Cell Membrane and Cytoskeleton - BMS100_PHL1-20_W23

Lipid components • Glycerophospholipids § Fatty acid “tail” – usually 16 – 18 carbons • Unbranched, may or may not be saturated • Usually even number of carbons § Glycerol backbone • Ester linkage to FA tails § Phosphate “head” • Usually the “R” is linked (ester) to another molecule § Choline (mos...

Lipid components • Glycerophospholipids § Fatty acid “tail” – usually 16 – 18 carbons • Unbranched, may or may not be saturated • Usually even number of carbons § Glycerol backbone • Ester linkage to FA tails § Phosphate “head” • Usually the “R” is linked (ester) to another molecule § Choline (most common) § Ethanolamine, glycerol, inositol, serine Anatomy and Physiology – 2nd ed. p. 83, fig. 3.2 https://openstax.org/books/anatomy-and-physiology-2e Lipid components • Cholesterol § A type of steroid § Intercalates between phospholipids with the –OH closest to the aqueous interface § Amount of cholesterol impacts membrane fluidity • Smaller amounts – “stiffens” the membrane à decreased fluidity • Larger amounts – interferes with the interactions between the lipid tails § Increases fluidity Harper’s Illustrated Biochemistry, 31 ed. Fig. 21-19 Lipid components • Sphingolipids § Different structure – “backbone” consists of sphingosine, not glycerol § Slightly different shape can decrease membrane fluidity § Often sphingolipids have sugar residues that can serve a number of functions • Cellular signaling – formation of lipid rafts and myelin (more later) • Contribution to the glycocalyx § Glycocalyx = proteins (glycoproteins) and lipids that are bound to carbohydrates that vary in size § More notable on the outer leaflet of the cell membrane § Many protective structural and signaling functions Glycosphingolipid structures Note: • How the sphingosine backbone differs from the glycerol backbone • The sugar groups on cerebrosides and gangliosides https://en.wikipedia.org/wiki/Sphingolipid#/media/File:Sphingolipids_general_structures.png Structure and function – membrane lipids Phospholipid • Amphipathic nature of lipids in the cell membrane à formation of micelles or formation of bilayers § Both are thermodynamically favourable –hydrophobic tails interact with each other and hydrophilic heads interact with the aqueous environment § As the concentration of phospholipids increase à bilayer formation is more favourable than micelle formation • The phospholipid bilayer is an effective barrier to: § Charged and polar molecules § Medium-sized and large non-polar molecules Harper’s Illustrated Biochemistry, 31 ed. Fig. 40-4 & 40-5 bilayer Micelle Structure and function – membrane lipids • The integrity of the plasma membrane is key to the survival and normal function of the cell § Ionic and fluid homeostasis § Keeps vital molecules needed for cellular metabolism within the cell § Cellular movement and shape are accomplished with interactions between the cell membrane and the cytoskeleton • The membrane system – plasma membrane, endomembrane system, and all membrane-bound organelles – have unique functions § The membrane is the most important way that these functions remain sequestered with these organelles • Loss of membrane integrity à threatened cell survival § The cell membrane can “re-seal” with minor mechanical disruptions – bilayers are thermodynamically favourable § However, severe mechanical or metabolic stresses can lead to degradation Structure and function – membrane lipids • Membranes inside the cell don’t need to: § Signal to other cells § Protect cells from harsh environments or microbes § Form a glycocalyx • Therefore no need for sphingolipids • Much less cholesterol in the membranes of organelles § Perhaps less need for alterations in membrane fluidity? Anatomy and Physiology – 2nd ed. p. 92, fig. 3.13 https://openstax.org/books/anatomy-and-physiology-2e Structure and function – membrane lipids Which intracellular membrane-bound components might have more cholesterol and/or sphingolipids? Why might this be? • Hint… how do you make more plasma membrane? Anatomy and Physiology – 2nd ed. p. 92, fig. 3.13 https://openstax.org/books/anatomy-and-physiology-2e Membrane proteins • Wide range of functions: § Signaling § Transport & General Homeostasis § Protection § Structure & Movement Anatomy and Physiology – 2nd ed. p. 84, fig. 3.4 https://openstax.org/books/anatomy-and-physiology-2e The membrane and cellular homeostasis Key forces at work across the cell membrane: • Diffusion § The movement of molecules from a region of higher concentration to lower concentration § Thermodynamics à spreading of molecules (energy) as they collide against each other • What aspect of Gibbs’ free energy? • To keep molecules close to each other, you would have to impose work on the system • Osmosis § Diffusion of water through a semi-permeable membrane • Semi-permeable = it allows water to pass through, but is impermeable to at least one solute Diffusion & Osmosis - models • Great Wikipedia demonstration of diffusion here: § https://en.wikipedia.org/wiki/Diff usion#/media/File:DiffusionMicro Macro.gif • Most-used model of osmosis discussed in the next slides Osmosis – U-tube • Imagine the model at the right, filled with pure water, with a semi-permeable membrane at the base § The membrane is permeable to water, but nothing else • Water is free to move across the membrane, and thus there is no difference between its “concentration” across either side of it Osmosis – U-tube Let’s add some sugar to the right side – the membrane is impermeable to the sugar 1. 2. What happened? Why? Why didn’t the solution overflow on the right side? Diffusion & Osmosis - Relevance • The cell membranes are semi-permeable – water + many other solutes can traverse them § Most cells express many aquaporins in the plasma membrane à high water conductance § Membranes are impermeable to many solutes especially larger ones • The interior of the cell has a higher concentration of large solutes than the exterior, therefore… • … plasma membrane must expend energy to regulate solute concentration and cell volume • The plasma membrane and organelle membranes rely on concentration gradients for important signaling and metabolic functions (more later) Importance of the Na+/K+ ATPase • The Na+/K+ ATPase is a key plasma membrane transporter – each “cycle” of transport involves § 3 Na+ out of the cytosol à into the extracellular fluid (ECF) § 2 K+ into the cytosol (ICF) à out of the ECF § Hydrolysis of one ATP à ADP + Pi • Responsible for up to 30% of ATP use in many cells § Establishes a gradient of sodium and potassium across the membrane • Prevents cell swelling due to osmosis • Establishes a gradient of charge across the membrane (more next day) • Na+ gradient can be used to transport other substances across the membrane Na+/K+ pump and ICF/ECF fluid ICF and ECF fluid Solute [ICF] [ECF] - blood Na+ 15 mM 142 mM K+ 120 mM 4 mM Cl- 20 mM 102 mM Ca+2 0.0001 mM - 0.05 mM ~ 1 - 2 mM HCO3- 16 mM 24 mM Protein 4 mM 1 mM Importance of the Na+/K+ ATPase • Prevention of cellular swelling § As soon as ATP levels fall to 10% of normal levels, cells start to swell • Why? • Important cellular signaling events can depend on movement of charged particles (Na+, K+, or others) across the membrane § Changes the charge across the membrane and can change cellular activity – more later • Since [Na+] in the ECF is high, there is a diffusional force that drives it into the cell § This force can be used to transport other substances into or out of the cell • Co- or countertransport Type of transport Process Example Active transport A protein moves a substance(s) across a membrane against a concentration gradient using ATP Na+/K+ ATPase Passive transport A protein forms a channel that allows a substance across the membrane, along its concentration gradient Aquaporins Facilitated transport A protein carrier binds to a substance and transports it across a membrane, allowing it to follow its concentration gradient Glucose transporter (GLUT) Cotransport The transport of two substances (X and Y) are Sodium-glucose coupled using the same protein. The concentration co-transporter gradient of X favours movement into the cell – Y is “pulled” along, even if the gradient for Y does not (SGLT-1 and -2) favour cell entry Countertransport X and Y move in opposite directions across the cell membrane – the gradient of one of the molecules supplies the energy to drive the transport Cl-/HCO3 countertransporter Transport and the Cell Membrane • Transport can be passive § In this situation, molecules move across the membrane from high to low concentration, driven by diffusion § Channels or transporters are required unless the molecule is small and hydrophobic (i.e. carbon dioxide, oxygen) • Transport can be active – energy is required § Energy from the hydrolysis of ATP § Energy from the gradient of another molecule § Either way, requires an integral membrane protein Other Cellular Membranes and Gradients • For each organelle, list at least one specific way that the membrane surrounding the organelle is important to the cell • Why not just have that organelle “open” to the cytosol? Plasma Membrane and Signaling • All cells respond to a wide range of extracellular signals § Growth factors and “anti-growth” signals § Signals that change cellular activity (hormones, neurotransmitters) • Most of these signals involve the binding of a chemical (the signal) to a high-affinity protein receptor on the cell membrane à a signal that’s propagated into the cytosol • Most of these receptors have: § hydrophobic domains that extend through the lipid bilayer (usually an alpha-helix) § An extracellular domain that binds to the message (i.e. hormone) § an intracellular domain that amplifies the signal A Typical Extracellular Signal • Whether the cell responds to the signal (the hormone) depends on whether it expresses the receptor Anatomy and Physiology – 2nd ed. p. 670, fig. 17.5 https://openstax.org/books/anatomy-and-physiology-2e Membrane proteins • Wide range of functions: § Signaling § Transport & General Homeostasis § Protection § Structure & Movement Anatomy and Physiology – 2nd ed. p. 84, fig. 3.4 https://openstax.org/books/anatomy-and-physiology-2e Membrane proteins - structure • Membrane proteins are important to the overall structure of the cell § Can link the cell membrane to important extracellular structures § Can link the cell membrane to the cytoskeleton • Membrane proteins that link the cell to extracellular structures (ECM, other cells) are known as junctions Tight junctions • Separate cells into apical and basal compartments § Remember epithelial cells? • The junction can be very selective and “leak-proof”, or be less selective • Purpose - commonly regulates movement across membranes and other epithelial structures Anchoring junction - desmosome • Intracellular component – a plaque formed of molecules that are associated with cadherins § Intermediate filaments (see cytoskeleton below) bind to the plaques • Extracellular component – cadherins on one cell interact with cadherins on a neighboring cell • Purpose – structural integrity for a wide range of cells and tissues Anchoring junction – hemi-desmosome • Similar to a desmosome, but extracellular component involves a protein known as an integrin § Plaque still binds to an intermediate filament § The integrin commonly binds to the basement membrane § Usually how epithelial cells stay anchored to the basement membrane and underlying connective tissue Anchoring junction – adherens junction • These junctions also contain a plaque and may connect to: § Another cell – cadherins § A basement membrane – integrins • However, they do not connect with intermediate filaments § Instead, they connect with microfilaments formed from actin (see cytoskeleton below) The Cytoskeleton • Performs the following functions: § cellular movement • cell can move through space (macrophage, fibroblast) or can change shape, performing work (muscle cells) § organization of cellular components/organelles • during mitosis and meiosis (interaction of DNA) • shuttling of organelles and membrane-bound vesicles § cellular structure • strength (skin), shape (absorptive surfaces) § communication • intracellular signals that regulate growth, general function The Cytoskeleton - Review Cytoskeletal structures and functions: • Microtubules • Trafficking of organelles and cell division • Organization of overall cellular structure • Cellular movement • Molecule – tubulin • Microfilaments • Cellular movement • Structural organization of the plasma membrane • Molecule - actin • Intermediate filaments • Overall structural integrity of the cell • Variety of molecules – keratins, desmin Openstax – Anatomy and Physiology 2e, page 92, fig. 3.13 Features of the cytoskeleton • Dynamic § filament subunits (monomers, heterodimers) are constantly “building themselves” into their polymer strands • Once monomers reach a critical concentration and interact with nucleating factors à polymer formation • Tightly regulated § filament assembly and disassembly is regulated by a huge array of intracellular signals § govern structure and formation of polymers • Can generate force § many proteins interact with the strands of the cytoskeleton: • move the cell or organelles along the axis of the strands • generate power or motion in cells through contraction Microfilaments - Actin • Monomer – G-actin; polymer – F-actin • Nucleating factors stimulate the formation of Factin § Linear arrangements – formin § Mesh-like nets – ARP 2/3 complex • When F-actin is formed, it spontaneously degrades § Each G-actin has an ATP bound • Over time, the G-actin hydrolyzes ATP to ADP, which makes it more likely that it will “fall off” the F-actin strand ATP ATP ATP ADP ADP Harper’s Illustrated Biochemistry, 31 ed. Fig. 51-3 Microfilaments - Actin • The stability of F-actin depends on a number of factors § Concentration of G-actin § “caps” that can be applied to the F-actin that prevent disassembly • Tropomodulin, capping proteins § Proteins that speed up or slow down the rate that G-actin hydrolyzes its ATP § Nucleating factors or inhibitory factors that modify the formation of F-actin (i.e. formin) ATP ATP ATP ADP ADP Harper’s Illustrated Biochemistry, 31 ed. Fig. 51-3 Microfilaments – actin in cellular structures Examples: • A muscle cell: § needs a stable, very organized system of parallel actin fibres § generates lots of force § Therefore actin filaments are “long-lasting”, very precisely organized, and strongly anchored to the cell membrane and other proteins • A fibroblast: § Needs to “crawl” to an area in order to deposit collagen/ECM § Needs to “explore” its environment in order to find the right places to deposit collagen/ECM § Therefore it organizes its actin filaments into meshes, fibres, and filopodium (kind of like feelers) under the cell membrane in order to accomplish these tasks Actin strategies The Sarcomere of a Skeletal Muscle Cell A Fibroblast Molecular Biology of the Cell, 6th ed., p. 911, fig. 16-21 Anatomy and Physiology – 2nd ed. p. 362, fig. 10.5 https://openstax.org/books/anatomy-and-physiology-2e Microtubules • More complicated than actin, but shares similar features: § Protein monomer = tubulin • Two types – alpha and beta – form dimers • Alpha-beta dimers organize themselves in a helical tube § Beta monomers also hydrolyze a nucleotide triphosphate • For tubulin, they cleave GTP to GDP + Pi • After GTP is cleaved, the dimer tends to “fall off” the microtubule and it falls apart § Known as dynamic instability Harper’s Illustrated Biochemistry, 31 ed. Fig. 51-13 Microtubules Important for: • Cellular organization – microtubules are formed from a microtubule organizing centre (MTOC) § MTOC composed of two centrioles that form the centrosome • Centrioles are also composed of microtubules, but have a unique shape – a tubulin triplet structure (9 tubulin triplets, known as the “9X3” structure) § The MTOC and the microtubules that radiate from it form a cellular scaffolding that • “moves” and “places” cellular structures at specific places in the cell • helps determine polarity of the cell – i.e. if a cell has a side that faces a lumen and a side that faces a basement membrane Microtubules Important for: • Cellular movement § Cilia and flagella are formed from microtubules – these structures rotate and move in a whip-like fashion • Flagella – present in sperm cells • Cilia – present in the respiratory mucosa • Cell division § The MTOC splits and pulls chromosomes to new daughter cells using newly-generated microtubules • Signaling – the primary cilium § primary cilium found on most cells § able to sense important stimuli in the extracellular environment to help with cellular localization or function Microtubules in cell division and cellular organization Microtubules in green Actin in red Microfilaments, Microtubules, and Cellular Motors • F-actin and microtubules form a network of dynamic filaments that “molecular motors” can move along, like a train moves along a track § F-actin à myosin is a protein that can “walk” and advance along microfilaments • Multiple different types of myosin exist • In muscle, myosin movement along an actin scaffold causes the entire cell to contract § Microtubules à dyneins and kinesins are other proteins that can move along microtubules and cause the “whipping” movements of cilia and flagella • Dyneins and kinesins move in opposite directions along microtubules § These molecular motors use ATP to move along the cytoskeleton, and they usually “drag” a structure along with them Molecular motors and microtubules Intermediate filaments • Many functions – commonly confer stability to cells • Much more diverse than actin & tubulin § encoded by 70 different genes § Long proteins with an alpha-helix conformation that coil around other monomers to form dimers • Often the dimers coil around each other as well – a “coiled coil” • Much more stable than actin or tubulin – do not hydrolyze GTP or ATP… so do not dissociate as readily § They can change their structure in response to cellular needs, though Molecular Biology of the Cell, 6th ed., p. 945, fig. 16-67 Intermediate Filaments Type of intermediate filament Location Lamins A network of filaments just under the nuclear membrane Keratins Epithelial cells, hair, nails Strong, modified to limit water permeability Vimentin family - Vimentin - Desmin - GFAP Confer stability and structure to: - Many mesenchymal cells - Muscle cells, some epithelial cells - Glial cells (a type of brain cell) Neurofilaments Intermediate filaments found in neurons The Cytoskeleton in an Idealized Epithelial cell Note: • Actin filaments are responsible for the shape of the microvilli • Desmin is organized to provide strength across the cell • The microtubules have a “+” and “-” end • How do you think this is related to the overall shape of the cell? Molecular Biology of the Cell, 6th ed. p. 893, fig. 16-4