Form 5 · Worked answers

Biodiversity, worked answers

Fully worked answers for Biodiversity, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Writing a scientific name correctly, using the exact binomial format with capitalisation and italics.
Using or constructing a dichotomous key to identify an unfamiliar set of organisms step by step.
Comparing viruses and bacteria in a table covering cell structure and the ability to reproduce independently.
Distinguishing genetic diversity, species diversity and ecosystem diversity with a correct, specific example of each.
Explaining why high biodiversity increases an ecosystem's stability and its resilience to disturbance.
Placing a familiar organism correctly within the classification hierarchy, from kingdom all the way down to species.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

State the seven levels of the taxonomic hierarchy in order, from the broadest to the narrowest, and name the human species at the two narrowest levels.

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The seven levels in order from broadest to narrowest are kingdom, phylum, class, order, family, genus and species. The human genus is Homo and the species is sapiens, so the full scientific name is written as the genus followed by the species, Homo sapiens, in italics.

kingdomphylumclassorderfamilygenusspeciesHomo sapiens

Explain why binomial nomenclature is used instead of common names, and state two formatting rules that must be followed when writing a scientific name.

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Binomial nomenclature is used because a single organism can have several different common names in different languages or regions, which would cause confusion; a scientific name is fixed and understood by biologists worldwide. Two formatting rules are that the genus name begins with a capital letter while the species name is entirely lower case, and that the whole name is written in italics when printed or underlined when handwritten.

binomial nomenclaturegenusspeciescapital letteritalics

A dichotomous key is used to identify four insects labelled P, Q, R and S. Explain, using the idea of paired statements, how such a key leads to a single correct identification.

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A dichotomous key presents a series of steps, each offering two contrasting statements about an observable feature of the insect, such as the presence or absence of wings. At each step the user selects the statement that matches the insect being examined and follows it either to the insect's name or to the next pair of statements. Repeating this process narrows the possibilities at every step until only one insect's identity remains.

dichotomous keypaired statementsfeaturenarrowsidentification

Distinguish between genetic diversity and species diversity, giving one correctly labelled example of each.

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Genetic diversity is the variation in genes between individuals of the same species, for example the different coat colours found within one species of domestic cat. Species diversity is the number of different species living together within a single habitat, for example the variety of fish, coral and molluscs found on one coral reef. The key difference is that genetic diversity is measured within one species, while species diversity is measured across many species in the same place.

genetic diversityspecies diversitywithin a specieshabitatexample

Explain why an ecosystem with high biodiversity is generally more stable than one with low biodiversity.

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An ecosystem with high biodiversity contains a greater variety of species and genes, which gives it more alternative feeding relationships and more genetic variation to draw on if conditions change. This means the loss of one species, or a disease affecting one gene variant, is less likely to disrupt the whole system. An ecosystem with low biodiversity depends more heavily on fewer species, so losing even one of them can cause the entire community to collapse.

biodiversitystabilityalternative pathwaysloss of one speciescollapse

State two roles of microorganisms in an ecosystem, and explain why not all microorganisms should be described as harmful.

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Microorganisms act as decomposers, breaking down dead organisms and waste to release nutrients such as nitrogen and carbon back into the environment, and some species also act as pathogens, causing disease in humans, animals or plants. Not all microorganisms should be described as harmful because the majority are either harmless or directly beneficial, such as yeast used in bread-making or the soil bacteria involved in decomposition and nutrient recycling; only a minority of species are pathogenic.

decomposernutrient recyclingpathogenbeneficialyeast

Compare a bacterium and a virus by stating one difference in cell structure and one difference in how each reproduces.

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A bacterium is a complete, single-celled organism with its own cytoplasm and cell membrane, whereas a virus is not a cell at all; it consists only of genetic material enclosed in a protein coat. A bacterium reproduces independently by simple cell division, while a virus can only reproduce by invading a living host cell and using that cell's machinery to make copies of itself. Because of this dependence on a host, a virus is often placed outside the five-kingdom classification system.

bacteriumviruscytoplasmprotein coathost cellindependent reproduction

Phrasing that earns marks

Classification: Organisms are grouped by shared features into kingdoms and smaller groups down to species. The full hierarchy runs kingdom, phylum, class, order, family, genus and species, with species being the smallest and most specific group and kingdom the broadest. Learning this order makes it far easier to answer questions that ask you to name the group immediately above or below a given level.
Binomial nomenclature: Each species has a two-part scientific name: genus (capitalised) then species, written in italics. This worldwide naming system avoids the confusion caused by different common names for the same organism in different languages or regions. Getting the capitalisation, word order or italics wrong is treated as an error even if the underlying organism is identified correctly.
Dichotomous key: A tool that identifies an organism through a series of paired either/or questions about its features. Following the matching statement at each step narrows the possibilities until only one organism's identity remains. A well-built key should never require more than a small number of paired steps to reach a single, unambiguous answer.
Levels of biodiversity: Genetic diversity within a species, species diversity within a habitat, and ecosystem diversity across regions. These three levels are often tested together, each needing a distinct, correctly labelled example rather than a general description. A common exam trap is describing species diversity when the question is actually asking about genetic diversity, or vice versa.
Microorganisms: Bacteria, fungi and protozoa are microscopic organisms with important roles such as decomposition and nutrient recycling. Some microorganisms are also pathogens, meaning they cause disease in humans, animals or plants. Not all microorganisms are harmful; many, including most bacteria and fungi found in soil, play a beneficial role in an ecosystem.
Viruses: Viruses are not cells; they can only reproduce inside a host cell, so they sit on the boundary of living and non-living. A virus is made of genetic material surrounded by a protein coat, with no cytoplasm of its own. Because a virus depends entirely on a host cell to multiply, most classification systems place it outside the five kingdoms altogether.
Importance of biodiversity: High biodiversity makes an ecosystem more stable, because a wider variety of species and genes gives it more ways to cope with disease, changing conditions, or the loss of a single species. Low biodiversity leaves an ecosystem more vulnerable to collapse from a single disturbance. This is why conservation programmes usually aim to protect several species together, not just one flagship species.
Taxonomic hierarchy in practice: Moving from kingdom to species narrows the group at every step: humans, for example, belong to the kingdom Animalia, the phylum Chordata, the class Mammalia, and finally the genus Homo and species sapiens. Placing a familiar organism at each level is a common exam task. The same logic applies to any organism, from a rice plant to a housefly, not only to humans.

Frequently asked questions

How do you write a scientific name correctly?

A scientific name has two parts, the genus and the species. The genus name starts with a capital letter and the species name is all lower case, and the whole name is written in italics (or underlined when handwritten). For example, the human is Homo sapiens, with a capital H for the genus and a lower-case s for the species. Even a small formatting error, such as a misplaced capital letter, still counts as a mistake even if the organism itself is correctly identified.

What is a dichotomous key?

A dichotomous key is a tool used to identify an organism. It presents a series of steps, each offering two contrasting statements about a feature, such as 'has wings' or 'has no wings'. You choose the statement that matches your organism and follow it to the next pair, repeating until you reach the organism's name. A good key is designed so that anyone can follow it without needing prior detailed knowledge of the organism.

Are viruses living or non-living?

Viruses sit on the boundary between living and non-living. They are not made of cells and cannot carry out life processes or reproduce on their own. However, once inside a host cell they can take over the cell's machinery to make copies of themselves, so they show one feature of living things, reproduction, only when inside a host. This debate over a virus's status shows that the definition of 'living' in biology is not always simple or clear-cut.