Question

Most of the energy that aerobic respiration releases from glucose ends up in ______. a. NADH b. ATP c. heat d. electrons

Short answer

b. ATP

Step-by-step solution

Understanding Aerobic Respiration

Aerobic respiration is a process that cells use to convert glucose and oxygen into energy, carbon dioxide, and water. This process ultimately results in the production of energy in the form of adenosine triphosphate (ATP).

Analyzing the Energy Carriers

During aerobic respiration, glucose is broken down in a series of steps. Initially, high-energy electrons are extracted from glucose and transferred to carrier molecules, such as NAD+ and FAD, forming NADH and FADH2. These molecules then carry electrons to the electron transport chain.

Role of the Electron Transport Chain

In the electron transport chain, the high-energy electrons from NADH and FADH2 are transferred through a series of proteins. This process creates a proton gradient across the mitochondrial inner membrane, which is used by ATP synthase to generate ATP.

Identifying the Main Energy Product

The primary purpose of the electron transport chain and oxidative phosphorylation in aerobic respiration is to convert the energy from electrons into a usable form, ATP. While heat is a byproduct of this process, the primary form of energy that is captured and used by cells is in the form of ATP.

Key concepts

ATP production

ATP, or adenosine triphosphate, is often called the "energy currency" of the cell. During aerobic respiration, ATP production is the main objective as cells break down glucose. This process is remarkably efficient and vital because ATP provides the energy needed for various cellular functions, like muscle contraction and active transport of molecules.
When glucose is broken down, the energy released is used to add an inorganic phosphate group to adenosine diphosphate (ADP), forming ATP. This process can be seen as charging a battery, where ADP is the low-energy state and ATP is the charged battery, ready to power cellular activities.
This is not a one-step process, but involves multiple stages such as glycolysis, the citric acid cycle, and oxidative phosphorylation. Each stage plays a critical role in contributing to the overall ATP yield.

electron transport chain

The electron transport chain (ETC) is a collection of protein complexes and other molecules embedded within the mitochondrial inner membrane. It plays a crucial role in converting energy stored in NADH and FADH2 into ATP.
In the ETC, electrons from NADH and FADH2 are passed along a series of proteins, gradually losing energy as each electron moves from one complex to the next. This energy is used to pump protons across the mitochondrial membrane, creating a gradient called the proton-motive force.
This process is meticulous. As the electrons move through the chain, oxygen plays a final role as the ultimate electron acceptor. After accepting electrons, oxygen combines with protons to form water, a safe and stable end product. Without oxygen, the entire ETC would halt, stopping ATP production.

oxidative phosphorylation

Oxidative phosphorylation is the final metabolic pathway of cellular respiration, taking place in the inner mitochondrial membrane. It is responsible for most of the ATP produced during aerobic respiration.
This process involves two key steps:

• The electron transport chain creates a proton gradient across the membrane by moving electrons through its complexes.
• ATP synthase, a special enzyme, uses this proton gradient to convert ADP into ATP by allowing protons to flow back across the membrane.

This is an elegant process as it couples the release of energy from electron transfers to ATP synthesis. If you think of the membrane as a hydroelectric dam and the protons as water, ATP synthase acts like the turbine generating electricity. Here, energy from the proton gradient is harnessed to synthesize ATP.
Oxidative phosphorylation is highly efficient, with each glucose molecule producing approximately 34 ATP molecules.

NADH

NADH is a critical high-energy molecule generated during glycolysis and the citric acid cycle. Its primary role is to transport electrons to the electron transport chain.
When glucose is broken down, NAD+ accepts electrons and hydrogen ions, forming NADH. This conversion stores energy that will eventually be used in ATP production. NADH acts like a shuttle, ferrying electrons from one stage of metabolism to another, ensuring energy transfer along the way.
In the electron transport chain, NADH donates its electrons to the first complex, starting the chain reaction that leads to ATP production. This donation also regenerates NAD+, making it available again to accept more electrons in the earlier stages of respiration. Without NADH, the pathway would stall, and ATP production would severely decline.

FADH2

FADH2 is another important electron carrier, similar to NADH, but with a few differences. It is primarily produced during the citric acid cycle when succinate is converted into fumarate.
Like NADH, FADH2 is responsible for transporting electrons to the electron transport chain, but it enters the cycle at a slightly later point, at Complex II. While FADH2 contributes less energy to the overall ATP yield than NADH, it is still essential for efficient energy conversion.
FADH2 can be thought of as the trustworthy backup to NADH. The electrons from FADH2 also contribute to the creation of the proton gradient necessary for ATP synthase to generate ATP. Every molecule of FADH2 is able to produce approximately 1.5 molecules of ATP. This makes FADH2 an integral part of the energy production process within the cell.