Fiche de révision : Cellular Sorting and Digestion Organelles

📋 Course Outline

1. Golgi apparatus structure and regions
2. Golgi vesicles and cis trans faces
3. Golgi membrane composition and asymmetry
4. Golgi functions: glycosylation and processing
5. Golgi sorting, SNAREs and vesicle fusion
6. Lysosomes: structure, enzymes and pH
7. Lysosomal membrane proteins and transporters
8. Lysosomal matrix and enzyme targeting
9. Lysosome classification: autophagy and heterophagy

📖 1. Golgi apparatus structure and regions

🔑 Key Concepts & Definitions

• Golgi apparatus : A eukaryotic organelle near the nucleus that processes and sorts proteins and lipids through stacked membrane cisternae.
• Dictyosome : A Golgi unit made of a stack of curved, membrane-bound sacs that are separated by hyaloplasmic bands and stabilized by cytoskeletal elements.
• Hyaloplasmic band : A cytosolic gap between adjacent Golgi sacs that separates the cisternae within a dictyosome.
• Microtubule stabilization : A structural support role where microtubules help maintain the organization of Golgi cisternae.
• Actin microfilament stabilization : A structural support role where actin filaments help stabilize the Golgi cisternae arrangement.

📝 Essential Points

• The Golgi apparatus is located near the nucleus and is specific to eukaryotes.
• A cell contains one or more dictyosomes, with an average of about 20, varying by cell type and physiological state.
• Each dictyosome has 4 to 8 curved membrane-bound sacs.
• The sacs are surrounded by vesicles and separated from each other by hyaloplasmic bands.
• Golgi sacs are stabilized by both microtubules and actin microfilaments.
• Under a light microscope the Golgi appears as small scales near the nucleus.

💡 Memory Hook

Dictyosomes are “stacked pancakes” (4–8 sacs) held by microtubules and actin, with gaps (hyaloplasmic bands) between layers.

📖 2. Golgi vesicles and cis trans faces

🔑 Key Concepts & Definitions

• cis-face saccules : The entry face of the Golgi that receives material from the ER via the ERGIC and is also called the CGN face.
• medial region saccules : The middle Golgi region where processing steps occur as cargo moves from the cis to the trans side.
• trans-face saccules : The exit face of the Golgi that connects to the TGN network of canaliculi for onward sorting.
• TGN : The trans Golgi network that forms from canaliculi continuous with the trans-face sacs and serves as a sorting hub.
• ERGIC : The ER–Golgi intermediate compartment that supplies cargo to the cis-Golgi network.

📝 Essential Points

• Transition vesicles lie between the ERGIC and the cis-Golgi network and are coated with coatomere.
• Transport vesicles lie between Golgi cisternae and are also coated with coatomere.
• Secretory vesicles bud from the TGN and contain the final product.
• Coatomere-coated secretory vesicles are linked to constitutive exocytosis and can include extracellular matrix components and peripheral plasma membrane proteins.
• Clathrin-coated secretory vesicles are linked to regulated exocytosis and form secretory granules with dense contents such as insulin and acid hydrolase vesicles.
• Caveolin-coated secretory vesicles are linked to membrane microdomain renewal pathways.

💡 Memory Hook

Cis = “come in” from ERGIC; Trans = “send out” into TGN; vesicle coats tell the route (coatomere, clathrin, caveolin).

📖 3. Golgi membrane composition and asymmetry

🔑 Key Concepts & Definitions

• Tristratified asymmetric membranes : A TEM appearance where Golgi saccular membranes show three-layered structure and asymmetry between cis and trans sides.
• cis saccular membrane thickness : The cis-side Golgi sac membrane thickness measured at about 60 Å under TEM.
• trans saccular membrane thickness : The trans-side Golgi sac membrane thickness measured at about 75 Å under TEM.
• Membrane lipid-protein intermediate composition : A Golgi membrane composition that lies between that of the endoplasmic reticulum and the plasma membrane.
• Membrane asymmetry by lumen-oriented carbohydrates : A Golgi membrane asymmetry created because carbohydrate groups on proteins and phospholipids face the lumen of the saccules.

📝 Essential Points

• Under TEM, Golgi saccular membranes appear tristratified and asymmetric.
• The cis saccular membranes have a thickness of 60 Å.
• The trans saccular membranes have a thickness of 75 Å.
• Golgi sac membranes contain about 30% to 40% lipids and 60% to 70% proteins.
• Carbohydrate content is negligible in the membrane composition overall.
• Unsaturated fatty acids and cholesterol levels are intermediate between ER and plasma membranes, and fluidity is also intermediate.

💡 Memory Hook

Cis is thinner (60 Å) and trans is thicker (75 Å); carbohydrates face the lumen to create asymmetry.

📖 4. Golgi functions: glycosylation and processing

🔑 Key Concepts & Definitions

• O-glycosylation : A co-transductional modification in the Golgi lumen that adds carbohydrates to serine or threonine -OH groups in peptide chains.
• O-glycosyltransferases : Golgi enzymes that catalyze O-glycosylation by transferring carbohydrates onto serine or threonine residues.
• Sulfation : A Golgi reaction that transfers sulfate groups (SO42-) to an acceptor using sulfotransferases.
• PAPS : Phosphoadenosine phosphosulfate, the sulfate donor synthesized in the hyaloplasm and used for sulfation in trans cisternae.
• Mannose-6-phosphate : A phosphorylation tag on lysosomal enzymes that enables recognition by mannose-6-phosphate receptors.

📝 Essential Points

• O-glycosylation occurs in the lumen of the medial and trans saccules.
• O-glycosylation attaches carbohydrates to the -OH residues of serine and threonine in peptide chains.
• O-glycosylation is sequential, building oligosaccharides sugar by sugar.
• In most cases, O-glycosylation occurs on proteoglycans.
• Sulfation occurs in trans saccules and targets extracellular-matrix-destined components such as glycoproteins, proteoglycans, and glycosaminoglycans.
• PAPS is synthesized in the hyaloplasm and enters the lumen of trans cisternae via a permease.

💡 Memory Hook

O-glyco = “Ser/Thr get sugars”; Sulfation = “Trans cisternae add SO4”; Lysosomal enzymes get “M6P” for delivery.

📖 5. Golgi sorting, SNAREs and vesicle fusion

🔑 Key Concepts & Definitions

• Centrifugal flow : A Golgi-to-target direction where vesicles bud from trans-sacculars and deliver cargo to endosomes, lysosomes, phagosomes, or the plasma membrane.
• Centripetal flow : A reverse direction where vesicles move from the plasma membrane to endosomes/caveosomes, from endosomes to the TGN, or from caveosomes to the RER.
• v-SNARE : A vesicle membrane SNARE protein that participates in fusion by pairing with a target SNARE.
• t-SNARE : A target membrane SNARE protein that recognizes the vesicle SNARE to enable membrane fusion.
• Coated vesicle budding from TGN : The step where vesicles bud from the TGN with a coat before they become fusion-competent.

📝 Essential Points

• Centrifugal flow transfers vesicles that bud from trans-sacculars carrying secretory products to endosomes, lysosomes, phagosomes, or the plasma membrane.
• Secretory stages include coated vesicle budding from the TGN.
• After budding, the coating is removed and naked vesicles form labeled with v-SNAREs.
• Vesicles are transported to receiving compartments equipped with t-SNARE receptor membrane proteins.
• Fusion occurs after recognition of v-SNAREs by t-SNAREs.
• Centripetal flow can move vesicles from the plasma membrane to endosomes or caveosomes, from endosomes to the TGN, or from caveosomes to the RER.

💡 Memory Hook

Fusion is a lock-and-key: v-SNARE on the vesicle meets t-SNARE on the target, then membranes fuse.

📖 6. Lysosomes: structure, enzymes and pH

🔑 Key Concepts & Definitions

• Lysosome : A membrane-bounded organelle in eukaryotic animal cells that performs intracellular digestion using acid hydrolases.
• Acid hydrolases : Lysosomal enzymes that digest cellular material and require an acidic environment to function.
• Lysosomal pH : The internal acidity level of lysosomes that supports acid hydrolase activity, ranging from 3.5 to 5.
• Lysosomal membrane thickness : The lysosome boundary membrane thickness measured at about 60 to 100 Å.
• Lysosome size : The typical lysosome diameter range of about 0.1 to 0.2 μm.

📝 Essential Points

• Lysosomes are 0.1 to 0.2 μm in diameter and bounded by a membrane 60 to 100 Å thick.
• They are present in the cytosol of all eukaryotic animal cells except red blood cells.
• Their main function is intracellular digestion, and extracellular digestion can occur via exocytosis in chondroblasts, osteoclasts, and macrophages.
• Lysosomes use various acid hydrolases to carry out digestion.
• The lysosomal lumen pH is between 3.5 and 5 for proper enzyme function.
• Lysosomes move within the cell by attachment to the cytoskeleton.

💡 Memory Hook

Lysosomes are “acid boxes”: 0.1–0.2 μm, 60–100 Å membrane, and pH 3.5–5 keeps hydrolases working.

📖 7. Lysosomal membrane proteins and transporters

🔑 Key Concepts & Definitions

• LAMP-1 : A lysosomal-associated membrane protein used as a marker of the lysosomal compartment.
• LAMP-2 : A lysosomal-associated membrane protein used as a marker of the lysosomal compartment.
• LAMP-3 : A lysosomal-associated membrane protein used as a marker of the lysosomal compartment.
• Proton pumps : Lysosomal membrane transport proteins that pump protons (H+) to maintain an acidic lumen.
• Chloride ion channels : Lysosomal membrane channels specific for Cl- that help maintain the acidic pH environment.

📝 Essential Points

• The lysosomal membrane is composed of lipids and proteins, primarily glycoproteins.
• Structural glycoproteins include LAMP-1, LAMP-2, and LAMP-3 as compartment markers.
• Enzymatic glycoproteins include acid phosphatase.
• Proton pumps for hydronium (protons H+) and chloride-specific ion channels (Cl-) maintain an acidic pH inside lysosomes.
• Import permeases bring hyaloplasmic molecules into the lysosomal lumen for degradation.
• Export permeases release catabolic end products from the lysosomal lumen into the cytoplasm, including permeases for amino acids, fatty acids, and sugars.

💡 Memory Hook

Lysosomal membrane has “keep-acid” tools: H+ pumps + Cl- channels, plus import/export permeases for cargo and waste.

📖 8. Lysosomal matrix and enzyme targeting

🔑 Key Concepts & Definitions

• Lysosomal matrix : The lysosome interior containing acid hydrolases and macromolecules that will be digested.
• N-acetylglucosamine phosphotransferase : A lysosomal targeting enzyme that attaches a GlcNAc-phosphate residue to mannose residues as a phosphorylation signal.
• Mannose-6-phosphate receptor : A receptor in the trans-Golgi network that recognizes mannose-6-phosphate on lysosomal enzymes.
• GlcNAc phosphoglucosidase : An enzyme that removes GlcNAc after the phosphorylation step, leaving mannose-6-phosphate on the enzyme.
• Enzyme targeting to specific lysosomes : The process by which each acid hydrolase is delivered to a particular lysosome after Golgi processing and receptor recognition.

📝 Essential Points

• Lysosomes contain acid hydrolases and macromolecules to be digested.
• Digestive enzymes require the acidic lysosomal environment to function.
• Acid hydrolases are produced by the endoplasmic reticulum, transported, and processed by the Golgi apparatus.
• Phosphorylation in cis cisternae is essential for maturation of soluble N-glycosylated proteins destined for lysosomes.
• The phosphorylation signal is created when GlcNAc-P-transferase attaches GlcNAc phosphate to carbon 6 of mannose residues.
• After GlcNAc is released by N-acetylglucosamine phosphoglucosidase, enzymes with mannose-6-phosphate are transported to the trans-Golgi network for receptor recognition and then targeted to the endosomal compartment.

💡 Memory Hook

M6P is the “address label”: ER makes enzymes → Golgi adds M6P → trans-Golgi receptor recognizes → vesicles target the endosomal route.

📖 9. Lysosome classification: autophagy and heterophagy

🔑 Key Concepts & Definitions

• Autophagic vacuole : A double-membrane structure formed around senescent organelles or partially degraded cytoplasmic molecules during autophagy.
• Autolysosome : A lysosome-containing compartment formed after fusion of an autophagic vacuole with a lysosome.
• Heterolysosome : A compartment formed when endocytic or pinocytic vesicles fuse with lysosomes loaded with hydrolases.
• Autophagy : A lysosomal pathway that degrades senescent cellular components by forming autophagic vacuoles that fuse with lysosomes.
• Heterophagy : A lysosomal pathway that degrades material taken up from outside the cell via endosomes that fuse with lysosomes.

📝 Essential Points

• In autophagy, senescent organelles or partially degraded cytoplasmic molecules become surrounded by a membrane originating from the endoplasmic reticulum.
• Autophagic vacuoles fuse with lysosomes to form autolysosomes.
• Autophagolysosomes are responsible for the mechanism of cellular autophagy.
• In heterophagy, endocytic or pinocytic products in endosomes fuse with lysosomal vesicles loaded with hydrolases to form heterolysosomes.
• In macrophages and neutrophils, endocytic vesicles can contain bacteria or viruses and are then called phagocytic vacuoles or phagosomes.
• Autophagy and heterophagy differ by whether the cargo originates from cytoplasm or from endocytosed material.

💡 Memory Hook

Auto = self (cytoplasm → autophagic vacuole → autolysosome); Hetero = outside (endosome → heterolysosome).

📊 Synthesis Tables

Golgi vesicle coats and exocytosis type

⚠️ Common Pitfalls & Confusions

1. Confusing cis and trans faces: cis is the entry face (CGN/ERGIC supply) while trans is the exit face connected to the TGN.
2. Mixing up vesicle coats: coatomere, clathrin, and caveolin correspond to different exocytosis/renewal contexts.
3. Assuming lysosomes are neutral: their pH must be acidic (3.5–5) for acid hydrolases to work.
4. Thinking M6P is added in the lysosome: phosphorylation and receptor recognition occur via Golgi processing and trans-Golgi network steps.
5. Swapping autophagy and heterophagy: autophagy targets cytoplasmic material, heterophagy targets endocytosed material.

✅ Exam Checklist

1. Describe the Golgi apparatus location, dictyosome organization, and how sacs are stabilized.
2. Identify the cis-face, medial region, trans-face, and link trans-face to the TGN canaliculi network.
3. Classify Golgi-associated vesicles (transition, transport, secretory) and state their coat and typical location.
4. State cis vs trans membrane thickness values and the intermediate lipid/protein composition ranges.
5. Explain how O-glycosylation works (serine/threonine, medial/trans lumen, sequential build) and name the key enzyme class.
6. Explain sulfation (trans cisternae, sulfate donor PAPS, sulfotransferase transfer, extracellular-matrix destinations).
7. Explain lysosomal enzyme phosphorylation for targeting (GlcNAc-P-transferase, GlcNAc release, M6P, mannose-6-phosphate receptor).
8. Outline centrifugal vs centripetal flow and list the main secretion stages using v-SNARE and t-SNARE.
9. Define v-SNARE and t-SNARE and state the fusion logic (recognition then membrane fusion).
10. Give lysosome size, membrane thickness, presence/absence in red blood cells, and the main digestion role.
11. State lysosomal pH range and connect it to acid hydrolase function.
12. List lysosomal membrane protein categories (LAMP markers, enzymatic glycoproteins) and the roles of H+ pumps and Cl- channels.
13. Differentiate import vs export permeases and give the types of molecules mentioned.
14. Describe lysosomal matrix contents and the ER→Golgi→targeting pathway for acid hydrolases at a high level without adding new steps not in the source.