Chapter 8: Membrane Systems: Structure, Function, and Membrane Trafficking Flashcards
Describe the endomembrane system.
A set of cytoplasmic organelles, including the ER, Golgi complex, endosomes, lysosomes, and vacuoles functioning as part of a coordinated unit
What’s the main function of organelles in the endomembrane system? What do they use to transport these materials?
To shuttle materials through, in, and out of the cell. They use membrane-bound transport vesicles.
Describe the process in which transport vesicles fuse to their recipient membrane.
- Bud from the donor membrane compartment
- Fuse with the membrane of the acceptor compartment
- The direction of the transported cargo protein is defined.
What is the exocytic pathway used for?
Materials move out of the cell via the endomembrane system.
What is the endocytic pathway used for?
Materials move from the outer surface of a cell into the endomembrane system.
Describe the characteristics of the exocytic pathway.
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)
What are the major organelles involved in the endocytic pathway?
the endosomes and the lysosome
Describe some characteristics of the Endoplasmic Reticulum
- 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
What are the two sub-components of the ER?
The Smooth ER and the Rough ER.
Describe the Rough ER.
- 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.
Describe the Smooth ER.
- 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
What does the ratio of RER (Rough ER) to SER (Smooth ER) depend on?
The type of cell and the activities of that cell
Specifically, what are the funcitons of the SER?
- 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
Specifically, what are the functions of the RER?
- 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
What are some general properties of the RER?
- The RER possesses a large surface area for ribosomes to attach to.
- The lumen of the RER cisternae provides a specialized local environment.
What is the function of the RER in relation to other organelles?
- 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
Approximately what percentage of polypeptides do RER membrane-bound ribosomes encode in the mammalian genome?
- about 33% (one-third of all polypeptides)
- this includes: secreted proteins, integral membrane proteins, and soluble proteins residing in the endomembrane system
For what proteins do “free” ribosomes encode for?
- Cytosolic proteins
- peripheral membrane proteins
- nuclear proteins
- proteins incorporated into chloroplats
- mitochondria
- peroxisomes
How do cells determine a site where proteins are made?
- Via the signal hypothesis.
What is the Signal Hypothesis? Further, explain the process the Signal Hypotheses states.
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
In what ways through the ER’s cisternal space can a polypeptide be synthesized?
It can be synthesized CO-TRANSLATIONALLY (while it’s being synthesized) or POST-TRANSLATIONALLY (after it has been made)
How are Secretory/Lysosomal Proteins made on Membrane-Bound Ribosomes?
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.
Once inside the lumen of the ER, how are the newly synthesized proteins processed?
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
The process of synthesizing Integral Membrane Proteins is similar to Secretory Protein synthesis, EXCEPT for one way. What is that?
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.
