This free quiz has 11 real AQA A-level Biology exam questions on mass transport in plants (28 marks, about 35 minutes). It marks itself: for each answer you check your work against the real AQA mark scheme and tick the points you made.
What the quiz covers (AQA 3.3.4.2)
the main carbohydrate transported in the phloem
why a cut shoot in a potometer may not behave like a whole plant
the measurements needed to work out a rate of water movement through xylem
why the xylem must stay open as a continuous tube, and how a porous pot models cohesion-tension
changing the environment to reduce transpiration in cut flowers
how pressure is generated inside the phloem
using standard deviations and radioactive carbon to interpret tracer experiments
evaluating a ringing experiment (4 marks) and describing the transport of carbohydrate in plants (5 marks)
How to use it
Write each answer in full before you open the mark scheme, then tick only the points you really made. Keep the two systems separate: water moves up the xylem by cohesion-tension, while sucrose moves through the phloem by mass flow. For the experiment questions, link each piece of evidence directly to the hypothesis instead of just describing the data.
Where students lose marks (from AQA examiners’ reports and mark schemes)
Each point below is followed by wording that gained marks.
Where tension comes from. Many students said cohesion or evaporation itself was the tension, described tension as a force that pulls up the column, or said water moves along the xylem by osmosis. Very few explained how tension is created.
✔ Gains credit: water evaporates from the leaves (transpiration), and this creates tension (negative pressure) that pulls water up the xylem.
Cohesion holds the column. Using adhesion to describe the attraction between water molecules was rejected, and a “chain of water molecules” was ignored. Some confused this with translocation, writing about sources, sinks and sugar moving in the xylem.
✔ Gains credit: cohesion from hydrogen bonds between water molecules maintains an unbroken column of water.
Explain transpiration rates. Many answers described the pattern in a graph without explaining it, or wrote “the sun” instead of light intensity. Some said stomata open to let water out.
✔ Gains credit: a named factor linked to its effect, e.g. a higher temperature gives more kinetic energy and lower humidity increases the water potential gradient, so more water evaporates; stomata open to allow gas exchange.
Measuring water movement. Answers such as “measure volume”, “the drop in water level” or “changes in the mass of the beaker” gained no mark. Only 17% of students noted that a cut shoot has no roots.
✔ Gains credit: record the initial and final mass of the beaker and all its contents; a whole plant has roots, which is where water normally enters.
Loading sucrose. Many students said sucrose enters the phloem without saying it is by active transport or co-transport, named glucose instead, or thought adding a solute increases water potential. Vague phrases such as “water just entering” lost marks.
✔ Gains credit: sucrose is actively transported (co-transported with H⁺) into the phloem by companion cells, which lowers the water potential so water enters from the xylem by osmosis, producing a high hydrostatic pressure.
Mass flow, not diffusion. Many described sugars diffusing along the phloem, often “along a pressure gradient”, or used cohesion-tension terms. Others wrote “respired at the sink” without mentioning cells.
✔ Gains credit: mass flow towards respiring cells or storage tissue, where sucrose is unloaded from the phloem by active transport or facilitated diffusion.
Tracers and ringing evidence. Fewer than 10% of students realised that radioactive carbon from ¹⁴CO₂ ends up in sugars. In ringing evaluations, many described the mass flow hypothesis without linking it to the results, and very few identified the source and the sink.
✔ Gains credit: radioactive carbon is converted into sugars in photosynthesis and moves in the phloem; each piece of evidence is linked to the hypothesis, e.g. removing the phloem stops movement, with the source and sink named.
Keep practising
The other AQA 3.3 self-marking quizzes: Surface area to volume · Gas exchange · Digestion and absorption · Mass transport in animals
Topic 3.2 quizzes start with cell structure
The AQA topic index
weekly AQA group classes: Year 12 on Tuesdays at 5.15pm, Year 13 on Wednesdays or Thursdays at 6.30pm
Questions © AQA, from past A-level and AS Biology papers, with their mark schemes and examiners’ reports.