Respiration in Plants Test - 9

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.

Mitochondria are cell organelles of eukaryotes. These are site of aerobic respiration, where Krebs' cycle occurs in matrix, while ETS and oxidative phosphorylation enzymes are located in inner membrane. They are called power houses of cell because they produce energy in the form of ATP. They are the major centres of release of energy in the aerobic respiration.

In the given image a represents fat, b represent proteins, c represents 3 phosphoglyceraldehydes while d- acetyl CoA.
So, the correct option is 'A- Fats; B- Proteins; C- 3-PGAL; D- AcetylCoA'.

Succinate dehydrogenase is the only enzyme of Krebs' cycle which is located in inner mitochondrial membrane in eukaryotes and in cytosol in prokaryotes while all other enzymes of this cycle are located in the mitochondrial matrix.

Glycolysis is an oxidative process in which one molecule of glucose partially oxidised into two molecules of pyruvate on a series of enzyme catalysed reactions. Glycolysis, the major pathway for glucose metabolism, occurs in the cytosol of all cells. It is a unique pathway that occurs aerobically as well as anaerobically and does not involve molecular oxygen.

The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid and water to yield citric acid. It is a tricarboxylic 6-carbon compound and undergoes reorganisation in the presence of iron containing enzyme aconitase first forming cis aconitate and releasing water.

Oxygen required for aerobic respiration is taken up from the air of the apparatus. Carbon dioxide released during aerobic respiration is absorbed by potassium hydroxide (KOH). Therefore, a partial vacuum is created in the conical flask. This causes the rise of water in the bent tube. That oxygen is consumed during aerobic respiration can be tested by taking a burning splinter into the conical flask, which gets extinguished.

Oxaloacetic acid is a 4C compound, HO₂CCH₂COCO₂H, that plays an integral role in the Krebs cycle. The anion, oxaloacetate, reacts with the acetyl group, from acetyl coenzyme A to form citrate.

Cyanide is an inhibitor of complex IV (in ETS) and other heme-containing enzymes of mitochondria. Thus, cyanide blocks ATP production in the cells.

There is a gain of 2 ATP molecules during glycolysis and 2 ATP(GTP) molecules during double Krebs cycle. A total of 10 NADH_2​ molecules are formed in aerobic respiration. Thus, the net gain from complete oxidation of a molecule of glucose in muscle and nerve cells is 36 ATP molecules (10 NADH₂ ​= 30ATP, 2FADH₂​ = 4ATP, four formed by substrate-level phosphorylation in glycolysis and Krebs' cycle and two consumed in transport of the NADH₂​ molecules into mitochondria). In aerobic prokaryotes, heart, liver and kidneys, 38 ATP molecules are produced per glucose molecule oxidised. passage of ATP molecules from inside of mitochondria to the cytoplasm is through facilitated diffusion.

Thus, there is a net gain of 36 or 38 ATP molecules depending upon the type of aerobic respiration.

[Note: 1NADH₂​ = 3ATP and 1FADH₂​ = 2ATP].

Photophosphorylation occurs in the grana and requires the direct sunlight energy to make energy-carrier molecules that are used in the dark reaction. The light energy is trapped by chlorophyll to make ATP and NADPH. Oxidative phosphorylation is the synthesis of energy-rich ATP molecules with the help of energy liberated during oxidation of reduced co-enzymes (NADH, FADH₂​) produced in respiration.

In electron transport chain, there are 7 electron acceptors, which are as follows Co − Q → Cyt b → Cyt c₁ → Cyt c → Cyt a → Cyt a₃ → O₂. Oxygen is the ultimate electron acceptor. These electron acceptors are present in a specific sequence along inner mitochondrial membrane.

The oxidative breakdown of respiratory substrates with the help of atmospheric O₂ is known as aerobic respiration. It involves complete breakdown of substrates into CO₂ and water, releasing energy. For example, the simple chemical equation for aerobic breakdown of glucose is

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 2870kJ/686KCal

While during photosynthesis chlorophyll a gets oxidised to transfer molecules to the electron transport chain.

A. i- Glucose molecule has 6 carbon atoms. It is a monosaccharide It is produced during the Calvin cycle.

ii. Pyruvic acid has 3 carbon atoms. It produces during the glycolytic breakdown of glucose into 2 pyruvic acids.

iii Pyruvic acid generated in the glycolysis enters in the mitochondrial matrix, undergoes oxidative decarboxylation and forms 2 carbon acetyl Co = A  which enters in Kreb's cycle.

B. The metabolic pathway through which the electron passes from one carrier to another is called an electron transport system. The steps in the respiratory process are to release and utilise the energy stored in NADH + H⁺ and FADH₂​. 

Adenosine triphosphate(ATP) is the energy currency of the cell. It is the most important energy carrier which carries energy in the two terminal phosphate bonds(called as high energy bonds or energy - rich bonds). Equal amounts of usable energy are released per mole of ATP or ADP hydrolysis:

ATP + H_2​O ⇌ ADP + P_i + 7.3 Kcal mol⁻¹

ADP + H₂​O ⇌ AMP + P_i + 7.3 Kcal mol⁻¹

Wounding or injuring an organ of a plant increases the rate of respiration. It is probably due to the formation of growth hormone, traumatic acid, which increases the meristematic activity, resulting in callus formation.

Site-specific inhibitors of electron transport prevent the passage of electrons by binding to a component of the chain, blocking the Oxidation/reduction reaction.

Antimycin A, an antibiotic, blocks electron transport at the level of the CoQH₂​-cytochrome c-reductase (Complex III). Cyanide, azide, and Co bind with cytochrome oxidase complex and inhibit the terminal transfer of electrons to oxygen.

36 ATP molecules are produced during complete oxidation of one molecule of glucose.
So, 40 molecules of glucose will produce (36×40) ATP = 1440 ATP.

The intermediate compound which links glycolysis with Krebs' cycle is acetyl CoA

Four major respiratory enzyme complexes of electron transport chain are located in the inner mitochondrial membrane :

Complex I (NADH dehydrogenase), complex II (succinate dehydrogenase), complex III (cytochrome be,) and cytochrome IV (cytochrome c oxidase)

There is a fifth complex called ATP synthase which is involved in ATP synthesis.

Common pathway of aerobic respiration consists of three steps-
(I) Glycolysis (II) TCA/Krebs' cycle and (III) ETS
It occurs in both prokaryotes and in eukaryotes.