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C3.1.2—Hierarchy in multicellular organisms

Hierarchy in multicellular organisms links named stimuli, coordinating structures and effectors so body systems or plant tissues produce an integrated biological response.

Syllabus
First assessment 2025
Objective
C3.1.2
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2015
Most common paperPaper1
Typical marks1

Common command terms

  • Explain

Recent exam appearances

May 2015Paper1 ["SL"] · TZ25[ 1 ]C3.1.2—Hierarchy in multicellular organisms
Practice this objective

Coverage 2015–2015 · Updated 16 Jul 2026

Organisation builds new properties at each level

Organisation builds new properties at each level.

Cells of one type form tissues, different tissues form organs, and organs form systems. Interactions between parts can produce an emergent capability that no isolated cell performs alone.

cell → tissue → organ → system → organism; at each step ask what cooperation adds.

Cardiac muscle, connective tissue and blood vessels combine in a heart that pumps blood; no one tissue performs the whole job.

Hierarchy is not just size ranking; it is a functional relationship between levels.

Hierarchy in multicellular organisms

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Explain.

Command terms

Explain

What earns marks

Mark schemes reward named structures, correct direction of information flow and explanations that keep the sequence causal.

Representative question

Question 1

[Maximum number: 1]

Why do multicellular organisms have emergent properties?

A

They have more genes than unicellular organisms.

B

Properties of unicellular organisms are enhanced by having many cells.

C

All of their genes are expressed whereas unicellular organisms express only some.

D

They show properties that can only result from the interaction of many cells.

Body-System Integration

  • Nervous signals, hormones and blood transport integrate organs into coordinated systems; emergent functions arise from their interactions.
  • Sensory neurons carry receptor input to the CNS; motor neurons carry output to effectors. Mixed nerves contain both fibre types.
  • Reflex arcs provide rapid involuntary responses through sensory, relay and motor neurons. The spinal cord also links the brain and peripheral nerves.
  • Cerebral hemispheres support conscious processing; the cerebellum coordinates movement and balance; the medulla adjusts ventilation and heart activity.
  • The hypothalamus links nervous and endocrine control through the pituitary. Pineal melatonin helps time sleep; adrenal epinephrine supports acute stress responses.
  • Baroreceptors and chemoreceptors provide feedback about pressure, CO2, pH and O2.
  • The CNS controls voluntary swallowing and egestion, while the enteric nervous system coordinates gut peristalsis.

Concept essentials

  • Hierarchy in multicellular organisms depends on linking the stimulus to the coordinating structure or signal.
  • The pathway for hierarchy in multicellular organisms must keep information flow and response direction clear.
  • Named tissues, hormones, neurons or effectors give hierarchy in multicellular organisms its biological specificity.
  • Evidence about hierarchy in multicellular organisms is strongest when structure, mechanism and outcome are connected.