Lecture 2

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).

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