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Intermediate of Krebs cycle that can form Chlorophyll?

Intermediate of Krebs cycle that can form Chlorophyll?



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This was a question in a test in my school today.

Q) Which intermediate of Krebs cycle further can form chlorophyll?

A) Oxaloacetic acid B) Citric acid C) Succinyl CoA D) Fumarate

Given answer was (C).

How can Succinyl CoA form chlorophyll? All I could find was related to porphyrine ring. Can anyone explain it or recommend a good source?


The standard treatment of this topic is covered in Section 24.4.3 of Berg et al., available freely online:

Succinyl CoA is a precursor for porphyrin in mammalian cells by condensing with glycine to form δ-aminolevulinate as shown in this diagram from that book:

This reaction is catalyzed by mitochondrial δ-aminolevulinate synthase.

To quote from that reference:

Two molecules of δ-aminolevulinate condense to form porphobilinogen, the next intermediate. Four molecules of porphobilinogen then condense head to tail to form a linear tetrapyrrole in a reaction catalyzed by porphobilinogen deaminase. The enzyme-bound linear tetrapyrrole then cyclizes to form uroporphyrinogen III, which has an asymmetric arrangement of side chains. This reaction requires a cosynthase. In the presence of synthase alone, uroporphyrinogen I, the nonphysiologic symmetric isomer, is produced.

This is illustrated in Fig. 24.35 of that section, which also shows that uroporphyrinogen I gives rise to protoporphyrin IX, the precursor of haem:

As regards the synthesis of plant chlorophyl (which also contains a porphyrin ring), the review quoted by March Ho contains the following statement:

The porphyrin ring with its conjugated double bonds is assembled in the chloroplast from eight molecules of 5-aminolevulinic acid…

(5-aminolevulinic acid is the chemically preferred name for δ-aminolevulinic acid.)

However the review goes on to explain that although 5-aminolevulinic acid synthesized in plant mitochondria uses the same pathway as mammals, that produced in the chloroplast and used for the synthesis of haem that gives rise to chlorophyl uses a different pathway, termed the C5 pathway, in which - as March Ho stated - glutamate is the precursor:

I suspect that the person who set the question is an animal or biochemist who thought he would try a variant of the standard question “which intermediate is a precursor of haem”, and ventured into waters where he was out of his depth.


This is a poorly set question. If any one member of the Krebs Cycle is used as the raw materials in a biosynthetic pathway, it will follow that all of the other members are also part of the same biosynthetic pathway, since they are part of the same cycle.

The full biosynthetic pathway of chlorophyll is a very complex process, and there are dozens if not hundreds of different steps and enzymes involved. This is one paper that attempts to cover the issue.

In short, the porphyrin ring region of chlorophyll is generated from glutamate, and the phytyl tail region of chlorophyll is generated via the phytol synthesis pathway, via the geranyl phosphate pathway


Respiration in Plants: Meaning and Mechanism | Botany

We know that during photosynthesis, light energy is converted into chemical energy, and is stored in carbohydrate molecules, such as glucose and starch. Organisms make use of such energy for their activities by oxidising these high energy food molecules into simple low energy molecules, i.e., carbon dioxide and water.

The reactions involved in process of oxidation are known as respiration. The compounds that are oxidised during process of respiration are called respiratory substrates.

Technically, Respiration is defined as follows:

This is a process by which living cells break down complex high energy food molecules into simple low energy molecules, i.e., CO2 and H2O, releasing the energy trapped within the chemical bonds.

The energy released during oxidation of energy rich compounds is made available for activities of cells through an intermediate compound called adenosine triphosphate (ATP).

During process of respiration, the whole of energy contained in respiratory substrates is not released all at a time. It is released slowly in several steps of reactions controlled by different enzymes.

Respiration takes place in all types of living cells, and generally called cellular respiration. During the process of respiration oxygen is utilised, and CO2 water and energy are released as products. The released energy is utilised in various energy-requiring activities of the organisms, and the carbon dioxide released during respiration is used for biosynthesis of other molecules in the cell.

As we know, important life processes, such as synthesis of proteins, fats and carbohydrates, require a certain expenditure of energy. Where does this energy come from, how is it stored, and how is it made available to the living cell, are some of the questions, which are to be answered by process of respiration.

The reaction that occurs in common respiration of glucose may be summed up as follows:

Here, 686 kcal or 2870 kJ of energy is liberated per molecule of glucose. Formerly, this calculated value was 673 kcal. One kcal is equal to 1000 calories. This means that one molecule of glucose on complete oxidation yields 686 kcal (kilocalories) of energy, (i.e., 686, 000 calories).

The main facts associated with respiration are:

a. Consumption of atmospheric oxygen.

b. Oxidation and decomposition of a portion of the stored food resulting in a loss of dry weight as seen in the seeds germinating in dark.

c. Liberation of carbon dioxide and a small quantity of water (the volume of CO2 liberated is equal to volume of O2 consumed).

d. Release of energy by breakdown of organic food, (such as carbohydrates).

Respiratory substrates are those organic substances which are oxidised during respiration. They are high energy compounds and are called respiratory substrates. They may be carbohydrates, fats and proteins. Carbohydrates, such as glucose, fructose (hexoses), sucrose (disaccharide) or starch, inulin, hemicellulose (polysaccharide), etc., are main respiratory substrates.

Besides, fats are used as respiratory substrates by a variety of organisms as they contain more energy than carbohydrates.

In rare circumstances, when carbohydrate reserves are exhausted, fats and proteins also serve as respiratory substrates. Blackman termed the respiratory oxidation of protoplasmic protein as protoplasmic respiration, while oxidation of carbohydrates as floating respiration.

There are two main types of respiration:

(i) Aerobic Respiration:

This type of respiration leads to a complete oxidation of stored food (organic substances) in the presence of oxygen, and releases carbon dioxide, water and a large amount of energy present in respiratory substrate. Such type of respiration is generally found in higher organisms.

The overall equation is:

(ii) Anaerobic respiration:

This type of respiration occurs in complete absence of oxygen. In the absence of free oxygen, many tissues of higher plants, seeds in storage, fleshy fruits, and succulent plants, such as cacti temporarily take to a kind of respiration, called anaerobic respiration. Such respiration generally occurs in lower organisms like bacteria and fungi.

This results in incomplete oxidation of stored food and formation of carbon dioxide and ethyl alcohol, and sometimes also various organic acids, such as malic, citric, oxalic, tartaric, etc. Very little energy is released by this process to maintain activity of protoplasm.

The equation is as follows:

This process of oxidation in microbes is known as fermentation. This is quite similar to that of anaerobic respiration in case of higher plants.

4. Mechanism of Respiration:

There are two major phases of respiration:

During process of respiration, carbohydrates are converted into pyruvic acid through a series of enzymatic reactions. This series of reactions is known as glycolysis which takes place in cytosol.

Now, pyruvic acid enters mitochondria, where several enzymes catalyse the reactions, and pyruvic acid finally converts into CO2 and water. This series of enzymatic reactions is known as Krebs cycle (after name of its discoverer Sir Hans Adolf Krebs (1900-1981), awarded Nobel Prize in 1953), or tricarboxylic acid (TCA) or citric acid cycle.

Glycolysis is a term used to describe the sequential series of reactions present in a wide variety of tissues that starts with a hexose sugar (usually glucose) and ends with pyruvic acid. This term has originated from Greek words, glycos = sugar and lysis = splitting.

The scheme of glycolysis was discovered by three German Scientists, Gustav Embden, Otto Meyerhof and J. Parnas, and therefore, referred as EMP pathway, after the abbreviation of their last names.

Glycolysis is the first stage in the breakdown of glucose and is common to all organisms. This means, glycolysis is common to both aerobic and anaerobic modes of respiration. In anaerobic organisms, this is only process in respiration. Glycolysis occurs in cytoplasm of cells. During this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.

In plants, glucose is derived from sucrose, which is the end product of photosynthetic carbon reactions (also known as dark reactions) or from storage carbohydrates.

Sucrose is converted into glucose and fructose by the enzyme invertase. Now, these two monosaccharides (i.e., glucose and fructose) enter glycolysis or EMP pathway.

The main steps of glycolytic pathway are as follows:

Glycolysis is carried out in following different steps:

a. Phosphorylation of Sugar (i.e., First Phosphorylation):

Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate and fructose-6-phosphate, respectively, by the activity of enzyme hexokinase, in presence of ATR The phosphorylated form of glucose then isomerises to produce fructose-6-phosphate. Isomerisation takes place with the help of enzyme phosphohexose isomerase.

Further steps of metabolism of glucose and fructose are quite similar.

Equations are as follows:

b. Phosphorylation of Fructose-6-Phosphate (i.e., Second Phosphorylation):

Now, fructose-6-phosphate is phosphorylated and fructose-1, 6-bisphosphate produced by the action of enzyme phosphofructokinase in presence of ATP.

Now, fructose- 1, 6-bisphosphate splits into two molecules of triose phosphate, i.e., 3-phosphoglyceraldehyde (PGAL) and dihydroxyacetone phosphate ( Di HAP ), which are interconvertible.

d. Oxidative Dehydrogenation:

After formation of 3-phosphoglycerldehyde (PGAL), the glycolytic pathway enters the energy conserving phase. Here, it is oxidized to a carboxylic acid, i.e., 1,3-bisphosphoglycerate, and NAD is reduced to NADH.

e. Formation of ATP:

In next step of glycolysis, 3-phosphoglycerate is formed from 1, 3-bisphosphoglycerate by enzymatic activity of phosphoglycerate kinase, and ATP is generated during this process. Direct synthesis of ATP from intermediate metabolites is called substrate level phosphorylsation.

This type of formation of ATP, where a phosphate group is directly transferred from a substrate to ADP to form ATP, is different from the ATP produced by ATP synthesis during oxidative phosphorylation in mitochondria or in chloroplasts (During photophosphorylation in photosynthesis).

f. Isomerisation:

In next step 3-phosphoglycerate converts into its isomer 2-phosphoglycerate by catalytic activity of enzyme phosphoglyceromutase.

In subsequent step 2-phosphoglycerate converts into phosphoenol pyruvate (PEP) in the presence of enzyme pyruvate kinase and liberates ATP.

h. Generation and Utilisation of ATP during Glycolysis:

During glycolytic pathway, the molecules of ATP are produced as follows:

(i) Direct transfer of phosphate to ATP.

(ii) Oxidation of NADH produced during glycolytic pathway to NAD + .

i. In the end of glycolysis net gain of ATP:

(i) During glycolysis two triose phosphate molecules are formed from one glucose molecule, and 4 ATP molecules are produced.

(ii) Out of 4 ATP molecules, 2 ATP molecules are utilised in first few steps in converting glucose to fructose-1, 6 bisphosphate.

(iii) Moreover, three ATP molecules are produced from oxidation of each of two molecules of NADH produced during catabolism of glucose.

(iv) In all, a net gain of 8 molecules occurs during process of glycolysis.

(v) However, in anaerobic respiration, NADH + H^ is not converted to ATP, and therefore, only 2 ATP molecules are produced.

5. Oxidative Decarboxylation Pyruvic Acid:

(Aerobic Oxidation of Pyruvic Acid)

Now, pyruvic acid generated in cytoplasm through glycolysis is transferred to mitochondria. This is initiation of second phase of respiration. As soon as, pyruvic acid enters the mitochondria, one of the three carbon atoms of pyruvic acid is oxidised to carbon dioxide in a reaction called oxidative decarboxylation.

Here, pyruvate is first decarboxylated, and thereafter oxidised by enzyme pyruvate dehydrogenase. This enzyme is made up of a decarboxylase, lipoic acid, TPP, transacetylase and Mg +2 .

Acetyl Co-A acts as substrate entrant for Krebs cycle.

The equation is as follows:

Acetyl Co-A can enter into mitochondria while pyruvate acid cannot.

6. Krebs Cycle :

Sir Hans Adolf Krebs, discovered role of pyruvate in conversion of glucose hydrogens into fumarate. He discovered, in 1937, tricarboxylic acid cycle (i.e., TCA cycle), also known as Citric acid cycle or Krebs cycle. Citric acid cycle occurs in matrix of mitochondria. This cycle involves two decarboxylations and four dehydrogenations.

Various steps of these reactions are as follows:

The starting point of Krebs cycle is entrance of acetyl Co-A into a reaction to form citric acid. Krebs elucidated this cycle, and explained how pyruvate is broken down to CO2and H2O. For this pioneer work Krebs was awarded Nobel Prize in 1953.

In the first reaction of Krebs cycle, one molecule of acetyl Co-A combines with 4-carbon oxaloacetic acid (OAA) with the result 6-carbon citric acid is produced, and Co-A is released. This reaction is catalysed by enzyme citrate synthase.

Now, citrate (citric acid) is isomerised to isocitrate (isocitric acid).

Cis-aconitic acid is converted into isocitric acid with the addition of water in the presence of iron containing enzyme aconitase.

During citric acid cycle (Krebs cycle) 3 molecules of NAD + and one molecule of FAD (Flavin Adenine Dinucleotide) are reduced to produce NADH and FADH2, respectively.

During citric acid cycle NADH and FADH, are produced. Now, they are linked with electron transport system (ETS) and produce ATP by oxidative phosphorylation.

This may be summarised in following equation:

In the end of Krebs cycle, glucose molecule is completely oxidised. From one glucose molecule, two pyruvic acid molecules are formed. After oxidation of one pyruvic acid molecule, three CO2 molecules are released. Thus, in all 6 molecules of CO2 are released.

Electron Transport System (ETS):

By the end of Krebs cycle, glucose molecule oxidises completely, but the energy does not release till NADH and FADH2 oxidise through electron transport system (ETS). The metabolic pathway through which electron passes from one carrier to another, is called electron transport system (ETS). The electron transport system is also known as electron transport chain or mitochondrial respiratory chain.

The electron transport system consists of a series of coenzymes and cytochromes that take part in passage of electrons from a chemical to its ultimate acceptor. The passage of electrons from one-enzyme or cytochrome to the next takes place with a loss of energy at each step. Electron transport system is operative in the inner mitochondrial membrane.

The electron carriers include flavins, iron sulphur complexes, quinones and cytochromes. Most of them are prosthetic groups of proteins.

Electron transport system in mitochondria consists of four complexes which are found in bases of stalked particles in the inner mitochondrial membrane, and also ubiquinone (UQ) or coenzyme Q and cytochrome c which are not bound to stalked particles but act as mobile electron carriers between the complexes.

Consists of NADH-dehydrogenase or NADH-Q reductase which contains a flavoprotein FMN (flavin mononucleotide) and is associated with iron-sulphur (Fe-S) proteins. This complex is responsible for passing electrons (also protons) from mitochondrial NADH to ubiquinone (UQ), located within inner mitochondrial membrane.

Consists of succinate dehydrogenase which contains a flavoprotein FAD (flavin adenine dinucleotide) in its prosthetic group and is associated with non heme iron-sulphur (Fe S) proteins.

This complex receives electrons (also protons) from succinic acid (which is oxidised in Krebs cycle to form fumaric acid) and passes them to ubiquinone (UQ). Ubiquinone also receives reducing equivalents via FADH2 that is generated during oxidation of succinate, through the activity of energy succinate dehydrogenase, in Krebs cycle.

Consists of ubiquinol, cytochrome c and cytochrome bc1 .The reduced ubiquinone is called ubiquinol. Here ubiquinol is oxidised with the transfer of electrons to cytochrome c via cytochrome bc1. Cytochrome c is a small protein attached to outer surface of the inner mitochondrial membrane and acts as a mobile carrier for transfer of electrons between complex III and complex IV.

This complex is called QH2-cytochrome c reductase complex. This bears three components, i.e., cytochrome b, non-heme iron sulphur (Fe – S), and cytochrome c1. Coenzyme Q is also involved between Fe-S and cytochrome c1.

The equations are as follows:

Now, cytochrome c, transfers electrons to cy c. Like coenzyme Q, cy c is also mobile carrier of electrons.

Is known as cytochrome c oxidase complex. This contains cytochromes a and a3, along with two copper centres. This complex receives electrons from cytochrome c and passes them to 1/2 O. Two protons are needed and Hp molecule is formed (terminal oxidation). Here, O2 is ultimate acceptor of electrons. It combines with protons to form metabolic water or respiratory water.

When electrons are transferred from one carrier to next carrier via complexes 1 to IV in electron transport system (ETS), they are coupled to ATP synthase enzyme complex for production of ATP from ADP and inorganic phosphate (iP).

Here, number of ATP molecules synthesised during ETS, depends on nature of electron donor. Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, and one molecule of FADH2 gives rise to 2 molecules of ATP. ATP synthase complex is called complex V.

During transportation of electrons, hydrogen atoms split into protons and electrons. The electrons are carried by cytochromes. Before last stage, where hydrogen atom is accepted by oxygen to form water, the electrons again recombine with their protons. Oxygen acts as final hydrogen acceptor.

Oxidative Phosphorylation:

The whole process, where oxygen effectively allows the production of ATP by phosphorylation of ADP, is called oxidative phosphorylation. In other words, synthesis of ATP is called phosphorylation, and as it takes place in presence of oxygen, it is called oxidative phosphorylation.

The enzyme required for synthesis of ATP, is called ATP synthase. This is located in F1, or head piece of F0 – F1 or elementary particles. ATP synthase enzyme becomes active in ATP formation, where there is a proton gradient saving higher concentration of H2.

ATP synthase, also known as complex V consists of two major components, i.e., F1, and F0. The F1 headpiece is a peripheral membrane protein complex and contains the site for ATP from ADP and inorganic phosphate (iP).

Whereas, F0 is an integral membrane mitochondrial-protein complex which forms the channel through which protons cross the inner membrane. The passage of protons through the channel is coupled to the catalytic site of the F1 component for the production of ATP.

Oxidation of one molecule of NADH2 produces 3 ATP molecules whereas a similar oxidation of FADH2 produces 2 ATP molecules.

Complete oxidation of glucose to CO2 and water shows that there is a net gain of 38 ATP. Each NADH + H + produces 3 ATP molecules, while FADH2 forms only 2 ATP molecules at the end of reaction.

Thus, total gain of ATP in aerobic respiration is as follows:

However, in most eukaryotic cells, 2 molecules of ATP are required for transport of NADH produced in glycolysis into mitochondrion for further oxidation, and therefore, net gain of ATP is 36 molecules.

Significance of Krebs Cycle:

a. During Krebs cycle, carbon skeletons are obtained for use in growth and maintenance of the cell.

b. Many intermediate compounds are formed which are used in synthesis of other biomolecules, such as amino acids, nucleotides, chlorophyll, cytochromes and fats.

c. During this pathway amino acids are synthesised from α-ketoglutaric acid, pyruvic acid and oxaloacetic acid.

d. Here succinyl Co-A acts as starting molecule for synthesis of chlorophyll.

e. Krebs cycle is major pathway for generation of ATP molecules, which make energy currency of the cell.

f. Energy is released from glucose, and is used in various biochemical reactions.

g. Phenol, anthocyanin, etc., are produced from acetyl Co-A, whereas fatty acids are formed from glycerol.

h. Glutamic acid is formed from α-ketoglutaric acid aspartic acid from oxaloacetic acid, and alanine from aspartic acid.

i. Amino acids are used in synthesis of proteins, nucleic acids, purines and pyrimidines.

j. Succinyl Co-A carries synthesis of pyrrole compounds of chlorophyll, cytochrome and phytochrome.

k. Krebs cycle is directly related to nitrogen metabolism, α-ketoglutaric acid, an intermediate of Krebs cycle is first acceptor molecule of NH3 forming an amino acid, the glutamic acid. From glutamic acid various transamination reactions begin to form different amino acids which ultimately condense to form proteins.

l. Krebs cycle is also intimately related with fat metabolism. Dihydroxyacetone phosphate produced in glycolysis may be converted into glycerol via glycerol-3-phosphate and vice versa. After β-oxidation, fatty acids give rise to active 2-C units, the acetyl Co-A which enters the Krebs cycle.


Krebs’ Cycle Acids

Alpha-ketoglutaric Acid, Malic Acid, Fumaric Acid, Succinic Acid, Citric Acid, Pyruvic Acid, Pantothenic Acid

These acids are intermediate compounds that are found in the Krebs’ cycle and are necessary to generate cellular energy for tissue fuel. Supplementing these essential Krebs’ cycle acids in the presence of nutrient cofactors can enable a partially completed Krebs’ cycle to go to completion. They can prevent and remove the harmful byproducts that are generated from abnormal energy production in the mitochondria. And they can stimulate a high yield of ATP from the mitochondria for tissue energy.

Supplementing these Krebs’ cycle fuel sources may be advisable for different purposes. They can help correct certain metabolic disorders that result from abnormal mitochondria energy production. They can provide an ergogenic edge in athletic performance by generating muscle energy, increasing aerobic capacity and preventing fatigue. They may be even more helpful for improving athletic performance when used in conjunction with alkalizers that buffer lactic acid build-up in muscle tissue and improve tissue oxygenation.

Alpha-ketoglutaric Acid (AKG)

Alpha-ketoglutaric acid plays a vital role in the Krebs’ cycle production of energy. As a precursor of the amino acid, glutamic acid, AKG stabilizes blood glucose levels during exercise. Alpha-ketoglutaric acid benefits the athlete by supporting protein synthesis, allowing for longer, more intense workouts, and by promoting healthy nitrogen balance.

Studies of patients given supplemental alpha-keto-glutarate following surgery found a nitrogen-sparing effect and a reduction in loss of lean body mass. Alpha-ketoglutaric acid helps reduce ammonium levels that may interfere with exercise performance. Studies have demonstrated that ammonia formed in the muscle, kidney and brain combines with alpha-ketoglutarate and L-glutamate to reduce ammonia toxicity. (31-33), (16-18)

Malic Acid

Malic acid acts as a catalyst in the Krebs’ cycle to increase energy production from the burning of pyruvic acid. Malic acid also aids in exercise recovery by counteracting the buildup of lactic acid. Supplementation of malic acid has been reported to be beneficial in Chronic Fatigue Syndrome by reducing symptoms of persistent fatigue, muscular myalgia and arthritic-like pains.

Fumaric Acid

Fumaric acid is the trans-isomer of malic acid that enters the citric acid cycle. It’s a byproduct at certain stages in the arginine-urea cycle and purine biosynthesis. In healthy individuals, fumaric acid is formed in the skin from exposure to sunlight. A deficiency of fumaric acid leads to the accumulation of metabolic half-products that may be responsible for causing the skin lesions of psoriasis. Sufferers of psoriasis have a biochemical defect in which they do not produce enough fumaric acid, requiring prolonged exposure to the sun. Administration of fumaric acid to individuals suffering from psoriasis has caused a gradual elimination of the symptoms. (40-47), (25-32)

Succinic Acid

Succinic acid, like other Krebs’ cycle intermediates, is an entry pathway for other metabolites into the cycle and is involved in a variety of important biological actions. In addition to its enzyme activity, it combines with protein to rebuild muscle fiber and nerve endings, and helps fight infection. Individuals with Chronic Fatigue Syndrome have shown low levels of succinic acid in their urine.
Several amino acids are metabolized into succinic acid, providing a source of anaerobic and aerobic energy. Amino acids that are metabolized into succinic acid have been shown to be important in supplying the heart with fuel for myocardium contractions under low oxygen conditions. The amino acid GABA can either be oxidized to succinic acid for cellular energy production, or reduced to GHB, depending on the metabolic needs of the body. (48-50), (33-35)

Citric Acid

Citric acid, a natural organic acid present to some extent in all plant and animal tissues, occupies a pivotal location in the Krebs’ cycle. After proteins, fats, carbohydrates and amino acids have been oxidized into acetyl coenzyme A, the acetic acid subunit of acetyl CoA is combined with oxaloacetate to form a molecule of citrate. The acetyl coenzyme A acts as a transporter of acetic acid from one enzyme to another.

First isolated by the German biochemist, Karl Wilhelm Steele in 1784, today citric acid is widely respected for relieving conditions of fatigue, poor digestion, cold and flu infections, asthma, hypertension and cholesterol deposits in blood vessels.

Pyruvic Acid

Pyruvic acid is a three-carbon ketoacid produced in the end stages of glycolysis. In the mitochondria, pyruvic acid is either reduced to lactate in the cytoplasm, or oxidized to acetyl CoA.

Research has shown that taking pyruvate (the salt of pyruvic acid) can increase muscle endurance and promote fat loss. Pyruvic acid also appears to increase the amount of glucose that enters muscle cells from the circulating blood. This ability of pyruvic acid leads to increases in immediate available energy, as well as increasing stored muscle glycogen levels for future energy. Research has shown that pyruvic acid increases muscle endurance and improves cardiac efficiency.

In one study pyruvic acid was found to increase glucose extraction by almost 300% and muscle glycogen by 50% after one hour of exercise. The researchers found that arm endurance increased by 150% and leg endurance by 60%. Another study conducted at the University of Pittsburgh School of Medicine found that pyruvic acid produced a significant amount of weight loss and fat loss in obese women on a low calorie liquid diet. Two potential mechanisms by which pyruvic acid enhances both fat and weight loss are through increasing both resting metabolic rate and fat utilization. (51-56), (36-41)

Pantothenic Acid

Vitamin B5 is required for the synthesis of coenzyme A. Supplementation of panthenine (pantothenate bound to cysteamine) has been shown to reduce elevated blood lipids in humans. It is postulated that this action is due to the accelerated synthesis of coenzyme A. It has also produced an anti-arrhythmic effect in animal hearts by increasing ATP synthesis. A study of elite distance runners who were given two grams of pantothenic acid daily for two weeks found a 17% reduction in lactic acid buildup and a seven percent reduction in oxygen consumption during prolonged, strenuous exercise. (57-61), (42-46)

Summary

The Krebs’ cycle is an eloquent and essential system designed to generate large amounts of cellular energy required for life. Disruption of the Krebs’ cycle, whether caused by deficiencies in energy substrates, acquired or inherited disease states, or physical stress, leads to an inhibition of normal energy production and contributes to a wide range of metabolic disturbances and symptoms.

The use of supplemental Krebs’ cycle acids and anti-fatigue buffers can assist in the management of mitochondrial energy substrates and increase cellular energy production. Such a nutritional approach can be of benefit to athletes, anyone who is aging, as well as those suffering from metabolic disturbances caused by inherited mitochondrial diseases or acquired diseases, such as Alzheimer’s disease and Chronic Fatigue Syndrome (CFS).


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Glycolysis

Glycolysis is the metabolic pathway where one molecule of glucose(C6H12O6) converts into pyruvic acid by the help of enzyme. Glycolysis occurs in the cytoplasm of the cell during both anaerobic and aerobic respiration. It is also known as EMP pathway i.e., Embden-Meyerhof-Parnas pathway named after German Biochemists Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas who first discovered the process of glycolysis in 1918. Glycolysis is also called Entner–Doudoroff pathway.

Features of Glyclysis

Step of Glycolysis

Reaction of glycolysis include the following three steps:

  1. Phosphorylation of glucose or Preparatory Phase
  2. Cleavage of Fructose-1, 6-diphosphate
  3. Formation of 3-carbon pyruvate or pyruvic acid

Phosphorylation of glucose or Preparatory Phase

1. At the first step, glucose undergoes phosphorylation by ATP(Adenosine triphosphate) in presence of Mg++ to form glucose-6-phosphate in the presence of hexokinase enzyme.

2. By the process of isomerization Glucose 6-phosphate is isomerized into fructose 6-phosphate with the help of phosphogluco isomerase enzyme.

3. Fructose 6-phosphate undergoes phosphorylation with the help of ATP and enzyme phosphofructokinase to form Fructose 1, 6-bisphosphate and ADP (Adenosine diphosphate).

Cleavage of Fructose-1, 6-bisphosphate

4. Fructose 1,6-diphosphate is broken down to two triose (3 carbon molecule) phosphate such as dihydroxyacetone phosphate and 3 phosphoglyceraldehyde with the help of the enzyme aldolase. The dihydroxyacetone phosphate is converted to 3 phosphoglyceraldehyde with the help of enzyme triose phosphate isomerase. In this case, reaction is reversible. Here two molecules of 3-phosphoglyceraldehyde are formed from the cleavage of one fructose 1, 6-biphosphate.

Formation of 3-carbon pyruvate or pyruvic acid

5. With the help of NAD (nicotinamide adenine dinucleotide), H3PO4(phosphoric acid) and the enzyme phosphoglyceraldehyde dehydrogenase, 3 phosphoglyceraldehyde is oxidized to 1, 3-diphosphoglyceric acid and NADH2.

6. In this step 1, 3 diphosphoglyceric acid transfers phosphoric acid to ADP with the formation of 3 phosphoglyceric acid and ATP with the help of enzyme phosphoglyceric acid kinase.

7. In the next step 3 phosphoglyceric acid is converted to 2 phosphoglyceric acid with the help of enzyme phosphoglyceromutase.

8. 2 phosphoglyceric acid is then converted to form 2 phosphoenol pyruvic acid with the help of enzyme enolase which gives out one molecule of water.

9. It is the last step of glycolysis where 2 phosphoenol pyruvic acid is converted to form pyruvic acid by the removal of phosphorus thus one molecule of ATP is synthesized from ADP. The enzyme catalyzing this step is pyruvic acid kinase.

So in the overall process, two molecules of pyruvic acid is formed from each molecule of glucose. In animals including human being glycogen is present in the muscle and liver cells, are phosphorylated by the glycogen phosphorylase enzyme in presence of inorganic phosphate into glucose 1 phosphate. Similarly starch of plant cells is converted to glucose 1-phosphate by the starch phosphorylase. Glucose 1-phospahte is then converted to glucose 6-phosphate by the enzyme phosphoglucomutase. Glucose 6-phosphate is then oxidized through the glycolytic path.

Thus when one molecule of glucose (6C) undergoes the reactions in glycolysis, the overall process may be represented as follows:

In this case, 2 molecules of ATP are used up in the phase of glycolysis.

Therefore the net gains of glycolysis are:

Significance of Glycolysis


Kreb`s cycle or citric acid cycle is an oxidation process which occurs stepwise. In this case, it includes four dehydogenase steps and two decarboxylation steps. It produces reduced co-enzymes and CO2.

Pyruvic acid is formed through the process of glycolysis in cell cytoplasm. After formation of pyruvic acid, it enters into the mitochondria. In the presence of six factors such as Mg ++ , NAD, TPP (Thiamine pyrophosphate), lipoic acid, FAD and coenzyme A, the pyruvic dehydogenase along with enzyme complex converts pyruvate to acetyle CoA.

Overall steps of citric acid cycle are described below:

Step-1: The first step is the condensation step. In this step, acetyle CoA mix with oxaloacetate and H2O in the presence of condensing enzymes citratrate synthetage and produce one molecule of citric acid. After reaction CoA is released out. In this case, acetyl CoA is two carbon molecule, oxaloacetate is 4 carbon molecule while cytric acid or citrate is 6 carbon molecule.

Step-2: It is the isomirization step. In this step, cytric acid is converted into its isomer isocitrate by completing the following two step reactions with the help of aconitase enzyme.

(i) Dehydration: In this case, one molecule of H2O is released out and citric acid is converted into cis-aconitic acid.

(ii) Rehydration: In this case, cis-aconitic acid joins with one molecule of H2O and produce isocitric acid.

Step-3: The third step is the dehydrogenation step. In this step, isocitrate /isocitric acid is dehydrogenated into oxalosuccinic acid by losing 2H - with the help of isocitrate dehydrogenase enzyme and Mn ++ . The enzyme isocitrate dehydrogenase catalyzes this step and this enzyme is responsible to regulate the speed of the citric acid cycle. During this step, NAD (Nicodinamide adenine dinucleotide) is reduced and forms NADH2.

Step-4: The fourth step is the decarboxylation step. In this step, oxalosuccinic acid is decarboxylated into α-ketoglutaric acid by losing CO2 with the help of enzyme, oxalosuccinate decarboxylase.

Step-5: It is oxidative decarboxylation step where α-ketoglutaric acid undergoes dehydrogenation and decarboxylation at the same time with the help of enzyme, ketoglutarate dehydrogenase. The enzyme, ketoglutarate dehydrogenase catalyzes and it is responsible for regulating the speed of the citric acid cycle. In this step, NAD+, Mg++, and CoA are required. Finally, succinyl CoA, NADH2 and CO2 are produced.

Step-6: It is the substrate level GTP or ATP synthesis step. In this step succinyl CoA is synthesized into succinic acid with the help of enzyme, succinyl-CoA synthatase. It is energy liberated step and during this step, one molecule of molecule of GTP is produced and CoA is released.

Step-7: This step is also known as dehydrogenation step. In this step, succinic acid is dehydrogenated into four-carbon molecule fumaric acid in the presence of succinate dehydrogenase enzyme. In this case, hydrogen is given out by succinic acid and is picked up by FAD(Flavin adenine dinucleotide) to form FADH2.

Step-8: In this step, fumaric acid is converted into a 4 carbon molecule malic acid. In this case, fumaric acid reacts with one molecule of H2O in the presence of enzyme fumarase.

Step-9: In this step, malic acid is dehydrogenated into oxaloacetic acid in the presence of malate dehydogenase enzyme. In this reaction, NAD + is reduced to form NADH2.

Oxaloacetic acid again joins with acetyle CoA and again begins a new citric acid cycle. The oxidative catabolism of pyruvate can be shown in the following equation:


Step 4 . Redox & Decarboxylation

  1. a NAD + is reduced to NADH, and
  2. the reactant loses a carboxyl group to produce carbon dioxide.

Notice the hydrogen (blue) on coenzyme A is released as a hydride ion (blue). As usual, this is picked up by the oxidizing agent NAD + thus reducing it to NADH. When the carboxyl group (green) is removed the oxidized coenzyme A (red) attaches to that site. The new product, succinyl CoA, is a still a two-carbon chain but it has only one carbonyl group on carbon #2 there are four carbons in all.


5.5 | Glycolysis

You have read that nearly all of the energy used by living cells comes to them in the bonds of the sugar, glucose. Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. Nearly all living organisms carry out glycolysis as part of their metabolism. The process does not use oxygen and is therefore anaerobic. Glycolysis takes place in the cytoplasm of both prokaryotic and eukaryotic cells. Glucose enters heterotrophic cells in two ways. One method is through secondary active transport in which the transport takes place against the glucose concentration gradient. The other mechanism uses a group of integral proteins called GLUT proteins, also known as glucose transporter proteins. These transporters assist in the facilitated diffusion of glucose.

Glycolysis begins with the six carbon ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate. Glycolysis consists of two distinct phases. The first part of the glycolysis pathway traps the glucose molecule in the cell and uses energy to modify it so that the six-carbon sugar molecule can be split evenly into the two three-carbon molecules. The second part of glycolysis extracts energy from the molecules and stores it in the form of ATP and NADH, the reduced form of NAD.

Figure 5.25 The first half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which is then split into two three-carbon molecules.

Figure 5.26 The second half of glycolysis involves phosphorylation without ATP investment (step 6) and produces two NADH and four ATP molecules per glucose.

Gain a better understanding of the breakdown of glucose by glycolysis by visiting this site(http://openstaxcollege.org/l/glycolysis) to see the process in action.

Outcomes of Glycolysis

The last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is not available in sufficient quantities. In this situation, the entire glycolysis pathway will proceed, but only two ATP molecules will be made in the second half. Thus, pyruvate kinase is a rate limiting enzyme for glycolysis.


7.3 Symbiosis And The Origin of Chloroplasts

Several groups of animals have formed symbiotic relationships with photosynthetic algae. These are most common in corals, sponges and sea anemones. It is presumed that this is due to the particularly simple body plans and large surface areas of these animals compared to their volumes. In addition, a few marine mollusks Elysia viridis and Elysia chlorotica also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies (see Kleptoplasty). This allows the mollusks to survive solely by photosynthesis for several months at a time. Some of the genes from the plant cell nucleus have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive.

An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with photosynthetic bacteria, including a circular chromosome, prokaryotic-type ribosome, and similar proteins in the photosynthetic reaction center. The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis) by early eukaryotic cells to form the first plant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like mitochondria, chloroplasts possess their own DNA, separate from the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those found in cyanobacteria. DNA in chloroplasts codes for redox proteins such as those found in the photosynthetic reaction centers. The CoRR Hypothesis proposes that this co-location of genes with their gene products is required for redox regulation of gene expression, and accounts for the persistence of DNA in bioenergetic organelles.


Question 1.
Maximum amount of energy/ATP is liberated on oxidation of:
(a) fats
(b) proteins
(c) starch
(d) vitamins

Question 2.
When one glucose molecule is completely oxidised, it changes:
(a) 36 ADP molecules into 36 ATP molecules
(b) 38 ADP molecules into 38 ATP molecules
(c) 30 ADP molecules into 30 ATP molecules
(d) 32 ADP molecules into 32 ATP molecules

Answer: (b) 38 ADP molecules into 38 ATP molecules

Question 3.
Glycolysis takes place in
(a) Cytoplasm
(b) Chloroplast
(c) Ribosome
(d) Mitochondria

Question 4.
Most of the energy of the carbohydrates is released by oxidation when
(a) Pyruvic acid is converted into CO2 and H2O
(b) Pyruvic acid is converted into acetyl Co-A
(c) Sugar is converted into pyruvic acid
(d) Glucose is converted into alcohol and CO2

Answer: (a) Pyruvic acid is converted into CO2 and H2O

Question 5.
ATP is injected in cyanide poisoning because it is:
(a) necessary for cellular functions
(b) necessary for Na+ – K+ pump
(c) Na+ – K+ pump operates at the cell membranes
(d) ATP breaks down cyanide

Answer: (a) necessary for cellular functions

Question 6.
Acetyl CoA combine with oxalo-acetate in presence of condensing enzyme citrate synthase to form 6-C compound called
(a) Malic acid
(b) Tartaric acid
(c) Pyruvic acid
(d) Citric acid

Question 7.
Number of oxygen atoms required for aerobic oxidation of one pyruvate
(a) 10
(b) 8
(c) 5
(d) 12

Question 8.
The TCA cycle is named after
(a) Robert Emerson
(b) Melvin Calvin
(c) Embden
(d) Hans Krebs

Question 9.
Most of the enzymes of the TCA cycle are present in
(a) Intermembrane space of mitochondria
(b) Mitochondrial matrix
(c) Inner membrane of mitochondria
(d) Cytoplasm

Answer: (b) Mitochondrial matrix

Question 10.
Energy obtained by a cell from catabolic reaction is stored immediately in the form of
(a) Glucose
(b) Pyruvic acid
(c) ADP
(d) ATP

Question 11.
Oxidative phosphorylation is production of
(a) ATP in photosynthesis
(b) NADPH in photosynthesis
(c) ATP in respiration
(d) NADH in respiration

Answer: (c) ATP in respiration

Question 12.
The net gain of ATP during glycolysis is
(a) 4
(b) 8
(c) 2
(d) 6

Question 13.
Complete oxidation of 1 gm mol of glucose gives rise to
(a) 6860000 cals
(b) 686000 cals.
(c) 68600 cals.
(d) 6860 cals.

Question 14.
The term ‘Glycolysis’ has originated from the Greek words
(a) Glucose and lysis
(b) Glyco and lysis
(c) Glycose and lysis
(d) Glykos and lysis

Answer: (d) Glykos and lysis

Question 15.
Which of the following is not correct about the Krebs cycle?
(a) It starts with a six-carbon compound.
(b) It occurs in mitochondria.
(c) It is also called the citric acid cycle.
(d) The intermediate compound which links glycolysis with the Krebs cycle is malic acid.

Answer: (d) The intermediate compound which links glycolysis with the Krebs cycle is malic acid.

Question 16.
End-product of citric acid/Krebs cycle is​
(a) Citric acid
(b) CO2 + H2O
(c) Lactic acid
(d) Pyruvic acid

Question 17.
Oxidative phosphorylation involves simultaneous oxidation and phosphorylation to finally form:
(a) pyruvate
(b) NADP
(c) DPN
(d) ATP

Question 18.
The respiratory ratio of protein is
(a) 0.2
(b) 0.9
(c) 1.0
(d) 0.7

Question 19.
In which of the following do the two names refer to one and the same thing?
(a) Krebs cycle and Calvin cycle
(b) Citric acid cycle and Calvin cycle
(c) Tricarboxylic acid cycle and citric acid cycle
(d) Tricarboxylic acid cycle and urea cycle

Answer: (c) Tricarboxylic acid cycle and citric acid cycle

Question 20.
Respiratory quotient (R.Q.) for fatty acid is:
(a) > 1
(b) < 1
(c) 1
(d) 0

Question 21.
The universal hydrogen acceptor is
(a) NAD
(b) ATP
(c) Co-A
(d) FMN

Question 22.
Out of 36 ATP molecules produced per glucose molecule during respiration:
(a) 2 are produced outside glycolysis and 34 during respiratory chain
(b) 2 are produced outside mitochondria and 34 inside mitochondria
(c) 2 during glycolysis and 34 during Krebs cycle
(d) All are formed inside mitochondria

Answer: (b) 2 are produced outside mitochondria and 34 inside mitochondria

Question 23.
The net gain of ATP molecules by glycolysis is
(a) Zero
(b) Two
(c) Four
(d) Eight

Question 24.
In anaerobic respiration seeds respire
(a) In presence of O2
(b) In presence of CO2
(c) In absence of O2
(d) In absence of CO2

Answer: (c) In absence of O2

Question 25.
End product of glycolysis is:
(a) acetyl CoA
(b) pyruvic Acid
(c) glucose 1-phosphate
(d) fructose 1-phosphate

Question 26.
Connecting link between glycolysis and Krebs cycle is/before entering Krebs cycle pyruvate is changed to:
(a) oxaloacetate
(b) PEP
(c) pyruvate
(d) acetyl CoA

Question 27.
Glycolysis is conversion of
(a) Glucose to citric acid
(b) Glucose to fructose
(c) Glucose to pyruvic acid
(d) Glucose to malic acid

Answer: (c) Glucose to pyruvic acid

Question 28.
Incomplete oxidation of glucose into pyruvic acid with several intermediate steps is known as
(a) TCA-pathway
(b) Glycolysis
(c) HMS-pathway
(d) Krebs cycle

Question 29.
Cytochromes are concerned with
(a) Protein synthesis
(b) Cellular digestion
(c) Cell division
(d) Cell-respiration

Question 30.
Common immediate source of energy in cellular activity is
(a) glucose
(b) aldohexose
(c) ATP
(d) NAD

Question 31.
The following is required both by the process of respiration and photosynthesis
(a) Carbohydrates
(b) Sunlight
(c) Chlorophyll
(d) Cytochromes

Question 32.
Out of 38 ATP molecules produced per glucose, 32 ATP molecules are formed from NADH/FADH2 in:
(a) respiratory chain
(b) Krebs cycle
(c) oxidative decarboxylation
(d) EMP

Answer: (a) respiratory chain

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