Cell Biology Chapter on Intracellular Transport

Questions and Answers

Which mechanism primarily directs proteins to lysosomes for degradation?

Mannose-6-phosphate tags (correct)

Signal recognition particles

Nuclear localization signals

Ribosome association signals

What role do nuclear pore complexes play in nuclear import?

They synthesize proteins in the nucleus.

They degrade misfolded proteins in the nucleus.

They store genetic material within the nuclear envelope.

They facilitate the transport of proteins across the nuclear envelope. (correct)

What is a key characteristic of receptor-mediated endocytosis?

It selectively takes up molecules via specific receptors. (correct)

It is an irreversible process.

It solely involves small molecules.

It does not require energy input.

Which statement best describes the endocytic pathway?

It encompasses the inward movement of various cargo from the extracellular space. Signup and view all the answers

How are proteins typically synthesized in the endoplasmic reticulum?

By ribosomes that are bound to the rough ER. Signup and view all the answers

Which of the following describes a role of the Golgi complex?

Modification and sorting of proteins for secretion. Signup and view all the answers

What is the function of GTPases in nuclear import?

They are involved in the import of proteins through nuclear pore complexes. Signup and view all the answers

Which of the following is NOT a type of transport mechanism for proteins?

Cytosolic diffusion pathway Signup and view all the answers

What is the primary role of sorting signals in protein targeting?

They direct proteins to their correct destinations within the cell. Signup and view all the answers

Which ribosome type synthesizes proteins destined for the lysosome?

Membrane-bound ribosomes attached to the endoplasmic reticulum. Signup and view all the answers

What initiates the binding of the ribosome complex to the SRP receptor during protein synthesis in the ER?

The SRP recognizing the ER signal sequence. Signup and view all the answers

Which process utilizes clathrin-coated vesicles for transport?

Endocytic transport to internalize membrane proteins. Signup and view all the answers

In the context of protein export from the ER, what is the vesicular transport mechanism called that moves proteins towards the Golgi?

Anterograde transport. Signup and view all the answers

Which protein coats are specifically associated with retrograde transport from the Golgi back to the ER?

COPI coats. Signup and view all the answers

What is the function of the translocon during protein synthesis in the ER?

It facilitates the insertion of polypeptides into the ER membrane. Signup and view all the answers

What characterizes the two types of ribosomes in terms of the locations of synthesized proteins?

Free ribosomes synthesize proteins for the cytosol, while ER-bound ribosomes synthesize proteins for secretion. Signup and view all the answers

What would happen to a protein without a sorting signal?

It would head to the cytosol. Signup and view all the answers

Which component is crucial for recognizing an ER signal sequence during the protein synthesis process?

Signal recognition particle (SRP). Signup and view all the answers

What is the first step in the lysosomal enzyme targeting process?

Mannose-6 is phosphorylated in the Golgi Signup and view all the answers

Which component mediates the selective import of proteins into the nucleus?

Nuclear pore complex Signup and view all the answers

What characterizes receptor-mediated endocytosis?

Selective uptake of specific molecules via receptors Signup and view all the answers

Which process involves the ingestion of fluids and small molecules?

Pinocytosis Signup and view all the answers

In the mechanism of nuclear import, what role does Ran-GTP play?

It displaces the cargo protein from importin Signup and view all the answers

What is formed when clathrin-coated vesicles fuse with late endosomes during lysosome formation?

Lysosomal enzymes Signup and view all the answers

Which signal assists in the import of proteins into the nucleus?

Nuclear localization signal (NLS) Signup and view all the answers

What is the pH level approximately at which lysosomal acid hydrolases function?

Acidic (pH ~ 5.5) Signup and view all the answers

How are proteins synthesized in the endoplasmic reticulum targeted for lysosomes?

Through Golgi apparatus processing Signup and view all the answers

What initiates the ingestion of large particles during phagocytosis?

Contact with macrophages Signup and view all the answers

Study Notes

Intracellular Transport Pathway

Intracellular transport involves movement of cargo within cells and between organelles and compartments.

Ribosomes and Protein Synthesis

Protein synthesis occurs on ribosomes, which are essential for translating mRNA into proteins.
Two types of ribosomes:
- Free ribosomes: Located in cytosol, synthesizing proteins for the nucleus, mitochondria, and peroxisomes.
- Membrane-bound (ER) ribosomes: Target proteins for the endoplasmic reticulum (ER), Golgi, endosomes, lysosomes, nuclear membrane, plasma membrane, and secretion.
Most animal cell proteins are synthesized in the ER.

Sorting Signals

Sorting signals are specific amino acid sequences that direct proteins to their correct destinations within the cell.
- Example: ER retention sequence KDEL (Lys-Asp-Glu-Leu).
Absence of a signal directs the protein to the cytosol.

Endoplasmic Reticulum (ER)

The ER is crucial for protein synthesis in the cell.
Components of ER synthesis:
- Signal Recognition Particle (SRP): Binds to the ER signal sequence.
- SRP Receptor: Located on ER membrane, facilitates ribosome binding.
Mechanism involves SRP recognizing the ER signal sequence, binding to the ribosome complex, inserting polypeptides into membrane channels, and releasing them into the ER lumen.

ER Export and Transport Types

ER Exit Site (ERES) specializes in vesicle budding for proteins with export signals.
Transport pathways:
- Anterograde: From ER to Golgi using COPII coats.
- Retrograde: From Golgi to ER using COPI coats.
- Clathrin-coated pathways: Involves endocytic and exocytic transport.

Vesicles for Protein Transport

Proteins are transported in vesicles through vesicle budding and fusion processes.
Vesicles utilize cytoskeletal elements for effective transport.

Clathrin Coats and Vesicle Budding

Vesicles have protein coats for budding from membranes (ER, Golgi, plasma membrane).
Clathrin-coated vesicles are involved in outward budding from the Golgi and inward budding from the plasma membrane.

Nuclear Import

Nuclear envelope structure supports selective import/export of proteins.
Nuclear pore complexes utilize nuclear localization signals (NLS) for targeting proteins.
Importins recognize NLS and transport proteins into the nucleus in conjunction with GTP-binding protein Ran.

Endocytic Pathway

The endocytic pathway permits the inward movement of cargo:
- Phagocytosis: Ingestion of large particles by macrophages.
- Pinocytosis: Ingestion of fluids and small molecules.
Receptor-mediated endocytosis selectively internalizes metabolites, hormones, and proteins through receptor-ligand interactions, often using clathrin-coated vesicles.

Lysosome Targeting

Sequential transport pathway: ER → Golgi → Endosomes → Lysosomes.
Lysosomal enzymes are tagged with mannose-6-phosphate (M6P) for targeting.
Clathrin-coated vesicles facilitate the transport of hydrolases to endosomes, maturing into lysosomes.

Protein Modifications

Proteins undergo modifications in the Golgi complex before being secreted or targeted to other organelles, playing a critical role in the secretory pathway.

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Description

This quiz focuses on intracellular transport pathways critical for movement of cargo within cells and between organelles. It also covers the role of ribosomes in protein synthesis, including the distinction between free and bound ribosomes. Test your knowledge on these essential concepts from Cell Biology.