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Characteristics and Classification of Living Organisms - Dichotomous keys

Grade 11IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A dichotomous key is a biological tool used to identify organisms based on a series of choices between alternative physical characteristics.

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The term 'dichotomous' means 'divided into two parts'; each step in the key presents two contrasting statements (a couplet) about a specific morphological feature.

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Identification relies on observable, permanent features such as the number of legs, wing structures, or leaf shapes, rather than variable behaviors or sizes.

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Classification within the IGCSE framework follows the hierarchical system: Kingdom, Phylum, Class, Order, Family, Genus, and Species.

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Binomial nomenclature is used for naming organisms identified by keys, formatted as Genus speciesGenus\ species. The Genus starts with a capital letter, the species with lowercase, and both are italicized (or underlined if handwritten).

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Biological drawings and identification often require calculating the actual size of a specimen using the magnification formula, where units are typically in mmmm or μm\mu m (1 mm=1000 μm1\text{ mm} = 1000\text{ }\mu m).

📐Formulae

Magnification=Measured Image SizeActual Size of Specimen\text{Magnification} = \frac{\text{Measured Image Size}}{\text{Actual Size of Specimen}}

Actual Size=Image SizeMagnification\text{Actual Size} = \frac{\text{Image Size}}{\text{Magnification}}

1000 nanometers (nm)=1 micrometer (μm)1000\text{ nanometers (nm)} = 1\text{ micrometer (}\mu m)

1000 micrometers (μm)=1 millimeter (mm)1000\text{ micrometers (}\mu m) = 1\text{ millimeter (mm)}

💡Examples

Problem 1:

Use the following dichotomous key to identify an arthropod with 8 legs and no wings:

  1. a) Wings present... Go to 2 b) Wings absent... Go to 3
  2. a) One pair of wings... Musca domesticaMusca\ domestica b) Two pairs of wings... Apis melliferaApis\ mellifera
  3. a) 8 legs present... Araneae sp.Araneae\ sp. b) More than 8 legs... CentipedeCentipede

Solution:

Araneae sp.Araneae\ sp.

Explanation:

Starting at step 1, the organism has no wings, so we follow 1b to step 3. At step 3, the organism has 8 legs, which leads to choice 3a: Araneae sp.Araneae\ sp. (a spider).

Problem 2:

An image of a bacterial cell measures 40 mm40\text{ mm} in length. If the magnification is ×5000\times 5000, calculate the actual length of the cell in μm\mu m.

Solution:

8 μm8\text{ }\mu m

Explanation:

Using the formula Actual Size=Image SizeMagnification\text{Actual Size} = \frac{\text{Image Size}}{\text{Magnification}}, we get 40 mm5000=0.008 mm\frac{40\text{ mm}}{5000} = 0.008\text{ mm}. To convert to micrometers, multiply by 10001000: 0.008×1000=8 μm0.008 \times 1000 = 8\text{ }\mu m.

Dichotomous keys - Revision Notes & Key Diagrams | IGCSE Grade 11 Biology