Respiration in Plants

• Plants require O2 for respiration to occur and they also give out CO2.
• Hence, plants have systems in place that ensure the availability of O2.
• Plants, unlike animals, have no specialized organs for gaseous exchange but they have stomata and lenticels for this purpose.
• There are several reasons why plants can get along without respiratory organs.
  • First, each plant part takes care of its own gas-exchange needs.
  • There is very little transport of gases from one plant part to another.
  • Second, plants do not present great demands for gas exchange.
  • Roots stems and leaves respire at rates far lower than animals do.
  • Only during photosynthesis are large volumes of gases exchanged and, each leaf is well adapted to take care of its own needs during these periods.
  • When cells photosynthesize, the availability of O2 is not a problem in these cells since O2 is released within the cell.
  • Third, the distance that gases must diffuse even in large, bulky plants is not great.
  • Each living cell in a plant is located quite close to the surface of the plant.
• In stems, the ‘living’ cells are organized in thin layers inside and beneath the bark.
  • ==They also have openings called== ==lenticels.==
  • The cells in the interior are ==dead and provide only mechanical support.==
• Thus, most cells of a plant have at least a part of their surface in contact with air.
• This is also facilitated by the loose packing of parenchyma cells in leaves, stems, and roots, which provide an ==interconnected network of air spaces.==
• The complete combustion of glucose, which produces ==CO2 and H2O== as end products, yields energy most of which is given out as heat.
  • If this energy is to be useful to the cell, it should be able to utilize it to synthesize other molecules that the cell requires.
  • The strategy that the plant cell uses is to catabolize the glucose molecule in such a way that not all the liberated energy goes out as heat.
• The key is to ==oxidize glucose== not in one step but in several small steps enabling some steps to be just large enough such that the energy released can be coupled to ATP synthesis.
  • During the process of respiration, ==oxygen is utilized, and carbon dioxide, water, and energy are released as products.==
• The combustion reaction requires oxygen.
  • But some cells live where oxygen may or may not be available.
• There are sufficient reasons to believe that the first cells on this planet lived in an atmosphere that lacked oxygen.
• Even among present-day living organisms, we know of several that are adapted to anaerobic conditions.
• Some of these organisms are facultative anaerobes, while in others the requirement for the anaerobic conditions is obligate.
• In any case, all living organisms retain the enzymatic machinery to partially oxidize glucose without the help of oxygen.
  • ==This breakdown of glucose to pyruvic acid is called glycolysis.==

Glycolysis:

• The term glycolysis originated from the Greek words, glycos for sugar, and lysis for splitting.
  • The scheme of glycolysis was given by ==Gustav Embden, Otto Meyerhof, and J. Parnas==, and is often referred to as the EMP pathway.
  • In anaerobic organisms, it is the only process in respiration.
  • ==Glycolysis occurs in the cytoplasm of the cell== and is present in all living organisms.
  • In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.
  • In plants, this ==glucose is derived from sucrose==, which is the end product of photosynthesis, or from storage carbohydrates.
  • ==Sucrose is converted into glucose== and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway.
  • ==Glucose and fructose are phosphorylated to give rise to glucose-6- phosphate== by the activity of the enzyme hexokinase.
  • This phosphorylated form of glucose then isomerizes to produce ==fructose-6- phosphate==.
  • Subsequent steps of metabolism of glucose and fructose are the same.
  • In glycolysis, ==a chain of ten reactions,== under the control of different enzymes, takes place to produce pyruvate from glucose.
• While studying the steps of glycolysis, please note the steps at which utilization or synthesis of ATP or (in this case) NADH + H+ takes place.
• ATP is utilized in two steps:
  • first in the conversion of ==glucose into glucose 6-phosphate==
  • second in the conversion of ==fructose 6-phosphate to fructose 1, 6-bisphosphate.==
  • The ==fructose 1, 6-bisphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).==
  • We find that there is one step where NADH + H+ is formed from NAD+; this is when ==3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (BPGA).==
  • Two redox equivalents are removed (in the form of two hydrogen atoms) from PGAL and ==transferred to a molecule of NAD+.==
    • ==PGAL is oxidized with inorganic phosphate to get converted into BPGA.==
    • The conversion of ==BPGA to 3-phosphoglyceric acid (PGA),== is also an energy-yielding process; this energy is trapped by the formation of ATP.
• Another ATP is synthesized during the conversion of ==PEP to pyruvic acid.==
  • ==Pyruvic acid is then the key product== of glycolysis.
• There are three major ways in which different cells handle pyruvic acid produced by glycolysis.
  • These are ==lactic acid fermentation, alcoholic fermentation, and aerobic respiration.==
  • Fermentation takes place under anaerobic conditions in many ==prokaryotes and unicellular eukaryotes.==
  • For the complete oxidation of glucose to CO2 and H2O, however, organisms adapt to Krebs’ cycle which is also called ==aerobic respiration.==
  • This requires O2 supply.

Fermentation:

• In fermentation, say by yeast, ==the incomplete oxidation of glucose== is achieved under anaerobic conditions by sets of reactions where ==pyruvic acid is converted to CO2 and ethanol.==
  • The enzymes, ==pyruvic acid decarboxylase, and alcohol dehydrogenase== catalyze these reactions.
  • Other organisms like some bacteria produce ==lactic acid from pyruvic acid.==
  • In animal cells also, like ==muscles during exercise==, when oxygen is inadequate for cellular respiration ==pyruvic acid is reduced to lactic acid by lactate dehydrogenase.==
    • The ==reducing agent is NADH+H+ which is reoxidized to NAD+== in both processes.
  • In both ==lactic acid and alcohol fermentation==, not much energy is released; less than ==seven percent of the energy== in glucose is released and not all of it is trapped as high energy bonds of ATP.
• Also, the processes are hazardous – either acid or alcohol is produced.
  • Yeasts poison themselves to death when the concentration of ==alcohol reaches about 13 percent.==
  • Aerobic respiration is the process that leads to the ==complete oxidation of organic substances in the presence of oxygen and releases CO2, water, and a large amount of energy== present in the substrate.
    • This type of respiration is ==most common in higher organisms.==

Aerobic Respiration:

• For aerobic respiration to take place within the mitochondria, the final product of glycolysis, ==pyruvate is transported from the cytoplasm== into the mitochondria.
• The crucial events in aerobic respiration are:
  • The ==complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO2.==
  • The passing on of the electrons removed as part of the hydrogen atoms to molecular O2 with the simultaneous synthesis of ATP.
  • What is interesting to note is that the first process takes place in the matrix of the mitochondria while the second process is located on the ==inner membrane of the mitochondria.==
• Pyruvate, which is formed by the ==glycolytic catabolism of carbohydrates in the cytosol==, after it enters the mitochondrial matrix undergoes oxidative decarboxylation by a complex set of reactions catalyzed by pyruvic dehydrogenase.
  • The reactions catalyzed by pyruvic dehydrogenase require the ==participation of several coenzymes, including NAD+ and Coenzyme A.==
  • During this process, ==two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid== (produced from one glucose molecule during glycolysis).
  • The acetyl CoA then enters a cyclic pathway, ==the tricarboxylic acid cycle, more commonly called Krebs’ cycle== after the scientist Hans Krebs who first elucidated it.

Tricarboxylic Acid Cycle:

• The TCA cycle starts with the condensation of the acetyl group with ==oxaloacetic acid (OAA) and water to yield citric acid==.
  • The reaction is catalyzed by the enzyme ==citrate synthase and a molecule of CoA== is released.
  • ==Citrate is then isomerized to isocitrate.==
  • It is followed by two successive steps of decarboxylation, leading to the formation of ==α-ketoglutaric acid and then succinyl-CoA.==
  • In the remaining steps of the ==citric acid cycle, succinyl-CoA is oxidized to OAA== allowing the cycle to continue.
    • During the conversion of ==succinyl-CoA to succinic acid, a molecule of GTP is synthesized.==
    • This is ==substrate-level phosphorylation.==
    • In a coupled reaction ==GTP is converted to GDP with the simultaneous synthesis of ATP from ADP.==
  • Also, there are three points in the cycle where ==NAD+ is reduced to NADH + H+ and one point where FAD+ is reduced to FADH2.==
    • The continued oxidation of ==acetyl CoA via the TCA cycle== requires the continued ==replenishment of oxaloacetic acid,== the first member of the cycle.
    • In addition, it also requires ==regeneration of NAD+ and FAD+ from NADH and FADH2== respectively.

Electron Transport System (ETS) and Oxidative Phosphorylation:

• The following steps in the respiratory process are to release and ==utilize the energy stored in NADH+H+ and FADH2.==
  • This is accomplished when they are oxidized through the electron transport system and the electrons are passed on to O2 resulting in the ==formation of H2O.==
  • The metabolic pathway through which the electron passes from one carrier to another is called the ==electron transport system (ETS) and it is present in the inner mitochondrial membrane.==
  • Electrons from NADH produced in the mitochondrial matrix during the citric acid cycle are oxidized by an ==NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone== located within the inner membrane.
  • ==Ubiquinone also receives reducing equivalents via FADH2== (complex II) that is generated during the oxidation of ==succinate in the citric acid cycle.==
  • The ==reduced ubiquinone (ubiquinol) is then oxidized with the transfer of electrons to cytochrome c via cytochrome bc1 complex== (complex III).
  • ==Cytochrome c is a small protein== attached to the outer surface of the inner membrane and acts as a mobile carrier for the ==transfer of electrons between complex III and IV.==
  • ==Complex IV refers to a cytochrome c== oxidase complex containing ==cytochromes a and a3==, and ==two copper centers.==
  • When the electrons pass from one carrier to another via ==complex I to IV in the electron transport chain, they are 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 3 molecules of ATP==, while that of ==one molecule of FADH2 produces 2 molecules of ATP.==
  • Although the aerobic process of respiration takes place only in the presence of oxygen, ==the role of oxygen is limited to the terminal stage of the process.==
• Yet, the ==presence of oxygen is vital==, since it drives the whole process by removing hydrogen from the system.
  • ==Oxygen acts as the final hydrogen acceptor.==
  • Unlike photophosphorylation where it is the light energy that is utilized for the ==production of the proton gradient== required for phosphorylation, in respiration it is the ==energy of oxidation-reduction utilized== for the same process.
  • It is for this reason that the process is called ==oxidative phosphorylation.==
  • As mentioned earlier, the energy released during the electron transport system is ==utilized in synthesizing ATP with the help of ATP synthase (complex V).==
• This complex consists of two major components, ==F1 and F0.==
  • The ==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 channel through which protons cross the inner membrane.
  • The ==passage of protons== through the channel is coupled to the catalytic site of the F1 component for the ==production of ATP.==
  • For each ATP produced, ==2H+ passes through F0 from the intermembrane space to the matrix down the electrochemical proton gradient.==

The Respiratory Balance Sheet:

• It is possible to make calculations of the net ==gain of ATP for every glucose molecule oxidized;== but in reality, this can remain only a theoretical exercise.
• These calculations can be made only on certain assumptions:
  • There is a sequential, orderly pathway functioning, with one substrate forming the next and with ==glycolysis, TCA cycle, and ETS pathway== following one after another.
  • The ==NADH synthesized in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.==
  • None of the intermediates in the pathway are utilized to synthesize any other compound.
  • Only glucose is being respired – no other alternative substrates are entering the pathway at any of the intermediary stages.
• Hence, there can be a ==net gain of 38 ATP== molecules during aerobic respiration of one molecule of glucose.
• Now let us compare fermentation and aerobic respiration:
  • Fermentation accounts for only a ==partial breakdown of glucose whereas in aerobic respiration it is completely degraded to CO2 and H2O.==
  • In fermentation, there is a ==net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid== whereas many more molecules of ATP are generated under aerobic conditions.
  • ==NADH is oxidized to NAD+ rather slowly in fermentation==, however, the reaction is very vigorous in case of aerobic respiration.

Amphibolic Pathway:

• ==Glucose is the favored substrate for respiration.==
  • All ==carbohydrates are usually first converted into glucose== before they are used for respiration.
• Other substrates can also be respired, as has been mentioned earlier, but then they do not enter the respiratory pathway at the first step.
  • ==Fats would need to be broken down into glycerol and fatty acids first.==
  • If fatty acids were to be respired they would ==first be degraded to acetyl CoA and enter the pathway.==
  • ==Glycerol would enter the pathway after being converted to PGAL.==
  • The proteins would be degraded by ==proteases and the individual amino acids (after deamination)== depending on their structure would enter the pathway at some stage within the ==Krebs’ cycle or even as pyruvate or acetyl CoA.==
• Since respiration involves the breakdown of substrates, the respiratory process has traditionally been considered ==a catabolic process and respiratory pathway as a catabolic pathway.==
  • ==Fatty acids would be broken down to acetyl CoA== before entering the respiratory pathway when it is used as a substrate.
  • But when the organism needs to synthesize ==fatty acids, acetyl CoA== would be withdrawn from the respiratory pathway for it.
    • Hence, the ==respiratory pathway comes into the picture both during the breakdown== and synthesis of fatty acids.
  • Similarly, during the ==breakdown and synthesis of protein== too, respiratory intermediates form the link.
  • ==Breaking down processes within the living organism is catabolism, and synthesis is anabolism.==
  • Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.

Respiratory Quotient:

• 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.
  • The respiratory quotient depends upon the type of respiratory substrate used during respiration.
  • When ==carbohydrates== are used as substrate and are completely oxidized, ==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 respiratory substrates the ratio would be about ==0.9.==
  • What is important to recognize is that in living organisms respiratory substrates are often more than one; ==pure proteins or fats are never used as respiratory substrates.==

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