Chapter 6 Plant Information Chloroplasts Capture of Light Energy chloroplasts contain the green pigment chlorophyll along with enzymes and other molecules that function in the photosynthetic production of sugar double membrane separated by a narrow intermembrane space inside the chloroplast is another membranous system in the form of flattened interconnected sacs called thylakoids a stack of thylakoids is called a granum the fluid outside the thylakoids is called the stroma contains the chloroplast DNA ribosomes and enzymes Chapter 8 An Introduction to Metabolism 8 1 An organism s metabolism transforms matter and energy subject to the laws of thermodynamics Metabolism the totality of an organism s chemical reactions it is an emergent property of life that arises from orderly interactions between molecules A metabolic pathway begins with a specific molecule which is then altered in a series of defined steps resulting in a certain product each step of the pathway is catalyzed by a specific enzyme Metabolism manages the material and energy resources of the cell Catabolic pathways release energy by breaking down complex molecules to simpler compounds ex cellular respiration Anabolic pathways consume energy to build complicated molecules from simpler ones sometimes called biosynthetic pathways ex synthesis of amino acids from simple molecules ex synthesis of proteins from amino acids Bioenergetics the study of how energy flows through living organisms Energy the capacity to cause change energy exists in various forms 1 Kinetic energy the relative motion of objects 2 Thermal energy heat kinetic energy associated with the random movement of atoms or molecules 3 Potential energy energy that matter possessed because of its location or structure 4 Chemical energy the potential energy available for release in a chemical light is also a type of energy that can be harnessed to perform work reaction Thermodynamics The study of the energy transformations that occur in a collection of matter An isolated system is unable to exchange either energy or matter with its surrounding In an open system energy and matter can be transferred between the system and its surroundings organisms are open systems There are two laws of thermodynamics 1 The First Law of Thermodynamics Energy can be transferred and transformed but it can not be created or destroyed known as the principle of conservation of energy 2 The Second Law of Thermodynamics Every energy transfer or transformation increases the entropy of the universe Entropy is a measure of disorder or randomness the more random the greater the entropy the concept of entropy helps us understand why certain processes occur without any input of energy a spontaneous process is one that can occur without an input of energy the process is energetically favorable a process that can not occur on its own is called nonspontaneous for a process to occur spontaneously it must increase the entropy of the universe Living systems increase the entropy of their surroundings 8 2 The free energy change of a reaction tells us whether or not the reaction occurs spontaneously Free Energy the portion of a system s energy that can perform work when temperature The change in free energy G can be calculated by G H T S This equation uses only properties of the system itself H change in the system s enthalpy total energy S change in the system s entropy T temperature in Kelvin Negative G spontaneous Positive G nonspontaneous G Gfinal state Ginitial state so G can only be negative if the process involves a loss of free energy during the change from the initial state to the final state We can think of free energy as a measure of a system s instability unstable systems higher G tend to change in such a way that they become more stable lower G Another term that describes a state of maximum stability is equilibrium as a reaction proceeds toward equilibrium the free energy of the mixture of reactants and products decreases for a system at equilibrium G is at its lowest possible value in that system systems never spontaneously move away from equilibrium so it can not do work A process is spontaneous and can perform work only when it is moving toward equilibrium An Exergonic reaction exothermic proceeds with a net release of free energy energy released because the chemical mixture loses free energy G is negative exothermic reactions occur spontaneously the magnitude of G for an exothermic reaction represents the maximum amount of work the reaction can perform the greater the decrease in free energy the greater the amount of work that can be done downhill reaction An Endergonic reaction endothermic absorbs free energy from its surroundings because this kind of reaction stores free energy in molecules G is positive endothermic reactions are nonspontaneous and the magnitude of G is the quantity of energy required to drive the reaction uphill reaction Metabolism is never at equilibrium because it could not do any work if it was 8 3 ATP powers cellular work by coupling exothermic reactions to endothermic reactions A cell does three main kinds of work 1 Chemical work the pushing of endothermic reactions that would not occur spontaneously such as the synthesis of polymers from monomers 2 Transport work the pumping of substances across membranes against the direction of spontaneous movement 3 Mechanical work the contraction of muscle cells and the movement of chromosomes during cellular respiration A key feature in the way cells manage their energy resources to do this work is energy coupling the use of an exothermic process to drive an endothermic one ATP is responsible for mediating most energy coupling in cells and it acts as the immediate source of energy that powers cellular work ATP adenosine triphosphate contains the sugar ribose with the nitrogenous base adenine and a chain of three phosphate groups bonded to it it is one of the nucleoside triphosphates used to make RNA the bond between the phosphate groups of ATP can be broken by hydrolysis with the addition of water a molecule of inorganic phosphate leaves the ATP and it becomes ADP adenosine diphosphate this reaction is exothermic because the hydrolysis releases energy the phosphate bonds of ATP are sometimes referred to as high energy phosphate bonds the reactants have high energy relative to the energy of the products the release of energy during hydrolysis of ATP comes from the chemical change to a state of lower free energy not from
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