How does a signal arriving at a cell get converted into a response inside the cell?
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.
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What this topic is asking
The College Board (Topic 4.2) wants you to describe signal transduction: how a signal arriving at a cell is converted into a cellular response through three stages, reception, transduction and response, and to explain the roles of receptors, ligands and second messengers.
The three stages
The three stages always run in this order. Reception happens first because the cell cannot respond to a signal it has not detected.
Where the receptor sits
The location of the receptor depends on whether the ligand can cross the membrane:
- Membrane receptors. Water-soluble (hydrophilic) ligands cannot cross the plasma membrane, so their receptors sit in the membrane. The ligand binds the outside of the receptor, and the resulting shape change starts a relay inside the cell. The ligand never enters.
- Intracellular receptors. Small, nonpolar (hydrophobic) signals, such as steroid hormones, can pass through the membrane and bind receptors inside the cytoplasm or nucleus, often directly affecting gene expression.
Transduction and second messengers
Transduction is often a multi-step relay: the activated receptor activates the next molecule, which activates the next, and so on. Each step can activate many molecules, so the signal is amplified, meaning one signal molecule can lead to a large cellular response. The detail of pathways and amplification is developed in Topic 4.3.
Try this
Q1. Identify the three stages of signal transduction in order. [3 points]
- Cue. Reception (ligand binds receptor), transduction (signal relayed and amplified), response (cellular action).
Q2. Explain why a steroid hormone can bind a receptor inside the cell while a protein hormone cannot. [2 points]
- Cue. Steroids are small and nonpolar, so they cross the membrane to reach intracellular receptors; protein hormones are large and hydrophilic, so they bind membrane receptors instead.
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 2018 (style)4 marksSection II (short FRQ). A water-soluble signal molecule cannot cross the plasma membrane, yet it triggers a response inside the cell. (a) Identify where its receptor is located. (b) Explain how the signal causes an intracellular response without entering the cell.Show worked answer →
A 4-point identify-and-explain FRQ on reception and transduction.
(a) Identify (1 point): the receptor is located in the plasma membrane (a membrane receptor).
(b) Explain (3 points): (1 point) the signal molecule (ligand) binds the membrane receptor on the outside, which changes the receptor's shape; (1 point) this activates a transduction pathway inside the cell, often a relay of molecules or a second messenger; (1 point) the relay produces the cellular response, so the ligand never needs to enter the cell.
Markers reward placing the receptor in the membrane and explaining that the shape change starts an internal relay that produces the response.
AP 2022 (style)1 marksSection I (multiple choice). What is the correct order of the stages of a signal transduction pathway? (A) Response, reception, transduction. (B) Reception, transduction, response. (C) Transduction, response, reception. (D) Reception, response, transduction.Show worked answer →
The correct answer is (B).
Signalling proceeds in three stages: reception (the ligand binds the receptor), transduction (the signal is relayed and often amplified through the cell), and response (the cellular action). The other orders are incorrect.
Related dot points
- 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 4.3 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.
- 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 2.4 Plasma Membranes: describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.
A focused answer to AP Biology Topic 2.4, covering the fluid-mosaic model, the phospholipid bilayer, membrane proteins, cholesterol and carbohydrates, and how each component maintains the cell's internal environment.
- Topic 2.6 Membrane Transport: describe the mechanisms that organisms use to transport large and small molecules across the membrane and the energy requirements of passive and active transport.
A focused answer to AP Biology Topic 2.6, covering passive transport (diffusion and osmosis) versus active transport, the role of concentration gradients and ATP, and bulk transport by endocytosis and exocytosis.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)