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IB Biology SL practice questions

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Question 1Unity and Diversity

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

  1. They have circular DNA not enclosed in a nuclear membrane. Correct answer.
  2. They possess membrane-bound organelles.
  3. They contain peptidoglycan in their cell walls.
  4. They reproduce by mitosis.

Explanation

Bacteria are prokaryotes, so their genetic material is a circular chromosome lying free in the cytoplasm without a nuclear envelope. This absence of a nucleus is the defining prokaryotic trait shared across the domain, distinguishing them from eukaryotes.

Question 2Unity and Diversity

A student observes an organism with cells containing a nucleus, mitochondria and a cell wall made of cellulose. To which group does the organism most likely belong?

  1. Plants Correct answer.
  2. Archaea
  3. Protozoa
  4. Fungi

Explanation

Cellulose cell walls together with a nucleus and mitochondria are characteristic of plant cells. Fungi have chitin walls, and protozoa generally lack a rigid cell wall, so the combination of features points to plants.

Question 3Unity and Diversity

Which characteristic is used to classify an organism as a member of the domain Archaea rather than the domain Bacteria?

  1. Presence of a cell wall
  2. Ability to survive in extreme environments
  3. Differences in the chemistry of membrane lipids and cell wall composition Correct answer.
  4. Absence of a nucleus

Explanation

Archaea and bacteria are both prokaryotic, so the nucleus and cell wall presence do not separate them. Their distinction rests on biochemical differences, notably ether-linked branched membrane lipids in archaea versus ester-linked lipids and peptidoglycan walls in bacteria.

Question 4Unity and Diversity

In the binomial system of nomenclature, which statement about the name Homo sapiens is correct?

  1. Homo is the family and sapiens is the genus, with only sapiens capitalised.
  2. Homo is the species and sapiens is the genus, both italicised.
  3. Homo is the genus and sapiens is the species, both italicised with Homo capitalised. Correct answer.
  4. Homo is the genus and sapiens is the species, both written in plain roman type.

Explanation

Binomial names consist of a genus name first, capitalised, followed by the species epithet in lower case, and the whole name is italicised or underlined. For Homo sapiens, Homo is the genus and sapiens the species.

Question 5Unity and Diversity

Two populations of a bird species become separated by a mountain range for many generations. Over time they accumulate different mating calls and no longer interbreed successfully. Which term best describes the outcome?

  1. Variation
  2. Adaptation
  3. Natural selection
  4. Speciation Correct answer.

Explanation

Speciation is the formation of new species, which occurs when populations become reproductively isolated and diverge over time. The loss of successful interbreeding between the two separated populations is the key evidence that speciation has occurred.

Question 6Unity and Diversity

Which sequence correctly orders the taxonomic ranks from most inclusive to least inclusive?

  1. Kingdom, phylum, class, order, family, genus, species Correct answer.
  2. Kingdom, class, phylum, order, family, genus, species
  3. Phylum, kingdom, class, family, order, genus, species
  4. Species, genus, family, order, class, phylum, kingdom

Explanation

The standard hierarchy runs from kingdom down to species: kingdom, phylum, class, order, family, genus, species. Each successive rank contains fewer organisms and groups that are more closely related.

Question 7Unity and Diversity

Which feature is present in a eukaryotic cell but absent from a prokaryotic cell?

  1. Ribosomes
  2. Cytoplasm
  3. A cell membrane
  4. A membrane-bound nucleus Correct answer.

Explanation

The defining difference between eukaryotes and prokaryotes is that eukaryotes enclose their DNA within a double membrane forming a nucleus. Ribosomes, cytoplasm and a cell membrane are found in both cell types.

Question 8Unity and Diversity

A dichotomous key separates four organisms. Organism P has a cell wall and a nucleus; organism Q has a cell wall and no nucleus; organism R has no cell wall and a nucleus; organism S has no cell wall and no nucleus. Which organism is most likely a bacterium?

  1. Organism Q Correct answer.
  2. Organism S
  3. Organism R
  4. Organism P

Explanation

Bacteria are prokaryotes, so they lack a nucleus, and they typically possess a cell wall. Organism Q is the only one combining a cell wall with the absence of a nucleus, matching the bacterial profile.

Question 9Unity and Diversity

Why are viruses not classified within the three domains of life?

  1. They cannot reproduce on their own and lack cellular structure and metabolism. Correct answer.
  2. They contain only RNA and never DNA.
  3. They are too small to be seen with a light microscope.
  4. They are always pathogenic to their hosts.

Explanation

Viruses are acellular and cannot carry out metabolism or replicate independently; they require a host cell's machinery. Because living organisms are defined by cellular organisation and independent reproduction, viruses fall outside the three-domain classification.

Question 10Unity and Diversity

Which pair of organisms both belong to the kingdom Fungi?

  1. Yeast and amoeba
  2. Mushroom and yeast Correct answer.
  3. Mushroom and moss
  4. Amoeba and moss

Explanation

Fungi are eukaryotic heterotrophs with chitinous cell walls, and this kingdom includes mushrooms and yeasts. Amoebae are protozoa and mosses are plants, so neither belongs to Fungi.

Question 11Unity and Diversity

Which observation provides the strongest evidence that two populations belong to different species rather than the same species?

  1. They look slightly different in colour.
  2. They can interbreed but their offspring are infertile. Correct answer.
  3. They live in different habitats.
  4. They eat different types of food.

Explanation

The biological species concept defines a species as a group whose members can interbreed to produce fertile offspring. Producing infertile offspring indicates reproductive isolation, so the populations are separate species.

Question 12Unity and Diversity

Which statement about the cell walls of bacteria is correct?

  1. They are made of chitin.
  2. They are made of cellulose.
  3. They are composed mainly of peptidoglycan. Correct answer.
  4. They are absent in all bacteria.

Explanation

The bacterial cell wall is composed mainly of peptidoglycan, a polymer of sugars cross-linked by short peptides. This wall maintains cell shape and provides protection, and its composition is a key feature distinguishing bacteria from other groups.

Question 13Unity and Diversity

A newly discovered organism is multicellular, obtains nutrients by absorbing them from its surroundings, and has cell walls made of chitin. To which kingdom should it be assigned?

  1. Animalia
  2. Plantae
  3. Fungi Correct answer.
  4. Protoctista

Explanation

Absorptive heterotrophic nutrition combined with chitinous cell walls is diagnostic of fungi. Animals ingest food and lack cell walls, and plants are photosynthetic with cellulose walls.

Question 14Unity and Diversity

What is the main purpose of classifying organisms into taxonomic groups?

  1. To group organisms based on shared characteristics and evolutionary relationships Correct answer.
  2. To prove that all organisms have identical DNA
  3. To rank organisms by their economic importance
  4. To arrange organisms by the size of their bodies

Explanation

Classification organises organisms into hierarchical groups according to shared features and inferred evolutionary relationships. This provides a stable, predictive framework that reflects common ancestry rather than arbitrary criteria.

Question 15Unity and Diversity

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

  1. Plants are unicellular while fungi are multicellular.
  2. Plants are eukaryotic while fungi are prokaryotic.
  3. Plants perform photosynthesis using chlorophyll, while fungi are heterotrophic. Correct answer.
  4. Plants have cell walls while fungi do not.

Explanation

Plants are autotrophs that photosynthesise using chlorophyll, whereas fungi are heterotrophs that absorb nutrients. Both groups are eukaryotic and both possess cell walls, so nutrition is the distinguishing feature.

Question 16Unity and Diversity

Which group of organisms is characterised by being unicellular eukaryotes, many of which are motile by means of cilia or flagella?

  1. Fungi
  2. Bacteria
  3. Protozoa Correct answer.
  4. Archaea

Explanation

Protozoa are unicellular eukaryotes, and many move using cilia, flagella or pseudopodia. Bacteria and archaea are prokaryotes, and fungi are typically multicellular with cell walls.

Question 17Unity and Diversity

Two organisms share the same class but belong to different orders. Which conclusion follows?

  1. They share a more recent common ancestor than organisms in different classes. Correct answer.
  2. They are more closely related than organisms in the same order.
  3. They must belong to the same species.
  4. They belong to different phyla.

Explanation

Taxonomic ranks reflect relatedness: organisms in the same class are more closely related than those in different classes, because they share a more recent common ancestor. Order is a narrower rank beneath class, so organisms in different orders are less closely related than those in the same order.

Question 18Unity and Diversity

Which statement about the genetic material of prokaryotes is correct?

  1. It consists only of RNA.
  2. It is enclosed within a nuclear envelope.
  3. It is linear and associated with histone proteins forming chromosomes.
  4. It is circular and lies free in the cytoplasm, sometimes with additional small plasmids. Correct answer.

Explanation

Prokaryotes typically have a single circular chromosome free in the cytoplasm, along with small circular plasmids carrying extra genes. There is no nuclear envelope and no histone-bound linear chromosomes.

Question 19Unity and Diversity

The scientific name of the domestic dog is Canis lupus familiaris. Which part of this name indicates the genus?

  1. Canis Correct answer.
  2. lupus
  3. familiaris
  4. The entire three-part name

Explanation

In a binomial or trinomial name, the first term is the genus and is capitalised and italicised. Here Canis is the genus, lupus is the species and familiaris denotes a subspecies.

Question 20Unity and Diversity

Which evidence would most strongly support the claim that two modern species share a recent common ancestor?

  1. They occupy the same habitat.
  2. Their DNA sequences show a high degree of similarity. Correct answer.
  3. They both reproduce sexually.
  4. They have similar body size.

Explanation

Molecular evidence such as DNA similarity reflects shared ancestry, since related species accumulate fewer genetic differences. Habitats, size and reproductive modes can be similar in unrelated organisms through convergence.

Question 21Unity and Diversity

Which feature is common to members of the kingdom Animalia?

  1. They are photosynthetic autotrophs.
  2. They possess cell walls made of cellulose.
  3. They are all unicellular.
  4. They are multicellular heterotrophs that typically ingest their food. Correct answer.

Explanation

Animals are multicellular, heterotrophic eukaryotes that generally ingest food and digest it internally. They lack cell walls and do not photosynthesise.

Question 22Unity and Diversity

A population of insects colonises a new island. Over many generations, individuals with thicker exoskeletons survive better in the windy conditions and produce more offspring. This is best described as an example of which process?

  1. Genetic drift
  2. Speciation
  3. Artificial selection
  4. Natural selection Correct answer.

Explanation

Natural selection occurs when heritable variation gives some individuals a survival and reproductive advantage in a particular environment. The thicker exoskeleton increases fitness in windy conditions, so its frequency rises over generations.

Question 23Unity and Diversity

Which combination of features is characteristic of all archaea?

  1. Multicellular organisation with chitin walls
  2. Eukaryotic cells with mitochondria
  3. Prokaryotic cells with ether-linked membrane lipids Correct answer.
  4. Cells containing a nucleus and chloroplasts

Explanation

Archaea are prokaryotes distinguished by ether-linked, branched membrane lipids. They lack a nucleus, mitochondria and chloroplasts, and they are not multicellular.

Question 24Unity and Diversity

Two strains of bacteria are compared. Strain X retains the crystal violet stain after decolourisation while strain Y does not. Which explanation is correct?

  1. Strain Y is Gram-positive because it loses the stain.
  2. Strain X has a thick peptidoglycan layer that retains the stain, identifying it as Gram-positive. Correct answer.
  3. Strain Y is Gram-positive because it has a thicker peptidoglycan layer.
  4. Strain X lacks a cell wall, so the dye cannot escape.

Explanation

In Gram staining, Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet-iodine complex, so they remain purple. Gram-negative bacteria have a thinner layer and an outer membrane, so they are decolourised and take up the counterstain.

Question 25Unity and Diversity

Which statement best explains why the biological species concept cannot be applied to all organisms?

  1. It applies only to organisms with cell walls.
  2. It requires the organisms to be multicellular.
  3. It relies on interbreeding, which cannot be assessed in asexually reproducing organisms or fossils. Correct answer.
  4. It depends on the physical appearance of the organisms.

Explanation

The biological species concept defines a species by reproductive compatibility, so it fails for organisms that reproduce asexually and for extinct forms known only from fossils. Such groups are instead classified using morphological or molecular criteria.

Question 26Unity and Diversity

In binomial nomenclature, how is the species name written in print according to the rules applied in IB Biology?

  1. Genus capitalised, species lowercase, whole name italicised Correct answer.
  2. Genus lowercase, species capitalised, whole name underlined
  3. Both words lowercase and enclosed in quotation marks
  4. Both words capitalised and written in bold

Explanation

Binomial names are Latinised and italicised in print, with the genus (first word) capitalised and the specific epithet (second word) in lowercase. For example, Homo sapiens follows this convention. Underlining is only a substitute used in handwritten work.

Question 27Unity and Diversity

Which characteristic is shared by all members of the domain Bacteria?

  1. They possess nuclei enclosed by a double membrane
  2. They possess membrane-bound organelles in their cytoplasm
  3. They possess histone proteins bound to their DNA
  4. They possess peptidoglycan in their cell walls Correct answer.

Explanation

Peptidoglycan (murein) is a defining component of bacterial cell walls and is not found in archaea or eukaryotes. Bacteria are prokaryotes, so their DNA is not wrapped in histones and they lack membrane-bound organelles and nuclei.

Question 28Unity and Diversity

A sample contains organisms that are unicellular, lack a nucleus, and live in extremely hot, salty springs. To which domain do they most likely belong?

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

Explanation

Organisms that are prokaryotic yet thrive in extreme environments such as hot springs and hypersaline lakes are characteristically archaea. Their cell walls lack peptidoglycan and their membrane lipids differ from those of bacteria. Viruses are not classified into domains at all.

Question 29Unity and Diversity

Which statement describes the role of the specific epithet in a scientific name such as Panthera leo?

  1. It identifies the species within the genus Correct answer.
  2. It identifies the genus within the family
  3. It identifies the kingdom of the organism
  4. It identifies the order to which the organism belongs

Explanation

A binomial name consists of the genus followed by the specific epithet, and together they designate one species. The epithet alone has no meaning without the genus, but within the genus it distinguishes the species. Higher ranks such as family, order and kingdom are not encoded in the binomial.

Question 30Unity and Diversity

Which feature is used to place an organism in the kingdom Protista rather than in the kingdom Fungi?

  1. Its cells are always multicellular and photosynthetic
  2. Its cells have chitin-containing walls and absorb nutrients
  3. Its cells are eukaryotic and it is mainly unicellular or colonial Correct answer.
  4. Its cells are prokaryotic and it lacks a nucleus

Explanation

Protists are eukaryotic organisms that are mostly unicellular or colonial and do not form the chitin-walled, absorptive bodies typical of fungi. Fungi are eukaryotic too, but they are multicellular (mostly) with chitin walls and heterotrophic absorption. Prokaryotic cells would place an organism in Bacteria or Archaea.

Question 31Unity and Diversity

Which statement about the classification hierarchy is correct?

  1. An order contains fewer organisms than a family
  2. A domain contains more organisms than a kingdom Correct answer.
  3. A family contains fewer organisms than a genus
  4. A species contains more organisms than a genus

Explanation

The hierarchy runs from domain (most inclusive) down through kingdom, phylum, class, order, family, genus and species (least inclusive). Therefore a domain contains more organisms than any kingdom it includes. Each lower rank is nested within the higher one.

Question 32Unity and Diversity

Which observation would provide evidence that a newly discovered organism is a prokaryote rather than a eukaryote?

  1. Its cells are surrounded by a plasma membrane
  2. Its cells divide to produce new cells
  3. Its cells contain ribosomes that synthesise proteins
  4. Its genetic material is a circular chromosome not enclosed by a membrane Correct answer.

Explanation

Prokaryotes typically have a single circular chromosome lying free in the cytoplasm, not enclosed by a nuclear envelope. Ribosomes, plasma membranes and cell division occur in both prokaryotes and eukaryotes, so they do not distinguish the two.

Question 33Unity and Diversity

Which feature distinguishes members of the kingdom Plantae from members of the kingdom Animalia?

  1. Plant cells have no nucleus and are heterotrophic
  2. Plant cells lack membrane-bound organelles
  3. Plant cells store genetic material outside the nucleus
  4. Plant cells have cellulose cell walls and are autotrophic Correct answer.

Explanation

Plants are multicellular autotrophs whose cells have cellulose cell walls and chloroplasts for photosynthesis. Animals are multicellular heterotrophs lacking cell walls. Both are eukaryotes, so both have nuclei and membrane-bound organelles.

Question 34Unity and Diversity

A biologist finds two organisms that share the same phylum. Which conclusion is best supported by this information alone?

  1. They belong to the same domain but may belong to different classes Correct answer.
  2. They belong to the same class but may belong to different orders
  3. They belong to the same species but may belong to different families
  4. They belong to the same genus but may belong to different species

Explanation

Sharing a phylum means the organisms share all higher ranks, including domain and kingdom, but ranks below phylum (class, order, family, genus) may differ. Sharing a phylum does not guarantee sharing a class, genus or species.

Question 35Unity and Diversity

Which pair of features would be most useful for separating an organism into the domain Archaea rather than the domain Bacteria?

  1. Presence of chloroplasts and presence of cellulose walls
  2. Absence of peptidoglycan in the wall and presence of ether-linked membrane lipids Correct answer.
  3. Presence of a nucleus and presence of linear chromosomes
  4. Presence of peptidoglycan in the wall and presence of ester-linked membrane lipids

Explanation

Archaeal cell walls lack peptidoglycan and their membrane lipids are ether-linked, whereas bacterial walls contain peptidoglycan and their lipids are ester-linked. These biochemical differences are key evidence for separating the two prokaryotic domains. Nuclei, chloroplasts and cellulose walls are eukaryotic features.

Question 36Unity and Diversity

Two populations of birds live in the same forest, look almost identical, and never interbreed because they sing different mating songs. Which conclusion is most consistent with the biological species concept?

  1. They are the same species because they occupy the same habitat
  2. They are the same species because they are morphologically identical
  3. They are separate species because reproductive isolation is maintained by behaviour Correct answer.
  4. They cannot be classified because the biological species concept requires physical separation

Explanation

Under the biological species concept, species are groups of interbreeding populations that are reproductively isolated from other such groups. Behavioural isolation through different mating songs prevents interbreeding, so the populations are separate species even though they look alike and share a habitat.

Question 37Unity and Diversity

Which statement best explains why the binomial system is useful to biologists worldwide?

  1. It provides a single universally recognised name for each species Correct answer.
  2. It replaces the need for any taxonomic hierarchy
  3. It ranks organisms by their economic importance
  4. It groups organisms according to their habitat

Explanation

Binomial nomenclature gives each species one Latinised name that is recognised internationally, avoiding confusion caused by different common names in different languages. It works alongside, not instead of, the taxonomic hierarchy. It is not based on habitat or economic value.

Question 38Unity and Diversity

Which evidence would most strongly indicate that two populations belong to the same species rather than different species?

  1. They have identical common names in one language
  2. They interbreed in the wild and produce fertile offspring Correct answer.
  3. They share the same genus and similar body shape
  4. They live in the same geographic region

Explanation

The biological species concept defines a species as a group of populations that can interbreed to produce fertile offspring. Producing fertile offspring in the wild is the strongest evidence of belonging to the same species. Shared genus, shared location or common names are weaker and can be misleading.

Question 39Unity and Diversity

A dichotomous key uses pairs of contrasting statements to identify organisms. Which feature makes a key reliable?

  1. The statements use subjective judgements such as beauty
  2. The statements rely on features visible only with a microscope
  3. Each pair of statements is mutually exclusive and clearly observable Correct answer.
  4. Each pair of statements overlaps so several paths are possible

Explanation

A dichotomous key works by offering two contrasting, mutually exclusive choices at each step, leading to a single identification. The features used should be observable and unambiguous. Overlapping statements, subjective criteria or features not practically observable reduce reliability.

Question 40Unity and Diversity

Which combination of features is characteristic of all viruses?

  1. A cell wall, ribosomes and a circular chromosome
  2. A nucleic acid core surrounded by a protein capsid and no cytoplasm Correct answer.
  3. Peptidoglycan wall, flagella and plasmid DNA
  4. A nucleus, mitochondria and a plasma membrane

Explanation

Viruses consist of genetic material (DNA or RNA) enclosed in a protein coat called a capsid, sometimes with an outer envelope. They lack cytoplasm, ribosomes and organelles, and they cannot reproduce independently. They are therefore acellular and not placed in any domain.

Question 41Unity and Diversity

Why are viruses described as obligate intracellular parasites?

  1. They can replicate only inside a host cell using the host's machinery Correct answer.
  2. They carry out photosynthesis inside host cells
  3. They digest their food externally before absorbing it
  4. They can reproduce freely in soil and water

Explanation

Viruses lack ribosomes and metabolic enzymes, so they cannot synthesise proteins or replicate their genomes on their own. They must enter a host cell and use its machinery to reproduce. This dependence on a living host defines them as obligate intracellular parasites.

Question 42Unity and Diversity

Which feature of Archaea supports the view that they are more closely related to eukaryotes than bacteria are?

  1. Their cells contain chloroplasts for photosynthesis
  2. Their RNA polymerase and histone-like proteins resemble those of eukaryotes Correct answer.
  3. Their cell walls contain peptidoglycan like bacteria
  4. Their cells contain membrane-bound nuclei

Explanation

Molecular comparisons show that archaeal RNA polymerase and histone-like DNA-binding proteins are more similar to eukaryotic versions than to bacterial ones. This supports a closer evolutionary relationship between archaea and eukaryotes. Archaea, however, remain prokaryotic and lack nuclei and chloroplasts.

Question 43Unity and Diversity

A scientist compares the amino acid sequences of a protein from four species and finds the following number of differences from a reference species: P = 2, Q = 8, R = 15, S = 21. Which species is most likely to share the most recent common ancestor with the reference species?

  1. Species P Correct answer.
  2. Species S
  3. Species Q
  4. Species R

Explanation

Fewer differences in amino acid sequence indicate a shorter time since divergence and therefore a more recent common ancestor. Species P, with only two differences, is the most similar and most likely to share the most recent common ancestor with the reference species. Greater differences indicate more distant relationships.

Question 44Unity and Diversity

Which statement about the kingdoms within the domain Eukarya is correct?

  1. Protista and Fungi both consist only of multicellular organisms
  2. Plantae and Animalia are both autotrophic and have cell walls
  3. Animalia and Fungi both have cellulose cell walls
  4. Fungi and Animalia are both heterotrophic but differ in how they obtain nutrients Correct answer.

Explanation

Fungi absorb nutrients from their surroundings, often after secreting enzymes, while animals ingest food and digest it internally; both are heterotrophic. Plants are autotrophic, animals lack cell walls, protists are mostly unicellular, and fungal walls contain chitin, not cellulose.

Question 45Unity and Diversity

A population of plants on a windy island includes individuals with thicker cuticles and more flexible stems. Over many generations, these individuals survive and reproduce more than others. Which process does this best illustrate?

  1. Classification of the population into a new kingdom
  2. Natural selection acting on variation within a population Correct answer.
  3. Formation of a symbiotic relationship with fungi
  4. Artificial selection by plant breeders

Explanation

Heritable variation exists within the population, and individuals with traits better suited to the windy environment survive and reproduce more successfully. This differential survival and reproduction is natural selection. It can lead to evolutionary change but is not classification or artificial breeding.

Question 46Unity and Diversity

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

  1. They belong to the same species
  2. They belong to the same genus
  3. They belong to the same class Correct answer.
  4. They belong to the same phylum but different kingdoms

Explanation

In the hierarchy, class is above order, so organisms sharing an order must also share the class that contains it. Family, genus and species are lower ranks and may differ. Organisms in the same order are also in the same phylum and kingdom, not different kingdoms.

Question 47Unity and Diversity

Which feature of a cell would allow a biologist to identify it as belonging to the kingdom Fungi rather than the kingdom Plantae?

  1. A cell wall made of peptidoglycan with no nucleus
  2. A cell membrane with no cell wall and no nucleus
  3. A cell wall made of chitin with no chloroplasts Correct answer.
  4. A cell wall made of cellulose with chloroplasts

Explanation

Fungal cell walls contain chitin and fungal cells lack chloroplasts because fungi are heterotrophic. Plant cell walls contain cellulose and plant cells generally contain chloroplasts for photosynthesis. Peptidoglycan walls and cells without nuclei indicate prokaryotes.

Question 48Unity and Diversity

A bacterium is treated so that its circular chromosome is destroyed but its plasmids remain. Which outcome is most likely?

  1. The cell becomes a eukaryote because it now lacks a chromosome
  2. The cell cannot carry out the essential functions encoded by its main chromosome Correct answer.
  3. The cell develops peptidoglycan-free walls and becomes an archaeon
  4. The cell continues normally because plasmids carry all essential genes

Explanation

The circular chromosome carries the genes needed for essential cell functions such as metabolism and replication. Plasmids are small extra circles carrying a few non-essential or advantageous genes, so they cannot replace the main chromosome. Destroying the chromosome would therefore be lethal.

Question 49Unity and Diversity

A researcher finds an organism with cells that contain nuclei, chitin-containing walls, and no chloroplasts, and that absorbs nutrients from decaying matter. Which kingdom classification is most appropriate, and why?

  1. Animalia, because it is heterotrophic and absorbs nutrients
  2. Fungi, because the organism is eukaryotic, has chitin walls and is absorptive Correct answer.
  3. Plantae, because it has cell walls and absorbs nutrients
  4. Protista, because it is eukaryotic and lacks chloroplasts

Explanation

Eukaryotic cells with chitin-containing walls and absorptive heterotrophic nutrition are diagnostic of the kingdom Fungi. Plants have cellulose walls and are autotrophic, protists are mostly unicellular and do not typically have chitin walls, and animals lack cell walls and ingest rather than absorb food.

Question 50Unity and Diversity

Two species of frog live in the same pond and are morphologically almost identical, but crosses between them produce sterile offspring. Which interpretation is most defensible?

  1. They are separate species because hybrid sterility shows reproductive isolation Correct answer.
  2. They are one species because their offspring are produced
  3. They are one species because they live in the same pond
  4. They cannot be classified because morphology is identical

Explanation

Producing sterile hybrids is a form of postzygotic reproductive isolation, meaning gene flow between the populations is blocked. Under the biological species concept this supports classifying them as separate species. Shared habitat or morphological similarity does not override reproductive isolation.