The Electron Transport System

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The electron transport system takes electrons that it receives from NADH and FADH2 and passes those electrons from one protein to another. The energy from those electrons is used to pump hydrogen ions into the mitochondrion's inter membrane space.  The reason for doing this is that the hydrogen ions then diffuse back to the inner compartment through special channels in a protein structure called ATP synthase. The hydrogen ions transfer energy to the ATP synthase which uses the energy to make ATP from ADP + inorganic phosphate. In many cells including those of our body this type of electron transport phosphorylation is the main way ATP is made.

NAD+,NADH,FAD+,FADH2. NAD+ and FAD are electron acceptors that carry electrons that are largely from the energy containing molecules that enter the Kreb's cycle. These electron acceptors pick up two electrons along with hydrogen ions yielding NADH and FADH2.

 

Kreb's cycle. The Kreb's cycle is the source of the hydrogen ions and the electrons needed to make ATP from the electron transport system. The Kreb's cycle takes place in the mitochondrion's inner compartment often called the matrix

 

As the electrons travel through the ETS the energy contained in the electrons is used to pump hydrogen ions into the mitochondrion's outer compartment or inter membrane space, the inter membrane space. The electron paths are shown in yellow.

 

Hydrogen pumps

The hydrogen pumps are shown in green integrated into the electron transport system. The path of hydrogen ions from the matrix to the inter membrane space is shown in black.

 

Hydrogen ions

The pumping of hydrogen ions into the outer compartment sets up a concentration gradient. Naturally the hydrogen ions should diffuse back into the mitochondrion's matrix. The black arrows represent the paths of hydrogen ions involved in the production of ATP using the electron transport system.

 

Inner mitochondria membrane

The inner mitochondria membrane is generally impermeable to the hydrogen ions except for ATP synthase channels(Blue flask shaped structure at right)that allow the hydrogen ions to diffuse through is a combination channel for diffusion of the hydrogen ions back to the matrix and enzyme for making ATP.

 

ATP Synthase

As the hydrogen ions diffuse through the ATP synthetase(Blue flask shaped structure at right) energy is transferred from the ions to the rest of the ATP synthase molecule. This energy is used to power the production of ATP from ADP and inorganic phosphate.

The process of making ATP in this way is called electron transport phosphorylation or chemio-osmosis. Note that most of the ATP we use is made in this way.
 

Inorganic phosphate

Its important to realize that ATP production by electron transport phosphorylation uses inorganic phosphate from the cytoplasm as the source of phosphate. In contrast substrate level phosphorylation uses phosphate transferred from other organic compounds to the ADP to make ATP.

 

The role of oxygen

As the electrons in the ETS are used to do work, the electrons lose energy and reach a point at the end of the ETS where they have to be gotten rid of. The scheme the cell uses to do this is to combine the electrons with hydrogen ions and oxygen to produce water. This is a sensible thing for our cells to do since oxygen is an excellent electron acceptor.

In anaerobic respiration, as opposed to aerobic respiration, other electron acceptors are used to accept electrons from the electron transport systems.

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pgd revised 02/20/00