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D2.3.5—Effects on cells without wall

Cells without walls can lyse in hypotonic solutions and crenate in hypertonic solutions because water movement changes cellular volume and shape.

Syllabus
First assessment 2025
Objective
D2.3.5
Level
HL

Exam analysis

Chance of appearing7%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper3
Typical marks1–2

Common command terms

  • Deduce
  • Outline
  • Predict
  • Explain
  • Describe
  • Suggest

Scoring notes

Common mistake
Calling crenated animal cells turgid, a term that applies to walled plant cells.

Recent exam appearances

November 2024Paper3 ["HL"] · TZ02(c)[ 1 ]D2.3.5—Effects on cells without wall
November 2024Paper3 ["HL"] · TZ02(b)[ 3 ]D2.3.5—Effects on cells without wall
May 2023Paper3 ["HL"] · TZ21(b)[ 2 ]D2.3.5—Effects on cells without wall
May 2022Paper1 ["HL"] · TZ22[ 1 ]D2.3.5—Effects on cells without wall
November 2021Paper2 ["HL"] · TZ03(a)[ 1 ]D2.3.5—Effects on cells without wall
Practice this objective

Coverage 2017–2024 · Updated 16 Jul 2026

Cells without Walls Can Swell or Shrink

Cells without a wall can burst in hypotonic solution or shrink in hypertonic solution because their plasma membrane cannot resist large volume changes.

Environment Net water movement Wall-less cell response
Hypotonic Into cell Swelling; excessive entry may cause lysis or haemolysis
Hypertonic Out of cell Shrinkage; animal cells such as red blood cells become crenated
Isotonic Balanced Stable average volume

Freshwater unicellular organisms continually gain water from their hypotonic environment, so contractile vacuoles collect and expel excess water. Multicellular animals instead maintain near-isotonic tissue fluid around cells.

A plasma membrane can deform but does not provide the rigid mechanical restraint of a cell wall; active water removal is an adaptation, not a reversal of osmosis.

Effects on cells without wall

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, multiple choice, commonly using Deduce / Outline / Predict.

Command terms

Deduce / Outline / Predict / Explain / Describe / Suggest

What earns marks

Build the answer around this relationship: Hypotonic solutions can cause animal cells to swell or lyse.

Watch for

Calling crenated animal cells turgid, a term that applies to walled plant cells.

Representative question

Question 1

[Maximum number: 3]

Explain the effect of placing red blood cells in distilled water (0.000 M NaCl).

Core Osmosis Effects

Water forms hydration shells around ions and polar solutes; hydrogen bonding and charge attraction reduce free water movement. Water moves by osmosis across partially permeable membranes from hypotonic/lower solute solutions toward hypertonic/higher solute solutions. Osmosis direction depends on internal and external solute concentration; isotonic conditions have dynamic water movement but no net osmosis. Plant tissue changes mass or length in sucrose solutions; percentage change graphs estimate isotonic or osmotic concentration. Animal cells can lyse in hypotonic solutions and crenate in hypertonic solutions; freshwater protists use contractile vacuoles to expel excess water. Plant cells become turgid in hypotonic solutions as vacuoles swell; hypertonic solutions cause flaccidity and plasmolysis from water loss. Isotonic saline prevents harmful water gain or loss in body cells; IV fluids and transplant organ baths must match tissue osmotic concentration.

Concept essentials

  • Hypotonic solutions can cause animal cells to swell or lyse.
  • Hypertonic solutions can cause animal cells to shrink or crenate.
  • Distilled water causes water entry into red blood cells by osmosis.
  • Contractile vacuoles expel excess water in freshwater unicellular organisms.