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Respiration in Plants

NCERT Textual Exercise (Solved)

1. Differentiate between respiration and combustion. Ans. Respiration Combustion (1) The breakdown of C–C bond (1) Conversion of total complex

of complex compounds inside compounds into energy (heat) the cells leading to release of is called combustion. considerable amount of energy is respiration.

(2) Enzymesareusedindifferent(2) Enzymesareabsent. steps.

(3) Energystoredintheform (3) Enzymesreleasedintheform of ATP. of heat.

2. What are respiratory substrates? Name the most common respiratory substrate.

Ans. The compounds that are oxidised during this process are known as respiratory substrates. Usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used as respiratory substances in some plants, under certain conditions.

3. Give the schematic representation of glycolysis. Ans. The term glycolysis has originated from Greek words, glycos of sugar and

cysis for splitting, the scheme of glycolysis was given by G. Embden, O. Meyerhof of and J. Parnas. 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 glycolysis, glucose undergoes partial oxidation to form two molecules of pyruvic acid.

4. What are the main steps in aerobic respiration? Where does it take place? Ans. v Pyruvic acid, generated is cytosol, is transported to mitochondria. v BeforepyruvicacidenterstheKrebs’cycle,itisfirstdecarboxylated

andthenoxidisedbyenzymepyruvatedehydrogenase(oxidativedecarboxylation).

v The remaining 2-C acetate unit is then readily accepted by a sulphur -containingcompound,coenzyme-A(CoA)toformacetylCoA.Thisis the connecting link between glycolysis and Krebs’ cycle.

v NAD+ is reduced to NADH. Pyruvic acid + CO2 acetyl CoA + CO2

v 2 molecules of NADH are formed from 2 molecules of pyruvic acid. So, a net gain of 6ATP occurs.

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5. Give the schematic representation of an overall view of Krebs’ cycle. Ans. v This cycle was elucidated by the British biochemist, Hans

Krebs in 1937, hence known as Krebs’ cycle. Also known as tricarboxylic acid cycle (TCA cycle).

v It occurs in mitochondrial matrix. v One molecule of acetyl CoA combines with 4-C oxalo acetic

acid (OAA), to form6-Ccitricacidby theenzymecitrate synthase .

v Citrate is then isomerised to isocitrate. v Then it is decarboxylated to a-ketoglutaric acid, which is then

decarboxylated to succinyl CoA. v Succinyl CoA is oxidised to oxalo acetic acid, allowing the

cycle to continue. v During this cycle, 3 molecules of NAD+ and one molecule of

FAD are reduced to produce NADH and FADH2, respectively. v These reduced electron carriers pass on hydrogen atoms to

oxygen through electron transport system, yielding 11 more molecules for each molecule of pyruvic acid.

v One more ATP molecule is generated directly to give rise a total of 12 molecules per pyruvic acid molecule (3C). So, from two molecules of pyruvic acid, a total of 24 molecules of ATP are formed.

v The summary equation for this phase of respiration is

Pyruvic acid + 4NAD+ + FAD + 2H2O + ADP + Pi mitochondrialmatrix → 3CO2 +

4NADH + 4H+ + FADH2 + ATP

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KREBS’ CYCLE

The citric acid cycle 6. Explain Electron Transport System (ETS). Ans. v The glucose molecule is completely oxidised by the end of citric

acid cycle. v But energy is released only when NADH and FADH2 are oxidised

through electron transport system. v The metabolic pathway through which the electrons passes from

one carrier to another is called the electron transport system (ETS). v It operates in inner mitochondrial membrane. v Electrons from NADH are oxidised by an NADH dehydrogenase

(complex I) and electrons are then transferred to ubiquinone located within the inner membrane.

v Ubiquinone also receives reducing equivalents via FADH2 through theactivityoftheenzyme,succinatedehydrogenase(complexII),in the citric acid cycle.

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v The reduced ubiquinone (ubiquinol ) i s then o x i d i s e d w i t h t h e transfer of electrons to cytochrome c via cytochrome by complex (complex III).

v Cytochrome c acts as a mobile carrier for transfer of electrons between complexes III and IV.

v Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3 and two copper centres.

v When the electrons pass form one carrier to another via complexes I to IV in the electron transport chain, they are coupled to ATP synthetase (complex V) for the production of ATP from ADP and inorganic phosphate.

Fig. 4.4 Electron Transport System (ETS)

NADH

+ H+

NAD+

2H+

2H+

HO2

2H+

2H+

2H+

2H+

½O2

2H

2H

FMN

OH2

OH2

2e

FeS FeS

FeS

2e–

2e–

2e–

2e–

2e–

O

Cy b

2e-

Cy c Cy a-a3

Cy

Inter-membranespace

Inner mitochondrialmembrane

Matrix

Electron Transport System (ETS)

v 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.

v During electron transfer, the hydrogen atoms split into proton and electron.

v The electron are carried by the cytochromes. v They recombinewith their protons before thefinal stage,when

hydrogen atoms is accepted by oxygen to form water. v The process by which oxygen effectively allows the production of

ATP by phosphorylation of ADP, is called oxidative phosphorylation. v Complex V consists of two major components F1 and F0.

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v F1 headpiece is a peripheral membrane protein complex and contain the site for synthesis of ATP from ADP and inorganic phosphate.

v F0 is an integral membrane protein complex that forms the channel through which protons cross inner membrane.

7. Distinguish between the following: (a) Aerobic respiration and anaerobic respiration. (b) Glycolysis and fermentation. (c) Glycolysis and citric acid cycle. Ans. (a) Aerobic Respiration Anaerobic Respiration 1. It occurs in majority of 1. It occurs in a few organism

organism (animals and (yeasts and some bacteria and plants). parasitic worms) some can resort to it temporarily.

2. It involves changes of gases 2. Exchange of gases does not between organism and occur. environment.

3. It uses O2. 3. It does not use O2. 4. It always releases CO2. 4. It may or may not release CO2. 5. It releases 686 kcal or 5. It produces 36 – 50 kcal

2870 kJ of energy per (150 – 210 kJ) of energy per mole of glucose. mole of glucose.

6. It occurs partly in cytoplasm 6. Mitochondria are not required and partly inside mitochondria. for this kind of respiration.

(b) Glycolysis Fermentation 1. It represents the initial 1. It represents mode of

non-oxygen requiring step of respiration. respiration.

2. Glycolysis is common to 2. Fermentation is anaerobic both aerobic and anaerobic mode of respiration. modes of respiration.

3. It occurs in each and every 3. It is restricted to microbes and cell of all organism. some cells of higher organism.

4. It breaks down glucose to 4. It breaks down pyruvic acid or pyruvic acid. glucose into lactic acid, ethyl alcohol, butyric alcohol, CO2 etc.

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5. It produces 2 NADH and 5. It does not produce NADH or 2ATP. ATP. Rather it may consume NADH produced in glycolysis.

(c) Glycolysis Citric Acid Cycle 1. Itisthefirststepofcellular 1. Itisthesecondstepofcellular

respiration and its substrate is respiration and its substrate is glucose. acetyl CoA.

2. Glucose in broken down 2. Activated acetate of acetyl incompletely to form two CoA is completely broken molecules of pyruvate. down to inorganic state.

3. Glycolysis occurs in 3. It generally occurs inside cytoplasm. mitochondria (exception aerobic prokaryotes).

4. It is a linear or straight 4. It is cyclic pathway. pathway.

5. It uses two molecules of ATP. 5. It does not consume any ATP molecules.

6. 8 molecules of ATP in all 6. 24 molecules of ATP, in all are are formed per one molecule produced per one molecule of glucose. of glucose.

8. What are the assumptions made during the calculation of net gain of ATP ? Ans. Net Gain of ATP: Stage of Respiration Source No. of ATP

Molecules Produced Glycolysis Direct 2 2 molecules of NADH (one 6 molecule of NADH yields 3 molecules of ATP) Pyruvic acid to acetyl 2 molecules of NADH 6 CoA citric acid cycle 6 NADH 18 2 FADH2 (FADH2 produces 4 only 2 molecules of ATP) Direct 2 Total yield of ATP molecules 38 In most eukaryotic cells, 2 molecules of ATP are required for transporting

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NADH produced in glycolysis to mitochondria for further oxidation. Hence, net gain of ATP is 36 molecules.

The calculations of net gain of ATP for every glucose molecule oxidised is made on certain assumptions that are as follows:

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

v The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxydative phosphorylation.

v None of the intermediates in the pathway are utilised to synthesise any other compound.

v Only glucose is being respired no other alternative substrate are entering in the pathway at any of the intermediary stages.

v But this kind of assumptions are not valid in living systems as all pathways work simultaneously.

v During aerobic respiration of one mol of glucose, there is net gain of 36 ATP molecules.

v In fermentation, there is a net gain of only two molecules of ATP of each molecule of glucose degraded to pyruvic acid where as many more molecules of ATP are generated under aerobic condition.

v NADH is oxidised to NAD rather slowly in fermentation, however the reaction is very vigorous in case of aerobic respiration.

9. Discuss ‘The respiratory pathway is an amphibolic pathway’. Ans. v Glucose is the favoured substrate for respiration. All carbohydrates

are usually first converted into glucose before they are used respiration, other substrate do not enter the respiratory pathway at firststep.

v Fatwouldneedtobrokendownintoglycerolandfattyacidfirst. v Fatty acids degraded to acetyl CoA and enter the pathway. v Glycerol enters pathway after being converted to PGAL. v Proteins degraded to amino acids that after de-amination enters the

pathway as pyruvate or acetyl CoA.

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Interrelationship among metabolic pathways showing respiration -mediated breakdown of different organic-molecules to CO2 and H2O

v Because many substage breakdown to enter the pathway it is called catabolism.

v Many compounds are also withdrawn from respiratory pathway for the synthesis substrates (e.g., acetyl CoA is withdrawn from pathway to synthesise fatty acid when needed). This is called anabolism.

v Because respiratory pathway is involved in both anabolism and catabolism, it is called as an amphibolic pathway.

10. DefineR.Q.Whatisitsvalueforfats? Ans. v The ratio of the volume of CO2 evolved to the volume of

O2 consumed in respiration is called respiratory quotient (R.Q.) or respiratory ratio.

R.Q. = volumeof CO evolvedvolumeof O consumed

2

2

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v R.Q. is different for different substrates. v Carbohydrates: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy as equal amounts of CO2 and O2 are evolved and consumed

respectively, the R.Q. will be 1.

R.Q. = 66

2

2

COO

= 1

v Fats: 2(C51H98O6) + 145O2 → 102CO2 + 98H2O + energy (tripalmitic acid)

R.Q. = 102145

2

2

COO = 0.7

As R.Q. of fats is less than 1, so they (fats) require relatively greater amount of O2 for oxidation.

v Organic acids: 2(COOH)2 + O2 → 4CO2 + 2H2O + energy (oxalic acid)

R.Q. = 4 2

2

COO

= 4

As R.Q. is 4 (more than 1), therefore, relatively less amount of oxygen is required for oxidation.

v In anaerobic respiration, as CO2 is evolved but no O2 is used, the R.Q.isinfinity.

C6H12O6 zymase → 2C2H5OH + 2CO2 + energy

R.Q. = 20

2

2

COO = ∞(infinity)

11. What is oxidative phosphorylation? Ans. The process of respiration in which synthesis of ATP occurs by utilising

the release during transfer of H+ and electrons through and series of H and electron acceptors is known as oxidative phosphorylation.

During respiration, NADH2 gives 3 molecules of ATP, whereas FADH2 produces 2 molecules of ATP which can be shown as follows.

State of Respiration Source No. of ATP Produced

1. Glycolysis Direct-2 ATP 2 2–NADH 6 2. Pyruvic acid of acetyl CoA 2–NADH 6

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3. Citric acid cycle 6 NADH 18 2 FADH 4 Direct - 2 2 38

12. Whatisthesignificanceofstep-wisereleaseofenergyinrespiration? Ans. Step-wisereleaseofenergyismostsignificantintheuseofdifferentcell

and facilitates next reaction.

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b/w øãç ø Ù ò€¦ · 2. Glycolysis is common to 2. Fermentation is anaerobic both aerobic and anaerobic mode of respiration. modes of respiration. 3. It occurs in each and