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For glycolysis to continue to continue, its products pf pyruvate and hydrogen must be removed.
Hydrogen must be released from the reduced NAD to regenerate NAD.
Without this, the already tiny supply of NAD in cells will be entirely converted to reduced NAD, leaving no NAD to take up the
hydrogen newly produced from glycolysis.
Glycolysis will then stop. The replenishment of NAD is achieved by the pyruvate molecule from glycolysis accepting the
hydrogen from reduced NAD.
In eukaryotic cells, only two types of anaerobic respiration occur with any regularity:
In plants, and in micro organisms such as yeast, the pyruvate is converted to ethanol and co2.
In animals, the pyruvate is converted to lactate.
Production of ethanol in plants and some micro organisms
Anaerobic respiration in certain bacteria and fungi leads to the production of ethanol. (e.g. yeast)
The pyruvate molecule formed at the end of glycolysis loses a molecule of carbon dioxide and accepts hydrogen from
reduced NAD to produce ethanol.
Pyruvate + reduced NAD ethanol + carbon dioxide + NAD
This form of anaerobic respiration has been used in the brewing industry for hundreds of years. In brewing, ethanol is the
most important product. Yeast is grown in anaerobic conditions, where it ferments natural carbohydrates in plant products
into ethanol. (eg. Grapes)
Production of lactate in animals
Anaerobic respiration leading to the production of lactate occurs in animals as a means of overcoming a temporary shortage
Instances like these include the period immediately after birth and in an animal living in water where the amount of oxygen
may be extremely low.
However, lactase production most commonly occurs in muscles as a result of strenuous exercise. In these conditions oxygen
may be used up more rapidly that it can be supplied and therefore an oxygen debt occurs.
It's important that the muscles still work because, for example, if the organism is fleeing from a predator.
In the absence of oxygen, glycolysis would normally cease as reduced NAD accumulates.
If glycolysis is to continue and release some energy, the reduced NAD must be removed.
To achieve this each pyruvate molecule produced takes up the two hydrogen atoms from the reduced NAD produced in
glycolysis to form lactate.
Pyruvate + reduced NAD lactate + NAD
At some point lactate produced needs to be oxidised back to pyruvate. This can then be either further oxidised to release
energy or converted into glycogen. This happens when oxygen is available.
Lactate causes cramp and although muscles have a certain tolerance to lactate, it's important it's removed by the blood and
taken to the liver to be converted to glycogen.
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Energy yields from anaerobic and aerobic respiration
Energy from cellular respiration is derived in two ways:
Substrate-level phosphorylation in glycolysis and the Krebs cycle. This is the direct linking of inorganic phosphate (Pi) to
ADP to produce ATP. ADP + Pi ATP
Oxidative phosphorylation in the electron transport chain. This is the indirect linking of inorganic phosphate to ADP to
produced ATP using the hydrogen atoms from glycolysis and the Krebs cycle that are carried on NAD and FAD.…read more