Bileagan Bith-eòlasach

• Biological membrane bilayers are also referred to as the cell membrane
• The membrane separates the interior of the cell from the outside world

Biological Membrane

• It is a selectively permeable membrane
• It separates the interior of a cell from the external environment or creates intracellular compartments
• Eukaryotic cell membranes consist of a phospholipid bilayer with embedded, integral, and peripheral proteins
• These proteins are used in communication and transportation of chemicals and ions
• Lipids in a cell membrane provide a fluid matrix for proteins to rotate and laterally diffuse
• Proteins are adapted to the high membrane fluidity environment with an annular lipid shell
• Cell membranes differ from isolating tissues like mucous, basement, and serous membranes
• Membranes act as a selective barrier around the cell and cell organelles
• Phospholipid bilayer is the basic structure of all biological membranes
• Besides phospholipids, glycolipids and cholesterol are generally present
• Membranes are essential for metabolism and energy usage
• An essential function is maintaining a defined chemical composition inside the membrane
• Selective passive diffusion and selective active transport maintain concentration differences

Membrane Proteins

• They translocate biologically important molecules through the membrane
• Selectivity is achieved by the composition and structure of the transporter
• Classified as peripheral or integral proteins
• Peripheral proteins are loosely bound and removed by mild treatments
• Integral proteins are embedded, difficult to remove, and have hydrophobic domains

Composition of Lipid Bilayer

• Consists of two layers: an outer leaflet and an inner leaflet
• Components are unequally distributed to create asymmetry
• Asymmetry is important for cell functions such as cell signaling
• Plasma and internal membranes have cytosolic and exoplasmic faces maintained during membrane trafficking
• New phospholipids are manufactured by enzymes in the endoplasmic reticulum
• Flippases catalyze the transfer of phospholipids to the opposite monolayer

Lipids in Membrane

• Made up of lipids with hydrophobic tails and hydrophilic heads
• Hydrophobic tails are hydrocarbon tails
• Their length and saturation are important in characterizing the cell
• Lipid rafts organize membrane components into localized areas for signal transduction
• Red blood cells have a unique lipid composition of cholesterol and phospholipids
• Phosphatidylserine is usually in the cytoplasmic side but flips to the outer membrane for blood clotting

Proteins in Membrane

• Phospholipid bilayers contain different proteins with various functions
• Integral proteins span the membranes with domains on either side, holding strong associations with the lipid bilayer
• Peripheral proteins hold weak interactions and are easily dissociated

Examples of Plasma Membrane Proteins and Functions

• Transporters: Na+/K+ pump actively pumps ions in/out of cells
• Anchors: Integrins link intracellular actin filaments to extracellular matrix
• Receptors: Platelet-derived growth factor receptor binds PDGF and generates intracellular signals to trigger cell growth/division
• Enzymes: Adenylyl cyclase catalyzes the production of cyclic AMP in response to extracellular signals

Oligosaccharides

• Sugar-containing polymers
• Covalently bound to lipids forming glycolipids, or to proteins forming glycoproteins
• In the bilayer, sugar groups are exposed at the cell surface, forming hydrogen bonds
• Glycolipids perform functions in cell recognition and adhesion
• Glycoproteins are integral proteins and play a role in immune response and protection

Formation of Phospholipid Bilayer

• Formed due to the aggregation of membrane lipids in aqueous solutions
• Aggregation is caused by the hydrophobic effect
• Hydrophobic ends come into contact and are sequestered away from water
• Arrangement maximizes hydrogen bonding between hydrophilic heads and water
• It also minimizes contact hydrophobic tails and water
• Increase in available hydrogen bonding increases the entropy

Function of Biological Molecules

• Amphiphilic or amphipathic
• Phospholipid bilayer contains charged hydrophilic headgroups interacting with polar water
• Hydrophobic tails meet with the hydrophobic tails of the complementary layer
• Interactions of lipids determine the lipid bilayer's physical properties, such as fluidity
• They define enclosed spaces or compartments in which cells may maintain a chemical or biochemical environment
• Cell membranes separate a cell from its surroundings

Selective Permeability of Biomembranes

• The size, charge, and other chemical properties determine success in crossing
• Essential for effective separation of a cell or organelle from its surroundings

Membrane Properties

• Biological membranes have mechanical and elastic properties to change shape and move
• Small hydrophobic molecules can readily cross phospholipid bilayers by simple diffusion

Membrane Transport

• Particles enter through membrane transport proteins or endocytosis
• Specialized plasma membranes can separate cells from the external environment
• Plasma membranes can also form supramembrane structures such as caveolae and cell junctions
• Distinct types of membranes create intracellular organelles
• Different types of biological membranes have diverse lipid and protein compositions
• The content of membranes defines their physical and biological properties

Membrane Fluidity

• The hydrophobic core of the phospholipid bilayer is constantly in motion because of rotations around the bonds of lipid tails
• Hydrophilic head groups exhibit less movement
• Below a transition temperature, a lipid bilayer loses fluidity and becomes a gel-like solid
• The transition temperature depends on hydrocarbon chain length and saturation of fatty acids
• Bacteria and cold-blooded organisms maintain constant fluidity by modifying membrane lipid fatty acid composition

Cholesterol in Membrane

• Modulates membrane fluidity in animal cells
• It constitutes approximately 20% of the lipids in the plasma membrane
• Cholesterol stiffens the bilayer, making it more rigid and less permeable

Importance of Membrane Fluidity

• Enables membrane proteins to diffuse rapidly and interact for cell signaling
• It permits membrane lipids and proteins to diffuse from synthesis sites
• It allows membranes to fuse and mix molecules
• Ensures molecules are distributed evenly in daughter cells during cell division

Membrane Signaling

• Synaptic transmission involves the release of neurotransmitters
• Vesicles fuse with the cell membrane and release their contents
• Lipid bilayers are involved in signal transduction as the home of integral membrane proteins
• CD59 protein identifies cells as "self" and inhibits their destruction by the immune system
• G protein-coupled receptors (GPCRs) relay signal events from outside to inside the cell
• Approximately 40% of drugs target GPCRs

Membrane-Mediated processes

• Lipid bilayers participate directly in signaling
• Phosphatidylserine-triggered phagocytosis removes dead or dying cells

Artificial Membranes

• Synthetically created for separation purposes
• Used in industrial processes
• Produced from organic (polymers, liquids) and inorganic materials
• Classified by surface chemistry, bulk structure, morphology, and production method
• Separation driving forces include pressure and concentration gradients
• Used in water purification, reverse osmosis, and removal of microorganisms

Types and Structure of synthetic membrane

• Fabricated from organic or inorganic materials
• Ceramic membranes are resistant to acids and strong solvents
• Liquid membranes include emulsion and immobilized liquid membranes
• Polymeric membranes are competitive in performance and economics

Membrane Polymer Selection

• Polymer has to have appropriate characteristics for intended application
• Should offer low binding affinity for separated molecules
• Should withstand harsh cleaning conditions
• Should be compatible with membrane fabrication technology
• Can range from amorphous and semicrystalline structures

Common Membrane Polymers

• Cellulose acetate, nitrocellulose, and cellulose esters (CA, CN, and CE)
• Polysulfone (PS)
• Polyether sulfone (PES)
• Polyacrilonitrile (PAN)
• Polyamide
• Polyimide
• Polyethylene and polypropylene (PE and PP)
• Polytetrafluoroethylene (PTFE)
• Polyvinylidene fluoride (PVDF)
• Polyvinylchloride (PVC)

Polymer Electrolyte Membranes

• Functionalized into ion-exchange membranes by adding acidic or basic groups
• Enables membranes to form water channels and selectively transport cations or anions
• Proton-exchange and alkaline anion-exchange membranes are important functional materials

Applications of Polymer Electrolyte Membranes

• Water treatment (reverse osmosis, electrodialysis)
• Energy storage (rechargeable metal-air electrochemical cells, flow battery)
• Energy generation (proton-exchange membrane fuel cells, alkaline anion-exchange membrane fuel cells, osmotic and electrodialysis-based osmotic power)

Ceramic Membranes

• Made from inorganic materials
• Used in separations where aggressive media are present
• Exhibit excellent thermal stability for high-temperature operations

Unit Membrane Model

• A concept that explains that every membrane has an underlying membrane of phospholipids
• Proposed by Danielli and Davson in 1935
• Characteristics include high electrical resistance and selective permeability to lipids

Robertson Model

• Given by Robertson in 1959
• Cell membrane consists of 3 layers: outer dark, middle light, and inner dark
• Outer dark layer is a protein layer
• Middle layer is a bilayer of phospholipid molecules
• Protein molecules are in extended form
• Cell membrane is a unit membrane, consisting of a bimolecular lipid layer between two protein layers
• Each protein layer is 20Å in thickness
• The polar heads of the phospholipids are 5Å thick each and the clear zone between them is 25Å thick
• The membranes of chloroplasts, mitochondria and nucleus are double membranes

Fluid Mosaic Model

• Explains various characteristics regarding the structure of functional cell membranes
• There is a lipid bilayer in which protein molecules are embedded
• Phospholipid bilayer gives fluidity and elasticity
• The biological model describes the cell membrane as a two-dimensional liquid where embedded proteins are generally randomly distributed