Cell Biology: Endoplasmic Reticulum and Lysosomes

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.