3.4.2Inquiry Process
Signalling Molecules — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 127 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 5Two Chemical Classes
Hydrophilic ligands
- Hydrophilic ligands Polar or charged molecules such as peptide hormones, epinephrine and neurotransmitters,
- which dissolve in blood plasma but cannot cross the phospholipid bilayer.
Hydrophobic ligands
- Hydrophobic ligands Non-polar molecules such as steroid hormones, which pass through the membrane by diffusion between the phospholipids.
Receptor position
- Receptor position Hydrophilic ligands bind transmembrane receptors on the cell surface, whereas hydrophobic ligands bind receptors inside the cell.
Transport in blood
- Transport in blood Hydrophobic hormones need carrier proteins in the plasma, but hydrophilic ones dissolve and travel freely.
| Property | Hydrophilic | Hydrophobic |
|---|---|---|
| Examples | Insulin, epinephrine, glucagon | Oestradiol, testosterone, progesterone |
| Cross membrane? | No | Yes, by diffusion |
| Receptor | Cell surface | Cytoplasm or nucleus |
| Speed of response | Seconds to minutes | Minutes to hours |
Section 2 of 5Examples of Signalling Molecules
Examples of Signalling Molecules
Peptides
- Peptides Hormones made of amino acid chains, such as insulin, bind receptors on the cell surface and act quickly on existing enzymes.
Steroids
- Steroids Lipid-derived hormones built from cholesterol, such as oestradiol and testosterone, usually alter which genes are transcribed.
Neurotransmitters
- Neurotransmitters Small molecules such as acetylcholine are released at synapses and act across a tiny gap on the next cell.
Cytokines
- Cytokines Protein signals released by immune cells to coordinate defence, which act locally or over longer distances.
Section 3 of 5Consequences of Chemistry
1
A hydrophilic ligand binds at the surface, so the message must be relayed inside by a second messenger because the ligand itself stays outside.
2
A hydrophobic ligand enters the cell and binds its receptor directly, so the ligand-receptor complex can influence DNA transcription.
3
Surface signals usually change enzyme activity already present, so the response is quick but short-lived once the ligand is removed.
4
Intracellular signals usually alter protein synthesis, so the response takes longer to appear but also lasts longer.
Section 4 of 5Worked Example
Question
- Insulin is a peptide and oestradiol is a steroid.
- Predict where each receptor is located and which response is faster, giving reasons.
- Insulin is hydrophilic, so it cannot diffuse through the phospholipid bilayer and must bind a transmembrane receptor at the cell surface.
- Oestradiol is hydrophobic and lipid-soluble, so it diffuses into the cell and binds an intracellular receptor.
- Insulin changes existing enzymes through a relay,
- which takes seconds to minutes; oestradiol alters transcription and protein synthesis,
- which takes longer.
Answer
- Insulin: surface receptor and the faster response;
- oestradiol: intracellular receptor and the slower, longer-lasting response.
Section 5 of 5Exam Checklist & Tips
3.4.2 – Key Points for the Exam
- Hydrophilic
- Water-loving;
- polar or charged molecules that dissolve in water but cannot easily cross a membrane.
- Water-loving;
- Hydrophobic
- Water-fearing;
- non-polar molecules that dissolve in lipids and diffuse through a bilayer.
- Water-fearing;
- Steroid hormone A lipid-based hormone derived from cholesterol that binds an intracellular receptor.
- Peptide hormone A hormone made of amino acids that binds a receptor at the cell surface.
Common Mistakes
- Claiming steroids need transmembrane receptors; being lipid-soluble, they bind inside the cell.
- Saying all hormones are proteins; steroid hormones are lipids.
- Assuming hydrophilic ligands enter the cell to start the response; they act from outside.
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