Topic 14: Endocytosis and Degradation by the Lysosome

Phagocytosis

“cell eating”, ingestion of large particles/cells

Autophagy

“self-eating”, used to recycle worn-out organelles

Endocytosis

uptake of molecules or solutions from outside the cell by invagination of the plasmla membrane, enclosing the material in a vesicle

Pinocytosis

“cell drinking”, uptake of bulk soluble molecules from external medium, indiscriminate

Receptor-mediated Endocytosis

uptake of specific molecules by binding to plasma membrane receptors

What are the three main pathways for organelles and molecules to reach the lysosome?

phagocytosis, autophagy, endocytosis

Where are metabolites produced in the lysosome sent?

exported to cytoplasm and recycled

Describe the three steps of phagocytosis

entrapment - pseudopods trap and surround “prey”

engulfment - pseudopods fuse to internalize prey in phagosome

digestion - phagosome fuses with lysosome

Are all bacteria susceptible to phagocytosis? If not, what types are immune?

no, some evolved to evade lysosome digestion and live as intracellular parasites or pathogens

Give some examples of bacteria that evolved to be immune from phagocytosis digestion

tuberculosis, listeria, plague

What organisms feed via phagocytosis?

amoeba, protists

What cells use phagocytosis to fight infection?

white bloods cells, these are specialized phagocytes

Give an example of cells that involve autophagy in their life cycles

liver mitochondria “live” about 10 days

What is done to replace a cell removed by autophagy?

a new membrane-bound organelle is made to replace the worn-out one engulfed by the ER and lysosome

What does endocytosis allow cells to balance?

allows cells to balance membrane loss during exocytosis

Give an example of a type of cell that uses endocytosis to regulate its membrane

fibroblasts use this process to help internalize their membrane at a rate of 2-3% of its membrane per minute

If you block endocytosis while exocytosis continues, then…

the membrane will grow

If you block exocytosis and endocytosis continues, then…

the membrane will shrink

Which types of endocytosis occur in clathrin-coated vesicles?

both pinocytosis and receptor-mediated endocytosis

What type of organelle evolved into a lysosome when it matures?

endosomes

Describe how lysosomes evolve/mature

endosome maturation to lysosome requires acidification of the organelle and delivery of acid hydrolases in Golgi vesicles

The change of early endosome to late endosome to lysosome is a…

continuum

Describe the steps of vesicle formation via receptor-mediated endocytosis

1. receptors in the plasma membrane bind specific cargo outside the cell

2. receptor-cargo complexes recruit adaptins

3. adaptins recruit clathrin coat proteins

4. clathrin assembly drives membrane curvature

5. dynamin constricts the “neck” of the budding vesicle

6. clathrin and adaptin are released

Adaptins

adaptors that mediate cargo selection

What phase is the cargo in when undergoing pinocytosis?

bulk aqueous phase

Where is cargo bound during receptor-mediated endocytosis and how does it affect efficiency?

bound to the receptor, increases efficiency of uptake by up to 1000x

What clathrin structures form the triskeleton on the outside of a vesicle?

3 clathrin heavy chains and 3 clathrin light chains form this structure

Describe the mass of clathrin heavy chains

~180-190 kDa

Describe the mass of light clathrin chains

~40 kDa

Does the assembly of triskeletons require energy or assistance from other proteins?

no, it is spontaneous

Describe the assembly movement of cargo receptors in the membrane

formed in the ER → moved to the Golgi → either secreted or put in a transport vesicle → moved to a membrane

Low-density Lipoprotein

structure in which cholesterol is transported throughout the body

Describe the structure of a lipid micelle

~800 phospholipids, ~500 cholesterol molecules, ~1500 cholesterol ester molecules, 1 apoprotein B copy