Translocation of food in phloem requires ATP energy, whereas the ascent of water through xylem does not directly require cellular energy. What does this difference indicate about the mechanism of each transport process? Name the physical forces that drive water movement in xylem.
Generated by claude-sonnet-4-6 · 2026-06-26 01:01 · grounding rag
Model Answer
Mechanism of Transport:
Translocation in phloem requires ATP energy, which means it is an active transport process — energy is used to load sugar into phloem cells against a concentration gradient. In contrast, water movement through xylem does not need cellular energy, indicating it is a passive process driven entirely by physical forces.
Physical forces driving water movement in xylem:
- Transpiration pull — loss of water vapour from leaves creates suction that pulls water upward.
- Root pressure — osmotic entry of water from soil into root cells pushes water up.
- Cohesion and adhesion — cohesion between water molecules and adhesion to xylem walls help maintain a continuous water column.
Source: Life Processes, Section 5 (Transport in Plants)
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Explanation
- The key contrast examiners look for: active transport (phloem, needs ATP) vs passive transport (xylem, no ATP).
- Name all three physical forces for xylem — transpiration pull, root pressure, and cohesion-adhesion — to secure full marks.
- Keep the phloem explanation brief (1 mark) and devote more space to naming and explaining xylem forces (2 marks).