Extracellular Matrix and Intercellular Junctions

Extracellular Structures in Plant Cells

• Cell Wall Functions: Protects the cell, maintains shape, and prevents excessive water uptake.

• Composition: Primarily cellulose fibers bundled and cross-linked by polysaccharides.

• Cell Wall Layers:

  • Primary Cell Wall: Thin and flexible layer secreted by young growing cells.

  • Middle Lamella: Thin layer between adjacent cells containing pectin to act as glue.

  • Secondary Cell Wall: Secreted between the plasma membrane and primary wall in some cells for added mechanical strength.

• Cellulose Synthesis: Cellulose synthase proteins move along underlying microtubules to extrude cellulose fibrils, creating parallel alignment between cellulose and microtubules.

The Extracellular Matrix (ECM) of Animal Cells

• Composition: Consists of glycoproteins and carbohydrate-containing compounds.

  • Collagen: The most abundant glycoprotein, comprising approximately $40\%$ of total protein in the human body.

  • Proteoglycan Complex: Consists of proteoglycan molecules attached to a long polysaccharide molecule.

  • Fibronectin: Attaches the cell to the ECM.

• Integrins: Surface receptor proteins that span the plasma membrane to link the ECM to the internal cytoskeleton (actin filaments) and transmit signals.

• Glycocalyx: A coat of lipids and polysaccharides on the plasma membrane involved in cell contact, recognition, and tissue strength.

• ECM Functions: Provides structural support, cell-to-cell adhesion, movement, and regulation of biochemical processes.

Intercellular Junctions

• Plasmodesmata (Plants): Channels $20-40\,nm$ in diameter that perforate cell walls, allowing water, small solutes, proteins, and RNA to pass between cells.

• Tight Junctions (Animals): Also called occluding junctions; they press neighboring membranes together to prevent leakage of extracellular fluid across a layer of cells.

• Desmosomes (Animals): Anchoring junctions that fasten cells into strong sheets using intermediate filaments (keratin) and transmembrane glycoproteins called cadherins.

• Gap Junctions (Animals): Communicating junctions that provide cytoplasmic channels through connexons (composed of connexins).

  • Pores are $1-2\,nm$ in diameter.

  • Allow passage of ions, sugars, and small molecules.

  • Regulated by substances such as $Ca^{2+}$ and $H^+$ ions.

Integrated Cellular Function

• Holistic Coordination: Cellular functions require the integration of organelles and extracellular structures.

• Example: A macrophage's ability to destroy bacteria depends on the coordinated activity of the cytoskeleton, lysosomes, and plasma membrane.