Cytoskeleton and Organelles: Structure and Function (Chapter 3: 3.4–3.5)

Cytoskeleton and Organelles

Overview

This section covers the structure and function of major eukaryotic cell organelles, the endomembrane system, and the cytoskeleton, including specialized cellular extensions. Understanding these components is essential for grasping how cells maintain organization, process materials, and move.

Cell Organelles

Nucleus

• Structure: Centrally located, surrounded by a double membrane called the nuclear envelope with nuclear pores. Contains chromatin (unspooled DNA) and a nucleolus.

• Function: Stores genetic information (DNA), controls cellular activities, and produces ribosomal RNA in the nucleolus.

• Key Terms: Chromatin (DNA + proteins), nucleolus (site of rRNA synthesis).

Mitochondria

• Structure: Double-membraned organelle with inner membrane forming cristae and an internal matrix. Contains its own circular DNA.

• Function: Site of ATP synthesis via aerobic respiration; considered the "powerhouse" of the cell.

• Endosymbiotic Theory: Suggests mitochondria originated from engulfed prokaryotic cells.

• Inheritance: Mitochondrial DNA is maternally inherited.

Ribosomes

• Structure: Composed of large and small protein subunits; can be free in cytosol or bound to rough ER/nuclear envelope.

• Function: Specialized for protein synthesis (translation of mRNA).

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; synthesizes and modifies proteins for secretion or membrane insertion.

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs, and stores calcium ions.

Golgi Apparatus

• Structure: Stack of flattened membranous sacs (cisternae).

• Function: Modifies, sorts, and packages proteins and lipids from the ER; synthesizes carbohydrates; forms glycoproteins.

Lysosomes

• Structure: Membrane-bound vesicles containing digestive enzymes (acid hydrolases).

• Function: Digests macromolecules, old organelles, and cellular debris (autophagy).

Peroxisomes

• Structure: Small, membrane-bound organelles formed by the rough ER.

• Function: Break down fatty acids, detoxify harmful substances (e.g., ethanol), and neutralize free radicals by converting hydrogen peroxide ($H_2O_2$) to water.

Endomembrane System

Definition and Function

• Definition: A group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Main Components: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Function: Coordinates the synthesis, modification, and transport of proteins and lipids within the cell.

Process Overview:

1. Proteins are synthesized and modified in the RER.

2. Lipids are synthesized in the SER.

3. Both are packaged into transport vesicles.

4. Vesicles merge with the Golgi apparatus for further modification and sorting.

5. Final products are packaged into vesicles for delivery to lysosomes, the plasma membrane, or secretion (exocytosis).

Cytoskeleton

Structure and Function

• Definition: A network of protein filaments providing structural support, shape, and facilitating movement within the cell.

• Functions:

  • Maintains cell shape and size

  • Anchors organelles

  • Enables cellular motion (e.g., mitosis, phagocytosis, cilia/flagella movement)

  • Facilitates intracellular transport

Types of Cytoskeletal Filaments

• Microfilaments (Actin Filaments):

  • Structure: Thin filaments composed of actin protein.

  • Function: Support cell shape, enable cell movement, and are involved in muscle contraction and cytokinesis.

• Intermediate Filaments:

  • Structure: Rope-like fibers; protein type varies (e.g., keratin in hair/nails).

  • Function: Provide mechanical strength and stabilize organelle position.

• Microtubules:

  • Structure: Hollow tubes made of tubulin protein.

  • Function: Form tracks for organelle movement, compose mitotic spindle, and are structural components of cilia and flagella.

Centrosome and Centrioles

• Centrosome: Microtubule-organizing center located near the nucleus; contains a pair of centrioles.

• Centrioles: Cylindrical structures composed of nine triplets of microtubules; critical for cell division (mitotic spindle formation).

Cellular Extensions

Microvilli

• Structure: Finger-like projections of the plasma membrane supported by actin filaments.

• Function: Increase surface area for absorption; found in intestinal and kidney tubule epithelial cells.

Cilia

• Primary (Non-motile) Cilia: Single, non-motile; sensory function (e.g., monitoring fluid flow in kidney tubules).

• Motile Cilia: Multiple, beat in coordinated waves to move substances (e.g., mucus in respiratory tract).

• Structure: "9+2" arrangement of microtubules (axoneme) anchored by a basal body.

• Clinical Note: Defects in cilia (e.g., cystic fibrosis, primary ciliary dyskinesia) can impair mucus clearance and cause disease.

Flagella

• Structure: Similar "9+2" microtubule arrangement as cilia, but much longer and usually singular (e.g., sperm cell tail).

• Function: Provides whip-like movement for cell motility.

Pseudopodia

• Structure: Temporary, actin-driven extensions of the cytoplasm.

• Function: Enable cell movement and phagocytosis (e.g., in macrophages).

Summary Table: Major Organelles and Their Functions

Organelle	Structure	Function
Nucleus	Double membrane, nuclear pores, nucleolus	Stores DNA, controls cell, rRNA synthesis
Mitochondria	Double membrane, cristae, matrix, mtDNA	ATP synthesis, metabolism
Ribosomes	Large/small subunits, free or bound	Protein synthesis
Rough ER	Membranous sacs with ribosomes	Protein synthesis/modification
Smooth ER	Membranous sacs, no ribosomes
Golgi Apparatus	Stacked cisternae	Protein/lipid modification, packaging
Lysosomes	Membrane vesicles, enzymes	Digestion, autophagy
Peroxisomes	Membrane vesicles, oxidative enzymes	Detoxification, fatty acid breakdown

Key Equations

• ATP Synthesis (Cellular Respiration):

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

• Hydrogen Peroxide Detoxification (Peroxisomes):

$2H_2O_2 \rightarrow 2H_2O + O_2$

Additional info:

• Some details, such as the clinical relevance of cilia defects and the endosymbiotic theory, were expanded for academic completeness.

• Table entries and some explanations were inferred and expanded for clarity and exam preparation.