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AS · Practice questions · From soil to xylem, and up the stem

Follow the water from soil to leaf.

Six original Cambridge-style questions on water uptake and transport in the xylem, from naming the pathways to explaining data on trunk diameter.

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

Two routes, one column.

01
Recall
[2 marks]

State two substances, other than water, that are transported in a plant dissolved in water, and name the tissue that carries each.

  • Mineral ions (for example nitrate or magnesium ions) are carried in the xylem. ✓
  • Organic compounds such as sucrose or amino acids are carried in the phloem. ✓
02
Explain
[4 marks]

Describe the apoplast pathway taken by water from the soil to the xylem of a root.

  • Water moves through the cellulose cell walls and the spaces between cells of the cortex. ✓
  • It does not cross any cell membranes; the cellulose walls are freely permeable. ✓
  • At the endodermis the Casparian strip (waterproof suberin) blocks the walls, so water must enter the cytoplasm of the endodermal cells (join the symplast). ✓
  • It then passes into the xylem through pits in the lignified walls. ✓
03
Explain
[3 marks]

Explain the importance of the Casparian strip to a plant.

  • It is a band of suberin, which is waterproof, in the walls of endodermal cells, so it blocks the apoplast pathway. ✓
  • All water and dissolved ions must therefore pass through the partially permeable cell surface membranes of endodermal cells. ✓
  • This lets the plant control (select) which mineral ions enter the xylem and keep out toxic substances. ✓
04
Explain
[5 marks]

Explain how hydrogen bonding allows water to move up the xylem from the roots to the leaves.

  • Water molecules are polar and form hydrogen bonds with each other. ✓
  • This gives cohesion, so the water in a xylem vessel forms a continuous column. ✓
  • Water molecules also form hydrogen bonds with the cellulose in the vessel walls: adhesion. ✓
  • Water evaporating from the leaves (transpiration) is replaced from the xylem, so the column is pulled upward under tension (transpiration pull). ✓
  • Lignified walls stop the vessels collapsing under this tension. ✓
05
Data
[3 marks]

The diameter of a tree trunk was measured every two hours. It was largest at 04:00 and smallest at 14:00. Explain these results.

  • Transpiration rate is highest in the early afternoon (warm, bright, stomata open) and lowest at night. ✓
  • A high transpiration rate pulls the water column up with greater tension. ✓
  • The tension pulls the xylem walls inward, so the trunk is narrowest at 14:00; at night tension is low and the trunk returns to its widest. ✓
06
Analysis
[3 marks]

A student sets up a potometer by cutting a leafy shoot in air and then fitting it into the apparatus. Very little water uptake is recorded. Suggest and explain why.

  • Cutting the shoot in air lets air bubbles into the xylem vessels. ✓
  • An air bubble breaks the continuous water column, because there are no hydrogen bonds across the gap. ✓
  • So the transpiration pull cannot draw water up from the potometer; the shoot should be cut and fitted under water. ✓

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.