Cells respond to signals by converting information at receptors into specific molecular changes. These changes can be fast and reversible or slower and longer-lasting, depending on whether gene expression is altered.

What a “cellular response” means

A signal transduction pathway ends when effector molecules produce a measurable change in the target cell, such as altered enzyme activity, secretion, movement, or transcription.

The response is determined by a cell’s expressed receptors and internal proteins, not just by the signal itself.

This diagram shows how a ligand activates a G-protein–coupled receptor (GPCR), triggering GDP→GTP exchange on the G protein and dissociation of its subunits. The activated subunits then interact with downstream targets to generate a cellular response, while GTP hydrolysis terminates the signal. It illustrates how receptor presence and intracellular relay proteins shape the final outcome of signaling. Source

Major categories of cellular responses

1) Changes in protein activity (common for rapid responses)

Many pathways modify pre-existing proteins, producing fast effects because the cell does not need to transcribe and translate new proteins.

This diagram shows a ligand-gated ion channel switching between closed and open states when a signaling molecule binds. The resulting ion movement across the membrane changes membrane potential and/or ion gradients, producing fast, reversible cellular responses. It provides a concrete model for how signaling can alter cell function by modifying existing proteins rather than changing gene expression. Source

• Enzyme activation/inhibition

  • Turning metabolic pathways on/off by changing enzyme shape or interactions

• Ion transport changes

  • Altered membrane permeability shifts ion gradients and downstream activities

• Cytoskeletal rearrangements

  • Changes in cell shape, motility, or intracellular transport

• Secretion and uptake

  • Increased exocytosis (release of molecules) or endocytosis (internalisation)

These responses are often reversible because proteins can be returned to their prior state or replaced.

2) Changes in gene expression (common for long-term responses)

Some pathways regulate transcription, creating sustained changes by adjusting which proteins are produced.

This figure illustrates a signaling-driven phosphorylation sequence that changes gene expression output at the level of translation: activated ERK phosphorylates MNK1, which phosphorylates eIF-4E to promote translation initiation. It emphasizes how signal transduction can produce longer-lasting cellular responses by altering protein production rather than only modifying existing enzymes. The pathway provides a concrete example of how signaling can reshape phenotype by changing what proteins a cell makes. Source

Key steps in a gene-expression response include:

• Activation of a regulatory protein that can influence transcription (often a transcription factor)

• Binding to specific DNA control regions to increase or decrease transcription of target genes

• Resulting changes in protein levels that alter cell structure and function

Gene-expression responses typically:

• Take longer to appear (minutes to hours)

• Persist longer because new proteins can maintain altered function

• Support cell specialisation and stable shifts in behaviour

How signalling produces new phenotypes vs alters existing activity

Producing new phenotypes

A phenotype can change when signalling causes the cell to make different proteins or different amounts of proteins, leading to new functional capabilities.

• Inducing expression of membrane transporters can change what nutrients a cell can import.

• Inducing expression of structural proteins can change cell morphology and tissue-level behaviour.

Altering existing cellular activities

A cell can also respond without creating a “new” phenotype by tuning activities it already performs.

• Increasing activity of an existing enzyme can increase pathway flux.

• Rearranging existing cytoskeletal proteins can temporarily change motility or adhesion.

• Modifying existing membrane proteins can quickly change electrical or osmotic properties.

Specificity: why the same signal can yield different responses

A single ligand can produce different outcomes in different cell types because:

• Only cells with the correct receptor can detect the signal.

• Different cells contain different relay proteins and effectors.

• The same pathway can connect to different target proteins or genes depending on cell context.

Key ideas to remember for AP Biology

• Signal transduction links reception to response, and the response may be at the level of protein function and/or gene expression.

• Rapid responses usually modify existing proteins; long-term responses often involve altered transcription and translation.

• Changes in signalling can create new phenotypes or adjust ongoing cellular activities.