NEET Biology Respiration in Plants Notes

NEET Biology notes on Chapter 14 Respiration in Plants, covering the definition of cellular respiration, the difference between respiration and combustion, aerobic and anaerobic respiration, glycolysis, the Krebs cycle, the electron transport system and oxidative phosphorylation, the respiratory balance sheet, the amphibolic pathway, the respiratory quotient, the hexose monophosphate pathway, fermentation and the factors affecting the rate of respiration.

AT A GLANCE

Terms and definitions to learn:
1. Cellular respiration
2. Respiratory substrates
3. Substrate level phosphorylation
4. Amphibolic pathway
5. Oxidative phosphorylation

Tables to learn:
1. Difference between cell respiration and combustion
2. Difference between aerobic and anaerobic respiration
3. Major steps involved in glycolysis
4. Difference between glycolysis and Krebs cycle

Flow chart to learn:
1. Types and mechanism of cellular respiration

Topics to learn:
1. Types of respiration
2. Aerobic respiration: glycolysis, Krebs cycle, electron transport chain
3. Hexose monophosphate pathway
4. The respiratory balance sheet
5. Amphibolic pathway
6. Respiratory ratio or respiratory quotient
7. Anaerobic respiration
8. Role of shuttle system in energy production

INTRODUCTION

Respiration is defined as the conversion of chemical bond energy of various organic molecules into energy of usable form within living cells, i.e., ATP.

The phenomenon of breaking of the C-C bond of complex organic molecules occurs through oxidation. The compounds that are oxidized are known as respiratory substrates.

The breakdown of respiratory substrates provides carbon skeleton for the synthesis of a large number of other essential plant products, such as polysaccharides, proteins, fats, nucleic acids, pigments, cytochromes, etc.

The specific organelle in a cell where most of the respiration occurs is the mitochondria.

During the process of respiration, oxygen is utilized and carbon dioxide, water and energy are released as products. This energy is utilized in various energy requiring processes of the organisms.

The reaction that occurs in common respiration of glucose can be summed up as:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

The mechanism of breakdown of food materials within the cell to release and trap this energy for ATP synthesis is known as cellular respiration. Hence, cellular respiration is an enzyme controlled process of biological oxidation of food materials in a living cell, using molecular oxygen (O₂), producing CO₂ and H₂O, and releasing energy in small steps and storing it in biologically useful forms, generally ATP.

Hence, respiration is a catabolic, exothermic and oxidative process.

DO PLANTS BREATHE?

Only green plants and cyanobacteria can prepare their own food by the process of photosynthesis. They trap light energy and convert it into chemical energy that is stored in the bonds of carbohydrates like glucose, sucrose and starch. They are called autotrophs.

Animals are heterotrophic, i.e., they obtain food from plants directly (herbivores) or indirectly (carnivores). Saprophytes, like fungi, are dependent on dead and decaying matter.

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.

Plants get along without a respiratory organ because each plant part takes care of its own gas exchange. There is little transport of gases from one part to another.

Roots, stems and leaves respire at rates far lower than animals do. Large volumes of gases are exchanged only during photosynthesis, and each leaf is well adapted to take care of its own needs during these periods.

RESPIRATION AND COMBUSTION

According to Lavoisier, cell respiration resembles combustion (e.g., burning of coal, wood, oil, etc.) in the breakdown of complex organic compounds in the presence of oxygen, with the production of carbon dioxide and energy. However, there are certain fundamental differences between cell respiration and combustion (refer Table 14.1).

Table 14.1: Differences between cell respiration and combustion

TYPES OF RESPIRATION

On the basis of the availability of oxygen and the complete or incomplete oxidation of the respiratory substrate, respiration may be either of the following two types: aerobic respiration and anaerobic respiration.

Aerobic respiration is essential for the survival of the vast majority of terrestrial and aquatic organisms, although some fungi (yeast) and many bacteria are facultative anaerobes (organisms which normally require oxygen but which can live anaerobically when grown on suitable media), e.g., butyric acid bacteria, lactic acid bacteria and Bacillus phosphorescens.

Obligate anaerobes live in the absence of oxygen or in the presence of a negligible concentration of this gas, e.g., Clostridia.

Table 14.2: Differences between aerobic and anaerobic respiration

[Figure: Flow Chart 14.1 Types and mechanism of cellular respiration. Respiration has two types, aerobic respiration and anaerobic respiration. Both start with glycolysis, which occurs in the cytoplasm and is the common pathway to aerobic and anaerobic respiration. Glycolysis partially oxidizes glucose into two molecules of pyruvic acid. In aerobic respiration this is followed by the citric acid cycle and then the electron transport chain, both occurring in the mitochondria. In anaerobic respiration it is followed by formation of ethyl alcohol or organic acid, occurring in the cytoplasm.]

Check Point
True or False
(i) Respiration is an anabolic, exothermic and reductive process.
(ii) Respiration occurs in the mitochondria.
(iii) ATP is the energy currency of the cell.
(iv) Plants require CO₂ for respiration and give out O₂.
(v) Aerobic respiration occurs in cytoplasm and in mitochondria.
(vi) Glycolysis is the common pathway to aerobic respiration and anaerobic respiration.

AEROBIC RESPIRATION

Aerobic respiration is the process that leads to a complete oxidation of organic substances in the presence of oxygen. It releases CO₂, water and a large amount of energy (in the form of ATP) present in the substrate. This type of respiration is most common in higher organisms.

Pyruvic acid does not directly enter the Krebs cycle. It enters the Krebs cycle in the form of acetyl CoA.

For aerobic respiration, pyruvic acid (generated in the cytosol) enters the mitochondria through a specific transport protein. It undergoes oxidative decarboxylation and dehydrogenation to produce CO₂ and NADH. The product combines with the sulphur containing coenzyme A to form acetyl CoA, or activated acetate. The reaction occurs in the presence of the enzyme complex pyruvate dehydrogenase (requires Mg²⁺):

Pyruvate + NAD⁺ + CoA → Acetyl CoA + NADH + H⁺ + CO₂ ↑

This step is called the link step, as it links glycolysis with the Krebs cycle. During this process 2 molecules of NADH are formed from the metabolism of 2 molecules of pyruvic acid. So a net gain of 6 ATP occurs.

Thus, the acetyl CoA enters the cyclic pathway, or Krebs cycle.

GLYCOLYSIS

Gustav Embden, Otto Meyerhof and J. Parnas (1930) discovered glycolysis. It is often referred to as the EMP pathway.

In anaerobic organisms, it is the only process in respiration.

Glycolysis occurs in the cytoplasm (cytoplasmic matrix of a living cell) and does not use oxygen. Thus, it is an anaerobic pathway.

In this process, glucose is partially oxidized, or converted, into two molecules of pyruvic acid.

In plants, 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.

The breakdown of glucose during glycolysis produces compounds for the synthesis of proteins, fats and nucleic acids.

Most of the steps of glycolysis are reversible.

Glycolysis is divided into two phases:
(i) Preparatory phase: breakdown of glucose and low energy phosphorylation occur, and energy is used.
(ii) Oxidative phase: high energy phosphate bonds are formed and energy is stored.

Connecting Concepts: Some molecules, such as phosphoenolpyruvate (PEP), possess a high energy phosphate bond similar to the bonds in ATP. When PEP's phosphate group is transferred enzymatically to ADP, the energy in the bond is conserved and ATP is created.

Steps of this pathway are as follows:

1. Glucose and fructose are phosphorylated into glucose-6-phosphate with the help of ATP, catalysed by the enzyme hexokinase.
2. Glucose-6-phosphate is converted into its isomer, fructose-6-phosphate. Isomerase catalyses this reaction.
3. Fructose-6-phosphate is phosphorylated into fructose 1,6-bisphosphate with the use of ATP, catalysed by phosphofructokinase.
4. Fructose 1,6-bisphosphate is split into one molecule of 3-phosphoglyceraldehyde and one molecule of dihydroxyacetone phosphate (DHAP). These two products are inter-convertible.
5. 3-phosphoglyceraldehyde is oxidized to 1,3-bisphosphoglycerate, where NAD⁺ is reduced to NADH.
6. 1,3-bisphosphoglycerate is converted into 3-phosphoglycerate, catalysed by phosphoglycerate kinase, along with the formation of ATP. This is called substrate level phosphorylation.
7. 3-phosphoglycerate is subsequently converted into 2-phosphoglycerate and then into phosphoenolpyruvate (PEP).
8. PEP is converted into pyruvate or pyruvic acid, catalysed by the enzyme pyruvate kinase, along with the formation of ATP.

In glycolysis, four molecules of ATP are formed in two ways:
(i) Direct or substrate level phosphorylation of ADP to ATP.
(ii) Another ATP is synthesized during the conversion of PEP to pyruvic acid.

Two ATP molecules are used up in the following steps:
(i) Phosphorylation of glucose to glucose-6-phosphate, and
(ii) Conversion of fructose-6-phosphate into fructose 1,6-bisphosphate.

Two redox equivalents are removed in the form of two hydrogen atoms and transferred to NAD⁺ from PGAL, which is reduced to NADH + H⁺.

The end products of glycolysis are two molecules of pyruvic acid, NADH, water and ATP.

Summary equation of glycolysis

C₆H₁₂O₆ + 2ADP + 2Pi + 2NAD⁺ → 2CH₃COCOOH + 2ATP + 2NADH + 2H⁺
(glucose) (pyruvic acid)

In all, there are four phosphorylations during glycolysis. Two phosphate groups are supplied by ATP and two by phosphoric acid.

The number of ATP used for phosphorylation of glucose in glycolysis is 2 (1st and 3rd reactions).

The number of ATP formed during glycolysis is 2 (4 - 2 = 2).

There are three major ways in which different cells handle the pyruvic acid produced by glycolysis. These are anaerobic respiration (lactic acid fermentation and alcoholic fermentation) and aerobic respiration. Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes.

Table 14.3: Major steps, enzymes involved, substrates and end products of glycolysis

In steps 1 and 3, ATP is utilized, whereas in steps 6 and 9, ATP is produced.

[Figure: Fig. 14.1 Schematic representation of glycolysis or EMP pathway]

Fig. 14.1 (labels in full):

• Preparatory phase: Glucose \u2192 (Hexokinase, Mg²⁺; ATP \u2192 ADP) Glucose-6-Phosphate \u21c4 (Phosphohexose isomerase, Mg²⁺) Fructose-6-Phosphate \u2192 (Phosphofructokinase, Mg²⁺; ATP \u2192 ADP) Fructose-1,6-bisphosphate \u2192 (Aldolase) Glyceraldehyde 3-Phosphate \u21c4 (Phosphotriose isomerase) Dihydroxyacetone 3-Phosphate (DHAP).
• Oxidative phase: Glyceraldehyde 3-Phosphate \u21c4 (Glyceraldehyde phosphate dehydrogenase; NAD + Pi \u2192 NADH) 1,3-bisphosphoglycerate \u21c4 (Phosphoglyceric acid kinase, Mg²⁺; ADP \u2192 ATP) 3-Phosphoglycerate \u21c4 (Phosphoglycerate mutase) 2-Phosphoglycerate \u21c4 (Enolase, Mg²⁺; H₂O released) Phosphoenolpyruvate (PEP) \u2192 (Pyruvate kinase, Mg²⁺, K⁺; ADP \u2192 ATP) Pyruvate.
• Pyruvate leads either to CO₂ + ethyl alcohol (fermentation) or to entrance into Krebs' cycle.

Check Point
Fill in the blanks
(i) Glycolysis takes place in __________.
(ii) During glycolysis ____________ molecules of NADH are formed.
(iii) In anaerobic respiration _________ is the only process in respiration.
(iv) 3-phosphoglycerate is converted into 2-phosphoglycerate in presence of ____________ enzyme.
(v) In glycolysis at step ____________ ATP is utilized and step no. ____________ ATP's are produced.
(vi) End products of glycolysis are 2 molecules of ____________, ____________, ____________, ____________.
(vii) Pyruvic acid enters Kreb's cycle in the form of ____________.
(viii) Breakdown of glucose and low phosphorylation occurs in ____________ phase.
(ix) Glycolysis is also called ____________ pathway.
(x) Glucose has ____________ carbon atoms and pyruvate has ____________ carbon atom.

KREBS CYCLE

The Krebs cycle is the second stage of plant respiration (the third step after glycolysis and the intermediate step). It is also called the TCA cycle, because of the formation of tricarboxylic acids in the cycle.

The Krebs cycle occurs in the mitochondrial matrix.

This cycle is named after the German biochemist Sir Hans Krebs, who discovered it in 1937. He won the Nobel Prize for this work in 1953. The Krebs cycle is also called the citric acid cycle, after one of the participating compounds.

Steps of this cycle are as follows:
1. One molecule of acetyl CoA combines with 4-C oxaloacetic acid (OAA) to form 6-C citric acid. This reaction is catalysed by the enzyme citrate synthetase.
2. Citrate is then isomerised to isocitrate. Then it is decarboxylated to alpha-ketoglutaric acid (the first dicarboxylic acid formed), which is then decarboxylated to succinyl CoA.
3. Succinyl CoA is oxidized to oxaloacetic acid, allowing the cycle to continue. During the conversion of succinyl CoA to succinic acid a molecule of GTP is synthesized, which is a substrate level phosphorylation. The reaction is catalysed by the enzyme succinyl CoA synthetase. The GTP produced in this step is the only high energy phosphate produced directly in the Krebs cycle.
4. Succinic acid undergoes oxidation or dehydrogenation to form fumaric acid. FAD is reduced to FADH₂ during this step, and the enzyme involved is succinic acid dehydrogenase.
5. Fumaric acid undergoes hydration to form malic acid in the presence of H₂O and the enzyme fumarase.
6. Malic acid undergoes final oxidation or dehydrogenation in the presence of malic dehydrogenase to produce oxaloacetic acid, and NAD⁺ is reduced to NADH + H⁺.

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

During this cycle, NAD⁺ and FAD are reduced to produce NADH + H⁺ and FADH₂, respectively.

Connecting Concepts: The pyruvate dehydrogenase complex includes Mg²⁺, TPP, lipoic acid, and transacetylase.

The summary equation for this phase of respiration is:
Pyruvic acid + 4NAD⁺ + FAD + 2H₂O + ADP + Pi → 3CO₂ + 4NADH + 4H⁺ + FADH₂ + ATP

All the enzymes, reactants, intermediates and products of the TCA cycle are found in aqueous solution in the matrix, except succinate dehydrogenase (the mitochondrial marker enzyme), which is located in the inner mitochondrial membrane.

The Krebs cycle is also called an amphibolic pathway, because it serves both anabolic and catabolic processes. It involves both the catabolism of carbohydrates and fatty acids, and the synthesis of anabolic precursors for amino acid synthesis, e.g., alpha-ketoglutarate.

[Figure: Fig. 14.2 Diagrammatic representation of oxidative decarboxylation of pyruvic acid and different chemical reactions in the Krebs cycle starting from acetyl CoA]

Fig. 14.2 (labels in full):

• Inputs: Fat \u2192 \u03b2-oxidation \u2192 2 Acetyl CoA \u2192 Aceto acetyl CoA \u2192 Acetyl CoA. Proteins \u2192 Amino acids \u2192 Glycine \u2192 Acetyl CoA; Amino acids \u2194 Alanine \u2194 Pyruvic acid. Pyruvic acid \u2192 (Pyruvic dehydrogenase; NAD \u2192 NADH₂, CoA, CO₂) Acetyl CoA (2C).
• Acetyl CoA (2C) + oxaloacetic acid \u2192 (Citrate synthetase; H₂O, CoA) Citric acid or citrate (6C) \u2192 (Aconitase, Fe⁺⁺; H₂O) Cis-aconitic acid (6C) \u2192 (Aconitase, Fe⁺⁺; H₂O) Isocitric acid (6C) \u2192 (Isocitric dehydrogenase, Mn⁺⁺; NAD \u2192 NADH₂) Oxalo succinic acid (6C) \u2192 (Decarboxylase, Mn⁺⁺; CO₂) \u03b1-Ketoglutaric acid (5C) (\u2194 Glutamic acid) \u2192 (\u03b1-Ketoglutaric dehydrogenase, Mg; NAD \u2192 NADH₂, CO₂) Succinyl CoA (4C) \u2192 (Succinic thiokinase; H₃PO₄, GDP \u2192 GTP, ADP \u2192 ATP) Succinic acid (4C) \u2192 (Succinic dehydrogenase; FAD \u2192 FADH₂) Fumaric acid (4C) \u2192 (Fumarase; H₂O) Malic acid (4C) \u2192 (Malic dehydrogenase; NAD \u2192 NADH₂) Oxalo acetic acid (4C) (\u2194 Aspartic acid) \u2192 back to citrate synthetase step.
• Oxidation of NADH₂ and FADH₂ (1/2 O₂ \u2192 H₂O) feeds the central ATP POOL (numbers 3, 3, 3, 2, 1 show ATP molecules formed at the dehydrogenase steps).

The significance of the Krebs cycle is:
1. It is the major pathway for the formation of ATP molecules.
2. Many intermediate compounds of this cycle are used in the synthesis of other biomolecules.
3. The Krebs cycle is useful for the conversion of fats into carbohydrates through the glyoxylate cycle.
4. Chlorophyll is synthesized from succinyl CoA.

Check Point
Give a scientific term for the following:
(i) First organic acid formed
(ii) Alternative name of TCA cycle.
(iii) Kreb's cycle enzymes occur in:
(iv) Enzyme used in the conversion of succinic acid to fumaric acid.

Table 14.4: Differences between glycolysis and Krebs cycle

ELECTRON TRANSPORT SYSTEM (ETS)

The electron transport system is a series of coenzymes and cytochromes that takes part in the passage of electrons from a chemical to its ultimate acceptor. These electron carriers are arranged according to their increasing positive potential.

Electron transport takes place on the cristae of mitochondria [oxysomes (F0-F1 particles) found on the inner surface of the membrane of mitochondria].

The electron transmitter system is also called the electron transport chain (ETC), or the cytochrome system (CS), as five out of these nine carriers are cytochromes.

It is the major source of the cell's energy. In the respiratory breakdown of simple carbohydrates, intermediates like phosphoglyceraldehyde, pyruvic acid, isocitric acid, alpha-ketoglutaric acid, succinic acid and malic acid are oxidized. The oxidation in all these is brought about by the removal of a pair of hydrogen atoms (2H) from each of them. This final stage of respiration is carried out in the ETS, located in the inner membrane of mitochondria (in prokaryotes, the ETS is located in the mesosomes of the plasma membrane).

The system consists of a series of precisely arranged nine electron carriers (coenzymes) in the inner membrane of the mitochondrion, including the folds or cristae of this membrane. These nine electron carriers function in a specific sequence and are: nicotinamide adenine dinucleotide (NAD), flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), coenzyme Q or ubiquinone, cytochrome b, cytochrome c1, cytochrome c, cytochrome a and cytochrome a3.

The ETC is comprised of four complexes and two mobile carriers. Coenzyme Q is a non-protein part of the chain; all the other members of the chain are proteins; the second mobile carrier is cytochrome c.

Complex I: consists of flavoproteins of NADH dehydrogenase (FPN).
Complex II: consists of flavoproteins of succinic dehydrogenase.
Between complexes II and III is the mobile carrier coenzyme Q (CoQ) or ubiquinone (UQ).
Complex III: consists of cytochrome b and cytochrome c1. Associated with cytochrome b is non-haem iron of complex III (Fe-NHR).
Complex IV: consists of cytochrome a and cytochrome a3 and bound copper, which are required for this complex reaction to occur.

The electrons either follow the pathway of complexes I, III and IV, or II, III and IV. Electrons from NADH produced in the mitochondrial matrix during the citric acid cycle are oxidized by NADH dehydrogenase (complex I), and the electrons are then transferred to ubiquinone located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH₂ generated during the oxidation of succinate by succinate dehydrogenase (complex II).

The reduced ubiquinone, called ubiquinol, is then oxidized by transfer of electrons to cytochrome c, through the cytochrome bc1 complex (complex III). Cytochrome c acts as a mobile carrier between complex III and complex IV. Complex IV refers to the cytochrome c oxidase complex containing cytochromes a and a3 and two copper centres.

As the electrons pass from one carrier to another via complexes I to IV in the electron transport chain, they are coupled to ATP synthetase (complex V) for the formation of ATP from ADP and Pi.

Oxygen functions as the terminal acceptor of electrons and is reduced to water along with the hydrogen atoms. It drives the whole process by removing hydrogen from the system.

In respiration, the energy of oxidation-reduction is utilized for the production of a proton gradient. So the process is called oxidative phosphorylation. Higher proton concentration in the outer chamber causes the protons to pass inwardly into the matrix or inner chamber through the inner membrane.

[Figure: Fig. 14.3 Mitochondrial electron transport system (ETS)]
[Figure: Fig. 14.4 A simplified system of terminal oxidation and oxidative phosphorylation]

The energy of the proton gradient is used in attaching a phosphate radical to ADP by a high energy bond.

Oxidation of one molecule of NADH₂ produces 3 ATP molecules, while a similar oxidation of FADH₂ forms 2 ATP molecules.

ATP synthetase (complex V) helps in ATP synthesis. It consists of two major components, F1 and F0. F1 (head piece) is a peripheral membrane protein complex and contains the site for ATP synthesis, while F0 is an integral membrane protein complex that forms the channel through which protons cross the inner membrane. For each ATP produced, 2H⁺ pass through F0 from the intermembrane space to the matrix down the electrochemical proton gradient.

Connecting Concepts: Peter Mitchell (1961) proposed the chemiosmotic mechanism of ATP synthesis (Nobel Prize in 1978), which states that ATP synthesis occurs due to H⁺ flow through a membrane. It involves two steps: development of a proton gradient, and proton flow.

[Figure: Fig. 14.5 ATP synthesis by F0-F1 particle]

Net gain of ATP

Total net gain of ATP = 36 or 38, depending upon the type of aerobic respiration.

Role of shuttle system in energy production

Glycolysis occurs in the cytoplasm, outside the mitochondrion, and 2 NADH₂ molecules are produced. The ETC is located along the inner mitochondrial membrane, so the NADH₂ of glycolysis must enter inside the mitochondrion to release energy. But the inner mitochondrial membrane is impermeable to NADH₂.

In the mitochondrial membrane there are two shuttle systems, each formed of carrier molecules. These shuttle systems are the malate-aspartate system and the glycerol phosphate shuttle system.

Malate-aspartate shuttle: When this electron shuttle operates, electrons are transferred from NADH₂ in the cytoplasm to NAD⁺ inside the mitochondria. This is more efficient and results in the production of 38 ATP molecules.

Glycerol-phosphate shuttle: In this shuttle, electrons are transferred from NADH₂ in the cytoplasm to FAD inside the mitochondria. It results in the production of 36 ATP molecules. It is less efficient and results in the reduction of FAD inside the mitochondrion.

Which shuttle predominates depends on the particular species and tissues involved. For example, 38 ATP molecules are formed in kidney, heart and liver cells, while 36 ATP molecules are formed in muscle cells and nerve cells. In these cells the glycerol-phosphate shuttle is predominant, and 2 ATP is formed from NADH₂.

THE RESPIRATORY BALANCE SHEET

There is a sequential, orderly pathway functioning, with one substrate forming the next: glycolysis, the TCA cycle and the ETS pathway follow one after another.

The NADH synthesized in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.

Hence, there can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose (38 ATP molecules with the malate-aspartate shuttle).

The calculations of the net gain of ATP for every glucose molecule oxidized are made on certain assumptions.

Comparison of fermentation and aerobic respiration:
1. Fermentation accounts for only a partial breakdown of glucose, whereas in aerobic respiration it is completely degraded to CO₂ and H₂O.
2. 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.
3. NADH is oxidized to NAD⁺ more slowly in fermentation than in aerobic respiration.

Check Point
Fill in the blanks
(i) ____________ cycle is involved in amphibolic pathway.
(ii) _________ shuttle system produces 38 ATP molecules.
(iii) ________ synthesized in glycolysis is transferred into ____________ and undergoes oxidative phosphorylation.

AMPHIBOLIC PATHWAY

Respiration involves the breakdown of organic compounds, so it is considered a catabolic process. But different substances enter the pathway at different steps as respiratory substrates.

Fats would need to be broken down into glycerol and fatty acids first. Fatty acids are degraded to acetyl CoA and enter the pathway. Glycerol enters the pathway after being converted to PGAL (glyceraldehyde 3-phosphate).

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.

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 is called an amphibolic pathway.

[Figure: Fig. 14.6 Inter-relationship among metabolic pathways showing respiration-mediated breakdown of different organic molecules to CO₂ and H₂O]

RESPIRATORY RATIO OR RESPIRATORY QUOTIENT

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

RQ = Volume of CO₂ evolved / Volume of O₂ absorbed

It is useful in knowing the type of respiration, the major transformations and the respiratory substrate.

RQ depends on:
• the nature of the respiratory substrate.
• the amount of CO₂ present in the respiratory substrate.
• the extent to which the substrate is broken down.
• the interconversion of one substrate into another in the cell.

RQ values of different respiratory substrates:

1. When carbohydrates are the respiratory substrate (e.g., germinating wheat, oat, barley, paddy grains or green leaves kept in dark, or tubers, rhizomes, etc.), RQ = 6 / 6 = 1 (unity).
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

2. When fats are the respiratory substrate (e.g., germinating castor, mustard, linseed and til seeds), the R.Q. is generally less than one.
(i) Stearic acid: C₁₈H₃₆O₂ + 26O₂ → 18CO₂ + 18H₂O; RQ = 18 / 26 = 0.7 (less than unity)
(ii) Tripalmitin: 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O; RQ = 102 / 145 = 0.7 (less than unity)

3. When proteins are the respiratory substrate (e.g., germinating gram, pea, bean and mung seeds), the RQ value is less than unity (0.5 to 0.9). When proteins are the respiratory substrate, the ratio would be about 0.9.

4. When organic acids are the respiratory substrate:
(i) Malic acid: C₄H₆O₅ + 3O₂ → 4CO₂ + 3H₂O; RQ = 4 / 3 = 1.33 (more than unity)
(ii) Oxalic acid: 2(COOH)₂ + O₂ → 4CO₂ + 2H₂O; RQ = 4 / 1 = 4 (more than unity)
Some other organic acids and their R.Q. values are: succinic acid (1.14), tartaric acid (1.6) and acetic acid (1).

5. When there is incomplete oxidation of carbohydrates (in the respiration of succulents, i.e., Bryophyllum and Opuntia):
2C₆H₁₂O₆ + 3O₂ → 3C₄H₆O₅ + 3H₂O; RQ = 0 / 3 = 0 (zero)

6. Respiration in the absence of O₂ (anaerobic respiration):
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (zymase); RQ = 2 / 0 = infinity

RQ is determined by Ganong's respirometer. A respirometer is an instrument to measure the rate of respiration.

HEXOSE MONOPHOSPHATE PATHWAY (HMP)

The hexose monophosphate pathway is an alternative method of aerobic respiration which occurs in the cytoplasm of mature plant cells. It accounts for 60% of total respiration in liver cells and also occurs in adipose tissue and lactating mammary glands.

It is also known as the phosphogluconate pathway, the hexose monophosphate shunt, the Warburg-Dickens pathway or the direct oxidation pathway. It provides an alternative pathway for the breakdown of glucose which is independent of the EMP pathway (glycolysis) and the Krebs cycle. Its existence was suggested for the first time by Warburg et al. (1935) and Dickens (1938). Most of the reactions of this cycle were described by Horecker et al. (1951) and Racker (1954).

Pentose phosphate pathway (PPP) takes place in the cytoplasm and requires oxygen for its entire operation.

Two types of evidence support the existence of this alternative pathway. These are the inhibiting action of malonic acid on the Krebs cycle, and studies with radioactive (C¹⁴) carbon.

Twelve molecules of NADPH₂ formed in the reaction can be oxidized back to 12 NADP⁺ with the help of the cytochrome system and oxygen of the air:
12 NADPH₂ + 6O₂ → 12H₂O + 12NADP⁺
In this electron transfer process, 36 molecules of ATP are synthesized.

[Figure: Fig. 14.6 Hexose monophosphate shunt]

Hexose monophosphate shunt (figure): glucose → glucose-6-P (ATP → ADP) → 6-phosphogluconic acid (NADP⁺ → NADPH+H⁺) → ribulose-5-P (NADP⁺ → NADPH+H⁺, CO₂ released); ribulose-5-P ⇄ xylulose-5-P and ribose-5-P; these interconvert via glyceraldehyde-3-P, sedoheptulose-7-P, fructose-6-P, erythrose-4-P, xylulose-5-P, and fructose-1,6-diP / dihydroxyacetone-P, returning to glucose-6-P.

Significance of PPP
1. It is the only pathway of carbohydrate oxidation that gives NADPH₂, which is needed for the synthesis of fatty acids (in adipose tissues) and amino acids (in the liver).
2. It synthesizes 3-carbon glyceraldehyde-3-P and dihydroxyacetone phosphate, 4-carbon erythrose-4-P, 5-carbon ribulose phosphate, xylulose phosphate and ribose phosphate, 6-carbon fructose 6-phosphate, and 7-carbon sedoheptulose-7-phosphate.
3. It is the major pathway by which the necessary ribose and deoxyribose are supplied for the biosynthesis of nucleotides and nucleic acids.
4. Erythrose-4-phosphate is used to synthesize lignin, auxin, anthocyanin and aromatic amino acids (phenylalanine, tyrosine and tryptophan).
5. Young growing tissues appear to use the Krebs cycle as the predominant pathway for glucose oxidation, while aerial parts of the plant and other tissues seem to use the PPP as well as the Krebs cycle.
6. It gives 6 CO₂ required for photosynthesis.
7. Ribulose-5-phosphate is used in photosynthesis to produce RuBP, which acts as the primary CO₂ acceptor in the C₃ cycle.
8. In this, for every 6 molecules of glucose, one molecule is completely oxidized into CO₂ and reduced coenzymes, while 5 are regenerated.

Connecting Concepts: The respiration that continues in the presence of cyanide (as in plants) is called cyanide resistant respiration. Respiration can continue in plants even when cytochrome oxidase is blocked, because such mitochondria have an alternative route allowing transport of electrons from ubiquinone to a flavoprotein, bypassing the oxidase.

Check Point
State true or false.
(i) Hexose monophosphate pathway is an alternative method of anaerobic respiration.
(ii) HMP is also known as pentose phosphate pathway.
(iii) HMP provides an alternative pathway for breakdown of glucose which is independent of glycolysis and Kreb's cycle.
(iv) HMP gives 8CO₂, required for photosynthesis.
(v) PPP occurs in cytoplasm of mature plant cells.
(vi) In HMP, for every 7 molecules of glucose, one molecule is completely oxidized into CO₂ and reduced coenzymes, while 6 are regenerated.

ANAEROBIC RESPIRATION

Anaerobic respiration was first studied by Kostychev (1902). It is an enzyme controlled, partial breakdown of organic compounds without using oxygen, and it releases only a fraction of the energy. It is also called intramolecular respiration (Pfluger, 1875).

It is a temporary process in higher plants and a permanent process in micro-organisms like bacteria and yeast.

In micro-organisms, anaerobic respiration is often called fermentation (Cruickshank, 1897). Fermentation, or anaerobic respiration, is completed in the cytoplasm.

Higher organisms like plants cannot perform anaerobic respiration for long. It is toxic, because the accumulation of end products and the insufficient amount of available energy stop many active processes.

In this process, pyruvate (formed by glycolysis) is metabolized into ethyl alcohol or lactic acid and CO₂ in the absence of oxygen. Glycolysis occurs in the cytoplasm, so the site of anaerobic respiration is the cytoplasm.

Reaction: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 52 kcal (218.4 kJ)

[Figure: Fig. 14.7 Summary of anaerobic respiration pathways]

Fig. 14.7: Glucose (C₆H₁₂O₆) + 2ADP + 2Pi → 2 × pyruvic acid (CH₃COCOOH) with a net gain of 2 ATP; in the presence of oxygen pyruvic acid goes to the Krebs cycle. In absence of oxygen (anaerobic respiration): in animals, pyruvic acid is reduced to lactic acid (2 × CH₃CHOHCOOH) with NADH+H⁺ → NAD⁺; in plants, pyruvic acid gives CO₂ and acetaldehyde (2 × CH₃CHO), which by alcoholic fermentation (NADH+H⁺ → NAD⁺) gives ethanol (2 × CH₃CH₂OH).

Fermentation

In fermentation, the incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions in which pyruvic acid is converted to CO₂ and ethanol. The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions. Other organisms, like some bacteria, produce lactic acid from pyruvic acid.

In animal cells, such as 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 re-oxidized to NAD⁺ in both processes.

Different types of fermentation or anaerobic respiration are:
(a) Alcoholic fermentation: It occurs in some fungi (yeast) and higher plants under anaerobic conditions. It is the most common type of fermentation.
(i) 2CH₃COCOOH → 2CH₃CHO + 2CO₂ (pyruvic acid decarboxylase; pyruvic acid to acetaldehyde)
(ii) 2CH₃CHO + 2NADH₂ → 2C₂H₅OH + 2NAD⁺ (alcohol dehydrogenase; acetaldehyde to ethyl alcohol)

(b) Lactic acid fermentation: It occurs in lactic acid bacteria (Lactobacillus) and in human muscles. Pyruvic acid produced in glycolysis is reduced by NADH₂ to form lactic acid, without producing carbon dioxide.
CH₃COCOOH + NADH₂ → CH₃CHOHCOOH + NAD⁺ (lactate dehydrogenase; FMN, Zn²⁺)
Pyruvic acid → Lactic acid

In both fermentation processes, less energy is released.

FACTORS AFFECTING THE RATE OF RESPIRATION

Temperature: With every 10°C rise of temperature from 0°C to 30°C, the rate of respiration increases 2 to 2.5 times (i.e., temperature coefficient Q₁₀ = 2 to 2.5), following Van't Hoff's law. The maximum rate of respiration occurs at 30°C. There is an initial rise, soon followed by a decline. The higher the temperature above this limit, the greater the initial rise, but also the greater and earlier the decline in the rate of respiration. This is probably due to the denaturation of enzymes at high temperature.

Supply of oxidisable food: An increase in the soluble food content, readily available for utilization as respiratory substrate, generally leads to an increase in the rate of respiration, up to a certain point when some other factor becomes limiting.

Oxygen concentration of the atmosphere: When the amount of oxygen in the environment of plants is increased or reduced up to quite low values, the rate of respiration is not affected. On decreasing the amount of oxygen to 1.9% in the environment, aerobic respiration becomes negligible (extinction point of aerobic respiration), but anaerobic respiration takes place.

Oxygen poisoning: A significant fall in the respiration rate was observed in many tissues in pure O₂, even at N.T.P. This inhibiting effect was also observed in green peas when they are exposed to pure oxygen at a pressure of 5 atm; the respiration rate falls rapidly.

Water: With an increase in the amount of water, the rate of respiration increases. In dry seeds, which have 8 to 12% water, the rate of respiration is very low, but as the seeds imbibe water, respiration increases, as water is necessary for the activity of enzymes.

Light: Respiration takes place at night also, which shows that light is not essential for respiration. However, light affects the rate of respiration indirectly by increasing the rate of photosynthesis, due to which the concentration of respiratory substrates is increased. The more respiratory substrate, the higher the rate of respiration.

Carbon dioxide (CO₂): If the amount of CO₂ in the air is more than the usual, the rate of respiration decreases. Germination of seeds is reduced and the rate of growth falls. Heath (1950) has shown that stomata are closed at higher concentrations of CO₂, due to which oxygen does not penetrate the leaf and the rate of respiration is lowered.

Inorganic salts: The chlorides of the alkali cations Na and K, as also the divalent cations of Li, Co and Mg, generally increase the rate of respiration as measured by the amount of CO₂ evolved. Monovalent chlorides of K and Na increase the rate of respiration, while divalent chlorides of Li, Ca and Mg cause less increase in respiration.

Injury and mechanical stimulation: Wounding or injury almost invariably results in an increase in the rate of respiration.

Effect of various chemical substances: Certain enzyme inhibitors, like cyanides, azides, carbon monoxide, iodoacetate and malonate, reduce the rate of respiration even when present in very low concentrations. However, various chemical substances such as chloroform, ether, acetone and morphine increase respiratory activity.

Pollutants: High concentrations of gaseous air pollutants like SO₂, NO_x and O₃ inhibit respiration by damaging the cell membrane. These gaseous pollutants cause an increase in pH, which in turn affects the electron transport system, thus inhibiting respiration. Heavy metal pollutants like lead (Pb) and cadmium (Cd) inhibit respiration by inactivating respiratory enzymes.

Protoplasm: The meristematic cells (dividing cells of the root and shoot apex) have more protoplasm than mature cells. Hence, meristematic cells have a higher rate of respiration than mature cells.

Respiratory substrate: With an increase in the amount of respiratory substrate, the rate of respiration increases.