Cell signaling
The process by which cells receive, interpret, and respond to chemical signals from other cells or the environment.
SignalingLigand
A signaling molecule that binds to a specific receptor to trigger a cellular response. Hormones, neurotransmitters, and growth factors are all ligands.
SignalingReceptor
A protein that specifically binds a ligand. Most receptors are in the plasma membrane; some are intracellular for hydrophobic ligands.
SignalingDirect contact signaling
Cells communicate by physically touching — through gap junctions (animals) or plasmodesmata (plants) that connect cytoplasms.
SignalingParacrine signaling
Local signaling where cells release a ligand that diffuses to nearby target cells. Example: growth factors during tissue repair.
SignalingSynaptic signaling
A specialized form of paracrine signaling — neurons release neurotransmitter into a synapse, which diffuses across a tiny gap to the target cell.
SignalingEndocrine signaling
Long-distance signaling using hormones that travel through the bloodstream to reach distant target cells anywhere in the body.
SignalingAutocrine signaling
A cell releases a ligand that binds to its own receptors, affecting the same cell. Common in immune cell regulation.
SignalingSignal transduction
The process of converting an extracellular signal into a specific intracellular response, usually through a cascade of molecular events.
TransductionReception
The first stage of signal transduction — the ligand binds to its receptor, causing a conformational change that begins the cascade.
TransductionTransduction
The middle stage where the signal is relayed (often through a phosphorylation cascade) and amplified inside the cell.
TransductionResponse
The final stage — the cell changes its behavior. Could be gene expression, enzyme activation, ion channel opening, or apoptosis.
TransductionG protein-coupled receptor (GPCR)
A common membrane receptor that activates an internal G protein when a ligand binds. Targeted by many drugs and hormones.
TransductionReceptor tyrosine kinase (RTK)
A membrane receptor that phosphorylates tyrosine residues on itself and other proteins when activated. Critical for growth factor signaling; often mutated in cancer.
TransductionIntracellular receptor
A receptor inside the cell (cytoplasm or nucleus). Used by small hydrophobic ligands like steroid hormones, which can cross the plasma membrane.
TransductionSecond messenger
A small molecule (like cAMP or Ca²⁺) that relays and amplifies a signal inside the cell after a receptor is activated.
TransductionProtein kinase
An enzyme that transfers a phosphate group from ATP to a target protein, often activating or deactivating it. The workhorses of signal transduction.
TransductionPhosphorylation cascade
A signaling relay where one kinase activates the next by phosphorylation, which activates the next, and so on — amplifying the signal at each step.
TransductionSignal amplification
The exponential magnification of a signal at each step of a cascade — one ligand can trigger millions of response molecules.
TransductionNegative feedback
A regulatory loop where the product of a process inhibits the process. Maintains homeostasis. Examples: blood glucose, body temperature.
FeedbackPositive feedback
A loop where the product amplifies the process. Drives systems toward completion. Examples: labor contractions, blood clotting, action potentials.
FeedbackHomeostasis
The active maintenance of stable internal conditions (temperature, pH, glucose, etc.) by an organism. Achieved largely through negative feedback loops.
FeedbackCell cycle
The ordered series of events from a cell's formation to its division. Phases: G₁ → S → G₂ → M (mitosis + cytokinesis).
Cell CycleInterphase
The portion of the cell cycle when the cell grows and copies its DNA. Includes G₁, S, and G₂. Takes up most of the cell's life.
Cell CycleG₁ phase
The first growth phase — the cell grows, performs its normal functions, and prepares to copy DNA.
Cell CycleS phase
DNA synthesis — each chromosome is replicated into two identical sister chromatids joined at the centromere.
Cell CycleG₂ phase
The second growth phase — the cell continues to grow and produces proteins needed for mitosis.
Cell CycleG₀ phase
A non-dividing 'resting' state cells enter when they're not preparing to divide. Mature neurons spend their lives in G₀.
Cell CycleMitosis (M phase)
The division of the nucleus into two identical daughter nuclei. Four phases: prophase, metaphase, anaphase, telophase.
Cell CycleProphase
Chromosomes condense and become visible; the nuclear envelope breaks down; the mitotic spindle forms from centrosomes.
Cell CycleMetaphase
Chromosomes line up along the cell's equator (the metaphase plate), with spindle fibers attached to each centromere.
Cell CycleAnaphase
Sister chromatids separate and are pulled to opposite poles of the cell by the shortening of spindle fibers.
Cell CycleTelophase
Two new nuclear envelopes form around the separated chromosomes; chromosomes decondense; the spindle breaks down.
Cell CycleCytokinesis
The physical division of the cytoplasm into two cells. Animal cells: cleavage furrow pinches inward. Plant cells: a cell plate forms in the middle.
Cell CycleSister chromatids
Two identical copies of a chromosome produced during S phase, joined at the centromere. Separated during anaphase.
Cell CycleCell cycle checkpoint
A control point where the cell verifies that conditions are right before proceeding. Three main ones: G₁, G₂, and M.
RegulationG₁ checkpoint
The most important 'go/no-go' checkpoint. Cell verifies size, nutrients, DNA integrity, and growth signals before committing to divide.
RegulationCyclin
A regulatory protein whose concentration rises and falls during the cell cycle. Binds to and activates CDKs.
RegulationCyclin-dependent kinase (CDK)
An enzyme that drives the cell into the next phase of the cell cycle when activated by binding to a cyclin. Phosphorylates target proteins.
Regulationp53
A tumor suppressor protein that monitors DNA damage. Stops the cell cycle for repair, or triggers apoptosis if damage is severe. Mutated in over half of human cancers.
RegulationApoptosis
Programmed cell death — a controlled self-destruction triggered when a cell is damaged, no longer needed, or potentially dangerous.
RegulationCancer
Uncontrolled cell division resulting from failed cell cycle regulation. Caused by mutations in proto-oncogenes (gain of function) or tumor suppressor genes (loss of function).
Regulation