Overview of Cellular Respiration, Photosynthesis and Redox Reactions Lecture 6 Fall 2008

Working Cells All organisms need ATP to do cellular work 1 All organisms need ATP to do cellular work Cells get ATP from converting carbon compounds (chemical energy) through cellular respiration Cellular Respiration: The conversion of chemical energy of carbon compounds into another form of chemical energy, ATP Where does an organism get the chemical energy (carbon compounds) used in cellular respiration?

Food Chains Producers Autotrophs (self-feeders) 2 Producers Autotrophs (self-feeders) Use photosynthesis or chemosynthesis to make organic food molecules Plants, algae (phytoplankton), cyanobacteria Basis for all other ecosystem growth Primary productivity Productivity: amount of biomass produced in a given area during a given period of time Biomass: dry weight of all organic matter in an organism Fig. 54.11

Cellular Respiration & Photosynthesis 3 Photosynthesis The process by which light energy from the sun is converted into chemical energy Chemical energy in the form of organic compounds

Food Chains Consumers Heterotrophs (other-feeders) 4 Food Chains Consumers Heterotrophs (other-feeders) Obtains organic food molecules by eating other organisms or substances derived from other organisms Primary = herbivores eat producers Secondary = carnivores eat herbivores Tertiary = top carnivores eat other carnivores Fig. 54.11

5 Metabolic Diversity Phototrophs: Organisms that obtain energy from light Chemotrophs: Organisms that obtain energy from inorganic or organic compounds in their environment Autotrophs: Organisms that need only inorganic molecules (CO2 or CH4 ) as a carbon source Heterotrophs: Organisms that require at least one organic nutrient (e.g., glucose) as a carbon source

Cellular Respiration & Photosynthesis 6 Cellular Respiration & Photosynthesis Cellular respiration requires carbon compounds & oxygen Carbon compounds & oxygen produced by photosynthesis See Fig. 9.2

Cellular Respiration Catabolic pathway Aerobic respiration 7 Catabolic pathway Aerobic respiration Consumes oxygen Anaerobic respiration Use other molecules (not oxygen) Catabolic pathway Breaks down a complex molecule into simpler compounds Creates energy harvesting of chemical energy from organic fuel molecules

Cellular Respiration 8 Cellular respiration is not the same as gas exchange (respiration) Gas exchange is the exchange of O2 and CO2 between an organism and its environment Cellular respiration is not the same as breathing “Breathing” refers specifically to process in lungs

Cellular Respiration Glucose molecule held together by covalent bonds 9 Cellular Respiration Glucose molecule held together by covalent bonds Covalent bonds share electrons Cellular respiration changes which atoms are bonded together ΔG = - 686 kcal/mol

Redox Reactions Redox reactions =oxidation-reduction reactions 10 Redox reactions =oxidation-reduction reactions Chemical reactions that transfer electrons from one substance to another Oxidation The loss of electrons Reduction The addition of electrons Reduction reduces the positive charge of an atom Reducing agent = electron donor Oxidizing agent = electron acceptor

Redox Reactions Energy needed to pull electrons away from an atom 11 Redox Reactions Energy needed to pull electrons away from an atom Requires more energy the more electronegative the atom Electronegativity : The tendency of an atom to attract electrons towards itself Electrons lose potential energy as they move from less electronegative to more electronegative atom Releases energy

Redox Reactions in Cellular Respiration 12 When glucose is broken down, hydrogen and its electrons change partners Electrons lose potential energy along the way and energy is released Oxidation Glucose oxidized Reducing agent - Loses electrons Reduction Oxygen reduced Oxidizing agent - Gains electrons

Electron Acceptors High electronegativity 13 High electronegativity Electrons move from molecule to molecule by being “passed” to a stronger electron acceptor Electron Acceptors NAD+ (nicotinamide adenine dinucleotide) Positively charged electron acceptor NAD+ is reduced to NADH NADP+ (nicotinamide adenine dinucleotide phosphate) Reduced to NADPH FAD (flavin adenine dinucleotide) FAD is reduced to FADH2 Oxygen Can accept (“pull”) electrons from both NADH & FADH2

Electron Transport Chain 14 Why can’t the electrons be transferred from glucose to oxygen in one step? Too explosive Energy released as heat and light energy Not readily usable to do work Breaking it down into steps allows the energy to be used to perform work = Electron Transport Chain Fig. 9.5

Electron Transport Chain 15 Electron Transport Chain Used to slow the fall of electrons from glucose to oxygen “chain” of protein complexes (and other molecules) Electrons pass from molecule to molecule in a series of redox reactions At each step, energy is released Used in the synthesis of ATP Fig. 9.5

Electron Transport Chain 16 At bottom of chain, the electron “drops” to oxygen Oxygen also picks up hydrogen and forms H20 Oxygen is driving the “fall” by attracting the electrons Fig. 9.5 Fig. 6.6