Plant Cell Walls Overview

Cell Walls of Plants

• Cell walls are extracellular structures in plant cells, distinct from animal cells.
• Functions include:
  • Protecting the plant cell.
  • Maintaining cell shape.
  • Preventing excessive water uptake.
• Prokaryotes, fungi, and some unicellular eukaryotes also possess cell walls.

Plant Cell Wall Composition

• Plant cell walls are significantly thicker than the plasma membrane.
• They're composed of cellulose fibers embedded within other polysaccharides and proteins.

Cell Wall Formation

• Young plant cells initially secrete a thin, flexible primary cell wall.
• The middle lamella, a sticky layer containing polysaccharides (pectins), joins adjacent cells.
• As the cell matures and growth stops, some strengthen their walls by incorporating substances into the primary wall.
• Certain cells develop a secondary cell wall between the plasma membrane and primary wall, like in wood.
• Channels called plasmodesmata perforate plant cell walls, connecting adjacent cells.

Animal Cell Extracellular Matrix (ECM)

• Animal cells have an extracellular matrix (ECM).
• Key components are glycoproteins like collagen and proteoglycans.

Collagen

• Collagen forms strong fibers outside animal cells.
• It's embedded within a network of proteoglycans secreted by cells.

Proteoglycans

• Proteoglycan molecules are small core proteins with extensive carbohydrate chains attached.
• They make up a significant portion of the ECM's carbohydrate content (sometimes up to 95%).

ECM Attachment and Signaling

• Some animal cells attach to the ECM via glycoproteins (e.g., fibronectin).
• These glycoproteins bind to cell-surface receptor proteins called integrins, embedded in the plasma membrane.
• Integrins connect to the cytoskeleton, transmitting signals between the ECM and cytoskeleton to coordinate cell behavior.

Cell Junctions in Plants

• Neighboring plant cells adhere, interact, and communicate through physical contact sites.
• Plasmodesmata are channels that perforate cell walls, connecting adjacent cells.
• Cytosol flows through the plasmodesmata, interconnecting the chemical environments of neighboring cells.
• Plasma membranes lining plasmodesmata provide a continuous pathway for molecules.
• Water, small solutes, and sometimes other molecules like proteins and RNA can pass through these channels

Cell Junctions in Animals

• Animals have three main types of cell junctions: tight junctions, desmosomes, and gap junctions.
• Tight junctions prevent fluid movement between cells, acting as barriers.
• Desmosomes or anchoring junctions fasten cells to each other using sturdy keratin proteins to form strong sheets.
• Gap junctions or communicating junctions provide cytoplasmic channels between cells enabling intercellular communication for transport of ions, sugars, amino acids, and other small molecules.
  Tight junctions are an important part of epithelial tissues. Desmosomes and Gap junctions are common in a variety of tissues, too.