Calculate the energy yield of glycolysis and cellular respiration per glucose molecule. Distinguish between substrate-level phosphorylation and oxidative phosphorylation.
To calculate the energy yield of glycolysis and cellular respiration per glucose molecule, we need to understand the different steps involved in each process. Let's break it down step-by-step:
1. Glycolysis:
Glycolysis is the first step in cellular respiration, which takes place in the cytoplasm. It involves the breakdown of glucose into two molecules of pyruvate.
The energy yield of glycolysis per glucose molecule is:
- 2 molecules of ATP (net gain of 2 ATP since 2 ATP are consumed initially)
- 2 molecules of NADH (reduced form of NAD+)
2. Cellular Respiration:
After glycolysis, if oxygen is available, the pyruvate molecules enter the mitochondria for further breakdown through three sequential processes:
a. Pyruvate decarboxylation:
Each pyruvate molecule is converted into acetyl-CoA, which enters the next step of cellular respiration, the citric acid cycle (also known as the Krebs cycle). This process produces:
- 2 molecules of NADH
b. Citric acid cycle (Krebs cycle):
The acetyl-CoA enters the citric acid cycle, which takes place in the mitochondria. During this process, the acetyl-CoA is completely oxidized, releasing energy. The energy yield per glucose molecule in the citric acid cycle is:
- 6 molecules of NADH
- 2 molecules of FADH2 (reduced form of FAD)
- 2 molecules of ATP (via substrate-level phosphorylation)
- 4 molecules of CO2 (byproduct)
c. Oxidative phosphorylation:
The electron carriers (NADH and FADH2) from glycolysis, pyruvate decarboxylation, and the citric acid cycle donate their electrons to the electron transport chain (ETC) located in the inner mitochondrial membrane.
As electrons pass through the ETC, energy is released and used to pump protons (H+) across the membrane, creating a proton gradient. The final acceptor of these electrons is oxygen (O2), resulting in the formation of water (H2O). This process is called oxidative phosphorylation because it generates energy in the form of ATP by using oxygen.
The energy yield of oxidative phosphorylation is:
- Approximately 28-32 molecules of ATP (the exact number varies depending on the source)
- Water (H2O) as a byproduct
Distinguishing between substrate-level phosphorylation and oxidative phosphorylation:
- Substrate-level phosphorylation: It occurs in both glycolysis and the citric acid cycle. During this process, a phosphate group is directly transferred from an organic molecule to ADP, resulting in the formation of ATP. This typically generates a small amount of ATP.
- Oxidative phosphorylation: It occurs during the electron transport chain in cellular respiration. It involves the transfer of electrons through a series of protein complexes in the inner mitochondrial membrane, creating a proton gradient. The energy released during this process is used by ATP synthase to convert ADP to ATP. Oxidative phosphorylation produces the majority of ATP in cellular respiration.
In summary, glycolysis produces 2 ATP and 2 NADH, while the complete breakdown of glucose through cellular respiration results in the production of approximately 28-32 ATP via substrate-level phosphorylation and oxidative phosphorylation.
To calculate the energy yield of glycolysis and cellular respiration per glucose molecule, we need to understand the different steps involved in these processes.
1. Glycolysis:
Glycolysis is the metabolic pathway that breaks down glucose into pyruvate. It occurs in the cytoplasm and does not require oxygen.
In glycolysis, one molecule of glucose undergoes a series of reactions and is eventually converted into two molecules of pyruvate. During this process, a small amount of energy is generated in the form of ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide).
Energy yield of glycolysis:
- Gross ATP: 4 ATP molecules are generated, but 2 ATP molecules are consumed in an early step of glycolysis, resulting in a net ATP production of 2 ATP molecules.
- NADH: 2 molecules of NADH are generated.
2. Cellular respiration:
Cellular respiration is a set of metabolic reactions that occur in the mitochondria and convert pyruvate into carbon dioxide, water, and large amounts of ATP. It can be divided into three main stages: pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation.
a. Pyruvate oxidation:
In this step, each molecule of pyruvate from glycolysis is converted into Acetyl CoA, releasing carbon dioxide and reducing NAD+ to NADH. Since each glucose molecule produces two pyruvate molecules in glycolysis, this step occurs twice per glucose molecule.
b. Krebs cycle (also known as the citric acid cycle or TCA cycle):
Acetyl CoA enters the Krebs cycle and undergoes a series of reactions. During this cycle, more NADH and FADH2 (flavin adenine dinucleotide) molecules are generated, along with some ATP.
c. Oxidative phosphorylation:
This is the final step of cellular respiration and involves the majority of ATP production. It occurs in the inner mitochondrial membrane.
Oxidative phosphorylation can be further divided into two processes: substrate-level phosphorylation and oxidative phosphorylation.
- Substrate-level phosphorylation:
During glycolysis and the Krebs cycle, some ATP molecules are produced directly by transferring a phosphate group from a phosphorylated intermediate to ADP (adenosine diphosphate). This is called substrate-level phosphorylation, as the phosphate group is directly transferred from a substrate to ADP to form ATP.
- Oxidative phosphorylation:
The majority of ATP is generated during oxidative phosphorylation. It involves the electron transport chain (ETC) and chemiosmosis.
In the ETC, NADH and FADH2 molecules from glycolysis and the Krebs cycle donate their high-energy electrons. These electrons pass through a series of protein complexes, creating a proton gradient across the inner mitochondrial membrane. The protons then flow back through ATP synthase, which generates ATP by phosphorylating ADP.
Energy yield of cellular respiration:
- Gross ATP: Approximately 36-38 ATP molecules are generated per glucose molecule, depending on the specific conditions and organism. This includes the 2 ATP molecules from glycolysis, 2 ATP molecules from substrate-level phosphorylation in the Krebs cycle, and the majority of ATP molecules from oxidative phosphorylation.
- NADH and FADH2: Several molecules of NADH and FADH2 are generated in glycolysis, pyruvate oxidation, and the Krebs cycle.
In summary, glycolysis produces a net yield of 2 ATP molecules and 2 molecules of NADH per glucose molecule. Cellular respiration, including glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation, results in a total ATP yield of approximately 36-38 ATP molecules per glucose molecule.
It's important to note that the energy yield can vary depending on the specific conditions and organism being studied.