4.2Cell Communication and Cell Cycle

Introduction to Signal Transduction — AP BIOLOGY Revision Notes & Topical Questions

5 min read 11 parts Lesson 2 of 6 1 quiz

0%read

Lesson summary

4.2 Introduction to Signal Transduction — AP Biology notes: Describe the three stages of a signal transduction pathway and the main receptor types.Signal transduction pathway = the sequence of events that links receipt of a cell signal with a cellular response.Three stages — rece…

Typical events
Response types
Specificity
Conformational change
Hydrophilic (polar) ligands
Hydrophobic (nonpolar) ligands

Section 1 of 11What a signal transduction pathway is

1Signal transduction pathway

Signal transduction pathway the sequence of events that links receipt of a cell signal with a cellular response.

2Three stages

Three stages reception, transduction and response.

3Typical events

Typical events protein modification and phosphorylation cascades.
- (e.g. cAMP activating enzymes)

4Response types

Response types changes to cell processes such as gene expression.

Section 2 of 11Reception

1Binding

Binding a ligand binds the ligand-binding domain of a receptor protein.
- (peptide, small chemical or protein)

2Specificity

Specificity receptors are specific to one type of ligand; a cell without the receptor does not respond.

3Conformational change

Conformational change binding changes the receptor's shape and triggers transduction.

4Hydrophilic (polar) ligands

Hydrophilic (polar) ligands e.g. peptide hormones;
- cannot cross the membrane, so bind membrane (cell-surface) receptors that relay the signal across.

5Hydrophobic (nonpolar) ligands

Hydrophobic (nonpolar) ligands e.g. steroid hormones such as testosterone; diffuse through the membrane and bind intracellular receptors,
- and the complex may act as a transcription factor.

6Hormone examples

Peptide hormones such as insulin, glucagon and ADH are water-soluble and act through surface receptors.
Steroid hormones such as testosterone and oestrogen are lipid-soluble and act through intracellular receptors.

Section 3 of 11Receptor types

Receptor types

G protein-coupled receptors

G protein-coupled receptors activate a G protein, which activates enzymes such as adenylyl cyclase.
- (GDP swapped for GTP)

Receptor tyrosine kinases

Receptor tyrosine kinases dimerise and phosphorylate each other, activating relay proteins.

Ligand-gated ion channels

Ligand-gated ion channels open or close when the ligand binds, letting specific ions (e.g. Na⁺, Ca²⁺) cross and causing a chemical change inside the cell.

Intracellular receptors

Intracellular receptors bind steroid or other hydrophobic ligands in the cytoplasm or nucleus.

Feature	Membrane receptor	Intracellular receptor
Ligand	Hydrophilic or large (peptides, epinephrine)	Hydrophobic or small (steroids, NO)
Location	Plasma membrane	Cytoplasm or nucleus
Mechanism	Transduction by second messengers and kinases	Complex acts as transcription factor
Example	Epinephrine, GPCR, cAMP	Testosterone alters gene expression

Section 4 of 11Transduction

Transduction

Transduction the series of chemical changes inside the cell that lead to the response.

Kinases

Kinases transfer phosphate groups to activate other molecules.

Phosphatases

Phosphatases remove phosphate groups to deactivate molecules.

Second messengers

Second messengers small molecules that relay and amplify the signal, e.g. cAMP made by adenylyl cyclase, and Ca²⁺.

Signalling cascade

Signalling cascade each change triggers the next, so a small signal produces a large response (amplification).
One ligand can produce thousands of product molecules.

Section 5 of 11Response

1Gene expression

Gene expression control of transcription or translation.

2Metabolic activity

Metabolic activity enzymes switched on or off.

3Other responses

Other responses secretion of enzymes or hormones, cell shape change, cell growth and cell death (apoptosis).

4Different cells

Different cells the same signal gives different responses because the cells have different receptors or relay proteins.

5Timescale

Changes in enzyme activity occur within seconds, for example glycogen breakdown or opening of channels.
Changes in gene expression take minutes to hours, and steroid hormones act directly this way.

6Cytoskeleton and movement

Cytoskeleton and movement responses include chemotaxis of white blood cells along a chemical gradient

7Apoptosis

Apoptosis a signal activates caspases, protein-cutting enzymes that dismantle the cell in a controlled way
It shapes development, for example separating the digits of the hand.
It removes damaged cells.

8Cell division

Cell division it is triggered by growth factors

Section 6 of 11Reception to response in order

Ligand is released and reaches the target cell.

Ligand binds a specific receptor (reception).

The receptor changes conformation.

Relay molecules and second messengers pass on and amplify the signal (transduction).

A specific cellular response occurs, e.g. altered gene expression.

GPCR example
- ligand, GPCR, G protein, adenylyl cyclase, cAMP, protein kinase A, response.

Section 7 of 11Stages compared

Stage	Location	Key molecules	Event	Example
Reception	Membrane or cytoplasm	Receptor	Ligand binds, shape change	Adrenaline binds a GPCR
Transduction	Cytoplasm	G protein, kinases, cAMP	Phosphorylation cascade, amplification	cAMP activates PKA
Response	Cytoplasm or nucleus	Enzymes, transcription factors	Metabolism or gene expression changes	Glycogen breakdown
Termination	Cytoplasm	Phosphatases	Signal switched off	Phosphate removed

Section 8 of 11Examples of cellular responses

Cytokines

Cytokines proteins that regulate gene expression for cell replication and division, for example in the immune response

Yeast pheromones

Yeast pheromones they bind G protein-coupled receptors and lead to transcription of mating genes

SRY gene

SRY gene on the Y chromosome, its protein starts transcription of genes for testis development

Ethylene

Ethylene a plant hormone that binds fruit-cell receptors and alters expression of ripening genes

Hox genes

Hox genes they code for proteins regulating transcription so body parts form in the right place, for example the head, thorax and abdomen of fruit flies, by controlling genes for division, differentiation and apoptosis

Section 9 of 11More from the specification

Pituitary gland

The master gland whose hormones control other glands.

Feedback

A change in a system that either amplifies or reduces the original change.

Glucose

A simple sugar; the unit that starch, glycogen and cellulose are built from.

Negative feedback

A mechanism in which a change from the set point triggers a response that returns the factor to the set point.

Positive feedback

A process whose output moves a system further from its equilibrium.

Population density

Number of individuals per unit area or volume.

Direct contact

Spread of pathogens by blood or other body fluids from one person to another.

Section 10 of 11Key terms

Term	Meaning
Signal transduction	converting a signal into a cellular response
Reception	ligand binds the receptor
Transduction	relay of the signal through a cascade
Second messenger	small molecule relaying the signal, e.g. cAMP or Ca²⁺
Protein kinase	enzyme that adds phosphate groups to proteins
Conformational change	change in the shape of a protein

Section 11 of 11Exam technique

Avoid these mistakes

Saying hydrophilic ligands enter the cell — they bind surface receptors; only hydrophobic ligands cross the membrane.

Confusing the stages — reception is binding, transduction is the relay, response is the cell change.

Forgetting amplification — each step activates many molecules of the next.

Last updated: 14 October 2026