Bisc 102, Week 4 Notes, Chapter 4: The Energy of Life
Bisc 102, Week 4 Notes, Chapter 4: The Energy of Life Bisc 102
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This 4 page Class Notes was uploaded by Alexis Neely on Wednesday September 7, 2016. The Class Notes belongs to Bisc 102 at University of Mississippi taught by Carla Beth Carr in Fall 2016. Since its upload, it has received 47 views. For similar materials see Inquiry Into Life Human Biology in College of Liberal Arts at University of Mississippi.
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Date Created: 09/07/16
4.1 All Cells Capture and Use Energy Energy Allows Cells to Do Life’s Work Energy: t he ability to do work, or move matter Potential energy: stored energy available to do work (e.g. bicycle at the top of hill, compressed spring) Kinetic energy: energy being used (by any moving object) to do work Calories: units used to measure energy. One (cal) is the amount of energy required to raise the temperature of 1 gram of water from 14.5°C to 15.5°C.The energy content of food is measured in k ilocalories (kcal) each of which equals 1000 calories. Energy is Converted from One Form to Another Law of energy conservation s tates that energy cannot be created or destroyed, although it can be converted to other forms. Most important energy transformations are photosynthesis and cellular respiration.. All energy transformations are inefficient because every reaction loses some energy to the surroundings as heat. Heat energy is disordered because it results from random molecular movements. Entropy is a measure of this randomness. (the more disordered, the higher) Organisms can increase in complexity as long as something else decreases in complexity by a greater amount. (e.g. sun and Earth) 4.2 Networks of Chemical Reactions Sustain Life Metabolism encompases all chemical reactions in cells, including those that build new molecules and those that break down existing ones. Each reaction rearranges atoms into new compounds, and each reaction either absorbs or releases energy. Chemical Reactions Absorb or Release Energy Two categories of metabolic reactions based on energy requirements: those that require energy to proceed, and those that release energy. If a reaction requires an input of energy, the products contain more energy than the reactants. Reactions that build complex molecules from simpler components require energy input. (e.g. photosynthesis, powered by sunlight) If a reaction releases energy, the products contain less energy than the reactants. Such reactions break large, complex molecules into their smaller, simpler components. (e.g. cellular respiration, the breakdown of glucose) Linked Oxidation and Reduction Reactions Form Electron Transport Chains Most energy transformations in organisms occur in o xidationreduction (“redox”) reactions, which transfer energized electrons from one molecule to another. (e.g. a person giving a gift) Oxidation means the loss of electrons and a corresponding loss of energy from a molecule, atom, or ion. Reduction m eans a gain of electrons (and their energy) Groups of proteins that are electronshuttling “specialists” often align in membranes. In an electron transport chain, each protein accepts an electron from the molecule before it and passes it to the next (e.g. basketball team passing a ball from player to player). As a result, each protein in the chain is first reduced and then oxidized. Small amounts of energy are released at each step, and the cell uses this energy in other reactions. 4.3 ATP is Cellular Energy Currency The molecule adenosine triphosphate (ATP) t emporarily stores energy in its covalent bonds, then uses the energy to power reactions that require energy input. In eukaryotic cells, organelles called mitochondria produce most of a cell’s ATP. Energy in ATP Is Critical to the Life of a Cell ATP is a type of nucleotide. All cells depend on the potential energy in ATP to power their activities. When a cell requires energy for a chemical reaction, it “spends” ATP by removing the endmost phosphate group. Coupled reactions are simultaneous reactions in which one provides the energy that drives the other. ATP hydrolysis is coupled to the reactions that require energy input, such as those that do work or synthesize molecules. A cell uses ATP as an energy source by transferring its phosphate group to another molecule. ATP Represents ShortTerm Energy Storage Though ATP is essential to life, cells do not stockpile it in large quantities. ATP’s highenergy phosphate bonds make the molecule too unstable for longterm storage. When supplies run low, cells divert some of their lipid and carbohydrate reserves to the metabolic pathways of cellular respiration producing additional ATP. 4.4 Enzymes Speed Reactions An enzyme is an organic molecule that catalyzes (speeds up) a chemical reaction without being consumed. Most enzymes are proteins, although some are made of RNA. Many of the cell’s organelles are specialized sacs of enzymes Enzymes copy DNA, build proteins, digest food, recycle a cell’s wornout parts, and catalyze oxidationreduction reactions. Enzymes Bring Reactants Together Enzymes speed reactions by lowering the a ctivation energy, t he amount of energy required to start a reaction. The enzyme brings reactants (also called substrates) into contact with one another, so that less energy is required for the reaction to proceed. The key to enzyme specificity lies in the shape of the enzyme’s a ctive site the region to which the substrates bind. Many Factors Affect Enzyme Activity One way to regulate a metabolic pathway is by n egative feedback ( also called feedback inhibition), in which a reaction’s products inhibit the enzyme that catalyzes the reaction. As the reaction products accumulate, the reaction rate slows or stops. But when the concentration of the reaction products falls, the block on the enzyme lifts, and the cell can once again carry out the reaction. In noncompetitive inhibition, product molecules bind to the enzyme at a location other than the active site in a way that alters the enzyme’s shape so that it can no longer bind the substrate. In competitive inhibition, the product of a reaction binds to the enzyme’s active site, preventing it from binding substrate. It is “competitive” because the product competes with the substrate to occupy the active site. If the pH or the salt concentration is too high or too low, an enzyme can become denatured and stop working. Temperature is also important. Enzyme action generally speeds up as the temperature climbs because reactants have more kinetic energy at higher temperatures. 4.5 Membrane Transport May Release Energy or Cost Energy Membranes are s electively permeable some substances pass freely through the bilayer, but others require help from proteins. Due to the regulation of membrane transport, the interior of a cell is chemically different from the outside. Likewise, the inside of each organelle in a eukaryotic cell may be chemically different from the solution in the rest of the cell. In a concentration gradient, a solute is more concentrated in one region than in another region. If a substance moves from an area where it is more concentrated to an area where it is less concentrated, it is said to be “moving down” or “following” its concentration gradient. As the solute moves, the gradient dissipates. Passive Transport Does Not Require Energy Input In passive transport, a substance moves across a membrane without the direct expenditure of energy. All forms of passive transport involve d iffusion, the spontaneous movement of a substance from a region where it is more concentrated to a region where it is less concentrated. Diffusion occurs because all substances have kinetic energy; that is, they are in constant, random motion. If diffusion lasts long enough, the gradient disappears, but the molecules do not stop moving. Instead, they continue to travel randomly back and forth at the same rate, so at equilibrium the concentration remains equal throughout the solution. In a form of passive transport called s imple diffusion, a substance moves down its concentration gradient without the use of a transport protein. Osmosis is this simple diffusion of water across a selectively permeable membrane. A human red blood cell demonstrates the effects of osmosis. The cell’s interior is normally isotonic to the surrounding blood plasma, which means that the plasma solute concentration is the same as the inside of the cell. In a hypotonic environment, the solute concentration is lower than it is inside the cell. Water therefore moves by osmosis into a blood cell placed into hypotonic surroundings; since animal cells lack a cell wall, the membrane may even burst. Conversely, hypertonic surroundings have a higher concentration of solutes than the cell’s cytoplasm in this environment, a cell loses water, shrivels, and may die for lack of water. Turgor pressure is the resulting force of water against the cell wall (e.g. limp lettuce versus placing lettuce in water and it becomes crisp) Facilitated diffusion is a form of passive transport in which a membrane protein assists the movement of a polar solute along its concentration gradient. This diffusion releases energy because the solute moves from where it is more concentrated to where it is less concentrated. Active Transport Requires Energy Input In active transport, a cell uses a transport protein to move a substance against its concentration gradient from where it is less concentrated to where it is more concentrated. Because a gradient represents a form of potential energy, the cell must expend energy to create it; this energy often comes from ATP. Endocytosis and Exocytosis Use Vehicles to Transport Substances In endocytosis, a cell membrane engulfs fluids and large molecules to bring them into the cell. When the cell membrane indents, a “bubble” of membrane closes in on itself. The resulting vesicle traps the incoming substance. In pinocytosis, the cell engulfs small amounts of fluids and dissolved substances. In phagocytosis, the cell captures and engulfs the large particles, such as debris or even another cell. Scientists now recognize a more selective form of the process. In receptormediated endocytosis, a receptor protein on a cell’s surface binds a biochemical; the cell membrane then indents, drawing the substance into the cell. Liver cells use receptormediated endocytosis to absorb cholesteroltoting proteins from the bloodstream. Exocytosis, the opposite of endocytosis, uses vesicles to transport fluids and large particles out of cells.
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