TheCell7e Ch15 Lecture

Cell Walls, Extracellular Matrix, and Cell Interactions

Introduction

• Cells are often surrounded by insoluble secreted macromolecules.

• Bacteria, fungi, algae, and higher plants have rigid cell walls, while animal cells are embedded in an extracellular matrix composed of proteins and polysaccharides.

• The extracellular matrix provides structural support and regulates cell behavior.

• Cell interactions are crucial for the organization and communication within plant and animal tissues.

Cell Walls

Structure and Function of Cell Walls

• Cell walls determine cell shape and prevent cells from swelling and bursting due to osmotic pressure.

• Bacterial and eukaryotic cell walls are structurally different.

Types of Bacterial Cell Walls

• Bacterial cell walls determine characteristic shapes:

  • Rod-shaped (e.g. E. coli)

  • Spherical (e.g. Pneumococcus, Staphylococcus)

  • Spiral-shaped (e.g. Treponema pallidum, causes syphilis)

• Bacteria classified into two groups based on cell wall structure:

  • Gram-negative: Dual-membrane system with a thin cell wall.

  • Gram-positive: Single membrane with a thicker cell wall.

Composition of Bacterial Cell Walls

• Main component is peptidoglycan: Linear polysaccharide chains cross-linked by short peptides.

• Structure makes bacteria susceptible to antibiotics, such as penicillin which inhibits cross-linking enzyme.

Cytoskeletal Regulation of Cell Wall Synthesis

• Bacterial cytoskeletal proteins like FtsZ (similar to eukaryotic tubulin) guide cell wall synthesis during division.

• Other proteins such as MreB contribute to determining cell shape.

Eukaryotic Cell Walls

• Eukaryotic cell walls are mainly composed of polysaccharides.

  • Fungi: Chitin (linear polymer of N-acetylglucosamine)

  • Plants: Cellulose (linear polymer of glucose with over 10,000 monomers).

• Chitin and cellulose feature β (1→4) linkages forming long, straight chains.

Plant Cell Wall Structure

• Cellulose chains join to form microfibrils that can extend for many micrometers.

• In plants, cellulose microfibrils are embedded in a protein matrix and other polysaccharides:

  • Hemicelluloses: Branched polysaccharides providing stability and mechanical strength.

  • Pectins: Branched polysaccharides with charged galacturonic acids, cross-linking cellulose and trapping water to form gels.

Layers of Plant Cell Walls

• Primary cell walls: Thin and flexible, allowing growth and expansion. Contain equal parts cellulose, hemicelluloses, and pectins.

• Secondary cell walls: Form after cell growth, thicker (50-80% cellulose), often reinforced with lignin (a complex polymer of phenolic residues).

• Cellulose fibers in secondary walls are oriented for strength, creating a laminated structure.

Cell–Matrix Interactions

Components of the Extracellular Matrix

• Most abundant in connective tissues:

  • Tough fibrous proteins embedded in a gel-like polysaccharide.

  • Collagen: Major structural protein forming triple helices of collagen fibers.

  • Composition varies among tissues: Tendons (high fibrous proteins), Cartilage (high polysaccharides), Bone (hardened by calcium phosphate crystals).

Cell–Matrix Adhesion Proteins

• Fibronectin: Main adhesion protein in connective tissues, linking components to cells.

• Laminins: Integral to basal laminae, consisting of three polypeptide chains, forming networks that bind surface receptors and proteoglycans.

Integrins and Cell Adhesion

• Integrins: Transmembrane proteins attaching cells to the extracellular matrix, involved in focal adhesions and hemidesmosomes.

• Focal adhesions: Anchoring actin filaments to integrins, key in cell movement and mechanical stability.

• Hemidesmosomes: Anchor epithelial cells to basal laminae, connecting to intermediate filaments.

Cell–Cell Interactions

Importance of Cell–Cell Adhesion

• Critical for development and tissue organization; mediated by cell adhesion molecules (selectins, integrins, cadherins).

• Interactions can be transient or stable, facilitating communication and structure.

Types of Cell Adhesion Molecules

• Selectins: Mediate transient adhesion, especially in immune responses.

• Cadherins: Mediate stable adhesion, linking the cytoskeleton of adjacent cells through adherens junctions and desmosomes.

• Ig Superfamily: Mediate diverse cell-cell interactions, usually involving ICAMs.

Junction Types

• Tight junctions: Prevent molecule passage and separate apical and basolateral domains of the plasma membrane.

• Gap junctions: Allow rapid intercellular communication via connexins, vital for synchronized cell activities, especially in muscular tissues.

• Plasmodesmata in plants: Functions like gap junctions, allowing communication through cytoplasmic connections.