Study Notes on Membrane-Bound Organelles

Overview of Membrane-Bound Organelles

Function: Protein processing and transport.
Discovery: Discovered in the 19th century by Camillo Golgi, an Italian scientist.
- Utilized heavy metals (silver, gold) to stain cells, visualizing tubular structures in neurons called the internal reticular apparatus.
- Awarded the Nobel Prize in Physiology or Medicine for this discovery.
- Initially named Golgi apparatus, now more commonly referred to as the Golgi complex.
Structure:
- Consists of flat membrane-bound structures called vesicles or cisternae.
- Types of Cisternae:
  - Cis Golgi Network: Closest to the ER.
  - Medial Cisternae: Intermediate compartment.
  - Trans Golgi Network: Closest to the plasma membrane.
Components:
- Each cistern has a lipid bilayer housing various proteins.
- Functionally active matrix proteins within the lumen.

Proteins synthesized on ribosomes attached to the rough endoplasmic reticulum (RER).
Proteins are transported from the ER to the Golgi.
- This process includes sorting and modification of proteins, including the addition of phosphate and oligosaccharides.
- Glycosylation Process: Addition of sugar molecules (e.g., glucose, mannose, galactose) to proteins, key at Asparagine residues.
- The addition of N-acetylneuraminic acid (NANA) increases negative charge on proteins, aiding in attracting positively charged molecules.
In the Trans Golgi Network, proteins are assigned to different pathways:
- Transport to lysosomes.
- Transport to the plasma membrane for exocytosis or integration into the membrane.
- Storage in secretory vesicles.

Vesicular Transport Hypothesis: Utilizes vesicles for transport.
Cisternae Maturation Hypothesis: Cisternae themselves mature and move forward.
Transport is bidirectional—from ER to Golgi and Golgi to ER, regulated by COPI and COPII proteins.
- COPII: Transports proteins from ER to Golgi (anterograde).
- COPI: Returns proteins to ER (retrograde).
Microtubules in the cytoskeleton facilitate vesicle movement via motor proteins.

Structure: Spherical organelles containing hydrolases (enzymes) that degrade macromolecules.
Functioning requires an acidic environment (pH ~ 5), maintained by proton pumps that actively transport protons into the lysosome.
Types of Enzymes in Lysosomes: Proteases (protein degradation), nucleases (nucleic acid degradation), lipases (lipid degradation), and glucosidases (carbohydrate metabolism).
Accumulate broken down materials for reutilization or exocytosis.
Lysosomal Storage Diseases: Genetic disorders from deficient enzymes leading to accumulation of metabolites, impacting organs like the nervous system, liver, and skeleton.
- Examples: Tay-Sachs and Gaucher's diseases.

Membrane-bound organelles involved in beta-oxidation of fatty acids and in detoxification processes (removal of hydrogen peroxide via catalase).
They do NOT contain their own DNA, replicating by importing proteins from cytosolic ribosomes.
Recent findings indicate roles in regulating cell differentiation and survival.
Diseases: Peroxisome biogenesis disorders due to mutations in PEX genes. Example: Zellweger syndrome, affecting organs like the liver, nervous system, and renal system.

Unique as double-membraned organelles.
Function in ATP production through oxidative phosphorylation, the citric acid cycle, and playing roles in apoptosis (programmed cell death).
Contain their own DNA and ribosomes, but most proteins are synthesized in the cytosol and imported via transporters (TOM and TIM).
ATP Synthesis Mechanism:
- Establishment of a proton gradient across the inner mitochondrial membrane, utilizing electron transport chain.
- Protons flow via ATP synthase, generating ATP from ADP and inorganic phosphate.
Involved in calcium storage and regulation, relevant for functions such as muscle contraction.

Mitochondrial release of cytochrome c activates caspases which cleave cellular components leading to cell death without inflammation.
Essential in development for removing excess cells and regulating organ formation.

Interconnections exist between organelles for normal cellular function, including transport systems, biochemical processes, and disease implications.