Module 3.4: Cytoplasmic Organelles

Overview of Cytoplasmic Organelles

Cytoplasmic organelles are specialized cellular structures that perform distinct functions necessary for cell survival and homeostasis. Some are separated from the cytosol by membranes, allowing compartmentalization of cellular processes.

• Membrane-bound organelles: Mitochondria, peroxisomes, endoplasmic reticulum, Golgi apparatus, lysosomes

• Non-membrane-bound organelles: Ribosomes, centrosomes

• Compartmentalization allows for specialized environments and prevents potentially destructive reactions from harming the rest of the cell.

Mitochondria

The mitochondrion is often called the "power plant" of the cell because it produces the majority of cellular ATP through aerobic respiration.

• Structure: Double phospholipid bilayer with a smooth outer membrane and a highly folded inner membrane (forming cristae).

• Components: Contains its own DNA, enzymes, and ribosomes.

• Function: Site of oxidative catabolism of organic fuels, producing ATP.

• Unique features: The inner membrane is selective, transporting only necessary solutes into the matrix using specific transport proteins.

Example: Muscle cells, which require large amounts of energy, contain many mitochondria.

Peroxisomes

Peroxisomes are small, membrane-bound organelles involved in the breakdown of organic molecules and detoxification.

• Function: Oxidize organic molecules, producing hydrogen peroxide (), which is then broken down to water and oxygen.

• Role in detoxification: Neutralize toxins such as alcohol in the liver and kidney.

• Lipid metabolism: Break down fatty acids for ATP production and synthesize certain phospholipids important for nervous system function.

Ribosomes

Ribosomes are the sites of protein synthesis in the cell and are composed of ribosomal RNA (rRNA) and proteins.

• Structure: Consist of large and small subunits.

• Types: Free ribosomes (in cytosol, synthesize proteins for use within the cell) and bound ribosomes (attached to membranes, synthesize proteins for export or for membranes/lysosomes).

The Endomembrane System

The endomembrane system is a group of interacting organelles and vesicles that synthesize, modify, and transport proteins and lipids.

• Main components: Plasma membrane, nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes.

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; synthesizes proteins for export, membranes, or lysosomes. Misfolded proteins are sent for recycling.

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

Golgi Apparatus

• Structure: Series of flattened membranous sacs.

• Function: Modifies, sorts, and packages proteins and lipids from the ER for secretion, membrane insertion, or delivery to lysosomes.

Lysosomes

• Function: Contain digestive enzymes (acid hydrolases) that break down worn-out organelles, cellular debris, and macromolecules.

• Clinical relevance: Deficiencies in lysosomal enzymes can lead to storage diseases (e.g., Gaucher's, Tay-Sachs, Hurler syndrome, Niemann-Pick disease).

Summary Table: Main Cytoplasmic Organelles

Module 3.5: The Cytoskeleton

Cytoskeleton Overview

The cytoskeleton is a dynamic network of protein filaments that provides structural support, maintains cell shape, and enables movement.

• Anchors organelles and supports plasma/nuclear membranes.

• Facilitates cellular movement and specialized functions (e.g., phagocytosis, muscle contraction).

Types of Cytoskeletal Filaments

• Microfilaments (Actin filaments): Thinnest; composed of actin subunits. Provide structural support, bear tension, and are involved in cell movement (with myosin).

• Intermediate filaments: Rope-like, made of various fibrous proteins (e.g., keratin). Provide mechanical strength and anchor organelles.

• Microtubules: Largest; hollow tubes made of tubulin. Maintain cell shape, serve as tracks for vesicle transport (with dynein and kinesin), and are essential for cell division.

Centrosome and Centrioles

• The centrosome is the microtubule-organizing center, containing a pair of centrioles (important for cell division).

• Centrioles are composed of nine groups of three microtubules.

Cellular Extensions

• Microvilli: Finger-like projections with actin core; increase surface area for absorption (e.g., in intestines).

• Cilia: Hair-like projections with microtubule core; move substances across cell surfaces (e.g., respiratory tract).

• Flagella: Longer, whip-like structure; propels entire cell (e.g., sperm cell).

Clinical relevance: Defects in cilia/flagella can cause primary ciliary dyskinesia, leading to respiratory and reproductive issues.

Module 3.6: The Nucleus

Nucleus Overview

The nucleus is the control center of the cell, housing genetic material and directing cellular activities.

• Contains DNA, which encodes instructions for protein synthesis.

• RNA molecules execute genetic instructions to build proteins.

Structure of the Nucleus

• Nuclear envelope: Double phospholipid bilayer; outer membrane continuous with ER, inner membrane supported by nuclear lamina (intermediate filaments).

• Nuclear pores: Large protein complexes allowing exchange between nucleoplasm and cytoplasm.

• Nucleoplasm: Gel-like substance containing water, nucleotides, enzymes, proteins, DNA, and RNA.

• Nucleolus: Region responsible for ribosomal RNA synthesis and ribosome assembly.

Chromatin and Chromosomes

• Chromatin: DNA-protein complex that organizes and condenses DNA within the nucleus.

• Nucleosome: DNA wrapped around histone proteins, resembling "beads on a string."

• During cell division, chromatin condenses into chromosomes for accurate segregation.

• Human cells have 46 chromosomes (23 pairs), each consisting of two identical chromatids joined at a centromere before division.