IB Biology Topic C2.1.7: Initiation of Signal Transduction Pathways by Receptors Notes

A Bound Receptor Changes Shape and Starts the Signal Inside the Cell

1. Signal transduction is how a cell converts a signal from outside into a specific response inside.
2. It starts when a signalling molecule (the ligand) binds to a receptor.
   1. Binding causes a conformational change, a change in the receptor's shape.
3. This shape change is what initiates the pathway, relaying the message from the receptor to the inside of the cell.
4. These pathways let cells respond accurately to their environment.

Transduction Happens in Three Stages: Reception, Transduction, Response

1. Reception: the signalling molecule binds to the receptor.
2. Transduction: the signal is relayed and often amplified through a series of molecules inside the cell.
   1. Small relay molecules called second messengers can spread the signal quickly through the cytoplasm.
3. Response: the cell carries out an action, such as switching on a gene or releasing a substance.

The Conformational Change Turns On the Receptor's Inside Face

1. A transmembrane receptor spans the plasma membrane, with one part facing outside and one part facing inside.
2. When a signalling molecule binds the outer part, the conformational change activates the inner part, which begins the pathway.
   1. The signal is passed across the membrane without the ligand ever entering the cell.

Different Receptor Types Start Different Pathways

1. The same general principle, binding then conformational change then relay, plays out through several receptor types you meet later.
2. Ligand-gated ion channels open a pore so ions flow and change the membrane potential (C2.1.8).
3. G protein-coupled receptors activate a G protein that relays the signal inside (C2.1.9), as in the epinephrine and cAMP cascade (C2.1.10).
4. Tyrosine kinase receptors add phosphate groups to trigger a cascade, as with the insulin receptor (C2.1.11).
5. Intracellular receptors bind ligands that cross the membrane and then change gene expression (C2.1.12 and C2.1.13).

Why Initiation Matters: One Signal Can Trigger a Large Response

Because each step can activate many molecules at the next step, a few ligands can produce a large, coordinated cellular response.