In this way electrons pass along a series of membrane-associated electron carriers called the respiratory chain.

The flow of electrons by way of a series of redox reactions causes the active transport of protons across the inner mitochondrial membrane and into the intermembrane space, creating a concentration gradient of H⁺ 's.

If these protons are then allowed to diffuse -through specific proton channels- back into the cell, down their concentration- (and electrical-) gradients, back into the matrix of the mitochondrion, then ATP can be created in the process through a  special ATP generating turbine system.

 

 

Nobel Prize 1997

The entire process of ATP synthesis by electron transport through the respiratory chain is called oxidative phosphorylation.

Thus, the respiratory chain transports electrons out of the and releases energy

NADH + H⁺ passes its hydrogen atoms to the NADH-Q reductase protein complex. The NADH-Q reductase passes the hydrogens on to ubiquinone(Q) forming QH₂.

The QH₂ passes electrons to cytochrome c reductase, which in turn passes them to cytochrome c. Next to receive them is cytochrome c oxidase. The cytochrome oxidase passes them on to oxygen.

Reduced oxygen unites with two hydrogens to form water.

FADH₂can also enter the chain through succinate-Q reductase.

As electrons pass through the respiratory chain, the associate protons are continuously pumped into the intermembrane space against a concentration and electrical gradient

The potential energy generated by this gradient of Protons is called the proton-motive force.

The subsequent movement of these proton (down their gradient, through the ATPase pump/cchannel causes the physical rotation of the core of the enzyme, which pushes the ADP and P_i so close together that they bond.

The synthesized ATP is transported out of the mitochondrial matrix almost as quickly as it is made, and is now available to the cell.

 

Contrasting Energy Yield:

A total net of 38 ATP molecules can be generated from each glucose molecule in glycolysis followed by complete aerobic respiration.

The inner mitochondrial membranes of most mitochondria are impermeable to NADH. To get into the matrix requires the energy of one ATP for each of the two NADH₂⁺ produced per molecule of glucose. This reduces the net yield somewhat.

In contrast, fermentation has a net yield of only 2 ATP molecules from each glucose molecule.

The end products of fermentation (such as lactic acid and ethanol) contain much more unused energy than the end products of aerobic respiration.

In aerobic respiration, each NADH + H⁺ generates three ATP molecules, and each FADH₂ generates two ATP molecules when consumed in the electron transport chain.

Coupled with glycolysis, aerobic respiration captures ~ 63 percent of the energy stored in glucose; fermentation captures only ~ 3.5 percent. Consequently, Aerobic respiration is 18 times more efficient at harvesting energy from glucose.

Metabolic Pathways.... It is no accident

 

Glucose utilization pathways can yield more than just energy. They are interchanges for diverse biochemical traffic.

Intermediate chemicals are generated that are substrates for the synthesis of lipids, amino acids, nucleic acids, and other biological molecules.

 

Catabolism and anabolism involve interconversions using "carbon skeletons"

 

  

 

Catabolic uses of molecules include the following:

In this way, catabolism and anabolism are integrated

The levels of the products and substrates of energy metabolism are remarkably constant.

Cells regulate the enzymes of catabolism and anabolism to maintain a balance.

What happens if inadequate fuel molecules are available?

Polysaccharides are an intermediate storage form of energy. A typical person has about one day's worth of energy stored as the carbohydrate glycogen.

A typical person has about a week of needed amino acids stored as protein, and over a month's energy stored as fats.

Fats are compact energy storage molecules because they exclude water and are particularly hydrocarbon rich.

If food is withheld, glycogen is used up first, then fats. As fats are depleted, in humans this leads to a build up of ketones in the body, which develops into a condition called ketosis which elicits a mild euphoric state, and dulls the senses....as in a diabetic shock.

Fats cannot cross the blood-brain barrier. To supply glucose to the brain, glucose must be synthesized by gluconeogenesis (which is a reversal of glycolysis). This requires the use of amino acids.

After fats are depleted, proteins alone provide the energy.

At this point, however, in humans the important disease-fighting proteins (antibodies) are also consumed, and the likeliness of severe illness increases.