• 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.