Cambridge International AS & A Level Biology 9700: Transport in plants – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover topic 7 of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: sections 7.1 (Structure of transport tissues) and 7.2 (Transport mechanisms). This is AS Level content, examined on Paper 1 and Paper 2; the syllabus says AS knowledge is also required on Paper 4. Questions 3 and 8 use the microscope measuring skills listed for Paper 3.

Before you start, you may want the study guide or the revision notes. Course links: Cambridge A Level Biology hub, printable 9700 checklist and the AS Level diagnostic.

Questions

1. Describe the distribution of xylem and phloem in a transverse section of the root of a herbaceous dicotyledonous plant. [2]

2. A student draws a plan diagram of a transverse section of a leaf. State three features the drawing should have. [3]

3. A student calibrates an eyepiece graticule. 25 divisions of a stage micrometer, each 0.01 mm, line up exactly with 40 eyepiece units.

(a) Calculate the length of one eyepiece unit in µm. Show your working. [2] (b) A sieve tube element is 12 eyepiece units wide. Calculate its actual width. [1] (c) In a photomicrograph the same sieve tube element is 30 mm wide. Calculate the magnification of the photomicrograph. [2]

4. Relate the structure of xylem vessel elements to their function. [5]

5. Explain how the structure of a sieve tube element and its companion cell suit them to the transport of assimilates. [5]

6. Describe how water moves across a root from the root hair cells to the xylem by the apoplast and symplast pathways. [4]

7. Explain how water moves from the xylem in a root to the air outside a leaf. In your answer, refer to hydrogen bonding, cohesion-tension and adhesion. [9]

8. A student compared the lower epidermis of a mesophyte leaf with the inner surface of a rolled xerophyte leaf. The circular field of view had a diameter of 0.40 mm. The student counted 36 stomata in one field for the mesophyte and 9 stomata for the xerophyte.

(a) Calculate the number of stomata per mm² for each leaf. Give your answers to the nearest whole number. [3] (b) Calculate the percentage decrease in stomatal density from the mesophyte to the xerophyte. [1] (c) Explain how two other features visible in a transverse section of the xerophyte leaf reduce water loss by transpiration. [4]

9. This question is about substances transported in plants.

(a) State the form in which mineral ions are carried in xylem. [1] (b) State two assimilates transported in phloem. [1] (c) Explain what is meant by a source and a sink, using a potato tuber as an example of an organ that can be both. [3]

10. Explain how companion cells transfer sucrose into phloem sieve tubes. Refer to proton pumps and cotransporter proteins. [5]

11. A leaf was supplied with carbon dioxide containing radioactive carbon. The label was detected in phloem 360 mm further down the stem 24 minutes later.

(a) Calculate the rate of movement in mm min⁻¹ and in m h⁻¹. [2] (b) Explain how the contents of phloem sieve tubes move from source to sink by mass flow. [6]

Answers

1. Xylem is in the centre of the root, in a star or cross shape [1]; phloem is in groups between the arms of the xylem [1]. [2] Examiner insight: The two positions are separate marks, so a correct xylem position still scores if phloem is misplaced; “xylem in the middle” without its star or cross shape is often not enough.

2. Any three: no individual cells drawn [1]; correct proportions of the tissue layers [1]; clear, continuous single lines with no shading, and label lines ruled to touch the tissue [1]. [3] Examiner insight: “Neat and labelled” is too vague for credit; each mark needs a specific, checkable feature.

3. (a) 25 × 0.01 mm = 0.25 mm = 250 µm [1]; 250 ÷ 40 = 6.25 µm [1]. (b) 12 × 6.25 = 75 µm [1] (c) 30 mm = 30 000 µm [1]; 30 000 ÷ 75 = ×400 [1]. Examiner insight: Allow error carried forward from (a) into (b) and (c); the most common loss is dividing mm by µm without converting.

4. Cells are dead with no cytoplasm, so there is an empty lumen and little resistance to flow [1]; end walls are absent, forming a continuous tube [1]; walls are thickened with lignin [1], which prevents collapse under tension (and waterproofs the wall) [1]; pits allow water to move sideways between vessels and to other cells [1]. [5] Examiner insight: Each structural feature must be matched to its function; a list of features with no “so that” link earns about half the marks.

5. Sieve tube elements have no nucleus and few organelles, leaving space for sap to flow [1]; sieve plates with pores allow sap to pass from element to element [1]; companion cells have many mitochondria to supply ATP [1] for active loading (proton pumps) [1]; many plasmodesmata connect the companion cell to the sieve tube element, allowing assimilates to pass across [1]. [5] Examiner insight: “Sieve tube elements have no organelles” is marked as incorrect; they have a few, so write “few organelles” and “no nucleus”.

6. Apoplast: water moves through the cellulose cell walls and spaces between cells [1]; symplast: water moves through cytoplasm from cell to cell via plasmodesmata [1]; at the endodermis the Casparian strip, made of suberin, blocks the apoplast [1]; so water must enter the cytoplasm of endodermal cells (symplast) before reaching the xylem [1]. [4] Examiner insight: The Casparian strip needs both its composition (suberin) and its effect (blocks the apoplast) – lignin in place of suberin loses that mark.

7. Water evaporates from the cell walls of mesophyll cells into the air spaces [1]; water vapour diffuses out through the stomata down a water vapour potential gradient [1]; water moves out of the xylem in the leaf to replace this water [1]; this lowers the pressure at the top of the xylem, putting the column under tension [1]; water molecules are held together by hydrogen bonds [1], so cohesion keeps the column continuous as it is pulled up (transpiration pull) [1]; water molecules also form hydrogen bonds with cellulose in the walls, which is adhesion [1]; the column moves up the continuous, empty xylem vessels by mass flow [1]; lignin prevents the vessels collapsing under tension [1]. [9] Examiner insight: Cambridge credits “evaporation” and “diffusion of water vapour” as separate points; “water evaporates out of the stomata” merges them and usually earns neither.

8. (a) Area of field = π × 0.20² = 0.1257 mm² [1]; mesophyte: 36 ÷ 0.1257 = 286 per mm² [1]; xerophyte: 9 ÷ 0.1257 = 71.6, so 72 per mm² [1]. (b) (286 − 72) ÷ 286 × 100 = 75% [1] (c) Two marks per feature, one for naming it and one for explaining it. For example: sunken stomata (or stomata in grooves) [1], which hold humid air above the stoma and reduce the water vapour potential gradient [1]; hairs [1], which trap still, humid air next to the stomata [1]. Also accept a rolled leaf (traps humid air inside the roll) or a thick waxy cuticle (less evaporation through the epidermis). Examiner insight: In (a) using the diameter (0.40 mm) instead of the radius in πr² is a common slip that loses all three marks; write the area down first so the method mark can be awarded, and keep at least 4 significant figures until the final step.

9. (a) As ions dissolved in water [1] (b) Sucrose and amino acids [1] (c) A source releases assimilates into the phloem; a sink uses or stores them [1]; a growing tuber is a sink, as it stores sucrose (as starch) [1]; in spring, when a new shoot grows, the tuber supplies assimilates, so it is a source [1]. Examiner insight: “Sink = where sucrose goes” is not enough; the definition needs “used or stored”.

10. Proton pumps in the companion cell membrane actively transport H⁺ out of the cell [1]; using ATP from the many mitochondria [1]; this creates a high H⁺ concentration outside the cell [1]; H⁺ diffuse back in through cotransporter proteins, carrying sucrose with them [1]; sucrose then passes into the sieve tube element through plasmodesmata [1]. [5] Examiner insight: Saying “sucrose is actively pumped into the companion cell” contradicts the mechanism and loses the first mark; the pumped substance is H⁺.

11. (a) 360 ÷ 24 = 15 mm min⁻¹ [1]; 15 × 60 = 900 mm h⁻¹ = 0.9 m h⁻¹ [1]. (b) Sucrose is loaded into sieve tubes at the source [1]; this lowers the water potential of the sap [1]; water enters by osmosis, so hydrostatic pressure rises at the source [1]; at the sink, sucrose is removed and used or stored [1]; water leaves, so hydrostatic pressure is lower at the sink [1]; sap moves by mass flow down the hydrostatic pressure gradient from source to sink [1]. Examiner insight: Both ends of the gradient must be explained; describing only the high pressure at the source caps the answer at about half marks.

Where marks are usually lost

• Drawing cells in a plan diagram or distorting tissue proportions.
• Putting phloem inside the xylem in a stem bundle.
• Using diameter instead of radius when finding a field-of-view area.
• Dividing mm by µm without converting.
• Writing “lignin” for the Casparian strip.
• Writing that water “evaporates from the stomata”.
• Leaving out “hydrogen bonds” in cohesion or adhesion answers.
• Saying sucrose itself is actively pumped.
• Explaining mass flow with concentration only, not hydrostatic pressure.

Next steps

• Recap the tables: transport in plants revision notes
• Relearn a weak area: transport in plants study guide
• Course overview: Cambridge A Level Biology hub
• Track every outcome: printable 9700 checklist
• Test the whole AS course: 9700 AS Level diagnostic, or see all free 10-minute diagnostics
• Book a free trial class

Official syllabus

Cambridge International, Cambridge International AS & A Level Biology 9700 syllabus for 2025, 2026 and 2027 (Version 1, published September 2022), Cambridge University Press & Assessment. Topic 7, Transport in plants: sections 7.1 and 7.2.