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?

    <p>Plasma membrane</p> Signup and view all the answers

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

    <p>Signal sequence</p> Signup and view all the answers

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

    <p>SRP receptor</p> Signup and view all the answers

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

    <p>Transitional ER</p> Signup and view all the answers

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

    <p>Cleavage</p> Signup and view all the answers

    What is the structure of SRP in animal cells?

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

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

    <p>To ensure protein is not released into the cytosol</p> Signup and view all the answers

    Where does SRP bind to the ribosomal subunit?

    <p>At the interface between the large and small ribosomal subunits</p> Signup and view all the answers

    What is the role of the SRP receptor?

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

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

    <p>It moves to a different site</p> Signup and view all the answers

    What is the function of the translocator?

    <p>To translocate proteins across the ER membrane</p> Signup and view all the answers

    What is the characteristic of membrane-bound ribosomes?

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

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

    <p>It slows down protein synthesis</p> Signup and view all the answers

    What is the purpose of ER tubes and sheets?

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

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

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

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

    <p>At the N-terminus of the polypeptide chain</p> Signup and view all the answers

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

    <p>They are removed from the finished protein by signal peptidases</p> Signup and view all the answers

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

    <p>It is composed primarily of positively charged amino acids</p> Signup and view all the answers

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

    <p>It is composed of three characteristic amino acids at the C-terminus</p> 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?

    <p>Around 50%</p> Signup and view all the answers

    What is the organization of the ER?

    <p>A netlike labyrinth of branching tubules and flattened sacs</p> Signup and view all the answers

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

    <p>To bring the ribosome to the membrane</p> Signup and view all the answers

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

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

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

    <p>To feed the completed polypeptide chain into the SecY translocator</p> Signup and view all the answers

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

    <p>The energy-driven cycle of binding and release</p> Signup and view all the answers

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

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

    Where is the SecA ATPase found?

    <p>Exclusively in bacteria</p> Signup and view all the answers

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

    <p>Protein synthesis and degradation</p> Signup and view all the answers

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

    <p>Intermediary metabolism</p> Signup and view all the answers

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

    <p>Receiving and dispatching lipids and proteins</p> Signup and view all the answers

    What is the function of lysosomes in eukaryotic cells?

    <p>Digesting defunct intracellular organelles</p> Signup and view all the answers

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

    <p>The protein's amino acid sequence</p> Signup and view all the answers

    What is responsible for recognizing sorting signals in proteins?

    <p>Sorting receptors</p> Signup and view all the answers

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

    <p>They remain in the cytosol as permanent residents</p> Signup and view all the answers

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

    <p>Mitochondria and chloroplasts</p> 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.

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