Structure of transport tissues
Cambridge A-Level Biology (9700) · Unit 7: Transport in plants · 8 flashcards
Structure of transport tissues is topic 7.1 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 7 — Transport in plants , alongside Transport mechanisms.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 8 key concepts — covering the precise wording mark schemes reward.
What the Cambridge 9700 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs
- describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants
- draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs
- relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Structure of transport tissues
- › Avoid 'concentration of water'. Always use 'water potential' and describe movement from a 'higher water potential' to a 'lower water potential'.
- › Remember that while sieve tube elements lose their nucleus and most organelles, they must retain a thin layer of cytoplasm.
- › Always use 'water potential' (Ψ) when discussing the movement of water in biological systems; 'concentration' is for solutes.
- › Transpiration is specifically the evaporation of water from mesophyll cell walls followed by the diffusion of water vapour through stomata.
- › Define the 'apoplast pathway' as movement through cell walls and 'symplast pathway' as movement through the cytoplasm and plasmodesmata.
Draw a plan diagram of a transverse section of a herbaceous dicotyledonous stem, labeling the epidermis, cortex, vascular bundles (including xylem and phloem) and pith.
Diagram should show a circular arrangement with the epidermis as the outermost layer, cortex filling the space between the epidermis and vascular bundles, vascular bundles arranged in a ring, and pith in the center. Xylem should be located towards the inside of the vascular bundle, and phloem towards the outside.
Describe the distribution of xylem and phloem in a transverse section of a herbaceous dicotyledonous root.
Xylem forms a central core (often a star shape) in the root. Phloem is located in discrete groups between the arms of the xylem core, often alternating with the xylem.
Describe the distribution of xylem and phloem in a transverse section of a herbaceous dicotyledonous leaf.
Xylem and phloem are located together in vascular bundles within the leaf veins. Xylem is usually positioned towards the upper surface of the leaf, and phloem towards the lower surface.
Draw and label a xylem vessel element.
Diagram should show elongated, hollow cells with thickened walls containing lignin. Pits (small unlignified areas) should be visible in the walls. End walls are absent or heavily modified (perforated).
How does the structure of xylem vessel elements relate to their function?
Lignified walls provide strength and prevent collapse under tension. The hollow lumen and absence of cell contents allow for efficient water transport. Pits allow lateral water movement between vessels. Perforations or absent end walls allow unimpeded vertical flow.
Draw and label a phloem sieve tube element and companion cell.
Sieve tube element should be elongated with sieve plates at the ends. Companion cell should be smaller and adjacent to the sieve tube element, containing a nucleus and dense cytoplasm. Plasmodesmata should be visible connecting the two cells.
How does the structure of phloem sieve tube elements relate to their function?
Sieve plates allow for the flow of assimilates (sugars) between cells. The lack of a nucleus and other organelles maximizes space for translocation. They rely on companion cells for metabolic support.
What is the function of companion cells in the phloem?
Companion cells provide metabolic support to sieve tube elements. They have a nucleus and many mitochondria to carry out active transport of sugars into and out of the sieve tube elements, maintaining the concentration gradient for translocation.
More topics in Unit 7 — Transport in plants
Structure of transport tissues sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
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