Samacheer Class 11 Bio Botany - Respiration

In succulent plants such as Opuntia and Bryophyllum, the respiratory quotient (RQ) is zero because these plants undergo a unique type of respiration in which carbohydrates are only partially oxidized to organic acids, particularly malic acid, without the corresponding release of carbon dioxide. During this process, oxygen is consumed as the plants break down carbohydrates, but since CO2 is not released in equivalent amounts, the ratio of CO2 released to O2 consumed becomes zero. This occurs because the organic acids produced accumulate in the plant tissues rather than being completely oxidized to CO2 and water. This adaptation is particularly advantageous for succulent plants living in arid environments, as it allows them to conserve water by closing their stomata during the day and opening them only at night to minimize water loss through transpiration. The accumulated organic acids are then oxidized during the day when stomata are closed, allowing the plant to maintain photosynthesis without excessive water loss.

Electron and hydrogen (proton) transport takes place across four multiprotein complexes (I-IV). They are.
1. Complex-I (NADH dehydrogenase).
It contains a flavoprotein (FMN) and associated with non-heme iron Sulphur protein (Fe-S). This complex is responsible for passing electrons and protons from mitochondrial NADFI (Internal) to Ubiquinone (UQ)
NADH+H + UQ ⇌ NAD – +UQH 2
2. In plants, an additional NADH dehydrogenase (External) complex is present on the outer surface of inner membrane of mitochondria which can oxidise cytosolic NADH + H +.
Ubiquinone (UQ) or Coenzyme Quinone (CoQ) is a small, lipid-soluble electron, proton carrier located within the inner membrane of mitochondria).
3. Complex-II (succinic dehydrogenase) It contains FAD flavoprotein is associated with non-heme iron Sulphur (Fe-S) protein. This complex receives electrons and protons from succinate in Kerbs cycle and is converted into fumarate and passes to ubiquinone.
Succinate + UQ Fumaraic LQH 2
4. Complex-III (Cytochrome bcj complex) This complex oxidises reduced ubiquinone (ubiquinol) and transfers the electrons through Cytochrome bc 1 Complex (Iron Sulphur centci bcl complex) to cytochrome c.
5. Complex IV (Cytochrome c oxidase) Complex IV is the terminal oxidase and brings about the reduction of 1/2 O 2 to H 2 O. TWO protons are needed to form a molecule of H 2 O (terminal oxidation).

Pentose phosphate pathway is the alternate pathway for breakdown of glucose.
* Pentose phosphate pathway was described by Warburg, Dickens and Lipmami (1938).
* It is also known as Hexose monophosphate shunt (HMP shunt) or Direct oxidative phase and non – oxidative phase.
* The oxidative phase convert six molecules of six carbon Glucose 6 phosphate to 6 molecules of five-carbon sugar Ribulose – 5 Phosphate with loss of 6CO 2 and generation of 12 NADPH + H +
Non oxidative pathway convert Ribulose – 5 – phosphate molecules to various intermediates such as
Ribose – 5 – phosphate (5C)
Xylulose – 5 – phosphate (5C)
Glyceraldehyde – 3 – phosphate (3C)
Sedoheptulose – 7 – phosphate (7C) and
Erythrose – 4 – phosphate (4C)
Finally five molecules of glucose 6 – phosphate is regenerated
6 x Glucose – 6 – phosphate + 12NADP + + 6H 2 O
↓
5 x glucose – 6 – phosphate + 6CO 2 + Pi + 12 NADPH + 2H +
The net result of complete oxidation of one glucose – 6 – phosphate yield 6CO 2 and 12 NADPH + H +. The oxidative pentose phosphate pathway is controlled by glucose – 6 – phosphate dehydrogenase enzyme which is inhibited by high ratio of NADPH to NADP +.

When the cost of transport of ATPs from the matrix into the cytosol is considered, the number will be 2.5 ATPs for each NADH + H + and 1.5 ATPs for each FADH 2 oxidized during the electron transport system. Therefore, in plant cells net yield of 30 ATP molecules for complete aerobic oxidation of one molecule of glucose. But in those animal cells (showing malate shuttle mechanism) net yield will be 32 ATP molecules. Since the sucrose molecule gives, two molecules of glucose and net ATP in plant cell will be 30 × 2 = 60.
In an animal cell, it will be 32 × 2 = 64.
Part-II.
11th Bio Botany Guide Respiration Additional Important Questions and Answers
I. Choose The Correct Answer

Respiration is a fundamental biological process that occurs in all living cells and involves the oxidation of various food substances such as carbohydrates, proteins, and fats. During respiration, these organic molecules are broken down in a controlled manner, and the chemical energy stored in their bonds is released and captured in the form of ATP (adenosine triphosphate), which serves as the energy currency of the cell. The process requires oxygen as the final electron acceptor in aerobic respiration, and carbon dioxide is liberated as a byproduct. Respiration is essential for providing energy for all cellular activities including biosynthesis, active transport, muscle contraction, and maintaining body temperature. The overall process can be represented by the general equation: organic molecules + O2 → CO2 + H2O + energy (ATP). Respiration occurs in two main forms: aerobic respiration, which requires oxygen and yields maximum ATP, and anaerobic respiration or fermentation, which occurs in the absence of oxygen and yields much less ATP.

The compensation point is a specific environmental condition at which the rate of photosynthesis exactly equals the rate of respiration in a plant. At this point, the amount of carbon dioxide released through respiration is precisely compensated by the amount of carbon dioxide fixed during photosynthesis, resulting in no net gaseous exchange between the plant and its environment. In other words, the oxygen produced by photosynthesis equals the oxygen consumed by respiration, and the CO2 consumed in photosynthesis equals the CO2 produced in respiration. The compensation point varies depending on the type of plant and environmental conditions such as light intensity, temperature, and CO2 concentration. For shade plants, the compensation point occurs at lower light intensities, while for sun plants, it occurs at higher light intensities. Below the compensation point, respiration exceeds photosynthesis, resulting in a net loss of organic matter, while above it, photosynthesis exceeds respiration, leading to net accumulation of biomass. Understanding the compensation point is important in agriculture and horticulture for optimizing plant growth conditions.

The Link reaction, also known as the transition reaction or oxidative decarboxylation of pyruvate, is a crucial intermediate step in aerobic respiration that connects glycolysis to the Krebs cycle. This reaction occurs in the mitochondrial matrix. During the Link reaction, each molecule of pyruvic acid, produced from glycolysis in the cytoplasm, is transported into the mitochondria and undergoes oxidative decarboxylation. In this process, pyruvic acid is converted into a two-carbon compound called acetyl coenzyme-A. This conversion involves the removal of a carbon atom as carbon dioxide (CO2) and the reduction of NAD+ to NADH + H+. For each glucose molecule, two molecules of pyruvic acid are produced, leading to the formation of two molecules of acetyl coenzyme-A, two molecules of NADH + H+, and two molecules of CO2. Acetyl coenzyme-A then enters the Krebs cycle for further oxidation.

Sir Hans Adolf Krebs was a renowned German-born British biochemist who made significant contributions to our understanding of cellular respiration. He was born on August 25, 1900, in Hildesheim, Germany. Krebs is most famous for his discovery and elucidation of the citric acid cycle, also known as the Krebs cycle or tricarboxylic acid cycle, which is a central metabolic pathway in all aerobic organisms. This cycle is crucial for the oxidation of acetyl-CoA and the generation of reducing equivalents (NADH and FADH2) that drive ATP synthesis through oxidative phosphorylation. His groundbreaking work on this cycle earned him the Nobel Prize in Physiology or Medicine in 1953, recognizing the fundamental importance of his discovery to our understanding of energy metabolism in living cells. Krebs' work laid the foundation for modern biochemistry and continues to be essential knowledge in the study of cellular respiration.

• TCA cycle starts with condensation of acetyl COA with oxaloacetate in the presence of water to yield Citri acid (or) Citrate.
• So it is also known as citric acid cycle (or) Tricarboxylic acid cycle.

NADH dehydrogenase, also known as Complex I of the electron transport chain, plays a crucial role in initiating the transfer of electrons from NADH to the electron transport system. This enzyme complex contains a flavoprotein cofactor called flavin mononucleotide (FMN) and is associated with non-heme iron-sulfur proteins (Fe-S clusters). The primary function of NADH dehydrogenase is to accept electrons and protons from mitochondrial NADH that is generated in the matrix during the citric acid cycle and glycolysis. These electrons are then passed through the FMN and iron-sulfur clusters to ubiquinone (UQ), also known as coenzyme Q, which is a lipid-soluble electron carrier embedded in the inner mitochondrial membrane. As electrons are transferred through this complex, protons are pumped from the mitochondrial matrix into the intermembrane space, contributing to the proton gradient that drives ATP synthesis. This process is essential for the efficient extraction of energy from NADH and the generation of the electrochemical gradient necessary for oxidative phosphorylation.

Two significant inhibitors of the electron transport chain are 2,4-Dinitrophenol (2,4-DNP) and Cyanide. 2,4-DNP acts as an uncoupler, meaning it prevents the synthesis of ATP from ADP by dissipating the proton gradient across the inner mitochondrial membrane. It achieves this by making the membrane permeable to protons, thereby directing electrons from Coenzyme Q to oxygen without the concomitant generation of ATP. Cyanide, on the other hand, is a potent inhibitor that directly blocks the flow of electrons. Specifically, it binds to the ferric iron (Fe3+) in cytochrome a3 of the cytochrome oxidase complex (Complex IV), preventing the transfer of electrons from cytochrome a3 to molecular oxygen. This inhibition halts the entire electron transport chain, leading to a rapid cessation of ATP production and cellular respiration.

The Respiratory Quotient (RQ) is a vital indicator in understanding cellular respiration, defined as the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during respiration (RQ = CO2 evolved / O2 consumed). Its significances are multifaceted. Firstly, the RQ value provides crucial information about the type of respiration occurring in living cells, indicating whether it is aerobic or anaerobic. For instance, an RQ of 1.0 suggests aerobic respiration with carbohydrate as the substrate, while an RQ greater than 1.0 or even infinity indicates anaerobic respiration. Secondly, the RQ value helps to identify the specific type of respiratory substrate being utilized by the organism. Different substrates like carbohydrates, fats, and proteins have distinct RQ values. For example, carbohydrates have an RQ of 1, fats typically have an RQ around 0.7, and proteins have an RQ of approximately 0.8-0.9. This allows scientists to infer the primary energy source being metabolized by a tissue or organism.

Two important internal factors that significantly affect the rate of respiration in plants are the amount of protoplasm and its state of activity, and the concentration of respiratory substrate. The amount of living protoplasm within the cells directly correlates with the respiratory capacity; cells with more active protoplasm, such as meristematic cells, generally exhibit higher rates of respiration due to increased metabolic demands. The physiological state of the protoplasm, including its hydration level and enzyme activity, also plays a critical role. Secondly, the concentration of respiratory substrate, such as glucose, is directly proportional to the rate of respiration, up to a certain saturation point. A higher availability of substrates means more raw material for the respiratory enzymes to act upon, leading to an increased rate of energy production. Conversely, a depletion of respiratory substrates will lead to a decrease in the rate of respiration.

Two significant aspects of the pentose phosphate pathway (PPP), also known as the Hexose Monophosphate (HMP) shunt, highlight its crucial role in cellular metabolism. Firstly, the HMP shunt is associated with the generation of two important products: NADPH and the precursor for nucleotide biosynthesis. NADPH, a reduced coenzyme, is vital for various anabolic processes, particularly reductive biosynthesis reactions such as fatty acid synthesis and steroid synthesis. Secondly, the coenzyme NADPH generated in the PPP is also critical for counteracting the damaging effects of oxygen-free radicals. It plays a key role in maintaining the reduced state of glutathione, which is essential for protecting cells from oxidative stress by neutralizing reactive oxygen species. Additionally, the pathway produces ribose-5-phosphate, a precursor for the synthesis of nucleotides, DNA, and RNA.

Plants and animals function as complementary systems in the biosphere that are intricately integrated to sustain life on Earth. In plants, oxygen enters through the stomata and is transported to cells where it is utilized during cellular respiration for energy production through the breakdown of organic molecules. Plants require carbon dioxide to survive and to produce carbohydrates through photosynthesis, and they release oxygen as a byproduct of this process. These oxygen molecules are inhaled by humans and animals through the nose, which reaches the lungs where oxygen is transported through the bloodstream and delivered to individual cells throughout the body. In return, animals produce carbon dioxide during cellular respiration, which is exhaled and becomes available for plants to use in photosynthesis. Cellular respiration takes place inside the cells of both organisms for obtaining energy in the form of ATP. This cyclical exchange of gases and nutrients demonstrates how plants and animals are mutually dependent, with plants providing oxygen and consuming carbon dioxide while animals provide carbon dioxide and consume oxygen, creating a balanced system that sustains life in the biosphere.

• Breaking of C-C bonds of complex organic compounds through oxidation within the cells.
• The energy released during respiration is stored in the form of ATP and heat is liberated.
• It occurs in all the living cells of organisms.

The compensation point in respiration is influenced by several environmental and physiological factors. The two most common factors associated with compensation points are carbon dioxide concentration and light intensity. Based on these factors, there are two main types of compensation points recognized in plants. The CO2 compensation point is the carbon dioxide concentration at which the rate of photosynthesis equals the rate of respiration, resulting in no net gas exchange. The light compensation point is the light intensity at which photosynthesis and respiration rates are equal. Different plant types show varying compensation points depending on their photosynthetic pathways. C3 plants, which use the Calvin cycle for carbon fixation, have CO2 compensation points ranging from 40 to 60 ppm (parts per million), reflecting their relatively higher respiration rates. In contrast, C4 plants, which have a more efficient photosynthetic pathway, have much lower CO2 compensation points ranging from 1 to 5 ppm CO2, allowing them to thrive in environments with lower carbon dioxide availability and making them more efficient at utilizing CO2 for photosynthesis.

Floating respiration and protoplasmic respiration differ in their respiratory substrates and products. In floating respiration, carbohydrates, fats, or organic acids serve as the respiratory substrate, and these are oxidized to produce energy in the form of ATP without generating toxic byproducts. This is the common mode of respiration in most plant and animal cells and does not produce any harmful substances. In contrast, protoplasmic respiration uses proteins as the respiratory substrate, which is a rare mode of respiration that occurs under specific conditions such as starvation or stress. When proteins are used as respiratory substrates, the deamination process liberates toxic ammonia as a byproduct, which must be either converted to less toxic compounds like urea or excreted from the organism. This fundamental difference in substrate utilization and product formation makes protoplasmic respiration an emergency metabolic pathway used only when carbohydrates and fats are depleted.

NAD + + 2e – + 2H + → NADH + H +
FAD + 2e – + 2H + → FADH 2
When NAD + (Nicotinamide Adenine Dinucleotide – oxidized form) and FAD (Flavin Adenine Dinucleotide) pick up electrons and one or two hydrogen ions (protons), they get reduced to NADH + H + and FADH 2 respectively. When they drop electrons and hydrogen off they go back to their original form. The reaction in which NAD + and FAD gain (reduction) or lose (oxidation) electrons are called redox reactions (Oxidation-reduction reactions). These reactions are important in cellular respiration.

• Electron transport chain and oxidative phosphorylation remove hydrogen atoms from the products of glycolysis, link reaction, and Kreb cycle.
• It releases water molecule with energy in the form of ATP molecules in the mitochondrial inner membrane.

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a central metabolic pathway with profound significance in cellular respiration and overall metabolism. Its primary significance is to provide energy in the form of ATP for metabolic activities in plants and other organisms through the complete oxidation of acetyl CoA, generating a large number of reduced coenzymes (NADH and FADH2) that feed into the electron transport chain. Beyond energy production, the Krebs cycle is a metabolic hub that provides carbon skeletons or raw materials for various anabolic processes. Many intermediates of the TCA cycle are further metabolized to produce essential biomolecules; for instance, alpha-ketoglutarate and oxaloacetate can undergo reductive amination to produce amino acids, which are then used to synthesize proteins. Succinyl CoA is a crucial raw material for the formation of important porphyrin compounds like chlorophylls, cytochromes, phytochrome, and other pyrrole substances. Therefore, the Krebs cycle acts as a metabolic sink, playing a central role in intermediary metabolism by linking carbohydrate, fat, and protein metabolism and providing precursors for biosynthesis.

Ubiquinone and Cytochrome C are both electron carriers in the electron transport chain, but they differ in their chemical nature, location, and function. Ubiquinone, also known as coenzyme Q, is a small, lipid-soluble molecule that serves as both an electron and proton carrier. It is located within the inner membrane of the mitochondria and is associated with Electron Transport System complex I, where it accepts electrons from NADH and transfers them to the next complex in the chain. Cytochrome C, in contrast, is a small protein molecule that is attached to the outer surface of the inner mitochondrial membrane. It functions as an electron carrier and is associated with Electron Transport System complex III and IV. While ubiquinone is hydrophobic and embedded in the lipid bilayer, allowing it to shuttle electrons between membrane-bound complexes, cytochrome C is a water-soluble protein that transfers electrons between the complexes. These differences in chemical composition and location reflect their specialized roles in facilitating electron transfer and maintaining the proton gradient necessary for ATP synthesis.

The characteristics of anaerobic respiration distinguish it significantly from aerobic respiration. Firstly, anaerobic respiration is considerably less efficient than aerobic respiration in terms of energy yield. It results in the generation of a very limited number of ATP molecules per glucose molecule, typically 2 ATPs, compared to the 36 or 38 ATPs produced in aerobic respiration. This inefficiency arises because glucose is only partially oxidized, and a significant amount of energy remains trapped in the end products like ethanol or lactic acid. Secondly, it is characterized by the production of carbon dioxide (CO2) in certain forms, such as alcoholic fermentation, where CO2 is released alongside ethanol. However, in lactic acid fermentation, CO2 is not produced. While CO2 is a product, it is important to note that this CO2 is not directly used for carbon fixation in photosynthesis within the context of anaerobic respiration itself; rather, CO2 is a byproduct of the breakdown of organic molecules in some anaerobic pathways.

• Red colour parts present in plants is due to the presence of anthocyanin
• Synthesis of anthocyanin require more O 2 than CO 2 evolved.
• So RQ will be less than one.

Three significant external factors that affect the rate of respiration in plants are temperature, oxygen concentration, and carbon dioxide concentration. Firstly, temperature has a profound impact on the rate of respiration. The optimum temperature for respiration in most plants is typically around 30°C. At temperatures below this optimum, the rate of respiration decreases due to reduced enzyme activity. Similarly, at very high temperatures, enzymes involved in respiration can denature, leading to a sharp decline and eventual cessation of respiration. Secondly, the concentration of oxygen is crucial for aerobic respiration. When a sufficient amount of O2 is available, the rate of aerobic respiration will be optimum, and anaerobic respiration is completely stopped. This point, where the oxygen concentration is just enough to prevent anaerobic respiration, is sometimes referred to as the extinction point for anaerobic processes. Conversely, low oxygen levels will promote anaerobic respiration. Thirdly, the high concentration of carbon dioxide in the surrounding environment can reduce the rate of respiration. Elevated CO2 levels can inhibit certain respiratory enzymes, particularly succinate dehydrogenase in the Krebs cycle, thereby slowing down the overall respiratory process. This inhibitory effect is part of the plant's response to environmental conditions.

• It is an alternate pathway for break down of glucose.
• It takes place in the cytoplasm of mature plant cells.
• In this pathway glucose 6 phosphate molecule is converted to Ribulose 5 phosphate with CO 2 and NADPH – + H +.

Alcoholic beverages like beer and wine are produced through a biological process called fermentation. This process involves the conversion of pyruvate, which is an end product of glycolysis, into ethanol. In the context of brewing beer, this conversion takes place in malted barley, while for wine production, it occurs in grapes. The key biological agent responsible for this transformation is yeast, a type of single-celled fungus. Yeast carries out this process under anaerobic conditions, meaning in the absence of oxygen. During fermentation, yeast enzymes convert pyruvate first into acetaldehyde and then into ethanol, simultaneously releasing carbon dioxide. As the fermentation progresses, the concentration of ethanol increases. However, ethanol is toxic to yeast cells. When the ethanol concentration reaches a certain level (typically around 13-15% for most yeast strains), it becomes lethal to the yeast, effectively stopping the fermentation process. The resulting liquid, after the yeast cells are removed or become inactive, is what we know as beer and wine, depending on the initial substrate.

Aerobic respiration
Anaerobic respiration
1. It occurs in all living cells of higher organisms.
It occurs yeast and some bacteria.
2. It requires oxygen for breaking the respiratory substrate
Oxygen is not required for breaking the respiratory substrate.
3. The end products are CO 2 and H 2 O
The end products are alcohol and CO 2 (or) lactic acid
4. Oxidation of one molecule of glucose produces 36 ATP molecules
Only 2 ATP molecules are produced.
5. It consists of four stages – glycolysis, link reaction, TCA cycle and electron transport chain.
It consists of two stages – glycolysis and fermentation.
6. It occurs in cytoplasm and mitochondria
It occurs only in cytoplasm

Two molecules of glyceraldehyde 3 – phosphate oxidatively phosphorylated into two molecules of 1-3
bisphospho glycerate.
* During this reaction 2 NAD + is reduced to 2NADH+ H + by glyceraldehyde 3-phosphate dehydrogenase.
* Further reactions are carried out by different enzymes at the end two molecules of pyruvate are produced.
* In this phase 4 ATPS are produced (at step 7 and step 10)
* Through Direct transfer of phosphate from substrate molecule to ADP and is converted into ATP is called substrate Phosphotylation. (or) Direct Phosphorylation (or) transphosphorylation.
* During the reaction (at step 9)2 phospo glycerate dehydrated into phosphoenol pyurvate, a water molecule is removed by the enzyme enolase.
* As a result enol group is formed within the molecule. This process is called Enolation.
Energy Budge of pay off phase:
* In the payoff phase totally 4 ATP and 2NADH + H + molecules are produced.
* Since 2 ATP molecules are already consumed in the preparatory phase the net products in glycolysis are 2ATP and 2NADH + H +

Glycolysis is a linear series of reactions in which 6- carbon glucose split into two molecules of 3 carbon pyruvic acid.
Preparatory phase:
* Glucose enters glycolysis which is the end product of photosynthesis.
* Glucose is phosphorylated into glucose 6 phosphate by the enzyme hexokinase and subsequent reactions are carried out by different enzymes.
* At the end of this phase fructose 1,6 – bisphote is cleaved into glyceraldehyde 3- phosphate and dihydroxyacetone phosphate by the enzyme aldolase.
* These two are Isomers.
* Dihydroxyacetone phosphate is isomerised into glyceraldehyde 3- phosphate by the enzyme triose phosphate isomerase.
* Now two molecules of glyceraldehyde 3 phosphate enter into pay off phase.
During the preparatory phase, two ATP molecules are àonsumed.

Pyruvate oxidation, also known as the link reaction or transition reaction, is a crucial step that connects glycolysis to the Krebs cycle in aerobic respiration. After glycolysis, two molecules of pyruvate are formed in the cytosol. These pyruvate molecules then actively enter the mitochondrial matrix, where the link reaction takes place. In this process, each pyruvate molecule undergoes oxidative decarboxylation, meaning it is oxidized and a carbon dioxide molecule is removed. This reaction is catalyzed by a multi-enzyme complex called the pyruvate dehydrogenase complex. This complex converts pyruvate into acetyl coenzyme A (acetyl-CoA). During this conversion, for each pyruvate molecule, one molecule of carbon dioxide is released, and one molecule of NAD+ is reduced to NADH + H+. Since two molecules of pyruvate are produced from one glucose molecule, the link reaction yields two molecules of acetyl-CoA, two molecules of CO2, and two molecules of NADH + H+. The pyruvate dehydrogenase complex is intricate, consisting of three distinct enzymes: pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase. It also requires five coenzymes for its activity: Thiamine Pyrophosphate (TPP), Nicotinamide Adenine Dinucleotide (NAD+), Flavin Adenine Dinucleotide (FAD), Coenzyme A (CoA), and lipoate.

• Two molecules of acetyl CoA formed from link reaction now enter into Kreb Cycle.
• It is named after its discoverer German Biochemist Sir Hans Adolf Kreb (1937).
• It is takes place in the mitochondrial matrix and inner membrane of mitochondria.
• The enzymes needed for TCA cycle are found in the mitochondrial matrix except for succinate dehydrogenase which is found in the mitochondrial inner membrane.
• First step starts with condensation of acetyl CoA with oxaloacetate in the presence of water to yield citric acid (or) citrate.
• It is followed by the action of different enzymes in cyclic manner.
• During the conversion of succinyl CoA to succinate by the enzyme succinyl CoA synthetase a molecule of ATP Synthesis from Substrate without entering the electron transport chain is called substrate-level phosphorylation.
• Kreb Cycle is repeated twice for every glucose molecule.
• Where two molecules of pyruvic acid produces six molecules of CO 2, eight molecules of NADH+H + two molecules of FADH 2 and two molecules of ATP.

The Krebs cycle, also known as the Citric Acid Cycle or TCA cycle, holds immense significance in cellular metabolism, particularly in providing energy and precursors for various anabolic processes. Its primary role is to generate ATP, the energy currency of the cell, through oxidative phosphorylation by producing reduced coenzymes (NADH and FADH2) that feed into the electron transport chain. Beyond energy production, the TCA cycle serves as a metabolic hub, providing carbon skeletons or raw materials essential for a multitude of anabolic processes. Many intermediates of the TCA cycle are siphoned off and further metabolized to produce vital biomolecules. For instance, alpha-ketoglutarate and oxaloacetate can undergo reductive amination to produce various amino acids, which are then used to synthesize proteins. Succinyl CoA is a critical precursor for the formation of important porphyrin compounds such, as chlorophyll, which is vital for photosynthesis, and cytochromes, which are essential components of the electron transport chain, as well as phytochrome and other pyrrole substances. Therefore, the Krebs cycle acts as a central metabolic sink, playing a pivotal role in intermediary metabolism by linking catabolic pathways (like carbohydrate and fat breakdown) with anabolic pathways (like amino acid and heme synthesis), ensuring a continuous supply of energy and building blocks for the cell.

• 2,4 DNP (Dinitrophenol) – It prevents the synthesis of ATP from ADP, as it directs electrons from CoQ to O 2
• Cyanide – It prevents the flow of electrons from Cytochrome a 3 to O 2
• Rotenone – It prevents flow of electrons from NADH + H + / FADH 2 to Co Q
• Oligomycin – It inhibits oxidative phosphorylation

Take small quantity of any seed (groundnut or bean seeds) and allow them to germinate by imbibing them.
* While they are germinating place them in a conical flask.
* A small glass tube containing 4 ml of freshly prepared Potassium hydroxide (KOH) solution is hing into the conical flask with the help of a thread and tightly close the one holed cork.
* Take a bent glass tube, the shorted end of which is inserted into the conical flask through the hole in the cork.
* The longer end is dipped in a beaker containing water.
* Observe the position of initial water level in bent glass tube.
* This experimental setup is kept for two hours.
* After two hours, the level of water rises in the glass tube. It is because the CO 2 evolved during aerobic
respiration by germinating seeds will be absorbed by KOH solution and the level of water will rise in the glass tube.
* CO 2 + 2KOH → K 2 CO 3 + H 2 O

Alcoholic fermentation and lactic acid fermentation are two distinct anaerobic pathways for ATP production, differing in their products, steps, enzymes, and intermediate compounds. Alcoholic fermentation, commonly observed in yeast, produces alcohol (ethanol) and releases carbon dioxide from pyruvic acid. This process occurs in two steps: first, pyruvate is decarboxylated to acetaldehyde, and then acetaldehyde is reduced to ethanol. It involves two key enzymes: pyruvate decarboxylase, which requires Mg++ as a cofactor, and alcohol dehydrogenase. Acetaldehyde serves as an intermediate compound in this pathway. In contrast, lactic acid fermentation, which occurs in bacteria, some fungi, and vertebrate muscle cells during strenuous activity, produces lactic acid directly from pyruvic acid without releasing carbon dioxide. This process takes place in a single step, where pyruvate is directly reduced to lactate. It utilizes one enzyme, lactate dehydrogenase, which requires Zn++ as a cofactor. Lactic acid fermentation does not form any intermediate compound like acetaldehyde. Therefore, the primary distinctions lie in the final products, the number of steps involved, the specific enzymes required, and the presence or absence of an intermediate compound and carbon dioxide release.

• In bakeries, it is used for preparing bread, cakes, biscuits.
• In beverage industries for preparing wine and alcoholic drinks.
• In producing vinegar and in tanning, curing of leather.
• Ethanol is used to make gasohol (a fuel that is used for cars in Brazil).

Glycolysis and fermentation are both crucial metabolic pathways, but they differ significantly in their starting points, oxygen requirements, and energy yields. Glycolysis is the initial stage of glucose breakdown, where a six-carbon glucose molecule is converted into two molecules of three-carbon pyruvic acid. This process can occur in the presence or absence of oxygen. A net gain of 2 ATP molecules is achieved through substrate-level phosphorylation, and 2 molecules of NADH + H+ are produced. Glycolysis is a universal pathway found in almost all living organisms. Fermentation, on the other hand, starts from the pyruvic acid produced by glycolysis and is an anaerobic process, meaning it strictly takes place in the absence of oxygen. Its primary purpose is to regenerate NAD+ from NADH + H+ so that glycolysis can continue. In fermentation, there is no net gain of ATP molecules; instead, the 2 NADH + H+ molecules produced during glycolysis are utilized to reduce pyruvic acid into either alcohol (as in alcoholic fermentation, common in yeast) or lactic acid (as in lactic acid fermentation, common in bacteria, some fungi, and vertebrate muscles). Thus, while glycolysis is the initial breakdown of glucose, fermentation is a subsequent anaerobic pathway that regenerates NAD+ to sustain glycolysis, with distinct products and energy outcomes.

Take a Kuhne’s fermentation tube which consists of an upright glass tube with a side bulb
* Pour 10% sugar solution mixed with baker’s yeast into the fermentation tube the side tube is filled plug the mouth with lid.
After some time, the glucose solution will be fermented. The solution will give out an alcoholic smell.
* The level of the solution in the glass column will fall due to the accumulation of CO 2 gas.
* It is due to the presence of zymase enzyme yeast which converts the glucose solution into alcohol and CO 2
* Now introduce a pellet of KOH into the tube, the KOH will absorb CO 2 and the level of solution will rise in the upright tube.
* This experiment proves during fermentation CO 2 gas is evolved.

External Factors:
* The optimum temperature for respiration is 30°C. At low temperatures and very high temperatures rate, respiration decreases.
* When sufficient amount of O 2 is available the rate of aerobic respiration will be optimum and anaerobic respiration is completely stopped. This is called Extinction point.
* The high concentration of CO 2 reduces the rate of respiration.
* A plant or tissue transferred from water to salt solution wi li increase the rate of respiration. It is called silt respiration.
* Light is an indirect factor affecting the rate of respiration.
* Wounding of plant organs stimulates the rate of respiration in that region.
Internal Factors:
* The concentration of respiratory substrate is proportional to the rate of respiration
* The amount of protoplasm and its state of activity influence the rate of respiration.

The pentose phosphate pathway was described by Warburg, Dickens, and Lipmann (1938). Hence, it is also called Warburg – Dickens Lipmann pathway.
* It takes place in the cytoplasm of mature plant cells. It is an alternate way for break4own of glucose.
* It consists of two phases, oxidative phase, and non-oxidative phase.
* The oxidative events concert six molecules of six carbon Glucose 6 phosphate to 6 molecules of five-carbon sugar Ribulose -5 phosphate with loss of 12 NADPH + H + (not NADH).
* The remaining reactions known as non oxidative pathway, covert Rihulose 5phosphate molecules to various intermediates such as Ribose – 5 – phosphate (5C), Xylulose – 5 – phosphate (5C), Glyceraldehyde – 7 – Phosphate (7C), and Eiythrose -4- phosphate (4C).
* Finally, five molecules of glucose -6- phosphate is regenerated. The overall reaction is:
6 x Glucose – 6 – Phosphate + 12NADP + + 6H 2 O
↓
5 x Glucose-6- Phosphate + 6CO 2 + Pi + 12NADPH + 12H +
* The net result of complete oxidation of one glucose-6-phosphate yield 6CO 2 and12NADPH+H +

The Pentose Phosphate Pathway (PPP), also known as the Hexose Monophosphate (HMP) shunt, is a significant metabolic pathway that runs parallel to glycolysis and plays a vital role in anabolic reactions by generating two important products: NADPH and pentose sugars. The coenzyme NADPH, generated in the oxidative phase of the PPP, is crucial for reductive biosynthesis reactions, such as fatty acid synthesis and steroid synthesis. It also plays a critical role in counteracting the damaging effects of oxygen-free radicals by maintaining the reduced state of glutathione, thereby protecting cells from oxidative stress. The pentose sugars produced, particularly ribose-5-phosphate, are essential precursors for the synthesis of nucleic acids like DNA and RNA, as well as important coenzymes such as ATP, NAD, FAD, and Coenzyme A. Furthermore, erythrose-4-phosphate, another intermediate of the PPP, is used for the synthesis of various aromatic compounds, including anthocyanins (plant pigments), lignin (a structural component of plant cell walls), and other secondary metabolites, highlighting its diverse contributions to cellular anabolism and overall plant physiology.