Golgi Apparatus and lysosomes

What is the Golgi complex and who discovered it?

The Golgi complex (or Golgi apparatus) is a structure discovered by Camilo Golgi in 1898; it consists of flattened membrane-bound sacs (cisternae) located near the nucleus and centrosome.

How are Golgi cisternae arranged and what is their approximate size?

The cisternae are stacked like plates, enclosing a lumen of about 1 µm in diameter, with dilated edges.

How many cisternae can each Golgi stack contain?

Each stack contains 3 to 20 cisternae depending on the cell type.

What are the main functions of the Golgi complex?

It receives proteins and lipids from the ER, modifies them, and dispatches them to destinations like lysosomes, plasma membrane, or for secretion. It is also a major site of carbohydrate synthesis.

What are the two faces of the Golgi stack?

Cis (entry) face: receives vesicles from the ER. Trans (exit) face: releases processed materials in vesicles.

List the five functional compartments of the Golgi apparatus.

1. Cis-Golgi network 2. Cis-cisternae 3. Medial cisternae 4. Trans-cisternae 5. Trans-Golgi network

How do proteins enter the Golgi complex?

Via COPII-coated transport vesicles from the ER. Only properly folded proteins are transported.

What happens to proteins with ER retention signals that enter the Golgi?

They are packed in COPI-coated vesicles that bud from the Golgi and return to the rough ER.

What are COPI and COPII vesicles?

COPI: retrograde (Golgi → ER) transport. COPII: anterograde (ER → Golgi) transport.

What happens to proteins and lipids as they move through the Golgi cisternae?

They travel sequentially via COPI/II vesicles and undergo further chemical modifications like glycosylation, phosphorylation, and sulfation.

What is N-linked glycosylation?

Addition of sugar residues to the NH₂ group of asparagine residues.

What is O-linked glycosylation?

Addition of sugar residues to the OH group of serine or threonine residues.

Where does phosphorylation of mannose residues occur and what is its importance?

In the cis-Golgi, it tags proteins destined for lysosomes with mannose-6-phosphate.

What modifications occur in the trans-Golgi network?

Sulfation of tyrosine residues and proteolytic cleavage of proenzymes.

Where do proteins and lipids exit the Golgi complex?

From the trans-Golgi network (TGN), where they are sorted into vesicles bound for lysosomes, plasma membrane, or secretion.

What happens to proteins with mannose-6-phosphate?

They are packed in clathrin-coated vesicles, which lose their coat and fuse with lysosomes.

What is the function of the 25kD receptor protein in the Golgi?

It binds hormones or neurotransmitters in the trans-Golgi for packaging into clathrin-coated vesicles (regulated exocytosis).

What is constitutive exocytosis?

Continuous vesicle fusion from the trans-Golgi to the plasma membrane, delivering new lipids/proteins or secreted materials.

What is regulated exocytosis?

Vesicles fuse with the membrane only upon specific extracellular signals (e.g., hormones, neurotransmitters); used by secretory cells.

What are clathrin-coated vesicles and their role?

Vesicles with a protein coat (clathrin) used for transport; they form by adaptins linking cargo receptors to clathrin and are pinched off by dynamin.

What is the role of adaptins?

They bind clathrin to the vesicle membrane and help select cargo molecules for transport.

What is the role of dynamin?

A GTP-binding protein that forms a ring around vesicle necks and uses GTP hydrolysis to pinch them off.

What is endocytosis?

Process by which cells take up fluids and molecules by forming inward-budding vesicles from the plasma membrane.

What are the two main types of endocytosis?

1. Phagocytosis – ingestion of large particles (>250 nm) like microorganisms. 2. Pinocytosis – ingestion of fluids and small molecules (<250 nm).

What is the function of phagocytosis in protozoa and animals?

In protozoa, it’s a feeding mechanism; in animals, it defends against infection and removes dead cells.

Which cells perform phagocytosis?

Macrophages and other white blood cells.

How do some bacteria avoid destruction after phagocytosis?

Salmonella enterica: prevents fusion with lysosomes. Mycobacteria: retain the phagosome’s protein coat to avoid degradation.

What is pinocytosis and what coats its vesicles?

Continuous ingestion of membrane and extracellular fluid by small clathrin-coated vesicles.

What are the two types of pinocytosis?

1. Non-specific – traps any molecule in extracellular fluid. 2. Specific (receptor-mediated) – selective uptake using specific receptors.

What substances enter via receptor-mediated endocytosis?

Cholesterol, Vitamin B12, and iron.

Name pathogens that exploit receptor-mediated endocytosis.

Epstein-Barr virus, Influenza virus, Listeria monocytogenes, Streptococcus pneumoniae.

What happens to LDL cholesterol during receptor-mediated endocytosis?

LDL binds to receptors, is internalized via clathrin-coated vesicles, uncoated, fuses with endosomes, and delivers cholesterol to lysosomes.

What are the two types of endosomes?

1. Early endosomes – beneath plasma membrane. 2. Late endosomes – near the nucleus.

What maintains the acidity of endosomes?

ATP-driven H⁺ pumps that move protons into the lumen.

What happens to receptors in endosomes?

They can be recycled, degraded, or sent across the cell by transcytosis.

What is the function of lysosomes?

They digest extracellular materials and worn-out organelles using hydrolytic enzymes.

What are the dimensions and pH of lysosomes?

0.2–0.5 µm in diameter; acidic pH ~5 maintained by H⁺ pumps.

Why are lysosomal membrane proteins highly glycosylated?

To protect them from degradation by lysosomal enzymes.

List four main types of hydrolytic enzymes in lysosomes.

Proteases, nucleases, glycosidases, phospholipases.

Where are lysosomal enzymes synthesized and how are they targeted?

Synthesized in the ER, modified in the Golgi with mannose-6-phosphate, sorted into vesicles, and delivered to lysosomes via late endosomes.

List the four routes by which materials reach lysosomes.

1. Phagocytosis 2. Endocytosis 3. Autophagy 4. Direct transport from Golgi.

What are lysosomal storage diseases?

Genetic disorders caused by defective lysosomal enzymes or trafficking, leading to accumulation of undigested substrates.

Describe Tay-Sachs disease.

Caused by mutation in β-N-acetylhexosaminidase → buildup of GM2 ganglioside → neurodegeneration, blindness, dementia.

Describe Sly syndrome.

Caused by defective β-glucuronidase → buildup of mucopolysaccharides → skeletal and CNS abnormalities.

Describe Inclusion-cell (I-cell) disease.

Lack of mannose-6-phosphate tagging → lysosomal enzymes secreted instead of stored → accumulation of undigested material in cells, causing severe deformities and psychomotor deficiencies.