BZYCT-135 Solved Assignment

1. a) How many molecules of ${\mathrm{CO}}_2,\mathrm{NADH}$${\mathrm{CO}}_2,\mathrm{NADH}$CO_(2),NADH\mathrm{CO}_2, \mathrm{NADH}${\mathrm{CO}}_2,\mathrm{NADH}$ and ${\mathrm{FADH}}_2$${\mathrm{FADH}}_2$FADH_(2)\mathrm{FADH}_2${\mathrm{FADH}}_2$ are produced in one citric acid cycle?
   b) Briefly discuss $\beta$$\beta$beta\beta$\beta$ oxidation of fatty acids.
2. a) Enlist the four major classes of digestive enzymes in animals.
   b) Describe the digestion of carbohydrates.
3. Write the differences between the following pairs:
   i) RBCs and WBCs
   ii) Membrane Potential and Action Potential
   iii) ‘Aerobic’ and Anaerobic’ respiration with reference to glucose catabolism
   iv) Molluscan kidney and Malphighian tubules
4. a) What are allosteric enzymes?
   b) Define enzyme inhibition. Explain any one type of enzyme inhibition.
5. a) Explain the composition of blood.
   b) Which hormones are secreted by the pancreas? Explain their functions.
6. Write short notes on the following:
   i) Bohr’s effect
   ii) Pheromones
7. a) Describe how renal tubule and collecting ducts produce dilute and concentrated urine.
   b) How does short-term regulation of the urea cycle differ from long-term regulation?
8. a) Explain oogenesis in human female.
   b) What is ageing? Mention any three theories of ageing.
9. a) Briefly discuss the Menstrual cycle.
   b) Explain the various mechanisms of enzyme regulation.
10. a) Explain the factors that affect the ${\mathrm{O}}_2$${\mathrm{O}}_2$O_(2)\mathrm{O}_2${\mathrm{O}}_2$ dissociation curve.
    b) Elaborate the chemical nature of hormones.

Answer:

1. a) How many molecules of ${\mathrm{CO}}_2,\mathrm{NADH}$${\mathrm{CO}}_2,\mathrm{NADH}$CO_(2),NADH\mathrm{CO}_2, \mathrm{NADH}${\mathrm{CO}}_2,\mathrm{NADH}$ and ${\mathrm{FADH}}_2$${\mathrm{FADH}}_2$FADH_(2)\mathrm{FADH}_2${\mathrm{FADH}}_2$ are produced in one citric acid cycle?

Answer:

1. Carbon dioxide (${\mathrm{CO}}_2$${\mathrm{CO}}_2$CO_(2)\mathrm{CO}_2${\mathrm{CO}}_2$): 2 molecules of ${\mathrm{CO}}_2$${\mathrm{CO}}_2$CO_(2)\mathrm{CO}_2${\mathrm{CO}}_2$ are produced. These come from the decarboxylation reactions in the cycle.
2. NADH: 3 molecules of NADH are produced. This occurs through the reduction of NAD${}^{+}$${}^{+}$^(+)^+${}^{+}$ to NADH in three different reactions in the cycle.
3. FADH${}_2$${}_2$_(2)_2${}_2$: 1 molecule of FADH${}_2$${}_2$_(2)_2${}_2$ is produced. This happens when FAD is reduced to FADH${}_2$${}_2$_(2)_2${}_2$ in one step of the cycle.

• 2 molecules of ${\mathrm{CO}}_2$${\mathrm{CO}}_2$CO_(2)\mathrm{CO}_2${\mathrm{CO}}_2$
• 3 molecules of NADH
• 1 molecule of FADH${}_2$${}_2$_(2)_2${}_2$
• 1 molecule of ATP (or GTP)

1. b) Briefly discuss $\beta$$\beta$beta\beta$\beta$ oxidation of fatty acids.

Answer:

1. 1 FADH2 – produced in the first dehydrogenation step.
2. 1 NADH – produced in the second dehydrogenation step.
3. 1 Acetyl-CoA – produced through thiolysis, which enters the citric acid cycle.

• 7 rounds of β-oxidation (for a 16-carbon fatty acid, 8 acetyl-CoA molecules are produced, requiring 7 cycles of β-oxidation).
• For each round:
  • 1 FADH2 → 1.5 ATP
  • 1 NADH → 2.5 ATP
  • 1 Acetyl-CoA → 10 ATP (from the citric acid cycle)

• 7 FADH2 × 1.5 ATP = 10.5 ATP
• 7 NADH × 2.5 ATP = 17.5 ATP
• 8 Acetyl-CoA × 10 ATP = 80 ATP

• Carnitine Acyltransferase I (CAT I): This enzyme plays a critical role in the transport of fatty acids into the mitochondria. It is inhibited by malonyl-CoA, a molecule that is produced when there is an abundance of glucose and fatty acid synthesis.
• Acetyl-CoA levels: High levels of acetyl-CoA, especially when there is a large amount of energy available, can inhibit the activity of key enzymes in β-oxidation, helping to balance the energy supply.
• Hormonal Regulation: Hormones like glucagon and epinephrine stimulate β-oxidation by promoting the release of fatty acids from adipose tissue, while insulin inhibits the process, favoring the storage of fats and glycogen.

2. a) Enlist the four major classes of digestive enzymes in animals.

Answer:

• Salivary Amylase: In humans and other mammals, salivary amylase (also known as ptyalin) is secreted by the salivary glands and begins the process of starch digestion in the mouth.
• Pancreatic Amylase: After food enters the small intestine, pancreatic amylase is secreted by the pancreas into the duodenum, where it continues the digestion of starch into maltose and other smaller sugar molecules.

• Pepsin: In the stomach, pepsin is secreted by the gastric chief cells in an inactive form called pepsinogen. When activated by the acidic environment of the stomach, pepsin starts to break down proteins into smaller peptides.
• Trypsin and Chymotrypsin: These proteases are secreted by the pancreas as inactive zymogens (trypsinogen and chymotrypsinogen) into the small intestine, where they are activated. Trypsin and chymotrypsin further digest proteins into smaller peptides and amino acids.
• Carboxypeptidase: This enzyme, also produced by the pancreas, breaks down peptides by removing amino acids from the carboxyl end of the peptide chain.

• Pancreatic Lipase: The major enzyme responsible for fat digestion in the small intestine is pancreatic lipase, secreted by the pancreas. It acts on triglycerides (the most common form of fat in the diet) to break them down into monoglycerides and free fatty acids.
• Gastric Lipase: In addition to pancreatic lipase, gastric lipase is secreted by the stomach and contributes to the digestion of fats, particularly in infants who have high fat consumption in their diet (such as milk).

• Deoxyribonuclease (DNase): DNase breaks down DNA into smaller nucleotides.
• Ribonuclease (RNase): RNase breaks down RNA into smaller nucleotides.

2. b) Describe the digestion of carbohydrates.

Answer:

• Maltase breaks down maltose into two molecules of glucose.
• Sucrase breaks down sucrose into glucose and fructose.
• Lactase breaks down lactose into glucose and galactose.

• Lactose Intolerance: This condition occurs when there is a deficiency of lactase, the enzyme responsible for breaking down lactose. As a result, lactose remains undigested in the intestines, leading to symptoms such as bloating, gas, and diarrhea.
• Celiac Disease: In this autoimmune disorder, the ingestion of gluten (a protein found in wheat, rye, and barley) damages the lining of the small intestine, impairing nutrient absorption, including carbohydrates.
• Diabetes Mellitus: Diabetes involves either insufficient insulin production or poor cellular response to insulin, resulting in impaired glucose metabolism and elevated blood sugar levels.

3. Write the differences between the following pairs:

Answer:

• RBCs are involved in oxygen and carbon dioxide transport, whereas WBCs are involved in immune defense.
• RBCs are anucleate and contain hemoglobin, while WBCs have a nucleus and do not contain hemoglobin.
• RBCs are uniform in shape and size, whereas WBCs vary in shape and size.

• Membrane potential is a steady state voltage across the membrane, while action potential is a rapid, temporary change that propagates along the cell.
• Membrane potential exists in all cells, while action potential occurs only in excitable cells like neurons and muscles.

• Aerobic respiration requires oxygen and produces more ATP (36-38 ATP per glucose), while anaerobic respiration does not require oxygen and produces only 2 ATP per glucose.
• Aerobic respiration occurs in the mitochondria, while anaerobic respiration occurs in the cytoplasm.

• Molluscan kidneys filter blood and reabsorb water and ions, whereas Malpighian tubules mainly excrete waste (uric acid or guanine) with little reabsorption.
• Molluscan kidneys are found in mollusks and resemble metanephridia, while Malpighian tubules are found in arthropods and are associated with the gut.

4. a) What are allosteric enzymes?

Answer:

1. The Concept of Allosteric Regulation

2. Allosteric Sites and Their Function

• Activators: Molecules that bind to the allosteric site and increase the enzyme’s activity.
• Inhibitors: Molecules that bind to the allosteric site and decrease the enzyme’s activity.

3. Mechanisms of Allosteric Regulation

• The Concerted Model (or MWC Model): In this model, all subunits of the enzyme exist in either an active (R) or inactive (T) state. When an allosteric effector binds to one subunit, it induces a conformational change in all subunits simultaneously, shifting the equilibrium between the R and T states.
• The Sequential Model: This model proposes that when one subunit binds to an allosteric effector, it induces a conformational change in that subunit, which is then transmitted to adjacent subunits. This model accounts for the fact that subunits do not have to change states simultaneously, as in the concerted model.

4. The Role of Allosteric Enzymes in Metabolism

• Feedback Inhibition: One of the most common forms of allosteric regulation is feedback inhibition, where the end product of a metabolic pathway binds to an allosteric site on an enzyme early in the pathway, inhibiting its activity. This prevents the overproduction of the product when it is already abundant, helping to maintain homeostasis in the cell. For example, in the synthesis of the amino acid isoleucine, isoleucine itself can act as an allosteric inhibitor of the enzyme that catalyzes the first step in its production.
• Feedforward Activation: In contrast to feedback inhibition, feedforward activation occurs when a molecule produced earlier in a metabolic pathway enhances the activity of an allosteric enzyme. This mechanism helps to speed up a reaction in response to an increase in substrate concentration.

5. Examples of Allosteric Enzymes

• Phosphofructokinase (PFK): This enzyme plays a key role in the glycolytic pathway, which is the breakdown of glucose to generate ATP. PFK is an allosteric enzyme that is activated by AMP (adenosine monophosphate), which signals low energy levels in the cell, and inhibited by ATP (adenosine triphosphate), which signals high energy availability. This ensures that glycolysis proceeds only when the cell requires more energy.
• Aspartate Transcarbamoylase (ATCase): This enzyme regulates the synthesis of pyrimidine nucleotides, which are necessary for DNA and RNA synthesis. ATCase is regulated by CTP (cytidine triphosphate), the end product of the pathway, which acts as an allosteric inhibitor, and by ATP, which acts as an activator.
• Acetyl-CoA Carboxylase (ACC): This enzyme is involved in fatty acid biosynthesis. It is regulated by citrate (an activator) and palmitoyl-CoA (a product of fatty acid synthesis that acts as an inhibitor). This regulation ensures that fatty acid synthesis occurs when necessary, but is turned off when sufficient fat stores are present.

6. Significance of Allosteric Enzymes in Cellular Regulation

• Coordination: Allosteric enzymes help coordinate metabolic pathways by regulating the rate of key reactions. By allowing enzymes to be activated or inhibited in response to metabolic demands, cells can quickly adapt to changes in energy availability, nutrient intake, or environmental conditions.
• Efficiency: The fine-tuning of metabolic pathways through allosteric regulation ensures that the cell does not waste energy or resources. By inhibiting the enzyme activity when the product is abundant or when energy levels are sufficient, cells conserve energy and maintain metabolic balance.
• Cell Signaling: Allosteric enzymes also play a role in cellular signaling. For example, enzymes involved in the second messenger systems (such as protein kinases) can be regulated allosterically by molecules like cAMP (cyclic adenosine monophosphate) or calcium ions.

7. Conclusion

4. b) Define enzyme inhibition. Explain any one type of enzyme inhibition.

Answer:

2. The Mechanisms of Enzyme Inhibition

• Competitive Inhibition: The inhibitor competes with the substrate for binding to the active site.
• Non-competitive Inhibition: The inhibitor binds to an allosteric site, not the active site, causing a conformational change in the enzyme.
• Uncompetitive Inhibition: The inhibitor binds only to the enzyme-substrate complex, preventing the reaction from proceeding.
• Irreversible Inhibition: The inhibitor binds permanently to the enzyme, often covalently, rendering it inactive.

3. Competitive Inhibition: Mechanism and Details

How Does Competitive Inhibition Work?

4. Characteristics of Competitive Inhibition

• Reversibility: Competitive inhibition is typically reversible. By increasing the concentration of the substrate, the effects of the inhibitor can be diminished or completely negated.
• Structural Similarity: The inhibitor must have a structure similar to that of the substrate to effectively compete for the active site. The more structurally similar the inhibitor is to the substrate, the more effective the inhibition.
• Effect on Km and Vmax: As mentioned, competitive inhibition increases the apparent Km (the substrate concentration required to reach half of Vmax), but Vmax remains unchanged. This means that although it takes more substrate to reach half-maximal activity, the enzyme can still achieve its maximum rate if sufficient substrate is present.
• Inhibition by Analogs: Some drugs or molecules act as competitive inhibitors by mimicking the structure of natural substrates. These inhibitors can block enzyme activity and are often used in pharmacology to treat diseases.

5. Example of Competitive Inhibition: The Role of Methotrexate in Cancer Treatment

6. Competitive Inhibition in Metabolic Pathways

• Regulation of the enzyme hexokinase: In the glycolytic pathway, glucose-6-phosphate (the product of the first step) acts as a competitive inhibitor of hexokinase, the enzyme that catalyzes the conversion of glucose to glucose-6-phosphate. This feedback mechanism prevents the overproduction of glucose-6-phosphate when glucose levels are high, helping maintain proper metabolic balance.
• Regulation of the enzyme phosphofructokinase (PFK): PFK, a rate-limiting enzyme in glycolysis, is regulated by several factors. ATP acts as a competitive inhibitor, reducing the enzyme’s activity when energy levels are sufficient, while AMP activates the enzyme to promote glycolysis during energy depletion.

7. Factors Influencing Competitive Inhibition

• Concentration of the inhibitor: As the concentration of the inhibitor increases, the inhibition becomes more pronounced. At sufficiently high inhibitor concentrations, the enzyme may become almost completely inactive.
• Concentration of the substrate: The effect of a competitive inhibitor is reduced as the concentration of the substrate increases. A higher substrate concentration shifts the equilibrium toward enzyme-substrate binding, thus overcoming the inhibitor.
• Affinity of the inhibitor for the enzyme: The stronger the binding affinity of the inhibitor for the active site, the more effective the inhibition. High-affinity inhibitors are more likely to bind and reduce enzyme activity at lower concentrations.

8. Conclusion

5. a) Explain the composition of blood.

Answer:

2. Plasma: The Liquid Component of Blood

Key Components of Plasma:

• Water: The largest component of plasma, water functions as a solvent for a variety of molecules, maintaining the fluidity and transport function of the blood.
• Proteins: Plasma proteins play a key role in maintaining blood pressure, immune function, and the transport of substances. The three primary types of plasma proteins include:
  • Albumin: The most abundant plasma protein, albumin is crucial for maintaining oncotic pressure (which helps retain fluid within blood vessels) and for the transport of fatty acids, hormones, and other molecules.
  • Globulins: These proteins have various functions, including transporting lipids and fat-soluble vitamins, as well as being involved in the immune response (e.g., antibodies or immunoglobulins).
  • Fibrinogen: This protein is essential for blood clotting. Upon activation, fibrinogen is converted into fibrin, which forms the meshwork of a blood clot.
• Electrolytes: Plasma contains several electrolytes, such as sodium, potassium, calcium, magnesium, chloride, and bicarbonate, which help regulate pH, maintain osmotic pressure, and support cellular functions.
• Nutrients: Plasma transports essential nutrients such as glucose, amino acids, lipids, and vitamins to cells for energy production and growth.
• Gases: Plasma carries gases like oxygen (O₂) and carbon dioxide (CO₂), which are essential for cellular respiration and the removal of metabolic waste products.
• Waste Products: Plasma is responsible for transporting waste products like urea, creatinine, and bilirubin to the kidneys and liver for excretion.
• Hormones: Blood plasma also carries a wide array of hormones that regulate various bodily functions, including insulin, thyroid hormones, adrenaline, and corticosteroids.

3. Blood Cells: The Cellular Component of Blood

Red Blood Cells (Erythrocytes)

• Structure: RBCs are biconcave discs, which increases their surface area for gas exchange. They lack a nucleus, which allows more space for hemoglobin, the protein responsible for oxygen binding.
• Hemoglobin: This iron-containing protein in RBCs binds oxygen in the lungs and releases it in tissues. It can also carry a small amount of carbon dioxide, though most CO₂ is transported in the plasma.
• Lifespan: RBCs have a lifespan of about 120 days, after which they are removed by the spleen and liver.

White Blood Cells (Leukocytes)

• Granulocytes: These cells contain granules that are visible under a microscope and play a role in defense against pathogens:
  • Neutrophils: The most abundant WBC, neutrophils are the first responders to bacterial infections. They are involved in phagocytosis, where they engulf and destroy pathogens.
  • Eosinophils: Eosinophils are involved in combating parasitic infections and modulating allergic reactions.
  • Basophils: These cells release histamine during allergic reactions, contributing to inflammation.
• Agranulocytes: These cells do not contain visible granules and play important roles in immunity:
  • Lymphocytes: These include B cells, T cells, and natural killer (NK) cells, which are involved in specific immune responses, such as the production of antibodies (B cells) or the destruction of infected cells (T cells).
  • Monocytes: These cells differentiate into macrophages after entering tissues, where they perform phagocytosis and help in immune surveillance.

Platelets (Thrombocytes)

• Function: When a blood vessel is damaged, platelets adhere to the exposed tissue and release chemicals that activate other platelets. This forms a platelet plug, which is further stabilized by fibrin threads (from fibrinogen).
• Lifespan: Platelets have a short lifespan of 7–10 days, after which they are removed by the spleen and liver.

4. Blood Cell Production (Hematopoiesis)

• Erythropoiesis: The production of red blood cells, stimulated by the hormone erythropoietin (produced by the kidneys), occurs in the bone marrow.
• Leukopoiesis: The formation of white blood cells occurs through differentiation of hematopoietic stem cells into the various types of WBCs.
• Thrombopoiesis: Platelets are produced by the fragmentation of megakaryocytes in the bone marrow, stimulated by thrombopoietin.

5. Conclusion

5. b) Which hormones are secreted by the pancreas? Explain their functions.

Answer:

• Insulin
• Glucagon
• Somatostatin
• Pancreatic Polypeptide

2. Insulin: The Key Regulator of Blood Glucose

Function of Insulin:

• Glucose Uptake: Insulin promotes the uptake of glucose from the blood into cells, particularly muscle cells, adipocytes (fat cells), and liver cells. This process is facilitated by insulin binding to specific receptors on cell membranes, which activates glucose transporters to bring glucose into the cell.
• Glycogen Synthesis: In the liver and muscles, insulin stimulates the conversion of glucose into glycogen, a storage form of glucose. This helps lower blood glucose levels after a meal.
• Fat Storage: Insulin also promotes the synthesis and storage of fats in adipose tissue. It inhibits the breakdown of fat (lipolysis) and encourages the storage of excess glucose as triglycerides.
• Protein Synthesis: In addition to its effects on carbohydrates and fats, insulin also promotes protein synthesis by enhancing the uptake of amino acids into cells and stimulating protein-building processes.

Overall Impact of Insulin:

3. Glucagon: The Counter-Regulator of Insulin

Function of Glucagon:

• Glycogenolysis: When blood glucose levels fall (e.g., between meals or during fasting), glucagon signals the liver to break down stored glycogen into glucose in a process called glycogenolysis. The released glucose is then released into the bloodstream, increasing blood glucose levels.
• Gluconeogenesis: In addition to glycogen breakdown, glucagon stimulates gluconeogenesis, the production of glucose from non-carbohydrate sources, such as amino acids and lactate. This helps maintain blood glucose levels during prolonged fasting or starvation.
• Fat Breakdown: Glucagon also promotes the breakdown of fat stores (lipolysis) to provide energy in the form of fatty acids, which the body can use when glucose availability is limited.

Overall Impact of Glucagon:

4. Somatostatin: The Inhibitor of Hormonal Secretion

Function of Somatostatin:

• Inhibition of Insulin and Glucagon Secretion: Somatostatin acts as a negative feedback regulator, inhibiting the secretion of both insulin and glucagon. By suppressing insulin and glucagon release, somatostatin helps to maintain the balance between blood glucose levels.
• Inhibition of Other Hormones: Somatostatin also inhibits the secretion of other hormones, including growth hormone (from the pituitary gland) and gastrin (a hormone involved in digestive processes).
• Regulation of Digestive Processes: Somatostatin reduces gastric acid secretion and slows down gastric emptying, which helps in the regulation of digestion. It can also reduce the release of certain gastrointestinal hormones, such as secretin and cholecystokinin.

Overall Impact of Somatostatin:

5. Pancreatic Polypeptide: The Regulator of Pancreatic Function

Function of Pancreatic Polypeptide:

• Regulation of Pancreatic Secretions: Pancreatic polypeptide is believed to regulate both exocrine and endocrine pancreatic secretions. It inhibits the secretion of digestive enzymes from the exocrine pancreas and may regulate the release of insulin and glucagon from the endocrine pancreas.
• Appetite Regulation: Studies have shown that pancreatic polypeptide can influence appetite by affecting hunger and satiety signals. It may reduce food intake, although its exact role in appetite control is still under investigation.

Overall Impact of Pancreatic Polypeptide:

6. The Interplay Between Pancreatic Hormones

• Insulin and Glucagon work in an antagonistic manner to maintain normal blood glucose levels: insulin lowers blood glucose, while glucagon raises it. These hormones are secreted in response to feedback loops that detect blood glucose levels.
• Somatostatin serves as an inhibitor, ensuring that insulin and glucagon secretion are not excessive or inappropriate. By modulating the release of these hormones, somatostatin prevents extreme fluctuations in blood glucose levels.
• Pancreatic Polypeptide helps in fine-tuning pancreatic function, preventing excessive digestive enzyme secretion and potentially influencing appetite and satiety.

7. Conclusion

6. Write short notes on the following:

Answer:

i) Bohr’s Effect

Mechanism of Bohr’s Effect:

• In the lungs: Oxygen binds more readily to hemoglobin due to the relatively high pH and low CO₂ levels, promoting the uptake of oxygen from the alveoli into the blood.
• In tissues: In metabolically active tissues, the production of CO₂ from cellular respiration lowers the pH (creating a more acidic environment). This acidic environment causes hemoglobin to release oxygen more easily to the tissues, where it is needed for energy production.

Physiological Importance:

ii) Pheromones

Types of Pheromones:

• Reproductive Pheromones: These are involved in mating behaviors and are often used to attract potential mates. For example, female moths release specific pheromones to attract male moths for mating.
• Alarm Pheromones: Released in response to danger, alarm pheromones trigger defensive or escape behaviors in other members of the species. For example, certain species of ants release alarm pheromones when they sense danger, causing the colony to become alert and respond.
• Trail Pheromones: Many animals, such as ants and termites, use trail pheromones to create paths leading to food sources or to mark territory.
• Territorial Pheromones: Used by some animals to mark the boundaries of their territory, these pheromones signal other individuals to stay away. Cats, for example, may rub their face on objects to deposit pheromones that mark their territory.
• Mother-Offspring Pheromones: These are used by mothers to recognize and communicate with their offspring. For example, many mammals release pheromones that help newborns locate their mother’s nipple.

Mechanism of Action:

Importance of Pheromones:

7. a) Describe how renal tubule and collecting ducts produce dilute and concentrated urine.

Answer:

2. The Renal Tubule and Its Role in Filtrate Processing

Proximal Convoluted Tubule (PCT)

• Reabsorption: The PCT is the first segment after the Bowman’s capsule and is responsible for the majority of solute reabsorption (about 65-70%). This includes sodium (Na⁺), glucose, amino acids, bicarbonate (HCO₃⁻), and a significant portion of water. Since water follows solutes passively (via osmosis), much of the filtrate’s water is reabsorbed in this segment.
• Secretion: The PCT also secretes waste products like hydrogen ions (H⁺) and certain drugs or toxins into the filtrate.

Loop of Henle

• Descending Limb: The descending limb of the loop is permeable to water but not solutes. As the filtrate descends deeper into the medulla, the osmolarity of the interstitial fluid increases due to the presence of concentrated solutes (mainly NaCl and urea). Water flows out of the descending limb through osmosis, concentrating the filtrate. By the time the filtrate reaches the hairpin turn of the loop, it is more concentrated than plasma.
• Ascending Limb: The ascending limb is impermeable to water but actively transports sodium chloride (NaCl) out of the filtrate into the surrounding interstitial space. This creates a concentration gradient in the renal medulla, with the interstitial fluid becoming increasingly concentrated as you move deeper. This gradient is essential for the kidney’s ability to concentrate urine in the collecting ducts.

3. The Collecting Ducts and Their Role in Urine Concentration

Concentrated Urine Production

• Presence of ADH: When the body is dehydrated or when blood osmolarity increases (such as in cases of high salt intake), the posterior pituitary gland secretes ADH into the bloodstream. ADH acts on the collecting ducts to make them more permeable to water by inserting aquaporin channels into the cells of the duct. This allows water to be reabsorbed from the filtrate back into the bloodstream due to the high osmolarity of the surrounding interstitial fluid created by the loop of Henle.
• Water Reabsorption: As water is reabsorbed from the collecting duct into the hyperosmotic medullary interstitial space, the filtrate becomes more concentrated, resulting in concentrated urine. The longer the collecting ducts are exposed to ADH, the more water is reabsorbed, and the more concentrated the urine becomes.
• Final Concentration: By the time the filtrate reaches the end of the collecting duct, it can be highly concentrated (even up to 4 times the osmolarity of plasma), depending on the level of ADH. The result is a small volume of highly concentrated urine, which helps the body conserve water.

Dilute Urine Production

• Absence of ADH: When the body is well-hydrated, or during periods of low blood osmolarity, ADH secretion is reduced or inhibited. Without ADH, the collecting ducts remain impermeable to water, meaning that water cannot be reabsorbed from the filtrate, and the filtrate remains diluted.
• Water Remains in the Filtrate: Since the filtrate is unable to reabsorb water in the collecting ducts, the urine remains dilute, and the kidneys excrete a larger volume of urine with a lower osmolarity (closer to that of plasma).
• Loop of Henle’s Role: The ability to produce dilute urine is partially due to the work done by the loop of Henle, which creates an osmotic gradient in the renal medulla. Even though the collecting ducts are impermeable to water in the absence of ADH, this gradient allows for the removal of sodium and chloride from the filtrate in the ascending loop, further diluting the urine.

4. Countercurrent Multiplier System: A Key to Urine Concentration

• Descending Limb: As water moves out of the descending limb and into the interstitial fluid, the filtrate becomes increasingly concentrated.
• Ascending Limb: The active transport of NaCl from the ascending limb into the interstitial space without water helps to build the osmotic gradient in the medulla, which is essential for water reabsorption in the collecting ducts under the influence of ADH.

5. The Role of Urea in Concentration

6. Conclusion

7. b) How does short-term regulation of the urea cycle differ from long-term regulation?

Answer:

2. Short-Term Regulation of the Urea Cycle

Substrate Availability

• Ammonia: Increased protein degradation leads to elevated ammonia levels, which stimulates the urea cycle. The presence of ammonia in the mitochondria activates the enzyme carbamoyl phosphate synthetase I (CPS1), the first and rate-limiting enzyme of the cycle, to initiate the conversion of ammonia to carbamoyl phosphate.
• Glutamine: Glutamine serves as a nitrogen donor and is converted into ammonia in the mitochondria. Elevated levels of glutamine can stimulate the urea cycle by providing more ammonia for detoxification.

Allosteric Regulation

• N-acetylglutamate (NAG), a derivative of glutamate, is a key allosteric activator of carbamoyl phosphate synthetase I (CPS1). When protein intake increases or when ammonia levels rise, the synthesis of NAG is increased. This leads to enhanced activation of CPS1, thereby increasing the rate of ammonia detoxification.
• ADP and ATP: The ATP levels in the liver also influence the urea cycle. High ATP levels favor the urea cycle by providing the necessary energy for the reactions, while low ATP levels may reduce the cycle’s activity, as energy-intensive processes are downregulated during periods of low energy availability.

Hormonal Regulation

• Glucagon: When blood glucose levels drop (such as during fasting or between meals), glucagon is released, which can indirectly stimulate the urea cycle by promoting the breakdown of proteins for energy. Glucagon activates enzymes involved in amino acid catabolism, increasing ammonia levels and stimulating the urea cycle to eliminate excess nitrogen.
• Cortisol: Cortisol, released during stress or in response to inflammation, can also increase the rate of protein catabolism, raising ammonia levels and stimulating the urea cycle. This helps eliminate the nitrogen produced from protein breakdown.

3. Long-Term Regulation of the Urea Cycle

Dietary Protein Intake and Adaptation

• Increased Protein Intake: When the body receives more protein, the liver responds by upregulating the synthesis of urea cycle enzymes such as CPS1, OTC, and arginase. This increase in enzyme production enhances the liver’s capacity to detoxify ammonia and convert it into urea for excretion.
• Decreased Protein Intake: Conversely, in periods of low protein intake (such as during fasting or malnutrition), the body reduces the synthesis of urea cycle enzymes. This downregulation of enzyme production helps to conserve energy and reduce the unnecessary excretion of urea.

Gene Expression and Transcriptional Control

• The transcription factor cAMP response element-binding protein (CREB) is involved in the upregulation of urea cycle enzymes in response to hormones like glucagon. When glucagon levels are elevated due to low blood glucose, CREB activation increases the transcription of urea cycle enzymes, promoting the production of enzymes that are involved in nitrogen detoxification.
• Sterol Regulatory Element-Binding Proteins (SREBPs) and other metabolic regulators are involved in the long-term adaptation of the liver to various metabolic states. These regulatory mechanisms adjust the gene expression of enzymes like CPS1 and OTC, which are involved in the urea cycle.

Fasting and Starvation

4. Key Differences Between Short-Term and Long-Term Regulation of the Urea Cycle

5. Conclusion

8. a) Explain oogenesis in human female.

Answer:

2. Stages of Oogenesis

Fetal Stage

• Primary oocytes are diploid (2n) and contain the full set of chromosomes.
• At birth, females have about 1 to 2 million primary oocytes in each ovary. However, the number decreases significantly over time.

Childhood (Arrested Oocytes)

Pubertal Stage and the Follicular Phase

• Primary oocytes resume their progress through meiosis, but they do not complete meiosis immediately. They enter metaphase II of meiosis, where they are arrested once again.
• Follicular development occurs as primary oocytes grow and form a primary follicle, which is characterized by the presence of multiple layers of granulosa cells around the oocyte. As the follicle grows, the oocyte undergoes further maturation.

Selection of the Dominant Follicle

• Antral follicle: The growing follicle eventually forms an antral follicle, where a fluid-filled cavity (antrum) forms around the oocyte.
• Secondary follicle: As the follicle matures, it develops into a secondary follicle, and its granulosa cells start secreting estrogen.
• Graafian follicle: The dominant follicle reaches the Graafian stage, which is the final stage of follicular maturation. It contains a large antrum and a fully mature oocyte that is now arrested in metaphase II.

3. Ovulation and the Release of the Oocyte

• The secondary oocyte is arrested in metaphase II of meiosis at the time of ovulation. It is released into the fallopian tube, where it is available for fertilization by a sperm.
• If fertilization occurs, the oocyte will complete meiosis II and form a mature ovum and a polar body. The polar body is a small, non-functional cell that contains discarded genetic material.

4. Meiosis in Oogenesis and the Formation of Polar Bodies

• During meiosis I, the primary oocyte divides into two cells: one larger cell (the secondary oocyte) and one smaller cell (the first polar body). The polar body contains very little cytoplasm and typically degenerates.
• If the secondary oocyte is fertilized, it undergoes meiosis II, which results in the formation of the mature ovum (egg) and another polar body.
• If fertilization does not occur, the secondary oocyte is eventually discarded during menstruation along with the unfertilized egg.

5. Hormonal Control of Oogenesis

• Gonadotropin-Releasing Hormone (GnRH): Secreted by the hypothalamus, GnRH stimulates the release of FSH and LH from the anterior pituitary gland.
• Follicle-Stimulating Hormone (FSH): FSH is responsible for stimulating the growth and maturation of ovarian follicles. It promotes the growth of granulosa cells and helps in the conversion of androgens to estrogens.
• Luteinizing Hormone (LH): LH triggers ovulation, causing the release of the secondary oocyte from the Graafian follicle. It also promotes the luteinization of the remaining follicular cells, transforming them into the corpus luteum, which produces progesterone.
• Estrogen: Produced by the granulosa cells, estrogen stimulates the growth of the follicle and the thickening of the uterine lining in preparation for potential pregnancy.
• Progesterone: After ovulation, the corpus luteum secretes progesterone, which helps maintain the uterine lining for potential implantation of a fertilized egg.

6. Conclusion

8. b) What is ageing? Mention any three theories of ageing.

Answer:

2. Theories of Ageing

a) The Genetic Theory of Ageing (Programmed Aging Theory)

• Hayflick Limit: This refers to the phenomenon where normal somatic cells can divide only a certain number of times before they enter a state called senescence. The limit is based on the cell’s telomere length, with each cell division shortening the telomeres until cell division stops. This is a part of the programmed process, ensuring that cells stop replicating after a set number of divisions.
• Telomeres and Telomerase: Telomeres are repetitive DNA sequences at the ends of chromosomes that protect them from damage. As cells divide, telomeres shorten, which eventually leads to cell death or senescence. The enzyme telomerase can replenish telomeres, but its activity is typically low in most somatic cells, contributing to the ageing process. Some researchers suggest that maintaining telomere length could delay ageing, but this remains a topic of debate.
• Genetic Regulation of Lifespan: Certain genes are believed to regulate processes like DNA repair, protein turnover, and stress responses. The activation or inactivation of these genes, possibly through environmental or lifestyle factors, could influence how long an organism lives.

b) The Free Radical Theory of Ageing

• Oxidative Stress: Free radicals are produced as a byproduct of normal metabolic processes, especially during mitochondrial respiration. These radicals, such as superoxide (O₂⁻) and hydrogen peroxide (H₂O₂), can damage tissues over time, leading to oxidative stress, a condition where the body’s ability to neutralize free radicals (via antioxidants) is overwhelmed. This oxidative damage is believed to contribute to the progressive functional decline seen in ageing.
• Mitochondrial Damage: Mitochondria, the energy-producing organelles in cells, are particularly vulnerable to free radical damage because they generate a significant number of these molecules. Over time, oxidative damage to mitochondrial DNA can impair cellular energy production and lead to cell death, accelerating the ageing process. This is known as the mitochondrial theory of ageing, which is often considered an extension of the free radical theory.
• Antioxidants and Ageing: Antioxidants such as vitamin C, vitamin E, and glutathione protect cells by neutralizing free radicals. The free radical theory implies that increasing antioxidant intake could slow down the ageing process, but evidence supporting this as a direct method of lifespan extension is inconclusive.

c) The Wear and Tear Theory of Ageing

• Cellular Damage and Accumulation: According to the wear and tear theory, over time, the cells of the body accumulate damage from physical stress (such as UV radiation, pollutants, and toxins) and metabolic byproducts (such as free radicals, advanced glycation end products, etc.). This damage reduces the cell’s ability to repair itself and function efficiently.
• DNA and Protein Damage: Cells constantly face damage to their DNA due to environmental factors like UV rays and toxins. Additionally, proteins undergo damage through oxidation, glycation, and misfolding, which reduces their functionality. Over time, this accumulated damage leads to cellular dysfunction and organ failure, contributing to the ageing process.
• Decline of Repair Mechanisms: While the body has mechanisms for repairing cellular damage, such as DNA repair enzymes and proteasomes that degrade damaged proteins, these systems become less efficient as we age. As a result, the body’s ability to repair cellular damage diminishes, leading to functional decline.

3. Other Notable Theories of Ageing

• Hormonal Theory of Ageing: This theory suggests that changes in hormones, particularly growth hormone, insulin-like growth factor (IGF-1), and sex hormones, play a significant role in the ageing process. As these hormones decline with age, cellular regeneration and tissue repair decrease, contributing to age-related decline.
• Inflammatory Theory of Ageing: This theory emphasizes the role of chronic low-grade inflammation in ageing. As we age, our immune system becomes less efficient, leading to inflammaging — a state of increased systemic inflammation that contributes to the development of age-related diseases such as cardiovascular disease, Alzheimer’s, and arthritis.
• Somatic Mutation Theory: According to this theory, ageing results from the accumulation of mutations in the genetic material of cells over time. These mutations can arise from environmental factors or errors in DNA replication, and they impair cellular function, leading to the eventual breakdown of tissues and organs.

4. Conclusion

9. a) Briefly discuss the Menstrual cycle.

Answer:

2. The Phases of the Menstrual Cycle

a) Menstrual Phase (Days 1-5)

• Hormonal changes: Levels of estrogen and progesterone are low during this phase because the corpus luteum from the previous cycle has degenerated, leading to a decline in these hormones.
• Endometrial shedding: Without the hormonal support from estrogen and progesterone, the endometrial lining becomes unstable and is shed through the vagina as menstrual blood. This marks the start of the next cycle.
• Signs and symptoms: During menstruation, individuals may experience cramps, bloating, fatigue, and headaches, as well as mood changes due to hormonal fluctuations.

b) Follicular Phase (Days 1-13)

• Hormonal changes: The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to release follicle-stimulating hormone (FSH). FSH promotes the growth of primary follicles into secondary follicles in the ovaries.
• Estrogen production: As the follicles grow, they secrete estrogen, which stimulates the endometrium to thicken in preparation for a potential pregnancy. The rise in estrogen also triggers a surge in luteinizing hormone (LH), leading up to ovulation.
• Follicular development: During this phase, one dominant follicle emerges from a group of developing follicles. This dominant follicle becomes the Graafian follicle, which will eventually release an egg during ovulation.

c) Ovulation (Day 14)

• Hormonal surge: The LH surge triggered by increasing estrogen levels from the maturing follicle causes the follicle to rupture and release the egg. FSH levels also increase but to a lesser extent.
• Egg release: The egg is released into the fallopian tube (oviduct), where it may be fertilized by sperm if intercourse has occurred.
• Signs and symptoms: Ovulation can cause slight abdominal pain or mittelschmerz, a temporary discomfort on one side of the lower abdomen. Cervical mucus becomes clear, stretchy, and slippery, indicating peak fertility.

d) Luteal Phase (Days 15-28)

• Hormonal changes: After ovulation, the ruptured follicle forms the corpus luteum, which secretes progesterone and estrogen. Progesterone prepares the endometrium for potential implantation of a fertilized egg by making the uterine lining more vascular and nutritious.
• Endometrial preparation: If fertilization occurs, the fertilized egg will implant in the thickened endometrial lining. If no fertilization occurs, the corpus luteum degenerates, and progesterone and estrogen levels decline, leading to the shedding of the endometrial lining.
• Signs and symptoms: The luteal phase is often marked by symptoms of premenstrual syndrome (PMS), including bloating, mood swings, irritability, and fatigue. If pregnancy does not occur, these symptoms gradually fade as hormone levels drop, leading to the onset of menstruation.

3. Hormonal Regulation of the Menstrual Cycle

• Hypothalamus: The hypothalamus secretes GnRH, which stimulates the pituitary gland to release FSH and LH. The levels of these hormones fluctuate throughout the cycle, regulating the growth of ovarian follicles and the release of the egg.
• Pituitary Gland: The anterior pituitary secretes FSH, which promotes follicular development, and LH, which triggers ovulation. The ratio of FSH and LH changes as the cycle progresses, with a peak in LH levels occurring right before ovulation.
• Ovaries: The ovaries secrete estrogen and progesterone. Estrogen is primarily responsible for the growth of follicles and the thickening of the endometrium, while progesterone prepares the uterus for pregnancy.

4. Menstrual Cycle and Fertility

5. Irregularities in the Menstrual Cycle

• Amenorrhea: The absence of menstruation, which may occur due to pregnancy, stress, excessive exercise, or underlying medical conditions like polycystic ovary syndrome (PCOS) or hypothalamic dysfunction.
• Oligomenorrhea: Infrequent menstrual periods, often associated with conditions like PCOS or weight changes.
• Dysmenorrhea: Painful menstruation, often due to uterine contractions caused by prostaglandins. It can also be caused by conditions like endometriosis.
• Menorrhagia: Abnormally heavy or prolonged menstrual bleeding, which may result from hormonal imbalances or conditions like fibroids.

6. Conclusion

9. b) Explain the various mechanisms of enzyme regulation.

Answer:

1. Allosteric Regulation
2. Covalent Modification
3. Enzyme Induction and Repression
4. Feedback Inhibition
5. Proteolytic Cleavage
6. Isoenzymes
7. Environmental Factors

2. Allosteric Regulation

• Mechanism: When an allosteric effector binds to the enzyme at the allosteric site, it induces a conformational change in the enzyme, altering the shape of the active site. This may increase or decrease the enzyme’s affinity for its substrate, thereby regulating its catalytic activity.
• Example: The enzyme phosphofructokinase-1 (PFK-1), which plays a key role in glycolysis, is allosterically regulated by ATP (inhibitor) and AMP (activator). High ATP levels signal that energy is abundant, inhibiting PFK-1, while AMP, a signal of low energy, activates the enzyme to accelerate glycolysis.

3. Covalent Modification

• Phosphorylation and Dephosphorylation: One of the most common forms of covalent modification is phosphorylation, where a phosphate group is added to an enzyme, often changing its conformation and activity. This process is catalyzed by enzymes called kinases. The reverse process, dephosphorylation, is catalyzed by phosphatases, which remove phosphate groups.
• Example: The enzyme glycogen phosphorylase, which is involved in glycogen breakdown, is regulated by phosphorylation. When phosphorylated by phosphorylase kinase, it becomes active, allowing glycogen to be broken down into glucose-1-phosphate.

4. Enzyme Induction and Repression

• Induction: When a substrate or molecule is present in high concentrations, the cell may increase the production of enzymes involved in metabolizing that substance. This is called enzyme induction and typically involves the activation of specific genes that encode for the enzyme.
• Repression: In contrast, enzyme repression occurs when the presence of a product or metabolite inhibits the expression of an enzyme. This helps prevent overproduction of certain metabolites. This form of regulation is often seen in pathways involving feedback inhibition.
• Example: In the case of lactose metabolism in bacteria, the presence of lactose induces the expression of the lac operon, which codes for enzymes involved in the breakdown of lactose. When lactose is not available, the expression of these enzymes is repressed.

5. Feedback Inhibition

• Mechanism: The final product of a biosynthetic or catabolic pathway binds to an allosteric site on an enzyme that catalyzes an earlier step in the pathway, inhibiting its activity. This type of regulation ensures that the cell does not waste resources by producing excess amounts of the end product.
• Example: In the synthesis of isoleucine in bacteria, isoleucine itself acts as a feedback inhibitor for threonine deaminase, the enzyme that catalyzes the conversion of threonine to isoleucine. When isoleucine levels are high, the enzyme is inhibited, preventing further synthesis of isoleucine.

6. Proteolytic Cleavage

• Mechanism: In this process, an inactive precursor enzyme or zymogen (e.g., pepsinogen) is cleaved by another protease to yield the active form of the enzyme (e.g., pepsin). This irreversible change is often essential for the enzyme’s function in certain environments, like the acidic stomach or during blood coagulation.
• Example: Trypsinogen, the inactive form of trypsin, is activated by enterokinase to form trypsin in the small intestine. Trypsin then goes on to activate other digestive enzymes.

7. Isoenzymes (Isozymes)

• Mechanism: The existence of isoenzymes allows an organism to regulate metabolic processes in a tissue-specific manner or in response to varying environmental conditions. Different isoforms may have different affinities for substrates or may be regulated differently by allosteric effectors or inhibitors.
• Example: Lactate dehydrogenase (LDH) exists as several isoenzymes, such as LDH-1 found primarily in the heart and LDH-5 found in the liver. These isoenzymes have slightly different kinetic properties and are adapted to meet the energy demands of different tissues.

8. Environmental Factors

• Temperature: Enzymes generally have an optimal temperature range where they function most efficiently. At higher temperatures, enzyme denaturation can occur, while lower temperatures may slow down the reaction rates.
• pH: Enzymes have an optimal pH at which they are most active. Deviation from this optimal pH can lead to denaturation or altered enzyme activity.
• Substrate Concentration: Enzyme activity increases with the concentration of substrate, up to a point where the enzyme becomes saturated and the reaction rate reaches a maximum (Vmax).

9. Conclusion

10. a) Explain the factors that affect the ${\mathrm{O}}_2$${\mathrm{O}}_2$O_(2)\mathrm{O}_2${\mathrm{O}}_2$ dissociation curve.

Answer:

1. Introduction to the Oxygen Dissociation Curve

2. Partial Pressure of Oxygen (pO₂)

• High pO₂ (in the lungs): At higher oxygen partial pressures, such as those found in the lungs, hemoglobin becomes more saturated with oxygen. The curve plateaus at high pO₂, indicating that the hemoglobin molecule is fully saturated.
• Low pO₂ (in the tissues): At lower oxygen partial pressures, such as those found in tissues during respiration, hemoglobin releases oxygen more readily. The curve steepens at lower pO₂, facilitating the release of oxygen to the tissues where it’s needed.

3. pH and the Bohr Effect

• Low pH (increased acidity): When tissues are metabolically active, they produce carbon dioxide (CO₂) and lactic acid, which lower the pH of the blood. This causes a rightward shift in the oxygen dissociation curve, meaning hemoglobin releases oxygen more readily.
• High pH (alkaline conditions): In contrast, when the pH increases (becomes more alkaline), hemoglobin’s affinity for oxygen increases, which leads to a leftward shift in the curve. This is advantageous in the lungs, where oxygen uptake needs to be enhanced.

4. Carbon Dioxide (CO₂) Concentration

• High CO₂: In tissues with high metabolic activity (such as muscles during exercise), CO₂ is produced as a waste product. This CO₂ dissolves in the blood and forms carbonic acid, which dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻), thus decreasing pH and promoting oxygen release. The increased CO₂ concentration also directly interacts with hemoglobin, reducing its oxygen affinity, a process known as carbamino effect.
• Low CO₂: In the lungs, where CO₂ is exhaled, the CO₂ concentration is low, raising the pH of the blood. This causes a leftward shift in the oxygen dissociation curve, enhancing hemoglobin’s affinity for oxygen, promoting oxygen uptake from the alveoli.

5. Temperature

• Higher temperature: During conditions like exercise, fever, or inflammation, body temperature rises, which leads to a rightward shift in the oxygen dissociation curve. This allows for greater oxygen release to tissues that are metabolically active and generating more heat.
• Lower temperature: In cold environments, the oxygen dissociation curve shifts to the left, meaning hemoglobin holds on to oxygen more tightly, which is beneficial in conserving oxygen when metabolic activity is reduced.

6. 2,3-Bisphosphoglycerate (2,3-BPG)

• High 2,3-BPG: Under conditions such as chronic hypoxia (low oxygen levels), high altitudes, or during exercise, the body produces more 2,3-BPG. This molecule binds to hemoglobin and decreases its affinity for oxygen, facilitating the release of oxygen to tissues. The rightward shift of the oxygen dissociation curve in these conditions helps meet the increased oxygen demand of tissues.
• Low 2,3-BPG: In contrast, lower levels of 2,3-BPG, such as those seen in fetal hemoglobin (HbF), shift the curve to the left, increasing hemoglobin’s affinity for oxygen. This helps fetal blood capture oxygen from the maternal circulation more efficiently.

7. Fetal Hemoglobin (HbF)

• Leftward shift: The oxygen dissociation curve for fetal hemoglobin is shifted to the left compared to adult hemoglobin. This means that at lower pO₂ levels, HbF binds oxygen more tightly, ensuring that oxygen is transferred from the mother’s blood to the fetus.
• Lower affinity for 2,3-BPG: Fetal hemoglobin binds less 2,3-BPG, which contributes to its higher affinity for oxygen and explains the leftward shift in the dissociation curve.

8. Altitude and Chronic Hypoxia

• Adaptation to low oxygen: In response to prolonged low oxygen levels (hypoxia), the body enhances the release of 2,3-BPG, promoting a rightward shift in the oxygen dissociation curve, thereby facilitating oxygen delivery to tissues.

_9. Carbon Monoxide (CO) and Hemoglobin Affinity

• CO effect: The presence of carbon monoxide shifts the oxygen dissociation curve to the left, as the binding of CO to hemoglobin increases its affinity for oxygen, but decreases the actual release of oxygen to tissues.

10. Conclusion

10. b) Elaborate the chemical nature of hormones.

Answer:

1. Introduction to Hormones

2. Types of Hormones Based on Chemical Structure

1. Peptide and Protein Hormones
2. Steroid Hormones
3. Amino Acid Derivatives (Amine Hormones)

3. Peptide and Protein Hormones

• Structure: These hormones are synthesized as preprohormones in the endoplasmic reticulum of endocrine cells. The preprohormones are then converted into prohormones (inactive forms) before being cleaved into their active forms. The active hormone is then stored in vesicles and secreted into the bloodstream.
• Examples:
  • Insulin (a peptide hormone) is composed of 51 amino acids and plays a crucial role in regulating glucose metabolism.
  • Growth hormone and prolactin are also protein hormones that regulate growth and lactation, respectively.
  • Oxytocin and vasopressin (antidiuretic hormone) are peptide hormones involved in processes like labor and water balance regulation.
• Mechanism of Action: These hormones are hydrophilic (water-soluble), meaning they cannot easily cross the lipid bilayer of cell membranes. As a result, peptide and protein hormones bind to specific receptors on the surface of target cells, typically located in the plasma membrane. The binding of the hormone to its receptor triggers a cascade of intracellular events, often involving second messengers like cyclic AMP (cAMP) or inositol trisphosphate (IP3), which leads to the desired cellular response.

4. Steroid Hormones

• Structure: Steroid hormones have a characteristic four-ring structure, which includes three six-membered carbon rings and one five-membered carbon ring. The structure of these hormones can be modified by the addition of functional groups such as hydroxyl (-OH), methyl (-CH₃), and carbonyl (C=O) groups. These modifications help determine the specific activity and function of each steroid hormone.
• Examples:
  • Corticosteroids (e.g., cortisol) regulate stress responses and immune function.
  • Sex hormones such as estrogen, progesterone, and testosterone are involved in regulating reproductive functions and secondary sexual characteristics.
  • Aldosterone, a mineralocorticoid, is important for regulating sodium and potassium balance.
• Mechanism of Action: Since steroid hormones are lipophilic, they can diffuse across the cell membrane and enter the target cell. Once inside, they bind to specific intracellular receptors, which are located in the cytoplasm or nucleus. The hormone-receptor complex then acts as a transcription factor, binding to specific regions of DNA and promoting or inhibiting the transcription of certain genes. This results in the synthesis of specific proteins, which carry out the hormone’s effects.

5. Amino Acid Derivatives (Amine Hormones)

• Structure: These hormones are derived from amino acids, primarily tyrosine and tryptophan. Tyrosine is the precursor for catecholamines (e.g., dopamine, epinephrine, and norepinephrine) and thyroid hormones (e.g., thyroxine and triiodothyronine). Tryptophan is the precursor for serotonin and melatonin.
• Examples:
  • Catecholamines: These include dopamine, epinephrine (adrenaline), and norepinephrine (noradrenaline), which are involved in the fight-or-flight response, regulation of blood pressure, and mood.
  • Thyroid hormones: Thyroxine (T₄) and triiodothyronine (T₃) regulate metabolism, growth, and development.
  • Melatonin: Produced by the pineal gland, melatonin helps regulate the sleep-wake cycle.
• Mechanism of Action:
  • Catecholamines are hydrophilic and thus act on the surface receptors of target cells, similar to peptide hormones. For example, epinephrine binds to adrenergic receptors on cell membranes and triggers a cascade of signaling events, leading to effects like increased heart rate and blood pressure.
  • Thyroid hormones are lipophilic, like steroid hormones, and enter cells to bind to intracellular receptors. The hormone-receptor complex then interacts with DNA to regulate gene expression, influencing metabolism and growth.

6. Hormone Transport

• Carrier Proteins: For example, thyroid hormones bind to thyroid-binding globulin (TBG), and steroid hormones bind to albumin or specific carrier proteins like sex hormone-binding globulin (SHBG). These carrier proteins not only help in the transport of hormones but also act as reservoirs, regulating the bioavailability of the hormones.

7. Hormone Receptors and Mechanisms of Action

• Surface receptors: For water-soluble hormones, binding to G-protein coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs) triggers second messenger pathways, such as the cAMP pathway or phosphoinositide pathway, leading to cellular changes.
• Intracellular receptors: For lipid-soluble hormones, the hormone-receptor complex enters the nucleus, where it directly influences gene expression, modulating the synthesis of specific proteins that mediate the hormone’s effects.

8. Hormonal Regulation and Feedback Mechanisms

• Negative feedback: Most hormonal systems are regulated by negative feedback, where an increase in the hormone’s activity leads to a signal that inhibits further hormone production. For example, the secretion of insulin is regulated by blood glucose levels: when glucose is low, insulin secretion decreases.
• Positive feedback: In some cases, the secretion of a hormone promotes its own release. An example is the oxytocin release during childbirth, which intensifies uterine contractions and promotes further oxytocin secretion, until delivery occurs.

9. Conclusion