Home · A-Level Biology 9700 · Transport in plants · Xylem and phloem: structure and distribution · Practice
AS · Practice questions · Xylem and phloem: structure and distribution

Find the tissue, then explain its structure.

Six original Cambridge-style questions on transport tissues, from describing the distribution of xylem in a root to relating companion cell structure to sucrose loading.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Position, then structure.

01
Recall
[3 marks]

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

  • The xylem is in the centre of the root (in the vascular tissue inside the endodermis and pericycle). ✓
  • It forms an X or star shape with several arms. ✓
  • The phloem is found in separate groups between the arms of the xylem. ✓
02
Recall
[4 marks]

List four rules to follow when drawing a plan diagram of a transverse section of a stem from a microscope slide.

  • Draw only the outlines of tissues; do not draw individual cells. ✓
  • Use clear, single, continuous lines and no shading. ✓
  • Keep the proportions of the tissues the same as in the specimen. ✓
  • Use a ruler for label lines, which touch the tissue and do not cross each other. ✓
03
Explain
[4 marks]

Relate the structure of a xylem vessel element to its function of transporting water.

  • Dead, with no cytoplasm or nucleus, so nothing obstructs the flow of water. ✓
  • No end walls, so the elements form a continuous tube from root to leaf. ✓
  • Lignified walls strengthen the vessel so it does not collapse under tension, and are waterproof. ✓
  • Pits allow water to move sideways into adjacent vessels or surrounding cells (and around an air bubble). ✓
04
Explain
[4 marks]

Explain how the structure of a companion cell is related to its role in transporting sucrose into a sieve tube element.

  • Many mitochondria carry out aerobic respiration and make ATP. ✓
  • The ATP is used by proton pumps in the companion cell membrane for active loading of sucrose. ✓
  • Many plasmodesmata link the companion cell to the sieve tube element, so sucrose can pass into it. ✓
  • A nucleus and dense cytoplasm (many ribosomes) make the proteins needed, including those for the sieve tube element, which has no nucleus. ✓
05
Analysis
[3 marks]

Compare the structure of a mature xylem vessel element with that of a sieve tube element.

  • Xylem vessel element is dead; sieve tube element is living (but has no nucleus). ✓
  • Xylem has lignified walls; sieve tube element has cellulose walls with no lignin. ✓
  • Xylem vessel element has no end walls; sieve tube element has end walls perforated as sieve plates. ✓
06
Data
[3 marks]

A transverse section of a plant organ shows vascular bundles arranged in a ring, with the xylem of each bundle facing the centre and a large region of pith in the middle. Identify the organ and give two reasons for your answer.

  • The organ is a stem. ✓
  • Vascular bundles in a ring near the outside (a root has a single central block of vascular tissue). ✓
  • Xylem on the inner side and phloem on the outer side of each bundle, around a central pith (a root has no pith and a leaf has a flat lamina with xylem on the upper side). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.