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UNIT I DIVERSITY IN THE LIVING WORLD
Chapter 2 : Biological Classification
UNIT II STRUCTURAL ORGANISATION IN PLANTS AND ANIMALS
Chapter 5 : Morphology of Flowering Plants
Chapter 6 : Anatomy of Flowering Plants
Chapter 7 : Structural Organisation in Animals
UNIT III CELL : STRUCTURE AND FUNCTIONS
Chapter 8 : Cell : The Unit of Life
Chapter 10 : Cell Cycle and Cell Division
UNIT IV PLANT PHYSIOLOGY
Chapter 11 : Photosynthesis in Higher Plants
Chapter 12 : Respiration in Plants
Chapter 13 : Plant Growth and Development
UNIT V HUMAN PHYSIOLOGY
Chapter 14 : Breathing and Exchange of Gases
Chapter 15 : Body Fluids and Circulation
Chapter 16 : Excretory Products and their Elimination
Chapter 17 : Locomotion and Movement
Exchange of Gases in Plants:
Plants do not have great demands for gaseous exchange. The rate of respiration in plants is much lower than in animals. Large amounts of gases are exchanged only during photosynthesis, and leaves are well equipped for that. The distance travelled by gases in plants is not much and hence diffusion is enough to meet the need. Hence, plants do not have specialized organs for exchange of gases. Lenticels and stomata serve as the openings through which exchange of gases takes place in plants.
Respiration:
The complete combustion of glucose yields energy during respiration. Most of the energy produced during respiration is given out as heat. CO2 and H2O are the end products of respiration.
The energy produced during respiration is also used for synthesizing other molecules. To ensure the adequate supply of energy for synthesis of different molecules; plants catabolize the glucose molecule in such a way that not all the liberated energy goes out as heat. Glucose is oxidized in several small steps. Some steps are large enough to ensure that the released energy can be coupled with ATP synthesis.
Steps of Respiration:
Respiration happens in two main steps in all living beings, viz. glycolysis and processing of pyruvate. Glycolysis involves breaking down glucose into pyruvate. This is common in all living beings. Further processing of pyruvate depends on the aerobic or anaerobic nature of an organism. In anaerobic respiration, pyruvate is further processed to produce either lactic acid or ethyl alcohol. There is incomplete oxidation of glucose in anaerobic respiration. In aerobic respiration, pyruvate is further processed to produce carbon dioxide and water; along with energy. There is complete oxidation of glucose in case of aerobic respiration.
GLYCOLYSIS
The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof and J Parnas. Due to this, it is also called the EMP Pathway. Glycolysis takes place in the cytoplasm.
Glucose undergoes partial oxidation in glycolysis; to form two molecules of pyruvic acid. Four molecules of pyruvic acid are formed after partial oxidation of one molecule of glucose during this process.
First, glucose and fructose undergo phosphorylation to produce glucose-6-phosphate. The enzyme hexokinase facilitates this process. Two molecules of ATP are utilised during phosphorylation of one molecule of glucose. Two molecules of fructose-6-phosphate are formed at the end of this step.
Fructose-6-phosphate is then converted into PGAL (Phosphoglyceraldehyde). Each molecule of PGAL then undergoes various steps to finally produce Pyruvic Acid. Four molecules of ATP are produced during this conversion. Since two molecules of ATP were utilised during phosphorylation of glucose, hence net production of ATP at the end of glycolysis is two for each molecule of glucose.
Fate of Pyruvic Acid: Pyurvic acid further undergoes subsequent processes which are different for anaerobic and aerobic conditions.
FERMENTATION
Endogenous electron acceptors are used for oxidation of organic compounds during fermentation. This contrasts with aerobic respiration in which exogenous electron acceptors are used. Anaerobic does not necessarily mean absence of oxygen, rather it can also take place even in the presence of oxygen.
Sugar is the most common substrate of fermentation. Ethanol, lactic acid and hydrogen are the common fermentation products.
However, other compounds can also be produced by fermentation, e.g. butyric acid and acetone. Apart from taking place in yeast and many other anaerobes, fermentation also takes place in mammalian muscles. In our muscle cells, fermentation takes place during intense exercise; to meet out the excess demand of oxygen.
AEROBIC RESPIRATION
Aerobic respiration takes place within the mitochondria. Following are the main steps in aerobic respiration:
Stepwise removal of all the hydrogen atoms leads to complete oxidation of pyruvate. This leaves three molecules of CO2. This step takes place in the matrix of mitochondria.
Electrons removed from hydrogen atoms are passed on to molecular O2. This happens with simultaneous synthesis of ATP. This step takes place in the inner membrane of mitochondria.
Pyruvate enters the mitochondria matrix and undergoes oxidative decarboxylation. This involves a complex set of reactions which are catalyzed by pyruvic dehydrogenase.
During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis).
After this, acetyl CoA enters a cyclic pathway. This pathway is called tricarboxylic acid cycle or Citric Acid Cycle or Krebs’ Cycle. This was first explained by Hans Krebs.
Kreb's Cycle
The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid. This reaction is catalyzed by the enzyme citrate synthase and a molecule of CoA is released. Citrate is then isomerized to isocitrate.
It is followed by two successive steps of decarboxylation. These steps of decarboxylation lead to the formation of α-ketoglutaric acid and then succinyl-CoA.
After that, succinyl-CoA is oxidised to OAA allowing the cycle to continue. During this step, a molecule of GTP is synthesised. This is a substrate level phosphorylation.
In a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP. Moreover, there are three points in the cycle where NAD+ is reduced to NADH+H+ and one point where FAD+ is reduced to FADH2.
The continued oxidation of acetic acid via the TCA cycle requires the continued replenishment of oxaloacetic acid. It also requires regeneration of NAD+ and FAD+ from NADH and FADH2respectively.
Electron Transport System (ETS) and Oxidative Phosphorylation
The next steps are to release and utilize the energy stored in NADH+H+ and FADH2. This is accomplished when they are oxidised through the electron transport system and the electrons are passed on to O2resulting in the formation of H2O.
The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS). This pathway is present in the inner mitochondrial membrane.
Electrons from NADH (produced in the mitochondria matrix) are oxidized by an NADH dehydrogenase (Complex I). After that, electrons are transferred to ubiquinone which is located within the inner membrane.
Ubiquinone also receives reducing equivalents via FADH2 (Complex II). FADH2 is generated during oxidation of succinate in the citric acid cycle.
The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via cytochrome bc1 complex (complex III).
Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV.
Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3, and two copper centres.
When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V). This coupling is necessary for the production of ATP from ADP and inorganic phosphate. The nature of the electron donor decides the number of ATP molecules synthesized.
Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while oxidation of one molecule of FADH2 produces 2 molecules of ATP.
Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. But since oxygen drives the whole process by removing hydrogen from the system, the presence of oxygen is vital.
Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.
During photophosphorylation, light energy is utilised to produce proton gradient. But in respiration, the energy of oxidation-reduction is utilised to produce proton gradient. Hence, this process is called oxidative phosphorylation.
The energy released during the electron transport system is utilised in synthesizing ATP with the help of ATP synthase (Complex V). This complex is composed of two major components, viz. F1 and F0. The F1 headpiece is a peripheral membrane protein complex. It contains the site for synthesis of ATP. F0 is an integral membrane protein complex which forms the channel through which protons cross the inner membrane. The passage of protons through the channel is accompanied by catalytic site of the F1 component to produce ATP. For each ATP produced, 2H+passed through F0 down the electrochemical proton gradient.
THE RESPIRATORY BALANCE SHEET
The respiratory balance sheet gives theoretical value about net gain of ATP for every glucose molecule oxidized. The calculations for respiratory balance sheet are based on some assumptions which are as follows:
There is a sequential and orderly pathway in which one substrate makes the next substrate. Glycolysis, TCA cycle and ETS pathway follow one after another.
NADH is synthesized in glycolysis and is transferred into the mitochondria. The NADH undergoes oxidative phosphorylation within the mitochondria.
None of the intermediates in the pathway are utilised to synthesise any other compound.
Glucose is the only substrate undergoing respiration. No other alternative substrates are entering in the pathway at any stage.
But these assumptions may not be valid in a living system because all pathways work simultaneously. There can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose.
AMPHIBOLIC PATHWAY
Glucose is the most favoured substrate for respiration. Other substrates can also be respired but they do not enter the respiratory pathway at the first step. Respiratory process involves both catabolism and anabolism; because breakdown and synthesis of substrates are involved. Hence, respiratory pathway is considered as an amphibolic pathway rather than a catabolic one.
RESPIRATORY QUOTIENT
The ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration is called the respiratory quotient (RQ) or respiratory ratio. The RQ for carbohydrates is 1. The RQ for fat and protein is less than 1.
NCERT Solutions
Plants' two primary functions are photosynthesis and respiration. The latter is introduced to the learner in this chapter. Aerobic and anaerobic respiration, glycolysis, fermentation, the electron transport system, and the tricarboxylic acid cycle are among the new names and concepts they encounter.
1. Differentiate between
(a) Respiration and Combustion
(b) Glycolysis and Krebs’ cycle
(c) Aerobic respiration and Fermentation
Solution:
a) Respiration and Combustion
Respiration
Combustion
It is a biochemical process
It is a physicochemical process.
Temperature stays low
Temperature drastically raises
Occurs in living cells
It is a non-cellular process
Energy entrapped in the form of ATP
ATP is not required for the combustion process
b) Glycolysis and Krebs’ cycle
Glycolysis
Krebs Cycle
The first step in respiration
The second step in respiration
Takes place in cytoplasm
Takes place in mitochondria
Occurs both aerobically and anaerobically
Occurs anaerobically
Two ATPs are consumed
ATPs are not consumed
The net gain is 8 ATP’s
The net gain is 24 ATP’s
It is a linear pathway
It is a circular pathway
c) Aerobic respiration and Fermentation
Aerobic respiration
Fermentation
Included in the exchange of gases
Does not include exchange of gases
Oxygen is necessary for aerobic respiration
Oxygen should be absent for the fermentation process
Respiratory material is completely oxidised
Respiratory material is incompletely oxidised
The end products are inorganic
At least one product is organic
2. What are respiratory substrates? Name the most common respiratory substrate.
Solution:
Organic substrates that are oxidised during respiration to liberate energy inside the living cells are respiratory substrates. Carbohydrates, proteins, fats and organic acids are the most common respiratory substrate.
3. Give the schematic representation of glycolysis?
Solution:
Schematic representation of glycolysis is as follows:
4. What are the main steps in aerobic respiration? Where does it take place?
Solution:
Main steps in aerobic respiration are as follows
Glycolysis: Occurs in the cytoplasm(cytosol) where glucose is broken down to pyruvic acid.
Oxidative decarboxylation of pyruvic acid to acetyl coenzyme-A: Takes place inside the mitochondrial matrix.
TCA or Krebs cycle takes place in Mitochondrial matrix where pyruvic acid is oxidized to transform the energy contained in these molecules into ATP.
Electron transport chain occurs in mitochondrial membrane involves ATP synthase complex.
5. Give the schematic representation of an overall view of Krebs’ cycle.
Solution:
The schematic representation of an overall view of Krebs’ cycle is as follows:
6. Explain ETS.
Solution:
Electron transport system(ETS) is found in the inner mitochondrial membrane and aids in liberating and using the energy stored in the NADH+H+ and FADH2
NADH+ H+ , formed while citric acid cycle and glycolysis occurs is oxidized by NADH dehydrogenase or complex I
Electrons hence produced are conveyed to ubiquinone via FMN
Similarly, the complex II or FADH2 synthesized during the citric acid cycle is conveyed to ubiquinone
From ubiquinone electrons are accepted by the complex III or cytochrome bc1 which furthermore gets conveyed to cytochrome c which serves as a mobile carrier between the cytochrome c oxidase complex and complex III comprising of cytochrome a and a3 with copper centers (complex IV) additionally
When electrons are transferred from each complex, simultaneously other processes occur such as production of the ATP from ADP and the inorganic phosphate through the action of ATP synthase(complex V)
This amount of ATP production is dependent on the molecule that has been oxidized. 3 ATP molecules are generated by the oxidation of 1 molecule of NADH while 1 FADH2 molecule upon oxidation produces 2 ATP molecules
7. Distinguish between the following:
(a) Aerobic respiration and Anaerobic respiration
(b) Glycolysis and Fermentation
(c) Glycolysis and Citric acid Cycle
Solution:
a) Aerobic respiration and Anaerobic respiration
Aerobic respiration
Anaerobic respiration
Occurs in the presence of the Oxygen
Occurs in the absence of Oxygen
Involves complete breakdown of respiratory materials.
Involves partial breakdown of the gases.
Carbon-di-oxide and water are the end products
Carbon-dioxide and ethanol are the end products.
Involves the exchange of gases
Does not include the exchange of gases
b) Glycolysis and Fermentation
Glycolysis
Fermentation
It is the first step in aerobic respiration, and it is common to both aerobic and anaerobic modes of respiration
It is anaerobic respiration which does not require Oxygen.
It produces pyruvic acid
It produces lactic acid and ethanol
It produces two molecules of NADH for every glucose molecule.
Uses NADH generated during glycolysis
It forms two ATP for every glucose molecule
It does not produce ATP.
c) Glycolysis and Citric acid Cycle
Glycolysis
Citric acid cycle
Occurs inside cytoplasm
Occurs inside mitochondria
It is a linear pathway
It is a cyclic pathway
In Glycolysis glucose is breakdown to pyruvate
Acetyl group is broken down completely.
The net gain is 8 ATP
Net gain is 24 molecules of ATP
8. What are the assumptions made during the calculation of net gain of ATP?
Solution:
Assumptions made during the calculation of net gain of ATP are as follows
NADH generated inside the mitochondria synthesizes 3 ATP molecules during its oxidation.
NADH formed during glycolysis sends its reducing power into mitochondria via the shuttle system.
During oxidation of FADH2, 2 molecules of ATP is produced inside mitochondria
Formation of 3 ATP in the malate-aspartate shuttle (heart, liver and kidney) and 2 ATP in the glycerol phosphate shuttle (muscles and nerve cells).
9. Discuss “The respiratory pathway is an amphibolic pathway.”
Solution:
Organic substances such as fats, carbohydrates, proteins etc liberate energy when they are disintegrated in the respiratory pathway. This phenomena is said to be catabolic in nature. The respiratory process that serves as a catabolic pathway for the respiratory substrates also serves as an anabolic pathway to produce different metabolic products and secondary metabolites. Thus, the respiratory pathway serves as a catabolic and anabolic pathway. Therefore, the respiratory pathway is an amphibolic pathway.
10. Define RQ. What is its value for fats?
Solution:
The ratio of volume of CO2 evolved to the volume of Oxygen consumed in respiration is called respiratory quotient (RQ) or respiratory ratio.
RQ is less than 1 when the respiratory substrate is either fat or protein
11. What is oxidative phosphorylation?
Solution:
Oxidative phosphorylation is the conversion of ADP into ATP by electron transport system. Phosphorylation takes place in the inner mitochondrial membrane via the ATP synthetase complex when the hydrogen protons pass through it. The energy essential for phosphorylation is derived from the oxidation-reduction phenomena in respiration. Thus the process is known as oxidative phosphorylation.
12. What is the significance of step-wise release of energy in respiration?
Solution:
During respiration single molecule of glucose is disintegrated to generate carbon dioxide and water along with the formation of ATP molecules. If the energy gets released at one go, then most of energy will be lost as heat. In order to synthesize new compounds, the cell should be able to utilize the energy. Hence step-wise release of energy in respiration is most efficient in the conservation of energy.
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