PPSC Past Papers 2026 Solved

The correct answer is SiO₂. Quartz is a hard, crystalline mineral composed entirely of silicon dioxide (SiO₂). It is the second most abundant mineral in Earth's continental crust, after feldspar.

Quartz and Common Mineral Formulas

• SiO₂ – Silicon Dioxide: Each silicon atom is covalently bonded to four oxygen atoms, forming a tetrahedral structure that links to create a continuous three-dimensional framework. This gives quartz its hardness (7 on the Mohs scale) and resistance to chemical weathering.
• CaCO₃ – Calcium Carbonate: This is the chemical formula of calcite, the main constituent of limestone and marble. It is not quartz.
• Al₂O₃ – Aluminium Oxide: This is corundum, the mineral that forms ruby and sapphire. It is extremely hard but distinct from quartz.
• Fe₂O₃ – Iron(III) Oxide: Commonly known as hematite, an iron ore. It gives a reddish streak and is often responsible for the red color of many rocks.

Thus, the chemical formula of quartz is SiO₂.

The correct answer is Restriction. The process of cutting DNA at specific sequences (often involving recognition sites that may be five nucleotides long, among other lengths) is performed by restriction enzymes in a process called restriction digestion.

Restriction Enzymes and DNA Cutting

• Restriction Process: Restriction enzymes are proteins that recognize specific short DNA sequences (typically 4–8 base pairs long) and cleave the phosphodiester bonds at or near that site. Some recognize pentanucleotide (5-base) sequences, making the question point to the restriction process.
• Biotechnological Importance: This cutting mechanism is the cornerstone of genetic engineering and molecular cloning. It allows scientists to insert genes into plasmids, create recombinant DNA, and perform gene editing.
• Difference from Other Processes: Transcription is the synthesis of RNA from a DNA template. Translation is protein synthesis from mRNA. Replication is the copying of entire DNA molecules. None of these involve cutting DNA at specific short recognition sites.
• Natural Function: In bacteria, restriction enzymes serve as a defense mechanism against invading bacteriophages by cutting foreign DNA. The bacterium protects its own DNA by methylation.

Therefore, the process involving DNA cutting at specific nucleotide sequences (including a five-nucleotide cut) is restriction.

The correct answer is Nucleolus. The nucleolus is a dense, specialized region within the nucleus of eukaryotic cells where ribosomal RNA (rRNA) is synthesized and ribosome subunits are assembled — effectively making it the "factory" of ribosomes.

Ribosome Biogenesis in the Nucleolus

• Function of the Nucleolus: It transcribes ribosomal DNA (rDNA) into rRNA, processes the rRNA, and combines it with ribosomal proteins imported from the cytoplasm. The assembled small and large ribosomal subunits are then exported through nuclear pores into the cytoplasm for final maturation.
• Why Not the Nucleus?: While the nucleolus is located inside the nucleus, the general nucleus as a whole is not the specific site of ribosome production. The ribosome assembly line is localized within the nucleolus.
• Cytoplasm and Endoplasmic Reticulum: Cytoplasm is where ribosomes function during protein synthesis, but not where they are manufactured. The Endoplasmic Reticulum (especially rough ER) is studded with ribosomes, but again, it does not build them.
• Significance: Actively growing cells, such as those in embryos or cancer cells, often have prominent and enlarged nucleoli due to high demand for protein synthesis and thus ribosome production.

Hence, the factory of ribosomes within the cell is the nucleolus.

The correct answer is Solid. Seismic S-waves (Secondary or Shear waves) are transverse waves that shake the ground perpendicular to their direction of travel. They can only propagate through solid materials and are unable to pass through liquids or gases.

Properties of S-Waves

• Shear Wave Mechanism: S-waves move by shearing or displacing material at right angles to the wave path. This requires the medium to have rigidity — a property only solids possess. Liquids and gases lack shear strength and will not transmit these waves.
• Evidence of Earth's Interior: The fact that S-waves cannot pass through the Earth's outer core (which is liquid iron and nickel) provides strong evidence that the outer core is molten. Seismologists observe an "S-wave shadow zone" on the side of the Earth opposite an earthquake.
• Comparison with P-Waves: Primary (P) waves are compressional and can travel through solids, liquids, and gases. S-waves are slower and arrive after P-waves on a seismogram.
• Why Not Other Options?: S-waves cannot travel through gas, liquid, or plasma because these states lack the necessary shear modulus. Only solids support shear stress.

Thus, S-waves in earthquakes can only pass through solid matter.

The correct answer is Biomass. The term biomass refers to any organic matter derived from plants, animals, and other living organisms that can be used as a renewable source of energy or material. It includes wood, crop residues, animal manure, and biodegradable waste.

Understanding Biomass and Its Related Terms

• Biomass – The Raw Material: Biomass is the collective term for recently living organic material. When plants undergo photosynthesis, they store solar energy in chemical form. Biomass can be burned directly for heat or converted into liquid biofuels, biogas, and other products.
• Biogas – A Product of Biomass: Biogas is a specific fuel (mainly methane and carbon dioxide) produced by the anaerobic digestion of biomass. It is not the broad category of waste and vegetation itself, but one of many derivatives.
• Compost – Processed Biomass: Compost is the nutrient-rich humus created by aerobic decomposition of organic waste. It is a product of managing biomass, not the overarching term for the raw material.
• Fossil Fuel – Ancient Biomass: Fossil fuels like coal, oil, and natural gas are also derived from ancient biomass that has been subjected to intense heat and pressure over millions of years. However, the phrase "plants, vegetation and waste from living things" refers to contemporary, renewable organic matter — i.e., biomass.

Thus, plants, vegetation, and waste from living things are collectively called biomass.

The correct answer is Hydrogen Bond. The attractive force that holds one water molecule to another is the hydrogen bond. This intermolecular bond is responsible for water's unique properties, including high surface tension, boiling point, and its ability to dissolve many substances.

Hydrogen Bonding in Water

• How It Forms: In a water molecule (H₂O), the oxygen atom is highly electronegative, pulling electrons away from the hydrogen atoms. This creates a partial negative charge on the oxygen and a partial positive charge on each hydrogen. The positively charged hydrogen of one molecule is attracted to the lone pair electrons of the oxygen on a neighboring molecule — this attraction is the hydrogen bond.
• Intermolecular, Not Intramolecular: The bonds within a single water molecule (O–H) are polar covalent bonds, where electrons are shared. The question asks about bonding between molecules, which is the hydrogen bond.
• Why Not Other Bonds?: Ionic bonds involve complete transfer of electrons between metal and non-metal atoms, not between neutral water molecules. Metallic bonds occur among metal atoms. The only accurate description of intermolecular bonding in water is hydrogen bonding.
• Significance of Hydrogen Bonds: These relatively weak bonds are collectively strong enough to give water a high specific heat, capillary action in plants, and ice's lower density than liquid water — all essential for life.

Therefore, the bonding between water molecules is a hydrogen bond.

The correct answer is Stroma. The stroma is the dense, colorless fluid that fills the inner space of chloroplasts, surrounding the thylakoid membranes. It is within this semi‑aqueous matrix that the Calvin cycle takes place—the set of light‑independent reactions that fix carbon dioxide and produce sugar.

Why the Stroma, Not Other Parts?

• Site of the Calvin Cycle: The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) captures CO₂ in the stroma. Through a series of reactions, carbon is reduced and ultimately converted into glucose and other carbohydrates. This is the true "sugar factory" of the chloroplast.
• Lumen – Proton Reservoir: The thylakoid lumen is the space inside the thylakoid sacs. Its primary role is to accumulate protons during the light reactions to generate a gradient for ATP synthesis, not carbon fixation.
• Thylakoid – Light Reactions: The thylakoid membrane houses the photosystems and electron transport chains that convert light energy into ATP and NADPH. These energy carriers are then consumed in the stroma during the Calvin cycle.
• Outer Membrane – Barrier Only: The outer membrane of the chloroplast is a porous envelope that allows passive transport of small molecules. It plays no direct role in photosynthesis or sugar manufacture.

Thus, CO₂ is fixed and sugar is manufactured in the stroma of the chloroplast.

The correct answer is Carbon Neutral. Ethanol produced from biomass is widely described as a carbon‑neutral fuel because the carbon dioxide it releases during combustion is roughly equivalent to the CO₂ absorbed by the plants during their growth.

Understanding Carbon Neutrality of Ethanol

• The Closed Carbon Cycle: Plants like corn, sugarcane, and switchgrass fix atmospheric CO₂ through photosynthesis. When ethanol derived from these plants is burned, the same CO₂ is returned to the atmosphere. In theory, there is no net addition of greenhouse gases over the entire life cycle, making it "carbon neutral."
• Carbon Positive vs. Negative: A carbon positive fuel adds more CO₂ to the atmosphere than it removes (e.g., most fossil fuels). A carbon negative fuel removes more CO₂ than it emits, which is possible with certain advanced biofuels combined with carbon capture. Standard corn ethanol is generally considered carbon neutral, although some life‑cycle analyses argue it has a small positive footprint.
• Renewable Fuel – Broader Category: Ethanol is indeed a renewable fuel because it is made from plants. However, the term most specifically used to describe its carbon balance relative to the atmosphere is "carbon neutral." The question asks for the specific name, not the general category.
• Real‑World Considerations: While often labeled carbon neutral, the full life‑cycle analysis (including fertilizers, land‑use change, and processing energy) can shift the balance. Nonetheless, in standard environmental terminology, bioethanol is called a carbon‑neutral fuel.

Therefore, ethanol as a biofuel is named carbon neutral.

The correct answer is Earth. Among all the planets in our Solar System, Earth is the densest, with an average density of approximately 5.51 grams per cubic centimeter. This is significantly higher than any of the other terrestrial or giant planets.

Why Earth is the Densest Planet

• Massive Iron Core: Earth's high density is primarily due to its large, heavy metallic core, composed mainly of iron and nickel. The immense pressure from the overlying layers compresses the core to a density greater than that of pure iron at the surface.
• Comparison with Mercury: Mercury is the second densest planet (about 5.43 g/cm³). Despite having a proportionally huge iron core, its much smaller size means gravitational compression is weaker, keeping its overall density slightly below Earth's.
• Venus and Mars: Venus has a similar size and composition to Earth, but its internal structure yields a density of roughly 5.24 g/cm³. Mars has a thinner, less compressed mantle and a smaller core, resulting in a density of only about 3.93 g/cm³.
• Giant Planets: The gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune) are composed predominantly of lighter elements like hydrogen and helium, making their densities far lower (e.g., Saturn could theoretically float in water).

Hence, Earth is the planet with the highest average density.

Sucrose is a disaccharide formed by the chemical combination of glucose and fructose.

Composition of Common Sugars

• Glucose + Fructose (Correct Answer): This combination forms sucrose, the common table sugar found in sugarcane, sugar beet, and many fruits.
• Glucose + Glucose: This combination forms maltose, commonly known as malt sugar.
• Glucose + Galactose: This combination forms lactose, the principal sugar found in milk.

The term for two or more populations of different species living and interacting in the same area is a Community.

Ecological Levels of Organization

• Community (The Correct Answer): This level focuses specifically on the biotic interactions between different species inhabiting the same geographic space.
• Ecosystem: This includes the biological community plus the non-living (abiotic) factors like climate, water, and soil that the organisms interact with.
• Biome: This is a large naturally occurring community of flora and fauna occupying a major habitat, such as a desert or tropical rainforest.
• Biosphere: This represents the highest level of organization, encompassing all ecosystems on Earth where life exists.

The correct answer is Anthracite. Among the four main types of coal, anthracite represents the highest rank and the purest form, distinguished by its extremely high carbon content, brilliant black lustre, and superior energy efficiency.

Understanding Coal Rank and Purity

• Anthracite – The Purest Form: Anthracite contains between 86% and 97% carbon. It has the lowest moisture content and very few volatile impurities compared to other coals. This makes it hard, slow-burning, and the cleanest burning coal, producing little smoke and a high heat output. It is primarily used in industrial furnaces and for domestic heating.
• Bituminous Coal (Medium-High Rank): This is the most abundant type of coal used for electricity generation and steelmaking. It has a carbon content of 45%–86% and a higher sulfur and moisture content than anthracite, making it less pure. It burns with a smoky flame.
• Lignite (Low Rank): Also known as brown coal, lignite has a carbon content of only 25%–35%. It is crumbly, contains high levels of moisture and volatile matter, and produces a lower amount of energy while emitting more pollutants. It is often used in nearby power plants because it is uneconomical to transport.
• Peat – Not Yet Coal: Peat is technically a precursor to coal, formed from partially decayed vegetation in waterlogged bogs. It has the lowest carbon content (below 60% when dry) and must be dried out for a long time before it can be burned. It is not considered true coal in the strictest geological sense.

Thus, the purest form of coal with the highest carbon content is anthracite.

The correct answer is 33%. According to scientific consensus and forestry policy, a minimum of 33% of a country's total land area should be covered by forests to maintain a healthy ecological balance, ensure biodiversity, and sustainably regulate the climate.

The 33% Rule for Ecological Stability

• Origin of the Standard: The target of 33% forest cover is widely referenced in international forestry guidelines, including the National Forest Policy of India (1952) and subsequent recommendations by the United Nations Food and Agriculture Organization (FAO). It aims to balance land use for agriculture, habitation, and ecological services.
• Ecological Functions: Forests act as carbon sinks, absorbing carbon dioxide and mitigating climate change. They regulate the water cycle, prevent soil erosion, maintain watersheds, and provide habitat for 80% of terrestrial species. A minimum threshold of one-third land cover is deemed necessary to keep these systems functional.
• Global and Regional Context: Many countries strive to meet this benchmark. For example, Pakistan's forest cover is estimated to be around 5% (far below the target), which has prompted massive reforestation drives like the Billion Tree Tsunami. In contrast, nations like Bhutan and Finland have forest cover well above the 33% threshold.
• Incorrect Percentages Explained: 20% and 25% are often cited as minimums for specific ecological zones or for combating desertification, but the globally recognized standard for overall ecological system maintenance is 33%. 30% is close but not the authoritative benchmark.

Therefore, the recommended percentage of land that should remain under forest cover to sustain a healthy ecological system is 33%.

The correct answer is AUG. In the genetic code, the codon AUG serves as the primary start signal for protein synthesis. It is the first codon of a messenger RNA (mRNA) transcript that is translated by a ribosome.

Understanding Start and Stop Codons

• Role of AUG: AUG has a dual function. It codes for the amino acid methionine in eukaryotes (or formylmethionine in prokaryotes) and simultaneously signals the beginning of the protein-coding sequence. The ribosome scans the mRNA until it finds this initiation signal.
• Stop Codons – The Opposite Signal: The other three options — UAA, UAG, and UGA — are stop codons. They do not code for any amino acid. Instead, they signal the termination of translation, telling the ribosome to release the newly made polypeptide chain.
• Importance of the Start Codon: Without a proper start codon, the ribosome cannot correctly assemble the translation machinery, and protein synthesis will not initiate. Mutations affecting the AUG sequence can lead to severe genetic disorders.
• Exceptions and Variations: While AUG is the universal start codon, a few organisms use alternative start codons like GUG or UUG on rare occasions. However, in standard examinations and general biology, AUG is the unambiguous primary start codon.

Thus, the primary start codon in protein synthesis is AUG.

The correct answer is Solar Wind. The solar wind is a continuous, high-speed stream of charged particles — primarily electrons and protons — that are ejected from the Sun's outermost atmosphere, the corona, and travel through interplanetary space.

Solar Wind vs. Other Solar Phenomena

• What is Solar Wind?: Composed mostly of plasma, the solar wind extends far beyond the planets, forming the heliosphere. It travels at speeds between 300 and 800 km/s and is responsible for phenomena like the auroras (Northern and Southern Lights) when it interacts with Earth's magnetic field.
• Distinction from Solar Flares: Solar flares are sudden, intense bursts of radiation and energy from the Sun's surface, not a continuous flow. They are explosive events, whereas the solar wind is a steady stream. Flares can accelerate particles but are not the same as the persistent solar wind.
• Cosmic Wind vs. Radiation: Cosmic wind is not a standard scientific term; the proper terminology is solar wind. General radiation encompasses many forms (electromagnetic, thermal, etc.), but the question specifically points to charged particles, which defines the solar wind.
• Impact on Earth and Technology: The solar wind can disturb satellite electronics, disrupt communications, and even affect power grids during geomagnetic storms. Space weather monitoring is crucial for predicting its effects.

Therefore, the charged particles coming from the sun are called solar wind.

The correct answer is Polypeptide. Chemically, the vast majority of enzymes are proteins, and proteins are polymers made of amino acids linked by peptide bonds. Thus, enzymes are fundamentally polypeptide chains folded into specific three-dimensional structures.

Enzymes as Biological Catalysts

• Polypeptide (Protein) Nature: Each enzyme's unique sequence of amino acids (a polypeptide chain) determines its active site shape, which binds to a specific substrate. This proteinaceous nature allows the immense diversity and specificity required for millions of biochemical reactions in living organisms.
• Exception – Ribozymes: A small number of catalytic RNA molecules, called ribozymes, exist (e.g., in ribosomes). However, in standard biochemical context, when asked about the "chemical nature" of enzymes, the answer is unequivocally proteins or polypeptides, as they dominate all metabolic functions.
• Why Not Other Biomolecules?: Carbohydrates are sugars used for energy and structure. Lipids are fats used for membranes and energy storage. Nucleic acids (DNA, RNA) store and transmit genetic information. None of these serve as enzymes in the broad sense, with the exception of the rare ribozyme, which is not the main answer.
• Enzyme Function: Enzymes lower the activation energy of reactions without being consumed. This function is entirely dependent on their polypeptide chain folding, which can be denatured by heat or pH changes — a classic property of proteins.

Hence, the chemical nature of enzymes is polypeptide (protein).