Science and Technology: UPSC Previous Year Questions (Prelims)

Explanation

The half-life of a radioactive substance is the time it takes for half of the substance to decay. After each half-life:

At the end of 1 half-life (4 months), 50% of the substance remains. At the end of 2 half-lives (8 months), 25% of the substance remains, meaning 75% (three-fourths) of the substance has decayed. Thus, it takes 8 months for three-fourths of the substance to decay.

Explanation

Astatine, Francium, and Tritium are radioactive elements:

Astatine is a halogen with no stable isotopes.

Francium is an alkali metal, highly unstable and radioactive. Tritium is a radioactive isotope of hydrogen. Zirconium is not radioactive. It is a stable, non-radioactive metal widely used in industries like nuclear reactors and construction.

Explanation

A is matched with 2: Neutrino is a particle with almost zero mass and no electric charge, making it the correct match for zero mass. B is matched with 3: Quarks are fundamental particles with fractional charges, such as +2/3 or -1/3. C is matched with 1: Positrons, the antimatter counterparts of electrons, possess fractional spin, matching this characteristic. D is matched with 4: Photons are elementary particles of light with an integral spin of 1, classifying them as bosons. Direction: The following item consists of two statements. One labeled as the ‘Assertion (A)’ and the other as "Reason (R). You have to answer this item using the code given below: (2000)

Explanation

Statement 1 is incorrect: Ionization potential (or Energy) increases across a period (left to right) because of increasing nuclear charge, which leads to a stronger attraction between the electrons and the nucleus, making electron removal harder. The effective nuclear charge (Z_eff) increases as protons are added, while shielding remains nearly constant.

Statement 2 is correct: Electron affinity generally becomes less negative down a group. As the atomic size increases, the added electron is farther from the nucleus and experiences greater shielding, reducing the energy released during electron addition. Across a period, electron affinity generally becomes more negative, reflecting stronger attraction to added electrons. Thus, in a group of elements, electron affinity decreases as atomic weight increases.

Statement 3 is incorrect: Electronegativity increases across a period due to increasing nuclear charge, which enhances the ability to attract bonding electrons. However, group-wise, it decreases as atomic size and shielding increase, reducing the effective pull of the nucleus on bonding electrons.

Explanation

The mitochondrion is an organelle in the cell that contains its own DNA. Unlike most of the cell’s DNA, which is found in the nucleus, mitochondria have their own small circular DNA. This mitochondrial DNA (mtDNA) encodes essential proteins involved in energy production through cellular respiration. Mitochondria are often referred to as the "powerhouses of the cell" due to their role in ATP synthesis. Options (a), (b) and (c) are incorrect:

Golgi Apparatus: Membrane-bound organelle; modifies, sorts, and packages proteins and lipids; consists of cisternae; involved in glycoprotein and glycolipid formation. Lysosomes: Membrane-bound vesicles; contain hydrolytic enzymes; digest carbohydrates, proteins, lipids, nucleic acids; play role in intracellular digestion; known as "suicide bags" due to potential self-digestion. Centrioles: Cylindrical structures; composed of microtubules; organize microtubules during cell division; form mitotic spindle; located near nucleus in animal cells; do not contain DNA.

Explanation

Athlete’s Foot, medically known as tinea pedis, is a fungal infection caused by a group of fungi called dermatophytes, which thrive in warm, moist environments like those found in shoes. It primarily affects the skin of the feet, causing itching, redness, and scaling. It is highly contagious and spreads through direct contact or contaminated surfaces. Options (a), (c) and (d) are incorrect:

Bacterial infections are caused by microorganisms like Staphylococcus or Streptococcus. These can lead to conditions like pneumonia, tuberculosis, and food poisoning. Unlike fungal infections, bacterial infections can usually be treated with antibiotics. Protozoans are single-celled organisms that can cause diseases like malaria (caused by Plasmodium), amoebic dysentery (caused by Entamoeba histolytica), and sleeping sickness (caused by Trypanosoma). They are typically transmitted through contaminated water or insect bites. Nematodes, or roundworms, are parasitic worms that can infect humans, causing diseases like ascariasis, filariasis, and hookworm infection. These are usually transmitted through contaminated soil or food.

Explanation

Epiphytes, commonly known as "air plants," are plants that depend on other plants primarily for mechanical support. They grow on the surface of other plants (like trees) to access better light and air circulation, but they do not take nutrients from the host plant. While they do not parasitize the host but they can absorb water and nutrients from rain, dew, and air humidity through specialized structures like spongy roots. Epiphytes are typically found in humid environments and include species like orchids, mosses, and ferns. They do not rely on the host for food or shade, as they can photosynthesize and derive most of their nutrients from the air and rain.

Explanation

In eye donation, the part of the eye that is transplanted is the cornea. The cornea is the clear, dome-shaped front surface of the eye that allows light to enter and is responsible for focusing light onto the retina. It can become damaged due to injury, disease, or aging. A corneal transplant replaces the damaged cornea with healthy tissue from a deceased donor.

Option (b) is incorrect: The lens is located behind the cornea and focuses light onto the retina. It is not transplanted during eye donation, as it can be replaced with an artificial lens in procedures like cataract surgery.

Option (c) is incorrect: The retina is a sensitive tissue lining the back of the eye that converts light into neural signals. Retinal damage cannot currently be treated through transplantation during eye donation.

Option (d) is incorrect: The whole eye is not transplanted due to the complexity of the organ and the high risk of rejection.

Explanation

Option (a) is correct: O, Rh- blood group is known as the universal donor because it does not have A, B, or Rh antigens. If the patient’s blood type is not O, receiving blood with A or B antigens can trigger an immune response, leading to serious complications. If the patient is Rh-, receiving Rh+ blood can lead to the development of Rh antibodies, which can cause problems in future pregnancies or transfusions. Therefore, O, Rh- blood group will be safe for transfusion if the blood group of the patient is not known.

Option (b) is incorrect: If the recipient is Rh-negative, transfusion of Rh+ blood can cause an immune reaction, leading to complications like hemolysis or sensitization.

Option (c) is incorrect: AB, Rh- has both A and B antigens, which can trigger an immune response in individuals with A, B, or O blood groups.

Option (d) is incorrect: AB, Rh+ blood group is highly specific and can only be safely given to individuals with the AB, Rh+ blood group. It is not suitable for transfusion in emergencies where the recipient’s blood group is unknown.

Explanation

A is matched with 3: Lederberg discovered bacterial conjugation (exchange of genetic material between bacteria) and transduction (transfer of DNA between bacteria via viruses). B is matched with 4: Thomas Hunt Morgan’s work with fruit flies (Drosophila) provided strong evidence for the chromosomal theory of inheritance and firmly established the concept of sex-linked genes. C is matched with 2: Kornberg isolated DNA polymerase I from Escherichia coli, which was crucial for understanding DNA replication. D is matched with 1: Har Gobind Khorana played a significant role in deciphering the genetic code, which specifies how the sequence of nucleotides in DNA translates into the sequence of amino acids in proteins.