VCE Biology — enrichment
Signal Transduction — Flashcards & Quiz
Signal transduction converts an external signal, such as a hormone binding to a receptor, into a response inside the cell through a chain of molecular events. It is not part of the current VCAA Biology Study Design (2022–2027); it appeared in the previous study design. Treat this page as enrichment: it explains how cells respond to hormones in the Unit 1 regulation of blood glucose, temperature and water balance, and how immune cells communicate in Unit 4 Area of Study 1.
Key Points
- Cell signalling has three stages: reception (a signalling molecule binds a receptor), transduction (relay molecules pass the signal on) and response (the cell changes its activity). Termination switches the signal off.
- Receptors are specific: only a molecule with a complementary shape (the ligand) binds.
- Hydrophilic signals such as peptide hormones (insulin, glucagon, adrenaline) bind receptors on the plasma membrane; hydrophobic signals such as steroid hormones cross the membrane and bind receptors inside the cell.
- G-protein-coupled receptors activate a G protein, which activates an enzyme that makes a second messenger such as cAMP.
- Amplification: each activated molecule activates many of the next, so a few hormone molecules can produce a large response.
- Termination: the ligand detaches and second messengers are broken down, so the response stops when the signal is removed.
Common Mistakes to Avoid
- Claiming receptors produce the response directly — they start a cascade that produces the response.
- Forgetting termination — without it the cell would keep responding after the signal has gone.
- Mixing up ligand and receptor — the ligand is the signal, the receptor is the protein it binds to.
- Placing all receptors on the membrane — steroid hormone receptors are inside the cell.
- Treating this as current study design content — it is enrichment; the study design asks about homeostasis and immune responses, not transduction pathways.
Exam Strategy
Signal transduction is enrichment rather than assessed content under the current study design. It is useful when you explain hormonal regulation: for example, glucagon or adrenaline binding to liver cell receptors triggers the breakdown of glycogen to glucose, raising blood glucose. Name the signal, the target cell and receptor, and the response, and keep the wording to the stimulus-response and feedback language that the study design uses.
Sample Flashcards
Q1: What are the stages of signal transduction?
Reception: a signalling molecule (ligand) binds a specific receptor protein on or in the target cell. Transduction: the receptor changes shape and passes the signal through relay molecules, often a cascade of enzymes. Response: the cell changes its activity, for example activating an enzyme or switching a gene on. Termination then switches the pathway off once the signal is removed.
Q2: Why do hydrophilic and hydrophobic signals use different receptor locations?
Hydrophilic signals, such as insulin, glucagon and adrenaline, cannot cross the hydrophobic interior of the phospholipid bilayer, so they bind receptors on the plasma membrane surface. Hydrophobic signals, such as steroid hormones like testosterone and cortisol, diffuse through the membrane and bind receptors in the cytoplasm or nucleus, where the hormone-receptor complex often changes gene expression.
Q3: How does a G-protein-coupled receptor pass on a signal?
When a ligand binds a G-protein-coupled receptor, the receptor changes shape and activates a G protein on the inside of the membrane. The G protein activates an enzyme such as adenylyl cyclase, which converts ATP into cyclic AMP (cAMP). cAMP acts as a second messenger, activating protein kinases that phosphorylate other proteins and so produce the cell's response.
Q4: What is signal amplification and why does it matter?
Amplification means each activated molecule in a pathway activates many molecules at the next step. One hormone-receptor complex can lead to many cAMP molecules, each activating kinases that in turn activate many enzyme molecules. As a result, a very small concentration of hormone in the blood can produce a large, rapid response, such as a liver cell releasing large amounts of glucose.
Q5: Outline how adrenaline causes liver cells to release glucose.
Adrenaline, a hydrophilic hormone, binds a G-protein-coupled receptor on the liver cell membrane. The receptor activates a G protein, which stimulates adenylyl cyclase to make cAMP. cAMP activates a kinase cascade that switches on the enzyme that breaks glycogen into glucose. Glucose leaves the cell and blood glucose rises, supplying energy for the fight-or-flight response.
Revision Tip
Draw one pathway (adrenaline acting on a liver cell) from receptor to response, then drill the stages and key terms with a short Revizi deck.
Related Concepts
Last updated: 9 October 2026 · 6 sample flashcards