UPSC CDS-2 Archives

1. Which one of the following is a saprophyte ?

Which one of the following is a saprophyte ?

Cuscuta

Rhizopus

Spirogyra

Grass

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UPSC CDS-2 – 2023

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A saprophyte is an organism that obtains its nutrition from dead and decaying organic matter.

Rhizopus is a genus of fungi, commonly known as bread mould. Fungi are heterotrophic organisms, and many, including species of Rhizopus, are saprophytes, digesting dead organic material externally before absorbing the nutrients.

Cuscuta is a parasitic plant that derives nutrients from a living host plant. Spirogyra is a filamentous green alga and an autotroph (producer) capable of photosynthesis. Grass is a plant and also an autotroph.

2. Energy and nutrients flow from one trophic level to another in an ecos

Energy and nutrients flow from one trophic level to another in an ecosystem. Which one of the following statements correctly describes the direction of flow of energy and nutrients in an ecosystem ?

Energy flow is cyclic and nutrient flow is unidirectional.

Energy flow is unidirectional and nutrient flow is cyclic.

Both energy and nutrient flow are unidirectional.

Both energy and nutrient flow are cyclic.

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UPSC CDS-2 – 2023

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In an ecosystem, energy flows in a unidirectional manner, while nutrients follow a cyclic path.

Energy enters an ecosystem primarily from the sun (for producers) and is transferred from one trophic level to the next (producers to consumers, etc.). At each transfer, a significant amount of energy is lost as heat, and this energy cannot be reused by organisms in lower trophic levels. Therefore, energy flow is unidirectional and non-cyclic. Nutrients, such as carbon, nitrogen, phosphorus, etc., are continuously cycled within the ecosystem, moving from the environment to organisms and back to the environment through processes like decomposition.

Biogeochemical cycles describe the pathways of nutrient circulation through the lithosphere, hydrosphere, atmosphere, and biosphere. These cycles ensure that nutrients are available for life to continue.

3. Water goes up through xylem vessels in tall trees. The process in plan

Water goes up through xylem vessels in tall trees. The process in plants and properties of water that help in upward movement are :

Respiration and cohesion

Transpiration pull, cohesion and adhesion

Root pressure, cohesion and adhesion

Transpiration and adhesion

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UPSC CDS-2 – 2023

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Water movement upwards through xylem vessels in tall trees is primarily explained by the cohesion-tension theory, which relies on transpiration pull, cohesion, and adhesion.

Transpiration pull is the force generated by the evaporation of water from the leaves (transpiration). This pull creates tension in the water column within the xylem. Cohesion is the property of water molecules to stick together due to hydrogen bonding, maintaining an unbroken water column. Adhesion is the property of water molecules to stick to the hydrophilic walls of the xylem vessels, preventing the water column from breaking and helping it climb upwards against gravity.

Root pressure is another mechanism that can push water upwards, but its effect is generally limited and insufficient to raise water to the tops of tall trees, especially during the day when transpiration rates are high. Respiration is a metabolic process for energy release and is not directly involved in the upward movement of water.

4. Cell wall is absent in which one of the following organisms ?

Cell wall is absent in which one of the following organisms ?

Bacteria

Diatom

Mushroom

Tapeworm

This question was previously asked in

UPSC CDS-2 – 2023

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A cell wall is a rigid layer found outside the plasma membrane of cells in plants, fungi, bacteria, algae, and some archaea. It provides structural support, protection, and helps maintain cell shape. Animal cells, however, do not have cell walls; they are surrounded only by a flexible cell membrane.

Let’s check the presence of cell walls in the given organisms:
A) Bacteria have cell walls typically made of peptidoglycan.
B) Diatoms are a type of algae belonging to Protista; they have unique cell walls made of silica, called frustules.
C) Mushrooms are fungi; fungal cells have cell walls made primarily of chitin.
D) Tapeworms are multicellular animals (Platyhelminthes). Animal cells characteristically lack cell walls.

The absence of a cell wall in animal cells allows for greater flexibility and motility compared to cells with rigid cell walls. It also facilitates processes like phagocytosis and tissue formation.

5. Which one of the following is the correct combination of organelles an

Which one of the following is the correct combination of organelles and their functions ?

Mitochondria – Respiration; Chloroplast – Photosynthesis; Ribosome – Protein Synthesis; Rough endoplasmic reticulum – Transport of proteins

Mitochondria – Respiration; Chloroplast – Photosynthesis; Ribosome – Transport of proteins; Rough endoplasmic reticulum – Protein synthesis

Mitochondria – Respiration; Chloroplast – Protein synthesis; Ribosome – Photosynthesis; Rough endoplasmic reticulum – Transport of proteins

Mitochondria – Photosynthesis; Chloroplast – Respiration; Ribosome – Protein synthesis; Rough endoplasmic reticulum – Transport of proteins

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UPSC CDS-2 – 2023

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Organelles are specialized structures within a cell that perform specific functions. Let’s examine the functions listed for each organelle:

Mitochondria are often called the “powerhouses” of the cell because they are the primary sites of cellular respiration, the process that generates ATP (energy) by breaking down glucose and other fuel molecules.
Chloroplasts (found in plant cells and algae) are the sites of photosynthesis, where light energy is converted into chemical energy in the form of glucose.
Ribosomes are responsible for protein synthesis, translating messenger RNA (mRNA) into polypeptide chains.
The endoplasmic reticulum (ER) is a network of membranes involved in protein and lipid synthesis. The Rough ER is studded with ribosomes and is particularly involved in synthesizing proteins that are destined for secretion, insertion into membranes, or transport to other organelles. It also plays a role in folding and transporting these proteins.

Based on these well-established functions, the combination in option A correctly matches each organelle with its primary role.

6. Which one of the following statements regarding acids is not correct ?

Which one of the following statements regarding acids is not correct ?

Hydrochloric acid is present in the gastric juice secreted by the stomach.

Acetic acid is the main constituent of vinegar.

Oxalic acid is found in tamarind paste.

Lemon and orange juices contain citric acid and ascorbic acid respectively.

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UPSC CDS-2 – 2023

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The question asks for the statement about acids that is NOT correct.

Let’s evaluate each statement:
A) Hydrochloric acid (HCl) is indeed a major component of gastric juice in the stomach, aiding digestion. (Correct)
B) Acetic acid (CH₃COOH) is the main active component of vinegar, typically present as a dilute solution (around 5-8%). (Correct)
C) Tamarind paste is known for its sour taste, which is primarily due to the presence of tartaric acid. While some other acids might be present in smaller quantities, oxalic acid is not the main acid found in tamarind. Oxalic acid is abundant in foods like spinach, rhubarb, and sorrel. (Incorrect)
D) Lemon and orange juices are rich in citric acid, which gives them their characteristic sourness. They also contain ascorbic acid (Vitamin C). The statement implies both acids are present, which is true. (Correct)

Tartaric acid is a common organic acid found in many fruits, most notably in grapes and tamarinds. It is also a component of cream of tartar.

7. What is the total number of naturally occurring elements ?

What is the total number of naturally occurring elements ?

114

This question was previously asked in

UPSC CDS-2 – 2023

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Naturally occurring elements are those found on Earth. This typically includes elements that exist in substantial quantities as well as those found in trace amounts resulting from natural decay processes.

Elements up to atomic number 92 (Uranium) are generally considered naturally occurring on Earth in macroscopic quantities. Elements with atomic numbers 93 (Neptunium) and 94 (Plutonium) also occur naturally, but only in trace amounts, primarily as products of neutron capture and radioactive decay of Uranium. Elements beyond 94 are predominantly synthetic, created in laboratories or nuclear reactions, although some very heavy elements might exist fleetingly in specific astrophysical events.

Based on the common understanding and options provided, the number of naturally occurring elements including those in trace amounts is considered to be 94 (up to Plutonium). Some definitions might limit it to 92 (up to Uranium) if excluding elements only found from decay chains or in trace quantities. However, 94 is the most appropriate choice among the given options.

8. Which one among the following substances is a compound ?

Which one among the following substances is a compound ?

Sugar

Air

Milk

Tea

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UPSC CDS-2 – 2023

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A compound is a substance formed when two or more different elements are chemically bonded together in a fixed ratio. A mixture is a substance composed of two or more components physically mixed, where each component retains its individual properties and the composition can vary.

Sugar (commonly sucrose, C₁₂H₂₂O₁₁) is made up of Carbon, Hydrogen, and Oxygen atoms chemically combined in a fixed ratio. Therefore, sugar is a compound. Air is a mixture of gases like nitrogen, oxygen, argon, carbon dioxide, etc. Milk is a complex mixture of water, fats, proteins, carbohydrates, minerals, etc. Tea is an aqueous mixture containing compounds extracted from tea leaves.

Elements are pure substances consisting only of atoms that all have the same number of protons in their atomic nuclei. Compounds can be broken down into simpler substances (elements) by chemical means, while mixtures can be separated into their components by physical means.

9. Which one of the following statements is correct about diamond and

Which one of the following statements is correct about diamond and graphite?

Diamond and graphite have similar physical and chemical properties.

Diamond is hard but graphite is smooth and slippery.

Diamond and graphite are both non-conductors of electricity.

Both diamond and graphite have similar structures.

This question was previously asked in

UPSC CDS-2 – 2023

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Diamond and graphite are allotropes of carbon, meaning they are different structural forms of the same element. Their vastly different physical properties stem from their different atomic structures.

In diamond, each carbon atom is bonded to four other carbon atoms in a rigid three-dimensional tetrahedral lattice, making it extremely hard and an electrical insulator. In graphite, carbon atoms are arranged in layers of hexagonal rings, with weak van der Waals forces between the layers. This layered structure allows the layers to slide past each other easily, making graphite soft, smooth, and slippery. Graphite also conducts electricity because of delocalized electrons within its layers.

While their physical properties are very different, their chemical properties (being forms of pure carbon) are similar, though reactivity can differ due to structural factors. For example, both burn in oxygen to form carbon dioxide, but diamond is much less reactive than graphite under normal conditions.

10. In which one of the following cases do both physical and chemical chan

In which one of the following cases do both physical and chemical changes take place?

Burning of candle

Freezing of water

Cooking of food

Rusting of iron

This question was previously asked in

UPSC CDS-2 – 2023

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A physical change involves a change in the form or appearance of a substance but not its chemical composition (e.g., changes of state). A chemical change involves a change in the chemical composition of a substance, resulting in the formation of new substances (e.g., combustion, rusting).

In the burning of a candle, the wax melts (physical change) and turns into liquid. This liquid wax is then drawn up the wick, vaporizes, and reacts with oxygen in the air (combustion), producing carbon dioxide, water vapor, heat, and light (chemical change). Thus, both physical (melting/vaporization of wax) and chemical (combustion of wax vapor) changes occur simultaneously as the candle burns.

Freezing of water is purely a physical change (liquid to solid). Rusting of iron is a chemical change (iron reacts with oxygen and water). Cooking of food involves primarily chemical changes in the food components (e.g., protein denaturation, starch gelatinization), although physical processes like boiling water might be involved as a heating method.