Which of the following structures is responsible for storing and releasing calcium ions that control the contraction-relaxation cycle of skeletal muscle?

Pepsin is a critical digestive protein that accelerates the breakdown of dietary proteins into smaller peptides. Its classification as an enzyme stems from its biological role as a catalyst, its proteinaceous nature, and its specific function in the stomach. Below is a detailed explanation of why pepsin is an enzyme and how it operates:

Definition and Role of Pepsin:

Enzyme Nature:

• • Pepsin is aproteolytic enzyme(a type of hydrolase) that cleaves peptide bonds in proteins.
  • Like all enzymes, itlowers activation energyfor protein digestion, speeding up the reaction without being consumed.

Production and Activation:

• • Secreted by gastric chief cells as inactivepepsinogen.
  • Activated byHClin the stomach (pH ~1.5–2), which unfolds pepsinogen to expose its active site.

2. Why It’s Not Other Options:

2. Carbohydrate:

• • Carbohydrates (e.g., sugars, starch) are energy sources or structural molecules (e.g., cellulose). Pepsin digests proteins, not carbs.

3. Nucleic Acid:

• • Nucleic acids (DNA/RNA) store genetic information. Pepsin has no role in nucleotide metabolism.

4. Lipid:

• • Lipids (fats) are broken down bylipases, not pepsin.

3. Key Characteristics of Pepsin as an Enzyme

• Substrate Specificity:
  Pepsin primarily targets peptide bonds next to hydrophobic or aromatic amino acids, such as phenylalanine and tyrosine.
• Optimal Conditions for Activity:
  • Functions best in an acidic environment (maintained by stomach acid).
  • Becomes inactive or denatured at neutral or alkaline pH, such as in the duodenum.
• Clinical Significance:
  • Low levels of pepsin or hydrochloric acid (HCl): Can cause protein malabsorption, often seen in conditions like hypochlorhydria (low stomach acid).
  • Excess pepsin: May contribute to GERD (gastroesophageal reflux disease) by damaging the esophageal lining during acid reflux.

4. Comparison with Other Digestive Enzymes



Solid

Reasoning:

The volume a substance occupies depends on its state of matter, with gases typically taking up the most space and solids the least. Carbon dioxide can exist in several states—gas (CO₂), liquid (under pressure), or solid ("dry ice")—depending on temperature and pressure.

1. States of Carbon Dioxide & Volume:
   • Gas: In this state, CO₂ molecules are far apart and move freely, so they occupy the largest volume.
   • Liquid: Requires high pressure and low temperature. Molecules are closer together, so the volume is smaller than gas.
   • Solid (Dry Ice): Molecules are packed tightly in a fixed structure, so it occupies the least volume.
   • Plasma: Not relevant for normal CO₂ behavior; plasma refers to an ionized gas state, not typical for CO₂ in natural conditions.

1. Why Option 3 is Correct:
   • In the solid state, carbon dioxide has minimal kinetic energy, and its molecules are tightly packed, resulting in the least volume among all options.
   • Dry Ice (Solid CO₂):
     In its solid form, carbon dioxide molecules are packed tightly in a rigid crystalline lattice, making it the densest state of CO₂.

• Density Comparison:
  • Solid CO₂: ~1.6 g/cm³
  • Liquid CO₂: ~1.0 g/cm³
  • Gaseous CO₂ at STP: ~0.0018 g/cm³
• Volume by Mass:
  • 1 kg of CO₂ gas occupies approximately 560 liters
  • 1 kg of liquid CO₂ occupies approximately 1 liter
  • 1 kg of solid CO₂ occupies approximately 0.6 liters

3. Why the Other Options Are Incorrect

• 1. Plasma:
  Plasma is an ionized gas that exists only under extreme conditions (e.g., high energy in labs or stars). It occupies a greater volume than solids or liquids and is not a natural state for CO₂ on Earth.
• 2. Liquid:
  Liquid CO₂ is more compressed than gas but still less dense than solid CO₂.
• 4. Gas:
  Gaseous CO₂ has the lowest density because its molecules are spread far apart, occupying the most space.

4. Real-World Applications

• Dry Ice for Storage and Transport:
  Solid CO₂ (dry ice) is ideal for refrigeration and shipping due to its high density and ability to sublimate directly into gas, avoiding liquid messes.
• Carbon Capture and Storage (CCS):
  In environmental technologies, captured CO₂ is often compressed into liquid or solid form to reduce storage volume and space required.

Yersinia pestis

Reasoning:

Yersinia pestis is the bacterium responsible for plague, including the bubonic plague. Its primary mode of transmission is through bites from fleas, particularly rat fleas (Xenopsylla cheopis) that have fed on infected rodents.

1. Pathogen Overview – Yersinia pestis:
   • Gram-negative bacterium.
   • Causes bubonic, septicemic, and pneumonic plague.
   • Historically associated with pandemics such as the Black Death.
2. Transmission Mechanism:
   • Fleas ingest the bacteria by biting infected rodents.
   • The bacteria multiply in the flea's gut, eventually blocking it.
   • When the flea bites a human, it regurgitates infected material into the bite wound.
   • Human infection then spreads from the bite site, typically to lymph nodes.

Why the Other Options Are Incorrect:

• 1. Corynebacterium diphtheriae
  • Causes diphtheria.
  • Transmitted via respiratory droplets, not fleas.
• 2. Neisseria meningitidis
  • Causes bacterial meningitis.
  • Spread by saliva and respiratory secretions.
• 3. Plasmodium falciparum
  • Causes the most severe form of malaria.
  • Transmitted by female Anopheles mosquitoes, not fleas or rats.

What Is Adhesion?

Adhesion is a property of water where water molecules are attracted to and stick to other substances—especially those with polar or charged surfaces, like glass, plant tissues, or metal. This occurs because water is a polar molecule, meaning it has a slightly positive end and a slightly negative end, which allows it to form hydrogen bonds with other polar surfaces.

Why 2 is Correct:

Raindrops stick to the outside of a window.

• When it rains, water molecules cling to the glass surface of the window.
• This happens because of adhesion—the attraction between the water molecules and the glass (a polar surface).
• It’s a classic example of how water interacts with other materials in the environment.

Why the Other Choices Are Incorrect:

1. Small water droplets cling together to make one large water droplet

• This demonstrates cohesion, not adhesion.
• Cohesion is when water molecules stick to each other, due to hydrogen bonding between water molecules.

3. Water molecules support the weight of a small insect

• This shows surface tension, which is a result of cohesion at the water's surface.
• Water molecules at the surface are tightly bonded together, forming a sort of “skin” that can support light objects (like a water strider).

4. Water and oil separate into two distinct layers

• This is due to differences in polarity, not adhesion.
• Water is polar, oil is nonpolar—they do not mix because there’s no attraction between them.

Diffusion down a concentration gradient

Reasoning:

The primary mechanism by which carbon dioxide (CO₂) moves from the blood into the alveoli of the lungs is diffusion. This occurs because of a concentration gradient between the blood (where CO₂ levels are higher) and the alveolar air (where CO₂ levels are lower).

This Is Correct because:

• Diffusion is a passive process that does not require energy.
• CO₂ moves from areas of high partial pressure in the blood to areas of low partial pressure in the alveolar air.
• This process occurs across the thin respiratory membrane in the alveoli.

Supporting Mechanisms of CO₂ Movement:

• Carbonic Anhydrase Role:
  Inside red blood cells, carbon dioxide (CO₂) combines with water to form bicarbonate ions (HCO₃⁻), aiding CO₂ transport in the bloodstream. In the lungs, this reaction is reversed—bicarbonate converts back to CO₂, which then diffuses into the alveoli for exhalation.
• Partial Pressure Gradient:
  • In venous blood (PvCO₂): ~45 mmHg
  • In alveolar air (PACO₂): ~40 mmHg
    This 5 mmHg difference creates the necessary gradient for CO₂ to move from the blood into the alveoli via diffusion.

Why the Other Options Are Incorrect:

• 2. Active transport using energy: CO₂ transport across the alveolar membrane does not involve active transport or ATP.
• 3. Conversion to carbon monoxide: CO₂ is never converted to carbon monoxide (CO); CO is a toxic gas and not part of normal respiratory physiology.
• 4. Passive transport using carrier proteins: While CO₂ can bind to hemoglobin in the blood, its movement into the alveoli happens by simple diffusion, not via carrier proteins.

Clinical Significance:

• Hypercapnia: An abnormal buildup of CO₂ in the blood, often due to impaired gas exchange as seen in conditions like emphysema.
• Hypoventilation: Reduced breathing efficiency (e.g., from opioid overdose) leads to CO₂ retention, potentially causing respiratory acidosis.

Swelling of the neck

Reasoning:

Endemic goiter is a condition resulting from iodine deficiency, which impairs the synthesis of thyroid hormones (T₃ and T₄). When the body senses low thyroid hormone levels, the pituitary gland secretes more thyroid-stimulating hormone (TSH) to compensate. This constant stimulation leads to hypertrophy (enlargement) of the thyroid gland, causing a visible swelling in the neck known as a goiter.

1. Cause of Endemic Goiter:
   • Iodine is essential for the production of thyroid hormones.
   • In iodine-deficient regions (often inland or mountainous), low iodine intake leads to reduced T₃ and T₄ levels.
   • The pituitary increases TSH secretion, stimulating thyroid growth in an attempt to normalize hormone levels.
2. Physical Symptom:
   • The thyroid gland enlarges, resulting in a swelling at the base of the neck, which may be clearly visible and even interfere with swallowing or breathing in severe cases.
3. Why the Other Options Are Incorrect
   • 1. Enlarged hands and feet:
     This symptom is characteristic of acromegaly, a condition caused by excessive growth hormone, not related to iodine deficiency or thyroid enlargement.
   • 2. Increased bone fractures:
     Frequently associated with osteoporosis or hyperparathyroidism, both of which affect calcium metabolism — not conditions linked to iodine deficiency.
   • 3. Rounded face (moon face):
     Typically seen in Cushing’s syndrome, which results from prolonged exposure to high cortisol levels. This is unrelated to thyroid or iodine disorders.
4. Additional Symptoms of Iodine Deficiency

• Hypothyroidism Symptoms:
  • Fatigue
  • Weight gain
  • Cold intolerance
  • Dry skin
• Severe Iodine Deficiency Outcomes:
  • Cretinism (in children): Delayed growth and cognitive impairment.
  • Myxedema (in adults): Puffiness of the skin, slowed metabolism, and mental sluggishness.

Urea

Reasoning:

When proteins are broken down in the body, a waste product called ammonia is formed. Since ammonia is toxic, the body converts it into urea, a less harmful substance. Urea is then eliminated from the body primarily by the kidneys through urine, but also in small amounts by sweat glands.

Here’s how it works:

1. Protein Catabolism
   • Proteins → Amino acids → Ammonia (NH₃)
   • Ammonia is highly toxic to cells and must be removed quickly.
2. Urea Formation
   • In the liver, ammonia is converted into urea via the urea cycle.
3. Excretion via Sweat
   • While the kidneys are the main organs responsible for filtering urea into urine, the sweat glands in the skin also excrete a small amount of urea.
   • This is why sweat can have a slightly ammonia-like odor during intense exercise or in people with kidney problems.
4. Other Options Explained:
   • Water: Also excreted in sweat, but not a direct byproduct of protein breakdown.
   • Sebum: An oily secretion from sebaceous glands, unrelated to nitrogen waste.
   • Lysozymes: Enzymes that kill bacteria, present in sweat but not related to protein catabolism.

Urea is the nitrogenous waste product excreted in small amounts by sweat glands after proteins are broken down and ammonia is formed. This helps the body safely eliminate excess nitrogen

Antimicrobial peptides

Reasoning:

Dermcidin and cathelicidin are part of the body's innate immune system. They are antimicrobial peptides (AMPs)—small proteins secreted by epithelial cells (especially in the skin) that help protect against a wide range of pathogens.

1. What Are Antimicrobial Peptides?

• Short proteins that disrupt microbial membranes.
• Active against bacteria, viruses, and fungi.
• Provide rapid, nonspecific defense as part of innate immunity.

2. Functions of Dermcidin and Cathelicidin:

• Dermcidin:
  • Secreted by sweat glands in the skin.
  • Kills bacteria on the skin surface by disrupting their membranes.
• Cathelicidin (LL-37 in humans):
  • Found in various tissues, including skin, lungs, and the gastrointestinal tract.
  • Neutralizes bacteria and modulates immune responses (e.g., reduces inflammation).

3. Why the Other Options Are Incorrect:

• B. Chemical messengers: Typically refers to hormones or cytokines, not AMPs.
• C. Neurotransmitters: Involved in nerve signaling (e.g., dopamine, serotonin), unrelated to innate immunity.
• D. Digestive enzymes: Break down food (e.g., amylase, pepsin), not involved in pathogen defense.

4. Clinical Relevance

• Wound Healing: Cathelicidin plays a vital role in promoting tissue repair and regeneration.
• Skin Disorders: Low levels of antimicrobial peptides are associated with conditions such as eczema and psoriasis.
• Infections: Some pathogens, like Streptococcus pyogenes, can evade these peptides, allowing them to cause infections.

Centromere

Reasoning:
During cell division, specifically in mitosis and meiosis, the spindle fibers play a crucial role in the accurate separation of chromosomes. These fibers are part of the mitotic spindle apparatus, which is composed of microtubules.

1. Centromere:
   The centromere is the region of a chromosome where the two sister chromatids are joined. It is also the specific location where the kinetochore forms—a protein structure that serves as the attachment point for spindle fibers.
2. Function of Spindle Fibers:
   Once attached to the kinetochores at the centromeres, spindle fibers pull the sister chromatids apart during anaphase, ensuring that each daughter cell receives an identical set of chromosomes.

Why Other Options Are Incorrect:

• Gene: A segment of DNA that codes for a specific protein. Spindle fibers do not attach to genes.
• Nucleosome: The basic unit of DNA packaging, consisting of DNA wrapped around histone proteins. It is involved in DNA compaction, not chromosome movement.
• Histone: Proteins that help package DNA into nucleosomes. These are structural, not involved in spindle attachment.

Key Visual:

• Centromere= The "waist" of the chromosome where spindle fibers pull chromatids apart.
• Kinetochore= Protein complex on the centromere that spindle fibers latch onto.

H₂O has stronger intermolecular bonds than H₂S.

Reasoning

Although hydrogen sulfide (H₂S) and water (H₂O) are chemically similar due to their group placement in the periodic table (Group 16: chalcogens), they exhibit very different physical states at room temperature—H₂S is a gas, while H₂O is a liquid. The key reason lies in the strength and type of intermolecular forces between their molecules.

1. Nature of Intermolecular Forces:
   • H₂O exhibits hydrogen bonding, a particularly strong type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom like oxygen.
   • H₂S, however, does not form hydrogen bonds. Sulfur is less electronegative than oxygen and too large in size to facilitate hydrogen bonding effectively. As a result, H₂S only exhibits weak van der Waals forces (London dispersion forces).
2. Impact of Hydrogen Bonding in Water:
   • In water, each molecule can form up to four hydrogen bonds with neighboring molecules, creating a tightly connected liquid network.
   • These strong intermolecular forces require more energy (heat) to break, resulting in higher boiling and melting points, and hence water remains a liquid at room temperature.
3. Why H₂S Is a Gas:
   • Lacking strong intermolecular forces, H₂S molecules separate easily and exist as a gas under the same conditions.
   • It has a significantly lower boiling point than water (-60°C vs. 100°C), confirming the weakness of its intermolecular interactions.
4. Incorrect Options Explained:
   • Option 1 (H₂S has stronger intermolecular bonds): Incorrect; its bonds are weaker than those in H₂O.
   • Option 2 and 4 (Ionic bonds): Both H₂O and H₂S are covalent, not ionic, compounds. These options are irrelevant to their physical states.