Problem 4 - Campbell Biology Concepts & Connections

Question

Most of the \(\mathrm{CO}_{2}\) from cellular respiration is released during a. stage 1: glycolysis. b. stage 2 : pyruvate oxidation. c. stage 2 : the citric acid cycle. d. stage 3: oxidative phosphorylation.

Step-by-Step Solution

Answer

Most \(\text{CO}_{2}\) from cellular respiration is released during stage 2: the citric acid cycle.

Step 1: Identify the Stages of Cellular Respiration

Cellular respiration consists of three main stages: glycolysis, pyruvate oxidation and the citric acid cycle, and oxidative phosphorylation.

Step 1: Glycolysis

Glycolysis occurs in the cytoplasm and involves the conversion of glucose into pyruvate, producing a small amount of ATP and NADH. No \(\text{CO}_{2}\) is released in this stage.

Step 2: Pyruvate Oxidation

Pyruvate oxidation takes place in the mitochondria where each pyruvate molecule is converted into Acetyl-CoA. This process releases \(\text{CO}_{2}\).

Step 3: Citric Acid Cycle

The citric acid cycle (also known as the Krebs cycle) occurs in the mitochondrial matrix. It further processes Acetyl-CoA, generating ATP, NADH, and FADH2, and releasing multiple \(\text{CO}_{2}\) molecules.

Step 4: Oxidative Phosphorylation

Oxidative phosphorylation happens across the inner mitochondrial membrane, using the electrons from NADH and FADH2 to produce ATP. This stage does not release \(\text{CO}_{2}\).

Key Concepts

glycolysis

Glycolysis is the first stage of cellular respiration and takes place in the cytoplasm. It involves the breakdown of one glucose molecule into two molecules of pyruvate. During this process, a small amount of ATP is generated along with NADH. The overall chemical equation for glycolysis is: \[ \text{Glucose} + 2 \text{NAD}^+ + 2 \text{ADP} + 2 \text{P}_i \to 2 \text{Pyruvate} + 2 \text{NADH} + 2 \text{H}^+ + 2 \text{ATP} + 2 \text{H}_2\text{O} \] This stage does not release any \text{CO}_{2}. Glycolysis is essential because it prepares glucose for further oxidation in subsequent stages of cellular respiration. If oxygen is scarce, cells can also convert pyruvate into lactate or ethanol through fermentation.

pyruvate oxidation

Pyruvate oxidation is the second step in cellular respiration and occurs in the mitochondria. Each pyruvate molecule, produced from glycolysis, is transported into the mitochondrion and converted into Acetyl-CoA. This process is coupled with the release of \text{CO}_{2}, and the reduction of NAD+ to NADH. The chemical reaction can be summarized as: \[ \text{Pyruvate} + \text{NAD}^+ + \text{CoA} \to \text{Acetyl-CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+ \] Here, the \text{CO}_{2} released accounts for some of the carbon dioxide produced during cellular respiration. This step is crucial as it links glycolysis to the citric acid cycle by providing Acetyl-CoA.

citric acid cycle

The citric acid cycle, also known as the Krebs cycle, takes place in the mitochondrial matrix. It processes Acetyl-CoA into \text{CO}_{2}, generating ATP, NADH, and FADH2. The complete cycle produces two \text{CO}_{2} molecules per Acetyl-CoA through a series of enzymatic reactions. The overall equation for one cycle is: \[ \text{Acetyl-CoA} + 3 \text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i + 2 \text{H}_2\text{O} \to 2 \text{CO}_2 + 3 \text{NADH} + 3 \text{H}^+ + \text{FADH}_2 + \text{ATP} + \text{CoA-SH} \] This stage is where the majority of the \text{CO}_{2} from cellular respiration is released, making it vital for eliminating carbon dioxide from the body. Additionally, the generated NADH and FADH2 will be used later in oxidative phosphorylation.

oxidative phosphorylation

Oxidative phosphorylation is the final stage of cellular respiration and occurs across the inner mitochondrial membrane. This stage utilizes the electrons from NADH and FADH2, produced in previous stages to drive the production of ATP. The electrons travel through a series of proteins known as the electron transport chain, eventually reducing oxygen to water. The flow of electrons creates a proton gradient that drives ATP synthesis through ATP synthase. The overall process can be summarized as: \[ \text{NADH} + \text{FADH}_2 + \text{O}_2 \to \text{ATP} + \text{H}_2\text{O} \] Unlike the earlier stages, oxidative phosphorylation does not produce \text{CO}_{2}. Instead, it generates the bulk of ATP, which cells use for energy. This stage highlights the importance of oxygen as the final electron acceptor, emphasizing its role in efficient energy production.