Biology Chapter 1: A Tour of the Cell (Part 4)

Biology (1) 1501143

• Course instructor: Dr Maysoun Qutob
• Academic year: 20241
• Course offered by: Applied Science Private University

A Tour of the Cell (Part 4)

• Extracellular components and connections between cells help coordinate cellular activities.

Cell Walls of Plants

• Plant cells have cell walls, a defining feature that differentiates them from animal cells.

• Functions of the Cell Wall:

  • Protection of the plant cell
  • Maintaining the plant cell's shape
  • Preventing excessive water uptake

• Other organisms with cell walls include prokaryotes, fungi, and some unicellular eukaryotes.

• Plant cell walls are much thicker than the plasma membrane.

• Composition: Cellulose fibers embedded in other polysaccharides and proteins.

Formation of Cell Wall

• Young plant cells initially secrete a relatively thin, flexible primary cell wall.
• Middle lamella: Located between the primary walls of adjacent cells, composed of sticky polysaccharides (pectins).
• The middle lamella glues adjacent cells together.
• Secondary cell wall: Some plant cells strengthen their primary wall by secreting substances into it. Other plant cells form a secondary cell wall between the primary cell wall and the plasma membrane.
• Plasmodesmata: Plant cell walls are perforated by channels called plasmodesmata, which connect adjacent cells.

The Extracellular Matrix (ECM) of Animal Cells

• Animal cells have an extracellular matrix (ECM).
• Main components: Glycoproteins and other carbohydrate-containing molecules (mostly collagen).
• Collagen: forms strong fibers embedded in the ECM network.
• Proteoglycans: ECM components comprised of a core protein with many carbohydrate chains covalently attached. Proteoglycans can make up to 95% carbohydrate by weight.
• Fibronectin: A glycoprotein in the ECM that connects to specific receptor proteins (integrins) in the plasma membrane.
• Integrins: Span the cell membrane and bind to associated proteins connected to the cytoskeleton, acting as a bridge between the ECM and the cytoplasm.

Integrins

• Signal Transmission: Integrins transmit signals between the ECM and the cytoskeleton.
• Cell Behavior Regulation: ECMs can regulate cellular behavior and influence the activity of genes in the cell's nucleus.

Cell Junctions

• Neighboring cells often adhere, interact, and communicate via direct physical contact.

Plasmodesmata in Plant Cells

• Plant cell walls are perforated with plasmodesmata—channels connecting cells.
• The cytosol passes through plasmodesmata, merging the internal chemical environments of adjacent cells.
• Plasma membranes of adjacent cells line the channels of plasmodesmata.
• Water and small molecules can pass freely between cells. In some cases, certain proteins and RNA molecules can pass.

Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells

• Animal cells have three main types of cell junctions:

  • Tight junctions
  • Desmosomes
  • Gap junctions (most similar to plasmodesmata in plants)

• All three types are common in epithelial tissues, lining external and internal surfaces of the body.