IGCSE Biology Revision Notes

Characteristics of Living Organisms

  • Movement: Actions that cause a change of position or location by an organism or part of an organism.

  • Respiration: Chemical reactions that break down nutrient molecules in cells to release energy for metabolism.

  • Sensitivity: The capability to detect stimuli in the internal or external environment and respond appropriately.

  • Growth: A permanent increase in size and dry mass due to an increase in either cell number, cell size, or both.

  • Reproduction: Processes that result in the production of more individuals of the same species.

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

  • Nutrition: The taking in of materials necessary for energy, growth, and development.

Mnemonic to Remember Characteristics: MRS. GREN

  • Movement

  • Respiration

  • Sensitivity

  • Growth and development

  • Reproduction

  • Excretion

  • Nutrition

Concept & Uses of Classification Systems

  • There are millions of species on Earth.

  • A species is a group of organisms capable of reproducing to produce fertile offspring.

  • Species can be classified by common features (e.g. mammals have hair, nurse young with mammary glands).

The Binomial System

  • Developed by Linnaeus, it classifies organisms into smaller, specialized groups based on shared features.

  • Organisms are named in Latin, with a two-part scientific name: the genus (capitalized) and species (lowercase).

  • Example: Homo sapiens (italicized when typed).

  • The classification sequence is: Kingdom, Phylum, Class, Order, Family, Genus, Species.

Dichotomous Keys

  • Keys are tools for identifying organisms based on a series of questions regarding their features.

  • Dichotomous means 'branching into two', offering two descriptions at a time for choice.

  • To use a dichotomous key:

    1. Start with a single organism.

    2. Follow statements/questions based on features from the provided information.

    3. Continue until identifying the organism.

Reflection on Evolutionary Relationships

  • Classification systems are designed to reflect evolutionary relations.

  • Traditional methods based on physical features have limitations and may lead to incorrect classifications.

  • Modern classification utilizes DNA sequencing to determine relationships among species: the more similar the DNA sequences, the more closely related the species are.

Evolutionary Classification Examples

  • For instance, species like Brachinus armiger and Brachinus hirsutus are closely related as indicated by the similarity in their DNA sequences, differing only by one base.

Features of Organisms

The Five Kingdoms

  • Organisms are divided into five kingdoms:

    • Animals: Multicellular, nucleus present, no cell walls or chloroplasts, heterotrophs.

    • Plants: Multicellular, nucleus, contain chloroplasts and cellulose cell walls, autotrophs.

    • Fungi: Usually multicellular, nuclei present, cell walls not made of cellulose, saprophytic or parasitic nutrition.

    • Protoctists: Mostly unicellular, some multicellular, have nuclei and potentially chloroplasts.

    • Prokaryotes: Usually unicellular with cells containing no nuclei, cell walls (not cellulose), and no mitochondria.

Key Distinctions in Animal Classification

  • Vertebrates: Possess a backbone; further classified into five classes (e.g., mammals, birds).

  • Invertebrates: Lack a backbone; classified by features like whether they possess legs. Arthropods are one of the biggest groups.

Plant Classification

  • Plants are characterized by the presence of chlorophyll essential for photosynthesis. They can be categorized into ferns and flowering plants:

    • Ferns: Reproduce via spores, have leaves called fronds.

    • Flowering Plants: Reproduce sexually through flowers and seeds, which can be classified into monocotyledons (e.g., wheat) and dicotyledons (e.g., sunflowers).

Comparison of Monocots and Dicots

  • Flowers: Monocots (multiples of 3) vs. Dicots (multiples of 4 or 5).

  • Leaves: Monocots have parallel veins vs. Dicots with a network of veins.

Viruses**

  • Not classified as living organisms since they do not carry out life processes independently.

  • Viruses consist of genetic material (RNA or DNA) encased in a protein coat and require host cells for reproduction.

  1. What are the seven characteristics of living organisms?

  2. How does the mnemonic MRS. GREN help in recalling the characteristics of living organisms?

  3. Explain the binomial system of classification.

  4. Describe the differences between vertebrates and invertebrates.

  5. What is a species, and how is it classified?

  6. How do dichotomous keys assist in identifying organisms?

  7. Compare and contrast monocots and dicots in terms of their features.

  8. Discuss the significance of modern classification methods like DNA sequencing in understanding evolutionary relationships.

  9. What are the five kingdoms of living organisms, and what are the main characteristics of each?

  10. Why are viruses considered non-living organisms?

  1. What are the seven characteristics of living organisms?

  2. How does the mnemonic MRS. GREN help in recalling the characteristics of living organisms?

  3. Explain the binomial system of classification.

  4. Describe the differences between vertebrates and invertebrates.

  5. What is a species, and how is it classified?

  6. How do dichotomous keys assist in identifying organisms?

  7. Compare and contrast monocots and dicots in terms of their features.

  8. Discuss the significance of modern classification methods like DNA sequencing in understanding evolutionary relationships.

  9. What are the five kingdoms of living organisms, and what are the main characteristics of each?

  10. Why are viruses considered non-living organisms?