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Fibers, starch and sugar are essential ingredients in our food and are important in our diet. These food ingredients are an important source of carbohydrates, or "carbs", that are converted to glucose, which provides energy to our body. Carbohydrates are biomolecules composed of carbon, hydrogen, and oxygen atoms, which are the main nutrients essential to our body. Carbohydrates are also known in biochemistry as "saccharides" which means "sugars."
Carbohydrates are classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides based on their structure. Carbohydrates perform multiple functions in living organisms. Plant cells are made of cellulose and the exoskeleton of insects is made of chitin. Therefore, carbohydrates are essential for the structural and functional role of the organism. We get a lot of carbohydrates from our diet, including grains, bread, potatoes, sugar, lactose from milk, honey, fruits, and glucose and fructose from vegetables.
Metabolism is the process in which your body converts the food that you eat into the form of energy by breaking down the large molecules into smaller ones.
Some MCQs
Which among the Following Enzymes does not Participate in Galactose Metabolism?
a) Glucokinase
b) Galactokinase
c) Galactose-1-Phosphate Uridyl transferase
d) UDP-Galactose 4- epimerase
Answer: Glucokinase
Which of the Following Enzymes is Considered as Defective in Galactosemia- a Fatal Genetic Disorder in Infants?
a) Glucokinase
b) Galactokinase
c) Galactose-1-Phosphate Uridyl transferase
d) UDP-Galactose 4- epimerase
Answer: Galactose-1-Phosphate Uridyl Transferase
Erythrocytes Undergo Glycolysis for Production of ATP. The Deficiency of Which Enzyme Leads to Hemolytic Anemia-
a) Glucokinase
b)Phosphofructokinase
c) Phosphoglucomutase
d) Pyruvate Kinase
Answer: Pyruvate Kinase
In the Liver, the Accumulation of which among the Following Metabolites Attenuates the Inhibitory of ATP on Phosphofructokinase?
a) Glucose-6-Phosphate
b) Citrate
c) Fructose-1,6-Bisphosphate
d) Fructose-2,6-Bisphosphate
Answer: Fructose-2,6-Bisphosphate
The Most Active Site of Protein Synthesis is the-
a) Nucleus
b) Ribosome
c) Mitochondrion
d) Cell sap
Answer: Nucleus
How many Total Molecules of ATP are Synthesized from ADP via Glycolysis of a Single Molecule of Glucose?
a) 36
b) 38
c) 2
d) 4
Answer: 36
Mutation in which of the Following Enzymes Leads to a Glycogen Storage Disease Known as Tarui’s Disease?
a) Glucokinase
b)Phosphofructokinase
c) Phosphoglucomutase
d) Pyruvate Kinase
Answer: Phosphofructokinase
Cancer Cells have High Energy Demands for Replication and Division. Increased Flux of glucose into Glycolysis Replenishes the Energy Demand. Which of the Following Enzymes Plays an Important Role in Tumor Metabolism?
a) Glucokinase
b)Phosphofructokinase
c) Phosphoglucomutase
d) Pyruvate Kinase M2
Answer: Pyruvate Kinase M2
The Rate of Absorption of Sugars by the Small Intestine is Highest for -
a). Pentoses
b). Disaccharides
c). Polysaccharides
d). Hexoses
Answer: Hexose
Which of the Following is not a Polymer of Glucose?
a) Glycogen
b) Cellulose
c) Amylase
d) Insulin
Answer: Insulin
An Essential for the Conversion of Glucose to Glycogen in Liver is -
A. UTP
B. GTP
C. Pyruvate kinase
D. Guanosine
Answer: UTP
Which of the Following Glucose Transporters (GLUT) are Important in Insulin-Dependent Glucose Uptake?
a) GLUT1
b) GLUT2
c) GLUT3
d) GLUT4
Answer: GLUT4
Which of the Following Metabolites Negatively Regulates Pyruvate Kinase?
a) Fructose-1,6-Bisphosphate
b) Citrate
c) Acetyl CoA
d) Alanine
Answer: Alanine
Which of the Following Glycolytic Enzymes is Inhibited by Accumulation of Long Chain Fatty Acid in the Liver?
a) Hexokinase
b) Glucokinase
c) Phosphofructokinase
d) Pyruvate kinase
Answer: Glucokinase
Glycogen Synthesis is Increased by -
a) Cortisone
b) Insulin
c) GH
d) Epinephrine
Answer: Insulin
Gluconeogenesis Occurs in the Liver and Which Organ -
a) Kidney
b) Muscle
c) Heart
d) Fat
Answer: Heart
Adrenaline acts on which Enzyme in Glycogenolysis?
a) Glucokinase
b) Hexokinase
c) Phosphorylase
d) Glucose diphosphate
Answer: Phosphorylase
Which among the Following Statements about Phosphofructokinase-2 (PFK-2) is False ?
a) PFK-2 is bifunctional enzyme having kinase domain, phosphatase domain and regulatory domain
b) Activated protein kinase A phosphorylates PFK-2 and activates phosphatase domain
c) PFK-2 catalyzes the conversion of fructose-6-phosphate to fructose 2-6 bisphosphate
d) PFK-2 phosphatase activity is activated by insulin signaling pathway
Answer: Activated Protein Kinase A Phosphorylates PFK-2 and Activates Phosphatase Domain
Which among the Following Hormones decreases Blood Glucose and Increases Uptake of Glucose in Various Tissues like Skeletal Muscle, Adipose Tissues?
a) Glucagon
b) Epinephrine
c) Cortisol
d) Insulin
Answer: Insulin
The First Product of Glycogenolysis is -
a)Glucose-6-phosphate
b)Glucose 1,6 diphosphate
c)Glucose-1-phosphate
d)Fructose-1-phosphate
Answer: Glucose-1-Phosphate
Enzymes Concerned with the Citric Acid Cycle are Found in the -
a) Nucleus
b) Ribosomes
c) Mitochondria
d) Nonparticulate cytoplasm
Answer: Mitochondria
Which among the Following Statements is True?
a) Glycolysis occurs only in mammalian cells
b) Glycolysis occurs in mitochondria
c) Glycolysis occurs in presence and absence of oxygen
d) Glycolysis occurs when ATP concentration is high
Answer: Glycolysis occurs in Presence and Absence of Oxygen
What high Energy Phosphate Compound is Formed in the Citric Acid Cycle through Substrate level Phosphorylation?
a) ATP
b) TTP
c) ITP
d) GTP
Answer: GTP
The Conversion of Glucose-6-Phosphate to Fructose-6-Phosphate is an Example of which of the Following Reactions?
a) Phosphate transfer
b) Isomerization
c) Dehydration
d) Aldol cleavage
Answer: Isomerisation
The Net Gain of ATP during Conversion of Glucose to Pyruvate is -
a) 1 ATP
b) 2 ATP
c) 1 ATP +1 GTP
d) 4 ATP
Answer: 2 ATP
During the Conversion of Glucose to Pyruvate, Two NADH Molecules are Generated. Which of the Following Steps Generates NADH?
a) Conversion of fructose-6-phosphate to fructose-1-6-bisphosphate
b) Conversion of glyceraldehydes-3-phosphate to 1-3-bisphosphoglycerate
c)Conversion of 3-phosphoglycerate to 2-phosphoglycerate
d) Conversion of phosphoenolpyruvate to pyruvate
Answer: Conversion of glyceraldehydes-3-phosphate to 1-3-bisphosphoglycerate
Which One of the Following is a Monosaccharide?
a) Maltose
b) Sucrose
c) Fructose
d) Starch
Answer: Fructose
Which among the Following Steps is Inhibited during Arsenate Poisoning?
a) Glyceraldehyde-3-phosphate dehydrogenase
b) 3-phosphoglycerate mutase
c) Enolase
d) Pyruvate kinase
Answer: Glyceraldehyde-3-phosphate dehydrogenase
The End Product of Glycolysis under Anaerobic Conditions is -
a) Lactic acid
b) Pyruvic acid
c) Acetoacetic acid
d) Oxaloacetic acid
Answer: Lactic acid
Gluconeogenesis is the Production of Glucose from Non-Carbohydrate Molecules. Which of the Following is not Substrate for Gluconeogenesis?
a) Lactate
b) Alanine
c) Glycerol
d) Acetyl CoA
Answer: Acetyl CoA
Carbohydrates Metabolism
Carbohydrate metabolism is nothing but a number of biochemical processes involved in the formation, breakdown and conversion of carbohydrates in the body. The most important carbohydrate is glucose. Glucose is a monosaccharide that is metabolized by almost all known organisms.
Dietary glucose is abundant in starch. Amylase is an enzyme that breaks down starch and helps metabolism. Glucose comes from many sources, including lactose (from milk), fructose (from fruits), and sucrose (from sugar). The active membrane transport system supports the uptake of fructose, glucose, and fructose, known as monosaccharide species. The monosaccharide component is formed by the division of disaccharides by a special intestinal glucosidase. Types of glucose, such as maltose, are hydrolyzed by isomaltase, reducing their ability to produce the desired results. Lactose intolerance is caused by a deficiency of lactase, an enzyme required to break down lactose in lactose and other dairy products.
The small intestine contains intestinal mucosal cells that transport monosaccharides to the circulation, where they reach the liver. Here, galactose and fructose are converted to glucose. The liver's main task is to regulate glucose levels in the blood, in other words, to act as glucose in the blood. The excess glucose molecules produced are mainly stored as glycogen in liver and muscle cells. It is also stored in adipocytes in the form of metabolized fat. When food intake is restricted, only glycogen, instead of fat, is used to maintain adequate levels of glucose in the bloodstream. Fat can be used for oxidative regeneration of Adenosine Triphosphate (ATP) and reducing power (NADH).
Since every digestible form of carbohydrates is in the end converted to glucose, it is important to understand how glucose can provide energy in the form of adenosine triphosphate (ATP) to various different cells and tissues. Glucose is usually metabolized in three stages of carbohydrate metabolism.
Glycolysis
Krebs Cycle
Electron transport chain/ Oxidative Phosphorylation
Importance of Carbohydrates in our Diet
Carbohydrates provide energy to our body and are an excellent source of vitamins, minerals and nutrients. They provide instantaneous energy in the form of glucose, which is the main source of energy for our body and can also be stored. These carbohydrates should be included in our daily diet in the form of bread, potatoes, fiber, fruits, vegetables and whole grains. People these days are so sensitive to nutrition that they avoid foods that contain carbohydrates, which is not good for our health. You need to have a balanced diet. You should try to eat fiber-rich foods such as fruits and vegetables. Complex carbohydrates are the ultimate source of energy and are low in calories. Therefore, we need to get "correct carbohydrates" in our diet for our good health.
Weight Loss Associated with Carbohydrates
Eating a low carb diet can help you lose weight better than eating a low fat diet. In a 2004 study published in Nutrition & Metabolism, researchers fed obese subjects a low-carbohydrate or low-fat diet which included included meat, oil, fish and vegetables, and low-fat diets included whole grains, fruits, vegetables, low-fat dairy products and meat. Researchers found that subjects on a low-carb diet had a significant weight and fat loss compared to a low-fat diet. Eating a low-carb diet helps reduce appetite by preventing blood sugar fluctuations. Also, the high protein content of a low carb diet may require more energy to digest and burn calories in the process.
FAQs on MCQs on Carbohydrates And Metabolism
1. What are carbohydrates and why are they essential for living organisms?
Carbohydrates are biomolecules composed of carbon, hydrogen, and oxygen atoms, often referred to as 'saccharides' or sugars. They are essential for two main reasons. Firstly, they are the body's primary source of energy, providing glucose which is metabolised to produce ATP. Secondly, they serve crucial structural roles, such as forming cellulose in the cell walls of plants and chitin in the exoskeletons of insects.
2. How are carbohydrates classified? Please provide examples for each type.
Carbohydrates are classified based on the number of sugar units they contain. The main types are:
- Monosaccharides: These are the simplest sugars. Examples include glucose, fructose, and galactose.
- Disaccharides: Formed when two monosaccharides are joined. Examples include sucrose (table sugar), lactose (milk sugar), and maltose.
- Polysaccharides: These are complex carbohydrates made of long chains of monosaccharides. Examples include starch (energy storage in plants), glycogen (energy storage in animals), and cellulose (structural component in plants).
3. What is carbohydrate metabolism and what are its main stages?
Carbohydrate metabolism is the entire set of biochemical processes involved in the formation, breakdown, and interconversion of carbohydrates in living systems. Its primary purpose is to convert dietary carbohydrates into glucose to produce energy. The three main stages of glucose metabolism are:
- Glycolysis: The initial breakdown of glucose into pyruvate.
- Krebs Cycle (or Citric Acid Cycle): Further oxidation of the products of glycolysis.
- Electron Transport Chain (Oxidative Phosphorylation): The final stage where the majority of ATP is produced.
4. What is the significance of Glycolysis in cellular respiration?
Glycolysis is a fundamental metabolic pathway with great significance. It is the first step in breaking down glucose to extract energy for the cell. A key feature is that it occurs in the cytoplasm and does not require oxygen, making it an anaerobic process. It generates a net gain of ATP and NADH, and produces pyruvate, which can then enter the aerobic Krebs cycle for much greater energy yield if oxygen is present.
5. How can solving MCQs on Carbohydrates and Metabolism help in my exam preparation?
Solving MCQs on this topic is an excellent way to prepare for exams. It helps you to actively recall and test your knowledge of complex pathways like glycolysis and the Krebs cycle. This practice improves both your speed and accuracy, familiarises you with the question patterns seen in exams like NEET, and helps you identify specific concepts that require more detailed revision.
6. What is the key difference between Glycolysis and Gluconeogenesis?
The key difference is that they are opposing metabolic pathways. Glycolysis is a catabolic pathway that breaks down glucose into pyruvate to generate energy (ATP). In contrast, Gluconeogenesis is an anabolic pathway that synthesises glucose from non-carbohydrate sources like lactate and certain amino acids. While many steps are reversible, key regulatory enzymes ensure that both pathways do not run simultaneously in a futile cycle. Glycolysis occurs in most cells, while gluconeogenesis is primarily active in the liver.
7. Why is the conversion of glucose to glycogen in the liver so important for the body?
The conversion of excess glucose to glycogen, a process called glycogenesis, is vital for maintaining blood sugar homeostasis. After a meal, blood glucose levels rise. The hormone insulin signals the liver to take up this excess glucose and store it as the polysaccharide glycogen. When blood sugar levels fall, the hormone glucagon signals the liver to break down glycogen (glycogenolysis) and release glucose into the blood, ensuring a steady energy supply for the brain and other tissues.
8. How do hormones like insulin and glucagon regulate carbohydrate metabolism?
Insulin and glucagon act as a push-pull system to maintain blood glucose balance.
- Insulin, secreted by the pancreas when blood sugar is high, promotes glucose uptake by muscle and adipose cells and stimulates the liver to store glucose as glycogen. It effectively lowers blood sugar.
- Glucagon, also from the pancreas, is released when blood sugar is low. It stimulates the liver to perform glycogenolysis (breaking down glycogen) and gluconeogenesis (creating new glucose) to raise blood sugar levels.
9. What are the different metabolic fates of pyruvate, the end product of glycolysis?
The fate of pyruvate depends on the presence or absence of oxygen.
- Under aerobic conditions (with oxygen), pyruvate enters the mitochondria, is converted to acetyl-CoA, and enters the Krebs cycle for complete oxidation, generating a large amount of ATP.
- Under anaerobic conditions (without oxygen), pyruvate undergoes fermentation to regenerate NAD+ for glycolysis to continue. In human muscle cells, it is converted to lactic acid. In yeast, it's converted to ethanol and CO2.
10. Can the human body produce glucose from fats? Explain the metabolic limitation.
The human body cannot perform a net conversion of fatty acids into glucose. This is due to a key metabolic block: the reaction converting pyruvate to acetyl-CoA (the product of fatty acid breakdown) is irreversible. Therefore, there's no pathway to turn acetyl-CoA back into pyruvate and then into glucose. However, the glycerol component of triglycerides can be used as a substrate for gluconeogenesis, but this constitutes only a small fraction of the fat molecule.
