Ch 5 Cell Function PDF

BIOL 1116: Ch 5 Cellular FunctionWhy are saturated fats bad for you? Membrane Structure The Cell (Plasma) Membrane The cell membrane serves a variety of functions: Acts as the boundary between interior of the cell and the extracellular environment...

BIOL 1116: Ch 5 Cellular FunctionWhy are saturated fats bad for you? Membrane Structure The Cell (Plasma) Membrane The cell membrane serves a variety of functions: Acts as the boundary between interior of the cell and the extracellular environment Controls what enters and exits the cell Organizes the chemical reactions of the cell Holds teams of enzymes that function in metabolism The Lipid Bilayer In water, phospholipids form a stable bilayer. The heads face outward (towards the water) and the tails face inward (away from the water). Membrane components are amphipathic. Amphipathic: molecules that have both hydrophilic and hydrophobic properties. Cell Membranes The cell membrane is a fluid mosaic of lipids and proteins. Lipid molecules form a flexible bilayer. Protein molecules are embedded in the plasma membrane. Carbohydrates act as cell identification tags on the surface of the plasma membrane. Membrane Fluidity Cell membranes have the consistency of salad oil at room temperature. Lipids have rapid lateral movement. Lipids flip-flop extremely rarely. Membranes must be fluid to function properly Fluid but not too fluid! Why would you expect flip-flops to be rare events? Fluidity depends on lipid composition Saturated fatty acids: All C-C bonds are single bonds. Straight chain allows maximum interaction of fatty acid tails. Make membrane less fluid (viscous = thicker) Solid at room temperature. e.g., "Bad Fats" that clog arteries (animal fats). Fluidity depends on lipid composition Unsaturated fatty acids: Some C=C bond (double bonds). Bent chain space tails apart. Make membrane more fluid. Liquid at room temperature. e.g., "Good Fats" which do not clog arteries (vegetable fats). Fluidity depends on lipid composition Cholesterol acts as a fluidity buffer Has different effects on membrane fluidity at different temperatures. At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids. *** At cool temperatures, it maintains fluidity by preventing tight packing of phospholipids. Membrane Proteins A membrane is a mosaic of different proteins embedded in the lipid bilayer Peripheral proteins Entirely on membrane surface Ionic and H-bond interactions with hydrophilic lipid and protein groups  Transmembrane Integral proteins Possess hydrophobic domains which are proteins anchored to hydrophobic lipids Span the membrane Contain both hydrophobic and hydrophilic regions Membrane Proteins Proteins determine most of the membrane’s specific functions. Membrane Carbohydrates Carbohydrates are found on exterior surface of cell membrane. Provide specificity for cell-cell or cell-protein interactions. Glycolipids: sugars attached to a lipid. e.g., blood antigens that determine blood type. Glycoproteins: sugars attached to a protein. e.g., protein receptors. Transport Across a Membrane Membranes are selectively permeable The cell membrane controls traffic into and out of the cell by being selectively permeable It allows some substances to cross but not others What makes the membrane selectively permeable? 1. Permeability of the lipid bilayer What types of substances would you expect to easily pass through the lipid bilayer?  Hydrophobic (nonpolar) molecules dissolve in the lipid bilayer and pass through the membrane rapidly. (e.g. CO2, O2, steroid hormones)  Polar molecules and ions do not cross the membrane easily Membranes are selectively permeable What makes the membrane selectively permeable? 2. Transport proteins Span the membrane. Each transport protein is specific to one solute. Some are channels and others are transporters. Two ways molecules can move through the membrane: Passive transport and Active transport… Passive Transport Diffusion = tendency for particles of any kind to spread out spontaneously from where they are more concentrated to where they are less concentrated In passive transport (simple diffusion), substances diffuse through membranes without work by the cell Substances spread from areas of high concentration to areas of lower concentration. Diffusion No energy is required No protein channel or carrier required Passive transport can occur with two solutes simultaneously  Osmosis Osmosis is the diffusion of water across a selectively permeable membrane. Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration. Or can think of it as… from high water conc to low water conc Cell behaviour in different environments Hypertonic: Higher solute concentration compared to the inside of the cell. Hypotonic: Lower solute concentration relative to the inside of the cell. Isotonic: Equal concentration of solutes. Many marine animals (sea stars, crabs) are isotonic to sea water Animals that live in hyper or hypotonic environments, need a mechanism to prevent excessive uptake or loss of water Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments. Organisms without Cell Walls e.g. Animal Cells Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution. The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump. Organisms with Cell Walls Hypotonic Isotonic Hypertonic Plant cells are happiest in hypotonic solutions because cell wall prevents membrane from rupturing, making them turgid Facilitated Diffusion Only small, non-polar molecules (e.g. O2, CO2) can diffuse freely through the lipid bilayer (simple diffusion) Other molecules pass through selective protein pores by facilitated diffusion Facilitated diffusion = substances moving down concentration gradients through transport proteins. Type of passive transport – does not require energy Active Transport Transport proteins can move solutes across a membrane against a concentration gradient. Active transport requires energy (in the form of ATP) The Na+/K+ pump is a very important example of active transport Active Transport: Bulk Transport Exocytosis: The movement of large molecules or particles to the outside of the cell. A membrane-bound vesicle fuses with the membrane and expels its contents. Exocytosis Active Transport: Bulk Transport Endocytosis: The movement of large molecules or particles to the inside of the cell. The membrane folds inward, trapping material from the outside. Endocytosis Passive vs. Active Transport Energy and the Cell Energy and the Cell Cell membranes are sites where chemical reactions can occur in an orderly manner. Living cells transform energy by means of enzyme-controlled chemical reactions. Energy is the capacity to perform work Two Forms of Energy Kinetic Energy: is energy that is actually doing work. The energy an object possesses due to its motion. Heat: the energy associated with motion of molecules. Light: another kind of kinetic energy. Potential Energy: is stored energy. The energy stored in a system due to its position. Potential energy is the most important energy to the cell Chemical Energy: the potential energy of molecules. Organisms transform energy to stay alive Light Energy  Chemical Energy  Work KE  PE  KE Chemical energy is utilized when a chemical reaction releases the potential energy stored in chemical bonds and it is transformed into kinetic energy. e.g. when a car burns gas releasing energy of the gasoline to drive the engine. e.g. when an organism undergoes chemical reactions to rearrange sugar molecules into other molecules. Some important definitions: Thermodynamics: is the study of energy transformations. System: the collection of matter under study. A system can be “open” or “closed”. A closed system is isolated from its surroundings. In an open system, energy and matter can be transferred between the system and its surroundings. Are organisms considered closed or open systems? Thermodynamics: Two Laws Govern Energy Transformations First law of thermodynamics Energy can be changed from one form to another. However, energy cannot be created or destroyed. Examples: A light bulb converts electrical energy into light energy. A plant converts light energy to chemical energy. Thermodynamics: Two Laws Govern Energy Transformations Second law of thermodynamics Energy changes are not 100% efficient. Energy conversions increase disorder, or entropy. Entropy: the energy of randomness. Entropy is increasing in the universe. If a system is becoming more ordered, then its surroundings become more disordered. Some energy is always lost as heat. Heat is disordered energy, or entropy. What happens to this heat??  Light Photosynthesis Potential energy Heat Heat Cellular Respiration Heat Work Heat Kinetic energy Heat Heat All of the reactions in a cell constitutes Cellular Metabolism Two general types of reactions: Catabolic pathways release energy by breaking down complex molecules into simpler compounds. “Exergonic” = release energy Anabolic pathways consume energy to build complex molecules from simpler ones. “Endergonic” = require energy Catabolic and Anabolic Reactions Which are catabolic? Anabolic? Which are endergonic? Exergonic? Anabolic and catabolic reactions are coupled together by ATP The energy released from exergonic reactions are used to drive endergonic reactions = energy coupling Most energy coupling in cells is mediated by ATP (adenosine triphosphate)  When ATP is hydrolyzed (split) into ADP + P, energy is released  ATP is used to drive almost all endergonic reactions in the cell The ATP Cycle ATP is a renewable resource When exergonic reactions release energy, that energy is stored in ATP molecules Then ATP is used to power endergonic reactions ATP is the “currency of the cell”! Cellular Reactions require Enzymes Enzymes are proteins that act as biological catalysts Catalyst = chemical that speeds up the rate of a reactions without itself being changed/consumed Are used over and over! Without enzymes, life’s reactions would take too long Enzymes speed up reactions by lowering the activation energy for the reaction Activation energy (EA) = amount of energy reactants must absorb to start a chemical reaction. Its essentially an energy barrier. Without this barrier, most biological molecules would spontaneously break down  Activation energy is often supplied in the form of heat from the surroundings.  Instead of heat, organisms use enzymes to speed up reactions Enzymes speed up the cell’s chemical reactions by lowering energy barriers Enzymes do not add energy or catalyze non-spontaneous reactions A specific enzyme catalyzes each cellular reaction Enzymes are selective This selectivity determines which chemical reactions occur in a cell Specificity is due to enzyme shape i.e. “Lock and key” Substrate: The specific reactant on which the enzyme acts Active site: The site on a enzyme to which the substrate binds A specific enzyme catalyzes each cellular reaction Each cell has thousands of different enzymes, each performing a specific chemical reaction Each enzyme functions optimally at a specific temperature, pH, and salt concentration