot afternoon the rate of water loss exceeds the rate of uptake [1]. The cells lose water by osmosis and become flaccid [1]. Without enough turgor pressure pressing outwards on the cell walls, the soft tissues cannot support themselves and the leaves droop [1].”

Key RuleWilting is always a balance problem, never a shortage in the soil when the question tells you the plant was watered. The marks live in the osmosis vocabulary you already have: flaccid, turgor pressure, turgid.

Exercise 5: “Define translocation. [2]”

Student’s Answer“Translocation is when the glucose made in the leaves is transported down the plant to the roots to be stored.”

The FlawThree problems. The sugar is sucrose, not glucose — the leaf converts it before loading. Amino acids are missing. And “down the plant to the roots” is true only sometimes: in spring a tuber sends sucrose up to a growing shoot.

Correct Answer“Translocation is the movement of sucrose and amino acids [1] in the phloem, from sources to sinks [1].”

Key RuleUsing source-and-sink language instead of “from the leaves to the roots” makes the definition correct in every season, and it is the phrasing the mark scheme uses.

Exercise 6: “A student measures the rate of transpiration with a potometer and gets 4.6 mm per minute. Evaluate this measurement. [2]”

Student’s Answer“The result is not reliable because the experiment could have gone wrong and there might have been mistakes in the readings.”

The FlawThis is a complaint, not an evaluation. It identifies nothing specific about a potometer and would apply equally to any experiment in any subject, so it earns nothing.

Correct Answer“A potometer measures the rate of water uptake, not the rate of transpiration [1]. A small proportion of the water taken up is used in photosynthesis and retained in the cells, so uptake slightly overstates the volume transpired — though over a short period it is a good estimate [1].”

Key RuleAn evaluation names what was actually measured, says in which direction the result is biased, and says whether the difference matters. Vague doubt is never credited.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions. Write your answer first, then reveal the model answer and the examiner’s notes.

Question 1

[6 marks]

(a) State the substances transported by xylem and by phloem, and give one further function of xylem. [3] (b) Describe how you would identify the xylem in a transverse section of a root, a stem and a leaf. [3]

Model Answer(a) Xylem transports water [1] and mineral ions; phloem transports sucrose and amino acids [1]; xylem also provides support [1].
(b) In a root the xylem is the central star-shaped mass [1]. In a stem it is the inner half of each of the bundles arranged in a ring [1]. In a leaf it is the upper half of the vein [1]. In all three it is the tissue nearer the centre of the plant, and it can be confirmed by its thick lignified walls.

Examiner’s Notes“Minerals” and “nutrients” are refused; the wording is mineral ions. In (b), a description of each organ earns the marks, but stating the underlying rule as well protects you if the section in the exam is drawn at an unfamiliar angle.

Question 2

[6 marks]

Relate the structure of a xylem vessel to its function, and explain why a xylem vessel being dead is an advantage rather than a defect. [6]

Model AnswerThick walls containing lignin [1], which are rigid and waterproof so the vessel does not collapse under the tension of the transpiration pull, and which also support the plant [1]. No cell contents [1], so the lumen is an open pipe offering very little resistance to flow [1]. No cross walls — the cells are joined end to end [1] — so many cells form one long continuous tube from root to leaf [1]. Being dead is what makes the tube empty, and the flow needs no energy from the plant because it is driven by evaporation at the leaves.

Examiner’s NotesThis is a three-features, three-consequences question, so a list of features alone scores half. The word lignin is required; “thick strong walls” is usually one mark short.

Question 3

[7 marks]

(a) Describe three adaptations of a root hair cell and state what each achieves. [3] (b) Explain how water and mineral ions enter the cell, naming the process in each case. [4]

Model Answer(a) A long thin projection giving a large surface area for absorption [1]; a thin cell wall, so a short distance to cross [1]; many mitochondria, releasing energy for active transport [1].
(b) Water enters by osmosis [1], because the dilute soil solution has a higher water potential than the cell sap and the membrane is partially permeable [1]. Mineral ions enter by active transport [1], against their concentration gradient, through protein carriers and using energy from respiration [1].

Examiner’s NotesNever write that water enters by active transport — it costs the mark here and makes the flooding and cold-soil questions impossible later. Do not offer chloroplasts as an adaptation: a root hair is underground and has none.

Question 4

[6 marks]

Describe the pathway of water from the soil to the air outside a leaf, naming every tissue and the process at each stage. [6]

Model AnswerWater enters the root hair cell by osmosis [1]. It crosses the root cortex cells, still by osmosis, down a gradient of falling water potential [1]. It enters the xylem in the centre of the root [1] and is drawn up in a continuous column by the transpiration pull [1]. In the leaf it reaches the mesophyll cells and evaporates from their walls into the air spaces [1]. The water vapour then diffuses out through the stomata [1].

Examiner’s NotesSix marks means six separate steps, so number them or start a new line for each. The cortex comes before the xylem, because in a root the xylem is right in the centre. Any route involving the phloem is wrong.

Question 5

[6 marks]

Explain how temperature, wind speed and humidity each affect the rate of transpiration. [6]

Model AnswerTemperature: a rise increases the rate [1], because water molecules gain more kinetic energy, so evaporation from the mesophyll walls and diffusion out of the stomata are both faster [1]. Wind speed: an increase raises the rate [1], because moving air removes the humid layer just outside the stomata, keeping the water vapour concentration gradient steep [1]. Humidity: an increase lowers the rate [1], because the air outside already contains a lot of water vapour, so the gradient is shallower and less diffuses out [1].

Examiner’s NotesEach factor needs a direction and an explanation, and the explanation should run through evaporation or through the steepness of the gradient. “Wind blows the water away” and “humid air stops the water escaping” describe results rather than mechanisms.

Question 6

[7 marks]

(a) Describe how you would set up a potometer to compare water uptake in still and moving air, including two precautions. [4] (b) The bubble moved 45 mm in 15 minutes in still air and 126 mm in 15 minutes with a fan. Calculate both rates and the percentage increase. [3]

Model Answer(a) Cut the shoot and assemble the apparatus under water, so no air enters the xylem and breaks the water column [1]. Seal the joint with petroleum jelly so the apparatus is airtight, and introduce one air bubble [1]. Allow it to equilibrate, then time how far the bubble moves over a set distance [1]. Repeat with a fan at a fixed distance, keeping temperature, humidity, light and the shoot the same, and take repeats to find a mean [1].
(b) 45 ÷ 15 = 3.0 mm per minute [1]; 126 ÷ 15 = 8.4 mm per minute [1]; increase = (8.4 − 3.0) ÷ 3.0 × 100 = 180 % [1].

Examiner’s NotesCutting under water is not a detail — a broken water column cannot transmit the pull, and the apparatus becomes meaningless. In (b), 280 % is the classic error: that is the new rate as a percentage of the old, not the increase.

Question 7

[6 marks]

Explain fully how and why wilting occurs, and explain why wilting is partly protective. [6]

Model AnswerWilting occurs when the rate of water loss exceeds the rate of uptake [1]. Cells lose water by osmosis and their vacuoles shrink [1], so the cells become flaccid [1]. There is then too little turgor pressure pressing outwards on the cell walls to support the soft, non-woody tissue, and the leaves and young stems droop [1]. It is damaging because drooping leaves intercept less light and the closed stomata cut off the carbon dioxide supply, so photosynthesis slows [1]. It is protective because the reduced exposed surface and the closed stomata greatly reduce further water loss, allowing uptake to catch up [1].

Examiner’s Notes“Explain fully” with six marks is inviting both sides. A plant that wilts at midday and recovers by evening has behaved correctly, and saying so is what turns a four-mark answer into a six-mark one.

Question 8

[6 marks]

(a) Define a source and a sink. [2] (b) Explain how a potato tuber can be each at different times of year, and state the direction of movement in the phloem in each case. [4]

Model Answer(a) A source releases sucrose or amino acids [1]; a sink uses or stores them [1].
(b) In summer the leaves photosynthesise and export sucrose, so the tuber is a sink [1] and movement in the phloem is downwards [1]. In early spring there are no leaves, so the store is converted to sucrose and exported to the growing shoot: the tuber is now a source [1] and movement is upwards [1].

Examiner’s NotesDefine both terms by the action, never by the organ. This question exists to test the claim that translocation is “downwards”, and the spring half is where the marks are usually lost.

Question 9

[7 marks]

A complete ring of bark is removed from a tree trunk in spring. (a) Predict and explain what happens at the ring and to the leaves over the next few weeks. [4] (b) Explain why the tree eventually dies. [2] (c) Suggest why removing only a narrow vertical strip of bark does much less harm. [1]

Model Answer(a) A swelling forms above the ring [1], because sucrose being translocated down from the leaves cannot pass and accumulates [1]. The leaves stay green and healthy for weeks [1], because the xylem is deeper in the trunk and still supplies water and mineral ions [1].
(b) The roots below the ring receive no sucrose, so they cannot respire enough to survive and they die [1]; without roots, water and mineral ion uptake stops and the whole tree dies [1].
(c) The phloem forms a complete ring, so a narrow vertical strip leaves the rest of it intact and sucrose can travel past the wound [1].

Examiner’s NotesThe tree dies from the bottom up, which surprises people. Part (c) is really a test of whether you picture the phloem as a cylinder rather than as a single tube — a direct check on whether the transverse sections in 8.1 were understood.

Question 10

[8 marks]

A grower removes many of the leaves near his developing tomatoes so that more light reaches the fruit, and closes the greenhouse vents on hot days to reduce wilting. The tomatoes turn out smaller than usual and some plants develop fungal disease. (a) Explain why the tomatoes are smaller. [4] (b) Explain how closing the vents reduces wilting, and suggest two disadvantages. [4]

Model Answer(a) A developing fruit is a sink: it cannot photosynthesise enough for itself and imports sucrose [1]. The nearby leaves are its main sources [1]. Removing them reduces the sucrose translocated to the fruit [1], so less material is available for growth and storage and the tomatoes stay small — extra light does not compensate, because a fruit contributes very little photosynthesis of its own [1].
(b) With the vents closed the air becomes more humid, so the water vapour gradient between the air spaces and the outside air is shallower and less vapour diffuses out, reducing transpiration and therefore wilting [1] [1]. Disadvantages: still, humid air favours fungal disease [1]; and with no ventilation the carbon dioxide concentration falls, so photosynthesis and yield are reduced (the temperature may also rise further) [1].

Examiner’s NotesThe phrase “so that more light reaches the fruit” is deliberate bait: it invites an answer about photosynthesis in the tomato when the limiting factor is supply. Notice that this single question draws on 8.3 and 8.4 together, which is exactly what a whole-topic challenge paper does.