Section 7F: Endocytosis and Phagocytosis

Question 1

Exocytosis

Answer 1

1. vesicle fusion with the plasma membrane
2. secretion

Question 2

Endocytosis

Answer 2

1. phagocytosis
2. pinocytosis (also simply called endocytosis)
3. transport from the plasma membrane into endosomes

Question 3

What are the two types of Secretory Pathways?

Answer 3

1. Constitutive Secretory Pathway
2. Regulated Secretory Pathway

Question 4

Constitutive Secretory Pathway

Answer 4

• 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

Question 5

Regulated Secretory Pathway

Answer 5

• 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

Question 6

Examples of exocytosis functions

Answer 6

• delivery of new membrane and proteins to the plasma membrane
• delivery of extracellular components like proteoglycans
• delivery of signalling molecules to the extracellular medium

Question 7

Examples of regulated secretory pathway: delivery of signalling molecules to the extracellular medium

Answer 7

• Proteases: remodel extracellular medium or fight infections
• Hormones and growth factors: signalling between cells
• Neurotransmitters: neuronal communication
• Immune and inflammatory factors: histamine and interleukins

Question 8

Examples of secretory vesicles

Answer 8

• Pancreatic β cells
• Histamines

Question 9

Pancreatic β cells

Answer 9

• 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

Question 9

What are histamines?

Answer 9

• 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)

Question 10

What are mast cells?

Answer 10

a type of immune cell that is loaded full of secretory vesicles, and those vesicles are full of molecules called histamines

Question 11

What do drugs do that prevent allergies?

Answer 11

the drugs prevent the action of the histamines releasing from the mast cell

Question 12

Breakdown of Endocytosis

Answer 12

• 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

Question 13

Process of Phagocytosis

Answer 13

• a receptor-driven process by which particulate matter is engulfed by a cell (not a random engulfment)
• protozoa usually phagocytose to obtain food

Question 14

In mammals, phagocytosis is mostly carried out by white blood cells:

Answer 14

macrophages and neutrophils:
1. bacteria and other foreign organisms
2. apoptotic/senescent cells

Question 15

What is a Phagosome?

Answer 15

• it is a membrane bound organelle that is the result of phagocytosis
• has a pathogen inside

Question 16

What is it called when a phagosome is fused with a lysosome?

Answer 16

Phagolysosome

Question 17

Lysosome is full of hydrolases, what are hydrolases?

Answer 17

enzymes that breakdown everything they come across

Question 18

What is the job of a Phagolysosome?

Answer 18

causes destruction of pathogen because lysosomal enzymes are released; this is how the immune system is able to get rid of invading bacterias

Question 19

Phagosome maturation:

Answer 19

fusion with endosomes and lysosomes needed to digest particle

Question 20

Process of Pinocytosis (or simply endocytosis)

Answer 20

• 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

Question 21

Why is the process of pinocytosis (or endocytosis) important?

Answer 21

this allows the cell to exchage material selectively with the outside

Question 22

Clathrin-coated pits drive pinocytosis
Generally, two sorts of molecules get taken up by pinocytosis:

Answer 22

1. those bound to a cell surface receptor (highly efficient)
2. those not bound to a receptor (fluid phase - much less efficient)

Question 23

Receptor-mediated endocytosis

Answer 23

• 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

Question 24

Example of Receptor-mediated endocytosis: Cholesterol

Answer 24

• 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

Question 24

Receptor-mediated endocytosis of LDL

Answer 24

• 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

Question 25

Why is receptor-mediated endocytosis important?

Answer 25

• 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