3.4.3Inquiry Process
Receiving the Signal — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 128 of 160
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.
Section 1 of 5Receptor Location
Cell-surface receptors
- Cell-surface receptors Embedded in the plasma membrane with a binding site facing outward, these detect ligands that cannot cross the membrane.
Intracellular receptors
- Intracellular receptors Found in the cytoplasm or nucleus, these detect lipid-soluble ligands that have diffused in through the membrane.
Binding site
- Binding site The site is a specific three-dimensional pocket formed by the folded polypeptide, matching one ligand like a key fitting a lock.
Reversible binding
- Reversible binding Ligands attach by weak non-covalent interactions, so they detach again and the receptor can respond to future changes.
Section 2 of 5From Binding to Response
1
A ligand finds a receptor whose binding site is complementary in shape and charge to it.
2
Binding causes a conformational change in the receptor, which is the key step in turning an outside event into an inside event.
3
The altered receptor activates the next component, such as a G protein, an enzyme or a transcription factor.
4
Signal relay molecules amplify the message, so one ligand can eventually affect thousands of target molecules.
5
The cell carries out its response, and when the ligand level falls the receptors return to their inactive state.
Section 3 of 5Why Different Cells Respond Differently
Why Different Cells Respond Differently
Receptor types
- Receptor types A cell responds only to ligands for which it has receptors, so the receptors it makes decide which signals it can read.
Same ligand, different effect
- Same ligand, different effect Epinephrine speeds the heart but causes glycogen breakdown in liver,
- because the downstream pathways differ between tissues.
Receptor number
- Receptor number Cells with more receptors for a ligand respond more strongly, and cells can change this number over time.
Gene expression
- Gene expression Differentiated cells express different receptor genes, which is one way cell specialisation affects behaviour.
| Feature | Cell-surface receptor | Intracellular receptor |
|---|---|---|
| Ligand type | Hydrophilic | Hydrophobic |
| Position | Plasma membrane | Cytoplasm or nucleus |
| Typical response | Change in enzyme activity | Change in transcription |
Section 4 of 5Worked Example
Question
- Epinephrine increases heart rate but also causes glycogen breakdown in the liver.
- Explain how one ligand can have two effects.
- Both tissues carry receptors that bind epinephrine, so the ligand is detected in each.
- After binding, the receptor activates different downstream pathways, because each cell type contains different relay proteins and enzymes.
- In heart cells the pathway raises contraction rate, while in liver cells it activates enzymes that break down glycogen.
Answer The response depends on the signalling pathway and enzymes of the target cell, not only on the ligand.
Section 5 of 5Exam Checklist & Tips
3.4.3 – Key Points for the Exam
- Receptor A protein that binds a specific ligand and changes shape to start a response.
- Binding site The region of a receptor with a shape complementary to its ligand.
- Conformational change A change in the three-dimensional shape of a protein, here caused by ligand binding.
- Amplification Increase in the size of a signal as each step activates many molecules in the next.
Common Mistakes
- Saying the same hormone always has the same effect; the response depends on the target cell.
- Believing binding is permanent; ligands detach so the signal can be switched off.
- Forgetting that receptors are proteins whose shape is set by their amino acid sequence.
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