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BIOL340: Cell & Molecular Biology Cell Membranes Week 2 Lecture 3 4th March, 2024 Anuk Indraratna [email protected] 1 Learning outcomes 1. Describe the general structure of the lipid bilayer and roles of the plasma membrane. 2. Explain membrane fluidity & flexibility and the importance of these phenom...

BIOL340: Cell & Molecular Biology Cell Membranes Week 2 Lecture 3 4th March, 2024 Anuk Indraratna [email protected] 1 Learning outcomes 1. Describe the general structure of the lipid bilayer and roles of the plasma membrane. 2. Explain membrane fluidity & flexibility and the importance of these phenomena. 3. Briefly outline the biosynthesis of cell membranes. 4. Describe, with examples, the importance of membrane asymmetry. 5. Outline the different classes of membrane proteins. 6. Explain, with examples, how transporters move components in and out of the cell across the plasma membrane. 7. Describe the general operation of ion channels and explain, with examples, their importance in disease and drug therapy. 2 Lecture outline 1. General membrane structure & function 2. Phospholipid structure and function 3. Membrane fluidity & flexibility 4. Membrane biosynthesis 5. Asymmetry of the membrane 6. Membrane proteins 7. Movement of substances across the membrane 3 Introduction to membranes ▪ Membranes are primarily lipid-based structures that enclose and envelop a compartment ▪ Plasma membrane encloses the cell and all its contents — Maintains the critical separation between the cytosol and extracellular environment — This separation is crucial for selective permeability, ion gradients, signal transduction ▪ Membranes surround internal cellular components — Nucleus, mitochondria, Golgi, endoplasmic reticulum, vacuoles, chloroplasts 4 5 The lipid bilayer ▪ Membranes are ~50% lipid by mass, and considerably more by surface area ▪ These lipids are amphiphilic — Hydrophilic (polar) head — Hydrophobic (non-polar) tail(s) ▪ Phospholipids are predominant class of lipid — Phosphatidylcholine — Phosphatidylserine (cytosolic) ▪ Cholesterol, glycolipids, sphingolipids are other common classes 6 7 The lipid bilayer ▪ Amphiphilic nature of lipids leads to spontaneous assembly into self-sealing compartments 8 Lecture outline 1. General membrane structure & function 2. Phospholipid structure and function 3. Membrane fluidity & flexibility 4. Membrane biosynthesis 5. Asymmetry of the membrane 6. Membrane proteins 7. Movement of substances across the membrane 9 Phospholipids ▪ Glycerol backbone — Diglyceride — Esterified to two fatty acid tails ▪ Phosphatidylcholine: Head group = choline, phosphate ▪ Fatty acid molecules are long, non-polar chains between 14-24 carbon atoms ▪ Glycerol backbone — — — — ▪ 14-24 carbons long, typically 18-20 Typically one is ‘saturated’ (with hydrogen; only single-bonds) Other contains one double-bond (‘unsaturated) The double-bond causes a kink in the molecule This unsaturation is critical: prevents tight packing of lipids, which results in more fluidity and flexibility of the membrane 10 Phospholipids 11 12 Lecture outline 1. General membrane structure & function 2. Phospholipid structure and function 3. Membrane fluidity & flexibility 4. Membrane biosynthesis 5. Asymmetry of the membrane 6. Membrane proteins 7. Movement of substances across the membrane 13 Membrane flexibility ▪ Plasma membranes are inherently flexible and can deform as necessary ▪ Cells often have a need to undergo transient changes in shape ▪ E.g. Division, secretion (next lecture), motility ▪ Flexibility is influenced by other factors, such as presence of cholesterol 14 Cholesterol ▪ Cholesterol is a smaller, more compact lipid ▪ Higher abundances of cholesterol will result in more ‘packed’ membranes, increasing rigidity 15 Membrane fluidity ▪ Fluidity refers to flow of individual molecules within the membrane moving or flowing past each other ▪ High fluidity means lipids and proteins can move more freely within the plane of the membrane ▪ The movement of receptors and other membrane proteins is crucial for many functions, such as cell signaling and transport ▪ Movement of molecules is interrupted by tight junctions 16 Membrane fluidity 17 Lecture outline 1. General membrane structure & function 2. Phospholipid structure and function 3. Membrane fluidity & flexibility 4. Membrane biosynthesis 5. Asymmetry of the membrane 6. Membrane proteins 7. Movement of substances across the membrane 18 Membrane biosynthesis ▪ Membrane lipids are primarily synthesised by enzymes in the smooth endoplasmic reticulum (ER) ▪ Assembled from glycerol, fatty acids, phosphate groups, other small molecules on the cytosolic surface of the ER ▪ Flippases then selectively transfer some molecules to achieve both even growth and deliberate asymmetry ▪ Newly assembled membrane pinches off from the ER to form vesicles ▪ Vesicles then fuse into the plasma membrane 19 Lecture outline 1. General membrane structure & function 2. Phospholipid structure and function 3. Membrane fluidity & flexibility 4. Membrane biosynthesis 5. Asymmetry of the membrane 6. Membrane proteins 7. Movement of substances across the membrane 20 Membrane asymmetry ▪ The lipid bilayer is asymmetrical — Interior (cytosolic) face is different to the side protruding into the extracellular space — Esterified to two fatty acid tails ▪ This asymmetry is established during synthesis at the ER by flippases ▪ Golgi apparatus also adds glycans (sugars) to lipids, but only on the ‘exterior’ face 21 Phosphatidylserine (PS) ▪ PS is predominantly found on the cytosolic face in most cell types, due to activity of ATP-dependent flippases ▪ Signal transduction — Docking site for cytosolic enzymes such as protein kinase C ▪ Membrane curvature — Smaller head group than phospholipids ▪ Translocated to other side during apoptosis — Scramblase enzymes redistribute PS — Serves as specific ‘eat me’ signal for phagocytes — Enables rapid, controlled clearance of dying cells 22 Membrane glycosylation ▪ A major example of membrane asymmetry ▪ Golgi apparatus receives lipids from ER — Glycosyltransferases sequentially add sugar moieties to the lipid backbone — Simple (a single sugar) or more complex oligosaccharides — Sugars are added only to the non-cytosolic face ▪ Important role in cell-cell communication — Such as host-recognition — Blood group antigens ▪ Protective glycocalyx — Mechanical and environmental protection — Prevents non-specific adhesion by other molecules 23 Glycosylation of endothelia 24 Lecture outline 1. General membrane structure & function 2. Phospholipid structure and function 3. Membrane fluidity & flexibility 4. Membrane biosynthesis 5. Asymmetry of the membrane 6. Membrane proteins 7. Movement of substances across the membrane 25 Membrane proteins Essential Cell Biology, 3rd edn, Alberts, Bray et al, Chapter 11 26 Membrane proteins ▪ Different classes of proteins are embedded in the plasma membrane (or organelle membranes) ▪ Defined orientation relative to the cytoplasm, and therefore distinct domains ▪ These can be categorised in different ways: — Spatial relationship to lipid bilayer — Mechanism of attachment — Function of the protein ▪ E.g. Spatial — Integral membrane proteins — Peripheral membrane proteins — Lipid-anchored proteins 27 Transmembrane proteins GLUT1 (glucose uptake) Na+/K+ ATPase Integrins G Protein coupled receptors Adenylyl cyclase 28 Other membrane proteins 29 Lecture outline 1. General membrane structure & function 2. Phospholipid structure and function 3. Membrane fluidity & flexibility 4. Membrane biosynthesis 5. Asymmetry of the membrane 6. Membrane proteins 7. Movement of substances across the membrane 30 Transport across the membrane ▪ In addition to retaining cellular contents, the plasma membrane functions to specifically allow necessary exchange of materials in and out of the cell ▪ There are two modes of transport: — Passive: by diffusion across gradient from high -> low — Active: energy-coupled transport ▪ Diffusion can occur through bilayer — Or via protein channel — Or via protein transporter ▪ Active transport requires energy — Referred to as ‘pumps’ 31 Membrane permeability Passive transport non-polar molecules ▪ Non-polar molecules are permeable through the mostly hydrophobic bilayer ▪ Smaller molecules are also more permeable ▪ Important examples include O2 and CO2 ▪ Water, despite being highly polar, is small enough to permeate through the membrane ▪ Its permeability is enhanced via aquaporins polar molecules solubility in oil 32 ▪ In addition to retaining cellular contents, the plasma membrane functions to specifically allow necessary exchange of materials in and out of the cell ▪ There are two modes of transport: — Passive: down the concentration gradient — Active: energy-coupled transport ▪ Passive transport occurs in different ways — Diffusion through lipid bilayer — Protein-lined channel — Transporter 33 Electrochemical gradients ▪ The deliberate separation of ions between cytosol and extracellular environment is critical in biology ▪ Concentrations of important ions are maintained at different levels via active transport ▪ Active transport requires energy (ATP), but this investment has a ‘pay-off’ in many different scenarios ▪ E.g. nutrient uptake, signal transduction, electrical conduction ▪ Also crucial in maintaining osmotic pressure 34 35 Extracellular space + Cell - (60-200 mV) High Na+ (145 mM) low Na+ (5-15 mM) Low K+ (5 mM) high K+ (140 mM) High Cl- (110 mM) low Cl- (5-15 mM) 36 Extracellular space + + - (60-200 mV) + + + Cell + + The possibility (potential energy) of ions to move through passively is essentially stored, like a battery. Electrochemical potential gradient 37 How is this potential maintained? 38 high Na+ Cell Extracellular space ATP high Na+ maintain electrochemical gradient ∆E (membrane potential) low Na+ + 2K+ Na+/K+ antiporter 3Na+ ADP + Pi ∆E = 58 mV high Na+ high Na+ 39 40 Example: glucose transport ▪ Glucose is a critical nutrient that must be absorbed from food across the gut ▪ Requires sodium electrochemical gradient which is actively maintained via Na+/K+ pumps ▪ This gradient is then implemented to drive Na+ into the cell (down its gradient) along with glucose — Via glucose-Na+ symporter — This occurs at the apical side ▪ At the luminal face, glucose exits via passive diffusion — Via facilitated diffusion — Known as glucose uniporter 41 high Na+ low Na+ 42 43 Ion channels ▪ Complexes that specifically allow movement of one kind of ion via its concentration gradient ▪ Different kinds of ion channels — Voltage-gated — Ligand-gated — Mechano-gated (stretch) ▪ Defects in ion channels can lead to disease ▪ Ion channels are useful drug targets 44 45 Example: cystic fibrosis 46 Example: lignocaine CNS Important role in action potentials 47 Lecture outline 1. General membrane structure & function 2. Phospholipid structure and function 3. Membrane fluidity & flexibility 4. Membrane biosynthesis 5. Asymmetry of the membrane 6. Membrane proteins 7. Movement of substances across the membrane 48 Learning outcomes 1. Describe the general structure of the lipid bilayer and roles of the plasma membrane. 2. Explain membrane fluidity & flexibility and the importance of these phenomena. 3. Briefly outline the biosynthesis of cell membranes. 4. Describe, with examples, the importance of membrane asymmetry. 5. Outline the different classes of membrane proteins. 6. Explain, with examples, how transporters move components in and out of the cell across the plasma membrane. 7. Describe the general operation of ion channels and explain, with examples, their importance in disease and drug therapy. 49 Example quiz questions 1. Describe the roles of the plasma membrane. [4 marks] 2. Explain how lipids assemble into self-sealing compartments. [4 marks] 3. Compare and contrast three different lipid molecules with regard to their distribution and roles in the plasma membrane. [6 marks] 4. With the aid of a labelled diagram, explain how active and passive transport are involved in the absorption of glucose across the intestinal epithelia. [10 marks] 50