Cellular Respiration: Harvesting Chemical Energy Chapter 9

All living things require energy Biological order (life) requires constant input of free energy(G) into the system. (Refer back to chapter 8) Total energy input(enthalpy or H) must exceed energy lost to entropy(S) to maintain life. G = H – TS Exergonic reactions (breakdown of ATP) must take place to fuel cellular work -- coupled reactions.

Use of free energy Organisms must maintain organization through metabolism. Endothermy (thermal energy from metabolism maintains body temperature) Ectothermy (external thermal energy regulates body temperature) Excess energy acquired can be used for growth and/or reproduction. These activities can vary in response to energy availability. Insufficient free energy results in lack of growth and reproduction, loss of biomass, and possibly death of the organism.

Cells are open systems Energy flows into most ecosystems as sunlight. Photosynthetic organisms trap light energy and transform it into chemical bond energy. Cells use chemical bond energy to make ATP. Chemical elements essential for life are recycled, but energy is not. How do cells harvest chemical energy?

“Spendable Energy”

Most efficient catabolic pathway is aerobic (with oxygen). Cellular respiration and fermentation are catabolic (energy-yielding) pathways Fermentation -- An ATP-producing process which is anaerobic (without oxygen). Cellular respiration -- An ATP-producing process which requires oxygen as a final electron acceptor. Most efficient catabolic pathway is aerobic (with oxygen). Carbohydrates, proteins and fats can all be metabolized as fuel, but cellular respiration is most often described as the oxidation of glucose: C6H12O6 + 6O2 ——>6CO2 + 6H2O + Energy (ATP + Heat)

Cells must recycle the ATP they spend for work Respiration transfers food energy to ATP. ATP (adenosine triphosphate) -- Nucleotide with unstable phosphate bonds that the cell hydrolyzes for energy; enzymes that catalyze this reaction are called ATPases. ATP + H2O  ADP + phosphate + energy ADP + H2O  AMP + phosphate + energy Removal of a phosphate yields 7 kcal of energy per mole of ATP. Phosphate groups from ATP are transferred to other compounds to do cellular work (phosphorylation); otherwise energy would be just be lost as heat.

Cells must recycle the ATP they spend for work (continued) The compound receiving the phosphate group from ATP is said to be phosphorylated and becomes energized; enzymes catalyzing these reactions are kinases. Cells must replenish the ATP supply to continue cellular work. Respiration provides the energy to regenerate ATP from ADP and inorganic phosphate.

An Introduction to Redox Reactions Oxidation/reduction reactions -- Chemical reactions which involve a transfer of electrons from one reactant to another (redox for short); use of chemical energy in living things involves redox rxns. Oxidation -- loss of electrons: Fe  Fe+3 + 3 e- (Iron has been oxidized) Reduction -- gain of electrons: O + 2 e-  O-2 (Oxygen has been reduced). LEO the lion says GER.

Electron Acceptors C6H12O6 + 6O2 ——>6CO2 + 6H2O + Energy (ATP + Heat) Hydrogens harvested from glucose are not transferred directly to oxygen, but are first passed to a special electron acceptor. Nicotinamide adenine dinucleotide (NAD+ / NADH) -- A dinucleotide that functions as a coenzyme in the redox reactions of metabolism.

Electron acceptors cont. Flavin adenine dinucleotide (FAD / FADH2) – see NADH. NAD+= Oxidized coenzyme (net positive charge); NADH = Reduced coenzyme (electrically neutral). Coenzyme -- Small nonprotein organic molecule that is required for certain enzymes to function. Dinucleotide -- A molecule consisting of two nucleotides.

NAD+ / FAD

Mitochondrion Review

Respiration: an overview There are three stages of cellular respiration: 1. Glycolysis 2. Krebs Cycle 3. Electron transport chain (ETC) Glycolysis occurs in the cytosol of the cell; splits glucose (6C) into two pyruvate (3C) molecules. Krebs Cycle occurs in the mitochondrial matrix; breaks down pyruvate into carbon dioxide. Electron transport chain is located at the inner membrane of the mitochondrion, where ATP synthesis or oxidative phosphorylation takes place.

Glycolysis: a closer look Glycolysis (Glyco = sugar; lysis = break) Occurs whether or not oxygen is present; yields 2 ATP. Overall reaction: glucose + 2 ATP  2 pyruvate + 4 ATP Substrate-level phosphorylation -- ATP production by direct enzymatic transfer of phosphate to ADP.

Glycolysis animations http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html http://glencoe.mcgraw-hill.com/sites/9834092339/student_view0/chapter7/how_glycolysis_works.html