5 Golgi Apparatus, Endocytosis, and Exocytosis

What is the main function of the Golgi apparatus?

Processing, modifying, sorting, and packaging proteins and lipids

What is the Golgi apparatus composed of?

A stack of flattened, membrane-bound sacs called cisternae

What are the four main compartments of the Golgi apparatus?

Cis-Golgi Network (CGN), Cis Cisternae, Medial Cisternae, Trans Cisternae, and Trans-Golgi Network (TGN)

What is the function of the Cis-Golgi Network (CGN)?

Entry point for proteins and lipids from the ER via transport vesicles

What occurs in the cis cisternae?

Early stages of protein modification

What occurs in the medial cisternae?

Further modifications like glycosylation

What is the function of the trans cisternae?

Final processing and sorting

What is the Trans-Golgi Network (TGN)?

Exit point; a network of tubules and vesicles that buds from the trans cisternae

What does the cisternal maturation model suggest?

Golgi cisternae are dynamic structures that move from the cis face to the trans face, carrying proteins and lipids

How are new cis cisternae formed?

By the fusion of vesicles from the ER

How do enzymes maintain proper enzymatic composition during cisternal maturation?

Enzymes for specific modifications are carried in retrograde direction via vesicles

What happens to trans cisternae at the TGN?

They break down into vesicles, delivering processed proteins and lipids to their destinations

What is glycosylation?

The addition of carbohydrate chains to proteins and lipids

Where does N-linked glycosylation begin and what is it important for?

Initiated in the ER, further modified in the Golgi; important for protein folding, stability, and function

Where does O-linked glycosylation occur and what is it involved in?

Sugar residues are attached to serine or threonine residues; involved in cell signaling, adhesion, and protein recognition

What role does the Golgi apparatus play in lipid metabolism?

Synthesis and modification of sphingolipids, which are important for membrane structure and function

How are hydrolytic enzymes tagged for lysosome formation?

In the cis Golgi, these enzymes are tagged with mannose-6-phosphate (M6P) residues

Where do M6P receptors bind to M6P-tagged enzymes?

In the TGN

What kind of vesicles are receptor-enzyme complexes packaged into?

Clathrin-coated vesicles

What do these vesicles fuse with?

Late endosomes

What does the late endosome mature into?

A lysosome as the pH becomes more acidic, activating the hydrolytic enzymes

What is the function of COP-II coated vesicles?

Transport cargo from the ER to the Golgi

What is the function of COP-I coated vesicles?

Mediate retrograde transport within the Golgi and from the Golgi back to the ER

What is the function of Clathrin-coated vesicles?

Transport cargo from the TGN to lysosomes, endosomes, and the plasma membrane

What is the role of adaptor proteins in clathrin-dependent vesicle formation?

Bind to clathrin and cargo receptors, selecting cargo for the vesicle

What is the role of dynamin in clathrin-dependent vesicle formation?

Constricts the neck of the budding vesicle and pinches it off

How are COP-I and COP-II coated vesicles formed?

By similar mechanisms to clathrin-coated vesicles, but use different coat and adaptor proteins

What is the role of Rab GTPases in vesicle targeting?

A Rab on the vesicle surface interacts with a specific effector protein on the target membrane, ensuring correct delivery

What is the role of SNARE proteins in vesicle fusion?

Mediate the fusion of vesicles with target membranes

Where are v-SNAREs located?

On the vesicle membrane

Where are t-SNAREs located?

On the target membrane

What is the function of lysosomes?

Degradation of cellular waste products, digestion of extracellular materials, and autophagy

What is autophagy?

Degrading the cell's own components to recycle nutrients and eliminate damaged organelles

What maintains the acidic environment in lysosomes?

Proton pumps

What type of enzymes do lysosomes have?

Hydrolytic enzymes, such as proteases, lipases, nucleases, and glycosidases

How do lysosomes and peroxisomes differ in enzyme content?

Lysosomes have hydrolytic enzymes, while peroxisomes have oxidative enzymes

How do lysosomes and peroxisomes differ in origin?

Lysosomes originate from the Golgi, while peroxisomes replicate by division

How do lysosomes and peroxisomes differ in function?

Lysosomes are for general degradation and recycling, while peroxisomes are for specific metabolic pathways

What are lysosomal storage diseases characterized by?

Accumulation of undegraded materials

What causes Tay-Sachs disease?

Deficiency in hexosaminidase A, leading to ganglioside accumulation in the brain

What causes Gaucher disease?

Deficiency in glucocerebrosidase, leading to glucocerebroside accumulation in tissues

What causes Pompe disease?

Deficiency in acid alpha-glucosidase, leading to glycogen accumulation in lysosomes

What is constitutive exocytosis?

Continuous process in all cells, delivering proteins and lipids to the plasma membrane

What is regulated exocytosis?

Triggered by specific signals, releasing hormones, neurotransmitters, and signaling molecules

What is receptor-mediated endocytosis?

Selectively uptakes molecules from the extracellular environment

What happens to receptor-ligand complexes in receptor-mediated endocytosis?

They cluster in clathrin-coated pits

What happens to vesicles after uncoating in receptor-mediated endocytosis?

They fuse with early endosomes

What happens to the receptor and ligand after dissociation in receptor-mediated endocytosis?

The receptor is recycled back to the plasma membrane, while the ligand is sorted for degradation or transport

What is pinocytosis?

Non-selective uptake of extracellular fluid and small molecules

What is macropinocytosis?

Formation of large vesicles that engulf large volumes of extracellular fluid

What is the function of caveolae in endocytosis?

Involved in a specialized form of endocytosis distinct from clathrin-dependent and clathrin-independent pathways

What are the two pathways for receptor recycling after endocytosis?

Direct recycling and recycling through the TGN

How does endocytosis affect cell signaling pathways?

Endocytosis can downregulate signaling pathways, internalize signaling complexes, and endosomes can act as signaling platforms