AP Biology: Cell Signaling and Feedback Mechanisms

Comprehensive vocabulary flashcards covering AP Biology Topics 4.1 through 4.5: cell communication, reception, signal transduction, cellular responses, pathway disruptions, and feedback mechanisms.

Cell Signaling

The series of steps that allow cells to respond to signals from their environment by initiating cellular activity, such as initiating the cell cycle.

Ligand

A chemical signal secreted by an initiating cell that travels to a target cell to initiate a response.

Reception

The step in cell signaling where a ligand binds to a specific receptor protein, causing the receptor to undergo a conformational (shape) change that transmits the signal into the cell.

Transduction

The pathway through which a signal is moved and amplified from the cell membrane to the nucleus via the sequential phosphorylation of a series of relay proteins.

Phosphorylation

The biochemical addition of a phosphate group to a protein or molecule to alter its activity.

Cellular Response

The end result of signal transduction, which can include transcription of target genes to produce proteins, stimulating cell growth and division, secreting molecules, or initiating apoptosis (cell death).

Cell-to-Cell Contact

The fastest means of transmitting a signal, requiring direct physical contact between membrane-bound ligands and receptor proteins on communicating cells.

Gap Junctions

Intercellular connections in animal cells where cell membranes fuse and open pores between adjacent cells, allowing signals to quickly diffuse and initiate a cellular response.

Plasmodesmata

Plant-specific microscopic channels that open pores through the cell wall and plasma membrane between adjacent cells, allowing signaling molecules to rapidly diffuse.

Paracrine Signaling

A mechanism of ligand delivery where a communicating cell secretes signaling molecules that diffuse across short distances to target receiving cells.

Membrane-Bound Receptors

Receptors embedded in the plasma membrane that bind hydrophilic ligands (which cannot cross the membrane), requiring a signal transduction cascade to relay and amplify the signal to the nucleus.

Intracellular Receptors

Receptors located inside the cytoplasm or nucleus that bind lipid-soluble ligands able to diffuse through the membrane; the resulting ligand-receptor complex travels directly to the nucleus, bypassing transduction.

Second Messenger

An intracellular molecule, such as cyclic AMP (cAMP), that is activated during transduction to amplify the signal message carried by the ligand and phosphorylate downstream proteins.

Transcription Factor

Usually the final protein activated in a signaling pathway that enters the nucleus to initiate the transcription of specific target genes into mRNA.

G Protein-Coupled Receptors (GPCRs)

Membrane receptors that, upon ligand binding, cause the activation and phosphorylation of an associated G protein, which then activates a second messenger to drive the cellular response.

Pathway Malfunction

The failure of a signaling cascade to produce a proper cellular response, often caused by mutations that alter the shape and chemical affinity of ligands, receptors, or transduction proteins.

Botulinum Toxin (Botox)

A bacterial toxin whose light chain cleaves specific SNARE proteins required for vesicle fusion, thereby preventing neurotransmitter release and causing muscle paralysis.

Feedback Mechanisms

Sensory pathways and physiological responses utilized by an organism's body system to regulate internal conditions and maintain homeostasis.

Negative Feedback

A regulatory mechanism that slows down or stops a physiological response, causing the body system to return to its normal set point and re-establishing homeostasis (e.g., sweating when body temperature exceeds $37^\circ\text{C}$).

Positive Feedback

A mechanism that amplifies and increases a biological process to produce large systemic changes (e.g., oxytocin release stimulating continuous uterine contractions during childbirth).

MDM2 and p53 Signaling Pathway

A cell cycle regulation pathway where DNA damage inhibits MDM2; because MDM2 normally inhibits p53, its inhibition allows p53 to form active tetramers that inhibit the cell cycle.

Allosteric Feedback Inhibition Pathway

A metabolic pathway where the end product (isoleucine) binds to an allosteric site on enzyme 1 to halt the pathway; inhibiting an intermediate enzyme causes its immediate preceding substrate to accumulate.