8 How Cells Make ATP: Energy-Releasing Pathways

Lecture Outline

I. Aerobic respiration is a redox process

A. Glucose contains energy that can be converted to ATP

B. The process uses oxygen; it= s called aerobic respiration

C. Aerobic respiration is a redox process

1. C6H12O6 + 6 O2 + 6 H2O± 6 CO2 + 12 H2O + Energy

2. Water is both a reactant and a product

3. Glucose is oxidized to form carbon dioxide

4. Oxygen is reduced, forming water

5. The electrons produced are used to form ATP

II. Aerobic respiration has four stages

A. Glycolysis

1. Glucose is converted to two 3-carbon molecules of pyruvate

2. ATP and NADH are formed

3. Glycolysis occurs in the cytosol

B. Formation of acetyl coenzyme A (acetyl CoA)

1. Pyruvate is converted into acetyl CoA

2. NADH is produced

3. Carbon dioxide is a waste product

4. The process occurs in the mitochondrion

C. The citric acid cycle

1. Acetyl CoA combines with oxaloacetate, forming citrate

2. Citrate undergoes conversions, ultimately reforming oxaloacetate

3. Carbon dioxide is a waste product

4. ATP, NADH, and FADH2 are produced

D. The electron transport system and chemiosmosis

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

2. Hydrogen ions are pumped across the inner mitochondrial membrane

3. Via chemiosmosis, ATP is produced

E. Reaction types

1. Dehydrogenation

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

2. Decarboxylations

a) Carboxyl groups are removed from the substrate as carbon dioxide

3. Preparation reactions

a) Molecules are rearranged in preparation for decarboxylations or dehydrogenations

F. In glycolysis, glucose yields two pyruvates

1. Glycolysis means A sugar splitting@

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

3. Glycolysis occurs in the cytosol

4. It occurs in aerobic or anaerobic conditions

5. It involves a series of reactions, each catalyzed by a different enzyme

a) The first phase of glycolysis requires an initial investment of ATP

(1) First steps of glycolysis

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

(3) 2 ATP molecules are invested in this step

b) The second phase of glycolysis yields NADH and ATP

(1) G3P is converted into 2 pyruvate molecules

(2) 4 molecules of ATP are produced (net yield 2)

(3) 2 molecules of NADH are produced

G. Pyruvate is converted to acetyl CoA

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

2. NADH is produced

3. The acetyl group joins with coenzyme A, forming acetyl CoA

4. Coenzyme A is made from pantothenic acid

H. The citric acid cycle oxidizes acetyl CoA

1. The citric acid cycle is also known as TCA cycle or Krebs cycle

2. It occurs in the mitochondrion

3. The cycle has 8 steps, all enzyme-mediated

a) Acetyl CoA combines with oxaloacetate ± citrate and CoA

b) It is a series of steps, ultimately reforming oxaloacetate

c) 6 NADH and 2 FADH2 are produced

d) 2 ATP molecules are produced

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

I. The electron transport chain is coupled to ATP synthesis

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

a) Electrons ± FMN ± a series of cytochromes and CoA

b) Electrons lose energy as they pass through the chain

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

d) Electrons are finally passed to oxygenC forming water

2. The chemiosmotic model explains the coupling of ATP synthesis to electron transport in aerobic respiration

a) A proton gradient is formed across the inner mitochondrial membrane

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

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

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

d) Movement of protons catalyzes production of ATP

 

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III. Aerobic respiration of one glucose molecule yields a maximum of 36 to 38 ATPs

A. Glycolysis produces 2 ATP molecules

B. 2 ATP molecules are produced in the citric acid cycle

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

1. Maximum yield of ATP from NADH is 3 per molecule

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

a) Different mitochondrial shuttle systems carry the electrons, form NADH produced in the cytosol, and are present in different kinds of cells

3. Maximum yield of ATP from FADH2 is 2

D. Total ATP yield per molecule of glucose is 36B 38

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

IV. Most organisms depend on nutrients other than glucose to provide energy

A. Humans gain more energy from oxidation of fatty acids than from glucose

1. Lipids contain 9 kcal per gram

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

B. Proteins are broken down to amino acids

1. Amino acids are deaminated (the amino group is removed)

2. Amino groups are converted to urea and excreted

3. The remaining carbon chain enters at various points

4. Proteins contain about 4 kcal per gram

V. Anaerobic respiration and fermentation do not require oxygen as the final electron acceptor

A. Anaerobic respiration

1. Various inorganic substances serve as the final electron acceptor

2. Yield is only the 2 ATP molecules from glycolysis

3. Seen in some bacteria, including some that are important in the global nitrogen cycle

B. Alcoholic fermentation and lactate fermentation are inefficient

1. Alcoholic fermentation produces ethanol

a) Pyruvate is converted to ethanol to regenerate NAD+

b) Ethanol is a potentially toxic waste product

c) Yeast carry out alcoholic fermentation when oxygen deprived

2. Bacteria and some fungi carry out lactate fermentation

a) Pyruvate is converted to lactate to regenerate NAD+

b) Strenuous exercise in mammals results in lactate fermentation as well

3. Yields only the 2 ATP molecules from glycolysis