The Cytoskeleton and Endomembrane System: Structure, Function, and Cellular Organization

The Cytoskeleton

Overview of the Cytoskeleton

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

• Structural support: Maintains the shape of the cell.

• Anchoring organelles: Positions organelles at specific locations within the cytoplasm.

• Intracellular transport: Provides a "roadway" for the movement of materials and organelles.

• Cell motility: Enables movement of the cell and its components (e.g., cytoplasmic streaming, muscle contraction).

Main Components of the Cytoskeleton

The cytoskeleton is composed of three main types of protein filaments, each with distinct structures and functions:

• Microfilaments (Actin filaments): Thin, flexible fibers involved in cell shape and movement.

• Intermediate filaments: Rope-like fibers providing mechanical strength and stability.

• Microtubules: Hollow tubes that serve as tracks for organelle movement and are involved in cell division.

Comparison of Cytoskeletal Components

Component	Diameter	Structure	Protein Subunits	Key Functions
Microfilaments (Actin)	7 nm	Thin, entwined threads, often bundled	Actin	Maintain cell shape Cell motility (e.g., muscle contraction, cytoplasmic streaming) Form cleavage furrow during cell division
Intermediate Filaments	8–12 nm	Stretchy, rope-like proteins	Various (e.g., keratin, vimentin, lamin)	Maintain cell shape Anchor nucleus and organelles Form nuclear lamina
Microtubules	25 nm	Hollow tubes	α- and β-tubulin	Maintain cell shape Cell motility (cilia, flagella) Intracellular transport Chromosome movement during cell division

Motor Proteins and Cytoskeletal Interactions

Motor proteins are specialized proteins that use ATP to move along cytoskeletal filaments, transporting cellular cargo.

• Myosin: Moves along actin filaments (microfilaments), important in muscle contraction and vesicle transport.

• Kinesin and Dynein: Move along microtubules. Kinesin generally moves toward the plus end (cell periphery), while dynein moves toward the minus end (cell center).

Example: Dynein motor proteins are essential for the movement of cilia and flagella. Mutations that result in missing dynein proteins can impair ciliary movement, leading to respiratory problems due to the inability to clear airways of particles.

Clinical Connections

• Ciliary dysfunction: Caused by missing dynein motor proteins, leading to impaired movement of cilia and associated health issues.

• Alzheimer's disease: Abnormal accumulation of Tau protein disrupts microtubule stability in neurons, impairing transport of neurotransmitters.

• Cardiomyopathy: Mutations in cytoskeletal proteins (e.g., actin, myosin) can affect heart muscle contraction and structure.

• Progeria: Mutations in the LMNA gene (coding for nuclear lamins, a type of intermediate filament) cause nuclear instability and premature aging.

The Endomembrane System

Overview of the Endomembrane System

The endomembrane system is a group of interconnected organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.

• Functions: Synthesis of proteins and lipids, detoxification, storage, and transport of cellular materials.

The Nucleus

The nucleus is the control center of the cell, containing most of the cell's genetic material in the form of DNA. It is surrounded by a double-membrane structure called the nuclear envelope, which contains nuclear pores for molecular exchange.

• Chromatin: DNA-protein complex that organizes genetic material.

• Nuclear lamina: Network of intermediate filaments (lamins) providing structural support.

• Nuclear pores: Protein complexes that regulate the passage of molecules between the nucleus and cytoplasm.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of membranes involved in protein and lipid synthesis. It is divided into two types:

• Rough ER (RER): Studded with ribosomes; site of protein synthesis and modification.

• Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Golgi Apparatus

The Golgi apparatus is a series of flattened membrane sacs that modify, sort, and package proteins and lipids for secretion or delivery to other organelles.

• Cis face: Receives vesicles from the ER.

• Trans face: Ships vesicles to their final destinations.

• Functions: Protein processing, sorting, and secretion.

Lysosomes, Peroxisomes, and Vacuoles

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules and cellular debris; function best at acidic pH.

• Peroxisomes: Break down fatty acids and detoxify harmful substances; contain enzymes that convert hydrogen peroxide ($H_2O_2$) to water and oxygen.

• Vacuoles: Storage organelles; in plants, the central vacuole maintains turgor pressure and stores nutrients and waste products.

Secretory Pathway

Proteins synthesized in the RER are transported to the Golgi apparatus for modification and sorting, then packaged into vesicles for delivery to their final destinations, including secretion outside the cell.

Genetic Mutations and Organelle Function

• Golgi apparatus mutations: Deletion of genes responsible for vesicle formation at the trans-Golgi can prevent lysosomes from receiving necessary hydrolytic enzymes, impairing their digestive function.

Semiautonomous Organelles

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are organelles with their own DNA and machinery for protein synthesis. They are considered semiautonomous because they depend on the cell for some proteins but can grow and divide independently.

• Mitochondria: Sites of cellular respiration; found in both plant and animal cells.

• Chloroplasts: Sites of photosynthesis; found only in photosynthetic organisms (e.g., plants, algae).

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence includes:

• Both organelles have double membranes.

• They contain their own circular DNA, similar to bacterial genomes.

• They have ribosomes more similar to those of bacteria than to eukaryotic cytoplasmic ribosomes.

• They can grow and divide independently of the host cell.

Table: Comparison of Mitochondria and Chloroplasts

Feature	Mitochondria	Chloroplasts
Function	Cellular respiration (ATP production)	Photosynthesis (glucose production)
DNA	Circular, prokaryote-like	Circular, prokaryote-like
Membranes	Double	Double (plus internal thylakoid membranes)
Found in	All eukaryotes	Plants and algae

Example: The presence of circular DNA and prokaryote-like ribosomes in mitochondria and chloroplasts supports their endosymbiotic origin.

Additional info: Some content was inferred and expanded for clarity and completeness, including clinical examples and the summary tables.