Section 7F: Endocytosis and Phagocytosis Flashcards
Exocytosis
- vesicle fusion with the plasma membrane
- secretion
Endocytosis
- phagocytosis
- pinocytosis (also simply called endocytosis)
- transport from the plasma membrane into endosomes
What are the two types of Secretory Pathways?
- Constitutive Secretory Pathway
- Regulated Secretory Pathway
Constitutive Secretory Pathway
- is not regulated in the cell, happens all the time
- proteins that are made in the golgi are packaged into vesicles and delivered to the plasma membrane
- there are 2 types of golgi derived vesicles: soluble and integral membrane proteins
- these will undergo vesicle fusion and 2 things will happen: inside content will be delivered to the outside, and integral membrane proteins will become a part of the plasma membrane
Regulated Secretory Pathway
- cell package into a specialized vesicle from the Golgi
- these secretory vesicles have some sort of lumen
- only undergo secretion when there is some sort of signal
Examples of exocytosis functions
- delivery of new membrane and proteins to the plasma membrane
- delivery of extracellular components like proteoglycans
- delivery of signalling molecules to the extracellular medium
Examples of regulated secretory pathway: delivery of signalling molecules to the extracellular medium
- Proteases: remodel extracellular medium or fight infections
- Hormones and growth factors: signalling between cells
- Neurotransmitters: neuronal communication
- Immune and inflammatory factors: histamine and interleukins
Examples of secretory vesicles
- Pancreatic β cells
- Histamines
Pancreatic β cells
- have pre-made vesicles loaded with insulin (in their lumen)
- sense an increase in blood glucose
- trigger exocytosis of vesicles that contain insulin when they sense an increase in glucose
- insulin triggers a lowering of blood glucose
What are histamines?
- small molecule responsible for itching and sneezing during allergic reactions
- immune signal the signals for your immune system that there’s something needed to be dealt with (e.g. foreign particle)
What are mast cells?
a type of immune cell that is loaded full of secretory vesicles, and those vesicles are full of molecules called histamines
What do drugs do that prevent allergies?
the drugs prevent the action of the histamines releasing from the mast cell
Breakdown of Endocytosis
- during endocytosis, extracellular content and plasma membrane are internalized
- internalization via small endocytic vesicles is called pinocytosis (or cell drinking)
- internalization of large particles including whole cells is called phagocytosis (or cell eating)
- endocytic organelles are targeted to endosomes for sorting, recycling and/or degradation
Process of Phagocytosis
- a receptor-driven process by which particulate matter is engulfed by a cell (not a random engulfment)
- protozoa usually phagocytose to obtain food
In mammals, phagocytosis is mostly carried out by white blood cells:
macrophages and neutrophils:
1. bacteria and other foreign organisms
2. apoptotic/senescent cells
What is a Phagosome?
- it is a membrane bound organelle that is the result of phagocytosis
- has a pathogen inside
What is it called when a phagosome is fused with a lysosome?
Phagolysosome
Lysosome is full of hydrolases, what are hydrolases?
enzymes that breakdown everything they come across
What is the job of a Phagolysosome?
causes destruction of pathogen because lysosomal enzymes are released; this is how the immune system is able to get rid of invading bacterias
Phagosome maturation:
fusion with endosomes and lysosomes needed to digest particle
Process of Pinocytosis (or simply endocytosis)
- all eukaryotic cells inget bits of the plasma membrane in small pinocytic (or endocytic) vesicles
- rate of pinocytosis varies on cells: macrophages internalize the equivalent of their plasma membrane in 30 mins
- therefore, pinocytosis must be balanced by exocytosis - otherwise the cell will shrink (in surface area)
- therefore, there is an endocytic-exocytic cycle
Why is the process of pinocytosis (or endocytosis) important?
this allows the cell to exchage material selectively with the outside
Clathrin-coated pits drive pinocytosis
Generally, two sorts of molecules get taken up by pinocytosis:
- those bound to a cell surface receptor (highly efficient)
- those not bound to a receptor (fluid phase - much less efficient)
Receptor-mediated endocytosis
- fluid-phase endocytosis does not enrich in cargo but often needs to be selective and enrich for certain types of cargo molecules: nutrients and signalling molecules
- receptor-mediated endocytosis permits selection and enrichment of specific cargo molecules
- cargo molecules (ligands) bind to receptors, which are then concentrated by adaptor protein complexes and clathrin-coat assembly into endocytic vesicle
Example of Receptor-mediated endocytosis: Cholesterol
- maintains membrane permeability
- cholesterol gets packaged by liver and other cells of the digestive track called Low-density lipoprotein (LDL)
- LDL is often called the “bad” cholesterol
Receptor-mediated endocytosis of LDL
- LDL receptor protein (an integral membrane protein) sits on the surface of the cell and binds to LDL
- on the cytosolic side the LDL receptor has a signal to bind to adaptor protein 2 (AP2) and trigger the formation of a clathrin coat
- allows nutrients to be captured into vesicles then, these vesicles go to the lysosome
- once vesicles are at lysosome the LDL will be degraded and release cholesterol for the cell to use as it needs
Why is receptor-mediated endocytosis important?
- helps to understand a disease: genetic form of high cholesterol (hypercholesterolemia)
- some people have a mutation or difference in gene for the LDL receptor which no longer is able to associate with AP involved in clathrin endocytosis (receptors are no longer packaged into vesicles by endocytosis)
- causing LDL particle not getting taken up by cells so, LDL particle and cholesterol build up in the bloodstream
- causing heart disease and myocardial infarctions
