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B3.2.18 (HL)—Phloem adaptations

Phloem sieve tubes and companion cells transport sucrose and amino acids from sources to sinks by active loading and pressure flow.

Syllabus
First assessment 2025
Objective
B3.2.18
Level
HL

Exam analysis

Chance of appearing17%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–4

Common command terms

  • Identify
  • Label
  • Outline
  • Describe
  • Explain

Scoring notes

Common mistake
Saying organic compounds move in xylem as well as phloem.

Recent exam appearances

November 2025Paper1A ["HL"] · TZ125[ 1 ]B3.2.18 (HL)—Phloem adaptations
May 2024Paper1 ["HL"] · TZ133[ 1 ]B3.2.18 (HL)—Phloem adaptations
May 2024Paper2 ["HL"] · TZ22(b)[ 2 ]B3.2.18 (HL)—Phloem adaptations
November 2023Paper1 ["HL"] · TZ229[ 1 ]B3.2.18 (HL)—Phloem adaptations
May 2023Paper2 ["HL"] · TZ18(b)[ 7 ]B3.2.18 (HL)—Phloem adaptations
Practice this objective

Coverage 2014–2025 · Updated 15 Jul 2026

Phloem Translocates Assimilates from Sources to Sinks

HL only

Phloem translocates sucrose, amino acids and other carbon compounds as sap from sources to sinks through sieve tube elements supported by companion cells.

Sieve elements align end-to-end with perforated sieve plates. They retain only a thin layer of cytoplasm, have few organelles and no nucleus, reducing resistance to mass flow while remaining living cells.

Companion cells contain many mitochondria for ATP-dependent loading and unloading and connect to sieve elements by plasmodesmata. Source loading draws water from xylem and raises pressure; sink unloading lowers pressure, driving bulk flow.

A mature leaf loads sucrose into nearby phloem, water enters from xylem, and high hydrostatic pressure drives sap toward a developing fruit where sucrose is unloaded.

Phloem movement is source-to-sink, not always upward. Sieve elements lack a nucleus but are alive because companion cells maintain them through plasmodesmata.

Phloem adaptations

HL only

Assessment in practice

1–4 marks
How it is assessed

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

Command terms

Identify / Label / Outline / Describe / Explain

What earns marks

Build the answer around this relationship: Phloem transports sucrose and amino acids from sources to sinks.

Watch for

Saying organic compounds move in xylem as well as phloem.

Representative question

Question 1

[Maximum number: 7]

Describe the transport of carbon compounds such as sucrose and amino acids in phloem.

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

  • Phloem transports sucrose and amino acids from sources to sinks.
  • Companion cells help load solutes into sieve tubes using energy.
  • Water enters loaded phloem by osmosis, raising hydrostatic pressure.
  • Pressure gradients drive mass flow through sieve tubes.
  • Sieve plates and plasmodesmata support movement through phloem tissue.