Lysosomes

Question 1

What is the secretory pathway?

Answer 1

a series of organelles that are very dynamically connected with each other through the secular transport, influence on endoplasmic reticulum, Golgi apparatus, secretory granule and the plasma membrane.

Question 2

what are hydrolytic enzymes?

Answer 2

catalyze reactions which break covalent bonds by utilizing water. Hydrolytic enzymes in a lysosome are acidic they only work at pH 4-5.

Question 3

What are heterogenous contents?

Answer 3

involved in the breakdown of many different substrates which includes red blood cells, white blood cells or bacteria taken out by macrophages. Substrates are in various stages of degradation – substances are heterogeneous. E.g. some lysosomes have other vesicles insides from degradation of another organelle.

Question 4

structure of lysosomes

Answer 4

Lysosomes in their lumen contain a lot of hydrolases – soluble enzymes, high proton conc, low pH

Certain enzyme activators and transport proteins. Variety of important + organelle specific membrane proteins inc. Vacuolar ATPase very large molecule proteins, protein complex sitting in the lysosomal membrane – pumps protons into the lumen of the lysosome- low pH. Lysosomes requires ion channels and transporters.

Products of hydrolysis reactions must be released so that they can be recycled and reused again by the cell.

Answer 5

Breakdown of lipids cause cholesterol – transporter. Release of cholesterol can be used for membrane biogenesis. The breakdown of carbohydrates by glycosidases produce monosaccharides which have to leave the lysosome through membrane sugar transporters. Nucleases break down RNA and DNA that produces nucleosides. Proteases break down proteins into amino acids. Lysosomes constantly exchange materials with other organelles through the secretory pathway system. Lysosomes have proteins on the surfaces including GTPases, ATPases, tethering factor, SNAREs to ensure that processes could occur.

Question 6

What are melanosomes?

Answer 6

they are produced by the melanocyte and in other cells of the body.

Mature melanosomes characterised by high conc. of melanin- can absorb UV light and it is critical for photoprotection and protects from the damaging effects of sunlight.

Question 7

What are endosomes?

Answer 7

require specific proteins which produce melanin and make up specific protein compositions through series of complex fusion and fission processes carried out by interactions with other organelles in secretory pathways.

Melanosomes are produced by melanocytes they are transferred to keratinocyte. - process in which this happens is still not understood.

Question 8

Substrate delivery to lysosomes:

Extracellular substrates : endocytosis

Answer 8

Fluid phase endocytosis of molecules and lipoproteins (includes receptor-mediated endocytosis)

Phagocytosis of particles >/ 0.5 um

Question 9

Intracellular substrates

Answer 9

Micro autophagy (invagination of the lysosomal membrane)

Macro autophagy (cytosol or organelles wrapped in ER membrane which then fuses with lysosomes)

Selective transport of proteins across the lysosomal membrane.

Question 10

What is LDL?

Answer 10

Building block of cellular membranes

(bad cholesterol) - very high conc in plasma of humans associated with increased risk of CVD.

Core- neutral lipids cholesteryl ester and triglycerides and are surrounded by a lipid monolayer that consists of unesterified cholesterol and phospholipids.

Cholesteryl ester is a cholesterol molecule that is esterified with a fatty acid.

Associated with a shell (apoprotein B-100) - important for recognition of cell surface receptors.

Question 11

Cholesteryl ester molecule

Answer 11

It is hydrolysed to produce cholesterol and fatty acid. Then they will be released for further metabolism outside the lysosomes.

Question 12

what does engulfment mean?

Answer 12

cell is eaten.

• bacteria is affected or inhaled from lungs
• dead cells
• ageing blood cells (RBC or WBC)

Question 13

Steps of phagocytosis

Answer 13

1. Particle will bind to cell surface then it will be engulfed
2. it occurs through the plasma membrane wrapping itself around the particle- phagosome
3. lysosome fuses with the phagosome to form a phagolysosome
4. lysosome delivers hydrolytic enzymes membrane that contains vacuolar atpase molecules protein pumps lead to acidification of inside of the phagolysosomes. low ph activates the hydrolytic enzymes will digest the enzyme.

Question 14

Autophagy

Answer 14

1. structure is formed - double membrane can wrap around into a cellular particle/ organelle. makes an autophagosome
2. it will fuse with the lysosome which will produce autophagolysosome- delivers hydrolytic enzyme pH will drop- enzyme will hydrolyze then digestion would occur.

Question 15

HIV

Answer 15

it binds to the cell surface of the plasma membrane fuses directly and delivers its content leading to infection.

Influenza virus - taken up for endocytosis.

Endosomes- proteins that mediate fusion of viral membranes which leads to release of viral contents leading to infection.