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.

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, protons are pumped into the intermembrane space against a concentration and electrical gradient

The potential energy generated is called the proton-motive force.

The movement of a proton through the channel 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 net total of 38 ATP molecules can be generated from each glucose molecule through glycolysis -followed by complete aerobic respiration.

The inner mitochondrial membranes of most mitochondria are impermeable to NADH. Consequently, to get into the matrix of the mitochondria energy is required (approximately one ATP for each of the two NADH₂⁺ produced per molecule of glucose). This additiona expenditure of energy) 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 that glucose metabolism plays a central role in all metabolic pathways...

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 many macromolecules... lipids, amino acids, nucleic acids, and other biological molecules.

 

    

 

Catabolism and anabolism involve interconversions using "carbon skeletons" where, for example, the carbon skeleton of Oxaloacetate can be transformed (through a process called transamination) into the amino acid, aspartic acid -see figure below, which can then be uused to build (anabolise) peptides and nucleic acids.

 

  

 

Catabolic uses of molecules include the following:

Thus, catabolism and anabolism are interwoven

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 of molecules in the cell.

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 in the form of the carbohydrate "glycogen".

A typical person has about one 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 invariably requires the use of amino acids.

After fats are depleted, proteins alone provide the energy.

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