Chapter 7. Membrane Structure & Function

Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, organization, and motility to eukaryotic cells. It is essential for maintaining cell shape, enabling intracellular transport, and facilitating cell movement.

• Microtubules: Hollow tubes composed of α-tubulin and β-tubulin subunits. They serve as tracks for motor proteins and are involved in cell division, organelle movement, and maintaining cell shape.

• Microfilaments: Solid rods made of actin protein. They bear tension, support cell shape, and are crucial for cell motility and division.

• Intermediate Filaments: Fibrous proteins (such as keratin) that provide mechanical strength and anchor organelles within the cytoplasm.

Example: Microtubules form the spindle apparatus during mitosis, while actin filaments enable muscle contraction and cell movement.

Centrosome and Centrioles

The centrosome is the main microtubule-organizing center (MTOC) in animal cells. It contains a pair of centrioles, which are cylindrical structures composed of microtubules arranged in a specific pattern.

• Centrosome: Organizes microtubules and regulates their distribution during cell division.

• Centrioles: Each centriole is made up of nine triplets of microtubules arranged in a ring. They play a role in forming cilia and flagella.

Example: During mitosis, centrosomes duplicate and help organize the mitotic spindle.

Cilia and Flagella

Cilia and flagella are motile cellular appendages that enable movement of cells or substances across cell surfaces. Both structures are composed of microtubules and are sheathed by the plasma membrane.

• Microtubule Function: Microtubules control the beating of cilia and flagella, which are locomotor appendages of some cells.

• Beating Patterns: Cilia and flagella differ in their beating patterns—cilia typically move with a back-and-forth motion (power stroke and recovery stroke), while flagella exhibit a whip-like motion.

• Ultrastructure: Both have a core of microtubules (the "9+2" arrangement), a basal body anchoring the structure, and motor proteins (dynein) that drive movement.

Example: Cilia line the respiratory tract to move mucus, while flagella propel sperm cells.

*Additional info: The cytoskeleton is also involved in intracellular transport, cell signaling, and the spatial organization of cellular components. Microtubules and microfilaments are highly dynamic, undergoing rapid assembly and disassembly in response to cellular needs.*