Cell–Cell Junctions and Extracellular Matrix (2A) - Bio & Biochem
- Identify the junction type: Ca-dependent cadherins connect cells to actin belts.
- Adherens junction. This junction forms a belt-like structure for circumferential adhesion and cytoskeletal linkage.
- What is the basement membrane (basal lamina) primarily composed of?
- Laminin, type IV collagen, proteoglycans, and nidogen. These components form a thin, organized sheet separating epithelia from connective tissue.
- Which protein subunits assemble to form a connexon in a gap junction?
- Connexins. Six connexins form a hemichannel (connexon) that docks with an adjacent cell's to create the pore.
- What is the primary function of tight junctions (zonula occludens) in epithelia?
- Seal adjacent cells to block paracellular transport and maintain polarity. They form a barrier at the apical-lateral border, preventing diffusion between cells and separating membrane domains.
- What is the primary function of desmosomes (macula adherens)?
- Strong cell–cell adhesion resisting mechanical stress. They distribute mechanical forces across tissues by anchoring intermediate filaments between cells.
- What is the primary function of hemidesmosomes?
- Anchor epithelial cells to the basement membrane (ECM). They provide stable attachment to the underlying ECM, maintaining epithelial integrity.
- What is the typical maximum size limit for molecules passing through gap junctions?
- Approximately kDa. The channel pore size restricts passage to small solutes, ensuring selective communication.
- Which cytoskeletal element is anchored at adherens junctions?
- Actin microfilaments. Catenins link cadherins to the actin cytoskeleton, enabling force transmission and cell shape maintenance.
- What is the key function of adherens junctions (zonula adherens) between cells?
- Cell–cell adhesion via actin-linked cadherins. They provide mechanical linkage and maintain tissue integrity by connecting cytoskeletons of neighboring cells.
- What cytoskeletal element is linked to tight junctions via ZO proteins?
- Actin microfilaments. ZO proteins act as scaffolds connecting transmembrane components to the cytoskeleton for structural support.
- Which basement membrane protein forms a sheet-like network and binds integrins?
- Laminin. It self-assembles into networks that organize basement membranes and support epithelial attachment.
- Which intermediate filaments are classically anchored to desmosomes in epithelia?
- Keratin intermediate filaments. These filaments provide tensile strength and resilience, linking to desmosomes via plaque proteins.
- What is the main function of the extracellular matrix (ECM) in animal tissues?
- Structural support, signaling, and regulation of cell behavior. ECM provides a scaffold for cells, influences differentiation, and modulates growth factor activity.
- What is a proteoglycan in the ECM?
- Core protein covalently linked to glycosaminoglycan (GAG) chains. This structure allows proteoglycans to attract water and ions, contributing to ECM hydration.
- Which transmembrane receptor links hemidesmosomes to the extracellular matrix?
- Integrins. These receptors bind ECM ligands like laminin, connecting to intracellular adapters.
- Which cadherin-family proteins are the transmembrane components of desmosomes?
- Desmoglein and desmocollin. These proteins form transcellular dimers, anchoring cytoskeletal elements for robust adhesion.
- Which ECM protein is the major tensile-strength component of connective tissue?
- Collagen. Its triple-helical structure resists stretching, forming fibrils that support tissue architecture.
- Identify the junction type: protein channels allow ion flow and electrical coupling.
- Gap junction. It facilitates rapid signaling and synchronization in tissues like cardiac muscle.
- Which Ca-dependent adhesion proteins mediate adherens junctions?
- Cadherins (for example, E-cadherin). These proteins homophilically bind across cells in a calcium-dependent manner to facilitate adhesion.
- What is the defining feature of integrins regarding ligand binding and signaling?
- Bidirectional signaling: inside-out and outside-in. Integrins transmit signals from ECM to cytoskeleton and vice versa, regulating adhesion and migration.
- Which transmembrane proteins form the sealing strands of tight junctions?
- Claudins and occludin. These proteins create the tight seal by forming extracellular interactions between adjacent cell membranes.
- Which ECM protein commonly mediates cell attachment by binding integrins (RGD motif)?
- Fibronectin. It links cells to collagen via integrin-binding domains, facilitating migration and organization.
- Which cytoskeletal element is anchored by hemidesmosomes inside the cell?
- Intermediate filaments (keratin). They link to plaque proteins like plectin, providing mechanical stability against shear forces.
- Which ECM component provides hydration and resistance to compression in cartilage?
- Proteoglycans and GAGs (for example, aggrecan, hyaluronan). Their negatively charged GAGs bind water, creating a hydrated gel that absorbs compressive forces.
- What is the primary function of gap junctions between animal cells?
- Direct intercellular passage of ions and small molecules. They enable metabolic and electrical coupling by forming aqueous pores between cytoplasms.