Lysosomes Flashcards
Where do ribosomes start translation? [1 mark]
At the inititation sequence (AUG)
What is the signal sequence and what is its purpose? [4 marks]
- 10-30 amino acid long sequence
- Rich in hydrophobic amino acids
- Guides ribosome complex to the ER surface
- Causes ribosome to be docked to the ER membrane
What does the signal recognition particle (SRP) do? [3 marks]
- Binds to the signal sequence and large ribosomal unit
- Stalls translation
- Guides ribesome to translocon
What happens once ribosome reaches the translocon [4 marks]
- Protein is threaded through the translocon into the lumen of ER
- Protein folds inside
- GTP is hydrolysed to provide energy for processes
- Then proteins are transported through vesicle into Golgi apparatus (cis to trans) then to lysosomes
Where are lysosomal enzymes made? [1 mark]
Endoplasmic reticulum
Structure of lysosome [3 marks]
- Single glycosylated membrane with lots of transmembrane proteins
- Heteromorphic
- Low pH
Substrate delivery to lysosomes for EXTERNAL substrates [2 marks]
- Fluid-phase endocytosis
- Phagocytosis
What does fluid phase mean? [2 marks]
- Fluids from outside are taken up
- Molecules destined for digestion are dissolved in the fluid
Endocytosis process for digestion [5 marks]
Pit in plasma membrane → pinches off to form vesicle → early endosome → late endosome → fuses with lysosome
Phagocytosis process [3 marks]
Particle engulfed by plasma membrane → phagosome formed → phagosome fuses with lysosome
What are the differences between phagocytosis and autphagy? [4 marks]
- Single membrane surrounds substrate in phagocytosis
- Double membrane for autophagy
- Substrate to undergo phagocytosis is wrapped in plasma membrane.
- For autphagy, ER membrane seals it off
Substrate delivery to lysosomes for INTERNAL substrates [1 mark]
- Autophagy
Receptor mediated endocytosis for LDL [4 mark]
- LDL binds to its receptor and forms a complex
- Complexes aggregate in coated pits and pinch off (forming a vesicle)
- pH drops and receptors dissociate
- Cholesteryl ester is hydrolysed with lipase (forms cholesterol and fatty acids)
What is glycosylation of lysosomal enzymes and why is it important? [3 marks]
- When lysosomal enzymes receive oligosaccharides into Asp residues
- Singal sequence cleaved off by signal peptidase
- Important as without it, some proteins can’t fold correctly
Mannose-6-phosphate pathway [2 marks]
M6P attaches to lysosomal enzyme → Vesicles form around modififed enzyme
