Solution-Class-11-Science-Biology-Chapter-13-Respiration and Energy Transfer-Maharashtra Board

(c) mitochondria

(d) 38 molecules of ATP

(b) Succinyl CoA → succinate

Pyruvic acid

2

Cytoplasm

ATP

2

• In absence of oxygen anaerobic respiration takes place in muscles of man.
• In absence of oxygen anaerobic respiration takes place in cytoplasm of the yeast.

• In anaerobic respiration, incomplete oxidation takes place which leads to release of less amount of energy.
• Some of the products of anaerobic respiration can be oxidised further to release energy which shows that anaerobic respiration does not liberate the whole energy contained in the respiratory substrate.
• NADH₂ does not produce ATP, as electron transport is absent.
• Only 2 ATP molecules are generated from one molecule of glucose during anaerobic respiration.

The respiration electron transport system is located in the inner mitochondrial membrane

Molecular oxygen is the terminal electron acceptor in aerobic respiration.

Ratio of volume of CO₂ released to the volume of O₂ consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio. It depends on the type of respiratory substrate.

R.Q. =

Fats (R.Q. is less than 1):

Substrates like fats are poorer in oxygen than carbohydrates. Thus, more oxygen is utilized for its complete oxidation.

2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + Energy

R.Q. = CO₂/O₂ = 102/145 = 0.7

RQ value for fats is 0.7

Respiratory substrates are the molecules that are oxidized during respiration to release energy which can be used for ATP synthesis. Carbohydrates, fats and proteins are the common respiratory substrate.

Glycolysis :

• Glycolysis is also known as EMP pathway.
• Glycolysis is a process where glucose is broken down into two molecules of pyruvic acid, hence called glycolysis (glucose-breaking).
• It is common to both aerobic and anaerobic respiration.
• It occurs in the cytoplasm of the cell. It involves ten steps.

Glycolysis consists of two major phases: (i) Preparatory phase (1-5 steps). (ii) Payoff phase (6-10 steps).

(i) Preparatory phase:

• In this phase, glucose is phosphorylatecl twice by using two ATP molecules and a molecule of fructose 1,6-bisphosphate is formed.

• It is then cleaved into two molecules of glyceraldehyde-3-phosphate and dihydroxy acetone phosphate. These two molecules are 3-carbon carbohydrates (trioses) and are isomers of each other.
• Dihydroxy acetone phosphate is isomerised to second molecule of glyceraldehyde-3-phosphate.
• Therefore, two molecules of glyceraldehyde-3- phosphate are formed.
• Preparatory phase of glycolysis ends.

(ii) Payoff phase:

• In this phase, both molecules of glyceraldehyde-3-phosphate are converted to two molecules of 1,3- bisphoglycerate by oxidation and phosphorylation. Here, the phosphorylation is brought about by inorganic phosphate instead of ATP.
• Both molecules of 1,3-bisphosphoglycerate are converted into two molecules of pyruvic acid through series of reactions accompanied with release of energy. This released energy is used to produce ATP (4 molecules) by substrate-level phosphorylation.

Fermentation by yeast (alcoholic fermentation) :

• Yeast shows both aerobic and anaerobic respiration depending upon the presence or absence of oxygen.
• In absence of oxygen anaerobic respiration takes place in cytoplasm of the yeast.
• In absence of oxygen, the pyruvate undergoes anaerobic respiration where it is decarboxylated to acetaldehyde. The acetaldehyde is then reduced by NADH+H⁺ to ethanol and carbon dioxide. This type of anaerobic respiration is termed alcoholic fermentation.
• Accumulation of ethanol by fermentation in a culture of yeast may stop further multiplication and lead to the death of cells..
• In the presence of oxygen however, it can respire aerobically to produce CO₂ and H₂O.

Electron Transport chain (Electron transfer system) :

• Wherever the NADH₂ (NADH+H⁺) and FADH₂ are produced during glycolysis, connecting link reaction and Krebs cycle, they are oxidised with the help of various electron carriers and enzymes.
• These carriers and enzymes are arranged on inner mitochondrial membrane in the form of various complexes as complex I, II, III, VI and V.
• NADH+H⁺ is oxidised by NADH dehydrogenase (complex I) and it's electrons are transferred to ubiquinone (coenzyme Q-CoQ) present on inner membrane of mitochondria. Reduced ubiquinone is called as ubiqunol.
• FADH₂ is oxidised by complex II (Succinate dehydrogenase) and these electrons are also transferred to CoQ.
• During oxidation of NADH+H⁺ and FADH₂, electrons and protons are released but only electrons are carried forward whereas protons are released into outer chamber of mitochondria (intermembrane space).
• Ubiquinol is oxidised by complex-III (Cytochrome bc 1 complex) and it's electrons are transferred to cytochrome C. Cytochrome C is a small, iron-containing protein, loosely associated with inner membrane. It acts as a mobile electron carrier, transferring the electrons between complex III and IV.
• Cytochrome C is oxidised by complex IV or cytochrome C oxidase consisting of cytochrome a and a₃. Electrons are transferred by this complex to the molecular oxygen. This is terminal oxidation.
• Reduced molecular oxygen reacts with protons to form water molecule called as metabolic water.
• Protons necessary for this are channelled from outer chamber of mitochondria into inner chamber by F₀ part of oxysome (complex V) present in inner mitochondrial membrane. This proton channelling by F₀ is coupled to catalytic site of F₁ which catalyses the synthesis of ATP from ADP and inorganic phosphate. This is oxidative phosphorylation. As transfer of protons is accompanied with synthesis of ATP, this process is named as ‘Chemiosmosis' by Peter Mitchell.

Significance of ETS :

• The electron transport system (ETS) or terminal oxidation generates major amount of energy in the form of ATP molecules, 34 ATP molecules out of total 38 ATP molecules are produced through ETS.
• It regenerates oxidized coenzymes such as NAD⁺ and FAD⁺ from their reduced forms (NADH+H⁺ and FADH²) for recycling.
• It also produces water molecules.
• It releases energy in a stepwise manner to prevent damage of cells.

Glycolysis, TCA cycle and electron transport chain link :

• The coenzymes are initially present in the form of NAD⁺ and FAD+ which latter get reduced to NADH+H⁺ and FADH+H⁺ by accepting the hydrogen from organic substrate during glycolysis, link reaction and Kerbs cycle.
• During glycolysis, glucose is oxidised to two molecules of pyruvic acid with net gain 2 molecules of NADH+H⁺.
• This pyruvic acid undergoes link reaction to form two molecules of acetyl CoA and two molecules of NADH+H⁺.
• Acetyl CoA, thus formed enters into the Krebs cycle and it gets completely oxidised to CO₂ and H₂O with a net gain of 6 NADH+H⁺ and 2 FADH+H⁺ are formed.
• During ETS, reduced coenzymes are reoxidized to NAD⁺ and FAD⁺ with a net gain of 34 ATPs.
• The ATPs thus formed are used during glycolysis.
• The oxidized NAD⁺ and FAD⁺ will again accept the hydrogen from organic substrate. Thus, reduced coenzymes are converted back to their oxidized forms by dehydrogenation to keep the process going.

(1) Anaerobic respiration in yeast:

• A pinch of dry baker’s yeast suspended in water containing 10ml of 10% glucose in a test tube (test tube A).
• The surface of the liquid is covered with oil to prevent entry of air and the test tube is closed tightly with rubber stopper to prevent leakage.
• One end of a short-bent glass tube is inserted through it to reach the air inside the tube.
• Other end of the glass tube is connected by a polyethylene or rubber tubing to another bent glass tube fitted into a stopper.
• The open end of the glass tube (delivery tube) is dipped into lime water containing in a test tube (Tube B).

• Stoppers of both the tubes are fitted tightly to prevent leakage of gases. First test tube is placed in warm water (37° C-38° C) in a beaker.
• Lime water gradually turns milky, indicating the evolution of carbon dioxide from the yeast preparation.
• Level of the lime water in the delivery tube does not rise, showing that there is no decline in volume of gas in test tube A and consequently no utilization of oxygen by yeast.
• Preparation is stored for a day or two.
• When we open the stopper of tube A we will notice a smell of alcohol indicating the formation of ethanol.
• From this activity it may be inferred that yeast respires anaerobically to ferment glucose to ethanol and carbon dioxide.

(2) Aerobic respiration in yeast: Experiment explained above can be carried out for demonstrating aerobic respiration in yeast.

• If the level of the lime water in the test tube B rises, indicating intake of oxygen, hence the level of volume of gas rises.
• The preparation tube is stored for a day or two, if no smell of alcohol is noticed it indicates that the yeast respires aerobically.

• In ETS energy is released in step wise manner to prevent damage of cells.
• A stepwise release of the chemical bond energy facilitates the utilization of a relatively higher proportion of that energy m ATP synthesis.
• Activities of enzymes for the different steps may be enhanced or inhibited by specific compounds. This provides a means of controlling the rate of the pathway and the energy output according to need of the cell.
• The same pathway may be utilized for forming intermediates used in the synthesis of other biomolecules like amino acids.

Electron Transport chain (Electron transfer system) :

• Wherever the NADH₂ (NADH+H⁺) and FADH₂ are produced during glycolysis, connecting link reaction and Krebs cycle, they are oxidised with the help of various electron carriers and enzymes.
• These carriers and enzymes are arranged on inner mitochondrial membrane in the form of various complexes as complex I, II, III, VI and V.
• NADH+H⁺ is oxidised by NADH dehydrogenase (complex I) and it's electrons are transferred to ubiquinone (coenzyme Q-CoQ) present on inner membrane of mitochondria. Reduced ubiquinone is called as ubiqunol.
• FADH₂ is oxidised by complex II (Succinate dehydrogenase) and these electrons are also transferred to CoQ.
• During oxidation of NADH+H⁺ and FADH₂, electrons and protons are released but only electrons are carried forward whereas protons are released into outer chamber of mitochondria (intermembrane space).
• Ubiquinol is oxidised by complex-III (Cytochrome bc 1 complex) and it's electrons are transferred to cytochrome C. Cytochrome C is a small, iron-containing protein, loosely associated with inner membrane. It acts as a mobile electron carrier, transferring the electrons between complex III and IV.
• Cytochrome C is oxidised by complex IV or cytochrome C oxidase consisting of cytochrome a and a₃. Electrons are transferred by this complex to the molecular oxygen. This is terminal oxidation.
• Reduced molecular oxygen reacts with protons to form water molecule called as metabolic water.
• Protons necessary for this are channelled from outer chamber of mitochondria into inner chamber by F₀ part of oxysome (complex V) present in inner mitochondrial membrane. This proton channelling by F₀ is coupled to catalytic site of F₁ which catalyses the synthesis of ATP from ADP and inorganic phosphate. This is oxidative phosphorylation. As transfer of protons is accompanied with synthesis of ATP, this process is named as ‘Chemiosmosis' by Peter Mitchell.

• Respiration is considered as a catabolic process; however, it is not entirely correct in case of Krebs cycle.
• Many reactions of Krebs cycle involve oxidation of acetyl CoA to release energy and CO₂.
• However, the breakdown of respiratory substrates provides intermediates like α-ketoglutarate, oxaloacetate are used as precursors for synthesis of fatty acids, glutamic acid and aspartic acid respectively.
• Thus, as the same respiratory process acts as catabolic as well as anabolic pathway for synthesis of various intermediate metabolic products, it is called amphibolic pathway.

• Many reactions of Krebs cycle involve oxidation of acetyl CoA to release energy and CO₂.
• However, the breakdown of respiratory substrates provides intermediates like α-ketoglutarate, oxaloacetate are used as precursors for synthesis of fatty acids, glutamic acid and aspartic acid respectively.
• Thus, as the same respiratory process acts as catabolic as well as anabolic pathway for synthesis of various intermediate metabolic products, it is called amphibolic pathway.

(a) Glycolysis

Krebs Cycle ( TCA cycle/ Citric Acid Cycle):

• Krebs cycle or citric acid cycle is the second phase of aerobic respiration which takes place in the matrix of the mitochondria.
• The acetyl CoA formed during the link reaction undergoes aerobic oxidation. .
• This cycle serves a common oxidative pathway for carbohydrates, fats and proteins.
• In mitochondria pyruvic acid is decarboxylated and the remaining 2-carbon fragment is combined with a molecule of coenzyme A to form acetyl-CoA.
• This reaction is an oxidative decarboxylation process and produces H* ions and electrons along with carbon dioxide.
• During the process NAD⁺ is reduced to NADH+H⁺.
• B-oxidation of fatty acids also produces acetyl-CoA as the end product.
• Acetyl-CoA from both sources is condensed with oxaloacetic acid to form citric acid. Citric acid is oxidized step-wise by mitochondrial enzymes, evolving carbon dioxide.
• Regeneration of oxaloacetic acid occurs to complete the cycle.
• There are four steps of oxidation in this cycle, catalyzed by dehydrogenases (oxidoreductases) using NAD⁺ or FAD⁺ as the coenzyme.
• The coenzymes are consequently reduced to NADH+H⁺ and FADH₂ respectively. These transfer their electrons to the mitochondrial respiratory chain to get reoxidised.
• One molecule of GTP (ATP) is also produced for every molecule of citric acid oxidized.