BES 107 Introduction to Cell Biology (Winter 2025) - PDF

BES 107 Introduction to Cell Biology Part 3 – MEMBRANE STRUCTURE AND FUNCTION Campbell’s Biology Chapter 7 Winter 2025 Sophon Bailey All figures are from Campbell’s Biology, © 2021 Pearson Press, un...

BES 107 Introduction to Cell Biology Part 3 – MEMBRANE STRUCTURE AND FUNCTION Campbell’s Biology Chapter 7 Winter 2025 Sophon Bailey All figures are from Campbell’s Biology, © 2021 Pearson Press, unless otherwise attributed. Course content, digital or otherwise, created and/or used within the context of the course is to be used solely for personal study, and is not to be used or distributed for any other purpose without prior written consent from the content author(s). LEARNING OBJECTIVES Understand cell theory and how it establishes the minimal functional unit of life Describe the structures of the lipid bilayer and how this establishes the nature and function of cells › Discuss membrane fluidity and the contribution of fatty acids and cholesterol to fluidity Identify challenges created by the presence of the lipid bilayer and the role of proteins in offering solutions to these problems › Fluidity, transport, signalling Describe the different forms of transport across the cell membrane and some examples of each › Passive transport –simple diffusion and facilitateddiffusion › Active transport & indirect active transport › Bulk transport –exocytosis and endocytosis 2 CELL THEORY & THE MEMBRANE The cell is the defining unit of life according to cell theory: 1. All living organisms* are composed of one or more cells 2. The cell is the basic unit of structure and organization in organisms 3. Cells arise from pre-existing cells The membrane is the defining featureof ALL cells Cell membrane or plasma membrane (PM) 3 Becker’s World of the Cell 9e, ©2016 Pearson Becker’s World of the Cell 9e, ©2016 Pearson CELL MEMBRANES Plants, bacteria and some fungi have a rigid cell wall outside the plasma membrane AND CELLWALLS Cell walls can protect the more delicate membrane from injury and stress, provide strong supportive structure 4 BILAYERS Amphipathic lipids will spontaneously form bilayers › Abiogenesis likely involved spontaneous capture of RNA within a lipid membrane Bilayers are semi-permeable –most polar molecules cannot cross Proteins with hydrophobic amino acids can be amphipathic and span bilayers 5 Becker’s World of the Cell 9e, ©2016 Pearson  FLUID MOSAIC MODEL Membranes have a diverse set of lipids and proteins in the bilayer Lipids and proteins are mobile within the plane of the membrane (lateral) Lipids and proteins cannot readily move acrossthe plane of the membrane (transverse) 6 Campbell’s Biology © 2021 Pearson Press CHALLENGES: Membranes have high lateral fluidity & mobility › How do membranes regulate fluidity across different temperatures? Membranes provide a barrier to exclude unwanted components › How does the cell import nutrients and export wastes? Membranes spontaneously form to contain and define the cell › How do cells identify other cells (self-identification or pathogen identification)? 7  MEMBRANE FLUIDITY Fluidity is a function of lipid packing › Tightly packed lipids are lessmobile, more viscous › Loosely packed lipids are less restricted, more fluid Varying fatty acid content canchange membrane fluidity 8 Campbell’s Biology © 2021 Pearson Press  HOWDOWE MEASURE MEMBRANE FLUIDITY? Bilayers can go through a phase transition with temperature › ‘Melting’ when transiting from a less fluid gel phase to a fluid phase Saturated fatty acids give bilayers a higher melting point than unsaturated fatty acids › Cells regulate fatty acid content to maintain the same fluidity at different temperatures » Aquatic organisms & plantsusually have higher unsaturated fatty acids » Mammals that thermoregulate have higher saturated fatty acids 9 Becker’s World of the Cell 9e, ©2016 Pearson  CHOLESTEROL The rigid ring structure of cholesterol disrupts packing of fatty acids Cholesterol and related sterols increase membrane fluidity 10 https://commons.wikimedia.org/w/index.php?curid=1997058 KEY POINTS Cell membranes are stable structures formed from assembly of amphipathic lipids Lipid bilayers are impermeable to most polar molecules Proteins occupy a large part of the membrane in a ‘fluid mosaic’ Membrane fluidity is regulated to maintain a constant viscosity › Fluidity increases with temperature, unsaturated fatty acids, cholesterol › Fluidity decreases with saturated fatty acids MEMBRANE PROTEINS Integral proteins or transmembrane proteins are those that span the bilayer Hydrophobic amino acid sidechains through the transmembrane region,hydrophilic sidechains on either side Membrane proteins facilitate transport, signal transduction, cell-cell recognition, intercellular contact, enzymatic functions, and attachment to cytoskeleton and extracellular matrix Other proteins can associate peripherally with either surface 12 Campbell’s Biology © 2021 Pearson Press  MEMBRANE PROTEINS Spanning membranes allows proteins to overcome many barriers the membrane presents Proteins also allow the membrane to act as a specific compartment to organize functions 13 Campbell’s Biology © 2021 Pearson Press TRANSPORT ACROSS THE CELLMEMBRANE 1. Passive transport –requires no energy expenditure but only moves substances down a concentration gradient › Simple diffusion › Facilitated diffusion 2. Active transport –requires energy to move substances up a concentration gradient › Transport ATPases (uniporters) › Indirect active transport –Co-transport (symport or antiport) 14 SIMPLE DIFFUSION - OSMOSIS The cell membrane is semi-permeable › Allows free passage of H2O, O2, CO2 › Blocks polar molecules (glucose) In a semi-permeable membrane water Osmotic pressure moves to equilibrate solute concentrations This movement of water can build pressure across a membrane –osmotic pressure 15 Campbell’s Biology © 2021 Pearson Press  TONICITY – PLANTS VS ANIMALS Animal cells require an external environment that has equal [solutes] to the cell interior Plant cells prefer a hypertonic environment and use osmotic pressure to make the cell and plant rigid Campbell’s Biology © 2021 Pearson Press 16  TRANSPORT - DIFFUSION Solutes diffuse from high to low concentration › Gases (O2, CO2) and hydrophobic compounds can freely diffuse across bilayers Transport proteins can provide specific (facilitated) passage for polar molecules across the membrane › Glucose transport into cells ispassive 17 Campbell’s Biology © 2021 Pearson Press Becker’s World of the Cell 9e, ©2016 Pearson GLUCOSE TRANSPORT Glucose transport is a specific form of facilitated diffusion Glucose only travels from high to low concentration BUT only glucose can pass through the GLUT transport protein 18  Na+/K+ACTIVE TRANSPORT Na+/K+ ATPase is a transporter in every (animal) cell ATP hydrolysis provides energy to transport both ions up a concentration gradient 3 Na+ are exported, 2 K+ imported per ATP hydrolyzed 19 Campbell’s Biology © 2021 Pearson Press  INDIRECT ACTIVE TRANSPORT Co-transport is the movement of one solute to drive a second Na+ or H+ gradients established using ATP Na + /H + moves down a gradient, carrying another solute up a gradient Campbell’s Biology © 2021 Pearson Press 20  MEMBRANE POTENTIAL Na+ (high) Cells maintain ions at an imbalance K+ (low) Cl- (high) across the plasma membrane extracellular › Na+/K+ in animal cells › 3Na+ per 2K+ transport creates a cytoplasm -60 mV charge imbalance Na + (low) Cl- (low) K+ (high) This creates a differential charge across the membrane  membrane potential proteins, DNA, RNA (trapped anions) Membrane potential is used to drive indirect active transport 21  HUMAN HEALTH – CYSTIC FIBROSIS Lung mucous is regulated partly by active transport of Cl- Cystic fibrosis is caused by a mutation in a lung Cl- transporter (CFTR –cystic fibrosis transmembrane conductance regulator) CF leaves an individual susceptible CFTR to lung infections and damage [PDB:5UAK] 22 Becker’s World of the Cell 9e, ©2016 Pearson KEY POINTS Membrane proteins associate with and functionalize the membrane › Integral membrane proteins span the bilayer –transport, signaling, cell-cell contact, cytoskeleton/extracellular matrix attachment, enzymes, etc › Peripheral proteins associate with one surface –signaling, enzymes Proteins provide transport routes for specific polar molecules › Passive transport uses simple diffusion down a concentration gradient › Active transport uses cellular energy to move solutes up a concentration gradient › Indirect active transport couples active transport of ions (Na+/H+) to drive other solutes up a concentration gradient (cells establish membrane potential) Membranes semi-permeable to H 2 O but not solutes are sensitive to osmotic pressure 23 BULK TRANSPORT Most macromolecules are too large for protein transporters › Cells have a need to both secrete soluble protein and deliver membrane protein to the PM Bulk export packages macromolecules into membrane bound vesicles and release by exocytosis › Vesicle membrane fuses with the PM bilayer › Delivers newly synthesized membrane proteins to PM › Secretes soluble contents to cell exterior 24  ENDOCYTOSIS Import of large particles and membrane retrieval is endocytosis Phagocytosis – very large particles (e.g. pathogens) Pinocytosis –bulk fluid uptake Receptor-mediated endocytosis – specific particle uptake (e.g. LDL) 25 Campbell’s Biology © 2021 Pearson Press  LOW DENSITY LIPOPROTEINS Cholesterol and triacylglycerols travel through blood as lipoprotein/fat droplets (LDL) Uptake of LDL is receptor mediated and driven by forming a clathrin coat Defects in the LDL receptor cause heart disease Campbell’s Biology © 2021 Pearson Press 26 KEY POINTS Macromolecules are too large to move through transport proteins and require bulk transport routes › Exocytosis –export of vesicle bound content through fusion with the PM › Endocytosis –import of soluble material or particles by capture into vesicles » Phagocytosis captures large particles (e.g. pathogens captured by immune cells) » Pinocytosis captures bulk soluble material » Receptor-mediated endocytosis captures specific large particles in coated vesicles (e.g. LDL uptake) 27