The end product pyruvate may then either undergo aerobic respiration in the mitochondria or anaerobic respiration (fermentation).
Anaerobic respiration : Fermentation In anaerobic respiration When oxygen is not available, pyruvate is converted to either ethanol or lactic acid:-
- Alcoholic fermentation:-Alcoholic fermentation is common in bacteria and yeast cells(unicellular fungus) In some plant cells and yeasts,alcoholic fermentation produces CO2 and ethanol. The conversion of pyruvate to acetaldehyde generates CO2 and the conversion of acetaldehyde to ethanol regenerates NAD+.
- Lactic acid fermentation.:- This pathway is common for animal cells ,bacteria, fungi, and protist cells and lactic acid bacteria. .During one form of anerobic glycolysis or fermentation, L-lactate is produced from pyruvate via the enzyme lactate dehydrogenase. This conversion also oxidizes one molecule of NADH to NAD+, and this is the reason for the conversion: NAD+ has to be regenerated so that glycolysis can continue.
Therefore, the NADH produced during glycolysis (2 molecules of NADH with 2 moles of pyruvic acid) is consumed during fermenation.
Aerobic respiration:-Aerobic respiration (when oxygen is present)comprises a series of enzymatic reactions. In which 3-c pyruvic acid (the end product of glycolysis) is broken down step by step to form the end products CO2 , H2 O and ATP energy,of aerobic respiration In eukaryotes, these processes occur in the mitochondria, while in prokaryotes they occur in the cytoplasm. It involves two stages:-
- The T.C.A. (Tricarboxylic acid) cycle (Krebs cycle or citric acid cycle) occurs in the matrix.
- Electron transport chain and oxidative phosphorylation occur deep in the cristae.
Formation of 2-C Acetyl Co-A :-The Transition Reaction:-The two pyruvate molecules formed from a single glucose molecule in glycolysis is transported to the matrix of the mitochondria undergoes oxidative de-carboxylation i.e
- The two 3-carbon pyruvate molecules are oxidized by dehydrogenation to two 2-carbon acetyl groups (CH3CO).
- Two molecules of CO2 are released from pyruvic acid .
- These two-carbon molecules are oxidized—that is, electrons are removed and used to form a total of two NADH molecules (to be used later).
- The two-carbon compound produced is attached to Coenzyme A ( CoAs-H) to produce acetyl CoA.and the cycle begins.
- The NADH so formed is channelled into the respiratory chain in the mitochondrion.
Thus, acetyl Co-A is the connecting link between glycolysis and Kreb’s cycle
T.C.A. (Tricarboxylic acid) cycle (Krebs cycle or citric acid cycle -Sir Hans Krebs, 1937)
image912:-Each turn of Krebs Cycle consumes only one of the two molecules of pyruvate formed via glycolysis therefore, each glucose molecule produces two turns of the cycle and twice the product.Two acetyl CoA molecules are consumed to produce 4 CO2, 2ATP, 6 NADH and 2 FADH2. The ATP molecules are produced by substrate-level phosphorylation
Acetyl CoA produced not only from pyruvic acid, but also that produced as a result of Beta-oxidation of fatty acids undergoes TCA cycle. All enzymes required for these reactions are present in the matrix of mitochondria.
The various steps in the T.C.A. Cycle are:-
- Acetyl-CoA --> Citric Acid :- The two-carbon fragment of Acetyl-CoA is added to oxaloacetate/oxaloacetic acid, a four-carbon compound to form a six-carbon molecule citric acid The acetyl coenzyme A acts only as a transporter of acetic acid from one enzyme to another. After this Step , the coenzyme is released by hydrolysis so that it may combine with another acetic acid molecule to begin the Krebs cycle again. Thus, the first product in Kreb’s cycle is citric acid, hence it is also called the citric acid cycle. As citric acid is a tricarboxylic acid (with 3-COOH groups), it is also called tricarboxylic acid (TCA) cycle.
- Citric Acid --> Isocitric Acid :- In the next step, 6-C citric acid is first converted into 6-C -aconitic acid (deletion of H2O) and then into 6-C isocitric acid (addition of H2O).
- Isocitric acid --> Oxalo succinic acid:- Dehydrogenation(of isocitric acid (by removal of hydrogen) to form 6-C oxalo succinic acid. NADH2 is formed from NAD in the process.
- Oxalo succinic acid ----> Alpha-Ketoglutaric acid:- 6-C Oxalosuccinic acid loses a molecule carbon dioxide (decarboxylation) produces, 5-C Alpha -Ketoglutaric acid (one carbon atom has bee removed in the form of CO2) .
- Alpha-Ketoglutaric Acid --> Succinyl CoA :-Alpha-Ketoglutaric acid (5-C) then undergoes oxidation (by removal of hydrogen) and decarboxylation to form 4-C succinyl Co-A in the presence of Co-A and NAD. NADH2 is formed and second molecule of carbon dioxide is released.
- Succinyl CoA --> Succinate/Succinic Acid :- Substrate-level Phosphorylation occurs in this step. 4-C Succinyl Co-A is hydrolyzed to succinic acid (4-C) and Co-A is regenerated. One molecule of H2O is used . The reaction is exergonic. Energy released is used for the formation of GTP (guanosine triphosphate) from GDP and inorganic phosphate. Subsequently, ATP is formed when GTP reacts with ADP
GTP + ADP ------->ATP + GDPThus, there is the direct formation of one ATP when 5-C acid is converted to 4-C acid.
- Succinate --> Fumarate/Fumaric Acid :-4-C succinate is oxidized(by removal of hydrogen)into 4-C fumaric acid in the presence of molecule of FAD (Flavin adenine dinucleotide). The FAD removes two hydrogen atoms to form FADH2 from the succinate as a result a double bond is formed between the two carbon atoms, thus forming fumarate.This coenzyme is similar to NADH but it stores less energy The enzyme that catalyzes this step, succinic dehydrogenase, is the only enzyme of the cycle that is embedded in the mitochondrial membrane .
- Fumarate --> Malate/Malic Acid :-Fumaric acid (4-C) is hydrated (addition of a water molecule) 4-C acid, malic acid.
- Malate --> Oxaloacetate :-In the final step of the TCA cycle, 4-C Malic acid is dehydrogenated (by removal of hydrogen) to 4-C Oxaloacetic acid.NAD is reduced to NADH. Oxaloacetate/Oxaloacetic acid is regenerated and is again accept a two-carbon fragment from acetyl CoA for another turn of the cycle.