Cellular Organelles: Structure, Function, and Disease

Introduction to Cellular Organelles

Cellular organelles are specialized subunits within eukaryotic cells, each performing distinct functions essential for cell survival and specialization. Understanding their structure, function, and relevance to human disease is fundamental in cell biology.

Eukaryotic vs. Prokaryotic Cells

Major Distinction

• Eukaryotic cells contain membrane-bound organelles, including a nucleus.

• Prokaryotic cells (Bacteria and Archaea) lack membrane-bound organelles.

The presence of organelles allows eukaryotic cells to compartmentalize functions, increasing efficiency and complexity.

Why Do Organelles Exist?

• Cells are large relative to molecules, making diffusion inefficient over long distances.

• Compartmentalization via organelles allows for specialized environments and processes within the cell, overcoming diffusion limitations and enabling complex metabolic pathways.

Structure of a Typical Cell

• Prokaryotic cell: Lacks nucleus and membrane-bound organelles.

• Animal cell: Contains nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and cytoskeleton.

• Plant cell: Contains all animal cell organelles plus chloroplasts, central vacuole, and cell wall.

The First Divide: Nucleus vs. Cytoplasm

• Cytoplasm: Region between the plasma membrane and nuclear envelope.

• Cytosol: Semifluid substance in which organelles are suspended.

• Cytoskeleton: Network of tubules and filaments providing structural support.

• Nucleus: Subdivided into nucleolus (site of ribosome synthesis) and nucleoplasm.

Packaging of DNA: Chromatin and Chromosomes

• Nucleus: The information center, storing genetic material (DNA).

• DNA associates with histone proteins to form chromatin.

• During mitosis, chromatin condenses into chromosomes.

• Chromatin is less condensed during interphase, resembling a spaghetti-like structure.

The Cytoplasmic Subdivide

• Endoplasmic Membrane System: Includes nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, endosomes, and lysosomes.

• Cytoskeleton: Provides structure and facilitates movement.

• Other Organelles: Mitochondria, peroxisomes, chloroplasts (in plants), and vacuoles.

• Organelles can be permanent or dynamic, universal or cell-specific, and have 2, 1, or 0 lipid bilayer membranes.

Endoplasmic Reticulum (ER): The Factory

• Rough ER: Studded with ribosomes; synthesizes membrane and secretory proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and steroids.

Golgi Complex: Processing Station

• Processes proteins from the ER via glycosylation.

• Synthesizes complex carbohydrates.

Process of Secretion in Eukaryotic Cells

1. Blueprint: Nucleus

2. Manufacture: Rough ER

3. Maturation: Golgi apparatus

4. Transport: Vesicles

5. Release: Plasma membrane

Lysosomes: Recycling Machinery

• Single-membrane organelles containing hydrolases (digestive enzymes).

• Digest macromolecules and recycle cellular components.

Lysosomal Disorders

• Heritable lysosomal storage diseases caused by defective lysosomal enzymes or their delivery.

• Leads to accumulation of polysaccharides or lipids, causing cellular dysfunction.

The Cytoskeleton

• Composed of microtubules (MT), microfilaments (MF), and intermediate filaments (IF).

• Functions: Cell motility, cell division, and intracellular transport.

Mitochondrion: The Energy Center

• Double-membrane organelle responsible for extracting energy from food and storing it as ATP.

• Contains its own DNA; inherited maternally.

Mitochondrial Disorders

• Primarily affect muscle and nerve cells.

• Result from mutations in mitochondrial genes.

Peroxisomes

• Single-membrane organelles involved in metabolism of hydrogen peroxide.

• Contain oxidases and catalase enzymes.

Key reactions:

• $\text{RH}_2 + \text{O}_2 \xrightarrow{\text{oxidases}} \text{R} + \text{H}_2\text{O}_2$

• $2\text{H}_2\text{O}_2 \xrightarrow{\text{catalase}} 2\text{H}_2\text{O} + \text{O}_2$

Chloroplasts: Photosynthesis (Plant Cells)

• Double-membrane organelles where photosynthesis occurs on thylakoid membranes.

• Involved in cytoplasmic streaming (cyclosis).

Vacuole in Plant Cells

• Large central vacuole provides pressure to keep cells swollen and plant tissue turgid.

Organelles Without a Membrane

• Formed via liquid-liquid phase separation (LLPS).

• Also called liquid droplets or biomolecular condensates.

• Examples: Nucleolus, stress granules.

Cell Type and Organelle Function Matching

Cell Type	Main Function	Relevant Organelle(s)
Muscle cells	Movement, work	Mitochondria (energy), cytoskeleton (contraction)
Pancreas cells	Secretion of digestive enzymes	Rough ER, Golgi apparatus, secretory vesicles
B lymphocytes	Antibody production	Rough ER, Golgi apparatus
Red blood cells	Oxygen transport	Hemoglobin (no nucleus in mature cells)
Neurons	Signal transduction	Plasma membrane, synaptic vesicles, cytoskeleton
Macrophages	Cell defense	Lysosomes, phagosomes
Sperm cells	Motility (win a swimming contest)	Mitochondria (energy for movement), flagellum (cytoskeleton)