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University of Waterloo
BIOL 130
Heidi Engelhardt

BIOL130 CHAPTER 6 – LIPIDS, MEMBRANES & THE FIRST CELLS  Membrane-forming lipids have a polar, hydrophilic head that interacts with water molecules when a phospholipid is placed in solution. In addition, a non-polar, hydrophobic fatty acid tail cannot form hydrogen bonds with the hydrocarbon tail. This characteristic is called being amphipathic (ie phospholipids, cholesterol).  Phospholipids do not dissolve when they are placed in water. Since they are amphipathic, they may form one of two structures: micelles or lipid bilayers.  Micelles: tiny droplets created when the hydrophilic heads of phospholipids face the water and the hydrophobic tails are forced together, away from the water. Lipids with compact/shorter tails form micelles.  Phospholipid bilayers: when two sheets of phospholipid molecules align. Lipids with longer tails form bilayers.  Micelles and bilayers form spontaneously (no energy input required, therefore exergonic). The formation of these structures decreases entropy. They are much more organized and energetically stable than independent molecules in solution. However they also have much lower potential energy than independent molecules. The loss of potential energy outweighs the decrease in entropy. Overall, free energy in system decreases.  Hydrocarbons are unstable in water because they disrupt hydrogen bonding in water molecules.  Permeability: tendency of a structure to allow a given substance to pass across it.  Liposome: artificial membrane bound vesicles, formed when shaking agitates lipid bilayers.  Planar bilayers: artificial membranes used to simulate what happens when a known ion or molecule is added to one side of a bilayer.  Selective permeability: some substances cross a membrane easily than others. Small, non-polar molecules (ie O 2 move across bilayers quickly. - Large molecules and charged substances (ie Cl ) cross the membrane slow or not at all. Small and uncharged molecules (ie H 2) can cross relatively rapidly, and small polar molecules (ie urea, glycerol) have intermediate permeability. Lipid bilayers are highly selective.  Theory: charged compounds and large polar molecules can’t pass through the non-polar hydrophobic tails. The electrical charge of an ion makes it more stable in solution where they form hydrogen bonds with water opposed to be interior of membranes where it is electrically neutral.  Two aspects of a hydrocarbon chain affect the way the chain behaves in a lipid bilayer: number of double bonds (saturation) and length. Double bonds create a kink in the tail because it no longer has free rotation and is also said to be unsaturated, meaning fewer than the maximum number of hydrogen atoms can be bonded. Saturated fats have more C-H bonds which have a higher bond energy than C=C bonds, meaning there is much more chemical energy.  Foods that have unsaturated fats are called polyunsaturated and are said to be healthier.  Double bonds create a kink in the tail, which produces spaces among the tightly packed tails. The interior of the membrane is glued less tightly together, which causes the structure to be more fluid and permeable.  Hydrophobic interactions become stronger as saturated hydrocarbon tails increase in length. Phospholipids with long, saturated tails form membranes that are much less permeable than membranes consisting of phospholipids with shorter, unsaturated tails.  Fluidity of a lipid depends on the length and saturation of its hydrocarbon chain. Butter consists of saturated lipids, and is solid at room temperature. Waxes are lipids with extremely long hydrocarbon chains and are stiff solids at room temperature. Oils are polyunsaturated, with hydrocarbon chains that contain multiple double bonds and are liquid at room temperature.  At room temperature (approximately 25 degree Celsius) phospholipids in bilayers are liquid. Fluidity and permeability decrease as temperature decreases. As temperatures drop, the individual molecules in the bilayer move slower and as a result pack together more tightly. At very low temperatures, liquid bilayers begin to solidify.  Membranes are dynamic. The fluid nature allows individual lipid molecules to move laterally within each layer with an average speed of 2 micrometers per second at room temperature. Phospholipid molecules move around each layer while water and small non-polar molecules shoot in and out of the membrane. How quickly molecules move within and across the membrane is a function of temperature and the structure of the hydrocarbon tails in the bilayer.  Dissolved molecules and ions, called solutes, have thermal energy and are in constant, random motion. Movement of molecules and ions that results from their kinetic energy is called diffusion. Solutes change position randomly due to diffusion and tend to move from a region of high concentration to a region of low concentration. A difference in solute concentrations creates a concentration gradient. There is a net movement from high concentration to low concentration. Diffusion along a concentration gradient is spontaneous process because it results in an increase in entropy.  At equilibrium, molecules move back and forth across the membrane at equal rates. This means there is no longer a net movement.  Osmosis: movement of water. It only occurs when solutions are separated by membranes that are permeable to some molecules but not others (semi permeable membrane). Water undergoes a net movement from the region of low concentration of solute/high concentration of water to the region of high concentration of solute/low concentration of water. The movement is spontaneous, driven by the increase in entropy achieved when solute concentrations are equal on both sides of the membrane.  Hypertonic solution: Outside solution containing more solutes than the solution on the other side of a membrane. Ex. Outside solution will come into vesicle and cause it to swell.  Hypotonic solution: Outside solution containing less solute than the solution on the other side of membrane. Ex. Inside solution will come out of vesicle to outside solution, causing it to shrivel.  Isotonic: Outside solution has same amount of solutes as inside solution. Ex. The cell maintains its shape.  Amphipathic proteins can be inserted into lipid bilayers and affect the permeability. Because amino acids can have a series of non polar amino acids in the middle of its primary structure but polar or charged amino acids on both ends, the non polar amino acids would be stable in the interior of a bilayer while the polar or charged amino acids would be stable alongside the polar heads and surrounding water. Also, because secondary and tertiary structures are varied and complex, it is possible for proteins to form tubes and function as some sort of channel or pore across a lipid bilayer.  Fluid mosaic membrane: Singer and Nicolson proposed that membranes are a mosaic of phospholipids and different types of proteins. Overall structure is dynamic and fluid.  Integral membrane proteins (transmembrane proteins): Some proteins that span the membrane and have segments facing both the interior and exterior surfaces.  Peripheral membrane proteins: proteins found on only one side of the membrane. They are often attached to an integral membrane protein. Membrane proteins are in constant motion, diffusing through the oily film.  Detergents are small amphipathic molecules that tend to form micelles in water. They break up bilayers when their hydrophobic tails interact with hydrophobic tails of lipids. Detergent molecules displace membrane phospholipids and end up forming water-soluble detergent-protein complexes. This is an effective way to isolate membrane proteins so they can be purified and studied.  Transport proteins: channels, transporters and pumps.  Facilitated diffusion via channel proteins: Ions move from regions of high concentration to regions of low concentration via diffusion and also flow from areas of like charge to areas of unlike charge. When ions build up on one side of a membrane, they establish a combined concentration and electrical gradient. Ions move in response to a combined concentration and electrical gradient, called an electrochemical gradient. The movement of ions creates an electrical current.  An ion channel is a peptide or protein that makes lipid bilayers permeable to ions.  Cells have many different types of channel proteins in their membranes that allow it to admit a particular type of ion or small molecule. Example: aquaporins (“water pores”) allow water to cross the plasma membrane over 10x faster than it does in its absence.  Aquaporins and ion channels are gated channels, meaning they open or close in response to the binding of a particular molecule or to a change in the electrical charge on the outside of the membrane.  Passive transport: the movement of substances through channels does not require a lot of energy. It is powered by diffusion along an electrochemical gradient.  Facilitated diffusion via carrier proteins: Facilitated diffusion is aided by the presence of specialized proteins in the membrane. Facilitated diffusion can also occur through carrier proteins (transporters) that change shape during the process.  GLUT-1 facilitates glucose diffusion into a cell. It is a transmembrane transport protein with its binding site facing outside the cell. It works when glucose binds to GLUT-1 from outside the cell. A conformational change results, transporting glucose into the interior by its concentration gradient and then released inside.  Active transport by pumps: transport against an electrochemical gradient. This task requires energy because the cell must counteract the entropy loss that occurs when molecules or ions are concentrated. A phosphate group usually provides the energy from ATP. The result of a lost phosphate group turns ATP into ADP. When one of the phosphate groups from ATP binds to a protein, two negative charges are added to the protein. These charges repel other charges on the protein’s amino acids. The protein’s potential energy increases and shape changes.  First pump discovered with the sodium-potassium pump or ATPase. Three binding sites within the protein have a high affinity for sodium ions, which are inside the cell. They bind to these sites and a phosphate group from ATP binds to the protein and changes its shape. The sodium ions diffuse to the exterior of the cell. In this conformation, the protein has binding sites, which have a high affinity for potassium ions. Two potassium ions bind to the pump and a phosphate group drops off the protein. The protein changes back to its original shape and the potassium ions diffuse into the cell. This movement of ions can occur even if an electrochemical gradient exists that favours the outflow of potassium and the inflow of sodium. By exchanging three sodium ions for two potassium ions, the outside of the membrane becomes positively charged relative to the inside. In this way, an electrical and chemical gradient are set up across the membrane.  The lipid bilayer and proteins involved in passive transport and active transport enable cells to create an internal environment that is much different from the external one. Membrane proteins allow ions and molecules to cross the plasma membrane even though they are not lipid soluble. CHAPTER 7 – INSIDE THE CELL  Prokaryotic cells: Phospholipid bilayer and proteins that span the bilayer or attach to one side. Inside membrane, contents are called cytoplasm. Hypertonic relative to surrounding environment. Water enters the cell via osmosis and makes cell’s volume expand. This pressure is resisted by a cell wall.  Glycolipids: lipids that contain carbohydrate groups.  Most dominant structure is the single circular chromosome that consists of a large DNA molecule (contains information) and small number of proteins (structural support).  Chromosomes contain DNA that contains genes. Prokaryotic chromosomes are found in localized area of the cell called the nucleoid. To fit in the cell, the DNA double helix coils itself with the aid of enzymes to form a highly compact structure, called plasmids. Plasmids contain genes but are physically independent of the main cellular chromosome.  Ribosomes: manufacture proteins. Flagella: rote to power swimming in aquatic species. Both lack a membrane.  Bacterial ribosomes: complex structures consisting of three RNA molecules and 50+ proteins. They are organized into a large subunit and small subunit. A cell may contain 10 000 of these.  Not all bacteria species have flagella. When present, they are few in number and on surface of cell. Over 40 proteins are involved in building and controlling bacterial flagella. At top speed, flagellar movement can drive a bacteria cell through water at 60 cell lengths per second.  Prokaryotes lack a nucleus but they do have membrane bound internal structures, organelles.  Organelle: membrane bound compartment in the cytoplasm that contains enzymes or structures specialized for a particular function.  Bacteria and archaea contain long thin fibres for structural support. Some species have protein filaments that help maintain cell shape called the cytoskeleton. Location of DNA Internal Cytoskeleton Overall Size Membranes and Organelles Bacteria and In nucleoid (not Extensive Limited in extent, Usually small Archaea membrane bound), internal relative to relative to plasmids common membranes only eukarotes. eukaryotes. in photosynthetic species, limited types and numbers of organelles. Eukaryotes Inside nucleus Large numbers of Extensive- Most are (membrane bound), organelles, many usually found larger than plasmids rare types of throughout prokaryotes. organelles. volume of cell  Eukaryotic Cells: ranges from uni cellular species to plants and animals.  Organelles in eurkaryotes can separate incompatible chemical reactions. Also the efficiency of chemical reactions is increased.  Two advantages of compartmentalization of eukaryotic cells: incompatible chemical reactions can be separated (ie fatty acids can be synthesized in one organelle while excess or damaged fatty acids are degraded and recycled in another); the efficiency of chemical reactions is increased (ie substrates for reactions can be localized and highly concentrated, if substrates are used up in part of organelle they can be replaced , groups of enzymes that work together can be clustered on internal membranes therefore increasing speed and efficiency of reaction).  Bacteria and archaea = machine shops, eukaryotic cells = industrial factories.  Nucleus: highly organized, enclosed in double membrane called nuclear envelope that is studded with pores and inside surface is associated with fibrous proteins that form a lattice-like sheet called nuclear lamina that stiffens structure; chromosomes are attached to nuclear lamina, contains distinct region called nucleolus where RNA molecules found in ribosomes are manufactured and small and large subunits are manufactured.  Cytoplasm: everything inside plasma membrane excluding nucleus. Cytosol: fluid portion of cytoplasm.  Ribosomes: found scattered throughout cytosol, comprised of two subunits (large and small – large composed of three RNA molecules while small is composed of one RNA molecule – both have protein as well, neither enclosed by membrane), synthesize proteins  Rough ER: ribosomes are associated with rough ER (studded on surface), responsible for synthesizing proteins to be inserted into plasma membrane, secreted to cell exterior, shipped to lysosome; ER is continuous with outer membrane of nuclear envelope; interior of sac like structure called lumen, where newly manufactured proteins undergo folding; these proteins have variety of functions – messengers, membrane transport proteins, enzymes.  Golgi Apparatus: prod
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