Ch. 15- Cell wall, ECM, and Cell Interactions

Cell Bio

Which eukaryotic cells use cell walls?

Fungi, algae, and higher plants

What is the purpose of bacteria cell walls?

Osmotic pressure and define shape

Which eukaryotic cells use cell walls?

Fungi, algae, and higher plants

What is the purpose of bacteria cell walls?

Osmotic pressure and define shape

What is the primary component of bacterial cell walls

Peptidoglycan - consists of linear polysaccharide chains cross-linked by short peptides

What do polysaccharide chains consist of?

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

• Joined by B-1,4 glycosidic bonds

  • Parallel chains are cross-linked by tetrapeptides attached to the NAM residues

What drug inhibits the enzyme responsible for forming cross-links between different strand of peptidoglycan?

Penicillin

• Does this by interfering with cell wall synthesis and blocking growth

What is the basic structural polysaccharide in fungi?

Chitin - linear polymer of N-acetylglucosamine

• Forms the shells of crabs and exoskeletons of insect and erthropods

What are the cell walls of algae and higher plants composed of?

Cellulose

• the most abundant polymer on Earth

  • Linear polymer of glucose residues often containing more than 10,000 glucose monomers

  • Form chains called microfibrils

What are the 2 polysaccharides that form the cellulose microfibril embedded matrix?

Hemicellulose - highly branched polysaccharides, H-bonded to cellulose microfibrils (stabilize microfibrils into a tough fiber = mechanical strength)

Pectin - branched polysaccharides containing a large number of negatively charge galacturonic acid residues

• Binds ions like Ca2+ and traps water molecules to form gels (due to - charge)

Plant cell wall structure

Cellulose - microfibrils oriented in layers.

Hemicelluloses- tightly associate with cellulose microfibrils

Both are embedded into a gel-like matrix of pectins

What causes cell expansion

A build up of internal turgor pressure when water fills the central vacuole.

What plant hormone triggers the expansion of the cell wall?

Auxins, which activate expansins - facilitate passive changes during cell wall expansion and is grown by the newly synthesized Hemicellulose and pectins (Synthesized in Golgi)

What is cellulose synthase and what is its role?

Transmembrane enzyme that synthesizes cellulose outside the cell from UDP-glucose in the cytosol

In which direction are cellulose fibrils deposited?

Perpendicular to the direction of cell elongation (provides strength during elongation)

Where are most animal cells?

Embedded in the extracellular matrix - most abundant in connective tissues

• ECMs have tough fibrous proteins embedded in a gel-like polysaccharide

What amount of each ECM component do tendons, cartilage, and bone have?

Tendons- High proportion of fibrous proteins

Cartilage- High level of polysaccharides that form compression-resistant gels

Bone matrix- hardened by Calcium Phosphate crystals

What is the structure of collagen?

form triple helices : three poly-peptide chains are wound together

• It is the major structural protein

• DO NOT synthesize inside the cell

Procollagen

Exists as a triple helices protein that is capped on the ends to prevent assembly of fibrils.

• Cleaved to collagen after its secretion (Procollagen peptidases)

  • assembly of collagen fibers only happens outside the cell

What is the most abundant type of collagen?

Type I that is found in skin, bones, tendons/ligaments, cornea, blood vessels, etc.

• Forms collagen fibrils

What are basal laminae made of?

Type IV collagen, that forms networks and can be more flexible.

• Contain Laminin adhesion proteins

What are elastic fibers made up of?

Elastin, a protein which is cross-linked into a network and behaves like a rubber band.

What are ECMs gels formed from?

Polysaccharides called glycosaminoglycans (CAGs) - repeating units of disaccharides

• sugars are modified with sulfate groups → CAGs highly negatively charged

  • Except for hyaluronan

• Bind positively charged ions and trap water molecules to form gels.

What do CAGs form when linked to proteins?

Proteoglycans, which interact with hyaluronan to form large complexes in the ECM

• 95% polysaccharide by weight

What is the main adhesion protein of connective tissues?

Fibronectin, which allows for proteins to be cross-linked into fibrils.

• Contains binding sites for both collagen and GAGs, enabling cross-linking

What are integrins?

Transmembrane proteins that attach cells to the extracellular matrix.

• Bind to various components of the ECM including collagen, fibronectin, and laminin

What are the 2 types of cell-matrix junctions?

1. Focal Adhesions: bundles of actin filaments anchored to B subunits of integrins via other proteins- a-actinin, talin, and vinculin

2. Hemidesmosomes: anchor epithelial cells to basal laminae

What is the role of metalloproteases?

Digest a mix of matrix proteins, like collagens, laminin, cell surface receptors, and adhesion molecules

• play important roles in normal movements of cells during development and growth of metastasis of cancers

Is cell-cell adhesion selective

Yes.

Cells adhere only to other specific cell types

• Mediated by cell adhesion molecules: selectin, integrins, immunoglobulin (lg) superfamily, and cadherins

  • Require Ca2+, Mg2+, and Mn2+

What is the role of selectins

Mediate transient interactions between leukocytes and endothelial cells or blood platelets

• followed by more stable interactions between integrins and intercellular adhesion molecules (ICAMs, members of the lg superfamily) on endothelial.

What is a homophilic interactions?

Adhesion molecule on one cell binds to the same molecule on another cell

• Cadherins mediate homophilic interactions

  • Adherens junctions - Link actin filaments of adjacent cells

  • Desmosomes

What is the role of B-catenin?

Bind to cadherin and help maintain stability

What is the role of desmosomes?

Link intermediate filament cytoskeletons of adjacent cells.