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OSAT Biological Sciences (110) study materials
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OSAT Biological Sciences (110) practice questions

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Question 1Foundations of Biological Science

Which property of water is most directly responsible for its ability to dissolve ionic compounds such as sodium chloride?

  1. Its low density as a solid
  2. Its high surface tension
  3. Its polar molecular structure Correct answer.
  4. Its high specific heat

Explanation

Water is a polar molecule with partial charges on its oxygen and hydrogen ends, so its dipoles orient around ions and shield them from one another. This ion-dipole interaction is what pulls ionic lattices apart and keeps the ions in solution. The other listed properties matter for heat regulation, ice floating, and cohesion, not for dissolving salts.

Question 2Foundations of Biological Science

A covalent bond in which electrons are shared unequally between two atoms of different electronegativity is best described as:

  1. nonpolar covalent
  2. polar covalent Correct answer.
  3. ionic
  4. metallic

Explanation

When two atoms differ in electronegativity but neither transfers electrons completely, the shared pair is pulled toward the more electronegative atom, producing a dipole. Such a bond is called polar covalent. Full electron transfer between a metal and a nonmetal produces an ionic bond instead.

Question 3Foundations of Biological Science

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

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

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. The proton gradient across that membrane drives ATP production. The other organelles listed handle protein processing, modification and sorting, and hydrolysis of macromolecules.

Question 4Foundations of Biological Science

During which phase of mitosis do sister chromatids separate and move toward opposite poles of the cell?

  1. Anaphase Correct answer.
  2. Metaphase
  3. Telophase
  4. Prophase

Explanation

Anaphase begins when cohesin is cleaved and sister chromatids are pulled apart by shortening kinetochore microtubules. The separated chromatids, now individual chromosomes, move toward the spindle poles. Chromosome condensation, alignment at the equator, and nuclear envelope reformation occur in other phases.

Question 5Foundations of Biological Science

In a population at Hardy-Weinberg equilibrium, the frequency of a recessive allele is 0.2. What proportion of the population is expected to be heterozygous?

  1. 0.32 Correct answer.
  2. 0.04
  3. 0.16
  4. 0.64

Explanation

With q = 0.2, p = 0.8, and the heterozygote frequency is 2pq = 2(0.8)(0.2) = 0.32. The homozygous recessive frequency q-squared equals 0.04, and the homozygous dominant frequency p-squared equals 0.64. These four frequencies sum to 1.

Question 6Foundations of Biological Science

An enzyme catalyzes a reaction, and a competitive inhibitor is added. Which change in kinetic parameters is expected when substrate concentration is varied?

  1. The apparent Km increases while Vmax is unchanged Correct answer.
  2. Both Km and Vmax decrease
  3. Vmax decreases while Km is unchanged
  4. Both Km and Vmax increase

Explanation

A competitive inhibitor binds the free enzyme at the active site, so more substrate is needed to reach half-maximal velocity, raising the apparent Km. Because sufficiently high substrate outcompetes the inhibitor, the maximal velocity Vmax is unchanged. Noncompetitive inhibition instead lowers Vmax without altering Km.

Question 7Foundations of Biological Science

Which statement best describes the role of the enzyme rubisco in photosynthesis?

  1. It fixes carbon dioxide onto ribulose bisphosphate in the Calvin cycle. Correct answer.
  2. It splits water to release oxygen in photosystem II.
  3. It transfers electrons from photosystem I to NADP+.
  4. It pumps protons across the thylakoid membrane.

Explanation

Rubisco catalyzes the attachment of CO2 to ribulose-1,5-bisphosphate, producing two molecules of 3-phosphoglycerate in the Calvin cycle. This carboxylation step is the entry point of inorganic carbon into organic molecules. Water splitting, terminal electron transfer to NADP+, and proton pumping belong to the light reactions.

Question 8Foundations of Biological Science

A mutation changes a single DNA base but the amino acid sequence of the encoded protein remains the same. This mutation is best classified as:

  1. silent Correct answer.
  2. missense
  3. nonsense
  4. frameshift

Explanation

Because the genetic code is degenerate, several codons specify the same amino acid, so a base substitution can leave the protein sequence unchanged. Such a change is a silent mutation. Nonsense and missense mutations alter the protein, and frameshifts shift the entire reading frame downstream.

Question 9Foundations of Biological Science

In a cross between two individuals heterozygous for a single gene (Aa x Aa), what is the expected phenotypic ratio when the allele shows complete dominance?

  1. 1:1
  2. 9:3:3:1
  3. 1:2:1
  4. 3:1 Correct answer.

Explanation

A monohybrid cross of two heterozygotes yields genotypes in a 1 AA : 2 Aa : 1 aa ratio. With complete dominance, both AA and Aa show the dominant phenotype, giving a 3:1 phenotypic ratio. The 1:2:1 ratio describes genotypes or incomplete dominance phenotypes.

Question 10Foundations of Biological Science

Which feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Prokaryotes lack ribosomes
  2. Prokaryotes lack nucleic acids
  3. Prokaryotes lack a plasma membrane
  4. Prokaryotes lack a membrane-bound nucleus Correct answer.

Explanation

Prokaryotes keep their genetic material in a nucleoid region without a nuclear envelope, whereas eukaryotes enclose chromosomes in a nucleus. Prokaryotes still possess ribosomes, a plasma membrane, and DNA or RNA. Therefore only the absence of a membrane-bound nucleus is a valid distinguishing feature among the choices.

Question 11Foundations of Biological Science

Which structure primarily regulates the passage of substances between adjacent plant cells through pores in their walls?

  1. Gap junctions
  2. Plasmodesmata Correct answer.
  3. Tight junctions
  4. Desmosomes

Explanation

Plasmodesmata are channels that traverse plant cell walls and connect the cytoplasm of neighboring cells, forming the symplast. They allow water, ions, and small molecules to move directly between cells. Desmosomes, gap junctions, and tight junctions are characteristic of animal tissues.

Question 12Foundations of Biological Science

A red blood cell is placed in a hypertonic solution. What is the expected immediate result?

  1. The cell swells and may lyse
  2. The cell maintains its volume unchanged
  3. The cell actively pumps solutes inward to equilibrate
  4. The cell shrinks as water leaves Correct answer.

Explanation

In a hypertonic solution, the solute concentration outside the cell exceeds that inside, so water moves out by osmosis. The cell loses volume and shrinks, a process called crenation in red blood cells. Swelling occurs in hypotonic solutions, and no active pumping is required for the immediate osmotic response.

Question 13Foundations of Biological Science

Which molecule serves as the final electron acceptor in the electron transport chain of aerobic cellular respiration?

  1. FAD
  2. Oxygen Correct answer.
  3. Pyruvate
  4. NAD+

Explanation

Oxygen accepts electrons at the end of the mitochondrial electron transport chain and combines with protons to form water. This keeps electrons flowing through the complexes and maintains the proton gradient that drives ATP synthesis. NAD+ and FAD are electron carriers that deliver electrons earlier in the chain.

Question 14Foundations of Biological Science

Which of the following best describes the process of natural selection?

  1. Allele frequencies change randomly due to chance events in small populations
  2. Individuals with heritable traits that improve fitness leave more offspring Correct answer.
  3. Gene flow between populations equalizes allele frequencies over time
  4. Organisms acquire traits during their lifetime and pass them to offspring

Explanation

Natural selection requires heritable variation in traits that affect reproductive success, so individuals with advantageous variants contribute more offspring to the next generation. Over time this changes allele frequencies in the population. Acquired traits are not inherited, and random change or migration describe other evolutionary mechanisms.

Question 15Foundations of Biological Science

Two species of finch occupy the same island but feed on different-sized seeds, and their beak sizes differ accordingly. This pattern is best described as:

  1. resource partitioning Correct answer.
  2. convergent evolution
  3. competitive exclusion
  4. sympatric speciation

Explanation

When coexisting species divide available resources, they reduce competition by using different portions of the resource spectrum, a pattern called resource partitioning. Here the two finches differ in beak size and seed preference, allowing coexistence. Competitive exclusion would drive one species to local extinction.

Question 16Foundations of Biological Science

In a food web, approximately what proportion of energy at one trophic level is typically transferred to the next?

  1. About 1 percent
  2. About 10 percent Correct answer.
  3. About 50 percent
  4. About 90 percent

Explanation

Ecological efficiency is generally around 10 percent, because most energy is lost to metabolism, heat, and waste at each transfer. This is why food chains rarely exceed four or five trophic levels. The remaining energy is not passed on to the next level.

Question 17Foundations of Biological Science

Which of the following is a primary function of the sodium-potassium pump in animal cells?

  1. Synthesizing ATP from ADP and phosphate
  2. Generating a resting membrane potential by moving three Na+ out and two K+ in Correct answer.
  3. Transporting glucose against its concentration gradient directly
  4. Serving as a receptor for steroid hormones

Explanation

The sodium-potassium pump uses ATP to export three sodium ions and import two potassium ions per cycle, creating an electrochemical gradient. This gradient underlies the resting membrane potential and drives secondary active transport. It consumes ATP rather than producing it and does not itself transport glucose.

Question 18Foundations of Biological Science

Which of the following correctly describes the relationship between DNA, mRNA, and protein during gene expression in eukaryotes?

  1. Protein is transcribed from DNA and then translated into mRNA
  2. mRNA is transcribed from DNA and then translated into protein Correct answer.
  3. DNA is translated directly into protein without an RNA intermediate
  4. mRNA is translated into DNA before protein synthesis

Explanation

In the central dogma, DNA is transcribed into messenger RNA, which is then translated by ribosomes into a polypeptide. This flow ensures that genetic information is expressed as protein. The reverse relationships described in the other options do not occur in normal gene expression.

Question 19Foundations of Biological Science

A student examines a pedigree in which a trait appears in every generation and affects both males and females equally, with an affected parent always having at least one affected child. Which mode of inheritance is most consistent with this pattern?

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

Explanation

A trait that appears in every generation, affects both sexes equally, and is transmitted by affected parents to offspring fits autosomal dominant inheritance. Affected individuals typically have an affected parent, and both sexes transmit the allele. Autosomal recessive traits often skip generations, and mitochondrial traits pass only through mothers.

Question 20Foundations of Biological Science

Which statement about the relationship between photosynthesis and cellular respiration is accurate?

  1. Photosynthesis and respiration both release carbon dioxide as a product
  2. Photosynthesis stores energy in organic molecules, while respiration releases it Correct answer.
  3. Respiration occurs only in animals and photosynthesis only in plants
  4. Both processes occur in the mitochondria of plant cells

Explanation

Photosynthesis uses light energy to build organic molecules, storing energy in chemical bonds, whereas cellular respiration oxidizes those molecules and releases energy as ATP. The two processes are complementary in their inputs and outputs. Respiration occurs in plants as well as animals, and photosynthesis occurs in chloroplasts.

Question 21Foundations of Biological Science

Which type of RNA carries amino acids to the ribosome during translation?

  1. Messenger RNA
  2. Small nuclear RNA
  3. Transfer RNA Correct answer.
  4. Ribosomal RNA

Explanation

Transfer RNA molecules have anticodons that pair with mRNA codons and carry the corresponding amino acid to the growing polypeptide. Ribosomal RNA forms part of the ribosome, and messenger RNA carries the coding sequence. Small nuclear RNA participates in splicing.

Question 22Foundations of Biological Science

A population of bacteria is exposed to an antibiotic, and a few cells survive because they carry a resistance mutation that existed before exposure. Which statement best explains the rise of the resistant population?

  1. The antibiotic caused all cells to become equally resistant
  2. The antibiotic induced resistance mutations in surviving cells
  3. The bacteria learned to resist the antibiotic and passed this on
  4. Pre-existing resistant variants were selected and reproduced Correct answer.

Explanation

Mutations arise independently of exposure, and the antibiotic acts as a selective pressure that favors pre-existing resistant variants. Those variants survive and reproduce, increasing resistance allele frequency in the population. This is directional selection rather than mutation induced by the drug.

Question 23Foundations of Biological Science

Which of the following contributes to genetic variation within a sexually reproducing population?

  1. Mitotic division of somatic cells
  2. Binary fission
  3. Crossing over during prophase I of meiosis Correct answer.
  4. Clonal reproduction

Explanation

Crossing over exchanges segments between homologous chromosomes during prophase I, producing new combinations of alleles on chromatids. Independent assortment and random fertilization further increase variation. Binary fission, cloning, and mitosis produce genetically identical or near-identical cells.

Question 24Foundations of Biological Science

In an experiment, a plant is grown in a sealed chamber with carbon dioxide labeled with a heavy isotope. After illumination, where is the label first expected to appear in Calvin cycle intermediates?

  1. In ribulose bisphosphate before carboxylation
  2. In oxygen released by photosystem II
  3. In glucose after the cycle completes
  4. In 3-phosphoglycerate immediately after carboxylation Correct answer.

Explanation

The labeled carbon enters the Calvin cycle when rubisco attaches CO2 to ribulose bisphosphate, and the first stable labeled product is 3-phosphoglycerate. Labeled carbon later appears in sugars such as glucose after further reduction and regeneration steps. Oxygen released in the light reactions comes from water, not CO2.

Question 25Foundations of Biological Science

Which of the following best explains why the enzyme-catalyzed reaction rate plateaus at high substrate concentration?

  1. The enzyme is permanently inactivated by the substrate
  2. All active sites of the enzyme are occupied by substrate Correct answer.
  3. The substrate becomes denatured at high concentration
  4. The reaction becomes endergonic at high substrate concentration

Explanation

At high substrate concentration, enzyme active sites are saturated, so the reaction rate reaches Vmax and cannot increase further unless more enzyme is added. The plateau reflects enzyme saturation rather than substrate denaturation or a change in reaction energetics. The reaction remains exergonic under these conditions.

Question 26Foundations of Biological Science

Which property of water allows insects such as water striders to move across the surface of a pond?

  1. Strong adhesion between water molecules and dissolved ions
  2. High surface tension resulting from cohesive hydrogen bonding Correct answer.
  3. High specific heat capacity that resists temperature change
  4. Low density of liquid water relative to ice

Explanation

Surface tension arises because water molecules at the air-water interface are pulled inward by hydrogen bonds to neighboring molecules, creating a cohesive film. This film can support small objects whose weight is less than the surface tension force.

Question 27Foundations of Biological Science

A polypeptide folds so that hydrophobic residues cluster in its interior while polar residues face the surrounding cytosol. Which interaction most directly drives this arrangement?

  1. Peptide bonds rotating to expose nonpolar groups
  2. Hydrophobic effect arising from water's tendency to exclude nonpolar groups Correct answer.
  3. Ionic bonds between oppositely charged side chains
  4. Covalent disulfide bonds forming between cysteine residues

Explanation

Water forms ordered hydrogen-bonded cages around nonpolar groups, which is entropically costly. Clustering hydrophobic residues in the interior minimizes this exposure, so the hydrophobic effect is a major folding driver.

Question 28Foundations of Biological Science

Which statement correctly distinguishes a competitive inhibitor from a noncompetitive inhibitor of an enzyme?

  1. A competitive inhibitor binds only the enzyme-substrate complex, whereas a noncompetitive inhibitor binds only free enzyme
  2. A competitive inhibitor permanently denatures the enzyme, whereas a noncompetitive inhibitor binds reversibly
  3. A competitive inhibitor lowers maximal velocity, whereas a noncompetitive inhibitor raises the substrate affinity of the enzyme
  4. A competitive inhibitor binds the active site and can be overcome by high substrate concentration, whereas a noncompetitive inhibitor binds elsewhere and lowers maximal velocity Correct answer.

Explanation

Competitive inhibitors resemble substrate and occupy the active site, so raising substrate concentration outcompetes them and restores the maximum rate. Noncompetitive inhibitors bind a separate site, distorting catalysis so that the maximum velocity falls regardless of substrate level.

Question 29Foundations of Biological Science

In a cell, a protein destined for secretion is synthesized on ribosomes attached to the rough endoplasmic reticulum. Which sequence of compartments does it most likely pass through?

  1. Golgi apparatus, rough ER, mitochondrion, plasma membrane
  2. Rough ER, Golgi apparatus, secretory vesicle, plasma membrane Correct answer.
  3. Smooth ER, lysosome, nucleus, plasma membrane
  4. Rough ER, peroxisome, Golgi apparatus, plasma membrane

Explanation

Secretory proteins enter the rough ER lumen, are packaged into transport vesicles to the Golgi for modification and sorting, then leave in secretory vesicles that fuse with the plasma membrane.

Question 30Foundations of Biological Science

During which phase of the cell cycle does DNA replication occur, and what is the resulting state of each chromosome?

  1. G2 phase; homologous chromosomes pair as tetrads
  2. S phase; each chromosome consists of two sister chromatids joined at the centromere Correct answer.
  3. M phase; chromosomes separate into daughter nuclei
  4. G1 phase; each chromosome consists of a single chromatid

Explanation

DNA synthesis occurs in S phase, producing two identical sister chromatids per chromosome that remain attached at the centromere until anaphase. G1 and G2 are gap phases, and tetrad pairing occurs in meiosis, not the standard cell cycle.

Question 31Foundations of Biological Science

A cell with genotype AaBb undergoes meiosis, and the two genes assort independently. What fraction of the gametes is expected to carry the genotype AB?

  1. 3/4
  2. 1/4 Correct answer.
  3. 1/2
  4. 1/8

Explanation

Independent assortment gives four equally likely gamete types (AB, Ab, aB, ab) from a double heterozygote, so each type occurs with probability 1/4.

Question 32Foundations of Biological Science

A woman who is a carrier of an X-linked recessive disorder has children with an unaffected man. What is the probability that a son will be affected?

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

Explanation

A son inherits his Y chromosome from his father and his X from his mother. Since the mother is a carrier, half of her X-bearing gametes carry the mutant allele, so each son has a 1/2 chance of being affected.

Question 33Foundations of Biological Science

Which observation would provide the strongest evidence that a trait in a population is influenced by genetic rather than purely environmental factors?

  1. The trait varies continuously across a geographic gradient
  2. Monozygotic twins raised apart show greater similarity for the trait than dizygotic twins raised apart Correct answer.
  3. The trait becomes more common as individuals age
  4. All members of a population express the trait after a change in diet

Explanation

Monozygotic twins share all alleles, so greater resemblance between them than between dizygotic twins, even when raised apart, points to a genetic contribution. Shared environment is partially controlled by the separation.

Question 34Foundations of Biological Science

Which feature of the genetic code explains why a single-base substitution can leave the amino acid sequence unchanged?

  1. The code is degenerate, with several codons specifying the same amino acid Correct answer.
  2. The code is overlapping, so one base affects several codons
  3. The code is read in the 3' to 5' direction on mRNA
  4. Each codon contains four nucleotides

Explanation

Degeneracy means multiple codons encode the same amino acid, often differing at the third position. A substitution that produces a synonymous codon therefore leaves the protein sequence unchanged.

Question 35Foundations of Biological Science

A frameshift mutation occurs near the beginning of a coding sequence. Which outcome is most likely?

  1. The mRNA is not transcribed at all
  2. A completely different and usually truncated polypeptide results Correct answer.
  3. A single amino acid is substituted with no other change
  4. The mutation is corrected by proofreading during translation

Explanation

Inserting or deleting a number of bases not divisible by three shifts the reading frame, so all downstream codons are reinterpreted and a premature stop codon often appears. The resulting protein is typically nonfunctional.

Question 36Foundations of Biological Science

Which statement accurately describes the difference between mitosis and meiosis II?

  1. Mitosis separates sister chromatids in a diploid cell, whereas meiosis II separates sister chromatids in haploid cells Correct answer.
  2. Mitosis produces four daughter cells, whereas meiosis II produces two
  3. Mitosis halves the chromosome number, whereas meiosis II maintains it
  4. Mitosis involves homologous chromosome pairing, whereas meiosis II does not

Explanation

Both mitosis and meiosis II separate sister chromatids, but mitosis occurs in diploid cells and preserves ploidy, while meiosis II follows meiosis I and separates chromatids in haploid cells. Homolog pairing occurs in meiosis I.

Question 37Foundations of Biological Science

In an operon regulated by a repressor, which condition leads to maximal transcription of the structural genes?

  1. The repressor is inactive and cannot bind the operator Correct answer.
  2. The repressor is bound to the operator
  3. The ribosome stalls on the mRNA
  4. The promoter is methylated at CpG sites

Explanation

Transcription occurs when RNA polymerase can access the promoter, which requires the operator to be free of bound repressor. An inactive repressor leaves the operator unoccupied, permitting transcription.

Question 38Foundations of Biological Science

Two populations of a lizard are separated by a mountain range for many thousands of years and accumulate different allele frequencies. They later come into contact and do not interbreed successfully. This is best described as which form of speciation?

  1. Sympatric speciation
  2. Allopatric speciation Correct answer.
  3. Parapatric speciation
  4. Polyploid speciation

Explanation

Allopatric speciation occurs when a geographic barrier physically separates populations, blocking gene flow while divergence proceeds. Reproductive isolation that arises after such separation fits this model.

Question 39Foundations of Biological Science

A population of insects colonizes a small island from the mainland. Which factor is most likely to increase the effect of genetic drift in this new population?

  1. A small effective population size Correct answer.
  2. High rates of gene flow from the mainland
  3. A high mutation rate
  4. Random mating across a large population

Explanation

Genetic drift is the random change in allele frequencies, and its effects are strongest when the effective population size is small. A founder population on an island typically has few individuals, amplifying drift.

Question 40Foundations of Biological Science

Which condition is required for a population to remain in Hardy-Weinberg equilibrium at a given locus?

  1. Strong directional selection favoring one allele
  2. No natural selection, no mutation, no migration, and random mating in a large population Correct answer.
  3. Assortative mating based on phenotype
  4. Frequent migration between populations

Explanation

The Hardy-Weinberg model assumes no selection, no mutation, no migration, random mating, and an effectively infinite population size. Deviations from these conditions cause allele or genotype frequencies to change.

Question 41Foundations of Biological Science

A researcher measures allele frequencies at a locus in a population and finds that genotype frequencies do not match Hardy-Weinberg expectations, with an excess of heterozygotes. Which explanation is most consistent?

  1. A recent population bottleneck
  2. Disassortative mating or heterozygote advantage Correct answer.
  3. Strong inbreeding within the population
  4. Complete random mating with no selection

Explanation

An excess of heterozygotes can result from disassortative mating, which pairs unlike genotypes, or from heterozygote advantage, which raises heterozygote survival. Both push heterozygote frequencies above Hardy-Weinberg prediction.

Question 42Foundations of Biological Science

Which structural feature allows a phospholipid bilayer to form spontaneously in an aqueous environment?

  1. A protein scaffold that holds each lipid in a fixed position
  2. Rigid cholesterol molecules forming covalent cross-links between leaflets
  3. Amphipathic molecules with hydrophilic heads facing water and hydrophobic tails sequestered inside Correct answer.
  4. Positively charged tails repelling water at both surfaces

Explanation

Phospholipids have a hydrophilic phosphate head and hydrophobic fatty acid tails. In water, the tails cluster away from water while heads face the aqueous surfaces, and this self-assembly closes into a bilayer.

Question 43Foundations of Biological Science

A mutation removes the signal peptide from a protein that normally functions in the lysosome. Where would the protein most likely end up?

  1. In the cytosol, because it cannot enter the endoplasmic reticulum Correct answer.
  2. In the nucleus, because nuclear pores admit all proteins
  3. In the mitochondrial matrix, because mitochondria import all cytosolic proteins
  4. In the lysosome, because the targeting signal is dispensable

Explanation

The signal peptide directs the ribosome to the rough ER, allowing the protein to enter the secretory pathway that leads to the lysosome. Without it, translation finishes in the cytosol and the protein lacks access to that pathway.

Question 44Foundations of Biological Science

Which process directly produces the proton gradient used by ATP synthase in mitochondria?

  1. ATP hydrolysis by pumps in the outer membrane
  2. Substrate-level phosphorylation in the cytosol
  3. Electron transfer through the respiratory chain pumping protons across the inner membrane Correct answer.
  4. The Krebs cycle releasing carbon dioxide in the matrix

Explanation

As electrons pass through complexes of the electron transport chain, energy released pumps protons from the matrix into the intermembrane space. The resulting electrochemical gradient drives ATP synthase.

Question 45Foundations of Biological Science

In the light-dependent reactions of photosynthesis, what is the immediate fate of the electrons that leave photosystem II?

  1. They are transferred directly to rubisco for carbon fixation
  2. They are used to phosphorylate ADP without a proton gradient
  3. They travel through an electron transport chain that pumps protons and ultimately reduce NADP+ Correct answer.
  4. They reduce oxygen to form water in the stroma

Explanation

Electrons excited at photosystem II move through the cytochrome complex and photosystem I, with proton pumping along the way, and end by reducing NADP+ to NADPH. Water splitting replaces the electrons lost from photosystem II.

Question 46Foundations of Biological Science

A plant is placed in a sealed, illuminated chamber. As photosynthesis proceeds, which change in the chamber atmosphere is expected?

  1. Both carbon dioxide and oxygen increase
  2. Carbon dioxide decreases and oxygen increases Correct answer.
  3. Carbon dioxide increases and oxygen decreases
  4. Both carbon dioxide and oxygen decrease

Explanation

Photosynthesis consumes carbon dioxide and releases oxygen as water is split and carbon is fixed into sugars. In a sealed chamber under illumination, net photosynthesis lowers carbon dioxide and raises oxygen.

Question 47Foundations of Biological Science

Which comparison of C3 and C4 photosynthesis is accurate?

  1. C4 plants initially fix carbon dioxide into a four-carbon acid before it enters the Calvin cycle Correct answer.
  2. C4 plants perform the Calvin cycle in mesophyll cells only
  3. C3 plants use PEP carboxylase as their primary carbon-fixing enzyme
  4. C3 plants separate initial carbon fixation from the Calvin cycle by cell type

Explanation

In C4 plants, PEP carboxylase fixes carbon dioxide into a four-carbon acid in mesophyll cells, which is then shuttled to bundle-sheath cells where the Calvin cycle operates. This spatial separation concentrates carbon dioxide and limits photorespiration.

Question 48Foundations of Biological Science

In a controlled experiment, a scientist wants to test whether a new fertilizer increases plant height. Which design best supports a valid conclusion?

  1. One group given fertilizer and measured weekly, with no comparison group
  2. Two groups, one given fertilizer and one given extra water, with temperatures allowed to vary
  3. A single plant given fertilizer and measured before and after treatment
  4. Two groups of similar plants, one given fertilizer and one given the same volume of water, with all other conditions held constant Correct answer.

Explanation

A valid test requires a control group differing only in the treatment variable, with other factors such as light, water volume, and temperature held constant. This isolates the fertilizer as the likely cause of any height difference.

Question 49Foundations of Biological Science

A population of birds shows stabilizing selection on beak size. Which pattern of change is expected over many generations?

  1. The intermediate beak size becomes more common and extreme sizes become rarer Correct answer.
  2. Both extreme beak sizes increase in frequency
  3. One extreme beak size becomes progressively more common
  4. Beak size becomes equally distributed across all values

Explanation

Stabilizing selection favors intermediate phenotypes and acts against both extremes, reducing phenotypic variance. The mean stays roughly the same while the distribution narrows.

Question 50Foundations of Biological Science

Two species of salamander live in the same forest and breed in different ponds at different seasons, so they never interbreed. This is an example of which reproductive isolating mechanism?

  1. Postzygotic isolation through hybrid sterility
  2. Prezygotic isolation through habitat and temporal differences Correct answer.
  3. Mechanical isolation through incompatible reproductive structures
  4. Postzygotic isolation through hybrid inviability

Explanation

Isolation that prevents mating or fertilization from occurring at all is prezygotic. Different breeding ponds and seasons represent habitat and temporal barriers that act before zygote formation.