BIO Unit 2 Cell- Cell Interactions & Cancer

Tight Junctions

Cell junctions formed by occludin and claudin proteins that seal adjacent cell membranes, preventing leakage of water and water-soluble molecules.

Occludin and Claudin

Proteins that form tight junctions between adjacent animal cells.

Function of Tight Junctions

Prevent substances from leaking between cells and help maintain barriers, such as the blood-brain barrier and intestinal barrier.

Anchoring Junctions

Mechanically strong cell junctions that connect cells to other cells or to the extracellular matrix (ECM).

Desmosomes

Cell-to-cell junctions that use cadherins connected to intermediate filaments; resist shearing forces in skin and cardiac muscle.

Cadherins

Calcium-dependent adhesion proteins that connect adjacent cells in desmosomes and adherens junctions.

Intermediate Filaments

Strong cytoskeletal fibers that provide mechanical strength; they connect to desmosomes and hemidesmosomes.

Adherens Junctions

Cell-to-cell junctions that use cadherins connected internally to actin microfilaments.

Hemidesmosomes

Cell-to-ECM junctions that use integrins connected internally to intermediate filaments.

Focal Adhesions

Cell-to-ECM junctions that use integrins connected internally to actin filaments.

Integrins

Proteins that connect cells to the extracellular matrix; they are used in hemidesmosomes and focal adhesions.

Gap Junctions

Communicating junctions between animal cells that form channels allowing ions, cAMP, and small metabolites to pass between cells.

Connexins

Proteins that assemble into channels forming gap junctions between adjacent animal cells.

Connexon

A channel formed when six connexin proteins in one cell align with six connexins in a neighboring cell.

Middle Lamella

A pectin-rich, gelatinous layer between plant cells.

Plasmodesmata

Channels through plant cell walls that connect adjacent cells and allow ions and molecules to pass between them.

Desmotubule

A central tube of smooth endoplasmic reticulum within a plasmodesma.

Symplast

The continuous network of shared cytoplasm between plant cells connected by plasmodesmata.

Apoplast

The continuous space outside plant cell plasma membranes, including cell walls and the middle lamella.

Direct Intercellular Signaling

Communication in which ions or signals move directly between cells through gap junctions or plasmodesmata.

Contact-Dependent Signaling

Signaling in which a membrane-bound signal on one cell binds to a receptor on a neighboring cell; example: Delta-Notch signaling.

Autocrine Signaling

A cell releases a ligand that binds to its own receptors or receptors on similar nearby cells, regulating growth or differentiation.

Paracrine Signaling

Short-range signaling in which a secreted signal diffuses locally to neighboring cells; its short half-life limits long-distance travel.

Endocrine Signaling

Long-distance signaling in which hormones travel through the bloodstream in animals or vascular tissue or the gas phase in plants.

Auxin

A plant hormone involved in growth responses, including phototropism.

Phototropism

Plant growth toward light, driven in part by auxin promoting cell elongation on the shaded side.

Three Stages of Cell Signaling

Receptor activation, signal transduction, and cellular response.

Receptor Activation

The first stage of cell signaling; a ligand binds reversibly and non-covalently to a receptor, changing its conformation and activity.

Ligand

A signaling molecule that binds specifically and non-covalently to a receptor.

Signal Transduction

The stage of signaling in which intracellular molecular cascades relay and often amplify a signal through second messengers or kinase cascades.

Cellular Response

The final stage of signaling; the cell changes enzyme activity, cytoskeletal organization, movement, or gene expression.

Hydrophilic Signals

Lipid-insoluble signals that generally require intracellular signal transduction after binding to cell-surface receptors.

Hydrophobic Signals

Lipid-soluble signals that can cross the plasma membrane and may bind intracellular receptors directly.

Cell-Surface Receptors

Receptors located in the plasma membrane that detect extracellular signals and transmit information into the cell.

Enzyme-Linked Receptors

Receptors with an extracellular ligand-binding domain and an intracellular domain that functions as a catalyst; most are protein kinases.

Receptor Tyrosine Kinases (RTKs)

A class of enzyme-linked receptors that often activate phosphorylation cascades controlling cell growth, division, survival, and differentiation.

EGF

Epidermal growth factor, a signaling molecule that binds its receptor tyrosine kinase and can stimulate cell growth and division.

RTK Ligand Binding and Dimerization

Two signaling molecules bind extracellular domains of neighboring RTK subunits, causing the receptors to associate into a dimer.

RTK Autophosphorylation

After dimerization, each RTK kinase domain uses ATP to phosphorylate tyrosine residues on the opposite receptor subunit.

Grb2 and Sos

Adaptor and relay proteins recruited to phosphorylated RTKs; Sos helps activate Ras by promoting GDP-GTP exchange.

Ras

A small, membrane-bound monomeric G-protein activated when it releases GDP and binds GTP.

Ras Activation

Sos stimulates Ras to exchange GDP for GTP, activating Ras and initiating downstream signaling.

MAP Kinase Cascade

A sequence of protein kinases that activate one another through phosphorylation downstream of Ras.

ERK

A kinase activated at the end of the MAP kinase cascade that enters the nucleus and activates transcription factors involved in cell growth and division.

RTK Signaling Sequence

Ligand binding and dimerization → autophosphorylation → Grb2/Sos recruitment → Ras activation → MAP kinase cascade → cellular response.

G-Protein-Coupled Receptors (GPCRs)

Cell-surface receptors with seven transmembrane segments that activate separate intracellular heterotrimeric G-proteins.

Heterotrimeric G-Protein

A signaling protein composed of alpha and beta/gamma subunits that interacts with activated GPCRs.

First Messenger

The extracellular signaling molecule that binds to a receptor; for example, epinephrine in the cAMP pathway.

Epinephrine

A hormone involved in the fight-or-flight response that can activate GPCR signaling.

GPCR Activation

The binding of a ligand changes the receptor's shape, allowing it to activate an intracellular G-protein.

G-Protein Activation

The activated GPCR promotes the exchange of GDP for GTP on the G-protein's alpha subunit.

G-Protein Subunit Dissociation

GTP binding causes the active alpha-GTP subunit to separate from the beta/gamma dimer.

Adenylyl Cyclase

A membrane enzyme activated by alpha-GTP that converts ATP into cyclic AMP (cAMP).

cAMP (Cyclic AMP)

A second messenger produced from ATP by adenylyl cyclase; it rapidly distributes signals and amplifies cellular responses.

Second Messenger

An intracellular signaling molecule, such as cAMP, that relays and amplifies a signal from an activated receptor.

Protein Kinase A (PKA)

A protein kinase activated by cAMP that phosphorylates specific enzymes, structural proteins, and transcription factors.

GPCR cAMP Pathway

Ligand binds GPCR → G-protein exchanges GDP for GTP → alpha-GTP activates adenylyl cyclase → cAMP forms → PKA activates → cellular response.

G-Protein Deactivation

The alpha subunit hydrolyzes GTP to GDP and recombines with the beta/gamma dimer, returning the G-protein to its inactive state.

Phosphodiesterase (PDE)

An enzyme that breaks down cAMP into inactive AMP, helping terminate GPCR signaling.

Caffeine and PDE

According to the study guide, caffeine blocks phosphodiesterase, allowing cAMP to persist abnormally in cells.

Ligand-Gated Ion Channels

Receptors in which ligand binding opens or closes an ion channel, allowing specific ions to cross the membrane.

Effects of Ligand-Gated Ion Channels

Ion flow rapidly changes a cell's electrical state, membrane potential, activity, or intracellular ion concentration.

GPCR vs. RTK

GPCRs activate G-proteins and often use second messengers; RTKs dimerize and activate phosphorylation cascades.

RTK Cellular Responses

Can promote cell growth, division, survival, differentiation, and changes in gene expression.

Intracellular Receptors

Receptors that bind small lipid-soluble signals that cross the plasma membrane; the receptor can regulate DNA transcription.

Example of an Intracellular Receptor

Steroid hormone receptor.

Unicellular Organism Signaling

Signaling in unicellular organisms often allows cells to respond to changes in their environment.

Quorum Sensing

Population-density-dependent communication in bacteria using secreted autoinducer molecules.

Autoinducer

A signaling molecule secreted by bacteria that accumulates as population density increases and triggers coordinated behavior.

Quorum Sensing Responses

Can coordinate biofilm secretion, virulence factor expression, and bioluminescence.

Chromosome

A long double helix of DNA wrapped around proteins called histones.

Histones

Proteins around which DNA wraps to form chromosomes.

Gene

A section of DNA that encodes genetic information.

Somatic Cells

Body cells that are generally diploid (2n), containing homologous chromosome pairs inherited from each parent.

Diploid (2n)

A cell with two sets of chromosomes, typically one set inherited from each parent.

Haploid (n)

A cell containing one set of chromosomes; human reproductive cells are haploid.

Human Chromosome Number

Humans typically have 46 chromosomes arranged into 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes.

Autosomes

The 22 pairs of human chromosomes that are not sex chromosomes.

Sex Chromosomes

The chromosome pair involved in determining biological sex; humans typically have one pair.

Karyotype

A display used to examine chromosome number, size, and form in a cell.

Homologous Chromosomes

A pair of chromosomes, one inherited from each parent, that have the same size, shape, and genes at corresponding loci.

Sister Chromatids

Identical copies of a replicated chromosome that remain joined at the centromere until separated during cell division.

Centromere

The chromosome region where sister chromatids are joined.

Cohesin Proteins

Proteins that hold sister chromatids together until they are separated during anaphase.

Kinetochore

A protein structure assembled at the centromere where spindle microtubules attach to chromosomes.

Spindle Fibers

Microtubules that attach to chromosomes and help separate them during cell division.

Centrosome

The cell's main microtubule-organizing center (MTOC); it duplicates before division and moves toward opposite poles to organize spindle fibers.

Microtubule-Organizing Center (MTOC)

A region where microtubules begin to grow; in dividing animal cells, the centrosome serves this role.

Centrioles

Two barrel-shaped structures within the centrosome, surrounded by a protein matrix.

Interphase

The part of the cell cycle consisting of G1, S, and G2, during which the cell grows, replicates DNA, and prepares for division.

G1 Phase

The first gap phase; the cell grows, performs normal metabolic functions, and makes proteins, RNA, and organelles needed for division.

G0 Phase

A quiescent, non-dividing state that a cell can enter when conditions are unsuitable for division or division is unnecessary.

Cell Cycle Arrest

A halt in cell-cycle progression, such as when a cell enters G0 or fails a checkpoint.

S Phase

The synthesis phase in which the cell replicates its DNA and duplicates its centrosome.

G2 Phase

The second gap phase; the cell continues growing, produces proteins needed for the mitotic spindle, and checks DNA replication and damage.

Chromatin Condensation Before Mitosis

Chromatin begins its initial condensation during late G2 and becomes fully condensed during prophase.

M Phase

The stage of the cell cycle involving mitosis, or nuclear division.

Prophase

Chromosomes fully condense through the action of condensin proteins, the nuclear envelope begins to dissociate, and centrosomes move toward opposite poles.

Condensin Proteins

Proteins that help chromosomes become tightly condensed during prophase.

Prometaphase

The nuclear envelope fragments completely, the spindle forms, and kinetochore microtubules attach to kinetochores.