Endocytosis

Question 1

what is the definition of endocytosis

Answer 1

Question 2

How is endocytosis balanced by the process of exocytosis

Answer 2

If exocytosis doesn’t occur:
- less plasma membrane so cell would shrink

• exocytosis is increased if a cell needs to increase the size of the plasma membrane for cell division

Question 3

Recall the 3 main types of endocytosis

Answer 3

Pinocytosis
Phagocytosis
Receptor mediated endocytosis

Question 4

What is pinocytosis

Answer 4

Endocytosis of fluids and solutes
Most eukaryotic cells do it continuously (doesn’t require a signal to occur
Removes damaged membrane
Endocytosis vesicle around 100nm in size

Question 5

Name the two types of pinocytic vesicle

Answer 5

Cathrin coated
Caveolae

Question 6

What is Clathrin

Answer 6

A protein that arranges itself to form a basket of proteins which fits over the pinocytic vesicle
They help to distort the plasma membrane and pinch the vesicle off the membrane

Question 7

How do Clathrin coated pinocytic vesicles work

Answer 7

Receptor Binding: Specific molecules in the extracellular fluid bind to receptors on the cell membrane. These receptors are often clustered in regions rich in clathrin.

Adaptor proteins, such as AP2 (Adaptor Protein 2), recognize and bind to the cytoplasmic tails of the receptors. These adaptors then recruit clathrin proteins to the membrane.
Clathrin Triskelion: Clathrin is a protein composed of three heavy chains and three light chains forming a triskelion shape. Multiple triskelions assemble into a lattice structure, creating a clathrin-coated pit on the plasma membrane.
Vesicle Formation:

Membrane Invagination: The clathrin lattice causes the membrane to fold inwards, forming a clathrin-coated pit.
Dynamin Action: The protein dynamin wraps around the neck of the budding vesicle and, through GTP hydrolysis, constricts and pinches off the vesicle from the plasma membrane, forming a clathrin-coated vesicle.
Vesicle Uncoating:

Uncoating Proteins: Once inside the cell, the clathrin coat is rapidly removed by uncoating proteins like Hsc70 (a heat shock protein) and auxilin. This uncoating process is necessary for the vesicle to fuse with early endosomes.
Fusion with Endosomes:

Delivery to Endosomes: The uncoated vesicle, now called an early endosome, can fuse with other endosomes, where the internalized molecules are sorted. Some molecules may be recycled back to the plasma membrane, while others are directed to lysosomes for degradation.

Question 8

What are caveolae pinocytic vesicles

Answer 8

Caveolins are in the plasma membranes of most cells
They form from lipid rafts
They are made up of caveolins and Calvin proteins (caveolins are embedded in the membrane of the pinocytic vesicle)
They don’t have a clathrin coat

Question 9

How do dynamin pinch off vesicles from the plasma membrane

Answer 9

Dynamin (protein) wraps around the neck of the vesicle and pinches it off the plasma membrane
It does this by making the lipids at the neck of the vesicle come close together, squeezing out the liquid so that the lipids fuse together to form the vesicle

Caveolins aren’t released

The vesicle then fuses with an endosome or transcytose

Question 10

What is macropinocytosis and how are macropinosomes formed

Answer 10

• is induced (doesn’t happen continuously) - by receptor activation (requires a signal to occur)
• Formed from cell surface ruffles which collapse back into the cell surface and trap extracellular fluid
• Macropinosomes- large fluid filled endocytosis vesicles (they transport fluid)
• Macropinosomes acidify and then fuse with late endosomes

Question 11

How is iron taken into a cell via receptor mediated endocytosis

Answer 11

Question 12

How do cells take up cholesterol

Answer 12

LDL Receptors: The primary mechanism for cholesterol uptake in most cells involves LDL receptors. LDL particles, which carry cholesterol through the bloodstream, bind to these receptors and are internalized by the cell.
Vesicle Formation: Once inside the cell, LDL particles are transported to lysosomes, where they are degraded to release free cholesterol.

Cells take up cholesterol via receptor-mediated endocytosis. Low-density lipoprotein (LDL) receptors bind to LDL particles, which contain cholesterol, and internalize them into the cell.

Synthesis of Bioactive Molecules -> vitD , steroid hormones

Membrane fluidity, structural component of membrane

Question 13

What is the result of a cell not being able to take up cholesterol

Answer 13

Question 14

What are some clinical relevances of endocytosis

Answer 14

Question 15

Outline the process of endocytosis after the endosome has formed

Answer 15

• when an endocytosis vesicle is formed it fuses with the early endosome which sorts the contents of the vesicle
• Some of the contents are returned to the plasma membrane directly of via a recycling endosome
• Other contents of the endosome are degraded in a late endosome
• Late endosomes fuse with each other and then with a lysosome

Question 16

What are caveolins

Answer 16

Are the major structural proteins in caveolae

Question 17

What is the role of receptor mediated endocytosis and what is its relevance to cholesterol

Answer 17

• macromolecules bind to complimentary transmembrane proteins which accumulate in coated pits and enter the cell in clathrin-coated vesicles
(Uses clathrin coated pits)
• Through this process cells uptake most of the cholesterol that they need
• If this process if blocked- cholesterol would accumulate in the blood

Question 18

How is cholesterol transported in the blood

Answer 18

In the form of LDLs

Question 19

How is cholesterol uptake into a cell via receptor mediated endocytosis

Answer 19

• a cell makes transmembrane receptor proteins for LDL when it needs cholesterol
• The receptors are inserted into the plasma membrane
• LDL receptors diffuse until they associate with clathrin-coated pits that are being formed
• An endocytosis signal in the cytoplasmic tail of the LDL receptors bind to the adaptor protein
• Coincidence detection signals clathrin to start endocytosis
• Any LDL particles bound to LDL receptors in the coated pits are put into coated vesicles
• The vesicles then shed their coats and deliver their contents to early endosomes which has a low pH
• LDL is released from its receptor and delivered to lysosomes via late endosomes
• LDL is then hydrolysed in the lysosome to release the cholesterol - cholesterol now available for the cell to use

Question 20

What is the role of phagocytosis

Answer 20

• a cell uses phagosomes to ingest large particles such as microorganisms and dead cells

Macrophages also engulf senescent cells and cells that have died by apoptosis

apoptosis cells gain their ‘eat me’ signals and lose their ‘don’t eat me’ signals

Question 21

How does phagocytosis occur (in general)

Answer 21

• Large particles taken up in phagosomes end up in lysosomes and the products of the break down of the contents of the phagosome pass into the cytosol
• Carried out by phagocytes

Question 22

How does phagocytosis occur for pathogens

Answer 22

• Is a signalled process- particles must bind to the surface of the phagocyte
• Antibodies coat a pathogen with antibody molecules which bind to the receptors on the surface of the macrophage and neutrophils ->they then engulf the pathogen

Question 23

What happens to indigestible substances in a phagosome after the phagosome fuses with a lysosome

Answer 23

• Indigestible substances from a phagosome stay within a lysosome forming residual bodies which are excreted via exocytosis

Question 24

What are the two types of phagocytes in mammals

Answer 24

• Two types of phagocytes in mammals= macrophages and neutrophils which engulf microorganisms

Question 25

What are senescent cells

Answer 25

Cells that haven’t died but have a low metabolism so aren’t very active

Question 26

Why can’t some cells take up cholesterol

Answer 26