A physicist claims to have developed friction-free tires for cars. Which of the following problems would result from using these tires?

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.

Hydrogen ions (H⁺) released from carbonic acid neutralize hydroxide ions (OH⁻) to resist change in blood pH.

Reasoning

The carbonic acid–bicarbonate buffer system is the body’s primary mechanism for maintaining blood pH around 7.4. When an alkaline substance like hydroxide ions (OH⁻) enters the bloodstream, this buffer system helps resist changes in pH by neutralizing the excess base.

1. Buffer System Overview:
The buffer relies on the following equilibrium:

CO2+H2O↔H2CO3↔HCO3−+H+

• Carbonic acid (H₂CO₃): a weak acid that can release H⁺.
• Bicarbonate (HCO₃⁻): a weak base that can accept H⁺.

2. Response to an Alkaline Input (OH⁻):

• Problem: OH⁻ increases pH by binding to free hydrogen ions:

OH−+H+→H2O

• Buffer Solution: The buffer system shifts to produce more H⁺. To restore balance, carbonic acid dissociates:

H2CO3→HCO3−+H+

This newly released H⁺ neutralizes the OH⁻, preventing the rise in pH.

• Final Step: Carbonic acid can also break down into carbon dioxide (CO₂) and water:

H2CO3→CO2+H2O

The CO₂ is then exhaled by the lungs, helping regulate the buffer system.

3. Why the Other Options Are Incorrect:

• 1 & 2: Incorrectly suggest that bicarbonate releases OH⁻. In reality, bicarbonate accepts H⁺, acting as a weak base.
• 4: Misstates the purpose of the buffer. It doesn’t aim to raise pH, but rather to maintain a stable pH by neutralizing either excess acid or base.

Points to Remember:

• H⁺ ions from carbonic acid neutralize incoming OH⁻, preventing alkalosis.
• Lungs help by removing CO₂ (driving the equilibrium left).
• Kidneys fine-tune pH by excreting or reabsorbing bicarbonate (HCO₃⁻).

A stem cell maturing to become a muscle cell that can contract.

Reasoning:

Cell differentiation is the biological process by which a less specialized cell (like a stem cell) becomes a more specialized cell type with a specific structure and function, such as a muscle cell, nerve cell, or blood cell.

1. What Is Cell Differentiation?
   • In multicellular organisms, stem cells give rise to different cell types during development or tissue repair.
   • Differentiation involves gene expression changes that lead to specialized structures and functions.
2. Why Option C Is Correct:
   • A stem cell becoming a muscle cell is a classic example of differentiation.
   • This transformation enables the cell to contract, a function unique to muscle cells.
3. Why Other Options Are Incorrect:
   • 1. Muscle cell producing more ATP is an example of cellular metabolism, not differentiation.
   • 2. A pancreatic cell releasing hormones reflects normal cell function, not a change in cell type.
   • 3. A mutation in a stomach cell is a genetic change, possibly harmful, but it is not differentiation.

Key Examples of Differentiation:

1. Embryonic Development:
   During early development, pluripotent stem cells (from the embryo) have the ability to become any cell type in the body. As development progresses, these stem cells differentiate into specialized cells such as:
   • Neurons: Specialized for transmitting electrical signals in the brain and nervous system.
   • Blood cells: Including red blood cells (which carry oxygen) and white blood cells (which fight infection).
   • Cardiomyocytes: Heart muscle cells that contract to pump blood.
2. Adult Tissues (Somatic Differentiation):
   In fully developed organisms, certain tissues still contain multipotent stem cells that can replenish specific cell types. A key example:
   • Hematopoietic Stem Cells (HSCs): Found in bone marrow, these stem cells differentiate into various blood cells, including:
     • Red blood cells (erythrocytes): Carry oxygen.
     • White blood cells (leukocytes): Defend against pathogens.
     • Platelets (thrombocytes): Help in blood clotting.

DNA is composed of two complementary strands arranged in an antiparallel fashion, meaning one strand runs 5' to 3', and the other runs 3' to 5'. The bases pair according to base-pairing rules:

• A (adenine) pairs with T (thymine)
• G (guanine) pairs with C (cytosine)

RNA uses uracil (U) instead of thymine, but since this question pertains to DNA, T is used, not U.

Step-by-Step Complementation:

Given DNA strand:
5' AGCTAGCGT 3'

Complement base by base (using A↔T and C↔G):

Use the base pairing rules:

A → T

G → C

C → G

T → A

Step-by-Step Pairing:

| Original (5'→3') | A | G | C | T | A | G | C | G | T |
| Complementary (3'→5') | T | C | G | A | T | C | G | C | A |

Thus, the complementary strand is:3' TCGATCGCA 5'

Why the Other Options Are Wrong:

2.Incorrect: Matches the original strand (no complementarity).

3.Incorrect: Uses "U" (uracil, found in RNA) and has typos ("UTCGCU").

4.Incorrect: Uses "U" (RNA) and has the wrong directionality (5'→3' instead of 3'→5').

Pancreas

Reasoning:

The pancreas plays a crucial role in digestion by releasing digestive enzymes and sodium bicarbonate (NaHCO₃) into the duodenum (the first section of the small intestine). Sodium bicarbonate helps neutralize the acidic chyme that enters the small intestine from the stomach.

1. Function of Sodium Bicarbonate:
   • The chyme from the stomach is highly acidic due to gastric hydrochloric acid (HCl).
   • The pancreas releases sodium bicarbonate to buffer this acid, raising the pH and creating a more alkaline environment ideal for enzyme activity in the small intestine.
2. Role of the Pancreas:
   • Part of both the endocrine and exocrine systems.
   • Exocrine function includes secreting:
     • Digestive enzymes (lipase, amylase, proteases).
     • Sodium bicarbonate via the pancreatic duct into the duodenum.

Why the Other Options Are Incorrect:

• 1. Liver:
  • Produces bile, which helps emulsify fats but does not release sodium bicarbonate.
• 2. Appendix:
  • A small, vestigial organ with no known role in digestion or pH regulation.
• 3. Gallbladder:
  • Stores and concentrates bile made by the liver, but does not produce sodium bicarbonate.

Mechanism of pH Regulation in the Small Intestine:

• Stomach Acid (HCl):
  The chyme entering the small intestine from the stomach is highly acidic due to hydrochloric acid.
• Pancreatic Bicarbonate (NaHCO₃):
  The pancreas secretes sodium bicarbonate, which neutralizes the acid through the following reaction:

NaHCO₃+HCl→NaCl+H₂CO₃

• Carbonic Acid (H₂CO₃):
  This intermediate breaks down into carbon dioxide (CO₂) and water (H₂O):

H₂CO₃→CO₂+H₂O

The CO₂ is exhaled via the lungs, and the water remains in the intestinal tract, helping to protect the intestinal lining from acid damage.

Clinical Relevance:

• Pancreatic Insufficiency:
  A decrease in bicarbonate and enzyme secretion (e.g., in chronic pancreatitis) can result in acidic intestinal contents and nutrient malabsorption.
• Cystic Fibrosis:
  Thick mucus obstructs pancreatic ducts, impairing bicarbonate delivery and enzyme flow into the small intestine, leading to digestive complications.

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.

A country with a growing population and high infant mortality typically experiences high birth rates that outpace death rates. This demographic pattern is common in developing countries, where families tend to have more children to compensate for the higher risk of infant and child mortality.

1. High Infant Mortality:
   • Increases the likelihood that families will have more children to ensure that some survive into adulthood.
   • This leads to elevated birth rates.
2. Growing Population:
   • Indicates that the number of people being born exceeds the number of people dying.
   • Even with high death rates (especially in infants), if the birth rate is even higher, the population will grow.
3. Demographic Transition Model:
   • Countries in Stage 2 (early industrializing) often have declining death rates due to improved healthcare but maintain high birth rates, resulting in rapid population growth.

Why the Other Options Are Incorrect:

• 2. Birth rates variable compared to death rates:
  Too vague and does not describe a consistent demographic pattern for population growth.
• 3. Birth rates lower than death rates:
  Would result in a declining population, which contradicts the condition that the population is growing.
• 4. Birth rates equal to death rates:
  Implies zero population growth, which is not the case here.

Milliliters

Reasoning:

Milliliters (mL) are the appropriate unit for measuring volume in the metric system, especially for fluids like blood.

Here's why:

• Milliliters (mL) measure liquid volume, which makes them ideal for medical samples such as blood, urine, or intravenous fluids.
  • For example, a typical blood sample drawn for lab analysis is often between 2 mL and 10 mL.
  • The milliliter is equivalent to 1/1000 of a liter.

Other Units Explained:

• Centimeters (cm):
  • A unit of length, not volume. It measures distance or size in one dimension.
• Milligrams (mg):
  • A unit of mass or weight, not volume. It is used to measure the weight of a substance, not how much space it occupies.
• Millimeters (mm):

Another unit of length, used to measure small distances (e.g., the diameter of a blood vessel), not volume.

Fallopian tubes

Reasoning:

Fertilization in humans typically occurs in the fallopian tubes, also known as uterine tubes or oviducts. These are the narrow tubes that connect the ovaries to the uterus and serve as the site where the sperm meets the egg.

Here's how fertilization happens:

1. Ovulation:
   • An ovary releases a mature egg (ovum) during ovulation.
2. Egg enters the fallopian tube:
   • The fimbriae (finger-like projections at the end of the fallopian tube) help guide the egg into the tube.
3. Fertilization:
   • If sperm are present, fertilization typically occurs in the ampulla, the widest section of the fallopian tube.
   • The sperm penetrates the egg, forming a zygote.
4. Zygote travels to uterus:
   • The fertilized egg continues down the tube and enters the uterus, where it may implant in the uterine lining and develop into an embryo.

Other Options Explained:

• Ovaries: Produce and release eggs but are not where fertilization takes place.
• Vagina: The entry point for sperm during intercourse; not involved in fertilization directly.
• Uterus: The site of implantation and development after fertilization, but fertilization itself does not occur here.

Clinical Relevance:

• Ectopic pregnancy: If the embryo implants in the fallopian tube (often due to scarring or blockage), it can rupture the tube—a medical emergency.
• IVF (In vitro fertilization): Eggs and sperm are combinedoutsidethe body (in a lab), then the embryo is placed directly into the uterus.

This is how natural selection works in response to environmental changes:

1. Initial Population Trait
   The majority of the beetles in the population are brown, which provides camouflage on brown trees and protects them from predators. White beetles, due to mutation, are not camouflaged and are quickly eaten by birds. Thus they are rare in the population.
2. Mutation and Variation
   Occasionally, a genetic mutation produces white beetles. Under normal conditions (brown trees), these white beetles are more visible and are quickly eaten by predators such as birds.
3. Environmental Change
   When all the trees are painted white, the environment changes dramatically. Now, brown beetles become highly visible, and white beetles blend in better with the surroundings.
4. Shift in Survival Advantage
   Birds will now easily spot and eat the brown beetles, reducing their numbers. White beetles will survive longer because they are camouflaged, increasing their chances of reproduction.

Population Change Over Time
Over time, the population will shift in favor of the white beetles as they survive and reproduce more than the brown ones. This is a classic case of evolution by natural selection