Plant Physiology :- Respiration In Plants

RESPIRATION IN PLANTS

POINTS TO REMEMBER :

• The breaking of C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called respiration.
• The compound that oxidized during this process is known as respiratory substrates.
• In the process of respiration the energy is released in a series of slow step-wise reactions controlled by enzymes and is trapped in the form of ATP.
• ATP acts as the energy currency of the cell.

Glycolysis : 

• The term has originated from the Greek word, glycos =glucose, lysis = splitting or breakdown means breakdown of glucose molecule.
• It is also called Embeden-Meyerhof-Paranus pathway. (EMP pathway)
• It is common in both aerobic and anaerobic respiration.
• It takes place outside the mitochondria, in the cytoplasm.
• One molecule of glucose (Hexose sugar) ultimately produces two molecules of pyruvic acid through glycolysis.
• Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate, catalyzed by hexokinase.
• This phosphorylated form of glucose is then isomerizes to produce fructose-6-phosphate.
• ATP utilized at two steps:
  • First in the conversion of glucose into glucose-6-phosphate
  • Second in fructose-6-phosphate→fructose 1, 6-diphosphate.
• The fructose-1, 6-diphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (DPGA).
• In one step where NADH + H⁺ is formed form NAD⁺; this is when 3-phosphogleceraldehyde (PGAL) is converted into 1, 3-bisphophoglyceric acid (DPGA).
• The conversion of 1, 3-bisphophoglyceric acid into 3-phosphoglyceric acid is also an energy yielding process; this energy is trapped by the formation of ATP.
• Another ATP synthesized when phosphoenolpyruvate is converted into pyruvic acid.
• During this process 4 molecules of ATP are produced while 2 molecules of ATP are utilized. Thus net gain of ATP is of 2 molecules.

FERMENTATION :

• There are three major ways in which different cells handle pyruvic acid produced by glycolysis:
  • Lactic acid fermentation.
  • Alcoholic fermentation.
  • Aerobic respiration.
• Alcoholic fermentation :
  • The incomplete oxidation of glucose to achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted into CO₂ and ethanol.
  • The enzyme pyruvic acid decarboxylase and alcohol dehydrogenase catalyze these reactions.
  • NADH + H⁺ is reoxidised into NAD⁺.
• Lactic acid fermentation:
  • Pyruvic acid converted into lactic acid.
    
  • It takes place in the muscle in anaerobic conditions.
  • The reaction catalysed by lactate dehydrogenase.
  • NADH + H⁺ is reoxidised into NAD⁺.
• Aerobic respiration:
  • Pyruvic acid enters into the mitochondria.
  • Complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO₂.
  • The passing on the electrons removed as part of the hydrogen atoms to molecular oxygen (O₂) with simultaneous synthesis of ATP.

AEROBIC RESPIRATION:

• The overall mechanism of aerobic respiration can be studied under the following steps :
• Glycolysis (EMP pathway)
• Oxidative Decarboxylation
• Krebs’s cycle (TCA-cycle)
• Oxidative phosphorylation

Oxidative decarboxylation:

• Pyruvic acid formed in the cytoplasm enters into mitochondria.
• Pyruvic acid is converted into Acetyl CoA in presence of pyruvate dehydrogenase complex.
• The pyruvate dehydrogenase catalyses the reaction require several coenzymes, including NAD⁺ and Coenzyme A.
  
• During this process two molecules of NADH are produced from metabolism of two molecules of pyruvic acids (produced from one glucose molecule during glycolysis).
• The Acetyl CoA (2c) enters into a cyclic pathway, tricarboxylic acid cycle.

Tri Carboxylic Acid Cycle (Krebs cycle) or Citric acid Cycle :

• This cycle starts with condensation of acetyl group with oxaloacetic acid and water to yield citric acid.  This reaction is catalysed by citrate synthase.
• Citrate is isomerised to form isocitrate.
• It is followed by two successive steps of decarboxylation, leading to formation of α-ketoglutaric acid and then succinyl-CoA.
• In the remaining steps the succinyl CoA oxidized into oxaloacetic acid.
• During conversion of succinyl CoA to succinic acid there is synthesis of one GTP molecule.
  
• In a coupled reaction GTP converted to GDP with simultaneous synthesis of ATP from ADP.
• During Krebs cycle there production of :
  • 2 molecule of CO₂
  • 3 NADH₂
  • 1 FADH₂
  • 1 GTP.
• During the whole process of oxidation of glucose produce:
  • CO₂
  • 10 NADH₂
  • 2 FADH₂
  • 2 GTP.( 2 ATP)

Electron transport system and oxidative phosphorylation :

• The metabolic pathway, through which the electron passes from one carrier to another, is called Electron transport system.
• it is present in the inner mitochondrial membrane.
• ETS comprises of the following:
  • Complex I – NADH Dehydrogenase.
  • Complex II – succinate dehydrogenase.
  • Complex III – cytochromes bc1
  • Complex IV – Cytochromes a-a₃ (cytochromes c oxidase).
  • Complex V – ATP synthase.
• NADH₂ produced in the citric acid cycle oxidized by NADH
  
  Dehydrogenase, and electrons are then transferred to ubiquinone located in the inner membrane.
• FADH₂ is oxidized by succinate dehydrogenase and transferred electrons to ubiquinone.
• The reduced ubiquinone is then oxidized with transfer of electrons to cytochrome c via cytochromes bc1complex.
• Cytochrome c is small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer electrons from complex III and complex IV.
• When electrons transferred from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase for the synthesis of ATP from ADP and Pi.
• One molecule of NADH₂ gives rise to 3 ATP.
• One molecule of FADH₂ gives rise to 2ATP.
• Oxygen plays a vital role in removing electrons and hydrogen ion finally production of H₂O.
  
• Phosphorylation in presence of oxygen is called oxidative phosphorylation.

Total ATP Production -

Process Total ATP produced :

• Glycolysis 2ATP + 2NADH₂ (6ATP) = 8ATP
• Oxidative decarboxylation 2NADH₂  (6ATP) = 6ATP
• Krebs’s Cycle 2GTP (2ATP) + 6NADH₂ (18ATP) + 2FADH₂ (4ATP) = 24 ATP
• Energy production in prokaryotes during aerobic respiration = 38 ATP
• Energy production in eukaryotes during aerobic respiration = 38 − 2 = 36 ATP
• (2ATP are used up in transporting 2 molecule of pyruvic acid in mitochondria.)

Abbreviations :

ATP −          Adenosine tri phosphate
ADP −         Adenosine di phosphate
NAD −         Nicotinamide Adenine dinucleotide
NADP −       Nicotinamide Adenine dinucleotide Phosphate
NADH −       Reduced Nicotinamide Adenine dinucleotide
PGA −          Phosphoglyceric acid
PGAL −        Phospho glyceraldehyde
FAD −          Flavin adenine dinucleotide
ETS −          Electron transport system
ETC −          Electron transport chain
TCA −          Tricarboxylic acid
OAA −          Oxalo acetic acid
FMN −          Flavin mono nucleotide
PPP −          Pentose phosphate pathway

 Amphibolic Pathway : A biochemical pathway that serves both anabolic and catabolic processes.
• An important example of an amphibolic pathway is the Krebs cycle, which involves both the catabolism of carbohydrates and fatty acids and the synthesis of anabolic precursors for amino-acid synthesis (e.g. α-ketogluturate and oxaloacetate).

Author- Azeem Farooqui (Biochemist)
Expert NEET/AIIMS Medical Biology Faculty Kota.