Question

Most of the energy that is released by the full breakdown of glucose to \(\mathrm{CO}_{2}\) and water ends up in _____. a. NADH c. heat b. ATP d. electrons

Short answer

Most energy ends up in ATP.

Step-by-step solution

Understanding the Process

The complete breakdown of glucose involves a process known as cellular respiration. This process occurs in several stages, including glycolysis, the Krebs cycle, and the electron transport chain, and it involves converting glucose ( C_6H_{12}O_6 ) into carbon dioxide ( CO_2 ) and water.

Energy Carriers Identification

During the breakdown of glucose, energy is temporarily stored in the form of high-energy molecules such as ATP and NADH. NADH is an electron carrier that transfers electrons to the electron transport chain, where the majority of energy is harnessed to produce ATP.

Energy Released and Utilization

The majority of energy released during the breakdown of glucose is ultimately used to produce ATP during oxidative phosphorylation in the electron transport chain. Although some energy is indeed lost as heat, ATP is the primary energy currency of the cell that directly powers cellular activities.

Concluding the Energy Destination

Considering the flow of energy through cellular respiration, the bulk of the energy from glucose breakdown is converted into ATP, which is used by the cell for various functions.

Key concepts

ATP Synthesis

ATP synthesis is a fundamental process in cellular respiration. It is essential because ATP, or adenosine triphosphate, serves as the main energy currency in cells. Most cellular activities, from muscle contraction to molecule transport across cell membranes, require ATP. The majority of ATP in cellular respiration is synthesized during the final stage in the electron transport chain.

During cellular respiration, ATP is generated through two distinct methods:

• Substrate-level phosphorylation: This occurs directly during glycolysis and the Krebs cycle. Here, a phosphate group is transferred directly from a substrate molecule to ADP, forming ATP.
• Oxidative phosphorylation: This occurs in the mitochondria at the electron transport chain. It's where most ATP is produced. The process involves the phosphorylation of ADP to ATP through a series of redox reactions.

Understanding ATP synthesis is crucial as it underpins the entire process of how cells harness energy from nutrients like glucose.

Krebs Cycle

The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid cycle, is a key stage in cellular respiration. It occurs in the mitochondria and plays a pivotal role in oxidizing acetyl-CoA, derived from carbohydrates, fats, and proteins.

This cycle generates high-energy molecules essential for the next stages of energy production:

• Production of NADH and FADH₂: These molecules are reduced coenzymes that carry high-energy electrons to the electron transport chain, critical for ATP production.
• Carbon Dioxide Release: The cycle results in the release of \( CO_2\), which is a waste product eliminated from the body.
• Regeneration of Oxaloacetate: The end of the cycle regenerates the molecule that starts the cycle, ensuring it can continue repeatedly.

Overall, the Krebs cycle does not produce a lot of ATP directly, but it is crucial for setting up the stage for ATP synthesis in the electron transport chain.

Electron Transport Chain

The electron transport chain (ETC) is the grand finale of cellular respiration, taking place within the inner mitochondrial membrane. It is where most ATP is produced, thanks to the electrons delivered by NADH and FADH₂.

The ETC is composed of a series of protein complexes and small molecules, which facilitate a chain of redox reactions:

• Electron transfer: Electrons from NADH and FADH₂ pass through the protein complexes. Each transfer releases energy used to pump protons across the mitochondrial membrane, creating a proton gradient.
• Proton Gradient and ATP Synthase: The gradient creates potential energy, driving protons back through ATP synthase. This enzyme facilitates the conversion of ADP into ATP through oxidative phosphorylation.
• Role of Oxygen: Oxygen acts as the final electron acceptor, combining with protons to form water—a critical step for the chain's continuation.

The electron transport chain's efficiency highlights why it is considered the powerhouse step of ATP production in cells.