Cytoskeleton and Cell Junction

Prokaryote vs Eukaryote Cell Walls

Prokaryotes

Peptidoglycan: A rigid cell wall

• Depending on the species, this wall is either the OUTERMOST layer (single membrane) or sits within the periplasmic space between and inner and outer membrane (double membrane)

Eukaryotes

Plants and Fungi

• Animal cells lack cell walls entirely.

• Plant cell walls are made primarily of cellulose (a polysaccharide)

• Fungal cell walls are composed of chitin and glucan

Proteoglycans vs Peptidoglycan

Feature	Proteoglycans	Peptidoglycan
Organisms Found In	Eukaryotes (Animal cells)	Prokaryotes (Bacteria)
Structure Location	Extracellular Matrix (ECM) surrounding animal cells	Bacterial cell wall
Molecular Structure	A core polypeptide heavily modified with attached polysaccharide chains	Polysaccharide chains cross-linked together by short peptide chains
Biosynthetic Pathway	Core polypeptide synthesized in the Rough ER, modified with sugars in the Golgi Apparatus, and secreted via vesicles	Synthesized and assembled into the external bacterial wall layer
Primary Function	Facilitates cell adhesion, extracellular support, and cell-cell communication	Provides structural rigidity and protection against osmotic pressure

3 Cytoskeletal Filaments in Eukaryotes

Microfilaments, Intermediate Filaments, and Microtubules

Microfilaments

Monomer = Actin Filaments

General Characteristics:

• Thinnest of the three filaments (~7 nm)

• Displays directionality with distinct plus (+) and minus (-) ends

• Highly dynamic: rapid assembly occurs mainly at the + end, and disassembly occurs at the - end.

General Functions:

• Maintains cell shape

• drives localized membrane changes like cell division cytokinesis (pinching membrane inward)

• cell crawling via pseudopodia

• cell contraction (e.g., muscle fibers).

Intermediate Filaments

Monomers: Diverse array of fibrous protein monomers (varies by cell/tissue type)

General Characteristics:

• Medium Thickness

• Stable and permanent structures that do not undergo continous assembly or disassembly

General Functions:

• Provides mechanical strength

• Maintains overall cell shape

• Anchors internal cell structures and organelles in place

Microtubules

Monomers: alpha tubulin and beta tubulin dimers

General Characteristics:

• Thickest of the three filaments

• Displays Directionality (+ and - ends)

• Dynamic filaments where both assembly and disassembly occur at the + end.

General Functions:

• Forms internal tracks for motor proteins walking cargo/vesicles through the cell

• Forms the 9+2 array structure inside eukaryotic cilia and flagella to drive bending and movement.

  
  

Types of Cell Junctions in Animals cells

Tight Junctions, Desmosomes, Gap Junctions

Tight Junctions

A tight seal between two adjacent cells, preventing the free movement of substances in between them (forcing substances to pass directly through the cell membranes)

Ex: cells in uyour blood vessels

Desmosomes

Structure & Function: Spot-weld connections spanning the membranes of neighboring cells.

Role: Mechanically links cells tightly together at specific points while leaving intercellular space between them, allowing fluids and molecules to pass through the gaps.

Gap Junctions

“tunnels” between linked cells, allowing for rapid communication and coordination between cells

Structure & Function: Transmembrane protein complexes that form open pores or channels between neighboring cells.

Role: Enables rapid, direct movement of small molecules and ions between adjacent cells to coordinate tissue-wide responses (e.g., synchronized heart muscle contractions).

Plasmodesmata (Plant Cells)

Open channels/gaps where the rigid cell wall did not close during cell division, leaving continuous plasma membrane and cytoplasm connections between adjacent plant cells.

Difference between Plasmodesmata and Gap Junctions

Both structures serve the same functional purpose—allowing direct intercellular communication and rapid transport of small molecules/ions—but plasmodesmata cross rigid plant cell walls, while gap junctions consist of protein channels between animal cell membranes.