Energy and Metabolism Chapter 6

Flow of Energy Energy: the capacity to do work -kinetic energy: the energy of motion -potential energy: stored energy Energy can take many forms: mechanical electric current heat light

Flow of Energy Most forms of energy can be converted to heat energy. Heat energy is measured in kilocalories. One calorie = the amount of heat required to raise the temp of water by 1oC 1 kilocalorie (kcal) = 1000 calories

Flow of Energy Potential energy stored in chemical bonds can be transferred from one molecule to another by way of electrons. oxidation: loss of electrons reduction: gain of electrons redox reactions are coupled to each other.

Laws of Thermodynamics First Law of Thermodynamics – energy cannot be created or destroyed -energy can only be converted from one form to another For example: sunlight energy chemical energy photosynthesis

Laws of Thermodynamics Second Law of Thermodynamics: disorder is more likely than order entropy: disorder in the universe The 2nd Law of Thermodynamics states that entropy is always increasing.

Laws of Thermodynamics Free energy: the energy available to do work -denoted by the symbol G (Gibb’s free energy) enthalpy: energy contained in a molecule’s chemical bonds free energy = enthalpy – (entropy x temp.) G = H - TS

Laws of Thermodynamics Chemical reactions can create changes in free energy: DG = DH - T DS When products contain more free energy than reactants – DG is positive. When reactants contain more free energy than products – DG is negative.

Delta G Gibbs Free Energy Equation ΔG = ΔH - TΔS where ΔG = Gibbs Free Energy, in kJΔH = enthalpy changeT = temperature, in KelvinΔS = entropy change (in kJ · K-1)

DELTA G IF ΔG < 0    The reaction is spontaneous ΔG > 0   The reaction is nonspontaneous ΔG = 0    The reaction is at equilibrium.Free energy change is the net driving force of a chemical reaction— whether the reaction will be spontaneous or not

DELTA G PROBLEM Notice the case where ΔG = 0. Equilibrium is a concept we will be discussing in depth in another module. Generally you will find that most exothermic reactions are spontaneous, even if entropy decreases (becomes more ordered), because enthalpy contributes more to ΔG than does entropy. The exceptions are reactions occurring at high temperatures. Example. Calculate ΔG for the following reaction at 25°C. Will the reaction occur (be spontaneous)? How do you know? NH3(g) + HCl(g) → NH4Cl(s) Also given for this reaction: ΔH = -176.0 kJΔS = -284.8 J·K-1 Solution We will calculate ΔG using the formula ΔG = ΔH - TΔS but first we need to convert units for ΔS and temperature to Kelvin: ΔS = -284.8 J·K-1 = -0.2848 kJ·K-1K = 273 + °C = 273 + 25 = 298 K Now we can solve our equation: ΔG = -176.0 - (298)(-0.2848)ΔG = -176.0 - (-84.9)ΔG = -91.1 kJ Since ΔG < 0 the reaction will be spontaneous.

Laws of Thermodynamics Chemical reactions can be described by the transfer of energy that occurs: endergonic reaction: a reaction requiring an input of energy - DG is positive exergonic reaction: a reaction that releases free energy - DG is negative

Laws of Thermodynamics Most reactions require some energy to get started. activation energy: extra energy needed to get a reaction started -destabilizes existing chemical bonds -required even for exergonic reactions catalysts: substances that lower the activation energy of a reaction

Energy Currency of Cells ATP = adenosine triphosphate -the energy “currency” of cells ATP structure: -ribose, a 5-carbon sugar -adenine -three phosphates

Energy Currency of Cells ATP stores energy in the bonds between phosphates. Phosphates are highly negative, therefore: -the phosphates repel each other -much energy is required to keep the phosphates bound to each other -much energy is released when the bond between two phosphates is broken

Energy Currency of Cells When the bond between phosphates is broken: ATP ADP + Pi energy is released ADP = adenosine diphosphate Pi = inorganic phosphate This reaction is reversible.

Energy Currency of Cells The energy released when ATP is broken down to ADP can be used to fuel endergonic reactions. The energy released from an exergonic reaction can be used to fuel the production of ATP from ADP + Pi.

Enzymes Enzymes: molecules that catalyze reactions in living cells -most are proteins -lower the activation energy required for a reaction -are not changed or consumed by the reaction

Enzymes Enzymes interact with substrates. substrate: molecule that will undergo a reaction active site: region of the enzyme that binds to the substrate Binding of an enzyme to a substrate causes the enzyme to change shape, producing a better induced fit between the molecules.

Enzymes Multienzyme complexes offer certain advantages: 1. The product of one reaction can be directly delivered to the next enzyme. 2. The possibility of unwanted side reactions is eliminated. 3. All of the reactions can be controlled as a unit.

Enzymes Not all enzymes are proteins. Certain reactions involving RNA molecules are catalyzed by the RNA itself. ribozymes: RNA with enzymatic abilities For example, the ribosome is a ribozyme.

Enzymes Enzyme function is affected by its environment. Factors that can change an enzyme’s 3-dimensional shape can change its function. -for example, pH, temperature, regulatory molecules

Enzymes Temperature -enzyme activity may be increased with increasing temp, up to the temp optimum -temperatures too far above the temp optimum can denature the enzyme, destroying its function pH – most enzymes prefer pH values from 6 to 8.

Enzymes Inhibitors are molecules that bind to an enzyme to decrease enzyme activity. -competitive inhibitors compete with the substrate for binding to the same active site -noncompetitive inhibitors bind to sites other than the enzyme’s active site

Enzymes Allosteric enzymes exist in either an active or inactive state. -possess an allosteric site where molecules other than the substrate bind -allosteric inhibitors bind to the allosteric site to inactivate the enzyme allosteric activators bind to the allosteric site to activate the enzyme

Metabolism Metabolism: all chemical reactions occurring in an organism Anabolism: chemical reactions that expend energy to make new chemical bonds Catabolism: chemical reactions that harvest energy when bonds are broken

Metabolism Some enzymes require additional molecules for proper enzymatic activity. These molecules could be: -cofactors: usually metal ions, found in the active site participating in catalysis -coenzymes: nonprotein organic molecules, often used as an electron donor or acceptor in a redox reaction

Metabolism Biochemical pathways are a series of reactions in which the product of one reaction becomes the substrate for the next reaction. Biochemical pathways are often regulated by feedback inhibition in which the end product of the pathway is an allosteric inhibitor of an earlier enzyme in the pathway.