Respiration in Plants Test - 17

Glucose is phosphorylated to glucose-6-phosphate by ATP in the presence of enzyme hexokinase or glucokinase and Mg²⁺.

In the initial phase (energy spendling phase) of glycolysis, the hexoses (i.e., glucose and fructose) ar phosphorylated in a reaction that uses ATP at two steps : First in the conversion of glucose into glucose-6-phosphate and second in the conversion of fructose-6-phophaste to fructose-1,6-biphosphate.
These reactions are catalysed by the hexokinase and phospho-fructokinase respectively

In the energy conserving phase of glycolysis, the conversion of BPGA to PGA is catalyzed by phosphoglycerate kinase. The phosphate on carbon 1 is transferred to a molecule of ADP, yielding ATP and 3-phosphoglycerate. This type of ATP synthesis, traditionally referred to as substrate-level phosphorylation, involves the direct transfer of a phosphate group from a substrate molecule to ADP, to form ATP.

Glycolysis is a biochemical reaction in which one glucose molecule into 2 pyruvate molecules. 
As shown in the diagram showing the step-by-step biochemical reaction in glycolysis. According to this A - Fructose-6-phosphate, B - Fructose-1,6-Biphosphate; C- 3-PGAL; D - 1,3-biphosphoglyceric acid.

During aerobic respiration, pyruvic acid which is formed during glycolysis enters mitochondrial matrix. It undergoes oxidative decarboxylation to produce CO₂ and NADH. The product combines with sulphur containing coenzyme A to form acetyl CoA. This reaction is catalysed by an enzyme complex pyruvate dehydrogenase. This step is called link reaction or gateway step as it links glycolysis with Krebs' cycle.

During Krebs' or citric acid cycle, succinyl-CoA is acted upon by enzyme succinyl-CoA synthetase to form succinate (a 4C compound). The reaction releases sufficient energy to form ATP (in plants) or GTP (in animals) by substrate-level phosphorylation. GTP can form ATP through a coupled reaction.

GTP/ATP

The first 5 carbon dicarboxylic acid in Krebs' cycle is α ketoglutaric acid. The α-ketoglutarate that accumulates when α-ketoglutarate dehydrogenase is inhibited can be used in nitrogen metabolism as a precursor for several amino acids and the purine bases.

α-ketoglutarate → Glutamate Other amino acids → Purines.

α-ketoglutarate of Krebs' cycle produces an important amino acid called glutamate on amination.

Krebs' cycle also known as TCA cycle or citric acid cycle, is a common pathway of oxidative breakdown of carbohydrates, fatty acids and amino acids. Amino acids enter the Krebs' cycle directly as glutamate (for α-Ketoglutarate) and aspartate (for oxaloacetate) after their deamination. Fats produce fatty acids and glycerol. Glycerol is phosphorylated and oxidized to form glyceraldehyde 3-phosphate. Fatty acids undergo β-oxidation to produce acetyl CoA. Acetyl CoA enters Krebs' cycle.

Krebs' cycle produces 2 GTP (or 2 ATP) through substrate level phosphorylation. Six molecules of NADH₂ and 2 molecules of FADH₂ for every two molecules of Acetyl CoA oxidised by it.

In glycolysis, two molecules of ATP are consumed during two phosphorylation reactions to form fructose 1, 6-biphosphate. In return four molecules of ATP are produced by substrate level phosphorylation (conversion of 1, 3-biphosphoglycerate to 3-phosphoglycerate and phosphoenol pyruvate to pyruvate). Two molecules of NADH₂ are formed at the time of oxidation of glyceraldehyde 3-phosphate to 1, 3-biphosphoglycerate. The net reaction of glycolysis is as follows :
Glucose + 2NAD⁺ + 2ADP + 2H₃PO₄ → 2 Pyruvate + 2NADH + 2H⁺ + 2ATP
Each NADH is equivalent to 2 ATP, so the net gain in glycolysis is 8 ATP.

Oxygen is required by the germinating seed during aerobic respiration, it is the main source of energy for the seedlings until the formation of leaves. Oxygen is an atmospheric gas that is found in soil pore spaces. If a seed is buried too deeply within the soil or’the soil is water logged, the seed can be oxygen starved. If germinating seeds do not get air for respiration, they are still capable of respiration in absence of oxygen. Anaerobic respiration takes place in the seeds in the absence of free oxygen.

In germinating seeds, imbibition takes place. Their kinetic energy is released in the form of heat (heat of wetting).

Krebs cycle(or TCA cycle) takes place in mitochondrial matrix. A 2-carbon acetyl CoA acts as substrate entrant for Krebs cycle and 4-carbon oxaloacetate acts as acceptor molecule.

Succinate undergoes dehydrogenation to form fumarate with the help of a membrane based enzyme succinate dehydrogenase. FADH₂ (reduced flavin adenine dinucleotide) is produced.

Acetyl CoA (2-carbon compound) combines with oxaloacetate (4-carbon compound) in the presence of condensing enzyme citrate synthase to form a tricarboxylic 6-carbon compound called citric acid. It is the first product of Krebs' cycle. CoA is liberated.

Krebs' cycle was discovered by Sir Hans Krebs in 1937 (Nobel Prize in 1953) and thus is named so. Krebs' cycle is also called as citric acid cycle or tricarboxylic acid cycle (TCA cycle) after the name of its first product. Citric acid is a tricarboxylic 6-carbon compound.

Glycolysis occurs in cell cytoplasm and Krebs' cycle occurs in mitochondrial matrix.

Glycolysis, also known as EMP (Embden, Meyerhof, Parnas) pathway, is the breakdown of glucose to pyruvic acid in the absence of oxygen. It occurs in the cytosol of the cell and completes in 10 steps. Net reaction of glycolysis is:
2ATP + 2H⁺

A molecule of pyruvic acid that enters a mitochondrion is completely oxidized to form 3 carbon dioxide in one pre-Krebs' cycle decarboxylation and two Krebs' cycle decarboxylations. Net reaction of Krebs' cycle is as:
Pyruvicacid + 4NAD⁺ + FAD+ + 2H2​O + ADP + Pl  → 3CO_2​ + 4NADH4H⁺ + FADH2 ​+ ATP

The same respiratory process which acts as catabolic pathway for respiratory substrates also acts as anabolic pathway for the synthesis of various intermediary metabolic products and secondary metabolites. The respiratory pathway, therefore acts both as catabolic as well as anabolic pathway. Thus, it is better to call it amphibolic pathway rather than a catabolic pathway alone.

TCA or Krebs' cycle is amphibolic (both catabolic and anabolic) because it provides a number of intermediates for anabolic pathways.