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Eukaryotic Cell Structures: Membranes, Cytoskeleton, and Organelles

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1. Eukaryotic Cell Membranes and External Structures

1.1 Plant Cell Walls

The cell wall provides structural support and protection for plant cells.

  • Function: Confers rigidity and prevents the cell from swelling or bursting in hypotonic environments.
  • Composition: Primarily composed of cellulose, pectin, and a hemicellulose matrix.

Layers of the Cell Wall

  • Middle Lamella: Integrated with pectins and proteins.
  • Primary Wall: Cellulose fibers cohesive with polysaccharides (hemicellulose and pectin).
  • Secondary Wall: May contain lignin, waxes, cutin, and suberin. It features narrow channels known as plasmodesmata.

1.2 Extracellular Matrix in Animal Cells

Also known as the glycocalyx, this structure is composed of glycoproteins and is essential for cell recognition and adhesion.

2. The Cell Cytoskeleton

A complex network of protein filaments consisting of microtubules, microfilaments, and intermediate filaments.

2.1 Microtubules

Composed of tubulin and organized by centrosomes.

  • Functions: Maintains cell shape, facilitates intracellular particle transport, and forms the spindle apparatus.
  • Movement: They form cilia and flagella.

Structure of Cilia and Flagella

  1. Axoneme: The inner structure (9+2 arrangement) containing dynein and nexin proteins.
  2. Basal Corpuscle: Located at the base of the cilium.
  3. Transition Zone: The basal plate.

2.2 Actin Microfilaments

Composed of globular actin proteins spirally coiled.

  • Muscle Contraction: Actin and myosin form myofibrils responsible for contraction.
  • Cellular Movement: Facilitates cyclosis and the formation of pseudopods.
  • Structural Support: Maintains the structure of microvilli and forms the contractile ring during division.

2.3 Intermediate Filaments

Structural filaments, such as keratin and neurofilaments, that provide mechanical strength to the cell.

3. Ribosomes

Composed of two subunits that form a groove. They exist either as free-floating structures or are associated with the Rough Endoplasmic Reticulum (RER) or the cell membrane.

4. Cytoplasmic Inclusions

4.1 Reserve Inclusions

  • Carbohydrates: Starch.
  • Fats: Stored in adipose tissue.

4.2 Pigments

  • Carotenoids: Orange pigments.
  • Hemoglobin: Found in red blood cells.
  • Melanin: Found in the skin.

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