Cell Walls and Extracellular Matrix

What is the main component of the bacterial cell wall?

Peptidoglycan (a linear polysaccharide chain cross-linked by short peptides).

What two alternating sugar residues make up the peptidoglycan backbone?

N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

What structure connects adjacent polysaccharide chains in the bacterial cell wall?

Cross-linked tetrapeptides.

What are three primary functions of the bacterial cell wall?

• Provide rigidity.

• Protect against osmotic pressure.

• Determine cell shape.

What is the main polysaccharide found in fungal/yeast cell walls?

Chitin (made of N-acetylglucosamine residues linked by $\beta$-1,4 bonds).

What is the main polysaccharide found in plant cell walls?

Cellulose (made of glucose residues linked by $\beta$-1,4 bonds).

What is the function of hemicellulose in plant cell walls?

It is a highly branched polysaccharide that binds to and stabilizes cellulose microfibrils, providing mechanical strength.

What is the function of pectin in plant cell walls?

It forms a gel-like matrix that traps positively charged ions and water molecules.

What force drives plant cell expansion and growth?

Turgor pressure (water uptake filling the central vacuole).

How does the orientation of cellulose microfibrils affect plant cell growth?

It restricts the direction of expansion, causing the cell to elongate along the plant's main axis.

What plant hormone initiates the Acid Growth Hypothesis?

Auxin (IAA).

How does auxin trigger the loosening of the cell wall during growth?

It stimulates proton pumps in the plasma membrane to pump $\text{H}^+$ into the cell wall, lowering the pH.

What enzymes are activated by low pH to loosen the plant cell wall?

Expansins (which separate microfibrils) and cell wall-loosening enzymes (which cleave cross-linking polysaccharides).

Where in the cell is cellulose synthesized?

At the plasma membrane by the enzyme complex cellulose synthase.

What structures in the cytoplasm guide the movement of cellulose synthase during synthesis?

Microtubules.

Where are hemicellulose and pectin synthesized before secretion?

In the Golgi apparatus.

What are the two main components of the animal extracellular matrix (ECM)?

Fibrous proteins (like collagen) and ground substance (gel-like polysaccharides).

What structural feature characterizes collagen molecules?

A triple helix formed by three polypeptide chains wound tightly around one another.

What are Glycosaminoglycans (GAGs)?

Gel-forming polysaccharides consisting of repeating disaccharide units (e.g., Hyaluronan, Chondroitin sulfate).

What is a proteoglycan?

A core protein covalently attached to multiple GAG chains.

What is the main function of ECM adhesion proteins?

To link components of the extracellular matrix to one another and to the surface of cells.

What are the binding domains found on a Fibronectin dimer?

Binding sites for proteoglycans, integrins, and collagen.

What is Laminin and what structure does it form?

A cross-shaped heterotrimer consisting of $\alpha$, $\beta$, and $\gamma$ chains that self-assemble into mesh-like networks.

What are integrins?

Transmembrane receptors consisting of $\alpha$ and $\beta$ subunits that link the intracellular cytoskeleton to ECM proteins.

What major cell-matrix junction components anchor epithelial cells to the basal lamina?

Integrins (e.g., Integrin $\alpha7\beta1$) binding to laminin, collagen IV, and nidogen in the basal lamina.