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Ways to transport. To make ATP, energy must be ‘‘transported’’ - first from glucose to NADH, and then somehow passed to ATP. How is this done? With an electron transport chain.
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?)
See //www.youtube.com/watch?v=1engJR_XWVU for an overview of the electron transport chain.
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.
Making ATP
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 acts as an enzyme, forming ATP from ADP and inorganic phosphate. After passing through the electron-transport chain, the “spent” electrons combine with oxygen to formwater. This is why oxygen is needed; in the absence of oxygen, this process cannot occur.
How much ATP is produced? The two NADH produced in the cytoplasm produces 2 to 3 ATP each (4 to 6 total) by the electron transport system, the 8 NADH produced in the mitochondriaproduces three ATP each (24 total), and the 2 FADH2 adds its electrons to the electron transport system at a lower level than NADH, so they produce two ATP each (4 total). This results in the formation of 34 ATP during the electron transport stage.
A summary of this process can be seen at the following sites: //www.youtube.com/watch?v=fgCcFXUZRk (17:16) and //www.youtube.com/watch?v=W_Q17tqw_7A (4:59).
Summary
- Electron transport is the final stage of aerobic respiration. In this stage, energy from NADH and FADH2 is transferred to ATP.
- During electron transport, energy is used to pump hydrogen ions across the mitochondrial inner membrane, from the matrix into the intermembrane space.
- A chemiosmotic gradient causes hydrogen ions to flow back across the mitochondrial membrane into the matrix, through ATP synthase, producing ATP.
- See Mitochondria at //johnkyrk.com/mitochondrion.html for a detailed summary.
Review
- Summarize the overall task of Stage III of aerobic respiration.
- Explain the chemiosmotic gradient.
- What is the maximum number of ATP molecules that can be produced during the electron transport stage of aerobic respiration?
Understanding:
• Transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is
coupled to proton pumping
The final stage of aerobic respiration
is the electron transport chain, which is located on the inner mitochondrial membrane
- The inner membrane is arranged into folds (cristae), which increases the surface area available for the transport chain
The electron transport chain releases the energy stored within the reduced hydrogen carriers in order to synthesise ATP
- This is called oxidative phosphorylation, as the energy to
synthesise ATP is derived from the oxidation of hydrogen carriers
Oxidative phosphorylation occurs over a number of distinct steps:
- Proton pumps create an electrochemical gradient (proton motive force)
- ATP synthase uses the subsequent diffusion of protons (chemiosmosis) to synthesise ATP
- Oxygen accepts electrons and protons to form water
Step 1: Generating a Proton Motive Force
- The hydrogen carriers (NADH and FADH2) are oxidised and release high energy electrons and protons
- The electrons are transferred to the electron transport chain, which consists of several transmembrane carrier proteins
- As electrons pass through the chain, they lose energy – which is used by the chain to pump protons (H+ ions) from the matrix
- The accumulation of H+ ions within the intermembrane space creates an electrochemical gradient (or a proton motive force)
Understanding:
• In chemiosmosis protons diffuse through ATP synthase to generate ATP
Step Two: ATP Synthesis via Chemiosmosis
- The proton motive force will cause H+ ions to move down their electrochemical gradient and diffuse back into matrix
- This diffusion of protons is called chemiosmosis and is facilitated by the transmembrane enzyme ATP synthase
- As the H+ ions move through ATP synthase they trigger the molecular rotation of the enzyme, synthesising ATP
Understanding:
• Oxygen is needed to bind with the free protons to maintain the hydrogen gradient, resulting in the formation
of water
Step Three: Reduction of Oxygen
- In order for the electron transport chain to continue functioning, the de-energised electrons must be removed
- Oxygen acts as the final electron acceptor, removing the de-energised electrons to prevent the chain from becoming blocked
- Oxygen also binds with free protons in the matrix to form water – removing matrix protons maintains the hydrogen gradient
- In the absence of oxygen, hydrogen carriers cannot transfer energised electrons to the chain and ATP production is halted
Summary: Oxidative Phosphorylation
- Hydrogen carriers donate high energy electrons to the electron transport chain (located on the cristae)
- As the electrons move through the chain they lose energy, which is transferred to the electron carriers within the chain
- The electron carriers use this energy to pump hydrogen ions from the matrix and into the intermembrane space
- The accumulation of H+ ions in the intermembrane space creates an electrochemical gradient (or a proton motive force)
- H+ ions return to the matrix via the transmembrane enzyme ATP synthase (this diffusion of ions is called chemiosmosis)
- As the ions pass through ATP synthase they trigger a phosphorylation reaction which produces ATP (from ADP + Pi)
- The de-energised electrons are removed from the chain by oxygen, allowing new high energy electrons to enter the chain
- Oxygen also binds matrix protons to form water – this maintains the hydrogen gradient by removing H+ ions from the matrix
Overview of Oxidative Phosphorylation