CLEP Biology — Mock Exam

Answer Key

1. A) Transcription. The enzyme that builds RNA from nucleotides is RNA polymerase, so blocking it halts transcription directly. Distractors: B: translation uses existing mRNA and would slow only later, as a downstream effect, not first. C: DNA replication adds DNA nucleotides via DNA polymerase, a different enzyme. D: glycolysis is metabolic, not nucleic-acid synthesis. E: active transport is a membrane process unrelated to RNA. Fix: match the blocked enzyme's substrate (RNA nucleotides) to the process it drives, then pick the process it acts on directly. [Apply]

2. E) Rough endoplasmic reticulum. Secreted enzymes are proteins made by ribosomes on the rough ER, so a secretory cell is rich in rough ER. Distractors: A: the central vacuole stores materials in plant cells and does not make protein. B: peroxisomes break down fatty acids and detoxify, not export enzymes. C: lysosomes digest material inside the cell, not for secretion. D: the nucleolus makes ribosome subunits but is not the site of protein synthesis or packaging. Fix: for exported proteins, trace the secretory pathway and pick its starting organelle (rough ER). [Apply]

3. D) Water leaves the cell and the membrane pulls away from the wall. A hypertonic solution draws water out osmotically; the shrinking membrane pulls away from the wall (plasmolysis). Distractors: A and B describe a hypotonic surrounding, the opposite condition. C assumes the cell responds by pumping solutes in, but osmosis of water, not solute transport, dominates here. E ignores the concentration gradient that must move water. Fix: compare external vs. internal solute concentration first, then move water toward the higher-solute (hypertonic) side. [Analyze]

4. E) The enzyme's tertiary structure was denatured. High temperature disrupts the bonds holding the enzyme's folded shape, denaturing it so the active site no longer functions. Distractors: A: substrate depletion would not depend on temperature this way. B: temperature change does not alter pH here. C: lower activation energy would speed the reaction, not stop it. D: no inhibitor is present in the scenario. Fix: when heat destroys activity, attribute it to denaturation of protein structure, not to substrate or pH. [Analyze]

5. B) At the inner mitochondrial membrane during oxidative phosphorylation. Oxidative phosphorylation on the inner mitochondrial membrane (electron transport chain plus chemiosmosis) yields most of the ATP. Distractors: A: glycolysis yields only a small net ATP. C: the Krebs cycle mainly produces electron carriers, not the bulk of ATP. D: fermentation is anaerobic and low-yield. E: the nucleus does not make ATP. Fix: assign the ATP majority to the electron transport chain/chemiosmosis at the inner membrane. [Understand]

6. D) ATP and NADPH. The light reactions supply the Calvin cycle with ATP and NADPH, which power carbon fixation; without them the cycle stops. Distractors: A: oxygen and water are not consumed by the Calvin cycle. B: carbon dioxide enters from the air and glucose is an output, not an input. C: NADP+ is the empty carrier returned to the light reactions, not delivered to the cycle. E: pigments capture light but are not passed to the cycle. Fix: the light reactions hand the Calvin cycle energy (ATP) and reducing power (NADPH). [Apply]

7. B) 5'-GCAT-3'. Pairing A-T, T-A, G-C, C-G gives the antiparallel complement 3'-TACG-5', which written 5' to 3' is 5'-GCAT-3'. Distractors: A: 5'-TACG-3' is the complement written without flipping direction. C: identical sequence ignores complementarity. D: reverses the wrong way. E: uracil belongs to RNA, not DNA. Fix: complement each base, then reverse the order to write it 5' to 3'. [Apply]

8. D) The genetic code is redundant, so different codons can specify the same amino acid. Because the code is degenerate, a base change can produce a synonymous codon coding the same amino acid (a silent mutation). Distractors: A: an intron change is possible but not required and is not the general reason a coding change is silent. B: repair would restore the original base, but the base is still altered here. C: ribosomes do not skip codons at will. E: tRNA reads codons, it does not proofread the DNA. Fix: a base change with no amino-acid change points to codon redundancy. [Analyze]

9. B) Meiosis includes synapsis and crossing over between homologous chromosomes. Only meiosis pairs homologs (synapsis) and exchanges segments (crossing over), generating genetic variation. Distractors: A describes mitosis, not meiosis. C describes mitosis; meiosis has one replication but two divisions. D is false: meiosis is limited to gamete/spore formation. E is false: meiosis halves the chromosome number. Fix: the meiosis-only fingerprints are synapsis, crossing over, and reduction to haploid. [Analyze]

10. D) Uncontrolled, continuous cell division. Ignoring stop signals removes the brakes on the cycle, producing continuous, uncontrolled division. Distractors: A: cancer cells often increase, not reduce, glucose use. B: tumor cells typically evade apoptosis rather than increase it. C: G0 arrest is the opposite of what unchecked cells do. E: they replicate DNA readily; that is why they keep dividing. Fix: no stop signal means the cycle runs continuously, so division becomes uncontrolled. [Analyze]

11. B) The bond linking the terminal phosphate group. Hydrolysis of ATP removes the terminal (third) phosphate, releasing usable energy. Distractors: A: base hydrogen bonds hold DNA strands, not ATP phosphates. C: peptide bonds join amino acids in proteins. D: glycosidic bonds link sugars. E: the ribose-adenine bond is not the one cleaved for energy release. Fix: ATP energy comes from cleaving its terminal phosphate bond. [Understand]

12. B) A nucleotide. AMP is a nucleotide, made of adenine, ribose, and one phosphate. Distractors: A: amino acids are protein monomers. C: monosaccharides are simple sugars. D and E are lipids. Fix: a base-sugar-phosphate unit is a nucleotide. [Remember]

13. D) Stabilize membrane fluidity across a range of temperatures. Cholesterol buffers fluidity, keeping the membrane from becoming too fluid when warm or too rigid when cold. Distractors: A: transport is handled by protein channels and pumps. B: receptors are membrane proteins, not cholesterol. C: cholesterol is not a fuel source here. E: catalysis is the job of enzymes. Fix: cholesterol is the membrane's fluidity stabilizer. [Understand]

14. E) A competitive inhibitor. Binding the active site and being overcome by excess substrate are hallmarks of competitive inhibition. Distractors: A: an activator increases activity, not blocks it. B: a noncompetitive inhibitor binds elsewhere and is not reversed by more substrate. C: denaturation would permanently destroy activity. D: a coenzyme assists catalysis rather than blocking it. Fix: active-site binding reversible by excess substrate equals competitive inhibition. [Analyze]

15. E) They inject their nucleic acid into a host bacterium to reproduce. A phage injects its nucleic acid into a bacterium and uses the host's machinery to make new viruses. Distractors: A: a virus carries only one type of nucleic acid, DNA or RNA. B: viruses lack metabolism and make no ATP. C: binary fission is how bacteria, not viruses, divide. D: viruses have no ribosomes and rely on the host's. Fix: viruses are acellular and hijack a host cell to replicate. [Understand]

16. D) Inactivates the repressor so it releases the operator. The inducer binds the repressor and changes its shape so it lets go of the operator, allowing transcription. Distractors: A: the inducer acts on the repressor, not directly on the promoter. B: gene copy number does not change. C: this operon controls transcription, not DNA replication. E: methylation is not the switching mechanism of the lac operon. Fix: induction works by knocking the repressor off the operator. [Analyze]

17. A) Ribosomes, chloroplasts, and mitochondria. A photosynthetic plant cell has ribosomes, chloroplasts, and mitochondria all at once. Distractors: B: a leaf cell has far more than ribosomes. C: leaf cells still need mitochondria for respiration even though they photosynthesize. D: contractile vacuoles are found in some protists, not typical leaf cells. E: plant walls are cellulose, not chitin. Fix: photosynthetic plant cells still respire, so they contain both chloroplasts and mitochondria plus ribosomes. [Understand]

18. A) Removal of a water molecule as the bond forms. Dehydration synthesis links monomers by removing water as the covalent bond forms. Distractors: B describes hydrolysis, the reverse reaction. C: no carbon dioxide is released when sugars are joined. D: no oxygen is consumed. E: a peptide bond joins amino acids, not sugars. Fix: building polymers removes water; breaking them adds water. [Apply]

19. C) Lactic acid (lactate). In oxygen-poor human muscle, pyruvate is reduced to lactate, regenerating NAD+ so glycolysis can continue. Distractors: A: ethanol fermentation occurs in yeast, not human muscle. B: carbon dioxide release is part of aerobic respiration, not this step. D: acetyl-CoA forms only when oxygen is available. E: citric acid forms in the aerobic Krebs cycle. Fix: human anaerobic muscle uses lactic acid fermentation; yeast uses alcohol fermentation. [Apply]

20. C) Transpiration pulling water through the xylem. Transpiration from leaf surfaces creates the tension that pulls a continuous water column up the xylem, so closing stomata slows the flow. Distractors: A: phloem carries sugars, not the main water column, and does not actively pump water up. B: root pressure is minor and cannot account for tall trees or the drought effect. D: osmosis into stomata is not the driving force. E: capillary action alone is too weak and occurs in xylem, not phloem. Fix: the transpiration-cohesion pull in the xylem drives upward water transport. [Analyze]

21. A) The small intestine. The small intestine's villi provide the surface for most nutrient absorption. Distractors: B: the stomach mainly begins protein digestion. C: the large intestine chiefly absorbs water and salts. D: the esophagus only transports food. E: the liver processes absorbed nutrients but does not absorb them from the gut. Fix: nutrient absorption is the small intestine's primary job. [Remember]

22. B) Right ventricle. The right ventricle pumps oxygen-poor blood to the lungs via the pulmonary artery. Distractors: A: the left ventricle pumps oxygenated blood to the body. C: the left atrium receives blood from the lungs, it does not pump to them. D: the right atrium receives blood from the body and passes it to the right ventricle. E: the aorta is a vessel, not a chamber. Fix: right side of the heart handles blood going to the lungs; left side handles blood going to the body. [Analyze]

23. D) The impulse jumping between the nodes of Ranvier. Saltatory conduction lets the action potential jump node to node, greatly increasing speed. Distractors: A: myelin insulates and prevents ion flow along its length, so no continuous signal passes through it. B: Schwann cells make myelin but do not transport the impulse. C: gap junctions occur at electrical synapses, not along myelinated axons. E: neurotransmitter acts at the synapse, not along the axon. Fix: myelin speeds conduction by forcing the impulse to leap between nodes of Ranvier. [Analyze]

24. C) Negative feedback. The response (insulin) counteracts the change (high glucose) to restore the set point, defining negative feedback. Distractors: A: positive feedback amplifies a change rather than reversing it. B: open circulation describes an insect body plan, not this regulation. D: a reflex arc is a rapid neural response, not this hormonal loop. E: countercurrent exchange is a fluid-flow arrangement, not blood-sugar control. Fix: a correction that opposes the original change is negative feedback. [Apply]

25. A) Blastula. Cleavage produces the blastula, a hollow ball of cells. Distractors: B: the gastrula forms later, when germ layers develop. C: the zygote is the single fertilized cell before cleavage. D: a larva is a later free-living juvenile stage. E: gametes are sex cells that fuse to make the zygote. Fix: order of stages is zygote, then blastula (hollow ball), then gastrula. [Remember]

26. E) A pollen tube delivering sperm nuclei to the ovule. After pollination, a pollen tube grows and delivers sperm nuclei to the egg in the ovule, achieving fertilization. Distractors: A: pollen must reach a stigma, not a leaf. B: fertilization joins sperm and egg, not two pollen grains. C: meiosis produces spores earlier; the mature seed does not undergo it. D: pollen is delivered to the stigma, not the root. Fix: fertilization needs a pollen tube carrying sperm to the ovule. [Understand]

27. A) Auxin. Auxin accumulates on the shaded side, promoting cell elongation there so the stem bends toward the light (phototropism). Distractors: B: ethylene mainly promotes fruit ripening and leaf drop. C and E are animal hormones with no such role in plants. D: abscisic acid promotes dormancy and stomatal closing, not bending. Fix: directional plant growth toward light is driven by uneven auxin. [Apply]

28. B) Reptilia. Snakes are reptiles (class Reptilia), with dry scaly skin and amniotic eggs. Distractors: A: amphibians have moist skin and aquatic larvae. C: mammals have hair and mammary glands. D: Aves are birds. E: Osteichthyes are bony fish. Fix: scaly-skinned, amniotic-egg tetrapods like snakes are reptiles. [Remember]

29. A) Regulation of water balance and removal of nitrogenous waste. The kidney regulates water and salt balance and excretes nitrogenous waste such as urea. Distractors: B: bile is made by the liver. C: oxygenation is the lung's role. D: glycogen storage is chiefly a liver and muscle function. E: digestive enzyme secretion is done by the pancreas and gut glands. Fix: kidney damage first hits water balance and nitrogenous-waste removal. [Apply]

30. B) Memory lymphocytes produced during the first exposure. The first exposure generates memory lymphocytes that mount a rapid, strong secondary response. Distractors: A: red blood cells carry oxygen and do not fight specific pathogens. C: stomach acid is a nonspecific barrier, not the memory of a specific pathogen. D: clotting stops bleeding, not infection. E: skin is a general barrier and does not explain the faster specific response. Fix: the faster, stronger second response comes from immunological memory cells. [Analyze]

31. A) The alveoli. Alveoli are the thin-walled air sacs where gas exchange with capillaries occurs, so destroying them reduces exchange. Distractors: B and C are conducting airways that carry air but do not exchange gases. D: the pleura is a lining membrane around the lungs. E: the diaphragm is a muscle that drives ventilation but is not the exchange surface. Fix: gas exchange happens across the alveoli, not the conducting airways. [Analyze]

32. D) Myosin heads pull actin filaments toward the center of the sarcomere. Myosin heads bind and pull actin toward the sarcomere's center, so the filaments slide past one another and the sarcomere shortens. Distractors: A: the filaments do not shorten; they overlap more. B: filaments are not rebuilt during a contraction. C: bone does not contract; muscle moves bone. E: calcium influx triggers contraction, and its removal causes relaxation. Fix: contraction is filaments sliding, driven by myosin pulling actin inward. [Analyze]

33. A) Epithelial tissue — covering and lining body surfaces. Epithelial tissue covers and lines surfaces and cavities. Distractors: B: fat storage is a connective (adipose) function, not nervous. C: muscle produces movement, not genetic transmission. D: conducting impulses is the nervous tissue role, not connective. E: pumping blood is a muscle (cardiac) function, not epithelial. Fix: match each tissue to its signature role: epithelium covers, connective supports, muscle moves, nerve signals. [Understand]

34. C) An ectotherm relying on external heat sources. Relying on the environment (sun and shade) for heat is the defining behavior of an ectotherm. Distractors: A: an endotherm generates its own heat metabolically and need not bask. B: homeotherms hold a nearly constant temperature; a basking lizard's temperature varies. D: warm-blooded is a synonym for endothermic, which the lizard is not. E: the lizard clearly does regulate temperature, just behaviorally. Fix: using external heat to set body temperature marks an ectotherm. [Apply]

35. D) Phloem. Phloem transports sugars from leaves to roots, so severing it starves the roots. Distractors: A: xylem carries water and minerals upward and would not cut off sugar to roots. B: the epidermis is a protective surface layer. C: cork cambium produces bark tissue, not sugar transport. E: pith is central storage tissue, not the sugar conduit. Fix: sugar transport to roots is the phloem's job. [Apply]

36. B) A fixed action pattern. An unchangeable stereotyped sequence that runs to completion once triggered is a fixed action pattern. Distractors: A: trial-and-error is learned and variable, not stereotyped. C: classical conditioning pairs stimuli to create a learned response. D: insight learning solves a novel problem, unlike this rigid routine. E: habituation is a decline in response to repeated stimulation. Fix: an innate motor sequence that completes even without the stimulus is a fixed action pattern. [Apply]

37. E) An open circulatory system in which hemolymph bathes the tissues directly. In an open system the heart pumps hemolymph into body cavities where it bathes tissues directly, so a dye spreads freely. Distractors: A describes a closed system, the opposite result. B: insects lack a full capillary network confining blood. C: many insects transport gases via tracheae, not hemoglobin-packed red cells. D: insect gas exchange occurs through tracheae, not across the heart. Fix: fluid that leaves vessels and bathes tissues indicates an open circulatory system. [Apply]

38. B) Express different subsets of the same genes. Differentiated cells share the same genome but turn on different gene subsets, giving them different structures and roles. Distractors: A: cells generally carry the same genes, not different ones. C: meiosis makes gametes and is not the basis of general differentiation. D: body cells retain their full DNA; they do not discard it. E: cells do not pick up chromosomes from the environment. Fix: differentiation is differential gene expression, not different genes. [Analyze]

39. C) Secreting enzymes and absorbing the digested nutrients. Fungi digest food externally by secreting enzymes and then absorb the products (absorptive heterotrophy). Distractors: A: fungi lack chlorophyll and do not photosynthesize. B: fungi have no gut; digestion is external. D: chemosynthesis is used by certain bacteria, not fungi. E: stinging cells belong to cnidarians. Fix: fungi feed by external digestion and absorption. [Apply]

40. E) 0.04. Homozygous recessive frequency is q² = (0.2)² = 0.04. Distractors: A: 0.2 is the allele frequency q, not the genotype frequency. B: 0.8 is p, the dominant allele frequency. C: 0.16 is 2pq halved or a miscalculation, not q². D: 0.32 is 2pq (the heterozygote frequency), not the homozygous recessive. Fix: homozygous recessive equals q squared under Hardy-Weinberg. [Apply]

41. C) Resistant individuals survived and reproduced, raising the resistance-allele frequency. Pre-existing resistant individuals survive the spraying and pass on resistance alleles, so resistance rises by natural selection. Distractors: A: individuals do not acquire and inherit toughness during life (Lamarckian error). B: the insecticide selects among existing variation; it does not direct mutations to occur. D: insecticide resistance is a physiological trait, not a taught behavior. E: the consistent selective pressure of spraying, not random drift, drives this directional change. Fix: attribute adaptation to selection acting on existing heritable variation. [Analyze]

42. D) Allopatric speciation. A physical barrier splits the population and the isolated groups diverge into separate species, defining allopatric speciation. Distractors: A: sympatric speciation occurs without geographic separation. B: convergent evolution makes unrelated species resemble each other, the opposite scenario. C: artificial selection involves human-directed breeding. E: genetic equilibrium means no evolutionary change, contradicting the speciation described. Fix: geographic barrier plus divergence equals allopatric speciation. [Apply]

43. D) 10%. About 10% of energy passes to the next trophic level, with the rest lost mainly as heat and through metabolism. Distractors: A: 1% is too low for the standard rule. B and C overstate the transfer efficiency. E: 100% would violate energy loss between levels. Fix: use the roughly 10% rule for energy transfer up a food chain. [Remember]

44. B) Exponential growth. With unlimited resources the growth rate accelerates, producing exponential (J-shaped) growth. Distractors: A: logistic growth levels off at carrying capacity, which requires limited resources. C: zero growth means no net change, contradicting rapid increase. D: a stable equilibrium is unchanging, not accelerating. E: a density-dependent decline would reduce the population, not grow it. Fix: unlimited resources yield exponential growth; limited resources yield logistic. [Apply]

45. C) The growth rate slows as resources become limiting. Near carrying capacity, limited resources and rising competition slow the growth rate. Distractors: A: birth rates fall, not rise without limit, as crowding increases. B: per-individual resources decrease as numbers climb. D: resource limitation is exactly what constrains the population near capacity. E: no such immigration pattern is implied by nearing carrying capacity. Fix: at carrying capacity, resource limits brake the growth rate toward zero. [Analyze]

46. C) Parasitism. One organism benefits while the host is harmed but usually not immediately killed, which defines parasitism. Distractors: A: mutualism benefits both partners. B: commensalism helps one and leaves the other unaffected. D: competition involves two species vying for the same resource, not one feeding on the other. E: a predator typically kills and consumes its prey outright, unlike a blood-feeding tick. Fix: benefit-to-one, harm-to-host without killing it is parasitism. [Apply]

47. C) Primary succession. Colonization of lifeless bare rock with no soil is primary succession, and lichens and mosses are typical pioneers. Distractors: A: secondary succession follows a disturbance that leaves soil intact. B: a climax community is a mature, stable end stage, not the beginning. D: eutrophication is nutrient enrichment of water, unrelated here. E: character displacement is an evolutionary divergence between competitors. Fix: starting on bare rock with no soil marks primary succession. [Apply]

48. E) Cellular respiration. Cellular respiration breaks down organic molecules and releases carbon dioxide back to the atmosphere. Distractors: A: photosynthesis removes carbon dioxide, the opposite direction. B: nitrogen fixation involves nitrogen, not carbon. C: transpiration releases water vapor from plants. D: condensation is part of the water cycle. Fix: respiration is the biological process that returns carbon dioxide to the air. [Apply]

49. E) The competitive exclusion principle. When two species use exactly the same niche, one outcompetes and excludes the other, the competitive exclusion principle. Distractors: A: mutualism is mutual benefit, not exclusion. B: the founder effect concerns allele frequencies in a new small population. C: predator-prey cycling involves one eating the other, not competition for shared resources. D: commensalism benefits one and is neutral to the other, unlike this competitive outcome. Fix: identical niches cannot be shared, so one competitor is excluded. [Analyze]

50. C) Genetic drift (bottleneck effect). A random, non-selective die-off that shifts allele frequencies by chance is genetic drift, specifically a bottleneck. Distractors: A and D require survival based on fitness differences, but the deaths here were random. B: gene flow involves migration of alleles between populations, not random death. E: sexual selection concerns mate choice, not a storm. Fix: random, chance-based changes in allele frequency are genetic drift, not selection. [Apply]

51. B) Homologous structures inherited from a common ancestor. The same underlying bone plan used for different functions signals common ancestry, so the limbs are homologous. Distractors: A: analogous structures share function but not underlying anatomy and origin. C: these limbs are fully functional, not vestigial. D: convergence produces similar function from different ancestry, the reverse of this case. E: these are living limbs, not fossil intermediates. Fix: same structure, different function points to homology and shared ancestry. [Analyze]

52. E) 3:1. Tt × Tt gives genotypes 1 TT : 2 Tt : 1 tt, so 3 tall to 1 short phenotypically. Distractors: A: 1:1 is a testcross result (Tt × tt). B: 1:2:1 is the genotype ratio, not the phenotype ratio. C: 9:3:3:1 is a dihybrid ratio, not a single-gene cross. D: 4:0 would require no recessive offspring. Fix: a monohybrid heterozygous cross gives a 3:1 phenotype ratio. [Apply]

53. E) Crossing an individual of dominant phenotype with a homozygous recessive individual. A testcross pairs an individual of dominant phenotype (unknown genotype) with a homozygous recessive to reveal the unknown genotype from the offspring. Distractors: A: crossing two unknowns cannot cleanly determine either genotype. B: two homozygous dominants yield only dominant offspring, revealing nothing about a hidden recessive allele. C: self-pollination is not a testcross. D: a heterozygote cross is a standard monohybrid cross, not a testcross. Fix: a testcross always uses a homozygous recessive partner to expose an unknown genotype. [Understand]

54. B) A rapidly growing population. Many young individuals relative to older ones signals a high birth rate and rapid future growth. Distractors: A: a declining population has a narrow base of young. C: zero growth shows fairly even age bands. D: exceeding carrying capacity is not directly read from a broad-based pyramid, which instead predicts growth. E: equal birth and death rates give a stable, column-like structure, not a broad base. Fix: a wide-based age pyramid predicts a rapidly growing population. [Analyze]

55. A) Producers. Organisms that convert sunlight into chemical energy and form the base of the food web are producers (autotrophs). Distractors: B: primary consumers eat producers. C and E are consumers that eat other animals. D: decomposers break down dead matter rather than capture sunlight. Fix: the sunlight-capturing base of a food web is the producer level. [Apply]

56. C) Energy is lost between levels, so each higher level supports less biomass. Because roughly 90% of energy is lost between trophic levels, each higher level can support less living mass. Distractors: A: individual size does not determine total biomass at a level. B: biomass is transferred and lost as energy, not simply destroyed and unmeasurable. D: decomposers act across all levels, not just the top. E: phytoplankton actually reproduce faster, and reproductive rate is not the cause of the biomass pyramid. Fix: declining biomass up a pyramid reflects energy loss between trophic levels. [Analyze]

57. A) Coevolution between the plant and its pollinator. Reciprocal adaptation in which each species exerts selective pressure on the other is coevolution. Distractors: B: drift is random and would not produce a precise reciprocal match. C: no human breeding is involved in a wild pollination relationship. D: convergence involves unrelated species evolving similar traits, not a partner-specific match. E: mutation supplies variation but cannot by itself produce a matched adaptation without selection. Fix: reciprocal trait matching between interacting species is coevolution. [Analyze]

58. D) Nitrogen fixation. Converting atmospheric N2 into ammonia is nitrogen fixation, carried out by bacteria such as those in legume nodules. Distractors: A: denitrification returns nitrogen to the atmosphere as N2, the reverse direction. B: ammonification releases ammonia from decaying organic matter, not from N2 gas. C: nitrification converts ammonia to nitrites and nitrates. E: decomposition is the general breakdown of dead matter. Fix: turning atmospheric N2 into a usable form is nitrogen fixation. [Understand]

59. D) binding to a site away from the active site and changing the enzyme's shape. Noncompetitive inhibitors bind an allosteric site, distorting the active site so the substrate no longer fits well. Distractors: A describes a competitive inhibitor (confuses the two inhibitor types). B assumes inhibition works by consuming substrate rather than affecting the enzyme. C mistakes an environmental variable for a molecular inhibitor. E reverses enzyme function, since enzymes lower activation energy. Fix: "non-competitive = binds elsewhere and reshapes; competitive = fights for the active site." [Analyze]

60. A) oxidative phosphorylation along the electron transport chain. Most ATP comes from the electron transport chain driving oxidative phosphorylation via the proton gradient. Distractors: B yields only a small net ATP and is not the main source. C produces mostly NADH/FADH2 and little direct ATP. D is anaerobic and gives no net electron-transport ATP. E belongs to photosynthesis, not respiration. Fix: "the electron transport chain is where the ATP payoff is; glycolysis and Krebs mostly load electron carriers." [Understand]

61. D) supply replacement electrons to photosystem II and release oxygen. Photolysis of water replaces electrons lost by photosystem II and releases O2 as a byproduct. Distractors: A and B belong to the Calvin cycle, which is light-independent. C is a downstream biosynthetic use of fixed carbon, not the role of water splitting. E is long-term storage, not the immediate function. Fix: "splitting water feeds electrons back into photosystem II and vents oxygen." [Apply]

62. D) one original parental strand paired with one newly made strand. Each parental strand serves as a template, so every daughter helix is half old, half new. Distractors: A describes a conservative-style result with all-new strands. B describes the retained parental duplex of a dispersive/conservative misconception. C misstates polymerase direction (synthesis is 5'-to-3'). E confuses replication with a halving of length. Fix: "semiconservative = one old strand kept in each daughter helix." [Apply]

63. C) Anaphase. Anaphase is defined by the separation of sister chromatids toward opposite poles. Distractors: A is chromosome condensation. B is alignment at the metaphase plate. D is nuclear reassembly after separation. E is the non-dividing growth and replication stage. Fix: "Anaphase = Apart; chromatids pull to the poles." [Remember]

64. A) lose water and shrink. Water leaves the cell by osmosis toward the higher solute concentration outside, so the cell shrinks. Distractors: B and E describe a hypotonic environment. C ignores the osmotic gradient. D invents an active-transport response that would not reverse the water loss. Fix: "hyper-tonic outside = water exits = cell shrivels." [Apply]

65. B) Golgi apparatus. The Golgi apparatus modifies, sorts, and packages proteins into vesicles for secretion. Distractors: A synthesizes polypeptides but does not package them. C digests material with hydrolytic enzymes. D produces ATP. E synthesizes lipids and detoxifies, not protein packaging. Fix: "Golgi = the cell's shipping-and-processing department." [Remember]

66. E) the redundancy (degeneracy) of the genetic code. Because multiple codons specify the same amino acid, a base change can be silent. Distractors: A would prevent the mutation, but the mutation is stated to persist. B is not part of normal eukaryotic gene expression here. C would only apply to intronic changes and is not the general explanation. D describes noncoding chromosome ends unrelated to codon reading. Fix: "same amino acid despite a base change = degenerate code (a silent mutation)." [Analyze]

67. C) exchanging segments between homologous chromosomes. Crossing over swaps homologous segments, creating new allele combinations on a chromosome. Distractors: A confuses recombination with a failure to reduce chromosome number. B describes mitosis, which yields identical cells. D is a separate meiotic/mitotic event, not recombination. E reverses the effect, since crossing over adds to variation alongside assortment. Fix: "crossing over = homologs trade pieces, mixing alleles." [Apply]

68. C) A single circular chromosome not enclosed in a nucleus. Prokaryotes carry a single circular chromosome in the cytoplasm with no nuclear envelope, unlike eukaryotes. Distractors: A, B, D, and E are shared by both cell types and so cannot distinguish them. Fix: "prokaryote = no nucleus; its circular DNA floats free in the cytoplasm." [Analyze]

69. E) They inject their nucleic acid into a bacterial host in order to replicate. Phages are viruses that inject nucleic acid into bacteria and hijack host machinery to make new phages. Distractors: A is false, since a given virus has either DNA or RNA. B and D wrongly credit viruses with independent metabolism. C misclassifies a virus as a cell. Fix: "a phage is a virus, not a cell; it needs a bacterial host to copy itself." [Apply]

70. C) Protein — amino acid. Proteins are polymers of amino acids. Distractors: A pairs a protein with the nucleic-acid monomer. B pairs a carbohydrate with the protein monomer. D pairs a nucleic acid with a carbohydrate monomer. E pairs lipids (not true polymers of nucleotides) with the wrong unit. Fix: "match the polymer to its unit: protein-amino acid, nucleic acid-nucleotide, starch-glucose." [Apply]

71. E) shows greatly reduced activity because its shape is altered. Moving away from the optimum pH disrupts bonds that maintain the active-site shape, so activity drops sharply. Distractors: A ignores that activity falls off both sides of the optimum. B and C invent transformations enzymes do not undergo. D restates a general enzyme property but does not explain the pH effect. Fix: "pH far from the optimum warps the active site and cripples the enzyme." [Analyze]

72. A) phosphate groups they contain. These molecules share adenine and ribose but carry one, two, or three phosphates respectively. Distractors: B and C are identical across all three (one base, one sugar). D and E name components these nucleotides do not contain. Fix: "AMP/ADP/ATP = same base and sugar, counting up phosphates." [Remember]

73. A) mitochondrial matrix. Krebs-cycle enzymes reside in the fluid matrix inside the inner mitochondrial membrane. Distractors: B is where glycolysis occurs. C houses the electron transport chain, not the cycle. D is unrelated to respiration. E belongs to photosynthesis. Fix: "glycolysis in cytoplasm, Krebs in the matrix, electron transport on the inner membrane." [Remember]

74. C) prevents wasteful overproduction of the end product. Feedback inhibition throttles the pathway once enough product exists, conserving resources. Distractors: A reverses the regulatory logic. B is not a real mechanism name. D confuses reversible inhibition with permanent destruction. E misapplies thermodynamics to a regulatory effect. Fix: "end-product feedback inhibition = the product signals the pathway to slow down." [Analyze]

75. B) a phospholipid bilayer with proteins embedded within it. The model pictures a flexible phospholipid bilayer studded with drifting proteins. Distractors: A confuses the plant cell wall with the membrane. C and E name molecules that do not form the bilayer. D ignores the "fluid" quality the model stresses. Fix: "fluid mosaic = a moving sea of phospholipids with proteins floating in it." [Understand]

76. A) simple diffusion down a concentration gradient. O2 diffuses passively from higher concentration in the alveoli to lower concentration in the blood. Distractors: B and E wrongly attach an energy cost to a passive process. C describes bulk uptake of large particles, not gas. D applies to water movement, not oxygen. Fix: "gas crossing a membrane down its gradient with no ATP = simple diffusion." [Apply]

77. C) transcription of the genes needed to metabolize lactose. Lactose (via allolactose) inactivates the repressor, freeing RNA polymerase to transcribe the lac genes. Distractors: A confuses gene regulation with gene loss. B is backwards, since lactose releases the repressor. D ignores that transcription must precede translation. E is unrelated to operon induction. Fix: "lactose present = repressor off = lac genes transcribed." [Apply]

78. E) pulmonary artery carrying blood toward the lungs. The right ventricle pumps oxygen-poor blood into the pulmonary artery toward the lungs. Distractors: A leaves the left ventricle. B returns oxygen-rich blood to the left atrium. C branches off the aorta. D delivers blood into the right atrium, before the ventricle. Fix: "right ventricle to lungs via the pulmonary artery, the only artery carrying oxygen-poor blood." [Apply]

79. C) increase heat loss from the body surface. Vasodilation brings warm blood near the surface so heat radiates away, cooling the body. Distractors: A describes vasoconstriction, the opposite response. B would worsen overheating. D removes a cooling mechanism rather than aiding it. E confuses the corrective response with resetting the target itself. Fix: "vessels dilate = more blood at the surface = more heat dumped." [Analyze]

80. D) transpiration pulling the water column up the stem. Evaporation from leaves (transpiration) creates tension that pulls a cohesive water column upward through the xylem. Distractors: A cannot account for movement up a tall trunk against gravity. B misassigns transport to the sugar-conducting phloem. C names only part of the process and omits the driving pull. E opposes upward flow. Fix: "transpiration pull plus cohesion draws water up the xylem." [Apply]

81. A) accumulates on the shaded side and promotes cell elongation there. Auxin migrates to the shaded side, where it stimulates elongation, so that side grows longer and the shoot bends toward light. Distractors: B and D misplace the growth on the lighted side. C would produce no bending. E misstates auxin's action as a growth block. Fix: "auxin gathers on the shady side, cells there stretch, and the shoot leans into the light." [Analyze]

82. A) Impaired absorption of digested nutrients. The small intestine is the principal site of nutrient absorption, so its loss impairs absorption. Distractors: B and C involve the mouth and pharynx, which are unaffected. D involves the salivary glands, upstream of the intestine. E involves the respiratory system, unrelated to digestion. Fix: "the small intestine is where most absorption happens; lose it and nutrients pass through unabsorbed." [Analyze]

83. E) changes in membrane permeability to sodium and potassium ions. Sequential opening of voltage-gated Na+ and K+ channels shifts membrane potential, propagating the impulse. Distractors: A and B name molecules unrelated to the electrical signal. C wrongly treats myelin as contractile. D confuses neurons with muscle or blood vessels. Fix: "the action potential is a wave of sodium-in, potassium-out permeability changes." [Understand]

84. B) promoting the uptake of glucose by body cells. Insulin increases glucose uptake by cells and promotes glycogen storage, reducing blood glucose. Distractors: A and D describe glucagon's effects, which raise glucose. C is not insulin's mechanism. E misattributes the effect to kidney filtration. Fix: "insulin opens cells to take up glucose, dropping blood sugar." [Apply]

85. E) recognize particular antigens by means of matching receptors. Specificity arises because each lymphocyte bears receptors that bind a particular antigen. Distractors: A describes nonspecific phagocytosis. B and D are general, nonspecific defenses. C is a barrier defense, not lymphocyte recognition. Fix: "specific immunity = lymphocyte receptors that match one antigen." [Analyze]

86. D) differentiation. Differentiation is the acquisition of specialized structure and function by cells. Distractors: A is the early rapid cell division of the zygote. B is fusion of gametes. C is the formation of germ layers. E is ordinary nuclear division without specialization. Fix: "differentiation = cells taking on specialized jobs." [Remember]

87. B) Fusion of a sperm and an egg to form a zygote. Fertilization unites sperm and egg into a diploid zygote, the first cell of the new organism. Distractors: A releases a gamete but is not yet fertilization. C is the shedding of the uterine lining. D is a hormonal event, not a developmental start. E is a later supporting structure. Fix: "a new individual begins at fertilization, when the zygote forms." [Understand]

88. B) self-pollination of that flower. Anthers produce pollen, so removing them stops that flower from fertilizing itself. Distractors: A, C, and E involve tissues unrelated to pollen production. D concerns already-formed seeds, not pollination. Fix: "no anthers = no pollen from that flower = no self-pollination." [Analyze]

89. B) reptiles. Dry scaly skin, shelled (amniotic) eggs laid on land, and ectothermy are hallmark reptile traits. Distractors: A amphibians have moist skin and lay eggs in water. C mammals have hair and bear live young or are endothermic. D birds are endothermic and feathered. E fish live in water and lack shelled land eggs. Fix: "dry scales plus shelled land eggs plus cold-blooded = reptile." [Analyze]

90. D) a branching network of tracheae. Insect tracheae open at spiracles and branch throughout the body, delivering air straight to cells. Distractors: A and E describe aquatic gill breathing. B assigns a vertebrate lung to an insect. C credits insect blood with an oxygen carrier it largely lacks. Fix: "insects pipe air to tissues through tracheae, bypassing the blood." [Analyze]

91. E) sliding past one another. The sliding filament model has actin and myosin overlap more without either shortening, so the sarcomere and muscle shorten. Distractors: A, B, C, and D invent length changes or turnover that the model rejects. Fix: "muscles shorten because filaments slide past each other, not because filaments shrink." [Analyze]

92. D) production of a larger volume of dilute urine. Less water reabsorption leaves more water in the filtrate, producing copious dilute urine. Distractors: A, C, and E involve organs unrelated to renal water handling. B misattributes a respiratory change to the kidney. Fix: "less water reabsorbed = more, more dilute urine." [Analyze]

93. A) phloem. Girdling severs the phloem, blocking downward sugar transport, which identifies phloem as the sugar conduit. Distractors: B carries water and minerals upward, not sugars. C, D, and E are not the long-distance sugar-transport tissue. Fix: "cut the phloem and sugar stops moving; phloem carries the food." [Analyze]

94. B) raise its body temperature. As an ectotherm, the lizard relies on external heat, so basking warms its body toward an active temperature. Distractors: A, C, and D assign unrelated functions to basking. E overstates a single minor effect while ignoring thermoregulation. Fix: "ectotherms bask to gain heat from the environment." [Apply]

95. D) carbon dioxide. Increased CO2 (and the resulting drop in blood pH) is the primary stimulus that raises breathing rate. Distractors: A is backwards, since falling oxygen is a weaker, secondary trigger. B, C, and E do not directly drive the respiratory center. Fix: "rising carbon dioxide, not falling oxygen, is the main signal to breathe faster." [Apply]

96. D) negative feedback that keeps hormone levels within a range. Rising thyroid hormone suppresses the pituitary and hypothalamus, holding levels within a range, the hallmark of negative feedback. Distractors: A describes amplification, the opposite pattern. B and E deny the regulation that clearly occurs. C is unrelated to endocrine signaling. Fix: "hormone output that shuts down its own trigger = negative feedback." [Analyze]

97. A) Crossing an organism showing the dominant phenotype with a homozygous recessive individual. A testcross pairs a dominant-phenotype individual with a homozygous recessive to reveal the unknown genotype. Distractors: B and D are used to study ratios but do not isolate one unknown genotype against a recessive tester. C involves no recessive tester. E lacks the known recessive parent that makes the cross informative. Fix: "testcross = mystery dominant crossed with a homozygous recessive to expose hidden alleles." [Understand]

98. C) 1/4. A Tt x Tt cross gives 1 TT : 2 Tt : 1 tt, so 1/4 are tt. Distractors: A ignores that both parents carry the recessive allele. B misapplies a dihybrid fraction to a single gene. D confuses the heterozygote fraction with the recessive fraction. E gives the dominant-phenotype fraction instead. Fix: "monohybrid Tt x Tt yields 1/4 homozygous recessive." [Apply]

99. E) 9:3:3:1. A dihybrid cross of two double heterozygotes yields the 9:3:3:1 phenotypic ratio. Distractors: A is a testcross-type ratio. B is a monohybrid phenotypic ratio. C is a monohybrid genotypic ratio. D is not a valid Mendelian outcome. Fix: "AaBb x AaBb gives 9:3:3:1 phenotypes." [Apply]

100. C) 0.04. Homozygous recessive frequency is q^2 = 0.2^2 = 0.04. Distractors: A mistakes the allele frequency q for the genotype frequency. B is 2pq, the heterozygote frequency. D squares the dominant allele frequency incorrectly. E is p^2, the homozygous dominant frequency. Fix: "homozygous recessive = q squared." [Apply]

101. A) No natural selection acts on the trait. Equilibrium requires no selection (along with no mutation, migration, drift, and random mating). Distractors: B introduces genetic drift, which violates equilibrium. C is nonrandom mating. D introduces mutation. E introduces gene flow. Fix: "Hardy-Weinberg needs no selection, no drift, no gene flow, no mutation, and random mating." [Analyze]

102. D) Half of the sons are expected to be color-blind. Sons receive one X from the carrier mother, so half inherit the recessive allele and are color-blind. Distractors: A overstates the fraction of affected sons. B and C require an affected father to make daughters affected. E ignores that the mother passes the allele to half her children. Fix: "for X-linked recessives, a carrier mother passes the trait to half her sons." [Analyze]

103. A) natural selection. Differential survival based on a heritable trait shifting allele frequencies is natural selection. Distractors: B and C are chance-based changes unrelated to survival advantage. D requires deliberate human breeding. E involves unrelated species evolving similar traits, not one population changing. Fix: "heritable trait plus survival advantage plus frequency shift = natural selection." [Analyze]

104. B) genetic drift. A chance event altering allele frequencies, especially in a small population, is genetic drift. Distractors: A, C, and E all require trait-based differential survival, but the deaths were random. D involves movement of alleles between populations, not a chance die-off. Fix: "random, trait-blind change in allele frequency = genetic drift." [Analyze]

105. E) allopatric speciation. A physical barrier splitting a population before divergence is allopatric speciation. Distractors: A occurs without geographic separation. B is human-directed breeding. C is unrelated lineages converging, not one splitting. D describes stability, not speciation. Fix: "geographic barrier splitting a population = allopatric speciation." [Apply]

106. E) homologous structures indicating common ancestry. Shared underlying bone structure across differing functions reflects descent from a common ancestor, the definition of homology. Distractors: A and C describe similar function from unrelated lineages, the opposite pattern. B misapplies the vestigial concept. D ignores that these animals live in very different environments. Fix: "same underlying build, different uses = homologous, pointing to common ancestry." [Analyze]

107. C) much energy is lost as heat at each transfer. Only about ten percent of energy passes to the next level, with most lost as metabolic heat. Distractors: A confuses body size with energy content. B violates conservation of energy. D wrongly claims producers spend nothing. E misplaces decomposers as energy sources for higher levels. Fix: "energy shrinks up the pyramid because most is lost as heat at each transfer." [Understand]

108. D) decomposers. Decomposers break down dead organisms and wastes, recycling nutrients into the soil. Distractors: A make their own food from sunlight. B and E eat producers. C are predators at the chain's top. Fix: "decomposers recycle dead matter back into nutrients." [Remember]

109. C) parasitism. One organism benefits while living on and harming a host, which defines parasitism. Distractors: A benefits both partners. B benefits one while the other is unaffected. D involves organisms vying for the same resource. E implies a predator killing prey outright, not prolonged feeding on a living host. Fix: "one benefits, the host is harmed but usually lives = parasitism." [Apply]

110. E) rapid exponential growth. With ample resources and no limits, the population grows exponentially at first. Distractors: A and B ignore the favorable conditions. C and D wrongly impose limits before the environment's carrying capacity is approached. Fix: "unlimited resources early on = exponential (J-shaped) growth." [Apply]

111. B) slows and levels off. As resources become limiting near carrying capacity, growth slows and the curve levels off (logistic growth). Distractors: A and C ignore limiting resources. D overstates the effect, since the population stabilizes rather than crashing indefinitely. E denies resource limitation. Fix: "near carrying capacity, growth brakes and the curve flattens." [Analyze]

112. E) primary succession. Colonization of lifeless bare rock with no existing soil is primary succession. B begins where soil already remains after a disturbance. Distractors: A is nutrient enrichment of water. C is toxin buildup up a food chain. D concerns population size, not community development. Fix: "starting from bare rock with no soil = primary succession." [Analyze]

113. B) carry out photosynthesis and take up carbon dioxide. Plants remove CO2 during photosynthesis, so fewer plants means less CO2 uptake and more remaining in the air. Distractors: A misstates photosynthesis as respiration. C and E name processes that add CO2, not remove it. D confuses living trees with fossil fuels. Fix: "fewer plants = less photosynthesis = less carbon dioxide pulled from the air." [Analyze]

114. A) a large drop in overall species diversity. A keystone predator holds dominant competitors in check, so its removal lets them take over and diversity falls. Distractors: B underestimates the outsized role of a keystone species. C and E misapply energy and producer effects to a predator's loss. D wrongly ties decomposers to a predator's removal. Fix: "lose the keystone predator and a few competitors dominate, so diversity collapses." [Analyze]

115. C) tundra. Permafrost, a short growing season, and low vegetation are defining features of tundra. Distractors: A is warm and highly productive year-round. B has seasonal broadleaf trees and no permafrost. D is defined by aridity, not cold and frozen soil. E is dominated by grasses without permafrost. Fix: "permafrost plus low mosses and shrubs plus short season = tundra." [Analyze]

Scaled Score Estimator

Approximate raw-score (out of 115) → 20–80 scaled-score conversion. CLEP's true conversion is proprietary and equated across forms; this piecewise estimate is only for self-assessment.

A scaled score of 50 — roughly 50% correct (~58/115) — is the usual credit-granting line. Below about raw 23 you are at random-guessing level (1 in 5), which floors near a scaled score of 20.