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IISER IAT practice questions

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Question 1Biology

In the fluid mosaic model of the plasma membrane, which component is primarily responsible for the selective permeability barrier to small polar solutes?

  1. Peripheral membrane proteins
  2. The hydrophobic core of the phospholipid bilayer Correct answer.
  3. The hydrophilic phosphate head groups
  4. Oligosaccharides of glycoproteins

Explanation

The interior of the bilayer is hydrophobic because the fatty acid tails face inward, so polar and charged molecules cannot readily diffuse across it. Peripheral proteins and sugar chains sit at the surfaces and do not form the barrier.

Question 2Biology

A cell is placed in a hypertonic solution. Which immediate response is expected in an animal cell lacking a rigid wall?

  1. Net water influx and swelling
  2. No net water movement across the membrane
  3. Net water efflux and shrinkage of the cell Correct answer.
  4. Active pumping of water out of the cell

Explanation

In a hypertonic solution the external solute concentration exceeds that inside the cell, so water leaves by osmosis down its potential gradient. Animal cells lack a wall to resist this loss and therefore shrink.

Question 3Biology

Which enzyme of the citric acid cycle is embedded in the inner mitochondrial membrane rather than dissolved in the matrix?

  1. Aconitase
  2. Citrate synthase
  3. Succinate dehydrogenase Correct answer.
  4. Malate dehydrogenase

Explanation

Succinate dehydrogenase is part of the inner membrane as Complex II of the electron transport chain and channels electrons directly to ubiquinone. The other listed enzymes are soluble matrix enzymes.

Question 4Biology

In the light reactions of photosynthesis, the immediate source of electrons that replace those lost by the reaction centre of photosystem II is:

  1. NADPH
  2. Plastoquinone
  3. Water Correct answer.
  4. ATP synthase

Explanation

Photolysis of water at the oxygen-evolving complex supplies electrons to the oxidised reaction centre of photosystem II, releasing oxygen as a by-product. NADPH and plastoquinone are electron carriers downstream, not the source.

Question 5Biology

A dihybrid cross between two heterozygous individuals (AaBb) yields offspring. Assuming independent assortment and complete dominance, what proportion is expected to show both recessive phenotypes?

  1. 3/16
  2. 1/16 Correct answer.
  3. 9/16
  4. 1/4

Explanation

Each locus independently gives a 1/4 chance of the recessive phenotype from an Aa x Aa cross. Multiplying the independent probabilities gives 1/4 x 1/4 = 1/16 for the double recessive phenotype.

Question 6Biology

In a pedigree, a trait appears in every generation and affects about half of the offspring of an affected parent, with both males and females affected. Which mode of inheritance best fits?

  1. X-linked recessive
  2. Mitochondrial
  3. Autosomal dominant Correct answer.
  4. Autosomal recessive

Explanation

Appearance in every generation and roughly equal transmission to sons and daughters from an affected parent is characteristic of autosomal dominant inheritance. Recessive traits often skip generations, and mitochondrial traits pass only through mothers.

Question 7Biology

Which process directly generates genetic recombination between homologous chromosomes during prophase I of meiosis?

  1. Cytokinesis
  2. Sister chromatid separation
  3. DNA replication in S phase
  4. Crossing over at chiasmata Correct answer.

Explanation

Crossing over exchanges segments between non-sister chromatids of homologous chromosomes at chiasmata, producing new allele combinations. Replication copies DNA but does not recombine loci between homologues.

Question 8Biology

A mutation changes a codon from UAU to UAA in the coding region of an mRNA. What is the most likely consequence for the polypeptide?

  1. No change to the polypeptide sequence
  2. Substitution of one amino acid by another
  3. Premature termination of translation Correct answer.
  4. A frameshift affecting all downstream residues

Explanation

UAA is a stop codon, so the ribosome will terminate translation at that point and release a truncated polypeptide. A single base change that creates a stop is a nonsense mutation.

Question 9Biology

In the lac operon of Escherichia coli, which condition produces the highest rate of transcription of the structural genes?

  1. Lactose present and glucose present
  2. Lactose absent and glucose absent
  3. Lactose present and glucose absent Correct answer.
  4. Lactose absent and glucose present

Explanation

Lactose (via allolactose) removes the repressor from the operator, while low glucose raises cAMP so CAP activates transcription. Both conditions together give maximal expression.

Question 10Biology

Which structural feature distinguishes archaeal membrane lipids from those of bacteria and eukaryotes?

  1. Ether linkages between glycerol and isoprenoid chains Correct answer.
  2. Ester linkages between glycerol and fatty acids
  3. Presence of peptidoglycan in the membrane
  4. A bilayer of phospholipids with cholesterol

Explanation

Archaeal lipids typically use ether bonds linking glycerol to branched isoprenoid chains, which increases chemical and thermal stability. Bacterial and eukaryotic lipids use ester-linked fatty acids.

Question 11Biology

A plant wilted after several days without water is watered and recovers turgor. Which change in the guard cells most directly explains stomatal reopening?

  1. Increased abscisic acid concentration in the leaf
  2. Closure of aquaporins in the guard cell membrane
  3. Efflux of potassium ions with loss of water
  4. Entry of potassium ions followed by osmotic water uptake Correct answer.

Explanation

Water availability allows guard cells to accumulate potassium ions, lowering their water potential so water enters osmotically and turgor pressure opens the stoma. Abscisic acid and potassium efflux promote closure instead.

Question 12Biology

Which tissue system in a dicot stem is responsible for the transport of sucrose from leaves to roots?

  1. Epidermis
  2. Phloem Correct answer.
  3. Xylem
  4. Pith

Explanation

Phloem sieve elements carry photosynthate from source leaves to sink tissues such as roots. Xylem conducts water and dissolved minerals upward from the roots.

Question 13Biology

In the mammalian heart, which structure delays the spread of the electrical impulse from the atria to the ventricles, allowing atrial emptying before ventricular contraction?

  1. Sinoatrial node
  2. Atrioventricular node Correct answer.
  3. Bundle of His
  4. Purkinje fibres

Explanation

The atrioventricular node introduces a delay in conduction so that the atria can complete contraction and fill the ventricles before ventricular systole. The sinoatrial node initiates the impulse and the bundles conduct it rapidly.

Question 14Biology

A person inhales deeply and holds the breath. Which change in blood chemistry most strongly drives the subsequent urge to breathe?

  1. Falling partial pressure of carbon dioxide
  2. Rising partial pressure of oxygen
  3. Rising partial pressure of carbon dioxide Correct answer.
  4. Rising blood pH

Explanation

Carbon dioxide accumulates during breath-holding and is converted to carbonic acid, lowering pH. Central chemoreceptors respond strongly to the resulting rise in CO2 and fall in pH, which drives the urge to breathe.

Question 15Biology

Which feature of the nephron is primarily responsible for creating the medullary osmotic gradient that allows urine concentration?

  1. The glomerulus with its filtration barrier
  2. The collecting duct with its smooth muscle
  3. The proximal convoluted tubule with its microvilli
  4. The loop of Henle with its countercurrent multiplier arrangement Correct answer.

Explanation

The loop of Henle acts as a countercurrent multiplier, with active salt transport from the ascending limb and water permeability of the descending limb establishing a gradient in the medulla. This gradient permits water reabsorption from the collecting duct under ADH.

Question 16Biology

In the sliding filament model of muscle contraction, which event immediately follows calcium binding to troponin?

  1. Release of calcium into the sarcoplasmic reticulum
  2. Hydrolysis of ATP by the myosin head
  3. Shortening of the actin filaments themselves
  4. Displacement of tropomyosin and exposure of myosin-binding sites Correct answer.

Explanation

Calcium binding to troponin causes tropomyosin to shift and uncover the myosin-binding sites on actin, allowing cross-bridge formation. ATP hydrolysis by myosin powers the power stroke but occurs on a different step of the cycle.

Question 17Biology

A population of insects is exposed to an insecticide over many generations, and resistance increases. Which mechanism best explains this observation in evolutionary terms?

  1. Natural selection acting on pre-existing heritable variation Correct answer.
  2. Directed mutation induced by the insecticide
  3. Random genetic drift alone
  4. Acclimatisation of each individual insect

Explanation

Resistant individuals already carrying favourable alleles survive and reproduce more, so allele frequencies shift over generations. The environment selects among existing variation rather than inducing adaptive mutations on demand.

Question 18Biology

In a food web, a snake eats frogs that eat insects that feed on grass. If the grass contains 1,000,000 kJ of energy, roughly how much energy is available to the snake, assuming 10% transfer efficiency per level?

  1. 1,000 kJ Correct answer.
  2. 100,000 kJ
  3. 100 kJ
  4. 10,000 kJ

Explanation

Energy passes grass to insects (100,000 kJ), insects to frogs (10,000 kJ), and frogs to snakes (1,000 kJ), with 10% retained at each transfer. The snake is at the fourth trophic level.

Question 19Biology

Which interaction between two species increases the fitness of both partners and is therefore classified as mutualism?

  1. A tapeworm absorbing nutrients from its host's intestine
  2. Mycorrhizal fungi supplying phosphorus to a plant that supplies sugars Correct answer.
  3. A barnacle attaching to a whale without affecting it
  4. A fox preying on a rabbit

Explanation

In mycorrhizae both organisms benefit: the fungus gains photosynthate and the plant gains mineral nutrients and water. This reciprocal benefit defines mutualism.

Question 20Biology

In an enzyme-catalysed reaction, a competitive inhibitor is added. Which kinetic change is expected?

  1. Both the Michaelis constant and maximum velocity decrease
  2. The maximum velocity decreases while the Michaelis constant is unchanged
  3. The apparent Michaelis constant increases while maximum velocity is unchanged Correct answer.
  4. The maximum velocity increases without limit

Explanation

A competitive inhibitor binds the active site reversibly, so more substrate is needed to reach half-maximal velocity and the apparent Km rises. At saturating substrate the inhibitor is outcompeted, so Vmax is unchanged.

Question 21Biology

Which statement correctly describes the role of the signal recognition particle in eukaryotic protein sorting?

  1. It transports folded proteins through nuclear pores
  2. It cleaves signal peptides after import into mitochondria
  3. It glycosylates proteins within the Golgi apparatus
  4. It binds a signal peptide on a nascent polypeptide and directs the ribosome to the endoplasmic reticulum Correct answer.

Explanation

The signal recognition particle recognises the hydrophobic signal peptide as it emerges from the ribosome and docks the complex at the ER membrane. The polypeptide is then translocated into the ER lumen or membrane.

Question 22Biology

Which feature of the genetic code allows a single tRNA to recognise more than one codon for the same amino acid?

  1. Wobble base pairing at the third codon position Correct answer.
  2. Proofreading by aminoacyl-tRNA synthetase
  3. Splicing of introns from pre-mRNA
  4. The presence of a poly-A tail on mRNA

Explanation

Wobble pairing permits non-standard hydrogen bonding between the third base of the codon and the first base of the anticodon, so one tRNA can decode several synonymous codons. Synthetases ensure correct amino acid attachment but do not explain multi-codon recognition.

Question 23Biology

A bacterial culture is grown in a medium containing lactose as the sole carbon source, then shifted to a medium with both glucose and lactose. What happens to lac operon expression immediately after the shift?

  1. It falls because glucose lowers cAMP and weakens CAP activation Correct answer.
  2. It stops because the repressor becomes permanently bound
  3. It rises because glucose is a gratuitous inducer
  4. It remains maximal because lactose is still present

Explanation

Glucose entry lowers cAMP, so CAP cannot activate the promoter efficiently even though lactose keeps the repressor off the operator. Expression therefore drops, illustrating catabolite repression.

Question 24Biology

Which feature of the mammalian small intestine most directly increases the surface area available for absorption?

  1. Villi and microvilli on the epithelial cells Correct answer.
  2. The circular muscle layer in the wall
  3. The serosa covering the outer surface
  4. The presence of goblet cells secreting mucus

Explanation

Finger-like villi and apical microvilli multiply the absorptive surface many times over. Goblet cells, muscle layers and serosa serve secretion, motility and protection respectively.

Question 25Biology

In a Hardy-Weinberg population, 16% of individuals show the recessive phenotype. What is the frequency of heterozygous carriers?

  1. 0.40
  2. 0.36
  3. 0.24
  4. 0.48 Correct answer.

Explanation

The recessive phenotype frequency q squared equals 0.16, so q = 0.4 and p = 0.6. The heterozygote frequency is 2pq = 2 x 0.6 x 0.4 = 0.48.

Question 26Biology

Which level of protein structure is stabilised mainly by hydrogen bonds between backbone carbonyl and amide groups, giving rise to alpha helices and beta sheets?

  1. Primary structure
  2. Tertiary structure
  3. Secondary structure Correct answer.
  4. Quaternary structure

Explanation

Secondary structure refers to local regular folding patterns such as alpha helices and beta pleated sheets, held together by hydrogen bonds between atoms of the polypeptide backbone rather than side chains. Primary structure is just the amino acid sequence, while tertiary and quaternary levels involve side-chain interactions and multiple subunits.

Question 27Biology

A cell is placed in a hypertonic solution. What is the most likely immediate effect on an animal cell?

  1. It maintains constant volume with no net water movement
  2. It loses water and shrinks Correct answer.
  3. It swells and may lyse
  4. It actively pumps water inward to swell

Explanation

In a hypertonic solution the solute concentration outside exceeds that inside, so water leaves the cell by osmosis and the cell shrinks (crenation). Swelling occurs in hypotonic solutions, and no net movement occurs only when the solution is isotonic.

Question 28Biology

Which organelle is the principal site of ATP synthesis by oxidative phosphorylation in a eukaryotic cell?

  1. Mitochondrion Correct answer.
  2. Lysosome
  3. Golgi apparatus
  4. Smooth endoplasmic reticulum

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. The other listed organelles perform processing, degradation, or lipid synthesis rather than bulk ATP generation.

Question 29Biology

In a dihybrid cross between two individuals heterozygous at both loci (AaBb x AaBb) with independent assortment, what proportion of offspring is expected to show both recessive phenotypes?

  1. 1/16 Correct answer.
  2. 9/16
  3. 1/4
  4. 3/16

Explanation

Each recessive phenotype has probability 1/4 from an Aa x Aa cross. Because the loci assort independently, the joint probability is the product, 1/4 x 1/4 = 1/16.

Question 30Biology

Which blood vessel type has the thinnest walls and is the main site of exchange of gases and nutrients with tissues?

  1. Aorta
  2. Artery
  3. Capillary Correct answer.
  4. Vein

Explanation

Capillaries have walls only one endothelial cell thick, minimising diffusion distance for exchange. Arteries, veins and the aorta have thicker muscular and connective tissue layers suited to transport and pressure handling.

Question 31Biology

Which molecule is the immediate electron acceptor at the end of the mitochondrial electron transport chain, allowing the chain to keep operating?

  1. NAD+
  2. Water
  3. Oxygen Correct answer.
  4. Carbon dioxide

Explanation

Oxygen accepts electrons at complex IV and combines with protons to form water, which keeps electrons flowing down the chain. Without oxygen the chain backs up and NADH cannot be reoxidised.

Question 32Biology

Which plant hormone is chiefly responsible for promoting cell elongation in response to directional light?

  1. Ethylene
  2. Cytokinin
  3. Abscisic acid
  4. Auxin Correct answer.

Explanation

Auxin redistributes to the shaded side of a shoot and promotes elongation there, causing the shoot to bend toward light. Abscisic acid and ethylene generally inhibit growth or promote responses such as ripening and stress closure.

Question 33Biology

Which term describes a relationship in which one organism benefits while the other is unaffected?

  1. Competition
  2. Commensalism Correct answer.
  3. Parasitism
  4. Predation

Explanation

Commensalism benefits one partner without measurably affecting the other, as when a bird nests in a tree. Parasitism and predation harm one partner, while competition reduces fitness of both participants.

Question 34Biology

During DNA replication, which enzyme removes the RNA primer and replaces it with DNA nucleotides?

  1. DNA polymerase I Correct answer.
  2. Primase
  3. DNA ligase
  4. Helicase

Explanation

DNA polymerase I has exonuclease activity that removes RNA primers and polymerase activity that fills the gap with DNA. Ligase then seals the remaining nick, helicase unwinds the duplex and primase synthesises the primer.

Question 35Biology

A woman who is a carrier of an X-linked recessive disorder marries an unaffected man. What is the probability that a son of this couple is affected?

  1. 0
  2. 1
  3. 1/4
  4. 1/2 Correct answer.

Explanation

A son inherits his X chromosome from his mother, and a carrier mother transmits the mutant allele with probability 1/2. All sons therefore have a 1/2 chance of being affected regardless of the father's genotype.

Question 36Biology

In the light-dependent reactions of photosynthesis, what is the direct role of water?

  1. It donates electrons to photosystem II and releases oxygen Correct answer.
  2. It transfers electrons to the cytochrome complex from photosystem I
  3. It fixes carbon dioxide into a three-carbon compound
  4. It accepts electrons at photosystem I to form NADPH

Explanation

Water is split at the oxygen-evolving complex of photosystem II, supplying electrons to replace those lost by P680 and releasing molecular oxygen as a by-product. Carbon fixation occurs later in the Calvin cycle, and NADP+ is the terminal electron acceptor.

Question 37Biology

Which statement best explains why a person with a deficiency of functional ADH (antidiuretic hormone) produces large volumes of dilute urine?

  1. The loop of Henle stops generating a medullary gradient
  2. The collecting duct becomes more permeable to water
  3. Aquaporin insertion into the collecting duct membrane is reduced Correct answer.
  4. Sodium reabsorption in the proximal tubule is increased

Explanation

ADH normally triggers insertion of aquaporin channels into collecting duct membranes, allowing water reabsorption. Without functional ADH, water remains in the duct lumen, so a large volume of dilute urine is excreted.

Question 38Biology

A reaction has a negative change in Gibbs free energy. Which conclusion about the reaction is correct?

  1. It is endergonic and requires continuous energy input
  2. It always proceeds at a very fast rate
  3. It must be catalysed by an enzyme to occur at all
  4. It is exergonic and can proceed spontaneously under the stated conditions Correct answer.

Explanation

A negative delta G means the reaction is exergonic and thermodynamically spontaneous under the given conditions. Spontaneity says nothing about the rate, which depends on activation energy and enzyme presence.

Question 39Biology

In a cross involving incomplete dominance, a red-flowered plant is crossed with a white-flowered plant and all offspring are pink. What phenotypic ratio is expected in the F2 generation when pink plants are crossed with each other?

  1. 9:3:3:1
  2. 1:2:1 red to pink to white Correct answer.
  3. 3:1 red to white
  4. 1:1 red to white

Explanation

In incomplete dominance the heterozygote has an intermediate phenotype, so a cross of two pink heterozygotes gives a 1:2:1 ratio of red, pink and white. The underlying genotypic ratio is also 1:2:1 because each genotype has a distinct phenotype.

Question 40Biology

Which structural adaptation of xerophytic plants most directly reduces water loss through transpiration?

  1. Reduced chlorophyll content in mesophyll
  2. Large, thin leaves with many stomata
  3. Extensive shallow root mats
  4. Sunken stomata in pits lined with hairs Correct answer.

Explanation

Sunken stomata create a humid microclimate in the pit, reducing the water vapour gradient and thus transpiration. Large thin leaves with many stomata would increase water loss rather than restrict it.

Question 41Biology

Which feature of the vertebrate small intestine directly increases the surface area available for absorption of digested nutrients?

  1. Secretion of bile into the duodenum
  2. Presence of gastric glands in the mucosa
  3. Rings of smooth muscle in the muscularis
  4. Villi and microvilli on the epithelial surface Correct answer.

Explanation

Finger-like villi and apical microvilli greatly amplify the absorptive surface of the intestinal epithelium. Gastric glands and bile secretion aid digestion rather than surface amplification, and muscular rings drive peristalsis.

Question 42Biology

A population of bacteria is exposed to a bacteriostatic antibiotic that inhibits protein synthesis. What is the expected short-term effect on the population?

  1. Bacteria are rapidly lysed by disruption of the cell wall
  2. Bacteria immediately develop resistance through directed mutation
  3. Bacterial growth is halted but cells may remain viable Correct answer.
  4. DNA replication is blocked and cells die within minutes

Explanation

Bacteriostatic agents inhibit growth and reproduction, leaving cells viable and able to resume growth when the drug is removed. Lysis is characteristic of bactericidal agents acting on the cell wall, and resistance arises through selection of pre-existing variants rather than directed mutation.

Question 43Biology

In a population at Hardy-Weinberg equilibrium, the frequency of the recessive allele is 0.3. What is the expected frequency of homozygous dominant individuals?

  1. 0.09
  2. 0.21
  3. 0.70
  4. 0.49 Correct answer.

Explanation

If q = 0.3 then p = 0.7, and the homozygous dominant frequency is p squared, which is 0.49. The value 0.09 is q squared and 0.42 would be the heterozygote frequency.

Question 44Biology

Which process produces the genetic recombination observed between linked genes on the same chromosome?

  1. Crossing over during prophase I of meiosis Correct answer.
  2. Separation of sister chromatids in anaphase II
  3. DNA replication during S phase
  4. Independent assortment of non-homologous chromosomes

Explanation

Crossing over between non-sister chromatids of homologous chromosomes exchanges segments and creates new allele combinations, breaking linkage. Independent assortment concerns different chromosome pairs and does not recombine alleles within one chromosome.

Question 45Biology

An enzyme shows a high Km for substrate A and a low Km for substrate B under identical conditions. What can be concluded about the enzyme's affinity for the two substrates?

  1. The enzyme has equal affinity for both substrates
  2. The enzyme has higher affinity for substrate B Correct answer.
  3. The enzyme has higher affinity for substrate A
  4. Affinity cannot be inferred from Km values

Explanation

Km is inversely related to affinity: a lower Km indicates that half-maximal velocity is reached at a lower substrate concentration, reflecting tighter binding. Thus the low Km for B means higher affinity for B.

Question 46Biology

A mutation in the operator region of the lac operon prevents the repressor protein from binding. In the presence of lactose, what is the expected expression pattern?

  1. No expression under any condition
  2. Expression only when glucose is absent
  3. Constitutive expression regardless of lactose presence Correct answer.
  4. Expression only when lactose is present

Explanation

If the repressor cannot bind the operator, negative control is lost and the structural genes are transcribed even without lactose. Positive control by CAP still modulates the level with glucose, but the operon is no longer inducible.

Question 47Biology

Which feature of the genetic code allows a single tRNA molecule to recognise more than one codon specifying the same amino acid?

  1. The code is universal
  2. The code is degenerate Correct answer.
  3. The code is non-overlapping
  4. The code is triplet-based

Explanation

Degeneracy means several codons can encode the same amino acid, and wobble base pairing at the third codon position lets one tRNA read more than one of these synonymous codons. Universality, non-overlap and triplet grouping describe other properties of the code.

Question 48Biology

A protein normally resident in the cytosol is engineered to carry a strong nuclear localisation signal. Where would it most likely accumulate?

  1. At the plasma membrane exterior
  2. In the nucleus Correct answer.
  3. In the lysosomal lumen
  4. In the mitochondrial matrix

Explanation

A nuclear localisation signal is recognised by importin receptors that carry the protein through nuclear pore complexes into the nucleus. Mitochondrial, lysosomal and secretory targeting require different signals such as presequences, mannose-6-phosphate tags or signal peptides.

Question 49Biology

In a food chain where grass fixes 100,000 kJ of energy and each trophic transfer is about 10 percent efficient, how much energy is available to a secondary consumer that eats primary consumers?

  1. 1,000 kJ Correct answer.
  2. 10 kJ
  3. 10,000 kJ
  4. 100 kJ

Explanation

Primary consumers receive about 10 percent of 100,000 kJ, which is 10,000 kJ. Secondary consumers receive about 10 percent of that, giving roughly 1,000 kJ.

Question 50Biology

Two enzymes catalyse the same reaction. Enzyme X has a much lower activation energy than enzyme Y but both reach the same Vmax. Which statement best describes their behaviour at low substrate concentration?

  1. Enzyme Y will produce product faster because its lower Km reflects tighter substrate binding Correct answer.
  2. Enzyme X will produce product faster because it lowers the activation barrier more
  3. Neither will produce product until substrate concentration saturates the enzyme
  4. Both will produce product at identical rates because their Vmax values are equal

Explanation

At low substrate concentration the rate depends strongly on Km, which reflects substrate affinity; a lower Km means more substrate is bound and the reaction proceeds faster toward Vmax. Lowering activation energy affects the catalytic step but does not by itself determine low-substrate velocity, and equal Vmax describes saturation behaviour only.