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BBCCT-109: Metabolism of Carbohydrates and Lipids

BBCCT-109: Metabolism of Carbohydrates and Lipids

IGNOU Solved Assignment Solution for 2023

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Assignment Code: BBCCT-109/TMA/2023

Course Code: BBCCT-109

Assignment Name: Metabolism of Carbohydrates and Lipids

Year: 2023

Verification Status: Verified by Professor


Attempt all questions. The marks for each question are indicated against it.


Q1) Define the terms:


(a) Autotrophs

Ans) Autotrophs are organisms that can produce their own food using energy from non-living sources such as sunlight or inorganic compounds. They are also known as primary producers because they form the basis of most food chains and ecosystems on Earth. Autotrophs can be further divided into two categories: photoautotrophs, which use light energy to produce food, and chemoautotrophs, which use energy from inorganic chemicals to produce food. Examples of autotrophs include plants, algae, and some bacteria.


(b) Amphibolic reactions

Ans) Amphibolic reactions are metabolic pathways that can both catabolize (break down) and anabolize (synthesize) molecules. These reactions play a crucial role in the metabolic pathways of living organisms, particularly in energy metabolism. Amphibolic reactions occur in many pathways, including the citric acid cycle, which is a central metabolic pathway in aerobic respiration. In this cycle, some of the intermediates produced through catabolism can be used to synthesize new molecules, such as amino acids and nucleotides.


(c) Fermentation

Ans) Fermentation is a metabolic process in which an organism converts organic compounds (usually carbohydrates) into simpler compounds without the involvement of oxygen. It is a type of anaerobic respiration and is used by many microorganisms, including bacteria and yeast, to produce energy when oxygen is not available. During fermentation, the organic compounds are broken down through a series of enzymatic reactions, producing energy in the form of ATP, as well as waste products such as ethanol or lactic acid. This process is used in the production of many food and beverage products, including beer, wine, bread, and yogurt. Fermentation can also occur in muscle cells when oxygen is not available, leading to the production of lactic acid, which can cause muscle fatigue and soreness. Overall, fermentation plays a crucial role in energy production and the metabolism of many organisms.


(d) Primary Metabolic Pathways

Ans) Primary metabolic pathways are the fundamental biochemical processes that are essential for the survival and growth of an organism. These pathways involve the conversion of small molecules such as glucose, amino acids, and fatty acids into various other molecules that are needed for cellular processes. They are called primary metabolic pathways because they are necessary for the basic functioning of the cell, as opposed to secondary metabolic pathways that are more specialized and produce specific compounds that are not essential for cell survival.


(e) Anabolism

Ans) Anabolism is the set of metabolic pathways that build larger and more complex molecules from smaller molecules. This process requires energy, typically in the form of ATP, and is the opposite of catabolism, which is the breakdown of larger molecules into smaller ones. Anabolism is essential for the growth and maintenance of all living organisms. Anabolic pathways involve a variety of biochemical reactions, including the synthesis of proteins, nucleic acids, and complex carbohydrates from their building blocks. These reactions require energy and reducing power, which is provided by ATP and reducing equivalents such as NADPH. Anabolic pathways are often regulated by hormones and enzymes, which control the rate and direction of the reactions.


Q2) (a) Explain metabolism and write its functions.

Ans) Metabolism refers to all the chemical processes that occur within an organism to maintain life. It involves the conversion of nutrients into energy and the synthesis of molecules necessary for cellular processes. Metabolism can be divided into two main processes: catabolism, which breaks down molecules into smaller units and releases energy, and anabolism, which builds larger molecules from smaller units and requires energy. The balance between these two processes is critical for maintaining the energy and nutrient requirements of an organism. The functions of metabolism include:

  1. The conversion of food into energy that is used to power cellular processes.

  2. The synthesis of biomolecules such as proteins, nucleic acids, and lipids that are needed for cellular processes and growth.

  3. The removal of waste products and toxins from the body.

  4. The maintenance of cellular homeostasis and the regulation of physiological processes such as temperature and pH.

  5. The storage of energy in the form of glycogen, fats, and other compounds for future use.


Q2) (b) What is substrate level phosphorylation? Write reactions of glycolysis in which substrate level phosphorylation takes place.

Ans) Substrate-level phosphorylation is a process in which ATP is synthesized directly from a phosphorylated intermediate molecule during a metabolic pathway. This occurs when a high-energy phosphate group is transferred from an organic compound to ADP, resulting in the formation of ATP. Unlike oxidative phosphorylation, which requires the presence of oxygen and involves the electron transport chain, substrate-level phosphorylation does not require oxygen and can occur during anaerobic metabolism.

In the glycolytic pathway, substrate-level phosphorylation occurs twice during the conversion of glucose into pyruvate. The first substrate-level phosphorylation occurs when the enzyme hexokinase transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate. The second occurs during the conversion of phosphoenolpyruvate (PEP) to pyruvate, when the enzyme pyruvate kinase transfers a phosphate group from PEP to ADP, forming ATP and pyruvate.


Q3) (a) With the help of a neatly labelled diagram, explain different steps of TCA cycle and its significance.

Ans) The TCA (tricarboxylic acid) cycle, also known as the Krebs cycle or citric acid cycle, is a fundamental metabolic pathway that is essential for the generation of ATP (adenosine triphosphate) and the synthesis of a variety of biomolecules. It takes place in the mitochondria of eukaryotic cells and the cytoplasm of prokaryotic cells.


The cycle starts with the formation of citrate, which is derived from the condensation of acetyl-CoA and oxaloacetate by the enzyme citrate synthase.


The following are the different steps of the TCA cycle:

  1. Citrate Isomerization: The citrate molecule undergoes isomerization to form isocitrate by the enzyme aconitase.

  2. Oxidative Decarboxylation of Isocitrate: Isocitrate is oxidatively decarboxylated and forms alpha-ketoglutarate by the enzyme isocitrate dehydrogenase. This reaction releases CO2 and generates NADH.

  3. Alpha-Ketoglutarate Decarboxylation: Alpha-ketoglutarate is oxidatively decarboxylated to form succinyl-CoA by the enzyme alpha-ketoglutarate dehydrogenase complex. This reaction releases CO2 and generates NADH.

  4. Succinyl-CoA Formation: Succinyl-CoA is formed from succinate thiokinase (also known as succinyl-CoA synthetase). This reaction generates ATP and CoA.

  5. Conversion of Succinate to Fumarate: Succinate is oxidized to form fumarate by the enzyme succinate Fumarate Conversion to Malate: Fumarate is hydrated to form malate by the enzyme fumarase.

  6. Oxidation of Malate to Oxaloacetate: Malate is oxidized to form oxaloacetate by the enzyme malate dehydrogenase. This reaction generates NADH.

Q3) (b) Describe the coordinated regulation of TCA and glyoxylate cycles.

Ans) The TCA (tricarboxylic acid) cycle and glyoxylate cycle are two essential metabolic pathways that are closely linked in plants, bacteria, and fungi. The TCA cycle is responsible for the oxidation of acetyl-CoA to CO2 and H2O, generating energy in the form of ATP, NADH, and FADH2. The glyoxylate cycle, on the other hand, is an anaplerotic pathway that enables the synthesis of carbohydrates from fatty acids in organisms that can use acetate or fatty acids as a carbon source.


The coordinated regulation of the TCA and glyoxylate cycles is essential to maintain energy balance and the appropriate use of metabolic intermediates. In plants and bacteria, the activity of isocitrate lyase, a key enzyme in the glyoxylate cycle, is regulated by the availability of glucose and other carbon sources. When glucose is abundant, the activity of isocitrate lyase is repressed, and the carbon flow is directed towards the TCA cycle. When glucose is limited, the activity of isocitrate lyase is induced, and the carbon flow is directed towards the glyoxylate cycle.


Q4) (a) Describe the process of glycogenesis.

Ans) Glycogenesis is the process of glycogen synthesis in the liver and muscle cells. It involves the conversion of excess glucose into glycogen, which is a branched polysaccharide that serves as a storage form of glucose. The process of glycogenesis is stimulated by the hormone insulin, which promotes the uptake of glucose by the liver and muscle cells. Once inside the cells, glucose is converted into glucose-6-phosphate by the enzyme hexokinase. Glucose-6-phosphate is then converted into glucose-1-phosphate by the enzyme phosphoglucomutase. Finally, glucose-1-phosphate is added to the growing glycogen chain by the enzyme glycogen synthase. The process of glycogenesis is essential for maintaining blood glucose levels and providing a source of energy during periods of fasting or exercise.


Q4) (b) What are glycogen storage diseases? Explain any two.

Ans) Glycogen storage diseases are a group of inherited metabolic disorders that affect the metabolism of glycogen in the body. GSDs are caused by mutations in genes that encode enzymes involved in glycogen metabolism, leading to abnormal glycogen storage in various tissues.

Two examples of GSDs are:

  1. Pompe Disease (GSD type II): Pompe disease is a rare genetic disorder caused by the deficiency of the enzyme acid alpha-glucosidase (GAA), which is responsible for breaking down glycogen into glucose. As a result, glycogen accumulates in various tissues, particularly in muscles and the heart, leading to muscle weakness, respiratory failure, and heart problems. Treatment of Pompe disease involves enzyme replacement therapy.

  2. Von Gierke Disease (GSD type I): Von Gierke disease is caused by the deficiency of the enzyme glucose-6-phosphatase, which is responsible for releasing glucose from glycogen in the liver. As a result, glycogen accumulates in the liver, leading to an enlarged liver, low blood sugar levels, and metabolic abnormalities. Treatment of Von Gierke disease involves frequent feeding of carbohydrates to prevent hypoglycemia and other complications.


Both GSDs demonstrate the importance of glycogen metabolism for normal cellular function and energy balance in the body. The severity of symptoms and prognosis of GSDs vary depending on the specific enzyme deficiency and the extent of glycogen accumulation in various tissues. Treatment options for GSDs include dietary modifications, enzyme replacement therapy, and gene therapy.


Q5) (a) Explain the mechanisms involved in concentration of CO2 in C4 plants and indicate the relevance of these adaptations for plant growth.

Ans) C4 plants have evolved a specialized mechanism to concentrate carbon dioxide (CO2) in their mesophyll cells to increase the efficiency of photosynthesis. The mechanism involves the initial fixation of CO2 into a four-carbon compound, which is then transported to the bundle-sheath cells for further processing via the Calvin cycle. The CO2 is then released from the four-carbon compound within the bundle-sheath cells and is used for photosynthesis.


This mechanism allows C4 plants to continue photosynthesis even at low atmospheric CO2 levels and under high light and temperature conditions. The higher efficiency of CO2 fixation in C4 plants leads to a higher rate of photosynthesis, resulting in faster growth and higher yields compared to C3 plants.


Therefore, the adaptation of concentrating CO2 in C4 plants is essential for their growth and survival in hot and dry environments. The mechanism allows C4 plants to maintain a high rate of photosynthesis while minimizing water loss through transpiration. The ability to concentrate CO2 also enables C4 plants to thrive in areas with low atmospheric CO2 levels, such as in arid regions, and to compete with other plants for resources.


Q5) (b) Compare the characteristics of C3, C4 and CAM plants.



Part B


Q6 (a) How are fatty acids activated and transported to the site of their oxidation?

Ans) Fatty acids are activated and transported to the site of their oxidation in a series of steps that involve different enzymes and transporters.


First, fatty acids are activated in the cytosol of the cell by conjugating them with coenzyme A (CoA), forming acyl-CoA. This process is catalysed by an enzyme called acyl-CoA synthetase and requires ATP.


Next, the activated fatty acids are transported across the mitochondrial membrane to the site of their oxidation, which occurs in the mitochondrial matrix. This is facilitated by a carnitine shuttle system, which involves the enzyme carnitine palmitoyltransferase I (CPTI) and carnitine-acylcarnitine translocase (CACT).


The carnitine shuttle system works as follows: CPTI catalyses the transfer of the fatty acyl group from CoA to carnitine, forming acylcarnitine. The acylcarnitine is then transported across the mitochondrial membrane by CACT, which exchanges it for a free carnitine molecule. Once inside the mitochondrial matrix, the acylcarnitine is converted back to acyl-CoA by the enzyme carnitine palmitoyltransferase II (CPTII).


Once inside the mitochondrial matrix, the activated fatty acids undergo β-oxidation, a process that breaks down the fatty acids into acetyl-CoA molecules, which can be used for energy production.


Q6) (b) Define ketogenesis, is it a normal, physiological process? Explain why it goes up in conditions of starvation and uncontrolled diabetes?

Ans) Ketogenesis is the process by which ketone bodies are synthesized in the liver from fatty acids during periods of low carbohydrate availability, such as during starvation or in uncontrolled diabetes. Ketone bodies, such as acetoacetate and beta-hydroxybutyrate, are water-soluble molecules that can be used as an alternative fuel source by the body when glucose is scarce.


Ketogenesis is a normal, physiological process that occurs in response to low carbohydrate availability. In healthy individuals, the production of ketone bodies is tightly regulated and only occurs when glucose levels are low.


In conditions of starvation or uncontrolled diabetes, however, the production of ketone bodies can become excessive, leading to a condition called ketosis. This occurs because the body is unable to use glucose for energy due to the low availability of insulin or glucose, and instead relies on the breakdown of fatty acids for energy production.


In uncontrolled diabetes, the lack of insulin leads to a state of insulin resistance, which can increase the breakdown of fatty acids and the production of ketone bodies. In starvation, the depletion of glycogen stores leads to the breakdown of fatty acids and the production of ketone bodies as an alternative fuel source.


While ketogenesis is a normal, physiological process, excessive production of ketone bodies can be dangerous and can lead to a condition called ketoacidosis. This is why it is important to monitor ketone levels in individuals with uncontrolled diabetes or other conditions that can lead to excessive ketone production.


Q7) (a) How does β oxidation of fatty acids in peroxisomes different from that occurring in mitochondria?

Ans) β-oxidation of fatty acids is a process by which fatty acids are broken down into acetyl-CoA molecules, which can be used for energy production. This process occurs in both peroxisomes and mitochondria, but there are some key differences between the two.


One major difference is the type of fatty acids that are oxidized. In peroxisomes, very-long-chain fatty acids (VLCFAs) are oxidized, while in mitochondria, medium- to long-chain fatty acids are oxidized. This is because peroxisomes contain a specific enzyme called acyl-CoA oxidase that can oxidize VLCFAs.


Another difference is the use of oxygen. β-oxidation in mitochondria requires the use of oxygen, while β-oxidation in peroxisomes does not. This means that peroxisomal β-oxidation can occur under anaerobic conditions, while mitochondrial β-oxidation cannot.


The final difference is the fate of the acetyl-CoA molecules that are produced. In mitochondria, the acetyl-CoA molecules are further oxidized in the TCA cycle to produce energy. In peroxisomes, however, the acetyl-CoA molecules are not oxidized further, but instead undergo other metabolic pathways, such as the synthesis of bile acids or the production of acetylcholine.


Q7) (b) Illustrate organization of various domains of animal fatty acid synthase and write their activities.

Ans) Animal fatty acid synthase is a large, multi-domain enzyme complex responsible for the de novo synthesis of fatty acids. FAS consists of several distinct domains, each of which plays a specific role in the fatty acid synthesis pathway.


The organization of the various domains of animal FAS is as follows:

  1. Acyl Carrier Protein (ACP) Domain: This domain is responsible for carrying the growing fatty acid chain during the synthesis process.

  2. Beta-Ketoacyl-ACP Synthase (KS) Domain: This domain catalyses the condensation of two acetyl-CoA molecules to form acetoacetyl-ACP, the first step in the fatty acid synthesis pathway.

  3. Beta-Ketoacyl-ACP Reductase (KR) Domain: This domain reduces the beta-keto group of the acetoacetyl-ACP to form (R)-3-hydroxybutyryl-ACP.

  4. Dehydratase (DH) Domain: This domain removes a water molecule from (R)-3-hydroxybutyryl-ACP to form crotonyl-ACP.

  5. Enoyl-ACP Reductase (ER) Domain: This domain reduces the double bond in crotonyl-ACP to form butyryl-ACP.

  6. Acyltransferase (AT) Domain: This domain adds a malonyl-CoA molecule to the butyryl-ACP to initiate the next cycle of fatty acid synthesis.


The activities of these domains are critical for the synthesis of fatty acids. The KS domain catalyses the first step in the pathway, while the KR and DH domains reduce and dehydrate the growing fatty acid chain, respectively. The ER domain then reduces the double bond in the chain, and the AT domain adds another malonyl-CoA molecule to initiate the next round of synthesis.


Q8) (a) Explain the allosteric regulation of fatty acid biosynthesis.

Ans) The process of fatty acid biosynthesis is regulated by allosteric regulation. Allosteric regulation involves the binding of regulatory molecules to enzymes, which can either enhance or inhibit their activity. In fatty acid biosynthesis, the enzymes that are involved in the process are regulated by various molecules such as malonyl-CoA, citrate, and AMP.


Malonyl-CoA is a potent inhibitor of fatty acid oxidation and an activator of fatty acid synthesis. It is synthesized by the enzyme acetyl-CoA carboxylase (ACC) and inhibits the enzyme carnitine palmitoyltransferase 1 (CPT1), which is involved in the transport of fatty acids into the mitochondria for oxidation. This inhibition prevents the oxidation of fatty acids and allows for their synthesis.


Citrate, on the other hand, is an activator of ACC and stimulates the synthesis of malonyl-CoA. It is produced during the TCA cycle and transported to the cytoplasm where it activates ACC, promoting the synthesis of malonyl-CoA and, subsequently, the synthesis of fatty acids.


AMP, a product of ATP hydrolysis, is an inhibitor of ACC and reduces the synthesis of malonyl-CoA, thereby inhibiting fatty acid synthesis.

Q8) (b) Write the role of various lipoproteins in lipid transport.

Ans) Lipoproteins are complexes of lipids and proteins that play a critical role in the transport of lipids, such as cholesterol and triglycerides, in the blood. There are several types of lipoproteins, including chylomicrons, very-low-density lipoprotein, intermediate-density lipoprotein, low-density lipoprotein, and high-density lipoprotein.


Chylomicrons are formed in the intestine and transport dietary lipids to the liver and peripheral tissues. VLDLs are synthesized in the liver and transport endogenous triglycerides to adipose tissue and muscle for storage and energy production. IDLs are intermediate particles formed during the metabolism of VLDLs and are either taken up by the liver or further metabolized to form LDLs.


LDLs are the primary carriers of cholesterol in the blood and are often referred to as "bad cholesterol" because high levels are associated with an increased risk of heart disease. HDLs, on the other hand, are often referred to as "good cholesterol" because they transport excess cholesterol from peripheral tissues back to the liver for processing and elimination.

Q9) (a) What are the four different pathways of TAG synthesis? Explain anyone.

Ans) Triacylglycerol (TAG) synthesis is a complex process involving several pathways. There are four main pathways of TAG synthesis:

  1. Acyl-CoA-dependent pathway,

  2. Acyl-CoA-independent pathway,

  3. Monoacylglycerol (MAG) pathway.

  4. Phosphatidic acid (PA) pathway.


Acyl-CoA-Dependent Pathway

The Acyl-CoA-dependent pathway is the most common pathway of TAG synthesis and involves the sequential acylation of glycerol-3-phosphate by acyl-CoA molecules. This pathway is regulated by the availability of acyl-CoA molecules, which are generated by fatty acid activation and β-oxidation.


In the Acyl-CoA-independent pathway, TAG synthesis occurs via the acylation of diacylglycerol (DAG) by fatty acyl groups. This pathway is less common and occurs primarily in the liver.


The MAG pathway involves the acylation of MAG by acyl-CoA molecules. This pathway is important for the synthesis of TAG in adipose tissue and is regulated by insulin and glucagon.


The PA pathway involves the dephosphorylation of PA to form DAG, which is then acylated to form TAG. This pathway is important for TAG synthesis in the endoplasmic reticulum and is regulated by ER stress and lipid metabolism.


One of the pathways that can be explained is the Acyl-CoA-dependent pathway. In this pathway, the acyl-CoA molecules are first activated by attachment to CoA, which is catalysed by acyl-CoA synthetase.


The activated acyl-CoA molecules are then transferred to G3P by glycerol-3-phosphate acyltransferase, forming lysophosphatidic acid (LPA). LPA is then acylated by acyl-CoA: LPA acyltransferase to form DAG, which is finally acylated by diacylglycerol acyltransferase (DGAT) to form TAG. This pathway is important for TAG synthesis in adipose tissue, where it provides an important energy source for the body.


Q9) (b) How is cholesterol biosynthesis regulated?

Ans) Cholesterol biosynthesis is regulated by a complex feedback mechanism that involves several regulatory enzymes. The key regulatory enzyme in cholesterol biosynthesis is 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase), which catalyses the rate-limiting step in cholesterol synthesis. HMG-CoA reductase is regulated at both the transcriptional and post-translational levels by a variety of factors, including cholesterol levels, insulin, glucagon, and sterol regulatory element-binding proteins.


High levels of intracellular cholesterol inhibit HMG-CoA reductase activity through a negative feedback mechanism, while low levels of intracellular cholesterol stimulate HMG-CoA reductase activity. Insulin stimulates HMG-CoA reductase activity, while glucagon inhibits it. SREBPs are transcription factors that activate the genes involved in cholesterol biosynthesis and are regulated by cholesterol levels.


Q10) (a) Which is the major phospholipid present in human lung surfactant and how is it synthesized?

Ans) The major phospholipid present in human lung surfactant is dipalmitoylphosphatidylcholine (DPPC). DPPC is synthesized through a multistep process that occurs primarily in the endoplasmic reticulum of lung epithelial cells.


The synthesis of DPPC involves the sequential acylation of lysophosphatidylcholine with palmitoyl-CoA by the enzyme LPC acyltransferase, followed by dephosphorylation of the resulting phosphatidylcholine molecule by the enzyme phosphatidylcholine-specific phospholipase C (PC-PLC).


The resulting LPC is then acylated with another molecule of palmitoyl-CoA to form DPPC. The synthesis of DPPC is tightly regulated and is critical for the proper function of lung surfactant, which helps to reduce surface tension in the lungs and prevent the collapse of alveoli during expiration.


Q10) (b) Name the lipid that accumulates in Faber’s disease and Tay-Sach’s disease. Explain the signs and symptoms of these diseases.


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