An Introduction to Metabolism • Overview: The Energy of Life  The living cell is a miniature chemical factory where thousands of reactions occur  The cell extracts energy and applies energy to perform work  Some organisms even convert energy to light, as in bioluminescenceAn organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics • Metabolism is the totality of an organism’s chemical reactions  A property of life that arises from interactions between molecules within the cell • A metabolic pathway begins with a specific molecule and ends with a product  Each step is catalyzed by a specific enzymePathway types • Catabolic pathways release energy by breaking down complex molecules into simpler compounds  Cellular respiration: the breakdown of glucose in the presence of oxygen, is an example of a catabolic pathway • Anabolic pathways consume energy to build complex molecules from simpler ones  The synthesis of protein from amino acids is an example of anabolic pathway • Bioenergetics is the study of how organisms manage their energy resources • Energy is the capacity to cause change  Energy exists in various forms, some of which can perform workForms of Energy • Kinetic energy is energy associated with motion  Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules • Potential energy is energy that matter possesses because of its location or structure – Chemical energy is potential energy available for release in a chemical reaction • Energy can be converted from one form to anotherThe Laws of Energy Transformation • Thermodynamics is the study of energy transformations  A closed system, such as that approximated by liquid in a thermos, is isolated from its surroundings  In an open system, energy and matter can be transferred between the system and its surroundings • Organisms are open systems • According to the first law of thermodynamics, the energy of the universe is constant:  Energy can be transferred and transformed, but it cannot be created or destroyed • The first law is also called the principle of conservation of energy • According to the second law of thermodynamics:  Every energy transfer or transformation increases the entropy (disorder) of the universe  During every energy transfer, some energy is unusable and is lost as heatThe free-energy change of a reaction tells us whether or not the reaction occurs spontaneously • Which reactions occur spontaneously and which require input of energy?  Need to determine energy changes that occur in chemical reactions • Free-Energy Change, ∆G  energy that can do work when temperature and pressure are uniform, as in a living cell • ∆G is related to the change in total energy or enthalpy (∆H), the change in the order or randomness or entropy (∆S), and temperature in Kelvin (T): ∆G = ∆H – T∆S  Only processes with a negative ∆G are spontaneous  Spontaneous processes can be harnessed to perform workFree Energy, Stability, and Equilibrium • Free energy is a measure of a system’s instability, its tendency to change to a more stable state  During a spontaneous change, free energy decreases and the stability of a system increases • Equilibrium is a state of maximum stability  A process is spontaneous and can perform work only when it is moving toward equilibriumFree Energy and Metabolism • The concept of free energy can be applied to the chemistry of life’s processes • Exergonic and Endergonic Reactions in Metabolism – An exergonic reaction proceeds with a net release of free energy and is spontaneous – An endergonic reaction absorbs free energy from its surroundings and is nonspontaneousEquilibrium and Metabolism • Reactions in a closed system eventually reach equilibrium and then do no work • Cells are not in equilibrium; they are open systems experiencing a constant flow of materials • A defining feature of life is that metabolism is never at equilibrium • A catabolic pathway in a cell releases free energy in a series of reactions – Closed and open hydroelectric systems can serve as analogiesATP powers cellular work by coupling exergonic reactions to endergonic reactions • A cell does three main kinds of work:  Chemical  Transport  Mechanical • To do work, cells manage energy resources by energy coupling.  Use of an exergonic process to drive an endergonic one • Most energy coupling in cells is mediated by ATP  ATP (adenosine triphosphate) is the cell’s energy shuttle  ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groupsThe Structure and Hydrolysis of ATP • The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis • Energy is released from ATP when the terminal phosphate bond is broken • This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselvesHow ATP Performs Work • The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP • Energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction  Overall, the coupled reactions are exergonic • ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule  The recipient molecule is now phosphorylatedHow ATP Performs WorkThe Regeneration of ATP • ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP)  The energy to phosphorylate ADP comes from catabolic reactions in the cell  The chemical potential energy temporarily stored in ATP drives most cellular workEnzymes speed up metabolic reactions by lowering energy barriers • A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction • An enzyme is a catalytic protein • Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reactionThe Activation Energy Barrier • Every chemical reaction between molecules involves bond breaking and bond forming • The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) • Activation energy is often supplied in the form of heat from the surroundingsHow Enzymes