Lecture 2

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Questions and Answers

Which of the following is NOT a fundamental component of all cellular membranes?

Proteins

Lipids

Carbohydrates

Nucleic Acids (correct)

What functional group is NOT typically found linked to the phosphate residue in phospholipids?

Ethanolamine

Glycerol (correct)

Serine

Choline

Which of the following best describes the structural composition of a sphingomyelin molecule?

Sphingosine, two fatty acids, phosphoric acid, and a functional group

Glycerol, sphingosine, and a carbohydrate

Sphingosine, one fatty acid, phosphoric acid, and a functional group (correct)

Glycerol, two fatty acids, phosphoric acid, and a functional group

What is the primary structural difference between cerebrosides and gangliosides?

Cerebrosides only have glucose or galactose, while gangliosides can have up to 7 sugar residues. Signup and view all the answers

Where are you most likely to find a high concentration of sphingomyelins?

Neural tissue Signup and view all the answers

What structural feature is characteristic of cholesterol?

A branched side chain Signup and view all the answers

If a lipid contains sphingosine, a fatty acid, and one or more sugar molecules but no phosphate group, to which class of lipids does it belong?

Glycolipids Signup and view all the answers

What distinguishes symport from antiport in coupled transport?

Symport involves the simultaneous movement of two molecules in the same direction, while antiport moves them in opposite directions. Signup and view all the answers

How does secondary active transport differ from primary active transport in terms of energy source?

Secondary active transport uses electrochemical gradients built up by primary active transport, while primary active transport uses ATP hydrolysis directly. Signup and view all the answers

In the context of a sodium-potassium pump, what is the direct role of ATP hydrolysis?

ATP hydrolysis results in the release of a phosphate group which temporarily binds to the pump, initiating a conformational change to facilitate ion transport against concentration gradients. Signup and view all the answers

How does the glucose transport system utilize the electrochemical gradient of sodium ions?

It uses the sodium electrochemical gradient to simultaneously transport glucose molecule into the cell against its concentration gradient. Signup and view all the answers

What is a key characteristic that differentiates bulk transport from other forms of membrane transport?

Bulk transport involves the use of membrane vesicles to move large molecules. Signup and view all the answers

What characteristic of fatty acid residues is most common in membrane lipids?

They generally contain an even number of carbon atoms, usually 16 to 18. Signup and view all the answers

What effect does the presence of unsaturated bonds in fatty acid residues have on the spatial arrangement of the molecule?

It causes the fatty acid to effectively take up more space. Signup and view all the answers

What is meant by the term 'amphiphilic' in the context of membrane lipids?

Exhibiting both a hydrophilic, polar end and a hydrophobic, nonpolar end simultaneously. Signup and view all the answers

How are membrane phospholipids arranged in the lipid bilayer?

Hydrophobic parts (hydrocarbon chains) face the interior of the bilayer, while hydrophilic parts (polar heads) are on the surface. Signup and view all the answers

What prevents the escape of membrane lipids from the lipid bilayer?

The aqueous environment outside and inside of the cell. Signup and view all the answers

Which of the following is NOT a category for membrane proteins based on their degree of binding to the lipid bilayer?

Globular Signup and view all the answers

What is the defining characteristic of transmembrane proteins?

They span across the entire thickness of the lipid bilayer. Signup and view all the answers

What part of integral membrane proteins interacts with the hydrophobic tails of the membrane lipids?

The hydrophobic amino acid side chains. Signup and view all the answers

According to the text provided, what is the role of the hydrophilic parts of integral membrane proteins within the lipid bilayer?

To allow passage of some polar molecules and water. Signup and view all the answers

Which type of integral membrane protein is located in the hydrophobic part of the plasma membrane?

Internal membrane proteins Signup and view all the answers

What type of bonding is NOT typically involved in the binding of peripheral membrane proteins to the cell membrane?

Covalent bonding Signup and view all the answers

Where are cell surface proteins located?

Only on the outer surface of the cell membrane Signup and view all the answers

Which function is NOT associated with membrane proteins?

Replication of cell DNA Signup and view all the answers

What is the primary component attached to proteins to form glycoproteins?

Short sugar chains like oligosaccharides Signup and view all the answers

Which of the following is a characteristic feature of the cell membrane?

Fluidity Signup and view all the answers

Where are the carbohydrate components (sugars) of glycoproteins, proteoglycans, and glycolipids located?

Only on the outer side of the cell membrane Signup and view all the answers

Which of the following is NOT a function of the glycocalyx?

Facilitating DNA replication Signup and view all the answers

What is the structural feature, used by cell-surface proteins to connect to the cell membrane called?

Anchor motif Signup and view all the answers

Which feature of the cell membrane refers to the unequal distribution of its components, like lipids and proteins, between the inner and outer layers?

Asymmetry Signup and view all the answers

What is the primary mechanism by which transverse movement of lipids occurs?

Enzymatic catalysis by flippases. Signup and view all the answers

Which types of movement are characteristic of integral membrane proteins within the lipid bilayer?

Rotational and lateral movements only. Signup and view all the answers

Lateral movement of membrane proteins can be restricted by attachments to which of the following?

The cell cortex, extracellular matrix, and other cell surface proteins. Signup and view all the answers

Which of the following statements accurately describes membrane asymmetry?

The outer and inner layers (leaflets) of the cell membrane have different lipid and protein compositions. Signup and view all the answers

What is the predominant composition of the outer layer of the cell membrane?

Mainly phosphatidylcholines, sphingomyelin, surface proteins, glycolipids, and glycoproteins. Signup and view all the answers

Which lipids are most abundant in the inner layer of the cell membrane, contributing to its asymmetry?

Lipids with electrically charged polar heads like phosphatidylserine, and lipids that form hydrogen bonds, like phosphatidylethanolamine. Signup and view all the answers

Which of these best describes the non-uniform nature of the cell membrane?

The cell membrane is composed of a lipid bilayer with independent structures like lipid rafts and caveolae. Signup and view all the answers

Which of the following best describes lipid rafts?

Flat, dynamic areas of the membrane rich in cholesterol and sphingolipids, involved in cell signaling and transport. Signup and view all the answers

What are caveolae, and what are their functions?

Bottle-shaped invaginations of cell membrane rich in cholesterol, sphingolipids and caveolin involved in signaling, endocytosis, and transcytosis. Signup and view all the answers

In what cell types are lipid rafts and caveolae generally not found?

Lymphocytes, erythrocytes, and nerve cells. Signup and view all the answers

Which factor does NOT directly influence the rate of simple diffusion across a cell membrane?

The presence of protein channels Signup and view all the answers

What is the primary role of aquaporins in cellular membranes?

To speed up the movement of water molecules Signup and view all the answers

Which statement accurately describes the relationship between concentration gradients and passive transport?

Passive transport moves substances from an area of high concentration to an area of low concentration. Signup and view all the answers

What is the net force driving the passive transport of ions across a cell membrane?

The electrochemical gradient Signup and view all the answers

Which of the following transport mechanisms does NOT require the direct input of external energy?

Facilitated diffusion through a protein transporter Signup and view all the answers

How does facilitated diffusion via a protein channel differ from simple diffusion?

It involves the use of a specific membrane protein. Signup and view all the answers

In passive transport, what determines the direction of ion movement across a membrane?

The electrochemical gradient Signup and view all the answers

What is the difference between simple diffusion and facilitated diffusion in terms of protein involvement?

Simple diffusion occurs directly through the membrane, and facilitated diffusion involves a membrane protein. Signup and view all the answers

Which of the following does not utilize passive transport mechanisms?

The sodium-potassium pump maintaining gradients Signup and view all the answers

An integral membrane protein that is classified as an 'outer monolayer protein' is characterized by which property?

It is attached to the outer leaflet of the cell membrane but does not penetrate it. Signup and view all the answers

Which of these combinations of forces is LEAST likely to be involved in the binding of peripheral membrane proteins to the cell membrane?

Covalent bonding and hydrophobic interactions Signup and view all the answers

A protein molecule on the cell membrane has multiple, short sugar chains attached to it. Which term most accurately describes this modification?

Glycoprotein Signup and view all the answers

Given that all carbohydrates in glycoproteins and glycolipids are located on the outer cell surface, what primary function does this arrangement support in cell biology?

Facilitating cell-cell recognition and interaction Signup and view all the answers

If a cell membrane exhibits 'asymmetry', this trait is best described by which of the following statements?

The membrane is composed of distinct types of lipids in the inner and outer layers. Signup and view all the answers

Which of the following scenarios correctly describes a structure that would be classified as a 'cell surface protein'?

A structural protein tethered to the outer cell membrane by a protein loop. Signup and view all the answers

How does the presence of sugar moieties on glycolipids and glycoproteins primarily contribute to the function of cellular membranes?

By creating a recognition layer for cell interactions and protection. Signup and view all the answers

If a cell membrane protein transports both glucose and sodium into the cell, and sodium moves down its concentration gradient, while glucose moves against its concentration gradient, what type of transport is being used?

Symport secondary active transport Signup and view all the answers

Which of these is the most direct energy source for a sodium-potassium pump?

ATP hydrolysis Signup and view all the answers

Which transport process relies on a pre-existing electrochemical gradient of one substance to power the transport of another substance against its gradient?

Secondary active transport Signup and view all the answers

If a cell needs to import a large protein, which mechanism would it primarily use?

Bulk transport Signup and view all the answers

In a symport transport system, if one of the transported substances is moving against its concentration gradient, what must be true of the other substance?

It must be moving down its concentration gradient. Signup and view all the answers

Which of the following best describes the function of the energy used by the Na+/K+ pump?

To establish an electrochemical gradient of sodium and potassium necessary for other transport mechanisms. Signup and view all the answers

What is the defining characteristic of all active transport mechanisms?

Requiring energy input to transport substances against their concentration gradient. Signup and view all the answers

A transport system moves two different molecules across the membrane: A into the cell down its electrochemical gradient, and B out of the cell against its electrochemical gradient. What is this system most likely an example of?

Antiport secondary active transport Signup and view all the answers

Which of the following occurs during primary active transport?

A direct coupling of transport with the process of energy release, typically through ATP hydrolysis, occurs. Signup and view all the answers

If the sodium-potassium pump were inhibited, what would be the most likely consequence for the cell?

Increase in the concentration of Na+ inside the cell. Signup and view all the answers

Which of the following accurately describes the process of endocytosis?

The uptake of substances into the cell by enclosing them in membrane-bound vesicles. Signup and view all the answers

What is a distinguishing characteristic of exocytosis, as compared to endocytosis?

It results in the release of substances from the cell through vesicle fusion with the cell membrane Signup and view all the answers

Which of the following is NOT directly involved in the process of phagocytosis?

The formation of an endosome. Signup and view all the answers

In pinocytosis, what is the purpose of a pinosome?

To transport fluids and dissolved substances to a primary lysosome Signup and view all the answers

What key step distinguishes receptor-mediated endocytosis from other forms of endocytosis?

The initial binding of a specific molecule to a receptor on the cell membrane. Signup and view all the answers

Within the stages of receptor-mediated endocytosis, what is the role of the endosome?

To progress from an early to a late stage and deliver cargo to specific cellular compartments or a lysosome Signup and view all the answers

What is the main function of lysosomes as described in the content provided?

To degrade macromolecules using specific hydrolases. Signup and view all the answers

In a healthy organism, what is a key reason proteins are degraded?

When their lifespan has ended, their structure is improper, or they are damaged or in excess. Signup and view all the answers

Under what conditions is protein degradation notably increased beyond baseline levels, as indicated in the content?

In a sick organism. Signup and view all the answers

What is the key requirement for protein metabolism, according to the text provided?

Strict control. Signup and view all the answers

Which of the following statements about lysosomal proteolysis is accurate?

It targets both exogenous and old endogenous proteins for degradation. Signup and view all the answers

What is the primary function of ubiquitin in the protein degradation process?

To facilitate the recognition of proteins by the proteasome. Signup and view all the answers

Which component is NOT part of the ubiquitin system?

Endoplasmic reticulum Signup and view all the answers

What role does the proteasome play in the degradation of proteins?

To unfold, degrade, and release peptides. Signup and view all the answers

Which of the following statements is true regarding the structure of proteasomes?

They are cylindrical structures composed of multiple proteases. Signup and view all the answers

Study Notes

Lecture 2: The Cell Membrane

Lecturer: Dr. Michelle Kuzma
Adapted from: Dr. Danuta Mielżyńska-Švach
Textbook: Essential Cell Biology, 6th ed. by Bruce Alberts

Housekeeping

Slides will be shared.
Recording is not permitted.
Videos will be shared online.
Email contact for lecture questions: [email protected]

Cell Membrane Functions

Receiving information: Information is received by the cell membrane.
Import/export of small molecules: Small molecules are imported and exported across the cell membrane.
Capacity for movement and expansion: Cell membranes can move and expand as needed.

Membranes in the Cell

Endoplasmic reticulum
Nucleus
Peroxisome
Endosome
Lysosome
Transport vesicle
Mitochondrion
Golgi apparatus
Plasma membrane

Membrane Structure

All membranes in cells are constructed using the same blueprint.
Lipids: Found in all membranes.
Proteins: Found in all membranes.
Sugars (carbohydrates): Bound to lipids (glycolipids) and proteins (glycoproteins).

Membrane Lipids

Lipids are divided into three groups based on chemical structure:
- Phospholipids
- Sphingolipids
- Sterols

Phospholipids

Lipids composed of:
- Two fatty acids
- Glycerol (an alcohol)
- Phosphoric acid
- A functional group attached to the phosphate (e.g., ethanolamine, choline, inositol, serine).

Sphingolipids

Lipids composed of:
- Sphingosine (a long-chain amino alcohol)
- A fatty acid
- Phosphoric acid (optional)
- A functional group (e.g., ethanolamine, choline, serine)
Divided into two subgroups:
- Sphingomyelins
- Glycolipids

Sphingomyelin

Made up of:
- Sphingosine
- A fatty acid
- Phosphoric acid
- A functional group (e.g., serine, ethanolamine, or choline)
Critical for:
- Brain matter
- Neural tissue
- Myelin sheath of nerve endings

Glycolipids

Made up of:
- Sphingosine
- A fatty acid
- One or more sugar molecules.
Simplest are cerebrosides (contain glucose or galactose)
More complex are gangliosides (contain up to seven sugar residues).

Sterols

Sterols are alcohols.
The most important animal sterol is cholesterol.
Cholesterol is a cyclic compound with a branched side chain.

Structure of Membrane Lipids

Membrane lipids contain one or two fatty acid residues.
Fatty acid residues contain an even number of carbon atoms (usually 16-18).
At least one bond in the fatty acid residue can be unsaturated.
Unsaturated bonds cause the fatty acid to take up more space.

Structure of Membrane Lipids (Amphiphilic)

Amphipathic (hydrophilic and hydrophobic)
- Hydrophilic ("water-loving") polar end.
- Hydrophobic ("water-fearing") nonpolar end.
Depending on its chemical structure, the hydrophilic part can:
- Be electrically charged.
- Have the polar character of an electric dipole.

Lipid Bilayer

The cell membrane is formed by two layers of phospholipids.
The hydrophilic parts (polar heads) are on the surface of the bilayer.
The hydrophobic parts (hydrocarbon chains) are on the interior of the bilayer.
The escape of membrane lipids from the bilayer is prevented by the aqueous environment.

Membrane Proteins

Categorized by the degree of binding to the lipid bilayer:
- Integral
- Peripheral
- Surface

Integral Membrane Proteins

Embedded within the plasma membrane.
Divided into:
- Monotopic (attached to one side)
- Transmembrane (span the entire thickness of the bilayer)
- Polytopic (span the membrane multiple times)

Membrane Proteins (Integral Proteins)

Reinforced by the highly hydrophobic nature of the lipid component of the membrane.
Hydrophobic amino acid side chains of integral membrane proteins interact with the hydrocarbon tails of the membrane lipids.
Hydrophilic parts of integral membrane proteins face internally, allowing passage of some polar molecules and water.

Non-penetrating Integral Membrane Proteins

Integral proteins that do not penetrate the plasma membrane.
Divided into:
- Outer monolayer
- Inner monolayer
- Internal monolayer

Peripheral Membrane Proteins

Found on both inner and outer surfaces of the cell membrane
Bound to the cell membrane by:
- Electrostatic/ionic bonding
- Hydrogen bonding
- Van der Waals forces

Cell Surface Proteins

Occur only on the outer surface of the cell membrane.
Connected to the cell membrane by an anchored element (e.g., protein loop or lipid).

Types of Membrane Proteins

Transmembrane
Monolater-associated
Lipid-linked
Protein attached

Functions of Membrane Proteins

Transport: Enable transport across the membrane.
Structural: Link cells together or to the extracellular matrix.
Receptor: Part of the signaling system.
Enzymatic: Catalyze chemical reactions.

Glycolipids and Glycoproteins

Some proteins and lipids in the outter layer of the cell membrane attach to sugars (covalently).
Most membrane proteins attach to short sugar chains (oligosaccharides) to form glycoproteins.
Some membrane proteins attach to long polysaccharide chains to from proteoglycans
A single protein can attach to multiple sugar chains, but a single lipid molecule can only attach to one sugar chain.

Glycocalyx

Sugars form a sugar coating called glycocalyx.
Involved in:
- Protecting the cell surface.
- Recognizing other cells.
- Forming contacts between cells.
- Merging cells into larger groups.

Cell Membrane Structure

Shows components of a cell membrane, like the phospholipid bilayer, cholesterol, glycolipids, peripheral proteins and integral proteins.

Cell Membrane Properties

Selective permeability: Allows certain substances pass through.
Fluidity: Membrane components can move.
Asymmetry: Lipid and protein composition varies between the inner and outer layers.
Heterogeneity: Contains different components.

Membrane Permeability

The membrane acts as a barrier to control molecule passage.
Small nonpolar molecules (e.g., oxygen, carbon dioxide) diffuse through the lipid bilayer.
Small uncharged polar molecules (e.g., water, ethanol) diffuse through the lipid bilayer.
Larger uncharged molecules (e.g., amino acids, glucose) do not diffuse through the lipid bilayer.
Ions and electrically charged molecules do not diffuse through the lipid bilayer.

Membrane Fluidity

Fluidity is how well membrane components move.
The cell membrane is an elastic, two-dimensional fluid.
Influenced by:
- Cholesterol (decreases)
- Unsaturated fatty acids (increase)

Membrane Fluidity (Movements)

Membrane lipid molecules perform various movements including: -Segmental movement (flexion) -Rotational movement -Translational movement: -Lateral movement -Transverse movement ("flip-flop")

Membrane Fluidity (Factors)

Factors influencing fluidity include: -Length of hydrocarbon chains -Number of unsaturated bonds -Amount of cholesterol -Temperature

Membrane Fluidity (Protein Movements)

Membrane integral proteins can undergo:
- Rotational movements
- Lateral movements (slower than lipids)
Membrane proteins do not exhibit transverse movement.

Restriction of Lateral Movement of Proteins

Lateral movement of proteins can be restricted due to attachments to:
- Cell cortex inside the cell
- Extracellular matrix molecules outside the cell
- Proteins on the surface of another cell

Membrane Asymmetry

The respective layers (leaflets) of the cell membrane have different lipid and protein compositions.
Outer layer: Mainly phosphatidylcholines, sphingomyelin, surface proteins, a large amount of glycolipids and glycoproteins.
Inner layer: Mainly phosphatidylserine, lipids that easily form hydrogen bonds (e.g., phosphatidylethanolamine).

Membrane Heterogeneity

Cell membrane is non-uniform.
Major components of a lipid bilayer: phospholipids, cholesterol, glycolipids and proteins.
Independent structures: lipid rafts, caveolae.

Lipid Rafts/Caveolae (Characteristics)

Lipid rafts: Flat and dynamic areas of the cell membrane, rich in cholesterol and sphingolipids. Involved in signaling and transport.
Caveolae: Bottle-shaped invaginations of the cell membrane, rich in cholesterol, sphingolipids, and caveolin. Involved in signaling, endocytosis, and transcytosis.

Membrane Transport

Small molecules: Passive transport (osmosis, simple diffusion, facilitated diffusion). Active transport (ATPases, co-transporters).
Large molecules: Bulk transport (endocytosis, exocytosis).

Passive Transport (Osmosis)

Water can diffuse directly, but slowly, through a lipid bilayer.
Osmosis is the movement of water across a selectively permeable membrane from an area of high water concentration to an area of low water concentration.

Passive Transport (Other aspects)

Passive transport does not require external energy input.
Depends on: -Concentration gradient -Membrane potential
Electrochemical gradient determines the direction of transport
Occurs from high to low concentration.

Passive Transport (Simple Diffusion)

Process by which solutes pass through a cell membrane along the concentration gradient of the solution.
Rate of diffusion depends on factors such as concentration differences, electric field, hydrostatic pressure gradient, permeability coefficient of substance, temperature.

Passive Transport (Facilitated Diffusion)

Passive-mediated transport that does not require external energy input.
Facilitated diffusion in cell membranes can occur via either:
- Protein channels
- Protein transporters.
Facilitating entity is a membrane protein.

Ion Channels

Facilitated diffusion can occur through ion channels.
Protein ion channels: connect intracellular and extracellular spaces, are filled with water.
Protein channels are selective depending on channels' diameter and shape, arrangement of charged amino acids and ion type.

Ion Channels (Function)

Function of ion channels is to temporarily increase membrane permeability to selected inorganic ions.
An ion channel can be:
- Open (allows ions to pass freely)
- Closed (allows ions to pass periodically)
Opening/closing of the channel is in response to stimuli (e.g., temperature, electrochemical gradient, mechanical stimuli, concentration gradient).
The concentration of the opening agent affects the number of open channels.

Transporters (Carrier Proteins)

Responsible for movement across cell membranes of mostly small water-soluble organic molecules, and some inorganic ions.
Each transporter is highly selective, often transporting only one type of solute.
Transporters open on only one side of the membrane at a time.

Transporters (Glucose)

Carrier protein for facilitated diffusion undergoes different conformations.
Glucose transport: outward open state (binding sites on the outside), closed state (binding sites inaccessible from both sides), inward open state (binding sites on the inside).

Facilitated Diffusion (Glucose)

Facilitated diffusion depends on the concentration gradient around the transporter, the rate of interactions between the carrier protein and the transported substance, the rate of conformational changes of the protein, and hormones (e.g. insulin).

Coupled Transport

Is a type of carrier transport under facilitated diffusion.
Transporter has binding sites for two substances.
Symport when both substances flow in the same direction.
Antiport when the flow of the substances are in opposite directions.

Active Transport

Occurs against the concentration gradient of the substance being transported.
Requires energy input.
Supplies the cell with substances like amino acids, sugars, sodium, potassium ions, etc.
Ensures appropriate osmotic pressure.
Distinguished as:
- Primary
- Secondary

Primary Active Transport (Sodium-Potassium Pump)

The pump uses energy released during ATP hydrolysis to move Na+ ions out of the cell and K+ ions into the cell.
During this process, a phosphate group is released from ATP and attached to the transporter.
The pump maintains a low concentration of Na+ and a high concentration of K+ inside the cell.

Secondary Active Transport (Glucose Transport)

The transport protein simultaneously allows sodium ions to move along their concentration gradient, transports a glucose molecule into the cell against its concentration gradient, using the electrochemical gradient of Na+ to drive glucose import.

Bulk Transport

Used to transport large molecules that can't pass directly through the cell membrane.
Completed via vesicles.
Types: endocytosis, exocytosis.

Endocytosis

Uptake of substances (viruses, bacteria, other cells) into the cell by enclosing them in a membrane-bound vesicle formed by the outer cell membrane.

Exocytosis

Removal of undigested waste or secretion of compounds (e.g., hormones) from the cell.
Exported via a membrane-bound vesicle that fuses with the outer cell membrane.

Types of Endocytosis

Phagocytosis: Uptake of macromolecules or bacteria. (Stages: uptake, phagosome formation, substance transport, and secondary lysosome formation.)
Pinocytosis: Uptake of fluids and substances dissolved in fluids. (Stages: uptake, pinosome formation, substance transport, and secondary lysosome formation.)
Receptor-mediated endocytosis: Uptake of specific molecules (ligands) by binding to receptors on the cell surface. (Stages: receptor location, ligand binding, endosome formation, endosome movement.)

Lysosomal Degradation

Degradation of macromolecules occurs within lysosomes.
Lysosomes contain numerous specific hydrolases (enzymes.)

Protein Degradation (General)

In a healthy organism, 3-5% of proteins are degraded (in sick organisms degraded more).
Protein metabolism must be under constant control.
Degradation occurs if lifespan is over, structure is improper, protein is damaged or an excessive amount is present.

Protein Degradation (Mechanisms)

Lysosomal proteolysis (non-selective): Degradation of exogenous or old endogenous proteins (e.g. structural proteins) within lysosomes.
Proteasomal proteolysis (selective): Degradation of ubiquitinated proteins involves the proteasome enzyme complex.

Ubiquination

Ubiquitin system consists of these elements: ubiquitin, ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), ubiquitin ligase (E3), proteasome, and ubiquitin-detaching enzyme (DUB).
Ubiquitin attaches to the protein intended for degradation, allowing the proteasome complex to recognize and degrade it.
Ubiquitin is comprised of alpha-helices and beta-sheets.

Proteasomes

Found in all eukaryotic cells.
Breakdown proteins bound to ubiquitin (present in cytoplasm and nucleus).
Number varies and depends on the cell's need for protein breakdown.
On average there are about 30,000 proteasomes in a single eukaryotic cell.

Proteasome Structure

Large, high-molecular-weight enzyme complexes.
Cylindrical structure made up of 28 proteases (centrally located).
Active sites of the proteases are directed towards the interior of the proteasome.
The ends are closed by large protein complexes acting like plugs.

Proteasome Functions

Bind to proteins to be degraded.
Unfold proteins and bring them into the "cylinder"
Cut proteins to into short peptides (Lyse them)
Release peptides from either end of the cylinder (this process requires energy from ATP hydrolysis).

Organelle Degradation (Autophagy)

Dying organelles send signals to form autophagosome membranes, which enclose the organelles from the cytosol.
Autophagosomes fuse with primary lysosomes to generate secondary lysosomes (autolysosomes) and degrade the organelle.

Organelle Degradation (Nucleus, Mitochondria, Lysosomes & Ribosomes)

Nucleus (nucleophagy): Degradation in the event of DNA damage or improper separation of chromosomes during cell division. Micronuclei form and contain parts/whole chromosomes, and fragments of the nuclear envelope.
Mitochondrial degradation (mitophagy): Primary signal is oxygen deprivation (hypoxia).
Lysosome degradation (lysophagy): Signals include increased lysosomal membrane permeability, appearance of lysosomal membrane proteins in the cytoplasm and ubiquitination of lysosomal surface proteins.
Ribosome degradation (ribophagy): Signal is demand for nitrogen (specific amino acids like Arg, Leu and nucleotides).
Proteasome degradation (proteaphagy): Occurs through binding of appropriate receptors to autophagosome proteins.

Literature

Essential Cell Biology, B. Alberts, D. Bray, K. Hopkin (Volume 2): Chapters 11, 12, and 15 (endocytosis only).

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Description

Test your knowledge on the fundamental components of cellular membranes and the roles of various lipids. This quiz covers key topics such as sphingomyelin, lipid classes, and transport mechanisms. It's essential for understanding the complex interactions within cell membranes