Cellular respiration included the metabolic pathways of glycolysis, the Krebs cycle, and the electron transport chain, as represented in the figures. In cellular respiration, carbohydrates and other metabolites are oxidized, and the resulting energy-transfer reactions support the synthesis of ATP. Using the information above, describe ONE contribution of each of the following in ATP synthesis. 1. Catabolism of glucose in glycolysis and pyruvate oxidation. 2. Oxidation of intermediates in the Krebs cycle. 3. Formation of a proton gradient by the electron transport chain.

1. Catabolism of glucose in glycolysis and pyruvate oxidation: Glycolysis is the process where one molecule of glucose is broken down into two molecules of pyruvate. This conversion occurs in the cytoplasm and involves several steps that lead to the production of ATP and the reduction of NAD+ to NADH. During glycolysis, a net total of 2 ATP molecules are produced per glucose molecule through substrate-level phosphorylation (the transfer of a phosphate group directly to ADP to form ATP). Following glycolysis, pyruvate oxidation takes place in the mitochondria (in eukaryotes), where each pyruvate is converted into acetyl-CoA. In this process, another molecule of NADH is generated from NAD+. Both ATP and NADH contribute to the total energy yield of cellular respiration. The NADH, in particular, carries high-energy electrons to the electron transport chain, where they play a crucial role in ATP synthesis.

2. Oxidation of intermediates in the Krebs cycle: The Krebs cycle (also known as the citric acid cycle or tricarboxylic acid cycle) is a series of enzyme-catalyzed reactions that further break down the acetyl-CoA produced from pyruvate oxidation. Throughout this cycle, the carbons from acetyl-CoA are released as CO2 while high-energy electron carriers, specifically NADH and FADH2, are generated through the oxidation of intermediates. Additionally, a small amount of ATP (or GTP) is produced through substrate-level phosphorylation. The NADH and FADH2 formed during the Krebs cycle are loaded with high-energy electrons crucial for the next stage of ATP synthesis, which occurs in the electron transport chain.

3. Formation of a proton gradient by the electron transport chain: The electron transport chain (ETC) is located on the inner mitochondrial membrane. High-energy electrons from NADH and FADH2 are transferred through a chain of proteins and other molecules in the ETC. As these electrons travel down the chain, they lose energy, which is used by the proteins in the chain to pump protons (H+ ions) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient, known as the proton gradient. This gradient is a form of potential energy, with a high concentration of protons outside the matrix and a low concentration inside. The protons flow back into the matrix through a protein called ATP synthase, and the energy released from the flow drives the synthesis of ATP from ADP and inorganic phosphate. This process of ATP generation powered by the proton gradient is termed oxidative phosphorylation.

Extra: Cellular respiration is a process that all living cells use to produce energy from organic molecules like carbohydrates. It's essential to understand that ATP synthesis is the primary goal of cellular respiration. Glycolysis, the Krebs cycle, and the electron transport chain are the main stages where chemical energy from food molecules is transformed into energy that the cell can use in the form of ATP.

Glycolysis is crucial because it is the first step in the breakdown of glucose, which is a key fuel for cells. It’s an anaerobic process, meaning it doesn’t require oxygen and is thus a quick source of ATP.

The Krebs cycle, which occurs after glycolysis if oxygen is present, fully oxidizes the energy-rich molecules formed during glycolysis, yielding more NADH and FADH2, which are essential for the next step.

The electron transport chain is the final and most energy-efficient stage. The energy carried by electrons from NADH and FADH2 is used to pump protons out of the mitochondrial matrix, generating the proton gradient used to produce a large amount of ATP. This stage is where oxygen is used as the final electron acceptor and is crucial for the aerobically respiring organisms as this stage yields the most ATP compared to glycolysis and the Krebs cycle.