Cell Biology 1

Question 1

Question: What is the significance of selective permeability in membranes?

Answer 1

Membranes exhibit selective permeability, regulating the passage of molecules within intracellular compartments

Question 2

What role do membranes play in intracellular compartments?

Answer 2

Cell and organelle membranes act as barriers within intracellular compartments.

Question 3

How is protein transport controlled within the cell?

Answer 3

The selective permeability of membranes controls protein transport within the cell.

Question 4

What is protein sorting or targeting, and why is it essential?

Answer 4

Protein sorting or targeting is the delivery of newly synthesized proteins to their correct cellular locations. It ensures the precise localization of proteins within the cell for efficient functioning.

Question 5

Briefly describe the two pathways for sorting proteins.

Answer 5

Pathway 1: Non-secretory proteins are targeted to the organelle membrane during or after translation, residing in the organelle’s interior.
Pathway 2: Secretory pathway begins in the endoplasmic reticulum (ER), not restricted to proteins for secretion, including those for the Golgi, endosomes, lysosomes, or cell membrane.

Question 6

Briefly describe Pathway 1 in protein sorting

Answer 6

Pathway 1: Non-secretory proteins are targeted to the organelle membrane during or after translation, residing in the organelle’s interior.

Question 7

Briefly describe Pathway 2 in protein sorting

Answer 7

Secretory pathway begins in the endoplasmic reticulum (ER), not restricted to proteins for secretion, including those for the Golgi, endosomes, lysosomes, or cell membrane.

Question 8

What is the role of a sorting signal in protein sorting?

Answer 8

A sorting signal directs proteins to the correct organelle, ensuring accurate protein localization within the cell.

Question 9

How are proteins in the cytosol dispatched to different locations?

Answer 9

Proteins in the cytosol are dispatched based on the address labels present in their amino acid sequence, with the sorting signal acting as the address label.

Question 10

What are the three mechanisms for importing proteins into membrane-enclosed organelles?

Answer 10

1. Transport through nuclear pores.
2. Transport across membranes using protein translocators.
3. Transport by vesicles from the endoplasmic reticulum (ER) onward.

Question 11

Describe the structure and function of nuclear pores.

Answer 11

Nuclear Pore Complex (NPC) is a multiprotein aqueous channel allowing the passage of molecules. It consists of 30 nucleoporins, serving as gates for molecular transport between the nucleus and cytosol.

Question 12

Where are histones, transcription factors, DNA polymerase, and RNA polymerase synthesized, and what are their functions?

Answer 12

Synthesized in the cytoplasm, these proteins function in DNA packaging, gene regulation, DNA synthesis, and RNA synthesis, respectively, within the nucleus.

Question 13

How do smaller molecules and larger molecules pass through nuclear pores?

Answer 13

Smaller molecules pass easily, while larger molecules require a Nuclear Localization Signal (NLS).

Question 14

Outline the key steps in protein import via nuclear pores.

Answer 14

1. Recognition of NLS by nuclear import receptors.
2. Receptor interaction with fibrils extending from the nucleus.
3. Attachment to short amino acid sequences in the pore’s centre.
4. Protein delivery to the nucleus, with the receptor returning to the cytosol.
5. The process requires energy through GTP hydrolysis.

Question 15

Name two types of proteins for transport, and describe their destinations.

Answer 15

1. Water-soluble Proteins: Completely translocated across organelle membranes, destined for secretion or organelle lumens.
2. Transmembrane Proteins: Partly translocated across membranes, becoming embedded, with destinations in the ER membrane, other organelle membranes, or the plasma membrane.

Question 16

What is an ER signal sequence, and what is its role?

Answer 16

An ER signal sequence is a hydrophobic segment of 8 or more amino acids, guiding translocation across the ER membrane during protein synthesis.

Question 17

How is protein import into the ER orchestrated?

Answer 17

Proteins with ER signals are directed to the ER during translation. Polyribosomes, when encoding such proteins, become riveted to the ER membrane.

Question 18

What guides the ER signal to the ER during soluble protein import?

Answer 18

ER signal is guided by the Signal-recognition Particle (SRP) and its receptor, present

Question 19

What is the function of Signal-recognition Particle (SRP) in soluble protein import into the ER?

Answer 19

SRP, present in the cytosol, binds to the ER signal sequence exposed on the ribosome during protein synthesis, initiating the import process.

Question 20

Describe the process of soluble protein import into the ER involving SRP.

Answer 20

1. SRP binds to the ER signal sequence and ribosome, slowing down protein synthesis.
2. The SRP-ribosome complex binds to the SRP receptor in the ER membrane.

3.The complex moves to the translocation channel, where SRP is released.

4. Synthesis resumes, and the polypeptide is threaded across the lipid bilayer.

Question 21

What is the protein that binds the ER signal sequence, and where does it bind?

Answer 21

The protein that binds the ER signal sequence is the SRP receptor, and it is embedded in the ER membrane.

Question 22

Differentiate between single-pass and double-pass transmembrane proteins.

Answer 22

Single-pass: Cross the membrane once.

Double-pass: Traverse the membrane in both directions, crossing back and forth.

Question 23

Single-pass transmembrane protein

Answer 23

Cross the membrane once.

Question 24

Double-pass transmembrane

Answer 24

Traverse the membrane in both directions, crossing back and forth.

Question 25

Explain the process of translocation for single-pass transmembrane proteins.

Answer 25

1. Initiated by an N-terminal signal sequence. 2. Halted by a stop transfer sequence. 3. α-helical membrane segment anchors the protein. 4. N-terminal signal is cleaved, placing N-terminus in the lumen and C-terminus in the cytosol.

Question 26

How does the translocation of double-pass transmembrane proteins differ?

Answer 26

Internal sequence: Start-transfer sequence initiates translocation. Process: Continues until a stop-transfer sequence is reached. Sequences: Both the start-transfer and stop-transfer sequences remain in the lipid bilayer as α-helices.

Question 27

In what form do transmembrane proteins cross the ER membrane?

Answer 27

Transmembrane proteins cross the ER membrane in an unfolded form.

Question 28

What is the purpose of disulphide bonds in proteins?

Answer 28

Disulphide bonds stabilize the conformation of proteins, crucial for maintaining their structure in dynamic cellular environments.

Question 29

How does glycosylation modify proteins in the ER?

Answer 29

Glycosylation involves the covalent attachment of short oligosaccharide side chains to proteins, contributing to their stability, proper folding, and functionality.

Question 30

What is the process of adding sugars during glycosylation, and where does it occur?

Answer 30

Pre-formed oligosaccharides, originating from dolichol in the ER membrane, are attached to proteins during translocation in the ER.

Question 31

Describe the catalysis of glycosylation and its further processing.

Answer 31

Catalysed by oligosaccharide protein transferase in the ER, further processing of oligosaccharides occurs in the Golgi apparatus, resulting in mature glycoproteins.

Question 32

What is the role of ER retention signals, and how are proteins transported to other locations from the ER?

Answer 32

ER retention signals keep proteins in the ER, while proteins for other locations are packaged into transport vesicles, which bud from the ER and fuse with the Golgi apparatus.

Question 33

How do chaperone proteins contribute to quality control in the ER?

Answer 33

Chaperone proteins prevent the exit of misfolded or partially assembled proteins from the ER, ensuring proper protein folding.

Question 34

What is the Unfolded Protein Response (UPR), and how does it contribute to cellular adaptation?

Answer 34

The UPR is triggered by an accumulation of misfolded proteins, leading to increased ER production and chaperone proteins to restore protein folding homeostasis.

Question 35

How does the size of the ER adjust to handle protein synthesis, and what happens in conditions like adult-onset diabetes?

Answer 35

The UPR allows the cell to adjust the size of the ER based on the volume of proteins entering the secretory pathway. In diseases like adult-onset diabetes, ER stress may trigger cell death if the ER capacity is exceeded.

Question 36

What distinguishes the secretory pathway from the non-secretory pathway in protein sorting?

Answer 36

The secretory pathway initiates in the endoplasmic reticulum (ER) and includes proteins for various destinations, not limited to secretion from the cell.