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Harvesting Chemical Energy: Cellular RespirationChapter 8
Cellular Respiration:THE BIG PICTURE
Cellular respiration is the process by which organisms can get energy (ATP) from their food (glucose)
Cellular respiration is CRITICAL for life Occurs in BOTH plants and animals Two main mechanisms
Aerobic cellular Respiration – Requires OxygenAnaerobic cellular Respiration – Does not
require Oxygen
Main Types of Cellular Respiration Pathways
Aerobic Respiration
Evolved later Require oxygen Start with glycolysis
in cytoplasm Completed in
mitochondria
Anaerobic Respiration
Evolved first Don’t require oxygen Start with glycolysis in
cytoplasm Completed in cytoplasm
Aerobic Respiration
Overall Equation:
C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water
dioxide
Several steps occur in the middle (intermediates) Each step (rxn) catalyzed by enzymes
Aerobic respiration Overview
Stage One: Glycolysis (cytoplasm)
Stage Two: Preparation for Krebs (mitochondrial matrix) Krebs Cycle (matrix)
Stage Three: Electron Transfer Chain (across inner
membrane of mitochondria)
The Role of Coenzymes Several oxidation-reduction rxns take place in
aerobic respiration (Glucose gets oxidized to carbon dioxide)
In order to aid in the redox rxns, enzymes use coenzymes NAD+ and FAD to carry electrons from glucose derivatives to the electron transfer chain
NAD+ and FAD accept electrons and hydrogen to become NADH and FADH2 during the first two stages of aerobic respiration (Glycolysis, Krebs) and deliver electrons and hydrogen to the Electron Transfer Chain to make ATP
Coenzymes: NAD+ & FAD
Nicotinamide adenine dinucleotide (NAD+)
Stage One: Glycolysis
Glucose (6-carbon) is broken down into 2 molecules of pyruvate (3-carbon)
Yields 2 ATP by substrate level phosphorylation
1 NADH
1 NADH
Glycolysis: Overall Reaction
Glucose(6C)
2 ATP 2 ADP
FructoseBisphosphate
(6C)
G3P(3C)
G3P(3C)
Pyruvate(3C)
Pyruvate(3C)
2 ADP 2 ATP
2 ADP 2 ATP
1 NAD+
1 NAD+
Aerobic Respiration:1. Glycolysis
Glycolysis: Net Yield Energy requiring steps:
2 ATP invested
Energy releasing steps:2 NADH formed 4 ATP formed
Net yield: 2 ATP + 2 NADH + 2 molecules of Pyruvate
What happens next?
Depends on the organism and the presence of oxygen
If oxygen is around: Aerobic respiration, proceed to Krebs cycle
If no oxygen: Anaerobic respiration, Proceed to Fermentation
Second Stage: Krebs cycle
Preparatory reactions: Oxidation of pyruvatePyruvate is oxidized into two-carbon acetyl units
and carbon dioxideNAD+ is reduced
Krebs cycleThe acetyl units are oxidized to carbon dioxideNAD+ and FAD are reduced
Oxidation of Pyruvate
Results of the Second Stage
All of the carbon atoms in pyruvate end up in carbon dioxide
Coenzymes are reduced (they pick up electrons and hydrogen)
One molecule of ATP forms Four-carbon oxaloacetate regenerates
Coenzyme Reductions during First Two Stages
Glycolysis 2 NADH Preparatory
reactions 2 NADH Krebs cycle 2 FADH2 + 6 NADH
Total 2 FADH2 + 10 NADH
Occurs in the mitochondria Coenzymes deliver electrons to electron
transfer chains Electron transfer sets up H+ ion gradients Flow of H+ down gradients powers ATP
formation
Electron Transfer Chain
Importance of Oxygen
Why does aerobic respiration require oxygen? Oxygen acts as the final electron (and
hydrogen ion) acceptor in ETCBinds to leftover e- and H+ to form water“Clean up crew”
ATP Actual Yield
Energy Harvest Varies
NADH formed in cytoplasm cannot enter mitochondrion
It delivers electrons to mitochondrial membrane
Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion
Electrons given to FAD yield less ATP than those given to NAD+
686 kcal of energy are released
7.5 kcal are conserved in each ATP
When 32 ATP form, 240 kcal (32 X 7.5) are
captured in ATP
Efficiency is 240 / 686 X 100 = 35%
Most energy is lost as heat
Efficiency of Aerobic Respiration
Do not use oxygen
Produce less ATP than aerobic pathways
Two types
Fermentation pathways
Anaerobic electron transport
Anaerobic Pathways
Fermentation Pathways
Begin with glycolysis
Do not break glucose down completely to carbon
dioxide and water
Yield only the 2 ATP from glycolysis
Steps that follow glycolysis serve only to
regenerate NAD+
When life originated, atmosphere had little oxygen
Earliest organisms used anaerobic pathways
Later, noncyclic pathway of photosynthesis
increased atmospheric oxygen
Cells arose that used oxygen as final acceptor in
electron transport
Evolution of Metabolic Pathways