Chapter 7:  How Cells Make ATP - Energy-releasing Pathways

Aerobic respiration is a redox process

  Glucose contains energy that can be converted to ATP

  The process uses oxygen; called aerobic respiration

  Aerobic respiration is a redox process

  C₆H₁₂O₆ + 6 O₂ + 6 H₂O® 6 CO₂ + 12 H₂O + Energy

  Water is both a reactant and a product

  Glucose is oxidized to form carbon dioxide

  Oxygen is reduced, forming water

  The electrons produced are used to form ATP

respiration has four stages

Glycolysis

  Glucose is converted to 2 3-carbon molecules of pyruvate

  ATP and NADH are formed

  Occurs in the cytosol

Formation of acetyl coenzyme A (acetyl CoA)

  Pyruvate is converted into acetyl CoA

  NADH is produced

  Carbon dioxide is a waste product

  Occurs in the mitochondrion

  The citric acid cycle

  Acetyl CoA combines with oxaloacetate, forming citrate

  Citrate undergoes conversions, ultimately reforming oxaloacetate

  Carbon dioxide is a waste product

  ATP, NADH and FADH₂ are produced

  The electron transport system and chemiosmosis

  Electrons that originated in glucose are transferred via NADH and FADH₂ to a chain of electron acceptors

  Hydrogen ions are pumped across the inner mitochondrial membrane       

  Via chemiosmosis, ATP is produced

  Reaction types

  Dehydrogenation

  Hydrogens are transferred to a coenzyme (NAD⁺ or FAD)

  Decarboxylations

  Carboxyl groups are removed from the substrate as carbon dioxide

  Preparation reactions

  Molecules are rearranged in preparation for decarboxylations or dehydrogenations

  In glycolysis, glucose yields two pyruvates

  Glycolysis means 'sugar splitting'

  One 6-carbon molecule is converted to two 3-carbon molecules

  Occurs in the cytosol

  Occurs in aerobic or anaerobic conditions

  A series of reactions; each catalyzed by a different enzyme

  The first phase of glycolysis requires an initial investment of ATP

  First steps of glycolysis

  Glucose ® fructose-1,6-bisphosphate® 2 glyceraldehyde-3-phosphate (G3P)

  2 ATP molecules are invested in this step   

  The second phase of glycolysis yields NADH and ATP

  G3P is converted into 2 pyruvate molecules

  4 molecules of ATP are produced (net yield 2)

  2 molecules of NADH are produced

  Pyruvate is converted to acetyl CoA

  A carboxyl group is removed from pyruvate (carbon dioxide is produced)

  NADH is produced

  The acetyl group joins with coenzyme A, forming acetyl CoA

  Coenzyme A is made from pantothenic acid

  The citric acid cycle oxidizes acetyl CoA

  Also known as TCA cycle or Krebs cycle

  Occurs in the mitochondrion

  8 steps, all enzyme-mediated

  Acetyl CoA combines with oxaloacetate® citrate and CoA

  Series of steps, ultimately reforming oxaloacetate

  6 NADH and 2 FADH₂ are produced

  2 ATP molecules are produced

  All of the energy of the glucose molecule is carried by NADH and FADH₂

  The electron transport chain is coupled to ATP synthesis

  The electron transport chain transfers electrons from NADH and FADH₂ to oxygen

  Electrons ® FMN® a series of cytochromes and CoA

  Electrons lose energy as they pass through the chain

  Hydrogen ions (protons) are passed into the intermembrane space of the mitochondrion

  Electrons are finally passed to oxygen-forming water

  The chemiosmotic model explains the coupling of ATP synthesis to electron transport

  A proton gradient is formed across the inner mitochondrial membrane

  In 1961, Peter Mitchell proposed the chemiosmotic model, for which he received the Nobel Prize in 1978

  A proton gradient is formed by the electron transport chain; protons are pumped into the intermembrane space of the mitochondrion  

  Protons diffuse through the channels formed by the enzyme complex ATP synthase

  Movement of protons catalyzes production of ATP

respiration of one glucose yields a maximum of 36 to 38 ATPs

  Glycolysis produces 2 ATP molecules

  2 ATP molecules are produced in the citric acid cycle

  Remainder of ATP is produced in the electron transport system (32 or 34)

  Maximum yield of ATP from NADH is 3 per molecule

NADHs from glycolysis may produce fewer ATPs due to the necessity of transport of NADH across the mitochondrial membrane

  Maximum yield of ATP from FADH₂ is 2

  Total ATP yield per molecule of glucose is 36-38

  Efficiency is about 40%; the remaining energy is disseminated as heat

other than glucose also provide energy

  Humans gain more energy from oxidation of fatty acids than glucose

  Lipids contain 9 kcal per gram

  Lipids are broken down and glycerol enters glycolysis; fatty acids are converted to acetyl CoA and enter the citric acid cycle

  Proteins are broken down to amino acids

  Amino acids are deaminated (the amino group is removed)

  Amino groups are converted to urea and excreted

  The remaining carbon chain enters at various points

  Proteins contain about 4 kcal per gram

regulate aerobic respiration

  ATP synthesis continues until ADP stores are depleted

  Enzyme regulation is important

  An important control point is phosphofructokinase

respiration and fermentation do not require oxygen

  Anaerobic respiration

  Various inorganic substances serve as the final electron acceptor

  Yield is only the two ATP molecules from glycolysis

  Seen in some bacteria

Alcoholic fermentation and lactate fermentation are inefficient

  Alcoholic fermentation produces ethanol

  Pyruvate is converted to ethanol to regenerate NAD⁺

  Ethanol is a potentially toxic waste product

  Yeast carry out alcoholic fermentation when oxygen deprived

  Bacteria and some fungi carry out lactate fermentation

  Pyruvate is converted to lactate to regenerate NAD⁺

  Strenuous exercise in mammals results in lactate fermentation as well

  Yields only the two ATP molecules from glycolysis