READ: Respiration Stage 3

Overview of Respiration Stage 3

Overview Screencast of Respiration Stage 3

Cellular Respiration Stage 3: Electron Transport

*Use the READING GUIDE as you read to help you identify and understand important concepts.

Cellular Respiration Stage III: Electron Transport

Electron transport is the final stage of aerobic respiration. In this stage, energy from NADH and FADH₂, which result from the Krebs cycle, is transferred to ATP. Can you predict how this happens? (Hint: How does electron transport occur in photosynthesis?)

An overview of the electron transport chain:

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Transporting Electrons

Transporting Electrons

High-energy electrons are released from NADH and FADH₂, and they move along electron transport chains, like those used in photosynthesis. The electron transport chains are on the inner membrane of the mitochondrion. As the high-energy electrons are transported along the chains, some of their energy is captured. This energy is used to pump hydrogen ions (from NADH and FADH₂) across the inner membrane, from the matrix into the intermembrane space. Electron transport in a mitochondrion is shown in Figure below.

Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.

The pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This chemiosmotic gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also transfers energy from the hydrogen ions to ATP. After passing through the electron-transport chain, the “spent” electrons combine with oxygen to form water. This is why oxygen is needed; in the absence of oxygen, this process cannot occur. You can see how all these events occur at the following link:

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

How Much ATP?

How Much ATP?

You have seen how the three stages of aerobic respiration use the energy in glucose to make ATP. How much ATP is produced in all three stages? Glycolysis produces 2 ATP molecules, and the Krebs cycle produces 2 more. Electron transport begins with several molecules of NADH and FADH₂ from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of aerobic respiration.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Lesson Summary

Lesson Summary

• Cellular respiration uses energy in glucose to make ATP. Aerobic (“oxygen-using”) respiration occurs in three stages: glycolysis, the Krebs cycle, and electron transport.
• In glycolysis, glucose is split into two molecules of pyruvate. This results in a net gain of two ATP molecules.
• Life first evolved in the absence of oxygen, and glycolysis does not require oxygen. Therefore, glycolysis was probably the earliest way of making ATP from glucose.
• The Krebs cycle and electron transport occur in the mitochondria. The Krebs cycle takes place in the matrix, and electron transport takes place on the inner membrane.
• During the Krebs cycle, pyruvate undergoes a series of reactions to produce two more molecules of ATP and also several molecules of NADH and FADH2.
• During electron transport, energy from NADH and FADH₂ is used to make many more molecules of ATP.
• In all three stages of aerobic respiration, up to 38 molecules of ATP may be produced from a single molecule of glucose.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/