Cellular Respiration
BIO100 Chp 7
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Cellular respiration
Mitochondria glycolysis
Inside the mitochondria
Fermentation
metabolism
Cellular Respiration
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Overall process
– Oxidation of glucose to carbon dioxide, water, and energy
– Exergonic reaction used to drive ATP synthesis
– 4 phases of respiration required for complete oxidation of glucose
– Oxidation involves the removal of hydrogen atoms from substrates by redox
coenzymes NAD+ and FAD
• Phases of cellular respiration
– Glycolysis
• Breakdown of glucose to 2 molecules of pyruvate
• Oxidation by removal of hydrogens releases enough energy to make 2
ATP
– Preparatory reaction
• Pyruvate oxidized to acetyl CoA and carbon dioxide is removed
• Prep reaction occurs twice because glycolysis produces 2 pyruvates
– Citric acid cycle
• Acetyl CoA is converted to citric acid and enters the cycle
• Cyclical series of oxidation reactions that produces 1 ATP and carbon
dioxide
• Citric acid cycle turns twice because 2 acetyl CoA’s are produced per
glucose
– Electron transport chain
• Series of electron carrier molecules
• Electrons passed from one carrier to another
• As the electrons move from a higher energy state to a lower one, energy
is released to make ATP
• Under aerobic conditions 32-34 ATP per glucose molecule can be
produced
– Pyruvate
• Pivotal metabolite in cellular respiration
• If no oxygen is available, pyruvate is reduced to lactate (in animals) or
ethanol and carbon dioxide (in plants) in a process called fermentation
Cellular respiration
Outside the mitochondria: glycolysis
• Energy-investment steps
– Energy from 2 ATP is used to activate glucose
– Glucose is split into 2 3-carbon G3P molecules
• Energy-harvesting steps
– Oxidation of G3P by removal of hydrogens
– Hydrogens are picked up by NAD+ to form NADH
– Oxidation of G3P and further substrates yields enough energy to produce 4
ATP by direct substrate phosphorylation
• Glycolysis yields:
– 4 ATP by direct substrate phosphorylation
• 2 ATP were consumed in the investments steps
• Net gain of ATP from glycolysis is therefore 2 ATP
– 2 NADH which will carry electrons to the electron transport chain
• When oxygen is available pyruvate will enter the mitochondria for further
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oxidation
If no oxygen is available, pyruvate will enter the fermentation pathway
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Breathing, eating, and cellular respiration
– Oxygen is taken in by breathing
– Digested food contains glucose
– Oxygen and glucose are carried to cells by the bloodstream
– Glucose and oxygen enter cells where respiration occurs
– Carbon dioxide is taken by the bloodstream to the lungs
Relationship between breathing, eating, and cell respiration
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Preparatory reaction
– Produces the molecule that will enter the citric acid cycle
– 3C pyruvate is converted to 2C acetyl CoA
– Carbon dioxide is produced
– Hydrogen atoms are removed from pyruvate and picked up to form NADH
– This reaction occurs twice per glucose
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Citric acid cycle
– 2C acetyl group from prep reaction combines with a 4C molecule to
produce 6C citrate
– Oxidation of citrate by removal of hydrogens
– Produces 3 NADH and 1 FADH2
– Produces 1 ATP by direct substrate phosphorylation
– Cycle turns twice per glucose
– Total yield: 6 NADH, 2 FADH2, 2 ATP, 4 CO
• Electron transport chain (ETC)
– 2 electrons per NADH and FADH2 enter ETC
– Electrons are passed to series of electron carriers called cytochromes
– Energy is captured and stored as a hydrogen ion concentration gradient
– For each NADH enough energy is released to form 3 ATP
– For each FADH2 there are 2 ATP produced
– the final electron acceptor is oxygen
– After receiving electrons oxygen combines with hydrogen ions to form water
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as an end product ½ O2+ 2 e- + 2H+  H2O
NAD+ and FAD recycle back to pick up more electrons from glycolysis, prep
reaction, and citric acid cycle
• The Efficiency of cellular respiration
– The difference in energy content of reactants (glucose and oxygen) and
products (carbon dioxide and water) is 686 kcal
– ATP phosphate bond has 7.3 kcal of energy
– 36 ATP are produced in respiration 36 X 7.3 = 263 kcal
– 263/686 = 39% efficiency of energy capture
– The rest of the energy is lost as heat
Fermentation
• Fermentation
– Occurs when O2 is not available
– Animal cells convert pyruvate to lactate
– Plant cells, yeasts convert pyruvate to ethanol and CO 2
– Fermentation regenerates NAD+ which keeps glycolysis going
• Advantages and Disadvantages of fermentation
– Provides a low but continuous supply of ATP when oxygen is limited and
only glycolysis can function
– Lactate is potentially toxic to muscles, lowering pH and causing fatigue
– Transported to liver where it is converted to pyruvate
• This process requires oxygen
• During exercise an oxygen debt is built up
• Oxygen debt is the amount of oxygen “owed” to the liver to convert
accumulated lactic acid to pyruvate
• Energy yield of fermentation
– Produces only a net of 2 ATP per glucose through direct substrate
phosphorylation by allowing glycolysis to continue
– Following fermentation most of the potential energy from glucose is still
waiting to be released
– Fermentation is a way to continue an ATP supply to cells when oxygen is in
short supply
Metabolism
• Catabolism-break down reactions
– Carbohydrates-digested to glucose for cell respiration
– Fats-digested to glycerol and fatty acids
• Glycerol can enter glycolytic pathway
• Fatty acids metabolized to acetyl CoA which enters citric acid cycle
– Proteins- deamination
• Amino acids can enter pathway at different points
• Anabolism- synthesis reactions
– Substrates of glycolysis and citric acid cycle can be substrates for synthesis
of macromolecules
• G3P can be converted to glycerol
• Acetyl groups can be converted to fatty acids
• Some citric acid intermediates can be converted to amino acids
– Anabolic reactions require the input of energy in the form of ATP generated
in catabolic reactions