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CLEP Natural Sciences practice questions

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Question 1Biological Science: Origin and evolution of life; classification of organisms

Which scientist proposed that populations change over generations through natural selection acting on heritable variation?

  1. Carolus Linnaeus
  2. Louis Pasteur
  3. Gregor Mendel
  4. Charles Darwin Correct answer.

Explanation

Darwin presented natural selection as the mechanism of evolutionary change, arguing that individuals with favorable inherited traits leave more offspring. Mendel established inheritance rules, Pasteur disproved spontaneous generation, and Linnaeus built the classification system.

Question 2Biological Science: Origin and evolution of life; classification of organisms

In the binomial name Panthera leo, what does Panthera represent?

  1. The family
  2. The species
  3. The genus Correct answer.
  4. The subspecies

Explanation

Binomial nomenclature gives each organism a two-part name in which the first word is the genus and the second is the specific epithet. Panthera is therefore the genus, while leo designates the species.

Question 3Biological Science: Origin and evolution of life; classification of organisms

Which kingdom-level grouping contains organisms whose cells have a nucleus and that absorb nutrients rather than ingesting them?

  1. Fungi Correct answer.
  2. Archaea
  3. Monera
  4. Protista

Explanation

Fungi are eukaryotic and heterotrophic, obtaining nutrients by absorption after secreting enzymes into their surroundings. Archaea and Monera are prokaryotic, and protists are a diverse eukaryotic group that includes photosynthetic and ingestive forms.

Question 4Biological Science: Origin and evolution of life; classification of organisms

A fossil is found in a sedimentary layer beneath another layer containing a distinct assemblage of fossils. What can be inferred about the two layers?

  1. The two layers were deposited at the same time
  2. The lower layer is older than the upper layer Correct answer.
  3. The lower layer is younger than the upper layer
  4. The upper layer has been radiometrically dated to the Archean

Explanation

The principle of superposition states that in undisturbed sedimentary sequences, deeper layers were deposited before shallower ones. Fossils in a lower layer are therefore generally older than those above them.

Question 5Biological Science: Origin and evolution of life; classification of organisms

Which feature is shared by all living organisms and is therefore used as a fundamental criterion of life?

  1. Presence of genetic material and metabolism Correct answer.
  2. Multicellular body plan
  3. Ability to photosynthesize
  4. Locomotion

Explanation

All known organisms store hereditary information in nucleic acids and carry out metabolic reactions to maintain themselves. Multicellularity, photosynthesis, and locomotion occur only in some groups.

Question 6Biological Science: Origin and evolution of life; classification of organisms

Structures such as the human appendix and the pelvic bones of some whales are best described as which kind of evidence for evolution?

  1. Vestigial structures Correct answer.
  2. Convergent structures
  3. Homologous structures
  4. Analogous structures

Explanation

Vestigial structures are reduced remnants of organs that had a function in ancestral forms but are no longer fully functional. Their persistence reflects shared ancestry rather than present-day need.

Question 7Biological Science: Origin and evolution of life; classification of organisms

Two species of squirrel live on opposite rims of a canyon and no longer interbreed. Which term describes this form of speciation?

  1. Artificial selection
  2. Polyploid speciation
  3. Allopatric speciation Correct answer.
  4. Sympatric speciation

Explanation

Allopatric speciation occurs when a geographic barrier separates populations, preventing gene flow and allowing divergence. The canyon is the physical barrier isolating the two squirrel populations.

Question 8Biological Science: Origin and evolution of life; classification of organisms

Which of the following best explains why antibiotic resistance spreads rapidly in bacterial populations?

  1. Antibiotics cause mutations that create resistance
  2. Exposure to antibiotics makes all bacteria equally fit
  3. Resistant variants survive treatment and reproduce Correct answer.
  4. Bacteria intentionally mutate in response to the drug

Explanation

Resistance arises from pre-existing genetic variation; when antibiotics kill susceptible cells, resistant variants survive and multiply. Selection acts on variation rather than creating it on demand.

Question 9Biological Science: Origin and evolution of life; classification of organisms

A population of insects is exposed to a pesticide for many generations, and average resistance increases. Which condition is required for this change to occur by natural selection?

  1. The population must be small
  2. All individuals must reproduce at equal rates
  3. Resistance must be heritable and vary among individuals Correct answer.
  4. The pesticide must be applied at a constant dose

Explanation

Natural selection requires heritable variation in a trait that affects survival or reproduction. If resistance were not inherited or did not vary, selection could not shift the population.

Question 10Biological Science: Origin and evolution of life; classification of organisms

Which pair of structures is homologous?

  1. The fin of a shark and the flipper of a dolphin
  2. The forelimb of a whale and the arm of a human Correct answer.
  3. The wing of a bat and the wing of a butterfly
  4. The eye of an octopus and the eye of a fish

Explanation

Homologous structures share a common evolutionary origin even when their functions differ. Whale forelimbs and human arms derive from the same ancestral forelimb skeleton, whereas the other pairs evolved independently.

Question 11Biological Science: Origin and evolution of life; classification of organisms

Which statement about the three-domain system is accurate?

  1. Fungi are classified within the domain Bacteria
  2. Archaea and Eukarya share features that Bacteria lack Correct answer.
  3. Bacteria and Archaea are more closely related to each other than either is to Eukarya
  4. All three domains contain membrane-bound organelles

Explanation

Molecular comparisons show Archaea and Eukarya are more closely related to each other than either is to Bacteria, and archaea share features such as histone-like proteins with eukaryotes. Bacteria and Archaea are both prokaryotic but distinct lineages.

Question 12Biological Science: Origin and evolution of life; classification of organisms

A virus is often excluded from standard biological classification of organisms because it lacks which property?

  1. Protein coat
  2. Ability to cause disease
  3. Genetic material
  4. Independent metabolism and reproduction Correct answer.

Explanation

Viruses carry nucleic acid and a protein coat but cannot metabolize or replicate without a host cell. This dependence on host machinery is why they are not classified as cellular organisms.

Question 13Biological Science: Origin and evolution of life; classification of organisms

Which observation would most strongly support the hypothesis that two similar-looking animal species are the result of convergent evolution rather than close common ancestry?

  1. They are the same size
  2. They occupy the same habitat
  3. They have similar diets
  4. Their DNA sequences differ substantially despite similar body form Correct answer.

Explanation

Convergent evolution produces similar traits in lineages that are not closely related, so large genetic differences alongside similar morphology support convergence. Shared habitat, diet, or size alone do not distinguish homology from analogy.

Question 14Biological Science: Origin and evolution of life; classification of organisms

In a phylogenetic tree, a clade is best defined as which of the following?

  1. Any two species that look alike
  2. An ancestor and all of its descendants Correct answer.
  3. A group defined by shared habitat
  4. A single species with no close relatives

Explanation

A clade is a monophyletic group consisting of a common ancestor plus all descendants of that ancestor. Appearance or habitat does not define clades; ancestry does.

Question 15Biological Science: Origin and evolution of life; classification of organisms

Which of the following is the most direct evidence that Earth's atmosphere lacked significant free oxygen early in its history?

  1. The abundance of modern forests
  2. The existence of fossilized fish
  3. The presence of banded iron formations Correct answer.
  4. The variety of flowering plants

Explanation

Banded iron formations formed when dissolved iron precipitated in oxygen-poor oceans, indicating low atmospheric oxygen. Later oxygenation changed the chemistry of iron deposition and is linked to such deposits.

Question 16Biological Science: Origin and evolution of life; classification of organisms

A population of plants becomes polyploid after a chromosome-doubling event and can no longer interbreed with the parent population. This is an example of which process?

  1. Behavioral isolation
  2. Sympatric speciation Correct answer.
  3. Temporal isolation
  4. Allopatric speciation

Explanation

Polyploidy can create instant reproductive isolation without geographic separation, which is a form of sympatric speciation. The chromosome doubling itself prevents fertile interbreeding with the diploid parent population.

Question 17Biological Science: Origin and evolution of life; classification of organisms

Which line of evidence most strongly indicates that mitochondria originated from an engulfed bacterium?

  1. Mitochondria are bounded by membranes
  2. Mitochondria have their own circular DNA and ribosomes Correct answer.
  3. Mitochondria are found in all eukaryotic cells
  4. Mitochondria produce ATP

Explanation

The endosymbiotic theory is supported by mitochondria possessing their own circular DNA and bacteria-like ribosomes, allowing them to synthesize some proteins independently. This reflects a prokaryotic ancestry rather than a purely eukaryotic origin.

Question 18Biological Science: Origin and evolution of life; classification of organisms

A gene is found in the same relative position on chromosomes of humans and mice but has diverged in sequence. What does this suggest about the gene?

  1. It arose independently in each species
  2. It is a vestigial structure
  3. It is homologous and inherited from a common ancestor Correct answer.
  4. It is a product of convergent evolution

Explanation

Conserved position and related sequence indicate that the gene was inherited from a shared ancestor and has diverged since the lineages split. Such homology is a strong signal of common descent.

Question 19Biological Science: Origin and evolution of life; classification of organisms

Which scenario best illustrates a postzygotic reproductive barrier?

  1. Two frog species breed in different ponds
  2. Two bird species have incompatible courtship songs
  3. Two plant species flower in different seasons
  4. Hybrid offspring of two species are sterile Correct answer.

Explanation

Postzygotic barriers act after fertilization, reducing the viability or fertility of hybrid offspring. Hybrid sterility prevents gene flow even when mating occurs.

Question 20Biological Science: Origin and evolution of life; classification of organisms

Which statement best describes the relationship between microevolution and macroevolution?

  1. Microevolution and macroevolution are unrelated processes
  2. Macroevolution is the accumulation of microevolutionary changes over long time spans Correct answer.
  3. Microevolution applies only to asexual organisms
  4. Macroevolution occurs only through sudden creation of new species in one generation

Explanation

Microevolution refers to changes in allele frequencies within populations, and macroevolution describes larger-scale patterns such as the origin of new taxa over long periods. The two are linked because accumulated population-level changes can produce major evolutionary divergence.

Question 21Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

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

  1. The mitochondrion Correct answer.
  2. The smooth endoplasmic reticulum
  3. The Golgi apparatus
  4. The lysosome

Explanation

The inner mitochondrial membrane carries the electron transport chain and ATP synthase, so oxidative phosphorylation occurs there. The other listed organelles handle modification and sorting of proteins, degradation, and lipid synthesis rather than bulk ATP production.

Question 22Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

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

  1. Water enters the cell and the cell swells
  2. The cell wall expands to resist the osmotic pressure
  3. Solutes move into the cell until concentrations equalize
  4. Water leaves the cell and the cell shrinks Correct answer.

Explanation

In a hypertonic solution the surrounding solute concentration exceeds that inside the cell, so water moves outward by osmosis and the cell loses volume. Animal cells lack a rigid wall, so they shrink rather than build turgor pressure.

Question 23Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

During which phase of the cell cycle is DNA replicated?

  1. G1 phase
  2. G2 phase
  3. M phase
  4. S phase Correct answer.

Explanation

DNA synthesis occurs during S phase, which lies between the two gap phases of interphase. G1 and G2 are periods of growth and preparation, while M phase is when chromosomes are segregated and the cell divides.

Question 24Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

Which statement correctly describes the outcome of meiosis in a diploid organism?

  1. Two diploid cells that are genetically identical to the parent cell
  2. Four diploid cells produced by a single round of chromosome separation
  3. Four haploid cells that are genetically distinct from one another Correct answer.
  4. Two haploid cells formed without any exchange of genetic material

Explanation

Meiosis has two divisions after one replication, yielding four haploid products. Crossing over in prophase I and independent assortment in metaphase I make those products genetically distinct.

Question 25Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

Which molecule serves as the immediate energy currency that drives most endergonic cellular reactions?

  1. ATP Correct answer.
  2. Glucose
  3. Pyruvate
  4. NADH

Explanation

Hydrolysis of ATP releases free energy that can be coupled to unfavorable reactions, making it the direct energy currency of the cell. NADH and glucose store energy but must first be processed to generate ATP.

Question 26Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

Which type of bond links the two strands of a DNA double helix?

  1. Peptide bonds between nucleotide bases
  2. Phosphodiester bonds between adjacent bases
  3. Hydrogen bonds between complementary bases Correct answer.
  4. Ionic bonds between the sugar-phosphate backbones

Explanation

Complementary base pairs are held together by hydrogen bonds, with A pairing with T and G pairing with C. Phosphodiester bonds instead link nucleotides into each strand's sugar-phosphate backbone.

Question 27Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

In glycolysis, one molecule of glucose is converted into two molecules of pyruvate. What is the net yield of ATP and NADH per glucose?

  1. 2 ATP and 4 NADH
  2. 2 ATP and 2 NADH Correct answer.
  3. 4 ATP and 2 NADH
  4. 4 ATP and 4 NADH

Explanation

Glycolysis consumes two ATP during the investment phase and produces four ATP during the payoff phase, for a net gain of two ATP. Two NAD+ are reduced to NADH when glyceraldehyde-3-phosphate is oxidized.

Question 28Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

A mutation changes a single DNA base but the encoded protein has the same amino acid sequence. What is this type of mutation called?

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

Explanation

Because the genetic code is degenerate, several codons specify the same amino acid, so a base change can leave the protein unchanged. Such a change is a silent mutation.

Question 29Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

Which process produces the greatest number of ATP molecules per glucose molecule in an aerobic eukaryotic cell?

  1. Glycolysis
  2. Lactic acid fermentation
  3. Oxidative phosphorylation Correct answer.
  4. The citric acid cycle

Explanation

Oxidative phosphorylation, driven by the proton gradient built by the electron transport chain, accounts for the large majority of ATP from glucose. Glycolysis and the citric acid cycle produce only small direct ATP yields and rely on NADH and FADH2 to feed the chain.

Question 30Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

Which feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Prokaryotic cells lack ribosomes
  2. Prokaryotic cells lack genetic material
  3. Prokaryotic cells lack a plasma membrane
  4. Prokaryotic cells lack a membrane-bound nucleus Correct answer.

Explanation

Prokaryotes keep their chromosome in a nucleoid region without a nuclear envelope, whereas eukaryotes enclose DNA in a nucleus. Prokaryotes still possess ribosomes, a plasma membrane, and DNA.

Question 31Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

A drug blocks the function of the mitotic spindle. At which point would a dividing cell most likely arrest?

  1. G1 phase, because DNA cannot be replicated
  2. Prophase, because chromosomes cannot condense
  3. Metaphase, because chromosomes cannot align and separate Correct answer.
  4. Telophase, because the nuclear envelope cannot reform

Explanation

Spindle microtubules attach to kinetochores and align chromosomes at the metaphase plate before pulling them apart. Without a functional spindle, the spindle assembly checkpoint halts the cell before anaphase.

Question 32Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

Which statement best describes the role of tRNA in translation?

  1. It forms the structural core of the ribosome and catalyzes peptide bond formation
  2. It removes introns from the mRNA transcript before translation
  3. It stores the genetic information that is copied into mRNA
  4. It carries amino acids to the ribosome and pairs its anticodon with the mRNA codon Correct answer.

Explanation

Each tRNA is charged with a specific amino acid and has an anticodon that base-pairs with a complementary codon on mRNA. This pairing places the correct amino acid in position for incorporation into the growing polypeptide.

Question 33Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

In the light-dependent reactions of photosynthesis, what is the main source of the electrons that reduce NADP+?

  1. Glucose
  2. Carbon dioxide
  3. Water Correct answer.
  4. ATP

Explanation

Photosystem II splits water to replace the electrons it loses to the electron transport chain, releasing oxygen and supplying electrons that ultimately reduce NADP+. Carbon dioxide is fixed later, in the Calvin cycle.

Question 34Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

A cell is treated with a drug that inhibits DNA ligase. Which process would be most directly impaired?

  1. Proofreading newly added nucleotides for errors
  2. Joining Okazaki fragments on the lagging strand during replication Correct answer.
  3. Unwinding the double helix ahead of the replication fork
  4. Synthesizing RNA primers on the template strand

Explanation

Ligase seals the nicks between Okazaki fragments once primers are removed and gaps are filled, completing the lagging strand. Without it, replication produces fragmented DNA.

Question 35Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

Which of the following best describes the relationship between the Calvin cycle and the light-dependent reactions?

  1. The Calvin cycle occurs in the thylakoid membrane and supplies oxygen to the light reactions
  2. The two processes are independent and neither supplies products to the other
  3. The light reactions supply ATP and NADPH that the Calvin cycle uses to fix carbon Correct answer.
  4. The Calvin cycle supplies ATP and NADPH that the light reactions use to split water

Explanation

The light reactions convert light energy into ATP and NADPH, which are consumed by the Calvin cycle to reduce carbon dioxide into sugar. The two stages are coupled by these energy carriers.

Question 36Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

Which structural feature of the plasma membrane is described by the fluid mosaic model?

  1. A phospholipid bilayer with proteins that move laterally within it Correct answer.
  2. A protein lattice that excludes all lipid molecules
  3. A continuous lipid monolayer with fixed protein channels
  4. A rigid layer of cellulose enclosing the cytoplasm

Explanation

The fluid mosaic model describes a dynamic phospholipid bilayer studded with proteins that can move laterally. This arrangement allows membrane fluidity and selective permeability.

Question 37Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

A culture of cells is supplied with a nucleotide analog that terminates chain elongation when incorporated. Which biosynthetic process would be halted most directly?

  1. Movement of ions through channel proteins
  2. Translation of mRNA into protein
  3. Formation of the lipid bilayer
  4. DNA replication Correct answer.

Explanation

Chain-terminating nucleotide analogs are incorporated by DNA polymerase and prevent further nucleotide addition, blocking replication. Processes that do not involve nucleotide polymerization are unaffected.

Question 38Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

An enzyme is added to a reaction mixture and the reaction rate increases. Which explanation is most consistent with enzyme function?

  1. The enzyme raises the activation energy so reactant collisions are more productive
  2. The enzyme lowers the activation energy required for the reaction Correct answer.
  3. The enzyme increases the free energy released by the reaction
  4. The enzyme changes the equilibrium constant of the reaction

Explanation

Enzymes stabilize the transition state and thereby reduce the activation energy, accelerating the reaction without altering its overall free-energy change. They do not change the equilibrium position of a reaction.

Question 39Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

A mutation in a gene encoding a DNA repair enzyme causes an increased rate of mutations in subsequent generations of cells. Which explanation best accounts for this observation?

  1. The mutation prevents chromosome condensation during mitosis
  2. The mutation increases the fidelity of DNA polymerase
  3. The repair enzyme directly proofreads mRNA during translation
  4. Unrepaired DNA lesions are copied during replication and become permanent mutations Correct answer.

Explanation

When repair pathways fail, damaged bases persist and are misread or bypassed by DNA polymerase during replication, converting lesions into heritable mutations. This produces a mutator phenotype that accumulates changes over generations.

Question 40Biological Science: Cell organization, cell division, chemical nature of the gene, bioenergetics, and biosynthesis

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

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

Explanation

Secretory proteins enter the rough ER lumen, are transported to the Golgi for modification and sorting, and are then packaged into vesicles that fuse with the plasma membrane for release. This is the standard secretory pathway.

Question 41Biological Science: Structure, function, and development in organisms; patterns of heredity

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

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

Explanation

The inner mitochondrial membrane carries the electron transport chain and ATP synthase, so oxidative phosphorylation occurs there. The other organelles listed handle protein modification, protein synthesis, and hydrolysis of macromolecules rather than bulk ATP synthesis.

Question 42Biological Science: Structure, function, and development in organisms; patterns of heredity

A human somatic cell in G1 phase contains 46 chromosomes. Immediately after the S phase of the same cell cycle, how many DNA molecules does the nucleus contain?

  1. 23
  2. 46
  3. 138
  4. 92 Correct answer.

Explanation

During S phase each chromosome is replicated into two sister chromatids, so the 46 chromosomes become 92 DNA molecules while the chromosome count stays 46. The DNA content doubles even though the number of centromeres is unchanged.

Question 43Biological Science: Structure, function, and development in organisms; patterns of heredity

In a dihybrid cross between two individuals heterozygous for both unlinked genes (AaBb × AaBb), what fraction of offspring is expected to show both recessive phenotypes?

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

Explanation

Independent assortment gives a 9:3:3:1 phenotypic ratio, and the double-recessive class occupies one part of sixteen. The probability is the product of the two independent single-gene recessive probabilities, 1/4 × 1/4.

Question 44Biological Science: Structure, function, and development in organisms; patterns of heredity

Which process directly produces the genetic variation that natural selection acts upon in sexually reproducing populations?

  1. Mitotic cell division in somatic tissue
  2. Cytoplasmic streaming during interphase
  3. Meiosis with crossing over and independent assortment Correct answer.
  4. DNA replication during S phase

Explanation

Crossing over in prophase I and independent assortment in metaphase I shuffle alleles into new combinations of gametes. Mutation is the ultimate source of new alleles, but the recombination of existing alleles in meiosis is the direct generator of the variation described.

Question 45Biological Science: Structure, function, and development in organisms; patterns of heredity

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 of this couple is affected?

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

Explanation

A son inherits his X chromosome from his mother, and a carrier mother transmits the mutant allele to half of her sons. Therefore each son has a 1/2 chance of being affected regardless of the father's genotype.

Question 46Biological Science: Structure, function, and development in organisms; patterns of heredity

Which structure in a flowering plant develops into the seed after fertilization?

  1. Receptacle
  2. Ovary wall
  3. Ovule Correct answer.
  4. Anther

Explanation

The ovule contains the egg and, after double fertilization, develops into the seed with its embryo and endosperm. The ovary wall becomes the fruit tissue surrounding the seed.

Question 47Biological Science: Structure, function, and development in organisms; patterns of heredity

Water and dissolved minerals move upward through the xylem of a tall tree primarily because of which mechanism?

  1. Pressure flow driven by sugar loading in roots
  2. Active transport by companion cells in the phloem
  3. Osmotic uptake by root hair cell walls
  4. Transpiration pull generated by evaporation from leaves Correct answer.

Explanation

Evaporation from stomata creates tension in the continuous water column of the xylem, and cohesion between water molecules transmits that pull up the stem. This transpiration-cohesion mechanism can lift water far above the height that root pressure alone could support.

Question 48Biological Science: Structure, function, and development in organisms; patterns of heredity

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

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

Explanation

Capillary walls are a single endothelial cell layer thick, which minimizes diffusion distance for exchange. Arteries and veins have thicker, layered walls suited to conduction, and arterioles regulate flow into capillary beds.

Question 49Biological Science: Structure, function, and development in organisms; patterns of heredity

A person hyperventilates, lowering blood carbon dioxide. Which immediate change in blood chemistry is expected?

  1. A rise in pH Correct answer.
  2. A rise in oxygen-carrying capacity of hemoglobin
  3. A fall in pH
  4. A rise in bicarbonate concentration

Explanation

Carbon dioxide combines with water to form carbonic acid, so removing CO2 by hyperventilation reduces hydrogen ion concentration and raises pH. This produces respiratory alkalosis.

Question 50Biological Science: Structure, function, and development in organisms; patterns of heredity

Which hormone is released by the posterior pituitary?

  1. Thyroid-stimulating hormone
  2. Prolactin
  3. Adrenocorticotropic hormone
  4. Antidiuretic hormone Correct answer.

Explanation

The posterior pituitary stores and releases antidiuretic hormone and oxytocin, which are synthesized in the hypothalamus. The other listed hormones are produced and secreted by the anterior pituitary.