Lysosomes Flashcards
where are lysosomes made?
in the ER
what is the pH inside the lysosome
pH 4.5
why is the pH low in the lysosomes?
- enzymes require low pH for optimum activity
- to regulate enzyme functions: if lysosomes were to break, enzyme will degrade in the cytoplasm due to high pH
describe the structure of lysosomes
- single membrane
- acidic
- heterogenous contents
- 100 lysosomes per cell
- 40 hydrolytic enzymes
what 2 ways do substrates enter a lysosome?
- extracellular substrates: endocytosis
- intracellular: microautophagy
give an example of when substrates would enter the lysosome by endocytosis
-phagocytosis
how do low density lipoproteins (LDLs) enter the lysosome?
- LDL is the main transport vehicle of cholesterol in bloodstream
- LDL binds to receptors on membrane
- receptors aggregate to form coated pits
- the pit folds inwards and the membrane detaches itself to form a closed coated vesicle.
- pH drops in vesicle so LDL dissociates from receptor and receptor returns to membrane
what is LDL hydrolysed into?
cholesteryl ester and fatty acid by lipase enzymes
what modification do lysosomal enzymes have that others don’t
mannose-6-phosphate
what is microautophagy?
when the lysosome directly enfgulfs the target particle or organelle
what is macroautophagy?
when cells form vesicles called autophagosomes and fuse with lysosomes to degrade cell contents
what is the name of the enzyme that adds the M6P modification
phosphoryl transferase
where do proteins that do not have the M6P modifications go
plasma membrane
what happens if there is no M6P modification
the enzyme gets diverted and is secreted, substrate builds up inside the lysosome.
describe the mannose 6-phosphate (M6P) pathway
- digestive enzymes & membrane proteins of the lysosome are synthesised in the ER & transported through the Golgi apparatus to the trans-Golgi network.
- while in the ER & cis-Golgi network, the enzymes are tagged with a specific phosphorylated sugar group (mannose 6-phosphate).
- when they arrive in the trans-Golgi network they can be recognized by an appropriate receptor, the mannose 6-phosphate receptor.
- the tagging permits the enzymes to be sorted & packaged into transport vesicles, which bud off & deliver their contents to the lysosome via late endosomes.
