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IB Biology HL practice questions

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Question 1Unity and Diversity

Which feature is shared by all members of the domain Archaea?

  1. Absence of ribosomes in the cytoplasm
  2. Absence of membrane-bound nuclei Correct answer.
  3. Presence of peptidoglycan in the cell wall
  4. Presence of linear chromosomes with histones

Explanation

Archaea are prokaryotes, so their genetic material is not enclosed by a nuclear envelope. They also lack peptidoglycan and typically carry circular chromosomes with histone-like proteins. Ribosomes are present in every living cell, so their absence cannot define a group.

Question 2Unity and Diversity

A newly discovered organism is unicellular, has a cell wall of cellulose, and carries out photosynthesis releasing oxygen. To which group does it most likely belong?

  1. Archaea
  2. Cyanobacteria
  3. Fungi
  4. Green algae Correct answer.

Explanation

Cellulose walls combined with oxygen-releasing photosynthesis point to a eukaryotic alga rather than a prokaryote. Cyanobacteria photosynthesise but have peptidoglycan walls and no cellulose. Fungi are heterotrophs, and archaea do not perform oxygenic photosynthesis.

Question 3Unity and Diversity

Which statement best describes the role of horizontal gene transfer in prokaryote evolution?

  1. It removes plasmids from a population under antibiotic stress.
  2. It moves genes between organisms of the same generation without reproduction. Correct answer.
  3. It doubles the chromosome number before every cell division.
  4. It converts circular chromosomes into linear ones during binary fission.

Explanation

Horizontal gene transfer is the movement of genetic material between organisms other than by descent, for example by conjugation, transformation or transduction. It allows traits such as antibiotic resistance to spread rapidly within a population. It is not tied to chromosome doubling or to plasmid loss.

Question 4Unity and Diversity

In a cladistic classification, which criterion is used to place organisms in the same group?

  1. Overall similarity of body form regardless of ancestry
  2. Shared derived characters inherited from a common ancestor Correct answer.
  3. Similarity of ecological niche in the same habitat
  4. Ability to interbreed and produce fertile offspring

Explanation

Cladistics groups organisms by synapomorphies, which are derived traits shared through common descent. Only monophyletic groups, containing an ancestor and all its descendants, are recognised. Convergent similarity or shared habitat can mislead and is not the basis of cladistic grouping.

Question 5Unity and Diversity

Two populations of the same bird species are separated by a mountain range for thousands of years. Which process most directly initiates speciation in this situation?

  1. Reproductive isolation evolves between the populations. Correct answer.
  2. Both populations merge into a single gene pool.
  3. Gene flow between the populations increases.
  4. The mutation rate of both populations falls to zero.

Explanation

Geographic separation reduces gene flow, allowing the populations to diverge genetically through mutation, drift and selection. Speciation occurs when reproductive isolation, such as incompatible mating behaviour, prevents interbreeding. Increased gene flow would instead oppose divergence.

Question 6Unity and Diversity

Which observation provides the strongest evidence that all known living organisms share a common origin?

  1. All organisms are composed of cells bounded by membranes.
  2. All organisms use the same genetic code for amino acid sequences. Correct answer.
  3. All organisms reproduce by forming gametes.
  4. All organisms obtain energy by aerobic respiration.

Explanation

The near-universal genetic code links organisms across all three domains, which is difficult to explain except by common ancestry. Cell theory is also universal, but the code gives direct molecular evidence of shared inheritance. Many organisms reproduce asexually or live anaerobically, so those options fail.

Question 7Unity and Diversity

What distinguishes eubacteria from archaea in terms of membrane lipids?

  1. Eubacteria lack membranes entirely, whereas archaea possess them.
  2. Eubacteria have ester-linked fatty acids; archaea have ether-linked isoprenoid chains. Correct answer.
  3. Eubacteria have ether-linked isoprenoid chains; archaea have ester-linked fatty acids.
  4. Both groups have identical membrane lipids but differ in ribosome size.

Explanation

Bacterial membrane lipids use ester bonds between glycerol and fatty acids, while archaeal lipids use ether bonds to branched isoprenoid chains. This difference affects membrane stability at extreme temperatures. Ribosome size does not distinguish the two groups, and both have membranes.

Question 8Unity and Diversity

A virus is considered non-living by most biologists mainly because it

  1. contains nucleic acid and protein.
  2. has a capsid surrounding its genome.
  3. cannot reproduce independently of a host cell. Correct answer.
  4. can cause disease in many organisms.

Explanation

Living things are generally defined by their ability to reproduce and metabolise independently. Viruses lack their own metabolism and can replicate only inside a host cell, so they are often classed as non-living. Having nucleic acid, protein or pathogenic effects does not satisfy the criteria for life.

Question 9Unity and Diversity

Which characteristic is used to place an organism in the kingdom Plantae rather than in the protist groups?

  1. Being multicellular and heterotrophic with chitin walls
  2. Being unicellular and heterotrophic with no cell wall
  3. Being multicellular and photoautotrophic with cellulose walls Correct answer.
  4. Being prokaryotic and capable of nitrogen fixation

Explanation

Plants are multicellular photoautotrophs with cellulose cell walls and are typically terrestrial. Unicellular photosynthetic forms such as Euglena are placed among the protists. Heterotrophic forms with chitin walls are fungi, and prokaryotes belong to the domains Bacteria or Archaea.

Question 10Unity and Diversity

A population of insects colonises an island and, after many generations, becomes reproductively isolated from the mainland population. Which term best describes this outcome?

  1. Convergent evolution
  2. Sympatric speciation
  3. Allopatric speciation Correct answer.
  4. Polyploidy

Explanation

Speciation that follows geographic separation of populations is allopatric. Sympatric speciation occurs without geographic isolation, for example through polyploidy in plants. Convergent evolution describes similar traits arising in unrelated lineages, not the formation of a new species.

Question 11Unity and Diversity

Which feature of the archaeal ribosome supports the hypothesis that archaea and eukaryotes are more closely related to each other than either is to bacteria?

  1. Archaeal ribosomal proteins and RNA sequences resemble eukaryotic ones. Correct answer.
  2. Archaeal ribosomes lack RNA entirely.
  3. Archaeal ribosomes are 70S, like bacterial ones.
  4. Archaeal ribosomes are 80S and membrane-bound.

Explanation

Molecular comparisons of ribosomal RNA and protein sequences place archaea and eukaryotes on a shared branch of the tree of life. Although archaeal ribosomes are 70S like bacterial ones, their molecular composition is more eukaryote-like. All ribosomes contain rRNA, and 80S ribosomes are characteristic of eukaryotes.

Question 12Unity and Diversity

Which type of evidence would most reliably resolve the evolutionary relationships among three closely related mammal species?

  1. Comparison of sequences of a conserved nuclear gene Correct answer.
  2. Comparison of homologous bone structure in the forelimb
  3. Comparison of body size and coat colour
  4. Comparison of the habitats in which each species lives

Explanation

DNA sequence comparisons provide many independent characters whose differences accumulate with time since divergence. They are more informative and less subject to convergence than body form or habitat. Skeletal homology is useful but offers fewer characters for close relatives.

Question 13Unity and Diversity

Which statement about the three-domain system is correct?

  1. It groups archaea with bacteria because both are prokaryotic.
  2. It recognises Bacteria, Archaea and Eukarya as domains. Correct answer.
  3. It classifies viruses as a fourth domain.
  4. It places all prokaryotes in the single kingdom Monera.

Explanation

The three-domain system, based largely on rRNA comparisons, divides life into Bacteria, Archaea and Eukarya. It replaced the earlier two-kingdom prokaryote grouping in which all prokaryotes were placed together. Viruses are not included because they are not considered cellular organisms.

Question 14Unity and Diversity

A bacterium acquires a gene for antibiotic resistance from a plasmid carried by another species. Which process does this describe?

  1. Osmosis
  2. Conjugation Correct answer.
  3. Transcription
  4. Binary fission

Explanation

Conjugation is the direct transfer of DNA, often a plasmid, between bacterial cells through a cytoplasmic connection. It is a form of horizontal gene transfer and can spread resistance genes between species. Binary fission is reproduction, while transcription and osmosis do not move DNA between cells.

Question 15Unity and Diversity

Which pair of organisms shows analogous rather than homologous structures?

  1. The scales of a reptile and the feathers of a bird
  2. The wings of a bird and the wings of an insect Correct answer.
  3. The forelimbs of a human and a bat
  4. The leaves of a fern and the leaves of a moss

Explanation

Bird and insect wings perform the same function but arise from different embryonic origins, so they are analogous and reflect convergent evolution. Human and bat forelimbs share a common skeletal plan, making them homologous. Fern and moss leaves are also derived from a shared ancestral structure.

Question 16Unity and Diversity

Which process increases genetic variation within a prokaryotic population most rapidly?

  1. Spontaneous mutation during DNA replication
  2. Growth of the population by binary fission
  3. Excision of a prophage from the chromosome
  4. Horizontal transfer of genes by transduction Correct answer.

Explanation

Transduction introduces whole genes from another cell, giving an immediate and substantial gain in variation. Mutation is important but occurs at a low rate per gene per generation. Binary fission produces clones, and prophage excision does not itself add new genes to the population.

Question 17Unity and Diversity

Which feature of the kingdom Fungi distinguishes it from the kingdom Plantae?

  1. Fungi are heterotrophic and have chitinous cell walls. Correct answer.
  2. Fungi are unicellular and lack a cell wall.
  3. Fungi are prokaryotic and lack membrane-bound organelles.
  4. Fungi are photoautotrophic and have cellulose walls.

Explanation

Fungi obtain organic nutrients by absorption and have cell walls made of chitin, whereas plants are photoautotrophs with cellulose walls. Fungi are eukaryotic and may be unicellular or multicellular. These differences place the two groups in separate kingdoms.

Question 18Unity and Diversity

Two species of frog live in the same pond but breed in different months of the year. Which type of isolating mechanism prevents interbreeding?

  1. Hybrid sterility
  2. Temporal isolation Correct answer.
  3. Mechanical isolation
  4. Geographic isolation

Explanation

Temporal isolation arises when species breed at different times, so gametes never meet. Geographic isolation involves physical separation of populations, and mechanical isolation involves incompatible reproductive structures. Hybrid sterility acts after fertilisation rather than preventing mating.

Question 19Unity and Diversity

Which statement about the classification of protists is correct?

  1. They are all unicellular and photoautotrophic.
  2. They are all prokaryotic and lack nuclei.
  3. They are a paraphyletic or polyphyletic assemblage of eukaryotic groups. Correct answer.
  4. They form a single monophyletic kingdom.

Explanation

Molecular data show that the organisms traditionally called protists do not share a single common ancestor exclusive to themselves. They form a convenience group scattered across several eukaryotic lineages. Protists include heterotrophs and unicellular forms but are not defined by those traits.

Question 20Unity and Diversity

A student compares the cytochrome c amino acid sequences of several species and finds fewer differences between species X and Y than between X and Z. What is the best interpretation?

  1. X and Y diverged from a common ancestor more recently than X and Z. Correct answer.
  2. X and Y live in more similar habitats than X and Z.
  3. Cytochrome c is not useful for evolutionary comparisons.
  4. X and Z are members of the same species.

Explanation

Amino acid differences in a conserved protein accumulate with time since divergence, so fewer differences indicate a more recent common ancestor. Habitat similarity does not directly determine sequence similarity. A larger difference between X and Z suggests greater divergence, not identity.

Question 21Unity and Diversity

Which feature is characteristic of the archaeal cell wall?

  1. It is absent in every archaeal species.
  2. It consists of cellulose microfibrils.
  3. It may contain pseudopeptidoglycan with N-acetyltalosaminuronic acid. Correct answer.
  4. It always contains peptidoglycan with N-acetylmuramic acid.

Explanation

Many archaea have pseudopeptidoglycan, which resembles peptidoglycan but contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid. Some archaea lack walls entirely, so absence is not universal. Cellulose walls occur in plants and some algae.

Question 22Unity and Diversity

Which comparison of archaea and bacteria is correct?

  1. Both are prokaryotic, but only bacteria possess membrane-bound organelles.
  2. Both are prokaryotic, but only archaea possess membrane-bound organelles.
  3. Both possess membrane-bound nuclei but differ in cell wall composition.
  4. Neither group possesses membrane-bound organelles. Correct answer.

Explanation

Neither bacteria nor archaea have membrane-bound organelles or a nuclear envelope; their genetic material lies free in the cytoplasm. They differ in cell wall chemistry and membrane lipid bonding, but not in the presence of organelles. A nuclear envelope is a eukaryotic feature.

Question 23Unity and Diversity

Which of the following best explains why analogous structures are misleading in classification?

  1. They arise from a recent common ancestor and reveal true relationships.
  2. They occur only in organisms that interbreed.
  3. They evolve independently in unrelated lineages under similar selection. Correct answer.
  4. They are always vestigial and carry no function.

Explanation

Analogous structures perform similar functions but evolved independently in response to similar selection pressures. Grouping by such traits would falsely unite unrelated organisms. Homologous structures, by contrast, indicate shared ancestry and are the basis of cladistic classification.

Question 24Unity and Diversity

A new organism is discovered that is multicellular, has cells with peptidoglycan-free walls, and lacks a nucleus. Which conclusion is most consistent with these observations?

  1. It is a virus because it has no nucleus.
  2. It is a eukaryote because it is multicellular.
  3. It belongs to the domain Bacteria because it is prokaryotic.
  4. It is an archaeon because it lacks a nucleus and peptidoglycan. Correct answer.

Explanation

Absence of a nucleus identifies the organism as prokaryotic, and the absence of peptidoglycan points towards archaea rather than bacteria. Some archaea form multicellular aggregates, so multicellularity alone does not make it eukaryotic. Viruses are not cellular and do not fit the description.

Question 25Unity and Diversity

Which statement about the biological species concept is correct?

  1. It applies equally well to asexual prokaryotes and to sexual eukaryotes.
  2. It defines species by their ability to interbreed and produce fertile offspring. Correct answer.
  3. It defines species solely by the number of chromosomes they possess.
  4. It defines species by the geographic region in which they live.

Explanation

The biological species concept groups organisms that can interbreed to produce fertile offspring and are reproductively isolated from other groups. It applies poorly to asexual organisms such as bacteria, which do not interbreed. Chromosome number or location alone does not define a species.

Question 26Unity and Diversity

Which feature of the binomial naming system, as used in Linnaean classification, identifies the genus of an organism?

  1. The full two-part name written in italics
  2. The second word of the two-part name, always capitalised
  3. The first word of the two-part name, always capitalised Correct answer.
  4. The name of the family to which the organism belongs

Explanation

In binomial nomenclature the first word is the genus name and begins with a capital letter, while the second word is the species epithet and is lower case. Both words together form the species name, so neither the family nor the italic formatting identifies the genus by itself.

Question 27Unity and Diversity

A dichotomous key separates four shells. Shell 1 is coiled and pointed; shell 2 is coiled and rounded; shell 3 is uncoiled and pointed; shell 4 is uncoiled and rounded. Which pair of features is used at the first division of the key?

  1. Uncoiled versus rounded
  2. Pointed versus rounded
  3. Coiled versus pointed
  4. Coiled versus uncoiled Correct answer.

Explanation

A dichotomous key splits the group using one feature at a time into two mutually exclusive categories. Coiled versus uncoiled divides all four shells into two groups of two, allowing the second feature to separate each pair. The other pairings mix the two features and would not give a clean split.

Question 28Unity and Diversity

Which statement describes the cell type found in all members of the kingdom Monera as traditionally defined?

  1. Prokaryotic cells containing histone-bound linear chromosomes
  2. Eukaryotic cells with a nucleus but no cell wall
  3. Eukaryotic cells with chitinous walls
  4. Prokaryotic cells lacking a membrane-bound nucleus Correct answer.

Explanation

The traditional kingdom Monera groups bacteria, which are prokaryotes: their genetic material is not enclosed by a nuclear envelope. Eukaryotic organisation with a nucleus defines the other kingdoms, and prokaryotes typically have circular DNA without histones.

Question 29Unity and Diversity

A sample of pond water contains an organism that is unicellular, photosynthetic, has a cellulose cell wall and a nucleus. To which group does it most likely belong?

  1. A cyanobacterium
  2. An archaeon
  3. A photosynthetic protist such as a euglenoid or diatom Correct answer.
  4. A fungus

Explanation

A unicellular, photosynthetic eukaryote with a cellulose wall fits the informal protist groups, which include many algal forms. Cyanobacteria and archaea are prokaryotes without a nucleus, and fungi are heterotrophic with chitinous walls.

Question 30Unity and Diversity

Which feature is shared by all viruses?

  1. A phospholipid bilayer envelope
  2. Ribosomes for protein synthesis
  3. Independent metabolism generating ATP
  4. A nucleic acid genome enclosed in a protein capsid Correct answer.

Explanation

All viruses consist of a nucleic acid genome packaged within a protein coat called a capsid. They lack ribosomes and metabolism of their own and must use a host cell, and only some viruses have an outer envelope.

Question 31Unity and Diversity

Which characteristic separates members of the kingdom Animalia from members of the kingdom Plantae?

  1. Animal cells contain a nucleus while plant cells do not
  2. Animal cells contain mitochondria while plant cells do not
  3. Animals are multicellular while plants are unicellular
  4. Animal cells lack a cell wall and animals are heterotrophic Correct answer.

Explanation

Animal cells are bounded only by a plasma membrane and animals obtain carbon by ingesting organic matter, whereas plant cells have cellulose walls and are photosynthetic autotrophs. Both groups are multicellular eukaryotes with mitochondria and nuclei.

Question 32Unity and Diversity

A researcher compares the DNA base sequences of the gene for a respiratory enzyme in four species. Species W differs at 2 sites from X, at 9 sites from Y and at 14 sites from Z. Which conclusion is best supported?

  1. W and X share the most recent common ancestor of the pairs compared Correct answer.
  2. W and Z are the most closely related pair
  3. The gene is not useful for comparing these species
  4. Y and Z are more closely related than W and X

Explanation

Fewer sequence differences imply a shorter time since divergence, so W and X are the most closely related pair among those compared. The number of differences increases in the order X, Y, Z, indicating progressively more distant relationships.

Question 33Unity and Diversity

Two organisms are classified in the same class but in different orders. Which statement must be true?

  1. They belong to the same family but not the same genus
  2. They belong to different phyla
  3. They belong to the same species
  4. They belong to the same phylum but not necessarily the same family Correct answer.

Explanation

Class is a higher rank than order and family, so organisms in the same class share the phylum above it but may be placed in different orders and families. They cannot be assumed to share a family, and they certainly need not be the same species.

Question 34Unity and Diversity

Which observation about the cell wall of a prokaryote would support placing it in the domain Bacteria rather than the domain Archaea?

  1. The wall is composed of pseudopeptidoglycan
  2. The wall is absent and the cell is bounded only by a membrane
  3. The wall is made of cellulose microfibrils
  4. The wall contains peptidoglycan (murein) Correct answer.

Explanation

Peptidoglycan is a defining component of bacterial cell walls and is not found in archaea, whose walls may contain pseudopeptidoglycan, protein or other polymers. Cellulose is a plant feature, and wall absence does not distinguish the two domains.

Question 35Unity and Diversity

A population of bacteria is exposed to a antibiotic. A few cells survive because they already carry a resistance gene, and these reproduce while the rest die. Which statement best describes this outcome?

  1. The antibiotic acted as a selective pressure on pre-existing variation Correct answer.
  2. The surviving cells acquired resistance by learning to degrade the drug
  3. The antibiotic induced new resistance mutations in the surviving cells
  4. The population as a whole gradually became resistant through use and disuse

Explanation

Variation in resistance exists before exposure, and the antibiotic changes the frequency of alleles by favouring individuals already carrying resistance genes. Selection acts on existing variation rather than creating directed mutations, and acquired characteristics are not inherited in this way.

Question 36Unity and Diversity

Which feature of the kingdom Fungi is directly related to their heterotrophic mode of nutrition?

  1. They possess cellulose cell walls
  2. They secrete digestive enzymes into their surroundings and absorb the products Correct answer.
  3. They contain chloroplasts for photosynthesis
  4. They store carbohydrate as starch in their cells

Explanation

Fungi are absorptive heterotrophs: hyphae release hydrolytic enzymes and take up the soluble products. They lack chloroplasts and store glycogen rather than starch, and their walls contain chitin.

Question 37Unity and Diversity

Two populations of a plant species are separated by a mountain range. After many generations they can no longer interbreed to produce fertile offspring. Which term best describes the barrier that initially allowed this divergence?

  1. Mechanical isolation
  2. Temporal isolation
  3. Behavioural isolation
  4. Geographic isolation Correct answer.

Explanation

A physical barrier such as a mountain range prevents gene flow between populations, allowing independent evolution and eventually reproductive isolation. The other mechanisms act without a geographic barrier, through timing, courtship or structural incompatibility.

Question 38Unity and Diversity

Which observation would provide evidence that two structures are homologous rather than analogous?

  1. The structures have a similar underlying arrangement of bones and similar embryonic origin Correct answer.
  2. The structures carry out the same function in both species
  3. The structures appear in unrelated lineages that convergently occupy similar niches
  4. The structures are found in organisms living in the same habitat

Explanation

Homologous structures share common ancestry, which is revealed by similar internal structure and embryonic development even when functions differ. Analogous structures share function but not ancestry, as produced by convergent evolution.

Question 39Unity and Diversity

Which statement correctly compares the domains Bacteria and Archaea?

  1. Both have peptidoglycan walls, but only archaea have circular DNA
  2. Both have membrane-bound nuclei, but only archaea have ribosomes
  3. Only bacteria are prokaryotes, since archaea have nuclei
  4. Both lack a membrane-bound nucleus, but only bacteria have peptidoglycan in their walls Correct answer.

Explanation

Members of both domains are prokaryotes without a nuclear envelope, but peptidoglycan is found in bacterial walls and is absent from archaeal walls. Both have ribosomes and typically circular DNA.

Question 40Unity and Diversity

A new unicellular organism is found in a hot spring. It has no nucleus, its membrane lipids contain ether linkages and its cell wall lacks peptidoglycan. To which domain does it most likely belong?

  1. Viruses
  2. Archaea Correct answer.
  3. Eukarya
  4. Bacteria

Explanation

Ether-linked membrane lipids, absence of peptidoglycan and a prokaryotic cell organisation together point to the domain Archaea, many of which inhabit extreme environments. Bacteria have ester-linked lipids and often peptidoglycan, while eukaryotes have nuclei and viruses are not cells.

Question 41Unity and Diversity

Which type of evidence is most useful for constructing a phylogenetic tree when morphological features have been lost or modified in several lineages?

  1. Comparison of habitat preferences
  2. Comparison of feeding habits
  3. Comparison of adult body size
  4. Comparison of conserved gene sequences Correct answer.

Explanation

Conserved sequences such as ribosomal RNA genes change slowly and provide many comparable characters that are less affected by morphological adaptation. Size, feeding and habitat are ecological traits that can change rapidly and converge independently.

Question 42Unity and Diversity

A fungus-like organism is discovered. Which feature would place it in the kingdom Fungi rather than among the protists?

  1. Presence of a nucleus and mitochondria
  2. Cells with cellulose walls and nutrition by photosynthesis
  3. Unicellular organisation with two flagella
  4. Cells with chitinous walls and nutrition by absorption Correct answer.

Explanation

Chitinous walls combined with absorptive heterotrophy are defining features of the true fungi. Cellulose walls with photosynthesis indicate a plant or algal protist, and a nucleus with mitochondria is shared by all eukaryotes.

Question 43Unity and Diversity

In a cladistic analysis, a shared derived character is used to define a clade. Which statement best explains why shared ancestral characters are less useful for this purpose?

  1. They always arise by convergent evolution
  2. They are only found in prokaryotes
  3. They are inherited from a distant ancestor and are shared by groups outside the clade Correct answer.
  4. They cannot be observed in fossils

Explanation

Ancestral characters are present in the common ancestor of a much larger group, so they do not distinguish the members of a particular clade from their relatives. Derived characters, by contrast, arose within the lineage being studied and mark its members.

Question 44Unity and Diversity

In a population of flowering plants, a mutation produces flowers that open a week earlier than those of the rest of the population. Over time, the early-flowering individuals mate only with each other. Which sequence best describes the likely outcome?

  1. Behavioural isolation leads to reduced gene flow and eventually to speciation
  2. Temporal isolation leads to reduced gene flow and eventually to speciation Correct answer.
  3. Mechanical isolation leads to reduced gene flow and eventually to speciation
  4. Geographic isolation leads to reduced gene flow and eventually to speciation

Explanation

Differences in flowering time are a form of temporal isolation, since the two groups are reproductively active at different times and cannot exchange gametes. Reduced gene flow then allows the groups to diverge until they become separate species.

Question 45Unity and Diversity

A phylogenetic tree shows that species P and Q are sister taxa, while species R branches off earlier. Which statement is correct?

  1. P is more closely related to R than to Q because R is drawn lower on the tree
  2. P and Q are more closely related to R than to each other
  3. The tree shows that P evolved directly from Q
  4. P and Q share a more recent common ancestor with each other than either does with R Correct answer.

Explanation

Relatedness in a phylogenetic tree is determined by the recency of common ancestry, not by vertical position or by one species giving rise to another. Sister taxa P and Q share an ancestor that is not shared with R, so they are more closely related to each other.

Question 46Unity and Diversity

Two enzymes in related species catalyse the same reaction but differ at several amino acid positions. Which explanation best accounts for this observation?

  1. The differences show that the species belong to different domains
  2. The enzyme in one species evolved the same sequence independently by convergence
  3. Neutral mutations accumulated after the species diverged, so fewer differences indicate closer relationship Correct answer.
  4. The differences prove the enzymes are not homologous

Explanation

After lineages diverge, mutations accumulate independently, so the number of differences in a conserved protein reflects the time since divergence. Convergence at many sites is unlikely, and the shared function and structure indicate homology rather than its absence.

Question 47Form and Function

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

  1. The rough endoplasmic reticulum
  2. The inner mitochondrial membrane Correct answer.
  3. The nuclear envelope
  4. The Golgi apparatus

Explanation

The electron transport chain and ATP synthase are embedded in the inner mitochondrial membrane, whose folded cristae increase surface area for chemiosmotic ATP production. The other listed structures are involved in protein processing, modification, or compartmentalisation, not oxidative phosphorylation.

Question 48Form and Function

A student examines a tissue and observes cells with a large central vacuole, cellulose cell walls and chloroplasts. Which conclusion is best supported?

  1. The tissue is plant tissue Correct answer.
  2. The tissue is prokaryotic
  3. The tissue is fungal tissue
  4. The tissue is animal epithelium

Explanation

Cellulose walls, chloroplasts and a large permanent central vacuole are characteristic features of plant cells. Fungi have chitin walls and no chloroplasts, animal cells lack walls and chloroplasts, and prokaryotes lack membrane-bound organelles such as chloroplasts.

Question 49Form and Function

Which property of the phospholipid bilayer directly explains why small nonpolar molecules cross membranes more readily than small ions?

  1. The hydrophobic core of the bilayer Correct answer.
  2. The fluidity of the fatty acid tails
  3. The presence of peripheral proteins
  4. The asymmetry of the two leaflets

Explanation

The interior of the bilayer is hydrophobic because the fatty acid tails face inward, so nonpolar molecules dissolve into and diffuse through it. Charged ions are repelled by this hydrophobic core and require channels or carriers.

Question 50Form and Function

During which phase of the cell cycle does DNA replication occur?

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

Explanation

S phase is the synthesis phase of interphase, when each chromosome is replicated to form two sister chromatids. G1 and G2 are gap phases of growth and preparation, while M phase is when mitosis and cytokinesis occur.