Unit 8: The Extracellular Matrix and Cell Junctions
What holds a tissue together from the outside in: the fibers cells build around themselves, and the junctions that seal, anchor, and connect them to each other.
6 topics · 21 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Collagen Structure and Assembly
- How it worksHow do three collagen chains wind together into a triple helix stronger than any one chain alone?
- How it worksWalk me through how collagen molecules assemble outside the cell into a staggered fibril.
- Common mistakeWhy isn't collagen assembled into its final fibrous form while it's still inside the cell?
- Going furtherWhat happens to a tissue's structural strength when a genetic defect misshapes its collagen triple helix?
Your turn1 of 2
Collagen molecules are secreted from cells as soluble triple helices and only afterward assemble into the strong, staggered fibrils found in tendons and skin. A student assumes collagen should already be built into its final fibrous form before it's ever secreted. Why isn't collagen assembled into its final fibrous form while it's still inside the cell?Proteoglycans and the Ground Substance
- How it worksHow do proteoglycans' brush-like sugar chains trap water to resist compression in cartilage?
- Common mistakeWhy isn't the extracellular matrix just an inert scaffold with no active role in a cell's behavior?
- Going furtherWhat happens to a joint's cushioning ability when its cartilage loses proteoglycan content with age?
Your turn1 of 2
A biologist describes the extracellular matrix as nothing more than an inert scaffold that passively holds cells in place, with no influence over what those cells actually do. Why is this description wrong?Integrins and Cell-Matrix Adhesion
- How it worksHow does an integrin span the membrane to link extracellular matrix fibers to the actin cytoskeleton inside?
- How it worksWalk me through how force on an integrin from outside can trigger a signal inside the cell, and a signal inside can strengthen the integrin's grip outside.
- Common mistakeWhy isn't cell adhesion to the matrix a passive, purely mechanical attachment with no signaling involved?
- Going furtherWhat happens to a cell's survival when it's detached from the extracellular matrix entirely?
Your turn1 of 2
A cell adheres to the extracellular matrix through integrin receptors, and that attachment also influences the cell's internal signaling and gene expression. A student assumes matrix adhesion must be a purely passive, mechanical attachment with no signaling involved. Why is this wrong?Tight Junctions and the Paracellular Barrier
- How it worksHow do strands of transmembrane proteins fuse the outer leaflets of adjacent cells to seal the gap between them?
- Common mistakeWhy isn't a tight junction a barrier that stops all molecules equally rather than a selective one?
- Going furtherWhat happens to gut permeability when tight junction proteins are disrupted by inflammation?
Your turn1 of 2
Tight junctions seal the space between neighboring intestinal cells, yet some small ions and molecules can still slip through this sealed space under certain conditions. A student assumes a tight junction should block all molecules equally if it's truly sealing the gap. Why is this wrong?Adherens Junctions, Desmosomes, and Cadherins
- How it worksHow do cadherins on neighboring cells bind each other through calcium-dependent connections?
- How it worksHow does a desmosome anchor intermediate filaments from two neighboring cells into one shared mechanical unit?
- Common mistakeWhy isn't an adherens junction the same structure as a desmosome, even though both rely on cadherins?
- Ties togetherHow does the cadherin-actin link at adherens junctions connect one cell's cytoskeleton mechanically to its neighbor's?
Your turn1 of 2
Both adherens junctions and desmosomes use cadherin proteins to link neighboring cells together. A student concludes the two structures must be the same thing. Why isn't an adherens junction the same structure as a desmosome?Gap Junctions and Direct Cytoplasmic Communication
- How it worksHow do six connexin proteins assemble into a connexon channel that lines up with one on a neighboring cell?
- Common mistakeWhy isn't a gap junction the same kind of connection as a tight junction or a desmosome?
- Going furtherWhat happens to heart muscle coordination when gap junction channels between cardiac cells fail to form?
Your turn1 of 2
Gap junctions, tight junctions, and desmosomes are all found between neighboring cells in many tissues. A student assumes they must all serve the same basic connective purpose. Why isn't a gap junction the same kind of connection as a tight junction or a desmosome?