3.4.4Inquiry Process
Transmembrane Receptors — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 129 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.
Part 1 of 5Structure of the Receptor
Extracellular domain
- Extracellular domain The part facing the outside contains the ligand-binding site, so it determines which signal the receptor can detect.
Transmembrane region
- Transmembrane region Hydrophobic amino acids span the phospholipid bilayer, anchoring the protein, and in many receptors this is made of α-helices.
Intracellular domain
- Intracellular domain The inner part interacts with a relay protein or has enzyme activity, and it changes shape when the ligand binds.
No ligand entry
- No ligand entry The ligand never crosses the membrane, because the receptor itself carries the message across by changing shape.
Part 2 of 5G-Protein Linked Receptors
1
A ligand binds the extracellular site of a G-protein-linked receptor, which has seven transmembrane helices.
2
The receptor changes shape and activates a G protein bound to its inner face, which swaps GDP for GTP.
3
The activated G protein, or part of it, separates from the receptor and moves along the inner membrane surface.
4
It activates an effector enzyme, such as adenylyl cyclase, which makes a second messenger inside the cell.
5
The G protein later hydrolyses GTP back to GDP and returns to its inactive state, ending that round of signalling.
Part 3 of 5Second Messengers and Amplification
Second Messengers and Amplification
Second messenger
- Second messenger A small diffusible molecule, such as cyclic AMP, carries the message through the cytoplasm after the ligand has stayed outside.
Cascade
- Cascade One activated enzyme activates many of the next, so each step multiplies the number of molecules involved.
Speed
- Speed Because existing proteins are modified rather than new ones made, responses can occur within seconds.
Variety
- Variety Many receptor classes exist,
- including G-protein-linked, enzyme-linked and ion channel receptors,
- but all pass information across the membrane.
| Receptor class | How the message crosses | Typical outcome |
|---|---|---|
| G-protein-linked | Activates G protein and enzyme | Second messenger made |
| Enzyme-linked | Inner domain becomes an active enzyme | Phosphorylation cascade |
| Ligand-gated channel | Channel opens when ligand binds | Ions flow across membrane |
Part 4 of 5Worked Example
Question Describe what happens to the G protein and the second messenger after a hormone binds a G-protein-linked receptor, in the correct order.
- The receptor changes shape and activates the G protein, which exchanges GDP for GTP.
- The activated G protein switches on an effector enzyme such as adenylyl cyclase.
- The enzyme makes many second messenger molecules, so the signal is amplified and spreads through the cytoplasm.
- The G protein hydrolyses GTP to GDP and becomes inactive again.
Answer Receptor activates G protein, which activates an enzyme that makes second messengers, then the G protein switches itself off.
Part 5 of 5Exam Checklist & Tips
3.4.4 – Key Points for the Exam
- Transmembrane receptor A protein spanning the plasma membrane with a ligand-binding site outside and a signalling domain inside.
- G protein A GTP-binding protein that relays a signal from receptor to effector enzyme.
- Second messenger A small molecule made inside the cell that spreads a signal after receptor activation.
- Adenylyl cyclase Enzyme that converts ATP into cyclic AMP.
Common Mistakes
- Saying the ligand passes through the transmembrane receptor; only the message is passed, by a shape change.
- Thinking the G protein is a hormone; it is a relay protein on the inside of the membrane.
- Mixing up first and second messengers; the first messenger is the ligand itself.
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