Endoplasmic Reticulum Structure and Function

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Questions and Answers

What is the primary function of ceramides in eukaryotic cells?

They synthesize cellular proteins.

They provide energy for cellular metabolism.

They form structural components of cell membranes. (correct)

They transport solutes across the membrane.

Where is the majority of cellular cholesterol found?

In the plasma membrane. (correct)

In the mitochondria.

In the endoplasmic reticulum.

Within cytosolic vesicles.

What are the primary components synthesized on the cytosolic side of the ER membrane?

Nucleic acids and ribosomes.

Proteins and carbohydrates.

Phospholipids from glycerol and fatty acids. (correct)

Cholesterol and sphingomyelin.

What role do carrier proteins play in eukaryotic cells?

They facilitate the movement of solutes across the membrane. Signup and view all the answers

Which of the following phospholipids is formed directly from phosphatidic acid?

Phosphatidyl inositol. Signup and view all the answers

What characterizes rough endoplasmic reticulum (RER)?

It has ribosomes on its outer surface. Signup and view all the answers

Which function is NOT associated with smooth endoplasmic reticulum (SER)?

Protein folding. Signup and view all the answers

Which type of cell would most likely have a well-developed smooth endoplasmic reticulum?

Muscle cells. Signup and view all the answers

What is one of the metabolic functions of smooth endoplasmic reticulum in liver cells?

Detoxification of harmful drugs. Signup and view all the answers

What is the main structural characteristic of endoplasmic reticulum?

It is a network of membrane-enclosed tubules and sacs. Signup and view all the answers

Which lipid is NOT synthesized by the endoplasmic reticulum?

Triglycerides. Signup and view all the answers

What distinguishes the smooth endoplasmic reticulum (SER) from the rough endoplasmic reticulum (RER)?

Involvement in lipid metabolism. Signup and view all the answers

Ceramides are composed of which two components?

Sphingosine and a fatty acid. Signup and view all the answers

What is the primary role of actin and myosin in muscle cells?

They work together to generate muscle contractions. Signup and view all the answers

What is the significance of the sarcomeric Z-disc?

It defines the lateral borders of the sarcomere. Signup and view all the answers

Which protein undergoes a structural change upon binding to calcium ions?

Troponin Signup and view all the answers

What initiates the process of muscle contraction?

The release of acetylcholine from a motor neuron. Signup and view all the answers

How does myosin contribute to muscle contraction?

It uses ATP energy to move its head groups towards the center of the sarcomere. Signup and view all the answers

What effect do calcium ions have on tropomyosin?

They move tropomyosin away from the myosin-binding sites. Signup and view all the answers

Which statement about actin and myosin filaments is correct?

Actin filaments are thinner and essential for muscle contraction. Signup and view all the answers

What role does the sarcoplasmic reticulum play in muscle contraction?

It releases calcium ions when stimulated. Signup and view all the answers

What is the primary function of the cytochrome P450 enzyme system in liver hepatocytes?

Metabolism of drugs Signup and view all the answers

Which enzyme is specifically involved in the breakdown of glycogen into glucose in the liver?

Glucose-6-phosphatase Signup and view all the answers

What is one of the main ways lipids leave the smooth endoplasmic reticulum (SER)?

By vesicular transport Signup and view all the answers

Granular endoplasmic reticulum (GER) is predominantly involved in which cellular process?

Protein synthesis Signup and view all the answers

In which type of cells is granular endoplasmic reticulum (GER) particularly well developed?

Protein-secreting cells Signup and view all the answers

Which of the following types of proteins is NOT synthesized in the granular endoplasmic reticulum (GER)?

Steroid hormones Signup and view all the answers

Which of the following statements about the rough endoplasmic reticulum (RER) is true?

RER is covered by ribosomes. Signup and view all the answers

What is the main source of fuel for cells when not stored as glycogen?

Glucose Signup and view all the answers

What is the primary destination for proteins that have a signal peptide during synthesis?

Endoplasmic reticulum Signup and view all the answers

What initiates the halt in protein synthesis during the targeting process?

Recognition of the signal peptide by SRP Signup and view all the answers

Which component is responsible for returning to the cytosol after attaching to the ribosome?

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

What is the function of the GTP binding during the protein targeting process?

Regulates the dissociation of SRP from the receptor Signup and view all the answers

Which of the following conditions leads to the synthesis of proteins in the cytoplasm on free ribosomes?

Absence of a signal sequence Signup and view all the answers

What is the approximate length of the signal peptide that is crucial for ER targeting?

16-20 amino acids Signup and view all the answers

What is the role of ribophorin proteins in the ER?

They form hydrophilic channels in the ER membrane Signup and view all the answers

Which of the following proteins is responsible for docked ribosomes during protein synthesis?

SRP receptor Signup and view all the answers

What is the initial state of proteins as they are translocated across the ER membrane?

Unfolded polypeptide chain Signup and view all the answers

What role do molecular chaperones play in the ER?

They assist in the folding of polypeptides Signup and view all the answers

Which chaperone is responsible for binding to unfolded polypeptides during translocation?

Hsp70 Signup and view all the answers

What is the significance of disulfide bonds in protein processing in the ER?

They increase protein stability Signup and view all the answers

What happens to improperly assembled proteins in the ER?

They are targeted for degeneration Signup and view all the answers

How do proteins travel from the ER to the Golgi apparatus?

Via transport vesicles Signup and view all the answers

What must occur for proteins to be correctly released from the ER?

They must be fully assembled Signup and view all the answers

What type of molecules are transported from the ER to the Golgi apparatus?

Proteins and lipids Signup and view all the answers

Study Notes

Endoplasmic Reticulum (ER) Structure and Function

The ER is a network of membrane-enclosed tubules and sacs (cisternae) that extends from the nuclear membrane throughout the cytoplasm.
It's the largest organelle in most eukaryotic cells.
The ER can comprise greater than 10% of the total cell volume.

Types of ER

Rough ER (RER) or Granular ER (GER): Covered by ribosomes on its outer surface.
- Involved in protein production, processing, folding, and quality control.
- Not associated with ribosomes.
Smooth ER (SER) or Agranular ER:
- Tubular or vesicular in form.
- Not involved in protein synthesis.
- Functions in lipid metabolism in different cell types, including lipid synthesis, metabolism, muscle contraction, and detoxification.

SER Functions

Lipid synthesis: Synthesizes phospholipids, cholesterol, and ceramide, the essential components of cell membranes.
Steroid hormone biosynthesis: Synthesizes steroid hormones.
Metabolism in liver cells: Metabolizes lipid-soluble compounds, harmful drugs, and alcohol, by converting them to water-soluble compounds for excretion in the urine.
Muscle contraction: Participates in the contraction process in muscle cells as sarcoplasmic reticulum (SR).
Lipid transport: Involved in lipid transport.

Phospholipids

Phospholipids are a group of lipids that consist of two hydrophobic fatty acids, a hydrophilic glycerol unit, and a phosphate group which is esterified to an organic molecule (e.g., choline, ethanolamine, inositol).

Sphingolipids

Ceramides are a family of waxy lipid molecules composed of sphingosine and a fatty acid.
Ceramides are found in high concentrations within the cell membrane of eukaryotic cells.
Component lipids of sphingomyelin, one of the major lipids in the lipid bilayer.

Cholesterol

Cholesterol is a sterol (or modified steroid) lipid.
More than 90% of cellular cholesterol is located at the plasma membrane.
High cholesterol can lead to fatty deposits in blood vessels.

Smooth ER and Lipid Synthesis

ER is the major site for synthesizing membrane lipids in eukaryotic cells.
Membrane lipids are synthesized in association with already existing membranes (hydrophobic) rather than in the aqueous environment of the cytosol.
Membrane lipids are then transported from the ER to their final destinations via vesicles or carrier proteins.

Smooth ER and Lipid Synthesis: Vesicles

A vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer.

Smooth ER and Lipid Synthesis: Carrier Protein

A carrier protein is a membrane protein that moves solutes across the membrane by creating conformational changes in the protein.

Smooth ER and Lipid Synthesis (Phospholipids): Chemical Process

Most phospholipids are derived from glycerol.
Synthesized on the cytosolic side of the ER membrane from water-soluble cytosolic precursors.
Resulting phospholipid (e.g., phosphatidic acid) is inserted into the membrane.
Enzymes on the cytosolic side of the ER membrane then modify phosphatidic acid or catalyze the addition of different polar head groups, including phosphatidyl choline, phosphatidyl serine, phosphatidyl ethanolamine, and phosphatidyl inositol.
These are formed from phosphatidic acid via a modified diacylglycerol.

SER and Steroid Hormones

SER contains the enzymes needed to synthesize steroid hormones, which are abundant in steroid-producing cells (e.g., Leydig's cells in testes, adrenal cortex cells, corpus luteum cells in ovaries).
Steroid hormones are synthesized from cholesterol.

SR and Muscle Contraction

SR (sarcoplasmic reticulum) is a smooth endoplasmic reticulum found within muscle cells.
The main function of the SR is to store calcium ions (Ca2+).
The SR specifically regulates Ca2+ ion flow; Ca2+ ions regulate muscular contraction.
Ca2+ ions bind to the Troponin C component of the actin filament, exposing binding sites for the myosin head to bind to; this stimulates muscle contraction.
SR membrane contains Ca2+-ATPase pumps.
SR system consists of a branching network of SER cisternae surrounding each myofibril.

Types of Muscle

Smooth muscle: Involuntarily contracts internal organs, blood vessels.
Cardiac muscle: Involuntarily contracts in the heart.
Skeletal muscle: Voluntarily contracts bones of the skeleton.

Muscle Structures

Myocytes are muscle cells.
Muscle tissue is composed of muscle fibers, which are bundles of tubular myofibrils.
Myofibrils are composed of repeating sarcomeres.
Sarcomeres are the functional units of a muscle fiber.
Actin and myosin are two proteins found in all muscle tissue. Myosin (15nm) is a thick filament, and actin (7nm) is a thinner filament.
Contraction is driven by the interaction of actin and myosin filaments.

Muscle Contractions

Actin and myosin work together for muscle contraction and movement; a motor neuron releases acetylcholine into the muscle cell.
Acetylcholine triggers the release of calcium ions from the sarcoplasmic reticulum.
Calcium ions bind to troponin, changing its shape and moving tropomyosin away from the myosin-binding sites on actin.
Myosin heads bind to actin, forming cross-bridges.
Myosin pulls actin filaments towards the center of the sarcomere (ATP energy).
The sarcomere shortens, causing the muscle fiber to contract.

SER and Liver Metabolic Reactions

Abundant in the liver, SER contains specific enzymes that metabolize lipid-soluble compounds and inactivates harmful drugs (e.g., phenobarbital), converting them into water-soluble compounds that are eliminated via the urine.
Also detoxifies alcohol and certain toxic compounds, certain hormones.
Cytochrome P450 is a large family of enzymes in the SER of liver cells; plays a significant role in drug metabolism.

Glycogen Breakdown

SER membranes contain Glucose-6-phosphatase in the liver.
This enzyme is involved in breaking down glycogen into glucose.

ER in Protein Synthesis

GER: Involved in protein synthesis and secretion.
- Prominent in cells specialized for protein synthesis.
Ribosomes: Attached to the GER's exterior.
- Detected by light microscope by staining basic dyes
Free ribosomes: Synthesize proteins destined to remain in the cytosol, nucleus, mitochondria, chloroplasts or peroxisomes.

Protein Synthesis in the GER(Detailed)

All protein synthesis begins on free ribosomes in the cytosol, protein synthesis is resumed.
Ribosomes synthesize secretory proteins and are subsequently targeted to the ER by a signal sequence (amino acid sequence) at the polypeptide chain's amino terminus.
The signal sequence is about 16–20 amino acids long and appears at the beginning of the growing polypeptide chain.
Signal peptides emerge; a special molecule (SRP; a protein-RNA complex) recognizes the sequence.
SRP binds to the signal peptide and ribosome. Protein synthesis halts.
SRP-ribosome complex attaches to the ER at specific sites (SRP receptors).
SRP is released, and synthesis resumes.
The growing polypeptide chain enters the translocon channel.
The signal peptide is removed by signal peptidase.
The polypeptide chain is released into the ER lumen.
The ribosome detaches.
Completed protein may be packaged into transport vesicles and move to Golgi. Further modifications may occur.

Post-translational Modifications

Newly synthesized polypeptides in the ER undergo five modifications:
1. Carbohydrate addition and processing
2. Disulfide bond formation
3. Specific proteolytic cleavages
4. Protein folding
5. Assembly into multimeric proteins

Protein Folding, Glycosylation

ER is the site of protein folding, assembly, disulfide bond formation, and initial glycosylation steps (modification of proteins with carbohydrates).
Luminal ER proteins' primary role is assisting newly translocated polypeptides' folding and assembly.
Translocated proteins enter the ER as unfolded peptides.
Chaperons (e.g., Bip/Hsp70) facilitate folding.
Disulfide bond formation enhances protein stability.

Export from ER

Correctly folded proteins are transported to the Golgi apparatus.
A protein export system in ER transports proteins through vesicles from the ER exit sites (ERES) through ER-Golgi intermediate compartment (ERGIC).
Incorrectly folded proteins are targeted for degradation.
ER exports proteins and lipids in vesicles, which carry them to the Golgi apparatus.

Additional Information

Details on video links provided at the end of the document.

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Description

This quiz covers the structure and functions of the Endoplasmic Reticulum (ER), focusing on its two types: Rough ER and Smooth ER. You'll explore the essential roles these organelles play in protein production and lipid metabolism. Test your knowledge on how the ER contributes to cellular functions and its significance in eukaryotic cells.