Energy The capacity to do work or cause particular changes Life is sustained by the trapping and use of energy Use of energy is made possible by the action of enzymes

Energy and work Chemical work The synthesis of complex biological molecules from simpler precursors

Energy and work Transport work The ability to transport molecules against a concentration gradient (uptake of nutrients, elimination of waste, maintenance of ion balance)

Energy and work Mechanical work Changing the location of organisms, cells and structures within cells

The flow of carbon and energy Source of most biological energy is sunlight Phototrophs trap light energy during photosynthesis

The flow of carbon and energy Chemolithoautotrophs derive energy from the oxidation of inorganic molecules Energy from photosynthesis and chemolithoautotrophy can then be used to transform CO 2 into organic molecules

The flow of carbon and energy Chemoheterotrophs can use organic molecules as carbon and energy sources Energy is released as organic molecules are oxidized to CO 2

Oxidation and reduction Loss of electrons is oxidation (LEO) Gain of electrons is reduction (GER) Aerobic respiration is when O 2 acts as the final electron acceptor (O 2  H 2 O)

Adenosine 5´-triphosphate (ATP) ATP serves as the major energy currency of cells Contains 2 high energy bonds ATP  ADP + Pi + Energy Energy + ADP +Pi  ATP Pi = orthophosphate

Energy cycle

The laws of thermodynamics 1.Energy can neither be created or destroyed The total amount of energy in the universe remains constant (although it can be redistributed)

The laws of thermodynamics 2. Physical and chemical processes proceed in such a way that the randomness of the universe increases to the maximum possible

Entropy A measure of the randomness or disorder of a system The greater the disorder the greater the entropy

Free energy and reactions  G =  H - T x  S  G = change in free energy (amount of energy available to do work)  H = change in enthalpy (heat content) T = temperature in Kelvin (  C + 273)  S = change in entropy

Free energy and reactions  G =  H - T x  S A reaction with a large positive change in entropy will result in a negative  G value and will occur spontaneously The change in free energy has an effect on the direction of a reaction

Free energy and reactions  G =  H - T x  S When  G is determined under standard conditions of, pressure, pH and temperature the  G is called the standard free energy change (  G  ) If the pH is set to 7, the standard free energy change is indicated by the symbol  G  ´

Free energy and reactions The change in free energy has an effect on the direction of a reaction K eq = the equilibration constant

Free energy and reactions When  G  ´ is negative, the K eq is greater than 1 and the reaction goes to completion as written The reaction is said to be exergonic

Free energy and reactions When  G  ´ is positive, the equilibrium constant is less than 1 and the reaction is not favored The reaction is said to endergonic

ATP and metabolism A major role of ATP is to drive endergonic reactions to completion ATP links energy-yielding reactions with energy-using reactions

Oxidation-reduction reactions The release of energy normally involves oxidation reduction reactions (redox reactions) Electrons move from an electron donor to an electron acceptor Acceptor +ne -  donor (n = number of electrons transferred)

Oxidation-reduction reactions 2H + + 2e -  H 2 The equilibrium constant of a redox reaction is called the standard reduction potential (E  ) The reference standard for reduction potentials is the hydrogen system with an E  ´ of volts Each hydrogen atom provides 2 protons and 2 electrons

Oxidation-reduction reactions Redox couples with more negative reduction potentials will donate electrons to couples with more positive potentials (and a greater affinity for electrons)

Oxidation-reduction reactions Electron tower with most negative reduction potentials at the top Electrons move from donors to acceptors from more negative to more positive potentials

Electron carriers Various carriers serve to transport electrons to different parts of the cell Example - Nicotinamide adenine dinucleotide NADH + H + + 1/2 O 2  H 2 O + NAD + NAD + / NADH is more negative than 1/2 O 2 / H 2 O, so electrons will flow from NADH (donor) to O 2 (acceptor)

Electron carriers

Structure of NAD

Flavin adenine dinucleotide (FAD) Proteins bearing FAD (or FMN) are referred to as flavoproteins

Coenzyme Q (CoQ) or ubiquinone Transports electrons and protons in respiratory electron transport chains

Cytochromes Cytochromes use iron atoms to transport electrons by reversible oxidation and reduction reactions Iron atoms in cytochromes are part of a heme group Nonheme iron proteins carry electrons but lack a heme group (e.g. Ferrodoxin)

Enzymes Enzymes can be defined as protein catalysts Increase rate of reactions without being permanently altered Reacting molecules = substrates Substances formed = product

Structure of enzymes Some enzymes are composed purely of protein Some enzymes contain both a protein and a nonprotein component The protein component = apoenzyme The nonprotein component = cofactor Apoenzyme + cofactor = holoenzyme

Structure of enzymes Cofactor tightly attached to apoenzyme = prosthetic group Loosely bound cofactor = coenzyme

Classification of enzymes Enzymes can be placed in one of six classes Usually named in terms of substrates and reactions catalyzed

Mechanisms of enzyme activity Enzymes serve to speed up the rate at which a reaction proceed to equilibrium by lowering the activation energy Activation energy required to from the transition state (AB)

Mechanisms of enzyme activity The enzyme may be rigid and shaped to precisely fit the substrate Binding to substrate positions it properly for reaction Referred to as the lock-and- key model

Mechanisms of enzyme activity Some enzymes change shape when they bind their substrate so that the active site surrounds and precisely fits the substrate Referred to as the induced fit model