Cell Biology: Endoplasmic Reticulum and Lysosomes

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Questions and Answers

What is the primary function of the smooth endoplasmic reticulum?

Synthesis of lipids and detoxification (correct)

Synthesis of proteins

Transport of proteins to the Golgi apparatus

Protein folding and quality control

Which of the following correctly describes the rough endoplasmic reticulum?

Detoxifies lipid-soluble drugs

Stores glucose as glycogen

Regulates calcium levels

Contains ribosomes for protein synthesis (correct)

How does the smooth endoplasmic reticulum contribute to calcium regulation?

By storing and releasing calcium ions (correct)

By detoxifying calcium-containing toxins

By synthesizing calcium-binding proteins

By converting calcium into glycogen

What occurs inside the rough endoplasmic reticulum after a polypeptide is synthesized?

It undergoes folding and quality control Signup and view all the answers

What is the role of the signal-recognition particle (SRP) in the rough endoplasmic reticulum?

To guide polypeptides to the ER membrane Signup and view all the answers

What is the primary function of free ribosomes in the cytoplasm?

Translate mRNA into proteins that remain in the cytoplasm. Signup and view all the answers

Which statement accurately describes the rough endoplasmic reticulum?

It has ribosomes attached, facilitating protein synthesis. Signup and view all the answers

What is produced when mRNA is translated by ribosomes?

Polypeptides which may fold into functional proteins. Signup and view all the answers

How does mRNA exit the nucleus to enter the cytoplasm?

Through nuclear pores. Signup and view all the answers

What is the primary role of the endoplasmic reticulum in the cell?

Assembly and transport of proteins and lipids. Signup and view all the answers

What is the primary function of lysosomes in the cell?

To digest macromolecules and recycle cellular components Signup and view all the answers

How do lysosomes maintain their acidic environment for optimal enzyme activity?

By actively transporting H+ ions into the lysosome Signup and view all the answers

Which type of material is NOT degraded by lysosomes?

Genetic material like DNA and RNA Signup and view all the answers

What role do tethering proteins play in the process of lysosome function?

They facilitate the fusion of vesicles with lysosomes Signup and view all the answers

What process allows cells to engulf large particles and transport them to lysosomes?

Phagocytosis Signup and view all the answers

What is the primary function of the cis face of the Golgi apparatus?

It receives vesicles from the endoplasmic reticulum. Signup and view all the answers

Which modification process in the Golgi apparatus involves the addition of sugars to proteins?

Glycosylation Signup and view all the answers

What feature of the Golgi apparatus allows proteins and lipids to be directed to their final destination?

The specific chemical modifications made Signup and view all the answers

What role do SNARE proteins play in vesicle fusion with the Golgi apparatus?

They ensure the vesicle properly docks to the target membrane. Signup and view all the answers

Which of the following best describes the Golgi apparatus structure?

A series of stacked, flattened membrane-enclosed sacs. Signup and view all the answers

What happens to proteins in the Golgi apparatus after being transported through the cisternae?

They are dispatched to their final destinations. Signup and view all the answers

What defines the trans face of the Golgi apparatus?

It dispatches proteins and lipids to their final destinations. Signup and view all the answers

Which component in the vesicle interacts with the target membrane to facilitate docking?

vSNARE Signup and view all the answers

What is the primary genetic cause of Tay Sachs disease?

Mutations in the HEXA gene Signup and view all the answers

Which population has a higher incidence of Tay Sachs disease?

Ashkenazi Jewish population Signup and view all the answers

What condition is associated with mitochondrial diseases?

Decreased ATP production Signup and view all the answers

Which symptom is NOT commonly associated with MELAS syndrome?

Visual loss Signup and view all the answers

Which clinical symptom is shared by both LHON and MELAS syndromes?

Cardiomyopathy Signup and view all the answers

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

Production of energy from nutrients Signup and view all the answers

Which of the following accurately describes the structure of the nucleus?

Composed of two lipid bilayers with pores Signup and view all the answers

How do small hydrophilic molecules primarily cross the cell membrane?

They require special transporter proteins Signup and view all the answers

What main role do lysosomes play in a cell?

Recycling damaged organelles Signup and view all the answers

What is the role of the Golgi apparatus in a eukaryotic cell?

Packaging and dispatching proteins and lipids Signup and view all the answers

Which organelle is the primary site for ribosome production?

Nucleolus Signup and view all the answers

What describes the function of the cell membrane?

It controls the import and export of various molecules. Signup and view all the answers

In which part of the cell does most metabolic activity occur?

Cytoplasm Signup and view all the answers

What is the primary component of the lipid bilayer of the cell membrane?

Phospholipids Signup and view all the answers

What occurs to chromatin when a cell prepares to divide?

It condenses into chromosomes. Signup and view all the answers

What does mRNA do after being transcribed from DNA?

It is translated into proteins at ribosomes. Signup and view all the answers

What type of molecules are embedded in the cell membrane to assist with transport?

Proteins Signup and view all the answers

Which part of the cell provides protection and maintains structure?

Cell membrane Signup and view all the answers

Which of the following accurately describes ribosomes?

Made of RNA and proteins Signup and view all the answers

Study Notes

Cellular and Molecular Biology MD105

Course taught by Dr. C. Michaeloudes at the European University Cyprus School of Medicine
Course focuses on cell structure and function, including organelles and associated functions

Lecture Objectives

Understand the structural organization of eukaryotic cells
Comprehend the structure and function of key intracellular organelles
Learn about intracellular protein transport from ribosomes to the endoplasmic reticulum and Golgi apparatus
Explore the role of lysosomes in recycling damaged organelles and extracellular materials
Study the role of mitochondria in energy production

Structural Organization of Eukaryotic Cells

Cells are microscopic factories, functioning like a factory.
The factory has different departments or organelles with specific roles.
The exterior defines the limits of the factory and controls what comes in and out

Cells are Microscopic Factories

The different "departments" (organelles) of the cell work together to produce and ship goods.
Each "department" performs a certain task (e.g. power plant creates power).

Cell Structure/Organelles

Mitochondrion: Produces energy from nutrients
Nucleus: Contains the DNA code
Lysosomes: Recycle damaged organelles and materials
Cell Membrane: Imports/exports molecules
Ribosomes/Endoplasmic Reticulum: Assembles and transports proteins/lipids
Golgi Apparatus: Packages and dispatches proteins/lipids

Cell Membrane and Cytoplasm

The cell membrane is a lipid bilayer
Semi-permeable - some molecules can pass through, some not
Contains proteins as channels, receptors, etc.
Protective layer around cells
Controls nutrient intake and waste removal
Acts as a sensor for environmental changes
Lipid bilayer structures made of phospholipids and cholesterol

Phospholipids and Cholesterol

Main components of cell membranes
Phospholipids have a hydrophilic head and hydrophobic tail
Cholesterol helps maintain membrane fluidity

Cell Membrane Lipid Bilayer

Membrane lipids face two opposing forces (attracted to water and repelled from water)
Hydrophilic heads face water on both sides of the bilayer
Hydrophobic tails stay in the bilayer interior forming the double layer

Cell Membrane Functions

Lipid bilayer controls what enters and leaves the cell
Small hydrophobic molecules (O2, CO2) pass rapidly
Small hydrophilic molecules (H2O) pass slowly
Large hydrophilic molecules (glucose, amino acids) and ions require special transporter proteins
Proteins control and perform other functions (transport, signaling, enzymes)

Cytoplasm

The part of the cell inside the cell membrane
Includes cytosol and intracellular organelles
Cytosol is an aqueous matrix with high concentrations of salts + macromolecules (proteins, sugars, lipids)
Majority of the cell's metabolic reactions occur in the cytosol

Nucleus

The control center of the cell
Information is stored in DNA
Separated from cytoplasm by the nuclear envelope
Consists of two lipid bilayer membranes with pores that allow movement of molecules in and out
Primary site of DNA and RNA synthesis
Nucleolus: structure involved in ribosome production
DNA is packaged in the nucleus as chromatin, fine threads disbursed throughout the nucleus
During cell division, chromatin condenses into chromosomes

Nucleus - DNA Replication and Transcription

DNA replication occurs in the nucleus
Genes are transcribed into mRNA in the nucleus
mRNA exits the nucleus through nuclear pores and is translated into proteins in ribosomes

Nucleolus - Ribosome Formation

Ribosome formation occurs in the nucleolus
Nucleolus is a non-membrane-bounded structure comprising RNA and protein
Contains genes for ribosomal RNA (a key part of ribosomes)
Formed transiently during cell division

Ribosomes

Protein production sites in the cell
Particles made up of RNA and proteins (ribonucleoproteins)
Not surrounded by a membrane
Found in two locations: cytoplasm (free ribosomes) and attached to the endoplasmic reticulum
mRNA code is translated into proteins; Free ribosomes translate proteins that remain in the cytoplasm (or incorporated into mitochondria and nucleus)
Endoplasmic reticulum ribosomes translate proteins incorporated into the cell membrane or destined for release

Ribosome Function

mRNA is released from the nucleus and attaches to ribosomes
Ribosomes translate mRNA into proteins
Free ribosomes translate proteins that stay in the cytoplasm
Endoplasmic reticulum ribosomes translate proteins that become part of the cell membrane or are released

Endoplasmic Reticulum

Assembly site for proteins and lipids
Continuous network of membranes (flattened sacs and tubes) attached to the nuclear membrane and extending into the cytoplasm
Two types: Rough ER (with ribosomes) and Smooth ER (no ribosomes)

Smooth Endoplasmic Reticulum Function

Lipid synthesis (e.g., cholesterol)
Steroid hormone production
Glycogen storage in liver, kidney, and muscle
Detoxification of lipid-soluble drugs and toxins via cytochrome P450 enzymes
Regulation of calcium levels (storage and release), crucial for bone structure and muscle/nerve function

Rough Endoplasmic Reticulum Function

Ribosomes attach if the polypeptide has a specific amino acid sequence (ER signal sequence)
Polypeptide synthesis is completed. Proteins are folded into 3D form; improper folding results in degradation
Proteins are packaged in vesicles and sent to the Golgi apparatus

Protein Synthesis and Folding in the ER

Ribosome translating polypeptide chain containing a signal sequence (amino acid sequence)
Signal recognition particle (SRP) binds to the signal sequence and the ribosome
SRP binds to the SRP receptor in ER membrane
Protein synthesized in the endoplasmic reticulum lumen
The polypeptide chain folds into its 3D structure

Folded Proteins

Folded proteins are packaged in vesicles and released from the endoplasmic reticulum
Vesicles are small fluid-filled structures enclosed by a lipid bilayer
Vesicles fuse with lipid membranes to release their contents

Vesicle Formation in the ER

Cargo protein recognized by cargo receptors in the ER membrane
Coat proteins assemble into spherical structures
Cargo is packaged into the formed vesicle and released from the ER

Golgi Apparatus

Protein/lipid modification and sorting
Sorting office of the cell
Receives proteins and lipids from the ER, modifies them, dispatches them to other destinations
Near the nucleus and endoplasmic reticulum
Stacks of flattened, membrane-enclosed sacs called cisternae, resembling a stack of pita breads
Cis face (entry) – vesicles from ER
Cisternae – contains enzymes for protein/lipid modifications (glycosylation and phosphorylation)
Trans face (exit) – dispatches proteins/lipids to other parts of the cell

Vesicle Fusion with Golgi

Tethering proteins on the target membrane make initial contact with incoming vesicles
SNARE proteins (v-SNAREs and t-SNAREs) dock the vesicle and target membranes
Vesicle and target membranes fuse releasing cargo proteins into the Golgi apparatus

Lysosomes

Recycling site of the cell
Contains enzymes that break down extracellular materials, damaged organelles and other materials
The resulting components are re-used or excreted
Lysosomes are membrane-enclosed organelles rich in hydrolytic enzymes (digestive enzymes) and are responsible for the breakdown of macromolecules.
Work best at acidic pH (maintained with an H+ pump)
Materials destined for degradation are transported in vesicles and fuse with lysosomes allowing for breakdown

Lysosome Function

Three types of materials are degraded:
- Extracellular large particles (phagocytosis)
- Extracellular macromolecules (endocytosis)
- Damaged organelles (autophagy)
Lysosomes contain hydrolases that break down various substances, such as proteins, lipids, carbohydrates, and nucleic acids

Mitochondria

The power generators of the cell
Release energy from nutrients (e.g., glucose, fatty acids)
Number of mitochondria varies between cells with high-energy needs (e.g., skeletal muscle) having more than those with lower energy needs (e.g., immune cells).
Mitochondria numbers increase when energy is needed.
Surrounded by two highly specialized membranes (outer and inner); outer membrane has pores (porins) for molecules to enter.
Inner membrane contains cristae, which increase surface area for ATP production
Matrix site of many metabolic reactions, similar to cytoplasm in composition
Mitochondria contain their own DNA and ribosomes. Encodes some proteins involved in oxidative phosphorylation (energy production). Mitochondrial DNA is transcribed into mRNA translated into proteins in the mitochondria

Mitochondrial Diseases

Genetic diseases characterized by dysfunctional mitochondria.
Gene mutations in mitochondrial and nuclear DNA.
Insufficient ATP production.
Cell dysfunction and death.

Lysosomal Storage Diseases

Diseases caused by mutations in genes encoding proteins involved in lysosome function (e.g., hydrolases). Lysosome fails to function.
Leads to accumulation of undigested materials (e.g., proteins, lipids), which can be toxic.
Cell dysfunction and death result
Tay-Sachs disease is a well-known example
Mutations in the HEXA gene, causing deficiency of the lysosomal enzyme beta-hexosaminidase A. GM2 ganglioside accumulates, damaging nerve cells and leading to neurological problems.

Organelle Dysfunction Diseases

Organelle dysfunction can cause various diseases
Dysfunction in any organelle affects cellular function and results in diseases.

Summary Video (Endomembrane System)

A video summarizing the endomembrane system is available.

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Description

Test your knowledge on the functions and roles of the endoplasmic reticulum and lysosomes in cells. Explore their unique contributions to cellular processes and understand the mechanisms of protein synthesis and degradation. This quiz covers essential concepts in cell biology.