AP® Biology · Unit 4 · Topic 4.3
Cellular Responses to Signals
A transduction pathway ends in a cellular response: sometimes a fast change to existing proteins, sometimes a switch that turns genes on or off, and sometimes a cell's orderly self-destruction.
A cell can respond to the same kind of input, a chemical signal binding a receptor, in wildly different ways because the response depends on which pathway the receptor triggers and which proteins are waiting downstream. One signal might activate kinases that turn on a transcription factor, which enters the nucleus and switches on genes for growth. Another signal might trigger a cascade that ends by activating enzymes that chop up the cell's own proteins and DNA, a controlled death called apoptosis. Same basic logic, receptor to relay proteins to response, completely different outcome, because the wiring differs.
Not every response touches the genome. Many pathways act directly on proteins already sitting in the cytoplasm: phosphorylating an enzyme to switch it on, opening an ion channel, or triggering actin filaments to rearrange so a cell changes shape or moves. These responses are fast, seconds to minutes, because no new proteins need to be built. Changing gene expression is slower, since it requires transcription and translation, so cells use that route when a lasting change, like differentiation or long-term growth, is called for.
Apoptosis shows transduction ending in demolition rather than construction. A signal, or the loss of one, activates a cascade of caspase enzymes that dismantle the cell from the inside: DNA gets fragmented, the cytoskeleton collapses, and the cell blebs into membrane-bound pieces that neighboring cells clean up, with no toxic spill of contents. This connects directly to tissue maintenance: many cells depend on a constant survival signal from neighbors just to keep their own death pathway switched off. Pull that signal away, as happens when blood supply is cut off or a cell detaches from its tissue, and apoptosis proceeds by default.
Key terms
- signal transduction pathway
- The series of molecular relay steps that converts a received signal into a specific cellular response.
- transcription factor
- A protein that binds DNA and helps switch a target gene's transcription on or off.
- apoptosis
- Programmed cell death in which a cell dismantles itself neatly without spilling harmful contents.
- caspase
- An enzyme activated during apoptosis that cuts apart proteins and helps break down the cell from within.
- survival signal
- A continuous signal from neighboring cells or the environment that actively suppresses a cell's default apoptosis pathway.
Watch it explained
- How it worksHow does a transduction pathway end up switching a gene on inside the nucleus?
- Common mistakeWhy doesn't every signal response involve changing which genes get expressed?
- How it worksWalk me through a signal triggering apoptosis and the cell taking itself apart neatly.
- Going furtherWhat happens to a tissue when cells stop receiving their normal survival signal?
Check yourself
End of topic.