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B3.2.17 (HL)—Root pressure generation

Root pressure is generated when active mineral ion transport into root xylem lowers water potential and water enters by osmosis.

Syllabus
First assessment 2025
Objective
B3.2.17
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1–2

Common command terms

  • Identify
  • Explain

Recent exam appearances

November 2025Paper2 ["HL"] · TZ34(b)(i)[ 2 ]B3.2.17 (HL)—Root pressure generation
May 2017Paper1 ["HL"] · TZ132[ 1 ]B3.2.17 (HL)—Root pressure generation
Practice this objective

Coverage 2017–2025 · Updated 15 Jul 2026

Root Pressure Can Push Water Upward

HL only

Root pressure is a positive pressure potential generated when root cells actively load mineral ions into xylem and water follows by osmosis.

ATP-powered ion transport raises solute concentration and lowers xylem water potential. Water enters from surrounding root tissue, creating hydrostatic pressure that can push xylem sap upward.

Root pressure is most useful when transpiration pull is weak—for example during high humidity, at night, or in spring before leaves of deciduous plants have opened.

Continued ion loading during a humid night can create enough positive pressure for xylem sap to emerge as guttation droplets at leaf margins.

Root pressure supplements transport when transpiration is insufficient but cannot by itself account for water reaching the tops of tall trees; it is positive pressure, unlike transpiration tension.

Root pressure generation

HL only

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Explain.

Command terms

Identify / Explain

What earns marks

Build the answer around this relationship: Active mineral ion transport into root xylem lowers xylem water potential.

Representative question

Question 1

[Maximum number: 2]

Explain how root pressure is generated to cause movement of water through seedlings.

Pressure, Heart, Lymph, And Phloem

HL only

HL transport adds pressure and route systems. Tissue fluid forms by capillary pressure and returns by osmotic pull or lymph. Double circulation separates pulmonary and systemic routes. The heart creates directional pressure with chambers, septum, valves, and cycle timing. Plants add root pressure and phloem pressure-flow translocation.

  • Tissue fluid: hydrostatic pressure out, osmotic pull back, lymph drains excess.
  • Heart: double circulation, one-way valves, thick left ventricle, cardiac cycle sequence.
  • Plant HL: root pressure by active ion loading and osmosis; phloem by source-to-sink pressure flow.

Concept essentials

  • Active mineral ion transport into root xylem lowers xylem water potential.
  • Water enters the xylem by osmosis in response to the solute gradient.
  • The resulting positive pressure can push water upward.
  • Root pressure is distinct from transpiration pull and capillary action.