LEPT Practice Questions: Science Specialization with Answers

Answer: C. The mitochondrion is the site of oxidative phosphorylation, the final stage of cellular respiration that produces the majority of ATP in eukaryotic cells. This process occurs along the inner mitochondrial membrane, where the electron transport chain and ATP synthase work together to generate approximately 34 ATP molecules per glucose molecule. Ribosomes are involved in protein synthesis, the Golgi apparatus in processing and packaging proteins, and the endoplasmic reticulum in protein folding and lipid synthesis.

Answer: B. Using a Punnett square for the cross Aa x Aa, the possible genotypes of the offspring are: AA (25%), Aa (50%), and aa (25%). Therefore, there is a 25% or 1-in-4 chance that the offspring will be homozygous recessive (aa). This is a fundamental principle of Mendelian genetics known as the Law of Segregation, which states that allele pairs separate during gamete formation and randomly unite at fertilization.

Answer: D. According to the 10% rule in ecology, only about 10% of the energy available at one trophic level is transferred to the next. Tertiary consumers, being at the top of the food chain, have the least available energy. For example, if primary producers capture 10,000 kcal of energy, primary consumers receive approximately 1,000 kcal, secondary consumers receive about 100 kcal, and tertiary consumers receive only about 10 kcal. This progressive energy loss explains why food chains rarely exceed four or five trophic levels.

Answer: B. A covalent bond forms when two nonmetal atoms share one or more pairs of electrons to achieve a stable electron configuration. This occurs because nonmetals have similar electronegativities, making electron transfer energetically unfavorable. Ionic bonds form through the transfer of electrons between a metal and a nonmetal. Metallic bonds involve a "sea" of delocalized electrons shared among metal atoms. Hydrogen bonds are weak intermolecular forces, not true chemical bonds between atoms within a molecule.

Answer: B. The balanced equation shows a 2:2 (or 1:1) mole ratio between H₂ and H₂O. This means that for every 2 moles of hydrogen gas consumed, 2 moles of water are produced. If 4 moles of H₂ react with excess oxygen, then 4 moles of H₂O are produced. Stoichiometry problems always require you to use the mole ratios from the balanced equation to convert between reactants and products.

Answer: B. Alcohols are organic compounds characterized by the presence of a hydroxyl group (-OH) bonded to a carbon atom. In ethanol (C₂H₅OH), the -OH group attached to the ethyl group (C₂H₅-) defines it as an alcohol. The carbonyl group is found in aldehydes and ketones, the carboxyl group in carboxylic acids, and the amino group in amines and amino acids.

Answer: B. Using Newton's Second Law of Motion (F = ma), the acceleration can be calculated as a = F/m = 10 N / 5 kg = 2 m/s². Newton's Second Law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This is one of the most fundamental equations in classical mechanics and is frequently tested on the LEPT.

Answer: B. The First Law of Thermodynamics is the law of conservation of energy. It states that the total energy of an isolated system remains constant — energy can be transferred or converted from one form to another, but it cannot be created or destroyed. The Zeroth Law deals with thermal equilibrium, the Second Law addresses entropy (the tendency of systems to move toward disorder), and the Third Law states that entropy approaches zero as temperature approaches absolute zero.

Answer: B. The Philippine archipelago sits at the convergent boundary where the Philippine Sea Plate is being subducted beneath the Eurasian Plate (and along the Philippine Trench, the Eurasian Plate subducts beneath the Philippine Sea Plate). This subduction process generates intense seismic activity and creates the volcanic arc that forms many of the country's islands. The Philippine Fault Zone, a major transform fault running through the archipelago, adds to the seismic hazard. This is why the Philippines experiences frequent earthquakes and has numerous active volcanoes, including Mount Mayon, Taal, and Mount Pinatubo.

Answer: B. Tropical cyclones require warm ocean water (at least 26.5°C) as their primary energy source, combined with atmospheric instability, sufficient Coriolis force (which is why they form at least 5° from the equator), low vertical wind shear, and pre-existing atmospheric disturbance. The warm water causes evaporation, which releases latent heat as the water vapor rises and condenses, fueling the cyclone's convective circulation. The Philippines, located in the western Pacific basin, is the most typhoon-prone country in the world, with an average of 20 tropical cyclones entering the PAR each year.