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

Question 1

Describe the endomembrane system.

Answer 1

A set of cytoplasmic organelles, including the ER, Golgi complex, endosomes, lysosomes, and vacuoles functioning as part of a coordinated unit

Question 2

What’s the main function of organelles in the endomembrane system? What do they use to transport these materials?

Answer 2

To shuttle materials through, in, and out of the cell. They use membrane-bound transport vesicles.

Question 3

Describe the process in which transport vesicles fuse to their recipient membrane.

Answer 3

• Bud from the donor membrane compartment
• Fuse with the membrane of the acceptor compartment
• The direction of the transported cargo protein is defined.

Question 4

What is the exocytic pathway used for?

Answer 4

Materials move out of the cell via the endomembrane system.

Question 5

What is the endocytic pathway used for?

Answer 5

Materials move from the outer surface of a cell into the endomembrane system.

Question 6

Describe the characteristics of the exocytic pathway.

Answer 6

As a Biosynthetic Pathway:
- it synthesizes, modifies, and transport proteins

As a Secretory Pathway:

• when proteins are discharged (secreted) from the cell
• it’s capable of Constitutive (in a continuous fashion) Secretion and Regulated Secretion (in response to a stimulus)

Question 7

What are the major organelles involved in the endocytic pathway?

Answer 7

the endosomes and the lysosome

Question 8

Describe some characteristics of the Endoplasmic Reticulum

Answer 8

• comprises a network of membranes that penetrates a lot of the cytoplasm
• Might have evolved from the invagination of the plasma membrane
• it is highly dynamic and constantly undergoing turnover and reorganization
• the luminal and cisternal space inside the ER membrane is different from the surrounding cytosolic space

Question 9

What are the two sub-components of the ER?

Answer 9

The Smooth ER and the Rough ER.

Question 10

Describe the Rough ER.

Answer 10

• This section of the ER has ribosomes bound to its cytosolic surface.
• It’s made of flattened sacs.
• It is continuous with the outer membrane of the nuclear envelope.

Question 11

Describe the Smooth ER.

Answer 11

• It has NO RIBOSOMES
• It’s made of highly curved and tubular membranes
• It is continuous just like the RER.
• It’s extensively made in a number of cell types: one specifically being smooth muscle cells

Question 12

What does the ratio of RER (Rough ER) to SER (Smooth ER) depend on?

Answer 12

The type of cell and the activities of that cell

Question 13

Specifically, what are the funcitons of the SER?

Answer 13

• to synthesize steroid hormones (i.e. endocrine cells, gonad, and adrenal cortex)
• to detox the liver of various organic compounds like ethanol (this is carried out by oxygenases like cytochrome P450 enzymes)
• sequester (isolate) calcium ions from the cytoplasm of muscle cells: therefore it contains a high-concentration of calcium-binding proteins

Question 14

Specifically, what are the functions of the RER?

Answer 14

• it synthesizes proteins for membrane-bound ribosomes as opposed to free ribosomes floating in the cytoplasm
• it synthesizes and processes secretory, lysosomal, or vacuolar proteins on membrane-bound ribosomes
• it synthesizes integral membrane proteins on membrane-bound ribosomes
• it functions as the site of glycosylation and membrane biosynthesis in the ER

Question 15

What are some general properties of the RER?

Answer 15

• The RER possesses a large surface area for ribosomes to attach to.
• The lumen of the RER cisternae provides a specialized local environment.

Question 16

What is the function of the RER in relation to other organelles?

Answer 16

• The RER and other organelles produce polarity for cells secreting large amounts of proteins. (This polarity reflects the flow from the site of protein synthesis to the site of discharge of the protein.)
• The RER is the starting point of the biosynthesis pathway for:
  1) Proteins
  2) Carb Cains
  3) Phospholipids

Question 17

Approximately what percentage of polypeptides do RER membrane-bound ribosomes encode in the mammalian genome?

Answer 17

• about 33% (one-third of all polypeptides)

- this includes: secreted proteins, integral membrane proteins, and soluble proteins residing in the endomembrane system

Question 18

For what proteins do “free” ribosomes encode for?

Answer 18

• Cytosolic proteins
• peripheral membrane proteins
• nuclear proteins
• proteins incorporated into chloroplats
• mitochondria
• peroxisomes

Question 19

How do cells determine a site where proteins are made?

Answer 19

• Via the signal hypothesis.

Question 20

What is the Signal Hypothesis? Further, explain the process the Signal Hypotheses states.

Answer 20

It states that the site of synthesis of a protein is determined by a sequence of amino acids in the N-terminus.

• A signal sequence at the N-terminus of a secretory protein directs the emerging polypeptide and ribosome to the RER.
• The polypeptide then moves into the ER cisternal space through a protein-lined pore

Question 21

In what ways through the ER’s cisternal space can a polypeptide be synthesized?

Answer 21

It can be synthesized CO-TRANSLATIONALLY (while it’s being synthesized) or POST-TRANSLATIONALLY (after it has been made)

Question 22

How are Secretory/Lysosomal Proteins made on Membrane-Bound Ribosomes?

Answer 22

1) The mRNA binds to free ribosomes in the cytosol.
2) A signal peptide is made (the stretch of 6-15 hydrophobic amino acid residues).
3) A Signal Reception Particle (SRP) recognizes the signal sequence and binds to both the signal sequence and the ribosome.
4) The SRP-nascent polypeptide-ribosome complex binds to the cytosolic surface of RER through binding of the SRP with SRP RECEPTOR and RIBOSOME and the translocon.

Question 23

Once inside the lumen of the ER, how are the newly synthesized proteins processed?

Answer 23

1) The signal is cleaved by a SIGNAL PEPTIDASE
2) Carbs are added to by the OGLIOSACCHARYLTRANFERASE ENZYME
3) MOLECULAR CHAPERONES assist in the appropriate folding of the proteins
4) PROTEIN DISULFIDE ISOMERASE catalyzes the formation of disulfide bonds between the cystines to stabilize proteins

Question 24

The process of synthesizing Integral Membrane Proteins is similar to Secretory Protein synthesis, EXCEPT for one way. What is that?

Answer 24

Integral Membrane synthesis is UNIQUE because it doesn’t enter the RER lumen. The hydrophobic transmembrane segments of the integral protein are shunted directly from the channel of translocon into the lipid bilayer via a lateral gate.

Question 25

What is the most common determinant of membrane protein alignment?

Answer 25

The presence of positively charged amino acids residues flanking the cytosolic end of a transmembrane segment.

Question 26

How are membranes synthesized in the ER?

Answer 26

- Just like with cells, membranes come from pre-existing. This happens via the insertion of newly made membrane proteins and lipids into existing membranes. - Membrane components move from the ER to almost every other compartment of the cell - As the membrane move from one compartment to the next, its proteins and lipids are modified.

Question 27

How is membrane asymmetry maintained?

Answer 27

- Domains facing the cytosolic side will always face the cytosolic side. - Domains facing the lumen of the ER will always face the extracellular side.

Question 28

What types of lipids are not made in the ER?

Answer 28

- sphingomyelin - glycolipids - some unique lipids of the mitochondria and chloroplast

Question 29

Specifically, explain the synthesis of membrane lipids.

Answer 29

- Newly synthesized phospholipids are inserted into the half of the bilayer facing the cytosol, and some may flip into opposite leaflet by flippases. - Different lipid composition of different organelles is generated as membrane flow through the cell.

Question 30

What THREE ways are glycolipids modified in order to function in a new environment (i.e. going from being part of the ER to becoming part of the plasma membrane)?

Answer 30

1. Enzymatic modification (head group) 2. Modification during vesicle formation 3. Modification by phospholipid transfer proteins

Question 31

What are some contributing factors to the variation of organelle lipids?

Answer 31

– Organelle-specific enzymes for lipid conversion. – Inclusion/exclusion process during vesicle formation. – Lipid-transfer proteins that bind and transport lipids without the use of vesicle transport.

Question 32

What type of protein do nearly all proteins made by the RER become?

Answer 32

Glycoproteins

Question 33

How is the addition of sugars catalyzed onto a RER proteins?

Answer 33

Via GLYCOSYLTRANSFERASE

Question 34

What is the function of glycosylation in the RER?

Answer 34

- Provide binding site for glycoprotein to interact with other macromolecules - Aid in proper protein folding - Help with protein stabilization

Question 35

Describe the features of glycosylation in the RER.

Answer 35

- A sequence of sugars forming the oligosaccharides of a glycoprotein is HIGHLY SPECIFIC - A single core oligosaccharide is linked to the asparagine (Asn or N) residues of the polypeptide through N-LINKE OLIGOSACCHARIDES

Question 36

The RER has a quality control system in place to properly maintain proteins synthesis. How does it regulate this?

Answer 36

- Misfolded proteins are tagged by a terminal glucose and recognized by chaperones for refolding. - If the protein does not correctly fold, it is translocated to the cytosol and destroyed.

Question 37

How is material transported from the RER to the Golgi Apparatus?

Answer 37

- The RER have specialized exit sites where transport vesicles are formed (no ribosomes). - Transport vesicles fuse with one another and form the ERGIC (endoplasmic reticulum Golgi intermediate compartment) toward the Golgi complex.