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?
Cellular respiration and fermentation are catabolic (energy-yielding) pathways • Fermentation -- An ATP-producing process in which both electron donors and acceptors are organic compounds; anaerobic process (without oxygen). • Cellular respiration -- An ATP-producing process in which the ultimate electron acceptor is an inorganic molecule, such as oxygen. • 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 the energy stored in food molecules 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.
An Introduction to Redox Reactions continued • Electron transfer requires both a donor and acceptor, so when one reactant is oxidized the other is reduced: • Fe + O2 Fe2O3 • In this case, Fe is the reducing agent or reducer; O is the oxidizing agent or oxidizer (has a high electronegativity). • Transfer of electrons may not be complete, but instead may just change the degree of sharing in covalent bonds: • CH4 + O2 CO2 + H2O
Electron Acceptors • C6H12O6 + 6O2 ——>6CO2 + 6H2O + Energy (ATP + Heat) • Hydrogens stripped 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.
Respiration: an overview • There are three metabolic 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 pyruvateinto 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 steps • Step 1: Glucose enters the cell, and carbon six is phosphorylated as 1 ATP is used. • Glucose glucose-6-phosphate • Step 2: Rearrangement of glucose-6-phosphate to its isomer, fructose-6-phosphate. • Step 3: Carbon one of fructose-6-phosphate is phosphorylated using another ATP to form fructose-1,6-diphosphate.
Glycolysis cont. • Step 4: Aldolase cleaves the 6-carbon fructose into two 3-carbon sugars. • Fructose-1,6-phosphate 2 3-phosphoglyceraldehyde • Step 5: Phosphoglyceraldehyde is phosphorylated on carbon one; NADH is formed. • 2 3-phosphoglyceraldehyde + 2 NAD+ 2 1,3 diphosphoglycerate + 2 NADH • Step 6: ATP is produced by substrate-level phosphorylation. • 2 1,3 diphosphoglycerate + 2 ADP 2 3-phosphoglycerate + 2 ATP
End of Glycolysis (really) • Step 7: Phosphate group on carbon three is transferred to carbon two. • 2 3-phosphoglycerate 2 2-phosphoglycerate • Step 8: Enzymatic removal of a water molecule. • 2 2-phosphoglycerate 2 2-phosphoenolpyruvate + H2O • Step 9: ATP is produced by substrate-level phosphorylation; pyruvate formed. • 2 2-phosphoenolpyruvate 2 pyruvate
Glycolysis animations • http://www.northland.cc.mn.us/biology/Biology1111/animations/glycolysis.html • http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html • http://student.ccbcmd.edu/courses/bio141/lecguide/unit6/metabolism/cellresp/glycol_an.html