General Biology/Cells/Energy and Metabolism

Energy The capacity to do work. Kinetic energy: energy of motion (ex. jogging). Potential energy: stored energy (ex. a lion that is about to leap on its prey). Many forms of energy: e.g., Heat Sound Electric current Light All convertible to heat Most energy for biological world is from sun Heat (energy of random molecular motion, thermal energy) Convenient in biology All other energy forms can be converted to heat Thermodynamics: study of thermal energy Heat typically measured in kilocalories Kcal: 1000 calories 1 calorie: amount of heat required to raise the temperature of one gram of water one degree Celsius (°C) Heat plays major role in biological systems Ecological importance Biochemical reactions Oxidation–Reduction Energy flows into biological world from sun Light energy is captured by photosynthesis Light energy raises electrons to higher energy levels Stored as potential energy in covalent C-H bonds of sugars Strength of covalent bond is measured by amount of energy required to break it 98.8 kcal/mole of C-H bonds In chemical reaction, energy stored in covalent bonds may transfer to new bonds. When this involves transfer of electrons, it is oxidation–reduction reaction Always take place together Electron lost by atom or molecule through oxidation is gained by another atom or molecule through reduction Potential energy is transferred from one molecule to another (but never 100%) Often called redox reactions Photosynthesis Cellular Respiration Chemiosynthesis Autotrophs Heterotrophs NAD+ Common electron acceptor/donor in redox reactions Energetic electrons often paired with H+ Free energy Energy required to break and subsequently form other chemical bonds Chemical bonds: sharing of electrons, tend to hold atoms of molecule together Heat, by increasing atomic motion, makes it easier to break bonds (entropy) Energy available to do work in a system In cells, G = H - TS G = Gibbs’ free energy H = H (enthalpy) energy in molecule’s chemical bonds TS (T, temperature in °K; S, entropy) Chemical reactions break and make bonds, producing changes in energy Under constant conditions of temperature, pressure and volume, ΔG = ΔH - TΔS ΔG, change in free energy If positive (+), H is higher, S is lower, so there is more free energy; endergonic reaction, does not proceed spontaneously; require input of energy (e.g., heat) If negative (–), H is lower, S is higher. Product has less free energy; exergonic; spontaneous Activation energy Reactions with –ΔG often require activation energy e.g., burning of glucose Must break existing bonds to get reaction started Catalysts lower activation energy Enzymes Biological catalysts Protein RNA (ribozyme) Stabilizes temporary association between reactants (substrates) to facilitate reaction Correct orientation Stressing bonds of substrate Lower activation energy Not consumed (destroyed) in reaction Carbonic anhydrase Important enzyme of red blood cells CO2 + H2O → H2CO3 -> HCO3 + H+ Carbonic anhydrase catalyzes 1st reaction Converts water to hydroxyl Orients the hydroxyl and CO2 Enzyme mechanism One or more active sites which bind substrates (reactants) Highly specific Binding may alter enzyme conformation, inducing better fit Factors affecting enzyme activity Substrate concentration Product concentration Cofactor concentration Temperature pH Inhibitors Competitive: bind to active site Noncompetitive: bind to 2nd site, called allosteric site; changes enzyme conformation Activators Bind to allosteric sites, increase enzyme activity Cofactors Required by some enzymes Positively charged metal ions e.g., ions of Zn, Mo, Mg, Mn Draw electrons away from substrate (stress chemical bonds) Non-protein organic molecules (coenzymes) E.g., NAD+, NADP+, etc. Major role in oxidation/reduction reactions by donating or accepting electrons ATP Adenosine triphosphate Major energy currency of cells, power endergonic reactions Stores energy in phosphate bonds Highly negative charges, repel each other Makes these covalent bonds unstable Low activation energy When bonds break, energy is transferred ATP → ADP + Pi + 7.3 kcal/mole Biochemical pathways Metabolism: sum of chemical reactions in cell/organism Many anabolic and catabolic reactions occur in sequences (biochemical pathways) Often highly regulated Evolution of biochemical pathways Protobionts or 1st cells likely used energy rich substrates from environment Upon depletion of a substrate, selection would favor catalyst which converts another molecule into the depleted molecule By iteration, pathway evolved backward References This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.