8 Cytoplasmic Membrane Systems: Structure, Function, and Membrane Trafficking

Question 1

What is endomembrane system?

Answer 1

An integrated network that allows the two-way transport of materials and information via various organelles

Question 2

How is materials shuttled between organelles

How does it finish the transport

Answer 2

• Transport vesicles

- The vesicle fuses into the membrane and the content is released

Question 3

Identify 2 pathways through the cytoplasm

Answer 3

• Biosynthetic pathway, also called the secretory pathway. It is the modification and transport of proteins.
• Endocytic pathway. materials moving from the outer surface of the cell to compartment within the cytoplasm

Question 4

What is endoplasmic reticulum?

Answer 4

A network of membranes that penetrates much of the cytoplasm. Its inner content has a different composition from the surrounding cytoplasm

Question 5

Smooth ER’s 3 functions?

Answer 5

• Synthesis of steroid hormones in endocrine cells
• Detoxification in the liver of various organic compounds
• Sequestration of calcium ion from muscle cell cytoplasm. (with a high concentration of calcium binding proteins)

Question 6

Rough ER’s structure

Rough ER’s functions

Answer 6

Structures
-A network of flattened sacs (cisternae)
-Continues with the outer membrane of the nuclear envelope and also has ribosomes on its cytosolic surface.
Functions
-Starting point of biosynthetic pathway
-Polarity of RER in some cells reflects the flow from the site of protein synthesis to the site of discharge of the protein

• Synthesis of Proteins on membrane-bound/free ribosomes
  a. 1/3 of the polypeptides are synthesized on ribosomes of RER including secreted proteins, integral membrane proteins and soluble proteins of organelles.
  b. the rest is synthesized on free ribosomes including: cytosolic proteins, peripheral membrane proteins, nuclear proteins, and mitochondria proteins.

Question 7

How is the synthesis site of protein determined?

Answer 7

It is determined by a sequence of amino acids in N-terminus (an address tag) called a “signal sequence”

Question 8

Protein processing in steps within RER?

Answer 8

1. The address tag is removed
2. Enzymes assist to add carbohydrates to nascent proteins
3. Other enzymes or chaperone proteins aid in protein folding
4. Hydrophobic portions of integral membrane proteins are inserted into the RER membrane immediately after translation
5. Membranes arise from other membranes (eg. RER membranes)

Question 9

How is protein folding quality control done?

Answer 9

Polypeptide is modified to become glycosylated.
Misfolded proteins are tagged by a terminal glucose and recognized by chaperones for refolding.
Once the protein is folded the glucose groups is removed
If the protein cannot correctly fold it will be moved to cytosol and destroyed.

Question 10

Golgi Complex Structure?

How is it integrated into the endomembrane system?

Answer 10

It is a stack of flattened cisternae.
Its cis face faces the specialized exit of RER where transport vesicles are created.
Its trans face faces the opposite side to the cytoplasm

Question 11

Significance of the folds in Golgi Complex?

Answer 11

Each fold is a functionally distinct compartment with different enzymes. In every fold the sugar groups are added to the protein in a stepwise fashion.

Question 12

2 historical Models of Material movement through Golgi?

What’s the current model?

Answer 12

1. Vesicular transport model. Golgi is stable, the cargo is shuttled from the cis to the trans through vesicles
2. Cisternal maturation model. Golgi is transient and dynamic. Each cistern matures as it moves from the cis face to the trans face.
3. Current model: Similar to cisternal maturation model but with vesicle retrograde (backwards) transport. Golgi cisternae serve as primarily anterograde (forward) carriers.

Question 13

What is exocytosis?

What controls it?

Answer 13

Exocytosis is the discharge of secretory vesicle or granule after fusion with plasma membrane.

It is controlled (triggered) by an increase in [Ca2+].
The vesicle fuses into the plasmic membrane.

Question 14

What is lysosomes’ function

Describe the pH inside lysosome, how is it maintained?

Answer 14

Lysosome can digest any type of biological molecule.
It plays an important role in organelle turnover.

The pH inside lysosome is low, which is maintained by a proton pump

Question 15

Describe the process lysosomes digest biomolecules

Answer 15

1. An organelle is surrounded by a double membrane to produce a structure called autophagosome
2. Merging of autophagosome with lysosome to produce autophagolysosome.

Question 16

3 types of endocytosis?

Answer 16

Pinocytosis: Nonspecific uptake of extracellular fluids (cell drinking)
Receptor-mediated endocytosis: uptake of specific extracellular ligands following their binding to receptor
Phagocytosis: uptake of particular matter (cell eating) Most common in destroying foreign particulate, such as in immune cells called macrophages

Question 17

Process of receptor-mediated endocytosis

Answer 17

1. Substances that enter the cell through clathrin-mediated RME become bound to coated pits on the plasma membrane.
2. Receptors and their ligands become 10-20 times more concentrated in coated pits
3. Clathrin coated regions invaginate into the cytoplasm and pinch free of the cytoplasm

Question 18

Describe clathrin’s structure

Answer 18

Contains three heavy and three light chains that form a triskelion.
The clathrin can come together to form coated vesicles that contains adaptors (eg. AP2 adaptor)

Question 19

Describe the endocytic pathway.

Describe the difference between the early and late endosome

Answer 19

Endocytic pathway
Materials are internalized and become bounded by vesicles and transported in the cytoplasm. (the vesicles are known as endosomes).
Early vs late endosome
Early endosomes are located near the periphery of the cell while the late endosomes are closer to the nucleus.

Question 20

2 Types of receptors that enter the cell via the receptor mediated endocytic pathway?

Answer 20

1. Housekeeping receptors: Responsible for uptake of materials
2. Signaling receptors: Responsible for dampening a signal transduction event

Question 21

How are the two types recycled?

Answer 21

1. Housekeeping receptors are recycled at the early endosome. The content is sent further to the late endosome. (or even to the lysosome, after the late endosome)
2. Signaling receptors (and their ligands) are transported from the early to the late endosome or lysosome for destruction

Question 22

What is phagocytosis?

Answer 22

It is the engulfment of large particles where plasma membrane takes up a particle and pinches off to form a phagosome, which fuses with lysosomes to further digest the large particles.