Cellular Endomembrane System and Cytoskeleton: Structure, Function, and Transport

Endomembrane System

Secretory Pathway

The endomembrane system is a network of membranes within eukaryotic cells that works together to modify, package, and transport lipids and proteins. The secretory pathway is a key process by which molecules are synthesized and exported from the cell.

• Key Organelles: Smooth Endoplasmic Reticulum (ER), Golgi apparatus (cis and trans faces), lysosomes.

• Process: Proteins and lipids are synthesized in the ER, processed and sorted in the Golgi apparatus, and then transported in vesicles to their destinations, including secretion out of the cell.

• Example: Hormones and neurotransmitters are packaged into vesicles and secreted from cells.

Additional info: The cis face of the Golgi receives vesicles from the ER, while the trans face is where vesicles exit toward the plasma membrane or other organelles.

Neurotransmitter Transport in Neurons

Vesicle Movement and the Cytoskeleton

Neurons are specialized cells that transmit signals via neurotransmitters. The transport of neurotransmitter-containing vesicles from the cell body to the synaptic cleft is a highly organized process involving the cytoskeleton.

• Neuron Structure: Includes the cell body, axon, and synaptic cleft.

• Challenge: Axons can be very long (up to 1 meter in humans), making passive diffusion (Brownian motion) inefficient for vesicle transport.

• Solution: Vesicles are actively transported along the axon using the cytoskeleton.

• Example: In the human brain, about half of the ATP used by neurons powers motor proteins for vesicle transport.

Additional info: Efficient vesicle transport is essential for rapid neurotransmitter release and proper nervous system function.

Cytoskeleton

Structure and Function

The cytoskeleton is a dynamic network of protein filaments that provides structural support, enables cell movement, and facilitates intracellular transport.

• Main Components: Microtubules, microfilaments (actin filaments), and intermediate filaments.

• Functions:

  • Maintains cell shape and structural integrity.

  • Enables movement of organelles and vesicles within the cell.

  • Facilitates cell motility (e.g., muscle contraction, amoeboid movement, cilia and flagella motion).

• Example: Microtubules serve as tracks for motor proteins to transport vesicles.

Motor Proteins and Vesicle Transport

Motor proteins are specialized proteins that convert chemical energy (ATP) into mechanical work, moving cargo along cytoskeletal fibers.

• Types: Kinesin and dynein (move along microtubules), myosin (moves along actin filaments).

• Mechanism: Motor proteins "walk" vesicles along cytoskeletal tracks toward specific cellular destinations.

• ATP Usage: Movement is powered by ATP hydrolysis.

• Example: Kinesin transports neurotransmitter vesicles from the neuron cell body to the synaptic terminal.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Additional info: Motor proteins are essential for long-distance transport in large cells, such as neurons.

Summary Table: Cytoskeletal Components and Functions

Component	Main Protein	Function	Associated Motor Protein
Microtubules	Tubulin	Cell shape, vesicle transport, chromosome movement	Kinesin, Dynein
Microfilaments	Actin	Cell movement, muscle contraction, cytoplasmic streaming	Myosin
Intermediate Filaments	Various (e.g., keratin)	Structural support, cell integrity	None