Eukaryotic Cell Membranes PDF

PUBLIC / CYHOEDDUS Cell Membranes – Lipids and Proteins  PUBLIC / CYHOEDDUS Learning Outcomes Understand the functions of cell membranes Explore the components of cell...

PUBLIC / CYHOEDDUS Cell Membranes – Lipids and Proteins  PUBLIC / CYHOEDDUS Learning Outcomes Understand the functions of cell membranes Explore the components of cell Membranes Explore the importance of membrane Lipids Understand the structure Understand how they form membranes Explain their distribution in Membranes Understand membrane Fluidity Understand the Fluid mosaic model Explore different types of proteins in the membrane Location Function Understand that the membrane is semi-permeable  PUBLIC / CYHOEDDUS Cell Membrane Cell Membrane Encapsulates Cell Protection Semi-permeable Controls movement of molecules in and out of the cell Maintains 2 environments Communication Proteins in membrane can act and sensors (receptors) Molecules can act as signals 3 main components Phospholipids Majority of membrane Arranged as a double layer ~5nm thick Proteins Cholesterol  PUBLIC / CYHOEDDUS What are lipids? Chemically relatively simple Mostly unreactive hydrocarbons Diverse group of water insoluble biological molecules Range from totally insoluble to partially soluble in water Partially soluble lipids are called amphiphatic Play a role in virtually every aspect of cell biology and in disease states in the body Nonpolar lipids – energy storage Polar lipids – membrane formation  PUBLIC / CYHOEDDUS Lipid Bilayer Hydrophilic Made up of mainly phospholipids 4 components Hydrophobic Fatty acid tails Backbone Phosphate Group Small Polar Group Amphipathic= both hydrophilic and hydrophobic sections  PUBLIC / CYHOEDDUS Phospholipids  PUBLIC / CYHOEDDUS Asymmetric Membrane 4 major phospholipids Phosphatidylcholine Phosphatidylethanolamine Phosphatidylserine Sphingomyelin These 4 account for majority of the lipids in most membranes Asymmetrically distributed Outer membrane= phosphatidylcholine and sphingomyelin Inner Membrane= phosphatidic acid, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine  PUBLIC / CYHOEDDUS Glycolipids Don’t contain phosphate group 3 Components Hydrophilic Fatty Acid Chains Serine Backbone Carbohydrate Amphipathic= both hydrophilic and hydrophobic sections Hydrophobic ~2% of lipids in membrane  PUBLIC / CYHOEDDUS Glycolipids Serve as recognition sites for cell-cell interactions Sugar group will bind to a specific complementary carbohydrate or to a lectin (carbohydrate-binding protein) of a neighbouring cell Interaction of cell surface markers is the basis of cell recognitions and initiates cellular responses Regulation Growth Immune responses Apoptosis When it goes wrong can result in conditions such as cancer and autoimmune diseases such as rheumatoid arthritis, MS and Type I diabetes  PUBLIC / CYHOEDDUS Cholesterol Major component of membranes 25% of membrane lipids in certain nerve cells A steroid lipid 4 linked hydrocarbon rings hydrophobic Hydrocarbon tail hydrophobic Hydroxyl (OH) group Polar= weakly hydrophilic Precursor to steroid hormones including Testosterone and oestrogen Signalling molecules All contain 4 hydrocarbon rings with distinct functional groups attached  PUBLIC / CYHOEDDUS The Picture So Far Phosphatidic acid  PUBLIC / CYHOEDDUS Membrane Fluidity- Types of Motion Lateral diffusion Rapid movement of lipids within the plane of one monolayer Transverse diffusion (Flip-flop) Lipids move from one layer of the membrane to the other layer  PUBLIC / CYHOEDDUS Membrane Fluidity Affected by temperature, cholesterol and saturated/unsaturated fatty acids. TEMPERATURE – affect how phospholipids move, and how close together they pack. COLD – closer together, HOT – move further apart.  PUBLIC / CYHOEDDUS Membrane Fluidity- Cholesterol Cholesterol molecules randomly distributed across the phospholipid bilayer. Stops phospholipids getting too close together preventing freezing They help the membrane stay stiff under different environmental conditions Holds phospholipids together so they don’t separate too far, or compacting too tightly  PUBLIC / CYHOEDDUS Membrane Fluidity- Saturated vs Unsaturated Phospholipid tails (Fatty Acids) Saturated – carbon atoms with single bonds, straight, and easy to pack. Unsaturated – carbon atoms with some double bonds = kinks. Harder to pack. Kinks = increase space between phospholipids, affecting fluidity PUBLIC / CYHOEDDUS  PUBLIC / CYHOEDDUS The Fluid Mosaic Model Proposed by Singer and Nicolson in 1972 Describes the arrangement of lipid and protein within a membrane According to the model:- Membrane is a dynamic structure Both proteins and lipids can rapidly and randomly diffuse laterally or rotate within the bilayer Movement depends on fatty acid chain Hydrocarbon length Degree of unsaturation  PUBLIC / CYHOEDDUS Membrane proteins Classed based on their mode of association with the lipid bilayer Responsible for the dynamic processes carried out by membranes Mediate nearly all other membrane functions Mass ratio of lipids to proteins ranges from 4:1 to 1:4 Myelinated neurons high % lipids because lipids play crucial role in insulation Mitochondria high % proteins because function is production of energy Cell membrane approx. 50% protein by mass  PUBLIC / CYHOEDDUS Integral membrane protein Contain hydrophobic and hydrophobic regions Mainly transmembrane proteins, Usually span the bilayer completely with hydrophobic core and hydrophilic termini Only removal by agents that interfere with hydrophobic interactions e.g. detergents  PUBLIC / CYHOEDDUS Integral Membrane Protein- Detergents Detergents are amphipathic Hydrophobic regions bind to hydrophobic regions of protein Hydrophilic regions surround complex Detergent/Protein complex= soluble  PUBLIC / CYHOEDDUS Peripheral membrane proteins Attach to the polar heads of phospholipids or onto polar region of an integral protein Associated with one face of the membrane through charge-charge interactions and hydrogen bonding Can dissociate from the membranes by changes in pH or ionic strength  PUBLIC / CYHOEDDUS Lipid anchored membrane proteins Tethered to a membrane by covalent bonding to a lipid anchor Most are permanently attached to the membrane Can be cytosolic or extrinsic  PUBLIC / CYHOEDDUS Glycocalyx Integral proteins extracellular regions or extracellular peripheral proteins often glycosylated Cell surface covered by carbohydrate coat known as glycocalyx Formed by associated oligosaccharides, glycolipids and glycoproteins  PUBLIC / CYHOEDDUS Membrane Fluidity Lipid and proteins freely diffuse laterally May be necessary to separate membranes into sections (domains) Separated by cell-cell junctions (eg Tight junctions) Prevents lateral diffusion between domains  PUBLIC / CYHOEDDUS What can cross the membrane? Phospholipids are always in motion – bouncing around. Creates incredibly small spaces in the membrane – still an effective barrier. Small molecules can sneak through the gaps. So it’s known as SEMI-PERMEABLE.  PUBLIC / CYHOEDDUS Movement of Molecules Small- non-polar molecules Gasses (O2 , CO2 , N2 ) Passive diffusion Fast Small- polar molecules  Hydrophobic Region Water, Ethanol Passive diffusion  Likes non-polar, Slow uncharged molecules Large, non-polar molecules  Tightly packed Benzene Passive diffusion phospholipids Slow  Likes small molecules Large, polar Glucose, Amino acids, Nucleotides No movement Charged molecules (ions) Cl-, Na+ No movement  PUBLIC / CYHOEDDUS Passive Diffusion Gases and uncharged molecules pass freely Termed passive diffusion From a region of high concentration to low concentration From outside the cell to the inside of the cell.  PUBLIC / CYHOEDDUS Proteins o Proteins help with transport of other molecules across the membrane  PUBLIC / CYHOEDDUS Getting through the cell membrane Diffusion Movement of molecules from a high concentration to a low concentration until it is equal Facilitated diffusion Transport of larger molecules through a protein channel Movement from high low Active transport Movement from low  high Uses a protein channel (pump) Requires energy from ATP  PUBLIC / CYHOEDDUS Transporters Allow molecules to cross the membrane Specific for particular substrates Uniport – transporters that carry only one substrate Symport – Carry 2 substrates moving simultaneously in the same direction Antiport – carry 2 substrates moving in opposite directions  PUBLIC / CYHOEDDUS Facilitated Diffusion Charged and larger molecules require assistance to move down their concentration gradient. – cannot get through the phospholipid bilayer! Proteins offer way for molecules to travel in an energetically favourable direction (down concentration/ electrochemical gradient) Even though they don’t need energy to drive their diffusion, they need a way of facilitating their crossing of the membrane. Requires specialist channels to produce a passageway. Allow multiple ions to move simultaneously.  PUBLIC / CYHOEDDUS Channel Proteins Multiple membrane spanning regions create a pore When pore is open molecules are free to flow across membrane Any molecules of appropriate size and charge Na+, K+, Ca2+, Cl- Selectively opened in response to signals  PUBLIC / CYHOEDDUS Carrier Proteins-Passive Transport Contain multiple membrane spanning regions Selectively bind specific molecules E.g. Glucose Induces a conformational change Molecules are released on the other side of the membrane  PUBLIC / CYHOEDDUS Active transport Transport of molecules across the membrane in an energetically unfavourable direction Against concentration gradient This requires energy ATP-driven (primary) – maintain the electrochemical gradients of several ions. Coupled transporters- (secondary) – the gradient is used by coupled transporters to move other solutes at the same time.  PUBLIC / CYHOEDDUS Active Transport- ATP ATP binding and hydrolysis provides energy Used to create electrochemical gradients Primary active transport  PUBLIC / CYHOEDDUS ATP-powered pumps There are four types P-class. F-calls V-class ABC P,F and V types transport only ions ABC (ATP Binding Cassette) – transport small molecules and ions. All use the energy from ATP to drive the process, but the architecture of the protein pumps is different  PUBLIC / CYHOEDDUS Active Transport-Coupled Transporters Transporter couples the movement of a molecule down electrochemical gradient to the movement of a molecule up its electrochemical gradient Driving ion= ion that is moving down electrochemical gradient Secondary active transport as electrochemical gradient of driving ion is created through primary active transport  PUBLIC / CYHOEDDUS Primary Transport- Na+/K+ Pump Exchanges 3Na+ for 2K+ Uses ATP as energy source Vital to numerous bodily processes Nerve cell signalling Heart contractions Kidney function  PUBLIC / CYHOEDDUS Secondary Transport- Na+ Gradient  PUBLIC / CYHOEDDUS Getting through the cell membrane Diffusion Movement of molecules from a high concentration to a low concentration until it is equal Facilitated diffusion Transport of larger molecules through a protein channel Movement from high low Active transport Movement from low  high Uses a protein channel (pump) Requires energy from ATP  PUBLIC / CYHOEDDUS Facilitated Diffusion vs Active Transport  PUBLIC / CYHOEDDUS Alternative for large molecules  Occurs through vesicles and vacuoles  Endocytosis  large molecules or whole cells by engulfing them  Phagocytosis  Exocytosis  removes or secretes substances such as hormones or enzymes  PUBLIC / CYHOEDDUS Summary Biological membranes are fluid structures which can alter under different conditions The fluid mosaic model describes this theory and is supported by the movement of the lipids Lateral Transverse They are vital structures for cell-cell interactions, cell communication and transport of ions and molecules into and out of cells There are 3 types of membrane proteins found at the cell surface Integral Peripheral Lipid anchored  PUBLIC / CYHOEDDUS Extra reading The Cell 2nd edition https://www.ncbi.nlm.nih.gov/books/NBK9898/ https://www.ncbi.nlm.nih.gov/books/NBK9879/ https://www.ncbi.nlm.nih.gov/books/NBK9928/ https://www.khanacademy.org/science/high-school-biolo gy/hs-cells/hs-the-cell-membrane/v/fluid-mosaic-model- of-cell-membranes https://www.khanacademy.org/science/ap-biology/cell-st ructure-and-function/membrane-permeability/v/cell-me mbrane-introduction