● In `color{violet}"fermentation"`, say by yeast, the `color{violet}"incomplete oxidation"` of glucose is achieved under `color{violet}"anaerobic conditions"` by sets of reactions where `color{violet}"pyruvic acid"` is converted to `CO_2` and `color{violet}"ethanol"`.

● The enzymes, `color{violet}"pyruvic acid decarboxylase"` and `color{violet}"alcohol dehydrogenase"` catalyse these reactions.

● Other organisms like some `color{violet}"bacteria"` produce `color{violet}"lactic acid from pyruvic acid"`.

● In `color{violet}"animal cells"` also, like `color{violet}"muscles during exercise"`, when `color{violet}"oxygen is inadequate"` for `color{violet}"cellular respiration"` pyruvic acid is reduced to lactic acid by `color{violet}"lactate dehydrogenase"`.

● The `color{violet}"reducing agent"` is `NADH`+`H^+` which is `color{violet}"reoxidised"` to `NAD^+` in both the processes.

● In both `color{violet}"lactic acid and alcohol fermentation"` not much energy is released; `color{violet}"less than seven per cent"` of the energy in glucose is released and `color{violet}"not all of it"` is trapped as high `color{violet}"energy bonds of ATP."`

● Also, the `color{Brown}"processes are hazardous"` – either `color{violet}"acid or alcohol"` is produced.

● `color{violet}"Yeasts poison themselves"` to death when the concentration of alcohol reaches about `color{violet}"13 per cent"`.

● The process by which organisms can `color{violet}"carry out complete oxidation"` of glucose and extract the energy stored to synthesise a `color{violet}"larger number of ATP"` molecules needed for `color{violet}"cellular metabolism"`.

● In `color{violet}"eukaryotes"` these steps take place `color{violet}"within the mitochondria"` and this requires `O_2`.

● `color{Brown}"Aerobic respiration"` is the process that leads to a `color{violet}"complete oxidation"` of organic substances in the `color{violet}"presence of oxygen"`, and releases `CO_2`, water and a large amount of `color{violet}"energy"` present in the substrate.

● This type of `color{violet}"respiration"` is most `color{violet}"common in higher organisms"`.

● For `color{violet}"aerobic respiration"` to take place within the `color{violet}"mitochondria"`, the `color{violet}"final product of glycolysis"`, `color{violet}"pyruvate is transported'` from the cytoplasm into the mitochondria.

● The `color{Brown}"crucial events"` in aerobic respiration are:

`star` The `color{violet}"complete oxidation of pyruvate"` by the step wise removal of all the `color{violet}"hydrogen atoms"`, leaving `color{violet}"three molecules"` of `CO_2`.

`star` The `color{violet}"passing on of the electrons"` removed as part of the `color{violet}"hydrogen atoms"` to molecular `O_2` with simultaneous `color{violet}"synthesis of ATP."`

● What is `color{violet}"interesting"` to note is that the `color{violet}"first process"` takes place in the `color{violet}"matrix of the mitochondria"` while the second process is located on the `color{violet}"inner membrane"` `color{violet}"of the mitochondria"`.

● `color{Brown}"Pyruvate"`, which is formed by the `color{violet}"glycolytic catabolism"` of carbohydrates in the `color{violet}"cytosol,"` after it enters `color{violet}"mitochondrial matrix"` undergoes `color{violet}"oxidative decarboxylation"` by a complex set of reactions catalysed by `color{violet}"pyruvic dehydrogenase"`.

● The reactions catalysed by `color{violet}"pyruvic dehydrogenase"` require the participation of `color{violet}"several coenzymes"`, including `NAD^`+ and `color{violet}"Coenzyme A"`.



● Pyruvic acid CoA NAD Pyruvatedehydrogenase Acetyl CoA CO NADH H


● During this process, `color{violet}"two molecules of NADH"` are produced from the metabolism of `color{violet}"two molecules of pyruvic acid"` (produced from `color{violet}"one glucose molecule"` during glycolysis).

● The `color{violet}"acetyl CoA"` then enters a cyclic pathway, `color{Brown}"tricarboxylic acid cycle"`, more commonly called as `color{Brown}"Krebs’ cycle"` after the scientist `color{violet}"Hans Krebs"` who first elucidated it.