How do signal transduction pathways amplify a signal, and what happens when they are altered?
Topic 4.3 Signal Transduction Pathways: explain how signalling pathways relay and amplify a signal to produce a response, and how mutations or chemicals that change the pathway affect the cell.
A focused answer to AP Biology Topic 4.3, covering relay molecules, phosphorylation cascades, signal amplification, the variety of cellular responses, and how mutations and chemicals alter pathways.
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What this topic is asking
The College Board (Topic 4.3) wants you to explain how signal transduction pathways relay a signal from the receptor to the response, how they amplify the signal, the variety of cellular responses they produce, and how mutations or chemicals that change a pathway affect the cell. Pathway-mutation questions are a favorite, especially the link to cancer.
Relay and phosphorylation
The relay separates reception from response, which lets the cell control, amplify and regulate the signal along the way.
Amplification
Amplification is why tiny amounts of a hormone can have a large effect on a target cell.
The variety of responses
The same idea of reception, transduction and response leads to many different outcomes depending on the pathway and cell:
- Activating or inhibiting an enzyme, changing metabolism (for example breaking down stored glycogen).
- Switching genes on or off, changing which proteins the cell makes.
- Changing the cytoskeleton or cell shape, or the cell's movement.
- Triggering cell division or, in some pathways, programmed cell death.
When pathways are altered
A stuck-on growth-signalling pathway makes a cell divide continuously without the normal signal, which is a feature of cancer. Many drugs work by deliberately blocking a specific pathway component, and many poisons act by jamming a pathway on or off.
Try this
Q1. Explain how a phosphorylation cascade switches a relay protein on. [2 points]
- Cue. An activated kinase transfers a phosphate to the next protein, changing its shape and activating it; that protein then activates the next.
Q2. Predict the effect of a chemical that permanently activates a relay protein in a cell-division pathway, and justify. [2 points]
- Cue. The cell divides continuously without the normal signal, because the downstream response is switched on regardless of whether the ligand is present; uncontrolled division is a feature of cancer.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)4 marksSection II (long FRQ excerpt). A mutation makes a relay protein in the middle of a signalling pathway permanently active, even with no signal present. (a) Predict the effect on the cellular response. (b) Explain your prediction, referring to how the pathway normally works.Show worked answer →
A 4-point predict-and-explain FRQ on altered pathways.
(a) Predict (1 point): the cellular response is switched on continuously, even without the signal.
(b) Explain (3 points): (1 point) normally the pathway is activated only when the ligand binds the receptor and the relay is switched on in sequence; (1 point) a permanently active relay protein activates the downstream steps regardless of whether the signal is present; (1 point) so the response (for example cell division or gene expression) occurs constantly, which can be harmful (uncontrolled division is a feature of cancer).
Markers reward explaining that a stuck-on relay produces the response without the normal signal, and linking continuous division to cancer.
AP 2018 (style)3 marksSection II (short FRQ). One signal molecule binding a single receptor can change the activity of millions of molecules in the cell. (a) Identify the property of signalling pathways responsible. (b) Explain how it is achieved.Show worked answer →
A 3-point identify-and-explain FRQ on amplification.
(a) Identify (1 point): signal amplification.
(b) Explain (2 points): (1 point) each activated molecule in the relay can activate many molecules at the next step (for example one enzyme activating many targets); (1 point) repeating this over several steps multiplies the effect, so one ligand leads to a very large response.
Markers reward describing a cascade in which each step activates many molecules, multiplying the signal.
Related dot points
- Topic 4.2 Introduction to Signal Transduction: describe the reception, transduction and response stages of a signalling pathway, and the roles of receptors, ligands and second messengers.
A focused answer to AP Biology Topic 4.2, covering the three stages of signal transduction (reception, transduction, response), membrane and intracellular receptors, ligands, relay molecules and second messengers.
- Topic 4.1 Cell Communication: describe the ways cells communicate, including direct contact and chemical signalling over short and long distances.
A focused answer to AP Biology Topic 4.1, covering direct contact signalling, paracrine, autocrine, synaptic and endocrine signalling, and how signal type relates to distance and target.
- Topic 4.4 Feedback: explain how negative feedback maintains homeostasis and how positive feedback amplifies a response, using examples from cellular and organismal systems.
A focused answer to AP Biology Topic 4.4, covering negative feedback and homeostasis, positive feedback and amplification, set points, and how feedback data are analyzed, with a worked chi-square example.
- Topic 4.6 Regulation of the Cell Cycle: explain how checkpoints and regulatory molecules control progression through the cell cycle, and how loss of control leads to cancer.
A focused answer to AP Biology Topic 4.6, covering cell-cycle checkpoints, cyclins and cyclin-dependent kinases, growth factors, the link to signal transduction, and how loss of regulation causes cancer.
- Topic 3.2 Enzyme Catalysis: explain how enzymes lower activation energy and how substrate concentration, enzyme concentration and inhibitors affect the rate of an enzyme-catalyzed reaction.
A focused answer to AP Biology Topic 3.2, covering activation energy, the transition state, saturation, the effect of substrate and enzyme concentration, and competitive versus noncompetitive inhibition, with a worked rate calculation.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)