AP Biology Unit 4: Mechanisms of Cell Signaling and Transduction

Cell Communication

The biological process by which cells detect and respond to signals in their environment.

Ligand

A signaling molecule that binds to a receptor to initiate a response.

Receptor

A protein that binds to a ligand and initiates a cellular response.

Signal Transduction Pathway

The mechanism by which a signal on a cell's surface is converted into a specific cellular response.

Reception

The stage where a target cell detects a signaling molecule.

Transduction

The conversion of a signal to a form that can bring about a specific cellular response.

Response

The specific cellular effect triggered by the transduced signal.

Juxtacrine Signaling

Direct contact signaling between adjacent cells.

Paracrine Signaling

Short-distance signaling where signals are released into the extracellular fluid to nearby target cells.

Endocrine Signaling

Long-distance signaling where signals travel through the bloodstream to distant targets.

Conformational Change

The change in shape of a receptor protein upon ligand binding.

Plasma Membrane Receptors

Receptors that bind to hydrophilic ligands that cannot cross the membrane.

Intracellular Receptors

Receptors located in the cytoplasm or nucleus that bind to hydrophobic ligands.

Protein Kinase

An enzyme that transfers phosphate groups from ATP to a protein, usually activating it.

Protein Phosphatase

An enzyme that removes phosphate groups from proteins, usually deactivating them.

Second Messengers

Small, non-protein, water-soluble molecules that help relay signals inside the cell.

Cyclic AMP (cAMP)

A second messenger formed from ATP in response to signaling that activates protein kinases.

Calcium Ions ($Ca^{2+}$)

Ions involved in various signaling processes and cellular responses, maintained at low concentrations in the cytosol.

Phosphorylation Cascade

A series of protein kinases that phosphorylate each other in sequence to amplify a signal.

Signal Amplification

The process where one signal molecule can trigger the production of many second messengers.

Constitutive Activation

A mutation that causes a receptor to remain active without a ligand, leading to uncontrolled signaling.

Negative Feedback

A regulatory mechanism where the output of a pathway inhibits upstream steps, promoting homeostasis.

Positive Feedback

A regulatory mechanism where the output amplifies the initial stimulus, moving the system away from a set point.

Inhibitors

Chemicals that block receptor sites to prevent ligand binding.

Activators

Chemicals that mimic ligands and trigger pathways artificially.

Homeostasis

The maintenance of a stable internal environment within an organism.

Hydrophilic Ligands

Water-soluble signaling molecules that cannot enter the cell and bind to surface receptors.

Hydrophobic Ligands

Lipid-soluble signaling molecules that can cross the cell membrane and bind to intracellular receptors.