Harvesting Energy CELLULAR RESPIRATION & FERMENTATION

Photosynthesis and respiration provide the energy needed for life This energy ultimately comes from the sun

RESPIRATION Harvesting of energy from food molecules Performed at the cellular level This energy can then be stored for later use

RESPIRATION Respiration is a catabolic process: large molecules are broken down and the energy released from bonds is used for: maintenance growth (anabolic process) reproduction The energy released is transformed into ATP

Summary Equation for Aerobic Respiration C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O glucose oxygen carbon water dioxide

What’s happening? C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O glucose oxygen carbon water dioxide Glucose is losing electrons - oxidation Oxygen is gaining electrons - reduction Energy released

This doesn’t happen at once Much energy lost as heat Energy conserved if smaller reactions take place

STAGES OF RESPIRATION Aerobic cellular respiration can be divided into three (or four) main stages: #1 Glycolysis - cytoplasm #1.5 Transition step cytoplasm  mitochondria #2 Krebs Cycle - inner compartment (matrix) #3 Electron Transport Pathway - Inner membrane

GLYCOLYSIS Occurs within eukaryotic cytoplasm Multi-step metabolic pathway Partial oxidation of glucose (6 carbon) No oxygen required Products: –2 ATP (net) –2 NADH –2 pyruvate (3 carbon)

NADH The reduced coenzyme NADH is also produced during cellular respiration –Nicotinamide adenine dinucleotide –High energy molecule –Can be “spent” to make more ATP later 

TRANSITION STEP The pyruvate produced in glycolysis (etc.) –Enters the mitochondria –Is converted into acetyl CoA (2 carbon) –Enters the Krebs Cycle Products: –2 NADH –2 CO 2 formed –2 acetly CoA

KREBS CYCLE a.k.a., Citric Acid Cycle Occurs within mitochondrial matrix Multi-step metabolic pathway Remnants of glucose completely oxidized Products: –2 ATP –6 NADH –2 FADH 2 –4 CO 2

GLYCOLYSIS and KREBS Several high-energy molecules are produced during glycolysis and the Krebs cycle –4 ATP –10 NADH –2 FADH 2 Most of the energy harvested from glucose is in the form of reduced coenzymes However, only ATP is readily usable to perform cellular work The Electron Transport Pathway oxidizes NADH and FADH 2 to produce more ATP

ELECTRON TRANSPORT PATHWAY Occurs within the inner mitochondrial membrane Electrons are removed from NADH and shuttled through a series of electron acceptors –Energy is removed from the electrons with each transfer This energy is used to make ATP –NADH  3 ATP –FADH 2  2 ATP –O 2 is the terminal electron acceptor ½O 2 + 2H + + 2e -  H 2 O

Generation of ATP Chemiosmosis Electrons attract H+ and pull them through transport proteins to outer- compartment of mitochondria H+ then diffuse back through ATP synthase channels making ATP and water

ENERGY YIELD 4 ATP (glucose, krebs) 10 NADH  30 ATP 2 FADH 2  4 ATP (electron transport) 38 ATP total This total yield depends on different things

THEORETICAL YIELD Theoretical yield of 38 ATP not generally reached because: –Intermediates in central pathways siphoned off as precursor metabolites for biosynthesis –Electrons of NADH generated in cytosol often shuttled into mitochondria as FADH 2 –Each NADH typically yields slightly less than 3 ATP

BURNING OTHER STUFF Glucose can be oxidized to yield ATP Other biomolecules can also be oxidized to yield ATP –These molecules are converted to either glucose or to an intermediate in the catabolism of glucose

O 2 REQUIREMENT ~38 ATP produced per glucose molecule –34 ATP from ETP Requires adequate supply of oxygen Under conditions of insufficient oxygen, ATP yields can be severely reduced

What happens when O 2 is unavailable? Some cells cannot obtain energy when deprived of O 2 –e.g., human heart cells –“Obligate aerobes” Some cells normally perform aerobic respiration, but can still obtain energy when O 2 is lacking –e.g., skeletal muscle cells, S. cerevisiae (yeast), E. coli –“Facultative anaerobes” Others do not use O 2 to obtain energy –e.g., Clostridium botulinum, an “obligate anaerobe” –e.g., Streptococcus pyogenes, an “aerotolerant anaerobe”

FACULTATIVE ANAEROBES In the absence of O 2, aerobic respiration is impossible –Glycolysis still occurs Net ATP production: 2 ATP –2 is significantly less than thirty-something NAD + is converted to NADH –NADH is not useful to the cell if energy is not extracted –The absence of NAD + is detrimental to the cell –NADH must be converted back to NAD + »“Fermentation”

FERMENTATION NADH is produced during glycolysis –Energy in NADH cannot be used –NADH must be oxidized to replenish NAD + No payoff –NADH is oxidized to NAD + –Pyruvate is reduced to _______ (Different substances in different organisms) Human muscle: pyruvate  lactic acid Yeast: pyruvate  ethanol & CO 2 Other cells  many other molecules –Total energy yield of fermentation is the 2 ATP generated in glycolysis

FERMENTATION Skeletal muscles normally undergo aerobic respiration During strenuous exercise, O 2 may be rapidly depleted –Fermentation can continue to provide energy –Pyruvate  lactic acid Lactic acid builds up Buildup causes muscle fatigue & pain Lactic acid ultimately removed

FERMENTATION Saccharomyces cerevisiae (yeast) normally undergoes aerobic respiration O 2 is not always available –Fermentation can continue to provide energy –Pyruvate  ethanol & CO 2 Ethanol ultimately toxic

FERMENTATION Many other organisms also undergo fermentation –Some are facultative anaerobes –Some are obligate fermenters Pyruvate is converted into a host of different molecules by a host of different organisms –Many of these molecules are commercially important