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Understand the process of respiration clearly

Senior Faculty, Hyderabad

UNIT-I Chapter 5 The 5th chapter of Plant Physiology is Respiration in Plants. This is the metabolic pathway that distinguishes living and non living. As for the photosynthesis this chapter also requires an understanding of biological oxidation processes. Compared to photosynthesis this chapter is much easily understood. Some students prefer to read this chapter before photosynthesis. This is advisable also. But in the text book reference of photosynthesis is mentioned for phosphorylation reactions; so the student is left with no choice of reading Photosynthesis first. Instead of by hearting the content student should understand the process of respiration clearly. Animals and plants both have to perform respiration for their energy requirements. Whereas animals have to depend entirely on the respiration but plants can obtain energy from photosynthesis also. The six carbon compound when oxidized or broken down it results in six molecules of carbon dioxide. If the change in the number of carbon atoms present in the resulting molecules after every step of respiration is followed the entire respiration is an easy metabolic pathway. The fundamental process of respiration is almost similar, if not identical, in both plants and animals. Photosynthesis and respiration are reversible processes in plants. But both processes release energy by the same method. Students are advised to read the photosynthesis chapter once again after reading the respiration chapter to understand the reversible nature of the process. Students have to remember that respiration is the process that also provides intermediate metabolites for the synthesis of other cellular compounds. Glycolysis and Krebs cycle is not the sole respiratory pathway in organisms. 30% od respiration is by HMP pathway in all organisms. For the IPE examinations one of the LAQ is from physiology. It is either from photosynthesis or respiration. Already we have discussed about photosynthesis. In respiration both the questions are important. Flow charts in the answers carry marks. The question on glycolysis has a breakup of three parts. All these parts without fail must be answered. Practice of these questions before examination is also important for proper presentation. For the EAMCET point of view one question may be given from this

chapter. Mostly the question will be of easy type. Sometimes numerical problems are also possible on the number of ATP molecules released in a particular condition. Overall this chapter cannot be a problematic to the student.

Important LAQs for IPE exam

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1. Give an account of glycolysis. Where does it occur? What are the end products? Trace the fate of these products in both aerobic and anaerobic respiration. 2. Explain the reaction of Krebs cycle. Two questions are there for IPE. For both the questions a Flow chart/Cycle must be drawn. Name of the enzymes are also to be written. For the first question answer must be to the breakup of the question.

Important SAQs for IPE exam 1. Why is the respiratory pathway referred to as an amphibolic pathway? Explain. 2. Write about two ATP yielding reactions of glycolysis. 3. The net gain of ATP for the complete aerobic oxidation of glucose is 36. Explain. 4. Define RQ. Write a short note on RQ. (Not for A.P students) 5. Describe briefly the process of fermentation. 6. Explain various complexes involved in electron transport system of respiration. Answers for all SAQs are brief and takes little time in answering. No diagrams are required.

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Respiration is oxidation of a respiratory substrate. In plants respiratory substrates can be starch, protein, fats, insulin, stachyose sugar, acids. In aerobic respiration complete oxidation takes place releasing carbon dioxide. The released electrons and protons reduce oxygen to form water. In anaerobic respiration takes place incomplete oxidation of respiratory substrate resulting in little amount of energy release. For respiration oxygen diffuses into the plant. In plants all living cells are conveniently placed in the outer regions for the purpose. Thick stems facilitated with central non-living regions. The starting substance for respiration is glucose.

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Sr. Inter Botany Botany

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Important information for

EAMCET

In plants aerobic respiration takes place and completes involving three compartments of the cell. The first compartment is the cytosol, the liquid portion of the cytoplasm excluding cell organelles. The second compartment is matrix of mitochondria.

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The third compartment is inner mitochondrial membrane. Glycolysis is break down of glucose in cytosol to form organic acids. Glycolysis requires ten steps. Without phosphorylation glucose cannot participate in the respiration. Only one oxidation reaction takes place mediated by G-3-P dehydrogenase. The electron carrier in respiration is NADH2. At two different sites 4 ATP molecules are formed. ATP formation is substrate level phosphorylation. Any phosphorylated molecule transfers its phosphate to ADP to form ATP is called substrate level phosphorylation. Per every glucose molecule 2 ATP are utilized in phosphorylation. At the end of the glycolysis 4 ATP molecules are formed and 2 ATP are utilized. Net gain is 2 ATP and 2 NADH2. Dihydroxy acetone phosphate do not participate in respiration. The 2 pyruvic acid molecules depending on the availability of oxygen may enter into mitochondria or undergo decarboxylation in cytosol to form ethyl alcohol. Ethyl alcohol formation is fermentation process. Pyruvic acid decarboxylates and reduced. NADH2 produced in glycolysis will be utilized. In lactic acid fermentation that occur in animal tissues no carbon dioxide is released. Many bacteria also undergo fermentation releasing alcohol or acetic acid.

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Pyruvic acid undergo a complex reaction before entering into Krebs cycle. This reaction takes place in mitochondria. Enzyme pyruvate dehydrogenase requires many cofactors (six). This reaction releases one CO2 molecule and One NADH2. Magnesium is required in phosphorylation of glucose in two steps and in dehydrogenation of pyruvic acid. Glucose enters as two carbon compound Acetyl CoA into Krebs cycle. Two CO2 molecules are released in Krebs cycle. Acetyl CoA is starting substance into Krebs cycle. As the first substance is Citric acid it is also called as Citric acid cycle. Citric acid is a tri-carboxylic acid. The cycle is also called as TCA cycle. At three instances 3 molecules of NADH2 and one FADH2 are released. One ATP molecule is also released. ATP formation is substrate level phosphorylation. Per every Acetyl CoA 3 NADH2 and one FADH2 are released. The enzyme involved in the release of FADH2 is membrane bound. Per every glucose after complete oxidation releases 6NADH2, 2FADH2 in Krebs cycle. In mitochondrial matrix beside these 2 more NADH2 are released during dehydrogenation of pyruvic acid. Each NADH2 can after electron transport can form 3ATP molecules. Each FADH2 molecules can release 2ATP molecules after electron transport. The NADH2 released in glycolysis can release only 2 ATP molecule as energy is consumed for NADH2 to enter mitochondria through mitochondrial membrane. A total of 32 ATP molecules are released through electron transport. 6 ATP are released through substrate level phosphorylation. Gross production of ATP is 38 molecules. Net gain is 36 molecules per every glucose. Electron transport takes place in the inner mitochondrial membrane. Four complex membrane proteins are involved. The first one is NADH dehydro-

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genase. It acts as proton transporter. The second one is Succinic dehydrogenase. It is part of TCA cycle enzymes. Succinic dehydrogenase do not help in proton transport. The third one is Cytochrome ‘bc1’ complex or Cytochrome reductase. The fourth one is cytochrome oxidase, a Cu containing protein. It shows affinity for O2. Proton concentration increase due to electron transport in the peri-mitochondrial space. Enzyme ATPase acts as channel for protons and phosphorylase ADP to form ATP. ATPase has F0 as membrane bound part and F1 exposed into matrix. For every 3 protons crosses the membrane through ATPase 3 ATP are formed. Respiration is considered as amphibolic pathway. It serves dual purpose of catabolism of respiratory substrate and anabolism of cellular substances. Fats enter respiratory cycle as Acetyl CoA. Proteins enter as Pyruvic acid or Acetyl CoA or citric acid into the respiratory chain. Respiratory Quotient is Volume of CO2 evolved to that of O2 consumed. RQ is used to identify the nature of the substrate. If substrate is carbohydrate RQ is equal to 1. In fats RQ is less than 1 and in acid respiration it is more than one. This depends on how many C atoms are present in the substrate.

TS EAMCET 2015 1. Which of the two statements together support that respiratory pathway is an ‘amphibolic pathway’? Ans : 4 (I) Fats breakdown to glycerol and fatty acids, subsequently yields acetyl co-enzyme ‘A’. (II) In respiration, C-C bonds of complex compounds breakdown through oxidation leading to release of energy. (III) Acetyl coenzyme ‘A’ from the respiratory pathway is withdrawn for the synthesis of fatty acids. (IV) Proteins are degraded by protease to amino acids and enter the respiratory pathway. (1) II, III (2) I, II (3) I, IV (4) II, IV