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3.4.7Inquiry Process

Regulating Cell Signalling — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 132 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 5Why Signals Must Stop

Prevent overstimulation
  • Prevent overstimulation A signal that never ended would keep a cell permanently active, for example holding blood glucose or heart rate high.
Allow new signals
  • Allow new signals A cell can only respond to a change if the previous message has been cleared first.
Short half-life
  • Short half-life Many ligands are broken down quickly, which is why hormones such as epinephrine have effects that fade within minutes.
Disease link
  • Disease link Faulty regulation, such as receptors stuck in an active state, can cause uncontrolled cell division and cancer.

Part 2 of 5Mechanisms of Regulation

1
The ligand is broken down by enzymes, for example acetylcholinesterase degrades acetylcholine and the liver breaks down hormones.
2
The ligand is removed from the surroundings, such as by reuptake into the sending neurone or by excretion from the body.
3
The second messenger is destroyed, for example phosphodiesterase converts cyclic AMP to AMP, stopping the cascade.
4
The G protein hydrolyses its bound GTP to GDP, switching itself off.
5
Receptors can be taken into the cell by endocytosis, reducing the number at the surface so the cell responds less to the ligand.

Part 3 of 5Adjusting Sensitivity

Adjusting Sensitivity
Receptor downregulation
  • Receptor downregulation After prolonged exposure to a ligand, cells may reduce receptor numbers so the same concentration produces a smaller response.
Receptor upregulation
  • Receptor upregulation If a signal is absent for a long time, a cell can add receptors, becoming more sensitive to small amounts.
Negative feedback
  • Negative feedback The response often reduces the original stimulus, so the signal declines automatically as the cell’s goal is achieved.
Antagonists
  • Antagonists Some molecules bind a receptor without activating it, blocking the natural ligand and reducing the signal;
    • many medicines work like this.
MethodLevel of regulationExample
Ligand breakdownOutside the cellAcetylcholinesterase
Second messenger breakdownInside the cellPhosphodiesterase on cyclic AMP
Receptor removalCell surfaceEndocytosis of receptors

Part 4 of 5Worked Example

Question

  • A drug blocks phosphodiesterase in liver cells that are being stimulated by epinephrine.
  • Predict the effect on cyclic AMP concentration and on glycogen breakdown, explaining your reasoning.

  1. Phosphodiesterase normally converts cyclic AMP to AMP, which removes the second messenger.
  2. If it is blocked, cyclic AMP is not removed, so its concentration stays high for longer.
  3. Cyclic AMP keeps protein kinase A active, so the cascade continues and glycogen breakdown is prolonged.

Answer Cyclic AMP stays high and glycogen breakdown continues for longer than normal.

Part 5 of 5Exam Checklist & Tips

3.4.7 – Key Points for the Exam

  • Desensitisation Reduced response of a cell to a signal after it has been exposed to it for a long time.
  • Phosphodiesterase Enzyme that breaks cyclic AMP down to AMP and so ends the second messenger signal.
  • Acetylcholinesterase Enzyme in the synaptic cleft that breaks down acetylcholine.
  • Receptor downregulation Decrease in the number of receptors on a cell, lowering its sensitivity.

Common Mistakes

  • Assuming a signal stops by itself without any removal step.
  • Saying the receptor is destroyed after each use; it usually detaches and is reused.
  • Mixing up upregulation (more receptors) and downregulation (fewer receptors).

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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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