A signaling pathway is useful only if it changes cell behavior. Two broad response modes differ mainly in what molecular machinery is modified.

Rapid responses modify proteins already present

A pathway can change the activity, localization or interactions of existing proteins within seconds to minutes.

Examples include

• opening or closing an ion channel;
• phosphorylating a metabolic enzyme;
• changing cytoskeletal organization;
• moving a transporter to or from a membrane.

Because the relevant proteins already exist, these responses do not require transcription and translation before they begin.

Slower responses change gene expression

Signaling pathways can also regulate transcription factors or chromatin-associated proteins. This changes which RNAs are produced and can ultimately change the abundance of many proteins.

The architecture is

receptor pathway
      ↓
transcriptional regulator
      ↓
gene-expression change
      ↓
new protein abundance
      ↓
cell-state change

Such responses often develop more slowly but can persist after the original signaling event because the cell's molecular composition has changed.

One pathway can produce both response types

An activated kinase might phosphorylate a cytosolic enzyme immediately and also phosphorylate a transcription factor that changes expression later.

Thus 'the cellular response' is rarely one terminal event. A signaling network can coordinate responses over multiple timescales.

Cell identity determines output

Two cell types can carry the same receptor yet respond differently because they contain different downstream enzymes, transcription factors and target genes.

The important distinction is therefore between signal recognition and response implementation. Receptors determine what a cell can detect; the downstream molecular context determines what the detected signal means for that cell.