Chapter 14 (Cell Biology)
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What is the main purpose of the endocytic pathway?
To internalize molecules and membrane components from the cell surface, balancing secretion and maintaining membrane homeostasis.
How does the endocytic pathway complement the secretory pathway?
The secretory pathway adds membrane to the plasma membrane, while the endocytic pathway removes it, keeping cell size constant.
What is endocytosis?
The process by which cells internalize substances from their surface into vesicles for processing or degradation.
What are the two main types of endocytosis?
Phagocytosis (“cell eating”) and Pinocytosis (“cell drinking”).
What is phagocytosis?
The ingestion of large particles such as microorganisms or cell debris into large vesicles called phagosomes.
Which cells specialize in phagocytosis?
Macrophages, neutrophils, and other immune cells.
What triggers phagocytosis?
The binding of antibody-coated particles or microbes to antibody receptors on the cell surface.
What are pseudopods?
Extensions of the plasma membrane that engulf large particles during phagocytosis.
What happens after pseudopods engulf a particle?
They form a phagosome, which fuses with lysosomes to digest the contents.
How many red blood cells do macrophages remove daily?
More than 10 billion per day.
How do some pathogens evade phagocytosis?
They block phagosome-lysosome fusion (e.g., Mycobacterium tuberculosis).
What is pinocytosis?
Continuous ingestion of extracellular fluid and small molecules through small vesicles (<250 nm).
What are the two types of pinocytosis?
Indiscriminate pinocytosis and receptor-mediated endocytosis.
What is indiscriminate pinocytosis?
Non-selective uptake of extracellular fluid and any dissolved solutes.
What is receptor-mediated endocytosis?
A highly selective process using receptors to internalize specific molecules efficiently.
What coats pinocytic vesicles during receptor-mediated endocytosis?
Clathrin, forming clathrin-coated pits.
What happens after clathrin-coated vesicles form?
They uncoat and fuse with early endosomes.
Why is receptor-mediated endocytosis efficient?
It can concentrate target molecules over 1000 times compared to extracellular fluid levels.
What is an example of receptor-mediated endocytosis?
LDL-cholesterol uptake into animal cells.
What does LDL stand for?
Low-Density Lipoprotein.
What is the role of LDL?
To transport cholesterol in the bloodstream.
How is LDL internalized by cells?
By binding to LDL receptors that cluster in clathrin-coated pits.
Where do LDL particles go after endocytosis?
To endosomes where the acidic pH causes LDL to dissociate from its receptor.
What happens to the LDL receptor after dissociation?
It recycles back to the plasma membrane.