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3.4.1Inquiry Process

Cell Signalling — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 126 of 160

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5The Basic Pathway

1
A cell releases a signalling molecule, called a ligand, which is made in response to a stimulus or internal change.
2
The ligand travels to a target cell, either by diffusing through tissue fluid or by being carried in the blood.
3
The ligand binds reversibly to a specific receptor on or in the target cell, which has a complementary shape to it.
4
Binding changes the receptor, starting a transduction pathway that passes the message into the cell.
5
The cell gives a response, such as a change in gene expression, enzyme activity or movement, and the signal is later switched off.

Part 2 of 5Ligands and Receptors

Ligand
  • Ligand Any molecule that binds specifically to a receptor, such as a hormone, neurotransmitter or growth factor, carrying information between cells.
Receptor
  • Receptor A protein with a binding site whose three-dimensional shape is complementary to one ligand, giving the target cell its specificity.
Specificity
  • Specificity Only cells that have the matching receptor respond, so one hormone in the blood can affect some tissues and ignore all others.
Conformational change
  • Conformational change Ligand binding alters the shape of the receptor, and this change is what starts the cell’s response.

Part 3 of 5Why Signalling Matters

Why Signalling Matters
Coordination
  • Coordination Cells far apart in the body can act together because signals link them, for example a hormone released from one gland affecting the liver.
Response to change
  • Response to change Signalling lets an organism detect changes in its internal or external environment and adjust its behaviour or physiology.
Development
  • Development Signals tell cells when to divide, differentiate or die, which shapes tissues and organs as an embryo grows.
Distance
  • Distance Signals act over different ranges: local signals reach neighbouring cells, while hormones travel through the blood to distant targets.
Type of signalRangeExample
AutocrineSame cellCell stimulates its own division
ParacrineNearby cellsNeurotransmitter across a synapse
EndocrineWhole body via bloodInsulin from the pancreas

Part 4 of 5Worked Example

Question

  • A hormone is injected into the blood and is found to change the activity of liver cells but not muscle cells.
  • Explain this observation.

  1. The hormone is carried everywhere in the blood, so both tissues are exposed to it.
  2. A response needs the ligand to bind a specific receptor with a complementary binding site.
  3. Liver cells must have this receptor, whereas muscle cells lack it, so binding and the conformational change occur only in the liver.

Answer Only the liver cells possess the matching receptor, so only they are target cells and show a response.

Part 5 of 5Exam Checklist & Tips

3.4.1 – Key Points for the Exam

  • Ligand A molecule that binds specifically to a receptor and triggers a change in the target cell.
  • Receptor A protein with a complementary binding site that detects a particular signalling molecule.
  • Transduction The series of events that relays a signal from the receptor to the cell’s response.
  • Target cell A cell that has the specific receptor and so responds to a given ligand.

Common Mistakes

  • Thinking every cell responds to every hormone; only cells with the right receptor respond.
  • Describing the ligand as entering the cell and being used up; most ligands simply bind and are released.
  • Forgetting that binding changes receptor shape; this conformational change starts the response.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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