UNIT – IV : PLANT PHYSIOLOGY CHAPTER 14 : RESPIRATION IN PLANTS. Exchange gases; Cellular respiration-glycolysis, fermentation (anaerobic), TCA cycle and electron transport system (aerobic); Energy relations-Number of ATP molecules generated; Amphibolic pathways; Respiratory quotient.

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CHAPTER14:RESPIRATIONINPLANTS.
1. Introduction:
Respirationin Plants
“Plant respiration is the chemical reaction by which plants cells stay alive.”
Do Plants Breathe?
The answer to this question is not direct. Yes, plants need oxygen for respiration but at the same time they also
give out carbon dioxide. Thus, plants have proper system to ensure the availability of oxygen. Unlike animals,
plants do not possess any specialized organs for exchange of gases but they have lenticels and stomata
(present in stems and leaves respectively) that carry out the function of gaseous exchange.
Plants do not have any specialized organ to respire and exchange gases because each part of the plant tak es
care of the need of gases themselves.
Glucose + Oxygen → Carbon Dioxide + Water (+ Energy)
CellularRespiration
 It isthe processof oxidation/breakdownof foodmaterialswithinthe cell torelease energy.
 Respiratorysubstrate tobe oxidized duringrespirationisusuallyglucose,butthese canalsobe proteins,fatsor
organicacids.
 In plantsrespirationgasexchange occursthroughstomataandlenticels.
Overall cellularrespirationis:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy(36 ATPs)
2.Types of respiration:
Respiration is of two types
A. Aerobicrespiration.
B. Anaerobic respiration

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A. Aerobicrespiration.
In this type of respiration, the food substances are completely oxidized into H2O and CO2 with the release of
energy. It requires atmospheric oxygen and all higher organisms respire aerobically. Following figure shows
the steps included in Aerobic Respiration.
A. Anaerobic respiration
In this type of respiration, partial oxidation of food takes place and energy is released in the absence of
oxygen. This type of respiration occurs in prokaryotic organisms like bacteria and yeast. Ethyl alcohol and
carbon dioxide are formed in this process.
Mechanismof respiration:

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Fermentation:
Incomplete oxidationof pyruvic acid,underanaerobicrespirationformslacticacid/ethyl alcohol. Itoccursin
bacteria,yeastandstriatedmuscles.
In yeast fermentation: Pyruvicacid→ Ethanol + CO2
o Enzymesinvolved−Pyruvicaciddecarboxylase,Alcohol dehydrogenas.
Only7% of energyof glucose isreleasedduringfermentation.
Yeastspoisonthemselvestodeathwhenalcohol concentrationreachesabout13%.
In bacterial fermentation: Pyruvicacid→ Lactic acid.
o Enzyme involved−Lactate dehydrogenase.
o While doingsevere exercise similarreactionoccursin animal musclesinanaerobic conditions.
 Glycolysis – it is common to both aerobic and anaerobic
respiration
 Citric acid cycle / Krebs cycle - Aerobic respiration in
mitochondria
 Electron transport system – in the inner membrane of
mitochondria
 Both aerobic and anaerobic respiration starts with
Glycolysis.
 In aerobic respiration Glycolysis is followed by Citric acid
cycle and ETS (bothoccur inmitochondria).
 In anaerobic respiration Glycolysis is followed by
formation of ethyl alcohol / lactic acid in the cytoplasm.

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3. Aerobic respiration:
Aerobicrespirationfollowsfive stepsetof reactions.
These are 3.1. Glycolysis
3.2. Linkreaction.
3.3. Kreb’s cycle/citricacidcycle.
3.4. Electrontransportsystem(ETS)
3.5. Oxidative phosphorylation.
3.1. Glycolysis:
It isthe processof breakingdownof glucose topyruvicacid.
It was givenbyEmbden,Meyerhof andParnas
A chainof 10 reactionsconvertsglucose intopyruvate.
NetATPsproduced= 4 (produced) −2 (consumed) =2 ATPs
The pyruvate,soproduced,mayundergo:
 Lactic acid fermentation
 Alcoholicfermentation
 Aerobicrespiration(Krebscycle)
So glycolysisiscommonpathwayforaerobicaswell asanaerobicrespiration.
3.2. Link reaction/ intermediate phase/Formation of Acetyl Coenzyme A
 Pyruvate fromglycolysisisactivelypumpedintomitochondria.
 One carbon dioxide molecule andone hydrogenmolecule are removedfromthe pyruvate (calledoxidative
decarboxylation) toproduce anacetyl group,whichjoinstoan enzyme called CoAtoform acetyl CoA.

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3.3. Citric acid cycle / Tricarboxylic acid cycle / Kreb’s cycle:
 TCA cycle – it takes place in the mitochondrial matrix – it is the process of complete oxidation of pyruvate by
stepwise removal of all hydrogenatoms,whichleavesthree moleculesof CO2.
 Electron Transport Chain and Oxidative phosphorylation – it takes place in the inner membrane of the
mitochondria–it isthe processof synthesisof ATPfronNADH2and FADH2.
 Overall reaction :

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2 acetyl CoA + 6NAD+
+ 2FAD + 2ADP + 2Pi → 4 CO2 + 6 NADH + 6H+
+ 2 FADH2 + 2ATP
3.4. Electron Transport Chain(ETS)
 NADH2 and FADH2 are oxidized torelease the energy storedinthemin the form of ATPs.
 Electronsare passedfromone carrier to another,andfinallytooxygen,resultinginthe formationof water.
 Oxidationof 1 NADHproduces3 ATPs.
 Oxidationof 1 FADH2 produces2 ATPs.

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3.5. Oxidative Phosphorylation
As itturns out,the reasonyouneedoxygenissoyourcellscan use thismolecule during oxidative phosphorylation,
the final stage of cellularrespiration. Oxidative phosphorylationismade upof twocloselyconnectedcomponents:
the electrontransportchainand chemiosmosis.

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4. Respiratory Balance Sheet:
Theoretically, we can calculate the net gain of ATP for every molecule of oxidized glucose and this calculation is
basedon followingassumptions –
 There is an orderly and sequential functioning of the pathway; with one substrate forming the next with
glycolysis,Krebscycle andElectronTransportSystemfollowingone afteranother.
 The NADH formedduringglycolysisistransferredintomitochondriaandoxidative phosphorylationtakesplace.
 None of the intermediatesinanyprocessisutilizedtosynthesize anyothercompound.
 No alternative substratesexceptglucoseare respired.
All of the pathways work simultaneously but none of the above mentioned assumptions are really valid in living
system. Substrate that enters the pathways are extracted as and when required, ATP is utilized as and when
required, the rate of enzyme is controlled by several means. On the other hand, doing this exercise is important
as it appreciate the efficiency and beauty of the living system in extracting and storing energy. Thus, there can be
netgain of 36 ATP moleculesfromone molecule of glucose incase of aerobicrespiration.
Followingfiguresexplainsthe netgainof ATP:
Theoretically, we can calculate the net gain of ATP for every molecule of oxidized glucose and this calculation is
basedon followingassumptions –
 There is an orderly and sequential functioning of the pathway; with one substrate forming the next with
glycolysis,Krebscycle andElectronTransportSystemfollowingone afteranother.
 The NADH formedduringglycolysisistransferredintomitochondriaandoxidative phosphorylationtakesplace.
 None of the intermediatesinanyprocessisutilizedtosynthesize anyothercompound.

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 No alternative substratesexceptglucoseare respired.
All of the pathways work simultaneously but none of the above mentioned assumptions are really valid in living
system. Substrate that enters the pathways are extracted as and when required, ATP is utilized as and when
required, the rate of enzyme is controlled by several means. On the other hand, doing this exercise is important as it
appreciate the efficiency and beauty of the living system in extracting and storing energy. Thus, there can be net gain
of 36 ATPmoleculesfromone moleculeof glucose incase of aerobicrespiration.
Followingfiguresexplainsthe netgainof ATP:
 Glycolysis2ATP+ 2NADH2 (6ATP) = 8ATP
 Oxidative decarboxylation2NADH2 (6ATP) =6ATP
 Krebs’sCycle 2GTP (2ATP) + 6NADH2 (18ATP) + 2FADH2 (4ATP) = 24 ATP
 Energyproductioninprokaryotesduringaerobicrespiration=38 ATP
 Energyproductionineukaryotesduringaerobicrespiration=38 − 2 = 36 ATP
 (2ATPare usedup intransporting2 molecule of pyruvicacidinmitochondria.)
5. Common pathways for Proteins, fats and carbohydrates:
PROTEIN AS AN ENERGY SOURCE
Proteins are used as an energy source only if protein intake is high, or if glucose and fat sources are depleted, in
which case amino acids from protein breakdown are converted into molecules that can enter the TCA Cycle. These
molecules are produced by either of two categories of reactions that alter the structure of amino
acids. Transamination transfers an amino group (NH2) from one amino acid to another,
whereas deamination removes an amino group from an amino acid. As they accumulate, the amino groups removed
by the process of deamination are altered to form a harmful waste product (ammonia), so are converted by the liver
intourea whichisexcretedbythe kidneys.
FAT AS AN ENERGY SOURCE
Fats (lipids) are stored in adipose tissue. These stored fat molecules are synthesized in the body from the breakdown
products of fat digestion (glycerol and fatty acids), in a process known as lipogenesis (p. 501). When needed as an
energy source, the fat reserves are mobilized, moved out of adipose tissue, and broken down into glycerol and fatty
acids in the liver by the process of lipolysis. Glycerol is changed into one of the intermediate products of glycolysis,
so enters the cell respiration pathway. Fatty acids are changed in a series of reactions called beta-oxidationinto

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acetyl CoA molecules, which enter cell metabolism at the Kreb's Cycle. When fats are being used as the pri mary
energy source such as in starvation, fasting or untreated diabetes, an excess amount of acetyl CoA is produced, and
is converted into acetone and ketone bodies. This produces the sweet smell of acetone on the breath, noticeable in
a diabeticstate.
GLUCOSEAS AN ENERGY SOURCE
The above notes describe the process of carbohydrate (glucose) catabolism for the production of ATP. When glucose
is in adequate supply, such as shortly after consumption of a meal, the hormone insulin from the pancreas
increases glycogenformation (glycogenesis) in the liver. When glucose levels drop between meals, the
hormone glucagon is released from the pancreas and stimulates the conversion of glycogen into glucose (by the
process of glycogenolysis). If all glycogen supplies are depleted, then other substances in the body are converted
into glucose or intermediate products that can enter the above-outlined cellular respiration pathway. The conversion
of fatty acids (from lipids) or amino acids (from proteins) into glucose or intermediate products is
calledgluconeogenesis.
6. Respiratory Quotient (RQ):
This is another aspect of respiration. “Respiratory quotient is the ratio of CO2 produced to O2 consumed while food is
being metabolized.”
Where,RQ standsforRespiratory Quotient
RQ depends on the type of respiratory substrate used in respiration. When carbohydrate is used as substrate and is
completely oxidized, RQ becomes 1. It implies equal amount of O2 and CO2 are consumed and evolved. This reaction
isdisplayedinthe figure below –
In case, fats are used during the process of respiration, RQ becomes less than 1. Following equation shows the
calculationforfattyacid andtripalmitinisusedassubstrate –
Whenproteinisusedas respiratorysubstratesthe ratio comesouttobe 0.9.
7. Significance of respiration.
Cellular respiration is important because it provides the energy for living organisms to perform all of the other
necessary functions to maintain life. Most single-celled organisms, such as bacteria, do not require much energy
and are able tosurvive onglycolysisandfermentation.

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8. Factors affecting Respiration in Plants
There are eightenvironmental factorsthathassignificantimpactonrespirationinplants –
 Oxygencontentof the atmosphere
 Effectof water content
 Effectof temperature
 Effectof availabilityof light
 Impact of respirable material
 Effectof concentrationof carbon dioxide inatmosphere
 Protoplasmicconditions,i.e.youngertissueshave greaterprotoplasmascomparedtooldertissues.
 Otherfactors,i.e.fluorides,cyanides,azides,etc.