Extracellular Matrices Flashcards
What is an extracellular matrix (ECM)
A 3D network of macromolecules assembled by the cell, surrounding it externally to provide structure, protection, and signaling
What are the four main functions of the ECM
- Structural support – cell adhesion, tissue integrity, organisation
- Protection – from mechanical, biochemical, and biotic stress
- Signalling – transmission of mechanical and chemical signals
- Regulation – cell growth, motility, and morphogenesis
What is the structure of fibre-reinforced ECMS
Fibrils: provide strength, stiffness, and elasticity
Fibril cross-linkages: organise and strengthen 3D networks
Gel: hydrophilic, protects against compression, maintains hydration
What are the main components of the ECM in animals
Fibrils: collagen, elastin
Cross-linkages: collagen domains
Gel: glycosaminoglycans (GAGs), proteoglycans
What is collagen
An insoluble, fibrous protein (most abundant in animals, ~30% of body)
Forms triple helices, every third amino acid is glycine, rich in proline and hydroxyproline
Key structural component of connective tissues
Describe the biosynthesis of collagen
- ER – Pro α-chain synthesis and hydroxylation (needs vitamin C)
- ER – Glycosylation of hydroxylysines
- ER – Self-assembly into procollagen triple helix
- Golgi – N-linked glycan processing
- Secretory Vesicles – Transport to PM
- ECM – Cleavage of propeptides
- ECM – Self-assembly into collagen fibrils
- ECM – Aggregation into collagen fibres
What are fibril-associated collagens
Collagens with interrupted triple helices that facilitate interaction with other ECM molecules - critical for cartilage, tendons, and ligaments
What is network-forming collagen
Collagen that assembles into meshworks (e.g. type IV collagen in basal laminae) with non-helical domains introducing flexibility
What is the basal lamina
A sheet-like meshwork of ECM under epithelial cells and surrounding muscle, fat, and Schwann cells - functions in filtration, polarity, signalling, and migration
What is elastin and what does it do
An elastic protein that forms unordered, cross-linked fibers (via tropoelastin)
Provides elasticity to tissues like skin, lungs, and blood vessels
What are GAGs
Glycosaminoglycans
Repeating disaccharide units, acidic and often heavily sulfated
Highly negatively charged and hydrophilic
Attract Na⁺ ions → osmotic effect → increased turgor pressure
Form gels even at low concentrations; resist compression
What is hyaluronic acid
A simple, abundant GAG with a key role in hydration and joint lubrication
What are proteoglycans
Core proteins with GAG chains attached via O-linked glycosylation
Synthesised in the Golgi
Up to 95% carbohydrate by weight (contrast with glycoproteins: 1–40%)
How is the plant ECM structured
Fibril: cellulose microfibrils
Cross-linkages: hemicellulose
Gel: pectin and lignin
What is cellulose
A β-1,4-glucan polymer forming straight chains
Synthesised at the PM by terminal rosettes
Forms crystalline microfibrils via H-bonding between 18–24 chains
Highly insoluble, acid-resistant, and tensile (strong like steel)
What are terminal rosettes and what do they do
Membrane protein complexes that synthesise cellulose microfibrils
Made of 6 subunits, each containing 3–6 cellulose synthase (CESA) proteins
Each rosette → 1 cellulose fibril
How is the orientation of cellulose fibrils regulated in plant cells
By microtubules in the cytoskeleton
Fibrils limit cell elongation perpendicular to their orientation
Rosettes weakly associate with microtubules (no motor proteins)
What is the role of hemicellulose
Polysaccharide with a glucose backbone and side chains
Side chains prevent aggregation; unbranched side can bind cellulose
Acts as a “glue” between cellulose microfibrils - mechanical hotspots
What is pectin and what are its functions
Acidic, Ca²⁺-regulated polysaccharide gel
Made in the Golgi and transported via secretory vesicles
Influences porosity, surface charge, ion balance, pathogen defense
Found mostly in the middle lamella (glue between cells)
What role does turgor pressure play in plant cell growth
Maintains rigidity by pressing against the cell wall
Drives expansion when wall yield threshold is exceeded
What is the Lockhart equation
Relative Growth Rate (R) = φ × (P - y)
Where:
φ = wall extensibility
P = turgor pressure
y = yield threshold pressure
What is the acid-growth hypothesis
Plant hormones (like auxin) lower apoplastic pH (<5)
Activates expansins → disrupt H-bonds between cellulose & hemicellulose
Softens wall and allows expansion
What are integrins
Heterodimeric (αβ) transmembrane receptors that bind ECM components
Found at focal adhesions - link ECM to actin cytoskeleton
Mediate adhesion, signal transduction, migration, shape
How do animal and plant ECMs respond to stress
Animal: Collagen reorients, stress fibers realign
Plant: Microtubules reorient, altering cellulose deposition
