Question

Which of the following molecules will produce the most ATP per mole? a. glucose or stearic acid \(\left(\mathrm{C}_{18}\right)\) b. glucose or two pyruvate c. two acetyl CoAs or one palmitic acid \(\left(\mathrm{C}_{16}\right)\) d. lauric acid \(\left(C_{12}\right)\) or palmitic acid \(\left(C_{16}\right)\) e. \(\alpha\) -ketoglutarate or fumarate in one turn of the citric acid cycle

Short answer

a. Stearic acid, b. Glucose, c. Palmitic acid, d. Palmitic acid, e. α-Ketoglutarate

Step-by-step solution

- Determine ATP Yield from Glucose

Calculate the ATP produced from the oxidation of one mole of glucose. Glucose undergoes glycolysis, the citric acid cycle, and oxidative phosphorylation, producing approximately 30-32 ATP.

- Determine ATP Yield from Stearic Acid

Calculate the ATP produced from the complete oxidation of one mole of stearic acid (C18). Stearic acid undergoes beta-oxidation, producing acetyl CoA which enters the citric acid cycle. The total ATP yield from one mole of stearic acid is about 120 ATP.

- Compare Glucose and Stearic Acid

Compare the ATP yield from glucose (30-32 ATP) with stearic acid (120 ATP). Stearic acid produces more ATP per mole.

- Determine ATP Yield from Pyruvate

Calculate the ATP production from one mole of pyruvate. Each pyruvate molecule produces about 12.5 ATP after entering the citric acid cycle. Therefore, two pyruvate molecules yield approximately 25 ATP.

- Compare Glucose and Two Pyruvate

Compare the ATP yield from glucose (30-32 ATP) with two pyruvate (25 ATP). Glucose produces more ATP per mole.

- Determine ATP Yield from Acetyl CoA

Calculate the ATP production from one mole of acetyl CoA. Each acetyl CoA produces about 10 ATP. Therefore, two acetyl CoA molecules yield approximately 20 ATP.

- Determine ATP Yield from Palmitic Acid

Calculate the ATP produced from the complete oxidation of one mole of palmitic acid (C16). Palmitic acid produces approximately 106 ATP.

- Compare Two Acetyl CoA and Palmitic Acid

Compare the ATP yield from two acetyl CoA (20 ATP) with palmitic acid (106 ATP). Palmitic acid produces more ATP per mole.

- Determine ATP Yield from Lauric Acid

Calculate the ATP produced from the complete oxidation of one mole of lauric acid (C12). Lauric acid produces approximately 80 ATP.

- Compare Lauric Acid and Palmitic Acid

Compare the ATP yield from lauric acid (80 ATP) with palmitic acid (106 ATP). Palmitic acid produces more ATP per mole.

- Determine ATP Yield from α-Ketoglutarate

Calculate the ATP produced from the oxidation of one mole of α-ketoglutarate in the citric acid cycle. α-ketoglutarate produces one ATP equivalent (GTP), one FADH2 (1.5 ATP), and one NADH (2.5 ATP) total of 5 ATP.

- Determine ATP Yield from Fumarate

Calculate the ATP produced from the oxidation of one mole of fumarate in the citric acid cycle. Fumarate produces one NADH which yields 2.5 ATP.

- Compare α-Ketoglutarate and Fumarate

Compare the ATP yield from α-ketoglutarate (5 ATP) with fumarate (2.5 ATP). α-ketoglutarate produces more ATP in one turn of the citric acid cycle.

Key concepts

Glycolysis

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. It takes place in the cytoplasm of the cell and does not require oxygen.

During glycolysis, one molecule of glucose (a six-carbon sugar) is split into two molecules of pyruvate (a three-carbon compound). This process results in the production of a net gain of 2 ATP molecules and 2 NADH molecules.

The main steps of glycolysis are:

• Glucose is phosphorylated by hexokinase to form glucose-6-phosphate.
• Glucose-6-phosphate is converted into its isomer, fructose-6-phosphate.
• Another phosphate group is added to form fructose-1,6-bisphosphate.
• Fructose-1,6-bisphosphate is split into two three-carbon sugars, dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.
• Glyceraldehyde-3-phosphate is then oxidized, leading to the reduction of NAD+ to NADH and the production of ATP.

Overall, glycolysis provides a quick way to produce ATP, although in lower amounts compared to other pathways.

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA.

The cycle takes place in the mitochondria and is crucial for the production of ATP, NADH, and FADH2.

Key steps in the citric acid cycle involve:

• Acetyl-CoA combines with oxaloacetate to form citrate.
• Citrate is then converted into its isomer, isocitrate.
• Isocitrate is oxidized, leading to the reduction of NAD+ to NADH and the production of carbon dioxide.
• Alpha-ketoglutarate is produced, which undergoes another oxidation step producing more NADH and carbon dioxide.
• Succinyl-CoA is converted into succinate, generating GTP (equivalent to ATP).
• Succinate is then converted to fumarate, producing FADH2.
• Fumarate is hydrated to malate, which is then oxidized to regenerate oxaloacetate and produce NADH.

Each turn of the citric acid cycle generates approximately one ATP (or GTP), three NADH, and one FADH2.

Beta-Oxidation

Beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the mitochondria to generate acetyl-CoA.

Each step of beta-oxidation shortens the fatty acid chain by two carbon atoms and produces one molecule of acetyl-CoA, NADH, and FADH2.

For a molecule like stearic acid (C18), beta-oxidation goes through several cycles to completely oxidize the fatty acid.

The main steps are:

• The fatty acid is activated and transported into the mitochondrion.
• The fatty acid undergoes dehydrogenation to form a trans double bond, producing FADH2.
• The double bond is hydrated to form a hydroxy group.
• The hydroxy group is oxidized to a keto group, generating NADH.
• The resulting keto acid is cleaved, producing acetyl-CoA.

Each molecule of stearic acid can yield up to 120 ATP molecules through complete oxidation.

Oxidative Phosphorylation

Oxidative phosphorylation is the process by which ATP is formed as electrons are transferred from NADH and FADH2 to oxygen by a series of electron carriers.

This process occurs in the inner mitochondrial membrane and consists of two main components: the electron transport chain (ETC) and ATP synthase.

The steps involved include:

• Electrons from NADH and FADH2 are passed through the ETC, which consists of a series of protein complexes.
• The flow of electrons through the ETC creates a proton gradient across the inner mitochondrial membrane.
• ATP synthase uses the energy from this proton gradient to convert ADP and inorganic phosphate into ATP.

Oxidative phosphorylation is the major source of ATP in aerobic organisms, producing about 26 to 28 ATP molecules from each glucose molecule.