Bio 150- Lecture 7 Edited- Cell membrane (Unit 2).pdf

Unit 2 Nucleic acids, proteins and inheritance Cell membranes Ch. 5 Objectives Discuss Discuss membrane basic functions Discuss Discuss components and structure of membranes Discuss Discuss passive transport Discuss Discuss active transport Discuss Discus...

Unit 2 Nucleic acids, proteins and inheritance Cell membranes Ch. 5 Objectives Discuss Discuss membrane basic functions Discuss Discuss components and structure of membranes Discuss Discuss passive transport Discuss Discuss active transport Discuss Discuss bulk transport Cells: Fundamental units of life All organisms are made of cells Cell: the simplest collection of matter that can be alive (Campbell Biology) Plasma membrane Selectively permeable, allowing some Define cell's borders materials to freely enter or leave cell, Flexible, allow certain cells (RBC and while others require a specialized WBC) to change shape as they pass Keep cell functional structure, and occasionally energy through narrow capillaries investment Integral proteins/receptors, transmit Carries markers allowing cells to signals. Act as extracellular input recognize one another, vital for tissue receivers, provide attachment sites and organ formation during early for effectors like hormones and development; and for immune growth factors and intracellular response processing activators (signal transduction/cascade)  Membrane’s components and structure  Microscopy: TEM Advances in microscopy, notably transmission electron microscopy (TEM), allowed researchers to see that plasma membrane's core consisted of a double layer.  Fluid Mosaic Model Describes membrane as a phospholipid bilayer—including: Lipids (phospholipids & cholesterol) Proteins Carbohydrates attached to some of the lipids (glycolipids) Attached to some proteins (glycoproteins) Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the membrane's outward-facing surface.  Main fabric of membrane. Phospholipid phosphate–containing group a three-carbon glycerol backbone two fatty acid molecules Amphiphilic molecule head area, polar, hydrophilic (water-loving) tail area, no charge, hydrophobic (water- Phospholipids: hating) Hydrophilic heads look like a collection of balls, in contact with aqueous fluid; face interior and exterior Hydrophobic areas, non-polar, interact with other non-polar molecules, not with water Forming a lipid bilayer—a double layered phospholipid barrier that separates water and other materials on one side from water and other materials on other side  Hydrophilic head: phosphate-containing group, attached to a glycerol molecule Two hydrophobic tails, each containing either a saturated or an unsaturated (=) fatty acid, are long hydrocarbon chains. In an aqueous solution, phospholipids form small spheres or droplets (micelles or liposomes): polar heads facing outward, hydrophobic tails facing inward Micelle= aggregate of amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension  Determine most of the membrane’s functions. Integral or peripheral Proteins: 2nd major Serve as: component Enzymes (catalysts of chemical reactions) Structural attachments for cytoskeleton's fibers (microscopic network of protein filaments and tubules in cytoplasm, giving cells shape and coherence) Part of the cell’s recognition sites (“cell-specific” proteins). Body recognizes its own proteins and attacks foreign proteins of pathogens  Aka integrins, integrate completely into membrane. Their hydrophobic membrane-spanning regions interact with phospholipid bilayer's hydrophobic region ✓ Single-pass: have a hydrophobic segment (20–25 amino acids). Some span Integral only part of membrane—associate with a single layer—others stretch from one side proteins to other ✓ Some complex proteins (up to 12 single segments), extensively folded and embedded in membrane, with protruding hydrophilic region(s) in contact with cytosol or EC fluid, with hydrophobic regions, adjacent to the phospholipids' tails  Integral proteins may have one or more alpha-helices that span the membrane (1 and 2) or may have beta- sheets that span the membrane (3). Alpha helix and beta plates are two different secondary structures of protein Proteins are made up of chemical 'building blocks' called amino acids. General formula of aa: R-CH(NH2)-COOH we need to remember this  Not embedded in the bilayer Appendages loosely bound Peripheral to membranes' exterior and proteins interior surfaces, attached most often to phospholipids or to exposed parts of integral proteins  This will be on the exam Form specialized sites/receptors on surface allowing cells to recognize each other (along with peripheral proteins) On cells' exterior surface, bound to proteins (forming glycoproteins) or to lipids (forming glycolipids) Chains of 2–60 monosaccharide units/monomers; straight or branched Carbohydrates: Glycocalyx “sugar coating” is the carbohydrate components of glycoproteins and glycolipids 3rd major Highly hydrophilic→ attracts water to surface, aids in component cell's interaction with watery environment and in ability to obtain substances dissolved in water Recognition function very important: immune system to differentiate between body cells and foreign cells or tissues Role in cell-to-cell attachments to form tissues, embryonic development How viruses infect specific organs? Glycoprotein and glycolipid patterns on cells' surfaces (binding sites) give many viruses an opportunity for infection Viruses can invade because they are compatible with those sites Antigens/markers on viruses' surface (recognition sites) interact with human immune system cells, causing them to produce antibodies which attach to sites of virus and destroy it or inhibit its activity. HIV can penetrate plasma membranes of a type of immunity cells, lymphocytes called T-helper cells, as well as some monocytes and central nervous system cells. Unfortunately, recognition sites on HIV change rapidly by mutations, making an effective vaccine very difficult (HIV evolves and adapts). Person infected with HIV quickly develop different populations/variants, of the virus This decreases effectiveness of immune system because antibodies will not recognize new variations. With HIV, problem is compounded because virus infects and destroys immunity cells (T-cells), further incapacitating host. Membrane fluidity Membrane fluidity Membranes are not static sheets of molecules locked rigidly in place Held together by hydrophobic weak electrostatic interactions which are much weaker than covalent bonds Most of the lipids and some of the proteins can shift about laterally Shift is very rapid Membrane fluidity it will be on the exam Due to 3 factors Mosaic characteristics: integral proteins and lipids loosely attached Nature of the phospholipids. In saturated form (only single bonds), fatty acids have straights tails. In unsaturated fatty acids, double bonds results in a bend in the carbon string. When T decrease, cold compress saturated fatty acids, making a dense rigid membrane (low fluidity). Unsaturated fatty acids, when compressed, “twists” in their tails due to double bonds maintain some space/fluidity. Many organisms adapt to cold environments by changing proportion of unsaturated fatty acids Cholesterol, alongside phospholipids, functions as a buffer, preventing lower T from inhibiting fluidity and preventing increased T from increasing fluidity too much. It reduces fluidity at moderate T by reducing phospholipids movement. ‫راكجنشن‬  Component Location Phospholipid Main membrane fabric Attached between Cholesterol phospholipids and between the two phospholipid layers Embedded within the Plasma membrane Integral proteins (for example, integrins) phospholipid layer(s); may or may not penetrate through both components layers On the phospholipid bilayer's inner or outer surface; not Peripheral proteins embedded within the phospholipids Carbohydrates (components of Generally attached to proteins glycoproteins and on the outside membrane layer glycolipids) Selective permeability of membranes- Semipermeable Some substances pass through, others cannot move freely, otherwise cell would no longer be able to sustain itself, would be destroyed Asymmetric: difference between array of phospholipids and proteins between two leaflets On the interior: some proteins serve to attach the membrane to cytoskeleton's fibers. On exterior: Peripheral proteins bind extracellular matrix elements. Carbohydrates, attached to lipids or proteins, help bind required substances in EC fluid. This adds to selectivity Selective permeability of membranes will be on the exam Plasma membranes are amphiphilic; move some materials and hinders others Small non-polar and lipid-soluble material easily slip through hydrophobic lipid core. Fat-soluble vitamins A, D, E, and K readily pass in the digestive tract; Fat-soluble drugs and hormone. O2 and CO2, nonpolar, pass by simple diffusion. Polar substances problematic. While some connect easily outside, cannot readily pass through the lipid core. Even if small ions could slip through, their charge prevents them Ions (charged) like sodium, potassium, chloride must have special means of penetrating Simple sugars and amino acids (polar) cannot pass; need help of various transmembrane proteins (channels) Video: https://www.youtube.com/watch?v=Ptml Cell transport vtei8hw (7 min)  Passive transport  Most direct forms of transport Naturally occurring phenomenon Does not require the cell to exert any energy to Passive transport move material Movement down concentration gradient Substances move from an area of higher concentration to an area of lower concentration ❖ Diffusion will be on the exam Passive transport, down concentration gradient Expends no energy (gradients are sort of potential energy) Diffusion through a permeable membrane moves a substance down its concentration gradient (into the cytoplasm) until concentration is equal across a space and not net movement of molecules; gradient then dissipated. (credit: modification of work by Mariana Ruiz Villareal) Factors affecting diffusion important Extent of concentration Mass of the molecules gradient: The greater the Temperature: Higher T diffusing: Heavier difference in increase molecule energy, molecules move more concentration, the more increasing diffusion rate slowly rapid the diffusion Surface area and Solvent density: As membrane thickness: Solubility: non-polar and density increases, diffusion Increased surface area lipid-soluble materials rate decreases. Molecules increases diffusion rate; pass more easily slow down thicker membrane reduces it Distance travelled: The A variation of diffusion is greater the distance, the filtration. This occurs in slower the diffusion rate. kidney, where blood This places an limitation pressure forces water and on cell size. solutes, out of the blood  Concentration gradient exists, materials diffuse without expending E. But polar molecules/ions, repelled by ❖ Facilitated membrane's hydrophobic parts diffusion/ Facilitating transport proteins shield materials from membrane's repulsive force, allowing them to diffuse facilitated Material first attaches to protein or glycoprotein receptors transport (polar Then pass to specific integral proteins that facilitate diffusion Some are collections of beta-pleated sheets that form a molecules) pore or channel through bilayer Others are carrier proteins, bind substance Channel proteins Have hydrophobic domains, and a hydrophilic channel allowing polar ions to pass through, avoiding repulsive hydrophobic core Highly specific. Gated. When closed, no ions can pass Aquaporins are channel proteins, allow water to pass through membrane Carrier proteins Bind a substance triggering a change of its own shape, moving the bound molecule from outside to interior Specific for a single substance→ adds to selectivity Since specific to one substance & there are a finite number of them→ when all are bound to their ligands, they are saturated, and rate of transport is at its maximum. Increasing concentration gradient will not result in an increased transport rate  ❖ Osmosis Special case of diffusion Movement of free water molecules through a semipermeable membrane down the water's concentration gradient, which is inversely proportional to the solutes' concentration In this example, solute cannot pass through the selectively permeable membrane, but water can Water moves from an area of higher water concentration to one of lower water concentration until the water's concentration gradient goes to zero  Tonicity Tonicity describes how an extracellular solution can change a cell's volume by affecting osmosis Osmolarity describes the solution's total solute concentration. ( will be on the exam) A solution with low osmolarity has low solute concentration; a greater number of water molecules relative to solute particles A solution with high osmolarity has fewer/less water molecules with respect to solute particles; more solute In situations where a membrane permeable to water, not to solute, separates two different osmolarities, water will move from the side with lower osmolarity (more water) to the side with higher osmolarity (less water) Extracellular fluid osmolarity Hypotonic solution: fluid has lower osmolarity compared to inside of the cell→ water enters the cell; swelling ( lss solute more water Hypertonic solution: extracellular fluid has higher osmolarity than cell’s cytoplasm; therefore, fluid contains less water than cell does→ water will leave cell; shrivels Isotonic solution: extracellular fluid has same osmolarity as cell→ no net movement of water  Active transport Active transport If a substance must move into the cell against its concentration gradient (to maintain homeostasis for ex), the cell (Energy must use energy to move this substance Energy needed in the form of adenosine dependent) triphosphate (ATP) Electrochemical gradient We discussed concentration gradient- a substance's differential concentrations across a membrane—but in living systems, gradients are more complex Electrochemical gradient Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is an electrical gradient, difference of charge, across plasma membrane. Crucial for homeostasis Electrochemical gradient Interior of living cells electrically negative compared to EC fluid in which they are bathed Additionally, cells interiors have higher concentrations of potassium (K+) and lower concentrations of sodium (Na+) than EC→ vital for normal cell functions. K+ homeostasis across membrane, is critical because a tilt in this balance can result in different life-threatening diseases Thus, concentration gradient of Na+ tends to drive it into cell, and its electrical gradient (positive ion) also drives it inward It is more complicated for K+. Electrical gradient of K+ drives it into cell; but its concentration gradient drives it out So???  How can this difference in concentration be maintained? Active transport mechanisms/pumps, work against electrochemical gradients Active transport: Maintains specific concentrations of ions and other moving substances substances that living cells require in opposition to passive movements against a gradient Active transport system drives Na+ sodium out of the cell and K+ into it. Cells spend much of its energy supply maintaining these processes  Because they depend on a cell’s metabolism for energy, they are sensitive to many poisons that interfere with ATP supply. Primary active transport moves ions across a Active membrane and creates a difference in charge across that membrane, directly transport dependent on ATP Secondary active transport does not directly require ATP: it is the movement of material due to the electrochemical gradient established by primary active transport. Primary active transport: transporter proteins 3 types of transporter proteins: A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in same direction An antiporter also carries two different molecules or ions, but in different directions  One of the most important pumps in animal cell. Sodium-potassium pump (Na+-K+ ATPase) maintains electrochemical gradient and adequate concentrations of Na+ and K+ in cells Primary active Moves, at same time, 3 Na+ out for every 2 K+ ions in. transport: Na-K pump An electrogenic pump (creates a charge imbalance; crucial for living cells) Exists in two forms, depending on its orientation to the cell's interior or exterior and its affinity for either Na+ or K+ Primary active transport: Na- K pump example Na-K pump: primary active transport process With enzyme oriented towards interior, carrier has a high affinity for Na+. 3 Na+ bind to it. The protein carrier hydrolyzes ATP, and a low-energy phosphate group P attaches to it. As a result, carrier changes shape and reorients itself towards exterior. Affinity for Na+ decreases; the 3 Na+ leave to outside Shape change increases carrier’s affinity for K+ ions, so 2 K+ ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier. Carrier protein repositions itself towards cell's interior. The carrier protein, in its new configuration, has a decreased affinity for K+; the 2 K+ moves into cytoplasm. The protein now has a higher affinity for sodium ions. Process starts again.  At this point, there are more Na ions outside the cell than inside and more K ions inside than out. Result of primary Results in interior being slightly active more negative relative to exterior. transport Difference in charge is important in creating conditions necessary for the secondary process.  Secondary active transport (Co-transport)- Glucose As Na ion concentrations build outside of membrane because of the primary active transport, this creates an electrochemical gradient If a channel protein exists and is open, sodium ions will move down their concentration gradient across membrane Movement of Na+ transports other substances that must be attached to the same transport protein. Many amino acids, as well as glucose, enter this way The Sodium-Potassium Pump - YouTube Fun to watch and learn Bulk transport Bulk transport In addition to moving small ions and molecules, cells also need to remove and take in larger molecules and particles Some cells are even capable of engulfing entire unicellular microorganisms When a cell uptakes and/or releases large particles, it requires energy (Active transport) A large particle, however, cannot pass through the membrane, even with energy that the cell supplies  Methods of Transport, Energy Requirements, and Types of Transported Material Transport Method Active/Passive Material Transported Diffusion Passive Small-molecular weight material Osmosis Passive Water Facilitated transport/diffusion Passive Sodium, potassium, calcium, glucose Primary active transport Active Sodium, potassium, calcium Secondary active transport Active Amino acids, lactose Phagocytosis Active Large macromolecules, whole cells, or cellular structures Pinocytosis and potocytosis Active Small molecules (liquids/water) Receptor-mediated endocytosis Active Large quantities of macromolecules ❖ Endocytosis Endocytosis: active transport that moves large molecules, parts of cells, and even whole cells, into a cell. Different variations Plasma membrane invaginates, forming a pocket around target particle Pocket pinches off, resulting in the particle containing itself in a newly created intracellular vesicle formed from the plasma membrane  Phagocytosis Phagocytosis (“cell eating”): takes in large particles or other cells Ex, when microorganisms invade human body, a type of WBC, a neutrophil, surrounds and engulf the microorganism, then destroys it A portion of membrane's inward-facing surface becomes coated with protein clathrin, which stabilizes this section Coated portion extends and surrounds the particle, enclosing it. Clathrin disengages and vesicle merges with a lysosome for breaking down the material in the newly formed compartment (endosome)  Pinocytosis “Cell drinking”, taking in fluid Including water, which cell needs from EC fluid Results in a much smaller vesicle than does phagocytosis, and vesicle does not need to merge with a lysosome  Receptor-mediated endocytosis Employs receptor proteins in membrane that have a specific binding affinity for certain substances: specifically targeted substances Clathrin attaches to membrane's cytoplasmic side. If a compound's uptake depends on RME and process is ineffective, material will not be removed from tissue fluid or blood. Instead, stays and increases in concentration Failure of RME causes diseases. Ex, receptor mediated endocytosis removes low density lipoprotein LDL ("bad" cholesterol) from blood. In genetic disease hypercholesterolemia, LDL receptors defective or missing. People with this condition have life-threatening levels of cholesterol in blood, because their cells cannot clear LDL particles.  ❖ Exocytosis Expel material from the cell into the EC fluid Waste material is enveloped in a membrane and fuses with the plasma membrane's interior Vesicle membrane becomes part of plasma membrane Fusion opens the membranous envelope on exterior, and waste material expels into the extracellular space End of lecture review questions Choose one correct answer Which plasma membrane component can be either found on its surface or embedded in the membrane structure? a. protein b. cholesterol c. carbohydrate d. phospholipid Choose one correct answer Which characteristic of a phospholipid contributes to the fluidity of the membrane? a. its head b. cholesterol c. a saturated fatty acid tail d. double bonds in the fatty acid tail Choose one answer What is the primary function of carbohydrates attached to the exterior of cell membranes? a.identification of the cell b.flexibility of the membrane c.strengthening the membrane d. channels through membrane Choose one answer The principal force driving movement in diffusion is the __________. a. temperature b.particle size c.concentration gradient d.membrane surface area Choose one answer What problem is faced by organisms that live in fresh water? a.Their bodies tend to take in too much water. b.They have no way of controlling their tonicity. c.Only salt water poses problems for animals that live in it. d.Their bodies tend to lose too much water to their environment. Choose one answer In what important way does receptor-mediated endocytosis differ from phagocytosis? a.It transports only small amounts of fluid. b.It does not involve the pinching off of membrane. c.It brings in only a specifically targeted substance. d.It brings substances into the cell, while phagocytosis removes substances. Choose one answer How does the sodium-potassium pump make the interior of the cell negatively charged? a.by expelling anions b.by pulling in anions c.by expelling more cations than are taken in d.by taking in and expelling an equal number of cations