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Characteristics and Classification of Living Organisms - Features of organisms

Grade 12IGCSEBiology

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

🔑Concepts

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Characteristics of Living Organisms: Defined by the mnemonic MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition.

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Concept of Respiration: A chemical reaction in cells that breaks down nutrient molecules to release energy for metabolism. Aerobic respiration: C6H12O6+6O2ightarrow6CO2+6H2OC_6H_{12}O_6 + 6O_2 ightarrow 6CO_2 + 6H_2O.

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Excretion: The removal of toxic materials, waste products of metabolism (e.g., CO2CO_2, urea), and substances in excess of requirements.

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Classification Systems: Traditionally based on morphology and anatomy, but modern classification uses sequences of bases in DNADNA and sequences of amino acids in proteins to determine evolutionary relationships.

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Binomial Nomenclature: A two-part naming system (introduced by Linnaeus) consisting of the Genus (capitalized) and species (lowercase), e.g., HomoHomo sapienssapiens.

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The Five Kingdoms: Animal (multicellular, no cell wall), Plant (multicellular, cellulose cell wall, chlorophyll), Fungus (multicellular/unicellular, chitin cell wall), Prokaryote (unicellular, no nucleus, e.g., bacteria), and Protoctist (complex unicellular or simple multicellular).

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Cellular Structures: All living cells contain DNADNA, cytoplasm, and a cell membrane. Ribosomes are present for protein synthesis using mRNAmRNA.

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Viruses: Non-living entities consisting of genetic material (DNADNA or RNARNA) surrounded by a protein coat (capsid); they lack cell membranes and cytoplasm.

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Dichotomous Keys: Tools used to identify organisms based on a series of choices between two alternative characteristics.

📐Formulae

Magnification=Image Size (I)Actual Size (A)\text{Magnification} = \frac{\text{Image Size (I)}}{\text{Actual Size (A)}}

Aerobic Respiration: C6H12O6+6O2→6CO2+6H2O+Energy\text{Aerobic Respiration: } C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}

Photosynthesis: 6CO2+6H2O→lightC6H12O6+6O2\text{Photosynthesis: } 6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2

💡Examples

Problem 1:

Calculate the actual length of a mitochondrion if its image size in a micrograph is 3030 mm and the magnification is ×15,000\times 15,000. Provide the answer in micrometers (μm\mu m).

Solution:

Using the formula A=IMA = \frac{I}{M}: A=30 mm15,000=0.002 mmA = \frac{30\text{ mm}}{15,000} = 0.002\text{ mm}. To convert mm to μm\mu m: 0.002×1000=2μm0.002 \times 1000 = 2\mu m.

Explanation:

The actual size is found by dividing the measured image size by the magnification factor, then converting units using 1 mm=103μm1\text{ mm} = 10^3\mu m.

Problem 2:

Two organisms have the following DNADNA base sequences for a specific gene: Organism A: ATCCGGTTAATCCGGTTA; Organism B: ATCCGGTTCATCCGGTTC. Organism C has: ATGCGGTTAATGCGGTTA. Which two organisms are most closely related?

Solution:

Organism A and Organism B are most closely related.

Explanation:

Closely related species share a more recent common ancestor and therefore have more similar DNADNA base sequences. Organism A and B differ by only 11 base (at the 9th position), whereas A and C also differ by 11 base (at the 3rd position), but modern phylogenetics looks for the highest percentage of similarity across the entire genome.

Features of organisms - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology