Preview:

• We shall begin by discussing how plants survive even without respiratory organs their requirement of gases for exchange process is very less except during photosynthesis; besides having special apparatus meant for gaseous exchange the stomata and the lenticels.
• The process by which C-C bonds of complex compounds are broken down through oxidation in the mitochondria of cells, leading to release of considerable amount of energy is what we call, respiration. Its types shall also be studied Aerobic and Anaerobic respiration which occur in the presence of oxygen and in the absence of oxygen respectively.
• Then we shall study stepwise a systematic process by which glucose is broken down to pyruvic acid called, Glycolysis. Then we shall learn a special process which occurs under anaerobic conditions called, Fermentation. There are two types of fermentation processes based on the product formed a) Alcoholic fermentation and b) Lactic acid fermentation. Then we will look at Aerobic respiration which occurs in a series of three complex cycles starting from glycolysis to Citric acid cycle to Electron transport process.
• We will then compare aerobic respiration and fermentation.
• We shall come to the end of the chapter with the study of Respiratory Quotient (RQ) which is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration.

14.0 Introduction:

• All the energy required for life processes is obtained by oxidation of some macromolecules that we consume as food .
• Only green plants and cyanobacteria can prepare their own food by the process of photosynthesis. During photosynthesis, plants trap light energy and convert it into chemical energy. They store the chemical energy in the bonds of carbohydrates like glucose, sucrose and starch. In green plants too, not all cells, tissues and organs photosynthesise; only cells containing chloroplasts carry out photosynthesis. Hence, even in green plants all other organs, tissues and cells that are non-green, need food for oxidation. Hence, food has to be translocated to all nongreen parts .
• Animals are Heterotrophic, that is, they obtain food from plants directly (herbivores) or indirectly (omnivores and carnivores). Saprophytes like fungi are dependent on dead and decaying matter.
• This chapter deals with cellular respiration or the mechanism of breakdown of food materials within the cell to release energy, and the trapping of this energy for synthesis of ATP.
• The breaking of the C-C bonds of complex compounds through oxidation within the mitochondria of cells, leading to release of considerable amount of energy is called respiration. It occurs only in eukaryotes. The compounds that are oxidised during this process are known as respiratory substrates.
• During oxidation within a cell, all the energy contained in respiratory substrates is not released into the cell, or in a single step. It is released in a series of slow step-wise reactions controlled by enzymes, and is trapped as chemical energy in the form of ATP until then. Hence, it is important to understand that the energy released by oxidation in respiration is not used directly but is used to synthesise ATP, which is broken down whenever (and wherever) energy needs to be utilised. Hence, ATP acts as the energy currency of the cell. This energy trapped in ATP is utilised in various energy-requiring processes of the organisms, and the other by-product carbon skeleton produced during respiration is used as precursors for biosynthesis of other molecules in the cell.

14.1 Do Plants Breathe?

• Plants intake $O_2$ and give out $CO_2$ during respiration. Plants use stomata and lenticels for this purpose.
• There are several reasons why plants can survive without respiratory organs such as,
• Each plant part takes care of its own gas-exchange needs.
• There is very little transport of gases from one plant part to another.
• Large amount of gases is not used during gaseous 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 photosynthesise, availability of $O_2$ is not a problem since $O_2$ is released within the cell.
• The distance that gases must diffuse even in large, bulky plants is not great.
• Thus, most cells of a plant have at least a part of their surface in contact with air, thus making it easy for plants to respire leaves have stomata that open up to receive gases while stems and thick bark have thin layers inside them called, lenticels.
• The cells in the interior are dead and provide only mechanical support. Hence, they do not have to breathe to survive.
• Gaseous exchange between parenchyma cells occur due to loose packing of these cells. The cells are present in leaves, stems and roots; and due to loose packing, provide an interconnected network of air spaces.

The Respiration process:

• Let us first recall the definition of respiration. The breaking of the C-C bonds of complex compounds through oxidation in the mitochondria of cells, leading to release of considerable amount of energy is called respiration. The complex compound here is glucose and the reaction that takes place is as follows:
• The complete combustion of glucose produces $CO_2$ and $H_2O$ along with energy which is liberated as heat, as depicted by the equation:
  $$ \ce { C_6H_{12}O_6 + 6O_2 -> 6CO_2 + 6H_2O + Energy }$$
• During the process of respiration, oxygen is utilised for combustion and; carbon dioxide, water and energy are released as products. The energy is utilised to synthesise other molecules that the cell requires.
• In order to not waste all the energy as heat, plants oxidise glucose not in one step but in several small steps enabling some steps to be just large enough to liberate enough energy required to synthesise ATP.

Aerobic and Anaerobic Respiration:

Aerobic respiration: Aerobic respiration is the process of producing energy within the mitochondria of a cell in the presence of oxygen.

Anaerobic Respiration: Incomplete intracellular breakdown of sugar or other organic compounds in the absence of oxygen to release energy is called Anaerobic respiration.

Some cells live where oxygen may or may not be available. Such living organisms are adapted to anaerobic conditions. Some of these organisms are facultative anaerobes (organisms that adapt to anaerobic conditions in the absence of oxygen and perform aerobic respiration in the presence of oxygen), while in others, the requirement for anaerobic condition is obligate. Therefore, all living organisms can partially oxidise glucose even without the help of oxygen.

Questions from section 14.1:

1. Comment Plants can survive without respiratory organs .

2. What is respiration? Explain with relevant equation.

14.2 Glycolysis

Definition box: Glycolysis: The breakdown of glucose to pyruvic acid is called 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.

Steps in Glycolysis:

• In this process, glucose undergoes partial oxidation to ultimately form two molecules of pyruvic acid in the cytoplasm of the cell.
• In plants, the glucose which is the end product of photosynthesis is derived from sucrose or from storage carbohydrates.
• Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides (glucose and fructose) readily enter the glycolytic pathway.
• Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate by the activity of the enzyme, hexokinase. ATP is used up in this conversion.
• This phosphorylated form of glucose then isomerises to produce fructose-6-phosphate.
• This fructose 6 phosphate is then converted to fructose 1, 6-bisphosphate with the utilization of ATP.
• The fructose 1, 6-bisphosphate is split into dihydroxyacetone phosphate (or triose phosphate) and 3-phosphoglyceraldehyde (PGAL).
• PGAL is oxidised with inorganic phosphate to get converted into 1, 3-bisphosphoglycerate (BPGA) while two redox-equivalents are removed, in the form of two hydrogen atoms from PGAL and transferred to a molecule of $NAD^+$ to form NADH + $H^+$; where NAD stands for Nicotineamide Adenine Dinucleotide.
• In the next step, conversion of BPGA to 3-phosphoglyceric acid (PGA) takes place. This is also an energy yielding process and the energy is trapped by the formation of ATP.
• From 3 PGA, 4 phosphoglycerate is formed the dehydration of which gives 2-phosphoenolpyruvate (PEP).
• PEP is converted to pyruvic acid during which another molecule of ATP is formed.
• Steps in metabolism of glucose and fructose are same. The various steps of glycolysis are depicted in Figure 14.1 below:

Note box: 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, we need to remember where there is utilisation of ATP and synthesis of NADH + $H^+$ take place.

There are three major ways in which different cells handle pyruvic acid produced by glycolysis. In other words, there are three different types of reactions that occur in living cells after the formation of pyruvic acid. These are:

Questions from section 14.2:

1. Define glycolysis. Explain the steps in glycolysis with flow diagram.

14.3 Fermentation

Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to $CO_2$ and $H_2O$, however, organisms adopt Krebs cycle which is also called as aerobic respiration. This requires $O_2$ supply.

Alcoholic Fermentation: Fermentation is the process during which incomplete oxidation of glucose is achieved under anaerobic conditions by a set of reactions during which pyruvic acid is converted to $CO_2$ and ethanol. The enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions.

Lactic acid Fermentation: Some organisms like 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 reoxidised to NAD⁺ in both the processes.

The processes are shown in Figure 14.2 below:

Note box: In both lactic acid and alcohol fermentation not much energy is released; less than seven per cent 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 per cent.

Questions from section 14.3:

1. Explain alcoholic and lactic acid fermentation.

14.4 Aerobic Respiration

• In eukaryotes, the process of breakdown of glucose takes place within the mitochondria in the presence of $O_2$ by aerobic respiration. Aerobic respiration is the process that leads to complete oxidation of organic substances in the presence of oxygen; releasing $CO_2$, water and a large amount of energy present in the substrate.
• For aerobic respiration to take place within the mitochondria, the final product of glycolysis, pyruvate is transported from the cytoplasm to mitochondria.
• The crucial events that take place in aerobic respiration can briefly be stated as:
• The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of $CO_2$.
• The electrons along with protons will be added to molecular $O_2$ with simultaneous synthesis of ATP.
• The first step takes place in the matrix of the mitochondria while the second step is located on the inner membrane of the mitochondria.

Process of Aerobic respiration:

• As mentioned earlier, Pyruvate, which is formed by the glycolytic catabolism (or glycolysis) of carbohydrates in the cytosol, enters mitochondrial matrix and undergoes oxidative decarboxylation by a complex set of reactions catalysed by pyruvic dehydrogenase. The reactions catalysed by pyruvic dehydrogenase require the participation of several coenzymes such as: NAD⁺ and Coenzyme A.
  $$ \ce{ Pyruvic acid + CoA + NAD^+ ->[{Mg^{2+}}][{Pyruvate dehydrogenase}] Acetyl CoA + CO_2 + NADH + H^+}$$
• 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, tricarboxylic acid cycle, more commonly called as Krebs cycle after the scientist Hans Krebs who first elucidated it.

14.4.1 Tricarboxylic Acid Cycle or Cytric Acid Cycle

The Tricarboxylic Acid Cycle (TCA) cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) to yield citric acid (Figure 14.3). The reaction is catalysed by the enzyme citrate synthase along with release of a molecule of CoA.

• Citrate is then isomerised to isocitrate.
• It is followed by two successive steps of: a) decarboxylation, leading to the formation of α-ketoglutaric acid and b) succinyl-CoA.
• In the remaining steps of citric acid cycle, succinyl-CoA is oxidised to Oxalo acitic acid (OAA) allowing the cycle to continue. During the conversion of succinyl-CoA to succinic acid, a molecule of Guanosine triphosphate (GTP) is synthesised.
• There are three points in the cycle where NAD+ is reduced to NADH + $H^+$ and one point where $FAD^+$ is reduced to $FADH_2$.
• 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 $FADH_2$ respectively.
• The summary equation for the TCA phase of respiration can be expressed as follows:
  $$ \ce { Pyruvis acid + 4 NAD^+ + FAD^+ + 2H_2O + ADP + Pi ->[{Mitochondiral Matrix}] 3CO_2 + 4NADH + 4H^+ + FADH_2 + ATP}$$
• To summarize the reactions so far, Glucose has been broken down to release $CO_2$ and eight molecules of NADH +$ H^+$; two molecules of $FADH_2$ have been synthesised, besides two molecules of ATP.

Note box: In a coupled reaction, GTP is converted to GDP with the simultaneous synthesis of ATP from ADP.

Let us now understand the role of O₂ in respiration and how ATP is synthesised.

14.4.2 Electron Transport System (ETS) and Oxidative Phosphorylation

The metabolic pathway by which the electron passes from one carrier to another, is called the electron transport system (ETS).

In the process, the electrons are passed on to$O_2$ resulting in the formation of $H_2O$. Also, the release and utilisation of energy stored in NADH+$H^+$ and $FADH_2$ is accomplished by oxidation through the electron transport system.

Steps in electron transport process:

• The electron transport process occurs in the inner mitochondrial membrane.
• Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by an NADH dehydrogenase (complex I) and transferred to ubiquinone located within the inner membrane. Ubiquinone also receives reducing equivalents from $FADH_2$ (complex II). The $FADH_2$ is generated during oxidation of succinate in the citric acid cycle.
• The reduced ubiquinone (ubiquinol) is then oxidised due to 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 of mitochondria and acts as a mobile carrier for transfer of electrons between complex III and IV. Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a₃, and two copper centres.]

• The energy released during the electron transport system is utilised in synthesising ATP with the help of ATP synthase (complex V) from ADP and inorganic phosphate. The number of ATP molecules synthesised 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.
• Complex V consists of two major components, $F_1$ and $F_0$. The $F_1$ headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. $F_0$ is an integral membrane protein complex that forms the channel through which protons cross the inner membrawne. The passage of protons through the channel is coupled to the catalytic site of the $F_1$ component for the production of ATP. For each ATP produced, $2H^+$ passes through $F_0$ from the inter-membrane space to the matrix down the electrochemical proton gradient.

Oxidative phosphorylation: 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 it acts as the final hydrogen acceptor. Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Unlike photophosphorylation where it is the light energy that is utilised for the production of proton gradient required for phosphorylation, in respiration, it is the energy of oxidation-reduction utilised for the same process. It is for this reason that respiration process is also called, oxidative phosphorylation.

Questions from section 14.4:

1. Explain the process of aerobic respiration with equation.

2. Explain the steps involved in citric acid cycle.

3. Explain the steps in ETS.

4. Comment: Respiration can be regarded as oxidative phosphorylation .

14.5 The Respiratory Balance Sheet

• It is possible to make calculations of the net gain of ATP for every glucose molecule oxidised during respiration.
• These calculations can be made only on certain assumptions, as listed:

Drawbacks of the assumptions: These assumptions are not really valid in a living system because:

There can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose if these assumptions hold good.

Comparing fermentation and aerobic respiration

• Fermentation accounts for only a partial breakdown of glucose whereas in aerobic respiration it is completely degraded to $CO_2$ and $H_2O$.
• 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 oxidised to $NAD^+$ rather slowly in fermentation, however the reaction is very vigorous in case of aerobic respiration.

Questions from section 14.5:

1. Explain the assumptions made in respiratory process.

2. State the differences between fermentation and aerobic respiration.

14.6 Amphibolic Pathway

Glucose is the favoured 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. Figure 14.6 below shows the points of entry of different substrates in the respiratory pathway.

• Fats would need to be broken down into glycerol and fatty acids first.
• Fatty acids will have to first be degraded to acetyl CoA and enter the pathway.
• Glycerol would enter the pathway after being converted to PGAL.
• Proteins would be degraded by proteases to individual amino acids (deamination). Depending on their structure, they will enter the respiratory pathway at Krebs cycle or even as pyruvate or acetyl CoA.

Is respiration an anabolic process or a catabolic process?

• Respiration involves breakdown of substrates and hence the respiratory process has traditionally been considered a catabolic process. But as we have discussed above, at different points in the respiratory pathway different substrates would enter if they were to be respired and used to derive energy.
• These very compounds are used for the synthesis of the other substrates. For example, fatty acids would be broken down to acetyl CoA before entering the respiratory pathway where it will be used as a substrate. But when the organism needs to synthesise fatty acids, acetyl CoA would be withdrawn from the respiratory pathway for it. Hence, the respiratory pathway comes into the picture both during breakdown and synthesis of fatty acids. Similarly, during 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.

Questions from section 14.6:

1. Why is respiratory pathway regarded as amphibolic pathway rather than a catabolic one?

14.7 Respiratory Quotient

During aerobic respiration, $O_2$ is consumed and $CO_2$ is released.

Definition box: Respiratory Quotient (RQ): The ratio of the volume of $F_0$ evolved to the volume of $O_2$ consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio. $$ RQ = \frac {Volume \space of \space CO_2 \space evolved}{ Volume \space of \space O_2 \space consumed}$$

The respiratory quotient depends upon the type of respiratory substrate used during respiration as explained in the examples:

1. When carbohydrates are used as substrate and are completely oxidised, the RQ will be 1, because the amound of oxygen consumed is equal to the amount of carbon dioxide evolved, as shown in the equation below:
   $$ \ce { C_6H_{12}O_6 + 6O_2 -> 6CO_2 + 6H_2O + Energy}$$
   $$ RQ = \frac { 6 CO_2}{6O_2} = 1.0 $$
2. When fats are used in respiration, the RQ is less than 1. For example, calculations for a fatty acid, tripalmitin, if used as a substrate is shown:
   $$ \ce { 2(C_{51}H_{98}O_6) + 145O_2 -> 102 CO_2 + 98 H_2O + energy } $$
   Tripalmitin
   $$ RQ = \frac { 102 CO_2}{145O_2} = 0.7 $$
3. When proteins are respiratory substrates, the ratio would be about 0.9.

Note box: In living organisms, respiratory quotient is often more than one except when pure proteins or fats come into picture. But, pure proteins and facts are never used as respiratory substrates.

Questions from section 14.7:

1. Define respiratory quotient. Explain with examples how it varies from one substrate to another.