Study Notes On Respiration in Plants

Study Notes On Respiration in Plants

  • Cellular respiration is the mechanism of breakdown of food materials within the cell to release energy and the trapping of this energy for the synthesis of ATP.
  • The breakdown of the complex molecules to synthesise energy takes place in the cytoplasm and in the mitochondria.
  • Respiration is breaking of the C-C l bonds of complex compounds through oxidation within the cells leading to release of a considerable amount of energy.
  • The compounds that are oxidized are known as respiratory substrates.
  • Usually, carbohydrates are oxidised to release energy.
  • Under certain conditions proteins, fats and organic acids can be used as respiratory substances in some plants.
  • During oxidation, the energy contained in respiratory substrates is released in a series of slow stepwise reactions controlled by enzymes.
  • The energy is trapped as chemical energy in the form of ATP.
  • Hence the energy released in respiration is not used directly.
  • It is synthesized to ATP which is broken down whenever energy needs to be utilised.
  • The carbon skeleton produced during respiration is used as precursors for biosynthesis of other molecules in the cell.
  • Plants also require O2 for respiration to occur and also give out CO2.
  • Plants have stomata and lenticels for the exchange of gaseous.
  • Each plant part takes care of its own gas exchange needs.
  • The complete combustion of glucose produces CO2 and H2O as end products.
  • This yields energy most of which is given out as heat.

C6H12O6 + 6O2 –-> 6CO2 + 6H2O + Energy

  • In plant cells, the glucose molecule catabolises in such a way that not all the liberated energy goes out as heat.
  • This takes place in several small steps.
  • In some steps, the energy which is released is coupled to ATP.
  • All living organisms retain the enzyme tech machinery to partially oxidized glucose without the help of oxygen.
  • This breakdown of glucose to pyruvate acid is called glycolysis.


  • The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J.Parnas.
  • This is also referred to as EMP pathway.
  • In anaerobic organisms, it is the only process in respiration.
  • It occurs in the cytoplasm of the cell in all living organisms.
  • Glucose undergoes partial oxidation to form two molecules of pyruvic acid.
  • In plants, this glucose is derived from sucrose or from storage carbohydrate.
  • first sucrose is converted into glucose and fructose by the enzyme invertase.
  • Then these two monosaccharides enter the glycolytic pathway.
  • In the presence of the enzyme hexokinase, glucose and fructose are phosphorylated to give rise to glucose 6 phosphates.
  • The phosphorylated form of glucose isomerase is to produce fructose 6 phosphate in the presence of the enzyme phosphoglucose isomerase.
  • Then fructose 6 phosphates are converted to fructose 1,6 biphosphate in the presence of phosphofructokinase.
  • The enzyme aldolase splits fructose 1,6 biphosphate into 2 sugars that are isomers of each other.
  • The sugars are dihydroxyacetone phosphate(DHAP) and glyceraldehyde 3 phosphate (GAP)
  • The enzyme trials phosphate isomerase rapidly interconverts the molecules DHAP and GAP.
  • Glyceraldehyde 3 phosphate dehydrogenase dehydrogenates and adds an inorganic phosphate to GAP.
  • This produces 1,3 biphosphoglycerate.
  • The enzyme phosphoglycerate kinase transfer of phosphate group from 1,3 biphosphoglycerate to ADP to form ATP and 3-phosphoglycerate.
  • The phosphate molecule from 3 phosphoglycerates from III carbon is relocated II carbon to form 2 phosphoglycerate by the enzyme phosphoglycerate mutase.
  • A molecule of water from 2 phosphoglycerates is removed by the enzyme enolase to form phosphoenolpyruvic acid (PEP).
  • In the final step, the enzyme pyruvate kinase transfers a phosphate molecule from phosphoenolpyruvate to ADP to form pyruvic acid and ATP.
  • Glycolysis is a chain of 10 reactions under the control of different enzymes.
  • In glycolysis ATP is utilised at two steps :
  • in the conversion of glucose into glucose 6 phosphate
  • In the conversion of fructose 6 phosphates to fructose 1, 6 diphosphate
  • 4 ATP molecules are directly synthesised in glycolysis per one molecule of glucose.
  • The metabolic fate of pyruvate depends on cellular need.
  • There are three major ways in which different cells handle pyruvic acid produced by glycolysis.
  • Lactic acid fermentation
  • Alcoholic fermentation
  • Aerobic respiration (Kreb’s cycle)


  • It is the incomplete oxidation of glucose achieved under anaerobic conditions.
  • Takes place in sets of reaction where pyruvic acid is converted to CO2 and ethanol
  • It takes place in many prokaryotes and unicellular eukaryotes.
  • The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyses these reactions.
  • Here the reducing is NADH+H+
  • In both lactic acid and alcohol fermentation, not much energy is released.
  • Less than 7% of energy in glucose is released.
  • This process is hazardous.
  • As either acid alcohol is produced.
  • The net ATP synthesised when one molecule of glucose is fermented to alcohol or lactic acid is two molecules of ATP.



  • Leads to complete oxidation of organic substances in the presence of oxygen and releases CO2 water and a large amount of energy present in the substrate.
  • Takes place in mitochondria.
  • For aerobic respiration the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria.
  • Events in aerobic respiration are:
    • Complete oxidation of pyruvate by step removal of all the hydrogen atoms, leaving three molecules of CO2
    • Passing on one of the electrons removed as part of the hydrogen atoms to molecular O2 with the simultaneous synthesis of ATP
    • The first step takes place in the matrix of mitochondria
    • The second step takes place on the inner membrane of the mitochondria.
  • Pyruvate, after it enters the mitochondrial matrix, undergoes oxidative decarboxylation by a complex set of reactions which is catalysed by the enzyme pyruvic dehydrogenase.
  • This reaction required the participation of several coenzymes, including NAD+ and coenzyme

Pyruvic acid + CoA + NAD+ —-> acetyl CoA + CO2 +  NADH +H+.

  • From one molecule of glucose two molecules of NADH are produced from this reaction.
  • Acetyl CoA then enters a cyclic pathway which is called a tricarboxylic acid cycle or Krebs cycle.
  • It is named after the scientist Hans Krebs.



  • This cycle starts with the condensation of acetyl group with oxalic acetic acid and water.
  • This step yield citric acid.
  • It is catalysed by the enzyme citrate synthase and a molecule of CoA is released.
  • Then the citrate is isomerized to isocitrate.
  • Then the two successive steps of decarboxylation take place.
  • This leads to the formation of alpha-ketoglutaric acid and then succinyl Co-A.
  • Then succinyl Co-A is oxidised to oxalic acetic acid.
  • This allows the cycle to continue.
  • A molecule of GTP is synthesized when succinyl Co-A is converted to succinic acid.
  • This step is a substrate-level phosphorylation.
  • As a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP.
  • NAD+ is reduced to NADH+ H+ in 3 steps of the cycle.



  • In this system, the NADH + H+ and FADH2 are oxidised and electrons are passed on to O2.
  • This results in the formation of H2O
  • The electron transport system is the metabolic pathway through which the electron passes from one carrier to another.
  • It takes place in the inner mitochondrial membrane.
  • NADH Dehydrogenase oxidizes the electrons from NADH produced during the citric acid cycle.
  • The electrons are then transferred to ubiquinone located within the inner membrane of mitochondria.
  • Ubiquinone also receives reducing equivalents via FADH2 (Complex II)
  • The reduced ubiquinone is then oxidized with the transfer of electrons to cytochrome c via cytochrome bc1 ( complex III)
  • Cytochrome c is a small protein attached to the outer surface of the inner membrane.
  • It acts as a mobile carrier for transfer of electrons between complex III and complex IV.
  • The complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3 and two copper centres.
  • The transfer of electrons from complex 1 to complex IV is coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate
  • The number of ATP molecules synthesized depends on the nature of the electron donor.
  • Oxidation of one molecule of NADH gives rise to three molecules of ATP.
  • Oxidation of one molecule of FADH2 produces two molecules of ATP.
  • In this system, oxygen acts as the final hydrogen acceptor.
  • In respiration, it is the energy of oxidation-reduction utilised for the production of proton gradient required for phosphorylation.
  • Due to this reason, the process is called oxidative phosphorylation.
  • The energy released during the electron transport system is utilised in synthesizing ATP with the help of ATP synthase.
  • The ATP synthase consists of two major components.
  • F1 and F0
  • F1 headpiece is a peripheral membrane protein complex and contains the site for the synthesis of ATP from ADP and inorganic phosphate.
  • F0 is an integral membrane protein complex that forms the channels through which protons across the inner membrane.
  • The passage of protons through this channel is coupled to the catalytic site of the F1 component for the production of ATP.
  • for 1 ATP produced 2H+ passes through from the intermembrane space to the matrix down the electrochemical proton gradient.



  • There can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose.
  • In fomentation, there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid.
  • NADH is oxidised to NAD+ slowly in fermentation. This reaction is very vigorous in the case of aerobic respiration.



  • Fats are first broken down into glycerol and fatty acids if required to respire
  • For the fatty acids to be respired they are first degraded into acetyl Co A and enter the pathway.
  • Glycerol enters the pathway after being converted to phosphoglyceraldehyde.
  • The proteins are degraded by proteases and individual amino acids depending on their structure enter the pathway at some stage within the kerb’s cycle.
  • Since respiration involves the breakdown of substrates the respiratory process has traditionally been considered as a catabolic process and the respiratory pathway as a catabolic pathway.
  • When the organism need to synthesise fatty acids acetyl CoA would be withdrawn from the respiratory pathway for it.
  • Respiratory intermediate form the link between breakdown and synthesis of any substrate.
  • The respiratory pathway is considered as an amphibolic pathway rather than as a catabolic one as because it is involved in both anabolism and catabolism.


  • The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.

RQ = volume of  CO2 evolved / volume of O2 consumed

  • RQ depends upon the type of respiratory substrate used during respiration.
  • When carbohydrate is used as the substrate and is completely oxidised the RQ will be 1 because equal amounts of CO2 and O2 are evolved and consumed
  • When fats are used in respiration the RQ is less than 1.
  • When proteins are the respiratory substrates the ratio would be about 0.9.
  • In living organisms, respiratory substances are often more than one. Pure proteins of fats are never used as respiratory substrates.