Biomedical Sciences I - Endomembrane System

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Questions and Answers

Which of the following organelles is NOT part of the endomembrane system?

Smooth endoplasmic reticulum

Rough endoplasmic reticulum

Mitochondria (correct)

Golgi apparatus

What is the main function of the Golgi apparatus in the endomembrane system?

Protein synthesis

Lipid synthesis

Protein modification and sorting (correct)

DNA replication

Which process describes how materials move between the organelles of the endomembrane system?

Direct contact between organelles

Active transport only

Passive diffusion

Transport vesicles (correct)

The biosynthetic pathway in the endomembrane system begins in which organelle?

Rough endoplasmic reticulum Signup and view all the answers

What role do motor proteins and cytoskeletal elements play in the endomembrane system?

They assist in the movement of transport vesicles Signup and view all the answers

Which of the following is NOT a function of the endomembrane system?

Energy production Signup and view all the answers

What type of proteins are synthesized in the rough endoplasmic reticulum?

Membrane proteins and lysosomal enzymes Signup and view all the answers

What is a key characteristic of the endomembrane system in eukaryotic cells?

It consists of distinct membrane-bound compartments Signup and view all the answers

In which way is the secretory pathway of the endomembrane system primarily utilized?

To transport proteins for secretion from the cell Signup and view all the answers

What happens to proteins as they pass through the Golgi apparatus?

They undergo extensive modification Signup and view all the answers

What is the primary role of the smooth endoplasmic reticulum in muscle cells?

Sequestering Ca2+ from the cytosol Signup and view all the answers

What process is responsible for the removal of excess smooth endoplasmic reticulum membrane after a drug has disappeared?

Autophagocytosis Signup and view all the answers

What is the primary function of the Golgi complex in a cell?

Modification and packaging of proteins Signup and view all the answers

What is the role of the enzymes present in different Golgi cisternae?

They facilitate distinct modifications of proteins. Signup and view all the answers

What mechanism enables proteins to be transported from the endoplasmic reticulum to the Golgi complex?

Vesicular transport Signup and view all the answers

Which of the following is a step in the glycosylation process of proteins in the Golgi complex?

Sequential removal and addition of specific sugar residues Signup and view all the answers

What type of proteins are specifically recognized by the mannose-6-phosphate receptor in the Golgi complex?

Lysosomal proteins Signup and view all the answers

During which phase of the Golgi function do vesicles bud from the peripheral tubular domain?

Trans phase Signup and view all the answers

What distinguishes the cis face of the Golgi complex?

It is the entry face for incoming vesicles. Signup and view all the answers

What is generated during the glycosylation of proteins in the Golgi?

Heterogeneous oligosaccharide structures Signup and view all the answers

What type of proteins are synthesized on ribosomes attached to the rough endoplasmic reticulum (RER)?

Water-soluble and integral membrane proteins Signup and view all the answers

What initiates the translocation of polypeptides into the rough endoplasmic reticulum?

The hydrophobic signal sequence recognized by the signal recognition particle (SRP) Signup and view all the answers

What role does glycosylation play in the function of proteins entering the rough endoplasmic reticulum?

Protects proteins from degradation and serves as a sorting signal Signup and view all the answers

How are integral membrane proteins integrated into the lipid bilayer of the rough endoplasmic reticulum?

They contain hydrophobic transmembrane segments that are shunted into the lipid bilayer from the translocon. Signup and view all the answers

What is the primary function of the smooth endoplasmic reticulum (SER)?

Detoxification of drugs and harmful compounds Signup and view all the answers

What happens to misfolded proteins within the rough endoplasmic reticulum?

They are degraded and their components recycled within the ER. Signup and view all the answers

What type of proteins undergo glycosylation within the rough endoplasmic reticulum?

Both water-soluble and integral membrane proteins Signup and view all the answers

What is the significance of the orientation of proteins synthesized in the rough endoplasmic reticulum?

It influences the protein's final destination and function. Signup and view all the answers

Which statement accurately describes glycoproteins produced in the endoplasmic reticulum?

Their carbohydrate chains are always found on the internal side of vesicles. Signup and view all the answers

What is a common outcome of congenital diseases of glycosylation?

Disruption of glycosylation pathways in the ER. Signup and view all the answers

What distinguishes the Smooth Endoplasmic Reticulum (SER) from the Rough Endoplasmic Reticulum (RER)?

Function in lipid metabolism and detoxification Signup and view all the answers

How does the ER identify the correct destination for proteins?

By amino acid sequence or attached oligosaccharides Signup and view all the answers

Which cellular compartments complete protein synthesis for proteins destined to the cytosol?

Free ribosomes in the cytosol Signup and view all the answers

What feature characterizes the lumen of the Endoplasmic Reticulum (ER)?

It is surrounded by the ER membrane and separate from the cytosol. Signup and view all the answers

What is the common length of the typical sorting signal on a protein?

15-60 amino acids Signup and view all the answers

What happens to the sorting signal of a protein after it has been successfully sorted?

It is often removed by specialized signal peptidases. Signup and view all the answers

What structure is continuous with the Endoplasmic Reticulum (ER)?

The outer nuclear membrane Signup and view all the answers

What property of the Endoplasmic Reticulum (ER) allows it to adapt to different cellular needs?

Its plastic and dynamic shape and size Signup and view all the answers

Which process does the Rough Endoplasmic Reticulum NOT participate in?

Synthesis of lipids Signup and view all the answers

Study Notes

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Biomedical Sciences I - Cellular Biology I

Course Instructor: Lorena Di Pietro
Email Address: [email protected]

The Endomembrane System

Found in all eukaryotic cells
Consists of distinct compartments, bounded by membranes.
Forms an extensive network of interconnected tubules and flattened sacs.
Includes: rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, lysosomes, and endosomes.

Endomembrane System - Summary

Organelles of the endomembrane system form a dynamic network.
Materials are continuously shuttled between compartments.
Materials move using small membrane-bound transport vesicles.
Transport happens in a directed manner.
Motor proteins and cytoskeletal elements play critical roles in transport vesicle movements.

Endomembrane System - Functions

Biosynthetic pathway: proteins are synthesized in the ER, modified in the Golgi, and sent to various destinations.
Secretory pathway: proteins are destined for secretion (e.g., extracellular matrix proteins, hormones).
Endocytic pathway: materials move from the cell surface to the intracellular space.

The Endoplasmic Reticulum (ER)

The ER is the largest subcellular structure in eukaryotic cells.
It's a network of membranous tubules and sacs (cisternae).
The interior space (lumen) is distinct from the cytosol.
The ER is continuous with the outer nuclear membrane.
The ER is dynamic and changes in shape and size.
The ER is divided into two distinct regions (rough and smooth ER) with different structures and functions.

Rough Endoplasmic Reticulum (RER)

The RER is continuous with the outer nuclear membrane.
Ribosomes are attached to the cytosolic surface of the RER membrane.
The presence of ribosomes gives the RER its rough appearance under electron microscopy.

Rough Endoplasmic Reticulum - Functions

Protein synthesis, folding, and initial glycosylation (adding sugar chains).
Protein quality control (detecting and addressing misfolded proteins).
Proteins are synthesized on the RER and often released into the lumen for further modification and transport.
Proteins destined for different organelles or secretion will use different pathways.
Proteins destined for cytosol, nucleus, mitochondria, and peroxisomes will be synthesized in free ribosomes outside ER and then directed to their specific locations.

Protein Synthesis

Proteins destined for the cytosol, nucleus, mitochondria, and peroxisomes are synthesized on free ribosomes.
Proteins destined for the ER, Golgi, lysosomes, plasma membranes, and secretion are synthesized on bound ribosomes and directed into the ER

Protein Maturation

Proteins are directed to their specific destinations by signals encoded in their amino acid sequences.
These signals allow the protein to be sorted and transported to the appropriate location.
Chemical modifications of newly translated polypeptides are sometimes needed during maturation.
Types of protein translocation are Co-translational and Post-translational

Protein Maturation - Mechanisms

Gated transport (for molecules moving through nuclear pores).
Protein translocation (for proteins transported across membranes).
Vesicular transport (for materials transported between organelles).

Rough Endoplasmic Reticulum - Functions cont

Synthesizes proteins for secretion and membrane insertion.
Modifies proteins (e.g., glycosylation).
Involved in protein quality control.
Synthesizes lipids, some carbohydrates.
Proteins that enter the RER are of two types (water-soluble and transmembrane).
Signals within proteins are recognized by the SRP and its receptor and direct the protein into the ER.
The signal sequences may be removed after protein translocation.

Rough Endoplasmic Reticulum - Glycosylation

Many proteins entering the RER are converted into glycoproteins, involving covalent attachment of branched oligosaccharide chains.
Very few cytosol proteins are glycosylated.
Oligosaccharyltransferase adds carbohydrates to nascent proteins.
Glycosylation plays crucial roles, like protection from degradation, folding assistance, and sorting.

Rough Endoplasmic Reticulum - Folding and Quality Control

Newly synthesized glycoproteins are screened for proper folding and quality control by molecular chaperones.
Misfolded proteins are recognized, diverted to the cytosol, and degraded.

Congenital Diseases of Glycosylation

Inherited metabolic disorders affect organ function.
Missing enzymes required for glycosylation result in partial disruption of the pathway.
Inherited diseases, such as cystic fibrosis, can stem from mutations.

Smooth Endoplasmic Reticulum (SER)

Lacks ribosomes.
Usually smaller than RER.
Located further from the nucleus compared to RER.
Forms tubular cavities, continuous with RER.

Smooth Endoplasmic Reticulum - Functions

Sequestering calcium ions within the cytoplasm.
Synthesizing steroid hormones (in endocrine glands).
Detoxifying drugs and harmful compounds (in liver cells).
Producing lipoprotein particles (in liver cells).
Modifies proteins and lipids after their synthesis.

Rough Endoplasmic Reticulum - Functions cont.

Synthesis of membrane lipids (excluding those also made in the Golgi).
Enzymes involved in lipid synthesis have their active sites facing the cytosol.
Detoxification of lipid-soluble molecules.
Synthesis and modification of phospholipids, cholesterol, and glycolipids.

Golgi Complex

Stack of flattened, membrane-bound sacs with associated tubules and vesicles.
Varying number (few to thousands) per cell.
Polarized organelle; categorized as cis-, medial-, and trans- Golgi.

Golgi Complex - Functions

Modification, sorting, and packaging of proteins and lipids.
Glycosylation of proteins by adding carbohydrates.
Synthesis of glycoproteins, glycolipids, and sphingomyelin.
Lipid metabolism; Modification of the carbohydrate portions of glycoproteins.
Processing proteins and lipids to make them suitable for transport and delivery to specific parts of the cell or secretion from the cell entirely.

Golgi Complex - Protein Modification

The Golgi modifies N-linked oligosaccharides added in the ER.
Removing and adding sugars in a particular order.
Modifying proteins for their final destinations.

Golgi Complex - Lysosomal Proteins

Some proteins targeted for lysosomes are modified by addition of mannose-6-phosphate.
This mark directs them to specific receptors in the Golgi, ultimately shipping them to lysosomes.
Modifications are essential for targeted protein trafficking.

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Description

This quiz covers the endomembrane system found in eukaryotic cells, including key organelles such as the rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, and endosomes. Understand the structure and function of this dynamic network essential for cellular processes.