Mass Transport in Plants: AQA A Level Biology Exam Questions by Topic

AQAMass Transport in PlantsExam Questions
Written by Tom Whitburn

Mass transport in plants (AQA 3.3.4.2) is a short section of the specification, but it produces some long answers on both Year 12 and A-level papers. You need to know xylem as the tissue that carries water and explain the cohesion-tension theory, know phloem as the tissue that carries organic substances and explain the mass flow hypothesis, and interpret tracer and ringing experiments, saying whether the evidence supports mass flow or not.

The examiners' reports keep finding the same gaps. Students name glucose instead of sucrose, say water moves up the xylem by osmosis, describe a water column without saying how the tension is created, write that sugars diffuse along the phloem instead of moving by mass flow, and describe tracer data without linking the radioactive carbon to sugars made in photosynthesis. In the evaluation questions, very few students name the source and the sink in the experiment in front of them.

Start with the xylem, then the phloem and mass flow, then the tracer and ringing experiments, and finish with the practical and data questions. Answer each question on paper before you open the mark scheme. If you'd like help with this topic, my AQA masterclasses run online on Tuesdays at 5.15pm for Year 12 and on Wednesdays or Thursdays at 6.30pm for Year 13.

What you need from GCSE and Year 12

Each question on this page assumes you're secure on these GCSE and Year 12 ideas. If any of them feels shaky, revise it before you start.

  • GCSE: xylem carries water and mineral ions from the roots to the leaves in the transpiration stream; phloem carries dissolved sugars around the plant (translocation).
  • GCSE: transpiration is faster at higher temperature, higher light intensity, lower humidity and more air movement, and can be measured with a potometer.
  • Year 12 (AQA 3.1.7): hydrogen bonds between water molecules make them cohesive.
  • Year 12 (AQA 3.1.2): sucrose is a disaccharide made from glucose and fructose.
  • Year 12 (AQA 3.2.3): water moves by osmosis to a lower water potential; active transport and co-transport move substances against a concentration gradient using carrier proteins.

Key terms

These words come up in the questions and mark schemes. Examiners expect you to use them exactly.

  • Xylem: the tissue that transports water (and mineral ions) in the stem and leaves; its vessels form continuous tubes.
  • Transpiration: evaporation of water from leaf cells and its loss as water vapour, mainly through the stomata.
  • Cohesion: water molecules sticking together by hydrogen bonds, so they form a continuous column in the xylem.
  • Tension: the pull, or negative pressure, on the water column created as water evaporates from the leaves.
  • Adhesion: attraction between water molecules and the walls of the xylem vessels.
  • Phloem: the tissue that transports organic substances such as sucrose; sieve tube elements are supported by companion cells.
  • Source and sink: the source is where sucrose is loaded into the phloem (such as a photosynthesising leaf); the sink is where it is unloaded and used or stored (such as roots).
  • Mass flow: movement of the whole solution along the phloem from high hydrostatic pressure at the source to lower pressure at the sink.
  • Translocation: the movement of organic substances through the phloem.
  • Ringing experiment: removing a ring of bark, and with it the phloem, from a stem to see whether substances still move past that point.
  • Tracer: a labelled substance, such as carbon dioxide containing radioactive 14C, that is followed through the plant to show where products of photosynthesis go.

What these questions cover (AQA 3.3.4.2)

The questions test these statements from the AQA AS and A-level Biology specification (7401/7402):

  • Xylem: Xylem as the tissue that transports water in the stem and leaves of plants. The cohesion-tension theory of water transport in the xylem. (the potometer and celery questions come from the AT b practical skill listed beside this section)
  • Phloem: Phloem as the tissue that transports organic substances in plants. The mass flow hypothesis for the mechanism of translocation in plants. The use of tracers and ringing experiments to investigate transport in plants.
  • Students should be able to recognise correlations and causal relationships (the cut flower question asks which factors are linked with shelf life, using standard deviations)
  • Students should be able to interpret evidence from tracer and ringing experiments and to evaluate the evidence for and against the mass flow hypothesis. (two evaluation questions ask whether ringing (2018) and tracer (2025) evidence supports the mass flow hypothesis)

How to use this page

Write your answer on paper before you open the mark scheme. Allow about 1.3 minutes per mark: the 18 questions are worth 53 marks, so about 69 minutes in total. Then mark your answer strictly against the mark scheme and read the examiners' tip underneath it. If you'd rather print the questions, download the PDF pack.

Xylem and cohesion-tension (3.3.4.2)

Question 1

AS Paper 2 · June 2017 · Q09.1 · 5 marks

Describe the cohesion-tension theory of water transport in the xylem.

Question text: describe the cohesion-tension theory of water transport in the xylem, 5 marks.
Show the mark scheme Mark scheme, max 5: water lost from leaves by transpiration or evaporation; lowers the water potential of leaf cells; water pulled up the xylem, creating tension; water molecules cohere by hydrogen bonds; forming a continuous water column; adhesion of water to the xylem walls.

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From the examiners' report: No examiners' report for this part. Start at the leaf: evaporation creates the tension, then cohesion by hydrogen bonds keeps the column unbroken. Adhesion to the xylem walls is a separate mark.

Question 2

A level Paper 1 · June 2023 · Q02.3 · 3 marks · Medium

Explain why it matters that lignin keeps the xylem open as a continuous tube.

Question text: lignin keeps the xylem vessel open as a continuous tube; explain the importance of the xylem being kept open as a continuous tube, 3 marks.
Show the mark scheme Mark scheme: allows an unbroken water column, no barrier to water movement; cohesion from hydrogen bonds between water molecules; evaporation or transpiration creates tension in the column.

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From the examiners' report: Fewer than 5% got all 3. About half mentioned a water column; very few said how the tension is created. Source, sink and sugar don't belong in the xylem.

Question 3

AS Paper 2 · June 2024 · Q01.2 · 3 marks · Hard

Explain how a porous pot model supports the cohesion-tension theory.

Figure 1: a porous pot on top of a water-filled glass tube standing in a beaker of water; arrows show water leaving the pot and moving up the tube.
Show the mark scheme Mark scheme: evaporation from the porous pot; tension created moves water upwards; cohesion maintains the column of water, or is due to hydrogen bonds between water molecules. Reject adhesion used for attraction between water molecules.

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From the examiners' report: Only a small minority said that evaporation creates tension. Explain the model in front of you; some just described what happens in a plant.

Question 4

AS Paper 2 · June 2019 · Q05.3 · 3 marks · Medium

Explain why coloured water moved up celery stalks.

Figure 6: slice of a celery leaf stalk with dots showing coloured water inside groups of xylem vessels around the edge; the stalks stood in coloured water for 20 minutes.
Show the mark scheme Mark scheme: water evaporates or is transpired; a water potential gradient creates tension or pulls up water; hydrogen bonds, cohesion or adhesion maintain the column.

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From the examiners' report: About a third scored zero. Start with evaporation; tension pulls the column up and cohesion holds it together. Water doesn't move up the xylem by osmosis.

Question 5

A level Paper 1 · June 2022 · Q06.1 · 4 marks · Medium

Explain the rise in transpiration rate of a mangrove tree from 5 am to midday.

Figure 9: rate of transpiration of a mangrove tree from 5 am to 7 pm, rising from about 0.38 cm³ per hour at sunrise to a peak of 0.84 at 11 am, dipping and then falling after 3 pm; tide height is also shown.
Show the mark scheme Mark scheme: transpiration increases due to higher temperature, light intensity, lower humidity or more air movement; so more kinetic energy or a bigger water potential gradient; stomata open after sunrise for gas exchange; some stomata close around 11 am to midday.

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From the examiners' report: Only two-thirds scored. Explain, don't describe: link each factor to its effect, say 'light intensity' not 'the sun', and remember stomata open for gas exchange, not to let water out.

Phloem and mass flow (3.3.4.2)

Question 6

AS Paper 1 · June 2025 · Q07.1 · 1 mark · Easy

Name the main carbohydrate transported in the phloem.

Question text: name the main carbohydrate transported through the phloem, 1 mark.
Show the mark scheme Mark scheme: sucrose.

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From the examiners' report: About 60% got it. Glucose was the common wrong answer.

Question 7

A level Paper 1 · June 2022 · Q09.1 · 5 marks · Medium

Describe the transport of carbohydrate in plants.

Question text: describe the transport of carbohydrate in plants, 5 marks.
Show the mark scheme Mark scheme, max 5: sucrose actively transported or co-transported with H+ into the phloem; by companion or transfer cells; lowers water potential so water enters from the xylem by osmosis; higher hydrostatic pressure; mass flow to respiring cells or storage tissue; unloaded by active transport.

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From the examiners' report: A third got 4 or 5. Name sucrose, use the words mass flow, and say where it goes: respiring cells or storage tissue. Sugars don't diffuse along the phloem or out of it.

Question 8

AS Paper 2 · November 2020 · Q08.2 · 3 marks · Hard

Use the mass flow hypothesis to explain how pressure builds up in a phloem tube.

Question text: the mass flow hypothesis is used to explain the movement of substances through phloem; use it to explain how pressure is generated inside a phloem tube, 3 marks.
Show the mark scheme Mark scheme: sucrose actively transported into the phloem; lowers the water potential; water moves into the phloem by osmosis from the xylem.

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From the examiners' report: Only 25% got a mark. Name sucrose and active transport; adding a solute lowers the water potential, it doesn't raise it. Leave cohesion-tension out.

Question 9

AS Paper 1 · June 2023 · Q09.2 · 2 marks · Hard

Explain why potato plants with fewer sucrose-transport proteins make fewer potatoes and keep sugar in their leaves.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Passage: carbohydrates move by mass transport in plants and mammals; plants transport most carbohydrate as sucrose using sucrose-transport proteins in the phloem; potato plants with fewer of these proteins produce fewer potatoes and accumulate sugars in their leaves.
Show the mark scheme Mark scheme: carbohydrates made by photosynthesis in the leaf; less sucrose transported into the phloem at the source, or to the sink or potato, or less stored as starch.

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From the examiners' report: This was poorly answered. Be precise about places: sugar is made in the leaf by photosynthesis, less is loaded into the phloem at the source, so less reaches the potato.

Question 10

AS Paper 2 · November 2020 · Q08.4 · 3 marks · Medium

Use graphs of phloem pressure and xylem flow to explain why phloem pressure falls in the hottest part of the day.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Figure 6: phloem pressure in a willow stem falls from about 11 to 9 arbitrary units between 08.00 and 12.00 and stays low until about 16.00, while the rate of water movement in the xylem rises to about 5 kg per hour over the same period, then falls.
Show the mark scheme Mark scheme: high rate of transpiration or evaporation; water lost through stomata, or high tension in the xylem; less water moves from the xylem into the phloem, or the phloem's water potential isn't low enough to draw water from the xylem.

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From the examiners' report: Over 60% got a mark, but very few got all 3. Go one step further: high tension in the xylem means less water moves into the phloem, so phloem pressure falls.

Tracer and ringing experiments (3.3.4.2)

Question 11

AS Paper 2 · June 2018 · Q06.2 · 3 marks · Easy

Use a ringing experiment to draw conclusions about substance X and root growth.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Figure 4 and Table 3: lengths of lemon stem with an agar block on top; D plain agar, intact stem, 5 roots; E substance X, intact, 11 roots; F substance X, ringed (phloem removed), 4 roots; G substance X, intact and cooled to 4 °C, 3 roots.
Show the mark scheme Mark scheme: D shows substance X is not needed for some root growth, or substances already in the stem cause some; substance X moves through the plant; E shows substance X increases or doubles root growth.

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From the examiners' report: Most got the mark for X increasing root growth. Say what the control shows (roots still grow without X), and state that X moves through the stem.

Question 12

AS Paper 2 · June 2018 · Q06.3 · 4 marks · Medium

Evaluate whether the ringing experiment supports the mass flow hypothesis.

Question text: the mass flow hypothesis is used to explain the movement of substances through phloem; evaluate whether the information from this investigation supports it, without statistical analysis, 4 marks.
Show the mark scheme Mark scheme, max 4: for, F shows phloem is involved; G shows respiration or active transport is involved, as cooling slows movement; agar block is the source; roots are the sink. Against, no bulge above the ring; no role shown for osmosis or pressure; gravity; roots still grow without phloem; no leaves.

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From the examiners' report: Only 0.6% got all 4. Link each result to for or against: there was no bulge in F, the agar is the source and the roots the sink, and G points to respiration.

Question 13

A level Paper 1 · June 2018 · Q08.1 · 2 marks

Explain how autoradiographs of barley plants show that heat treatment damaged the phloem.

Figure 9: barley plants given 14CO2 in a boxed area of one leaf; after 4 hours, radioactivity in the heat-treated plant stays in the leaf above the steamed section, while in the control plant it has spread down to the base and roots.
Show the mark scheme Mark scheme, either pair: labelled carbon is made into sugar in photosynthesis, and mass flow in the phloem moved it through the plant only in the control; or phloem transport needs living cells and respiration, and heat killed the cells.

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From the examiners' report: Nearly 60% made no headway, mostly by describing the images. Explain: the 14C goes into sugar by photosynthesis, and the sugar moves in the phloem, which stopped at the heat-treated section.

Question 14

AS Paper 1 · June 2025 · Q07.3 · 1 mark · Hard

Suggest why less radioactivity was found in the stem than the leaf absorbed.

Figure 9 and Table 4: a geranium leaf above a section where an impermeable layer separates phloem and xylem was given 14CO2 for 15 hours; radioactivity in phloem and xylem falls from 41 100 and 24 210 counts per minute in section A to about 100 to 160 and 50 to 75 in sections C to E.
Show the mark scheme Mark scheme, any one: some sugars converted to cellulose or starch; some used in respiration; some not moved down, or remained in the leaf; some moved further down than E.

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From the examiners' report: Under 10% scored. The 14C ends up in sugars, so your answer must be about sugars, not 'radioactive material' or 'they were respired'.

Question 15

AS Paper 1 · June 2025 · Q07.4 · 4 marks · Hard

Evaluate whether the geranium tracer data support the mass flow hypothesis.

Question text: the mass flow hypothesis states that organic substances are transported through the phloem from leaves to all parts of plants; evaluate whether this investigation supports it, without referring to statistical tests or sample size, 4 marks.
Show the mark scheme Mark scheme, max 4: for, radioactivity higher in phloem than xylem; found in sections below the leaf so it moved down; leaf is the source. Against, max 3, some radioactivity in the xylem; only one species; no evidence of transport above the leaf; movement could be due to gravity.

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From the examiners' report: Under 1% got all 4 and just over half scored zero. More in the phloem than the xylem supports it; some in the xylem counts against. Then look for what the data don't show, such as movement upwards.

Practical skills and data (3.3.4.2)

Question 16

AS Paper 2 · June 2019 · Q05.1 · 2 marks · Hard

Give two measurements needed to find the rate of water movement in a celery stalk.

Figure 5: a celery stalk with leaves standing in a beaker of water under a layer of oil, on a digital balance; the rate is to be found in grams per hour per group of xylem vessels.
Show the mark scheme Mark scheme: initial and final mass of the beaker and all its contents; number of groups of xylem vessels. Ignore volume; reject change in mass of the celery, beaker or water alone.

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From the examiners' report: Only about 2% got both marks. Use the apparatus and the units: the balance gives mass, and 'per group of xylem vessels' means count them.

Question 17

A level Paper 3 · June 2017 · Q06.3 · 1 mark

Suggest why water uptake by a cut shoot in a potometer may differ from a whole plant.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Figure 5: a potometer with a leafy shoot sealed into a water-filled tube, an air bubble in a capillary tube and a water reservoir.
Show the mark scheme Mark scheme: the plant has roots, or xylem cells are very narrow; accept xylem damaged when cut. Ignore air bubbles, mass flow and photosynthesis.

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From the examiners' report: Only 17% noted that the shoot in the potometer has no roots, which is where water normally enters a plant.

Question 18

AS Paper 2 · June 2022 · Q07.2 · 4 marks · Hard

Use means and standard deviations to conclude which factors affect the shelf life of cut flowers.

Table 4: cut roses from December had 23 (±2) stomata per mm², transpiration rate 22 (±3) cm³ per day and shelf life 5 (±1) days; April roses had 20 (±3), 15 (±2) and 16 (±2) days. Values are means ±2 SD.
Show the mark scheme Mark scheme: SDs don't overlap for shelf life or transpiration rate, and do overlap for stomata; shelf life significantly longer in April; no significant difference in stomata so not linked to shelf life; significant difference in transpiration rate so likely linked.

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From the examiners' report: Less than 2% got all 4, though most spotted the SD overlap. Say whether each difference is significant and so whether that factor is linked with shelf life; don't bring in temperature or daylight.

Where students lose marks (from AQA examiners' reports and mark schemes)

Each point below is followed by wording that gained marks.

  • Say what creates the tension: Fewer than 5% got all 3 marks on why xylem must be a continuous tube: about half mentioned a water column, but very few said how tension is created. On the porous pot, only a small minority linked tension to evaporation.

    ✔ Gains credit: "Evaporation / transpiration creates tension (in column)"; "cohesion from H bonds between (all) water (molecules)".

  • No osmosis, source or sink in the xylem: About a third scored nothing on why coloured water moved up celery stalks. Many said water moved along the xylem down an osmotic gradient, or mixed in source and sink from translocation.

    ✔ Gains credit: "Water evaporates / is transpired; water potential gradient creates tension / pulls up water; hydrogen bonds / cohesion / adhesion maintains column."

  • Sucrose, not glucose: About 60% named the main carbohydrate carried in the phloem. Glucose was the common wrong answer, and weaker answers to the 5-mark sugar transport question had glucose, dead phloem cells or sugar moving in the xylem.

    ✔ Gains credit: "Sucrose"; "sucrose actively transported into phloem (cell) by companion / transfer cells".

  • Adding sucrose lowers the water potential: Only 25% got a mark for how pressure is generated in a phloem tube. Few said sucrose, some thought adding a solute raises the water potential, and many used cohesion-tension words.

    ✔ Gains credit: "Sucrose actively transported (into phloem); lowering / reducing water potential; water moves (into phloem) by osmosis (from xylem)."

  • Mass flow along the phloem, not diffusion: Many answers on carbohydrate transport described sugars diffusing along the phloem 'along a pressure gradient', said vaguely that sugar is 'respired at the sink', or had it diffuse out of the phloem.

    ✔ Gains credit: "Mass flow to respiring cells / storage tissue"; "unloaded / removed (from phloem) by active transport".

  • Turn the labelled carbon into sugar first: Under 10% scored on why the radioactivity in the stem was lower than the leaf took in, and nearly 60% made no headway on the heat-treated barley. Students didn't say the 14C ends up in sugars made in photosynthesis.

    ✔ Gains credit: "The radioactively labelled carbon is converted into sugar / organic substances during photosynthesis"; "some sugars used in respiration".

  • Name the source and sink in the experiment: Only 0.6% got all 4 marks evaluating the ringing experiment. Many described the hypothesis without using the results, mentioned a bulge that wasn't in the figure, or called leaves the source when there were none.

    ✔ Gains credit: "The agar block is the source"; "roots are the sink"; "(G shows) respiration / active transport is involved"; "no bulge above ringing (in F)".

  • Let the units tell you what to measure: Only about 2% got both marks for the measurements needed for a rate in grams per hour per group of xylem vessels. 'Measure volume' and 'change in mass of the beaker' didn't score, and few counted the xylem vessels.

    ✔ Gains credit: "Initial and final mass (of beaker and all contents)"; "number of (groups of) xylem vessels".

Download the questions

Download the PDF pack (18 questions, mark schemes at the end)

Keep practising

Questions, mark schemes and examiners' report extracts © AQA.

Tom Whitburn
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