Cell Biology Chapter 12

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Questions and Answers

What is the primary function of the ER in most eukaryotic cells?

To sequester Ca2+ from the cytosol (correct)

To synthesize proteins

To regulate cell growth

To store genetic information

What happens to the signal sequence after it serves its function?

It is translated into a functional protein

It is embedded in the ER membrane

It is transported to the Golgi apparatus

It is cleaved off by a signal peptidase (correct)

What is the function of the Signal-Recognition Particle (SRP)?

To translate mRNA into protein

To direct proteins to the Golgi apparatus

To recognize the ER signal sequence (correct)

To regulate gene expression

What is the name of the organelle that the ER is in close contact with to regulate phosphoinositide levels?

Plasma membrane Signup and view all the answers

What is the term for the sequence of amino acids that directs a protein to the ER?

Signal sequence Signup and view all the answers

What is the name of the receptor that the SRP guides the signal sequence to?

SRP receptor Signup and view all the answers

What is the type of ER involved in transporting newly synthesized proteins and lipids?

Transitional ER Signup and view all the answers

What is the term for the process by which the signal sequence is removed?

Cleavage Signup and view all the answers

What is the structure of SRP in animal cells?

Six different polypeptide chains bound to a single RNA molecule Signup and view all the answers

What is the purpose of SRP binding to the signal sequence?

To ensure protein is not released into the cytosol Signup and view all the answers

Where does SRP bind to the ribosomal subunit?

At the interface between the large and small ribosomal subunits Signup and view all the answers

What is the role of the SRP receptor?

To bring the SRP-ribosome complex to an unoccupied protein translocator in the ER membrane Signup and view all the answers

What happens to the part of SRP bound near the ribosomal tunnel during protein translocation?

It moves to a different site Signup and view all the answers

What is the function of the translocator?

To translocate proteins across the ER membrane Signup and view all the answers

What is the characteristic of membrane-bound ribosomes?

They are attached to the cytosolic side of the ER membrane and are involved in protein synthesis concurrent with translocation Signup and view all the answers

What is the result of SRP binding to the signal sequence on protein synthesis?

It slows down protein synthesis Signup and view all the answers

What is the purpose of ER tubes and sheets?

To wrap around decondensing chromosomes and form a sealed double-membrane envelope Signup and view all the answers

What is the main difference between signal sequence and batch sequence?

Signal sequence is linear, while batch sequence is three-dimensional Signup and view all the answers

Where are the linear signal sequences for protein translocation often found?

At the N-terminus of the polypeptide chain Signup and view all the answers

What happens to the N-terminal signal sequences once the sorting process is complete?

They are removed from the finished protein by signal peptidases Signup and view all the answers

What is characteristic of the signal sequence for protein import into the nucleus?

It is composed primarily of positively charged amino acids Signup and view all the answers

What is the characteristic of the signal sequence for proteins destined for peroxisomes?

It is composed of three characteristic amino acids at the C-terminus Signup and view all the answers

What is the approximate percentage of the total membrane of an average animal cell that is constituted by the ER membrane?

Around 50% Signup and view all the answers

What is the organization of the ER?

A netlike labyrinth of branching tubules and flattened sacs Signup and view all the answers

What is the role of the SRP and SRP receptor in co-translational translocation?

To bring the ribosome to the membrane Signup and view all the answers

What is the function of the Sec62–Sec63 complex in post-translational translocation in eukaryotic cells?

To position BiP molecules where they can bind to the translocating chain Signup and view all the answers

What is the role of the SecA ATPase in post-translational translocation in bacteria?

To feed the completed polypeptide chain into the SecY translocator Signup and view all the answers

What is the driving force for protein import after a precursor has initially inserted into the Sec61 translocator?

The energy-driven cycle of binding and release Signup and view all the answers

What is the function of BiP molecules in post-translational translocation in eukaryotic cells?

To bind to the translocating chain as it emerges from the translocator Signup and view all the answers

Where is the SecA ATPase found?

Exclusively in bacteria Signup and view all the answers

What is the primary function of the cytosol in eukaryotic cells?

Protein synthesis and degradation Signup and view all the answers

What is the main function of the rough ER in eukaryotic cells?

Intermediary metabolism Signup and view all the answers

What is the function of the Golgi apparatus in eukaryotic cells?

Receiving and dispatching lipids and proteins Signup and view all the answers

What is the function of lysosomes in eukaryotic cells?

Digesting defunct intracellular organelles Signup and view all the answers

What determines the final location of a protein in a eukaryotic cell?

The protein's amino acid sequence Signup and view all the answers

What is responsible for recognizing sorting signals in proteins?

Sorting receptors Signup and view all the answers

What happens to proteins that do not have any sorting signals?

They remain in the cytosol as permanent residents Signup and view all the answers

Which organelles are responsible for generating most of the ATP in eukaryotic cells?

Mitochondria and chloroplasts Signup and view all the answers

Study Notes

Eukaryotic Cell Structure and Function

The nucleus contains the genome and is surrounded by cytoplasm, which consists of cytosol and cytoplasmic organelles.
The cytosol is the main site of protein synthesis and degradation, accounting for more than half of the cell's total volume.
The endoplasmic reticulum (ER) performs most of the cell's intermediary metabolism, with about half of the total membrane area in a eukaryotic cell enclosing the ER's labyrinthine spaces.

Organelle Functions

The Golgi apparatus receives lipids and proteins from the ER and dispatches them to various destinations, often covalently modifying them en route.
Lysosomes contain digestive enzymes that degrade defunct intracellular organelles.
Mitochondria and chloroplasts generate most of the ATP used by cells to drive reactions requiring an input of free energy.
Peroxisomes are small vesicular compartments that contain enzymes used in various oxidative reactions.

Protein Sorting and Transport

Nearly all proteins, except those inside mitochondria and plastids, begin their synthesis on ribosomes in the cytosol.
The final location of each protein depends on its amino acid sequence, which can contain one or more sorting signals that direct its delivery to different parts of the cell.
Sorting signals are recognized by complementary sorting receptors that mediate movement between compartments.
Proteins that do not have any sorting signals remain in the cytosol as permanent residents.

Signal Sequences and Sorting Receptors

Signal sequences are usually composed of amino acid side chains and come in two general varieties: signal patch and signal sequence.
Signal sequences direct proteins to different intracellular locations, such as the ER, mitochondria, nucleus, and peroxisomes.
The signal for protein import into the nucleus is composed primarily of positively charged amino acids.
A sorting signal for any particular destination needs to be sufficiently distinctive from all other sequences to permit its selective recognition by the appropriate sorting receptor.

Endoplasmic Reticulum (ER)

The ER membrane typically constitutes more than half of the total membrane in an average animal cell.
The ER is organized into a netlike labyrinth of branching tubules and flattened sacs that extend throughout the cytosol.
The ER is specialized in regions that make intimate contacts with other organelles, such as mitochondria, plastids, endosomes, and the plasma membrane.
The ER can sequester Ca2+ from the cytosol, and its release and reuptake occur in many rapid responses to extracellular signals.

Signal Sequence Discovery

Signal sequences were discovered in secreted water-soluble proteins that are first translocated across the ER membrane.
The signal hypothesis was formulated to explain these observations, proposing that the mRNA for the secretory protein codes for a protein that is bigger than the protein that is eventually secreted.

Signal-Recognition Particle (SRP) and Protein Translocation

The ER signal sequence is guided to the ER membrane by the SRP, which binds to the signal sequence, and an SRP receptor in the ER membrane.
SRP is a large complex that consists of six different polypeptide chains bound to a single RNA molecule in animal cells.
The SRP exposes a binding site for an SRP receptor, which brings the SRP-ribosome complex to an unoccupied protein translocator in the ER membrane.
The translocator transfers the growing polypeptide chain across the membrane, and SRP and SRP receptor are then released, allowing protein synthesis to resume at full speed.

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Description

This quiz covers the major intracellular compartments common to eukaryotic cells, including the nucleus, cytoplasm, cytosol, and endoplasmic reticulum. Learn about protein synthesis and degradation in cells.