Transport Vesicles in Eukaryotic Cells

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Questions and Answers

What role do transport vesicles play in eukaryotic cells?

They generate energy through ATP synthesis.

They bud from one compartment and fuse with another. (correct)

They metabolize nutrients directly in the cytoplasm.

They transport ions across the plasma membrane.

Which of the following organelles are isolated from the vesicular traffic in eukaryotic cells?

Endosomes and lysosomes

Ribosomes and rough ER

Nucleus and rough ER

Mitochondria and plastids (correct)

What structure is characterized by its ribosome-studded membrane and is involved in the secretion of digestive enzymes?

Rough ER (correct)

Nuclear envelope

Smooth ER

Lysosome

How do organelles communicate with the cell exterior?

By using transport vesicles. Signup and view all the answers

What is the primary function of the endoplasmic reticulum (ER) in eukaryotic cells?

Protein synthesis and processing. Signup and view all the answers

What are microsomes derived from after homogenization of tissues or cells?

Closed vesicles representing the ER Signup and view all the answers

What distinguishes rough microsomes from smooth microsomes?

Rough microsomes contain ribosomes on their surface Signup and view all the answers

What do membrane-bound ribosomes primarily synthesize?

Proteins meant for translocation across the ER membrane Signup and view all the answers

How do scientists typically separate rough and smooth microsomes?

Through density-based centrifugation Signup and view all the answers

What is a polyribosome?

A ribosome attached to mRNA for concurrent protein synthesis Signup and view all the answers

What is a characteristic of the nucleolus?

It functions as a biomolecular condensate. Signup and view all the answers

How does the total membrane surface area of the endoplasmic reticulum in liver and pancreatic cells compare to the plasma membrane?

It is larger, being 25 times and 12 times, respectively. Signup and view all the answers

What was a major evolutionary event in the formation of early eukaryotic cells?

The merging of an anaerobic archaeon with an aerobic bacterium. Signup and view all the answers

What is the relationship of the lumen of internal compartments in eukaryotic cells to the extracellular space?

The lumen is topologically equivalent to the extracellular space. Signup and view all the answers

During the evolution of eukaryotic cells, what was the fate of the endosymbiont?

It escaped into the cytosol and evolved into modern-day mitochondria. Signup and view all the answers

Study Notes

Intracellular Organization and Protein Sorting

The thousands of macromolecules and biochemical activities in a cell are spatially organized into different regions.
Eukaryotic cells are subdivided into membrane-enclosed compartments (organelles) unlike bacteria.
Key organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, plastids, and mitochondria.
Some organelles are further subcompartmentalized by internal membranes.
Biomolecular condensates (reversible assemblies) are specialized factories or temporary storage depots.
The nucleolus is an example of a biomolecular condensate, not enclosed by a membrane.

Animal Cell Protein Molecules

An animal cell contains about 10 billion protein molecules (perhaps 10,000 kinds).
Protein synthesis almost always begins in the cytosol (cytoplasm outside membrane-enclosed organelles).
Newly synthesized proteins are delivered to the specific organelles needing them.
Organelles have a unique protein and lipid composition to guide protein and lipid delivery.

Compartmentalization of Cells

Intracellular membrane systems create enclosed compartments separate from the cytosol.
This compartmentalization creates specialized aqueous spaces.
Subsets of molecules (proteins, reactants, ions) are concentrated in these spaces to optimize biochemical reactions.
Multiple types of compartments allow simultaneous biochemical reactions needing distinct conditions.

Relative Volumes of Intracellular Compartments in a Liver Cell

Cytosol occupies 54% of the total cell volume.
Mitochondria occupy 22%.
Rough ER cisternae occupy 9%.
Smooth ER cisternae occupy 5%.
Golgi cisternae occupy 6%.
Nucleus occupies 1%.
Peroxisomes occupy 1%.
Lysosomes occupy 1%.
Endosomes occupy 1%.

Relative Amounts of Membrane Types

In liver cells, plasma membrane constitutes only 2% of the total cell membrane.
Rough ER membrane makes up 35%.
Smooth ER membrane makes up 16%.
Golgi apparatus makes up 7%.
Intermediate filaments, microtubule, Mitochondria, secretory vesicle, lysosomes, peroxisomes, and endosomes membranes are present in significant amounts in liver cells as well. The percentage differs in cells of other tissues.

Evolutionary Origins of Organelles

Eukaryotic cells likely arose from the joining of an ancient anaerobic archaeon and aerobic bacterium.
Membrane expansion through protrusions and blebs may have stabilized the nascent nuclear pores.
The added surface area facilitated metabolite exchange.
Symbiotic relationship with the aerobic bacterium allowed the archaeon to increase in volume.

Topological Relationships of Organelles

The lumen of internal compartments are topologically equivalent to extracellular space.
Endosymbionts (evolved into mitochondria) escaped the enclosing membrane.
Expanded compartments became specialized in modern-day eukaryotic cells.
Cytosol and nucleus were originally the same.

Topologically Equivalent Compartments

Compartments were derived from the same primordial internal compartment.
Compartments can exchange material with one another via vesicular transport pathways; these pathways are consistent amongst cells.

Topologically Equivalent Pathways

Molecules can be transferred between compartments by transport vesicles.
Membrane budding and fusion permit communication between organelles and the cell's exterior.

Major Intracellular Compartments

Nucleus and cytosol (topologically equivalent).
Organelles in the secretory and endocytic pathways: ER and Golgi, endosomes, lysosomes, and transport vesicles.
Endosymbiont-derived organelles: mitochondria and, in plants, plastids.

Macromolecules Segregated Without Membranes

Macromolecule subsets can segregate within cells without a membrane barrier.
Nucleic acids or proteins can act as scaffolds in biomolecular condensates.
Scaffold molecules attract client molecules in a concentrated region.
Client molecules do not diffuse away readily under high concentration of binding sites for clients.

Examples of Biomolecular Condensates

Nucleolus (rRNA and ribosome assembly).
Pyrenoid (carbon fixation in algae).
Stress granules (temporary storage).
P-granules (RNA metabolism).
Balbiani body (RNA and organelle localization).
Cajal body (mRNA processing).
Paraspeckles (regulation of gene expression).
RNA transport granule (RNA localization).
PML body (regulation of gene expression).
Postsynaptic density (neuronal transmission).

Biomolecular Condensates as Biochemical Factories

Some condensates concentrate molecules for specific biochemical reactions.
Condensates are used for temporary storage and specialized chemical processes.
Examples from various cell types.

Biomolecular Condensates Form and Disassemble

Weak interactions are necessary for condensate formation and stability from a well-mixed system.
Changes in interaction strength can influence condensate properties (formation and physical properties).
The change can regulate the biological processes; an example is responding to the receptor-ligand interaction.

Proteins Move Between Compartments

Nearly all proteins are synthesized on ribosomes in the cytosol.
Protein sorting signals (in the amino acid sequence) direct proteins to their final location.
Four main ways proteins move between compartments: protein translocation, gated transport, vesicular transport, engulfment.

Protein Translocation

Transmembrane proteins cross the membrane directly.
Transmembrane protein molecule must unfold to snake through the translocator.
Translocation into the ER lumen, ER membrane, or mitochondria occurs in this way.

Gated Transport

Proteins and RNA molecules move between the cytosol and nucleus.
Nuclear pore complexes act as selective gates for macromolecule transportation.

Vesicular Transport

Membrane-enclosed transport intermediates, vesicles, or fragments of organelles transfer proteins between compartments.
Transport intermediates are loaded with molecules from the beginning compartment.
The contents are released at the destination compartment that it fuses with.

Endocytosis/Envelopment

Sheets (similar to vesicles) enclose parts of the cytoplasm to form a compartment.

Sorting Signals and Receptors

Sorting signals are amino acids in a protein (linear or three-dimensional arrangement).
Sorting signals direct proteins to the correct cellular location.
Signal sequences usually are at the N terminus, and they often are removed by enzymes.

Signal Sequence Specificity

Each signal sequence directs a protein to a particular cellular destination.
The sequences have specific characteristics distinguishing them. (positively charged, hydrophobic amino acids).

Transmembrane Protein Insertion

Transmembrane segments are recognized by SRP and Sec61.
Segments insert into lateral gate of Sec61 in particular orientations based on their characteristics.

Multipass Transmembrane Proteins

Multipass proteins comprise multiple transmembrane segments.
Orientation of each transmembrane segment depends on the adjacent segments and flanking regions.

Post-translational Translocation

Some proteins are completely synthesized completely prior to import into the ER.
Protein molecules are recognized by a pre-targeting complex.
Protein is transported by cycles of BiP binding and release.

Covalently Attached GPI Anchor

Some proteins acquire a covalently attached GPI anchor.
A hydrophobic segment near the C-terminus is recognized and cleaved.
The GPI anchor is attached to the rest of the protein.

Translocation Polypeptide Chains Folding-ER Lumen

Proteins enter ER lumen as unfolded polypeptides and must fold and assemble.
ER lumen contains high chaperone concentration to assist folding.

Oxidized Protein PDI

Protein disulfide isomerase (PDI) catalyzes disulfide bond formation in proteins.
This ensures correctness in protein folding in a harsh environment.

Proteins Synthesized in the Rough ER Are Glycosylated

Oligosaccharides are often added to proteins, primarily N-linked.
Oligosaccharides are attached at the asparagine side chains.
Precursor oligosaccharides are synthesized on membrane-bound dolichol or a lipid.
Oligosaccharides are flipped to the lumenal side of the ER.
They are modified in the Golgi.

###Oligosaccharides as Protein Folding Tags

Chaperone proteins (calnexin and calreticulin) recognize and bind oligosaccharides on incompletely folded proteins.
Prevents aggregation and promotes correct folding.
Glucose addition-removal cycle helps in determining proper folding.

Improperly Folded Proteins Exported

Misfolded proteins are exported from the ER to the cytosol
Chaperones help prevent aggregation and maintain unfolded conformation.
E3 ubiquitin ligase marks for degradation

Misfolded Proteins in the ER Activate Protein Response

Accumulation of misfolded proteins initiates ER unfolded protein response (UPR).
UPR modulates transcription processes in the nucleus.

ER Assembles Most Lipid Bilayers

The ER synthesizes nearly all major lipid classes for membranes.
Includes phospholipids, cholesterol and ceramides.
Phospholipid synthesis occurs in the cytosol.
Phospholipids can move between leaflets through scramblases.

Membrane Protein Contacts

ER and other organelles have contact sites for selective lipid transfer and protein transport.
Specialized lipid and protein channels allow for passage between compartments.

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Description

This quiz explores the crucial functions of transport vesicles in eukaryotic cells. Understand their role in intracellular transport and how they contribute to cellular organization and function. Test your knowledge on the mechanisms and processes involving these essential cellular components.