Lec 3 Cell Membrane Biochemistry - Feb 24, 2024 PDF

# OBJECTIVES * Basics of Cell * Visualization * Cell Theory * Cell Membrane and its structure * Cell Membrane Components * Lipids * Proteins * Carbohydrates * Mosaic Model * Functions of Cell * Transport (Passive & Active) * Receptor Hormone, * Biochemical Reacti...

# OBJECTIVES * Basics of Cell * Visualization * Cell Theory * Cell Membrane and its structure * Cell Membrane Components * Lipids * Proteins * Carbohydrates * Mosaic Model * Functions of Cell * Transport (Passive & Active) * Receptor Hormone, * Biochemical Reaction # CELL * Structural and functional unit * Most biochemical reactions take place here > Organelles of the Cell (This is a diagram) > * A cell is shown with a nucleus, a nucleolus, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, and vacuoles. # Visualizing Cells | Object | Size | |-------------------|---------| | Giant sequoia | 100m | | Elephant | 10m | | Adult human | 1m | | Hen egg | 10cm | | Paramecium | 1cm | | Human egg | 1mm | | Human red blood | 100µm | | Bacterium | 10µm | | Large Virus (HIV) | 1µm | | Ribosome | 100nm | | Protein | 10nm | | Amino acid | 1nm | | Hydrogen atom | 0.1nm | > Human Eye > Light Microscope > Electron Microscope # Major Events in Cell Biology and Imaging | Event | Year | |--------------------------------------------|-------| | Robert Hooke observes cells of a cork tree | 1655 | | Leewenhoek discovered bacteria | 1683 | | Leewenhoek discovered protozoa | 1674 | | Kolliker described mitochondria in muscle | 1857 | | Schleiden and Schwann proposed the cell theory| 1838 | | Koch identified bacteria causing TB and cholera | 1882 | | Ruska built the first transmission electron microscope | 1931 | | Golgi stained cells with silver nitrate discovering the Golgi apparatus | 1898 | | Sheep cloned | 1997 | | First commercial scanning election microscope | 1965 | # Cell Theory > Developed By > 1. Theodor Schwann > 2. Matthias Schleiden > 3. Rudolf Virchow > A photo of Rudolf Virchow is shown. # Cell Theory 1. All organisms are made of 1 or more cells. 2. Cells are the basic building blocks of life. 3. All cells come from existing cells. > A picture of a microscope and Theodre Schwann is shown. > A photo of Rudolf Virchow is shown. > A handwritten note is visible on this image: "Theodore Schwann", "Rudolf Virchow", "Microscope", "Theodore Schwann" # CELL MEMBRANE STRUCTURE * 50-80 Angstrom thick, lipid bilayer with polar heads outside and apolar hydrocarbon tails inside * Highly fluid, dynamic structure * Selective permeability * Barrier- Different concentrations outside and inside the cell * Asymmetric lipid bilayer with distinct inner and outer surface # Structure of the Cell Membrane * Phospholipid bilayer * Hydrophobic Hydrocarbon Tail * Hydrophilic Phosphate Head * Cholesterol * Glycoproteins * Integral proteins * Peripheral Proteins > A diagram shows the cell membrane with the following layers: > * Exterior layer- Hydrophilic Phosphate Head > * Middle layer - Hydrophobic Hydrocarbon Tail > * Inner layer - Hydrophilic Phosphate Head > Between the layers Cholesterol and Glycoproteins are embedded > Exterior layer - Integral proteins > Interior layer- Peripheral proteins # Davson and Danielli * Saw 2 dark lines with a lighter band in between * Proteins usually look dark on micrographs and lipids look lighter > A diagram of the Davson-Danielli model is shown. > A diagram of a cell membrane is shown which shows evidence of the Davson-Danielli model # Falsification of Davson and Danielli > **Falsification of the Davson-Danielli Model** - freeze fracturing. > A diagram of a cell membrane is shown which shows evidence of the Davson-Danielli model. > * The fracture occurs along lines of weakness, including the center of membranes. > * The fracture reveals an irregular rough on the phospholipid bilayer. > * The globular structures were interpreted as transmembrane proteins. > A Handwritten note is visible: "Analysis of the falsification...., "The technique involved..." > **Conclusion:** > * This is contrary to the Davison-Danielli model which only involves proteins coating the surface of the membrane. > * A new model is needed to explain the presence of as transmembrane proteins. # Cell Membrane Theory * Singer and Nicolson (1966) * Proteins occupy a variety of positions in the membrane * They are also capable of moving their location * Like tiles in a mosaic * Fluid-Mosaic Model > A diagram of the cell membrane model is shown in this image. # Fluid Mosaic Model * Phospholipids in membranes are in a fluid state. * This allows membranes to change shape. * Proteins are seen to be sporadically placed in the membrane. * Proteins float within the membrane as well as on the surface. > A handwritten note: "24/02/2" # FLUID MOSAIC MODEL * Lateral Diffusion: - * Integral protein and phospholipids- rapid redistribution in the lateral plane of membrane * Proportional to lipid composition * ↑ Temperature → Transition from order to disorder →↑fluidity (Transition temperature) * ↑ Unsat. FA → cis form →↑fluidity of bilayer # "CIS" CONFORMATION INTRODUCES KINKS > A diagram of a cell membrane is shown. Phospholipids are shown with "cis" confirmations, causing kinks in the hydrocarbon chain. # The Fluid-Mosaic Membrane Model * Membranes are not static; they have a fluid consistency. * Most membrane lipids and proteins can drift about laterally in the plane of the membrane. * Cholesterol enhances membrane fluidity, allows animal membranes to function in a wide range of temperatures and also makes the membrane less permeable to biological molecules. # Membrane Components > A diagram of a cell membrane is shown. > * Phospholipids are shown in blue > * Cholesterol is shown in yellow > * Proteins are shown with purple heads and green tails > * Glycocalyx is shown in green > * Cytoskeleton is shown in purple | Component | Description | |---------------------|----------------------------------------------------| | Phospholipids | Bilayer | | Cholesterol | | | Proteins | Peripheral and Integral | | Carbohydrates (glucose)| | # Phospholipids * Partly hydrophilic and hydrophobic (amphipathic) * Phospholipid phosphate groups are attracted to water where as their hydrocarbon tails are not * Hydrocarbon tails are attracted to one another just as the phosphate heads are attracted to other phosphate heads. * This attraction makes for a strong but fluid membrane. > A diagram of Phospholipids is shown in this image. # Structure of Phospholipids > A diagram of a Phospholipid is shown in this image. > * The head is shown with the following components: > * Polar group > * Phosphate > * Glycerol > * The tail is shown with the following components: > * Fatty acid chain # CELL MEMBRANE LIPIDS * Phospholipids (e.g. phosphoglyceride, sphingomyelin), glycosphingolipids and cholesterol * Amphipathic lipids (e.g. lecithin, cephalin) * Saturated fatty acids have straight tails, Unsaturated having cis double bonds → kinks-fluidity * Lipid soluble material easily enters the cell # Membrane Fat Composition Varies * Fat composition affects flexibility * Membrane must be fluid & flexible * About as fluid as thick salad oil * % Unsaturated fatty acids in phospholipids * Keep membrane less viscous > Two diagrams are shown in this image: > * **Fluid:** Unsaturated hydrocarbon tails with kinks > * **Viscous:** Saturated hydrocarbon tails # Cholesterol In Membrane * Contributes to fluidity * Tucked in between phospholipid molecule * Prevent packing and crystallization of F.A. # Cholesterol In Membranes * Found in animal cells * Types of lipid called a steroid * Mostly hydrophobic so it attaches to the lipid portion of the membrane * It does have a small hydrophilic portion that is attached to the phosphate head * It is usually found between phospholipids # More Than Lipids * In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer > A diagram shows a hydrophilic region of a protein embedded in a phospholipid bilayer. The hydrophobic region of the protein is shown embedded in the hydrocarbon tails. # Role of Cholesterol in Membranes * Controls the fluidity of the membrane * Too fluid and it cannot control what comes in/out * Not fluid enough and it restricts cell movement * Cholesterol disrupts the packaging of the hydrocarbon tails (so not so rigid) * Reduces the permeability of some ions # Membrane Proteins 1. Hormone Binding Site: * Site exposed on the outside of the membrane allows one specific hormone to bind thus transmitting a signal to the inside of the cell 2. Enzymes: - * Enzymes located in the membranes either catalyze reactions inside or outside the cell (this depends on where their active site is) 3. Cell Adhesion: * Forms tight junctions between cells and tissues. 4. Cell to Cell communication: * Cell receptors receive signals via hormones or neurotransmitters # Membrane Proteins 5. Passive Transport Protein: * Channels or passages through the center of the membrane proteins. Each channel is specific for certain substances. 6. Protein Pumps for Active Transport: * Pumps release energy from ATP and use it to move specific substances across the membrane. # CELL MEMBRANE PROTEINS (1) Amphipathic * Form integral part of membrane * Interact with phospholipids—globular * Anchored to 1 leaflet/span the bilayer (2) Peripheral proteins * Bound to hydrophilic regions of phospholipid/integral protein # MEMBRANE PROTEINS * Proteins determine membrane's specific functions * Cell membrane & organelle membranes * Each have unique collections of proteins * Membrane proteins: * Peripheral proteins * Loosely bound to the surface of membrane * Cell surface identity marker (antigens) * Integral proteins * Penetrate lipid bilayer, usually across the whole membrane * Transmembrane protein * Transport proteins * Channels, permeases (pumps) > A diagram shows 2 cell membranes structures. One is shown with peripheral proteins attached to the exterior of the cell membrane. The other diagram shows an integral protein embedded within the cell membrane. # Proteins Domains Anchor Molecule * Within membrane * Nonpolar amino acids * Hydrophobic * Anchors protein into membrane * On outer surfaces of membrane * Polar amino acids * Hydrophilic * Extend into extracellular fluid & into cytosol > A diagram shows a cell membrane and protein with polar and nonpolar regions. # Signal Transduction ## Many Functions of Membrane Proteins > 6 diagrams are shown. All show the cell membrane with proteins embedded in them with different functions > * **Transporter:** A protein is shown with an opening through the center of the molecule, going through the membrane from exterior to interior. > * **Enzyme activity:** A protein is shown with the exterior region interacting with a molecule. > * **Cell surface receptor:** A protein is shown with an exterior region interacting with a molecule. > * **Cell surface identity marker:** A protein is shown with an exterior sugar chain on the top. > * **Cell adhesion:** Two proteins are coming together in the middle of the cell membrane. > * **Attachment to the cytoskeleton:** A protein is shown interacting with filaments on the interior of the cell membrane. # Membrane Carbohydrates ## Sugar Coated Cells ### GLYCOCALX * Usually branched molecules of 15 or less sugar units. * Some are bonded to lipids: Glycolipids. * Most are bonded to proteins: Glycoproteins * Play a key role in cell-cell recognition * Ability of a cell to distinguish one cell from another * Antigens * Important in organ & tissue development * Basis for rejection of foreign cells by the immune system * Blood Grouping > A diagram shows a cell membrane with a glycocalyx layer. # Functions of the Plasma Membrane (PM) * Forms a physical barrier (that separates): * Separates inside of cell from outside (forms a compartment). * Selectively Permeable allows some things through but not others * Regulation of movement * Actively regulates or influences what can enter/exit the cell * Connection (Attachment) * Connects cells to other cells and/or surrounding structures * Connects to internal cell parts * Communication → regulation/coordination * Allows cell-cell communication signaling for coordination of activity * Chemical reactions * Chemical reactions take place on the PM * Cell recognition * The PM "labels/identifies" the cell # FUNCTIONS OF CELL MEMBRANE * TRANSPORTERS:- * Transport ions across the membrane to maintain the concentration of electrolytes, water, etc. inside and outside the cell * RECEPTORS:- * For hormones/drugs etc * BIOCHEMICAL REACTIONS # Transport Processes ## Movement across the cell membrane * **Passive Transport** * Substances move from high concentration to low concentration * No energy input required * 3 Types: 1. Simple Diffusion 2. Facilitated Diffusion 3. Osmosis * **Active Transport** * Substances move from low concentration to high concentration * Energy input required * 3 Types: 1. Protein Assisted 2. Endocytosis 3. Exocytosis # Types of Passive Transport | Transport Type | Description | |----------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | Simple Diffusion | Molecules simply diffuse through a semipermeable membrane without the aid of transport proteins. (Oxygen, carbon dioxide) | | Facilitated Diffusion | The transport of polar substances or ions across a semipermeable membrane that requires transport proteins. (ions, salts, potassium, etc.) | | Osmosis | The diffusion of water across a semipermeable membrane. | > A Handwritten note: "OTF 13" # TRANSPORT MECHANISMS 1. PASSIVE DIFFUSION:- * (A) SIMPLE:- * Moves many small molecules across the membrane * Small solutes/gases, enter the cells passively * No energy required * Very slow process * Diffusion along the concentration gradient (high-low) and electrochemical gradient-- equilibrium * ↑ Temperature - ↑diffusion # Facilitated Diffusion * *Is a type of passive transport* * Doesn't require energy * Uses transport proteins to move molecules from high to low concentrations > A diagram shows facilitated diffusion occurring. > * The passage of materials is activated by a concentration gradient and by a transport protein. > **Examples:** Glucose or amino acids moving from blood into a cell. # TRANSPORT THROUGH THE CELL MEMBRANE > 4 diagrams are shown, depicting types of transport through the cell membrane. > * Simple Diffusion: Through the phospholipid bilayer. > * Carrier-mediated Facilitated Diffusion: A protein carrier specific for one chemical binding of a substrate changes shape to transport protein. > * Channel-mediated Facilitated Diffusion: Through a channel protein, mostly ions selected based on size and charge. > * Osmosis: Water diffusing across a semipermeable membrane either from a higher to lower water concentration, or from a lower to higher solute concentration. # I. PASSIVE DIFFUSION (B) FACILITATED DIFFUSION * Carrier mediated process * Carrier can be saturated like Enzymes * Structurally similar solutes → competitive inhibition * Bi-irectional * No energy required * Faster than simple diffusion * Carrier is in Ping-Pong state * Aqua-porins are water channels in RBC/Collecting ducts of kidneys * Mutations in their gene - Nephrogenic Diabetes Insipidus # I. PASSIVE DIFFUSION (B) FACILITATED DIFFUSION * 3 types: * (a) UNIPORT—moves 1 type of molecules bidirectionally * (b)CO-TRANSPORT—2 solutes transferred e.g. Na and glucose * (c)ANTIPORT—moves 2 molecules in opposite directions * Hydrophilic molecules pass in this way * Movement is proportional to transmembrane gradient * Ion channels = transmembrane protein pore—very selective > A diagram shows the 3 different types of facilitated diffusion. # I.PASSIVE DIFFUSION (B) FACILITATED DIFFUSION * Ligand gated channels: Gates are opened by binding of Effectors to Receptor site on the channel. * Ligand may be extracellular signal molecule/intracellular messenger * E.g. * (a)Acetylcholine receptor present in post synaptic membrane:- > Acetylcholine released from the presynaptic region binds with receptors on the postsynaptic region—opens channel—influx of Sodium > (b) Calcium channels: - opened - ↑Ca in muscle cells. > * Ionophores e.g. Valinomycin - Ion gradient lost. # OSMOSIS IN CELLS * The movement of water into and out of a cell is vital to living things, and it is driven by imbalances in concentration * Ideally, the solute concentration outside the cell is equal to that inside the cell. * Sugars, salts and proteins are common solutes on cells. Water is solvent * When water moves inside or out of a cell, the concentration of the solution inside the cell changes. # Active Transport * Requires the cell to use energy * Molecules move against the concentration gradient— from an area of low concentration to an area of high concentration * Active transport is used to: * Move large molecules * Concentrate molecules within the cell * Remove waste from the cell > A diagram shows a message bubble. "This is how we move molecules against a concentration gradient". # ACTIVE TRANSPORT * Requires energy- 40% of total energy expenditure by the cell is for this process * Unidirectional * Requires transport proteins * Transporters are saturated at high solute concentration * Transporters can be inhibited * Movement of solute against concentration/electrical gradient # ACTIVE TRANSPORT E.g. (a) Sodium Pump: - ANTIPORT * Cells have low intracellular sodium and high potassium caused by active extrusion of sodium which enters a cell. * Na+/K+/ATPase enzyme—hydrolyses 1 ATP—ADP+Pi—→efflux of 3 Na+ and influx of 2 K+—↑ electronegativity inside the cell (b) Calcium pump— * Regulates muscle contraction (Ca++ in the sarcoplasmic reticulum regulated). * Each ATP → ADP+Pi—→ 2Ca++ transported—Muscle contraction (c) Secretion of H+ by Parietal cells of the stomach (Proton pumps push out H+) (d) Uptake of Iodide by cells of the Thyroid gland (e) Glucose transported by several mechanisms:- * Glucose enters the skeletal muscles through Transporters (Insulin - ↑ effect) * In small intestine—Glucose and Sodium symport/Glucose uniport. # Receptor G-Protein > A diagram shows a cell membrane with a receptor, a G-protein, and an enzyme or ion channel. > * The receptor is shown with 7 transmembrane domains (7 TMD) > * The G-protein is shown with alpha, beta, and gamma subunits. GDP is bound to the alpha unit. # RECEPTOR-G-PROTEIN-ENZYME > A diagram show 5 steps of a receptor signaling pathway. > * Step 1: A nonsteroid hormone binds to the receptor. > * Step 2: The receptor activates G-protein. The alpha subunit of the G-protein releases GDP and binds GTP. > * Step 3: The activated G-protein activates an enzyme. The enzyme is shown as adenylyl cyclase. > * Step 4: The enzyme produces a second messenger, cAMP, which activates a protein kinase. > * Step 5: The protein kinase produces a cellular effect. # BIOCHEMICAL REACTION > A diagram shows the mitochondrial inner membrane with a protein complex, electron carriers, and ATP synthase. > * The electron carriers shown are embedded within the inner membrane, and they are involved in the electron transport chain. > * The ATP synthase protein is shown protruding from the inner membrane into the intermembrane space. > * The flow of electrons through the electron transport chain is shown as a series of red arrows. > * The electrochemical gradient of protons across the inner membrane is shown as a series of green arrows. > * The production of ATP by ATP synthase is shown as a blue arrow. # EXOCYTOSIS AND ENDOCYTOSIS > 3 diagrams are shown to depict exocytosis and endocytosis. > * **Endocytosis:** A cell is shown taking in a food particle. > * **Exocytosis:** A cell is shown excreting waste. > * **Fusion forms secondary lysosome:** A cell is shown with a vesicle containing a food particle fusing with a lysosome. Both are involved in the process of digestion. # CLINICAL ASPECT OF ENDOCYTOSIS * Receptor mediated endocytosis with viruses are responsible for many diseases. * Hepatitis virus affecting liver cells * Poliovirus affecting motor neurons * AIDS affecting T cells. * Iron toxicity also occurs with excessive uptake due to endocytosis. # CLINICAL CORRELATES OF CELL MEMBRANE PROTEIN 1. Hereditary Spherocytosis 2. Cystic Fibrosis # HEREDITARY SPHEROCYTOSIS * The abnormal erythrocytes are sphere-shaped (spherocytosis) - normal biconcave disk shaped. * Dysfunctional membrane proteins interfere with the cell's ability to be flexible to travel from the arteries to the smaller capillaries. # Hereditary Spherocytosis * Abnormality of erythrocytes. * Caused by mutations in genes relating to membrane proteins, allow for erythrocytes to change shape. * This difference in shape also makes the red blood cells more prone to rupture. Cells with these dysfunctional proteins are taken for degradation at the spleen. This shortage of erythrocytes results in hemolytic anemia. # Cystic Fibrosis * The inherited CF gene directs the body's epithelial cells to produce a defective form of a protein called CFTR (or cystic fibrosis transmembrane conductance regulator) found in cells that line the lungs, digestive tract, sweat glands, and genitourinary system. > A handwritten note: "24/02/2" # QUESTIONS 1. Who proposed the Cell Theory? 2. What are the major components of a cell membrane? 3. What are the main types of membrane proteins? 4. What is meant by amphipathic and peripheral proteins? 5. What are the different types of active transport and passive transport? 6. What are the three types of facilitated diffusion? 7. Give some examples of facilitated diffusion and active transport? 8. Which viruses are affected by endocytosis? > A handwritten note: "fed" # FILL IN THE BLANK 1. Singer and Nicolson proposed that proteins were embedded within and ______ the membrane 2. Cholesterol is also embedded in the membrane, which helps _____ membrane 3. Increase unsaturated Fatty acids and Cis Form will lead to _____ the fluidity of bilayer 4. ____ soluble material easily enters the cell membrane. 5. The human blood groups differ in the external _____ on red blood cells. 6. Substance move from high concentration to low concentration is called as ____. 7. The diffusion of water across a semipermeable membrane is called as _____. 8. The transport of polar substances or ion across a membrane through a transport protein is called as ____. 9. Aqua-porin water channels, in kidney mutation, can lead to ____ 10. CFTR transmembrane protein is affected in _____. # IMPORTANT POINTS * Cell Membrane → cell theory → Function of Cell Membrane * Components of Cell Membrane and its characteristics * Cell transport system → Active and Passive transport * Cell Membrane disorder