The Nature and Variety of Living Organisms

Biology is the study of living organisms. To make sense of the immense variety of life on Earth, biologists look at the fundamental characteristics shared by all living things, as well as the structural features that distinguish major groups of organisms: eukaryotes, prokaryotes, and non-cellular entities like viruses.

1. Characteristics of Living Organisms

All living organisms share eight basic life processes and characteristics:

  • Require nutrition: Organisms need food/nutrients to provide energy and raw materials for growth and repair (either by making their own food via photosynthesis or by consuming other organisms).
  • Respire: Organisms release energy from their food (such as glucose) through cellular respiration to produce ATP.
  • Excrete waste: Organisms remove metabolic waste products from their bodies (e.g. carbon dioxide, urea).
  • Respond to surroundings: Organisms detect changes in their external environment (stimuli) and produce appropriate responses.
  • Move: Organisms can change their position or orientation (animals move their whole body; plants show growth movements such as phototropism).
  • Control internal conditions: Organisms maintain a steady internal environment within narrow limits, a process known as homeostasis (e.g. controlling body temperature or water content).
  • Reproduce: Organisms produce offspring to ensure the continuation of their species (sexually or asexually).
  • Grow and develop: Organisms increase in size and dry mass by increasing cell number or cell size, and undergo changes in structure and function over their lifetime.

2. Eukaryotes and Prokaryotes

All cellular organisms are classified as either eukaryotic or prokaryotic:

  • Eukaryotes: Organisms whose cells contain a distinct nucleus enclosed within a nuclear membrane, as well as membrane-bound organelles (e.g. mitochondria and chloroplasts). Eukaryotes include plants, animals, fungi, and protoctists.
  • Prokaryotes: Single-celled organisms that do not possess a nucleus or membrane-bound organelles. Their genetic material is free in the cytoplasm. Bacteria are prokaryotic organisms.

3. The Main Groups of Living Organisms

3.1 Plants

  • Organization: Multicellular organisms.
  • Cell Structure: Cells contain chloroplasts and carry out photosynthesis. Cells have a cell wall made of cellulose, a permanent vacuole, and a nucleus.
  • Carbohydrate Storage: Store carbohydrates as starch or sucrose.
  • Examples: Flowering plants such as a cereal (e.g. maize) or a herbaceous legume (e.g. peas or beans).

3.2 Animals

  • Organization: Multicellular organisms.
  • Cell Structure: Cells do not have chloroplasts, cannot carry out photosynthesis, and do not have cell walls.
  • Co-ordination and Movement: Usually possess nervous co-ordination and are able to move from one place to another.
  • Carbohydrate Storage: Often store carbohydrates as glycogen.
  • Examples: Mammals (e.g. humans) and insects (e.g. housefly and mosquito).

3.3 Fungi

  • Organization: Mostly multicellular with a body organised into a mycelium made from thread-like structures called hyphae, which contain many nuclei (multinucleate). Some fungi are unicellular (e.g. yeast).
  • Cell Structure: Do not contain chloroplasts and cannot photosynthesise. Cells have cell walls made of chitin.
  • Nutrition: Saprotrophic nutrition — they feed by extracellular secretion of digestive enzymes onto dead or decaying organic material, then absorb the digested organic products.
  • Carbohydrate Storage: Store carbohydrates as glycogen.
  • Examples: Mucor (which has the typical hyphal structure) and yeast (single-celled).

3.4 Protoctists

  • Organization: Microscopic, single-celled (unicellular) organisms.
  • Diversity: Highly diverse group; some resemble animal cells, while others have chloroplasts and resemble plant cells.
  • Examples:
    • Amoeba: Lives in pond water and has features like an animal cell.
    • Chlorella: Contains chloroplasts and has features more like a plant cell.
    • Plasmodium: A pathogenic protoctist responsible for causing malaria.

3.5 Bacteria

  • Organization: Microscopic, single-celled (unicellular) prokaryotic organisms.
  • Cell Structure: Possess a cell wall, cell membrane, cytoplasm, and plasmids (small circular loops of DNA). They lack a nucleus; instead, their genetic material is present as a single circular chromosome of DNA.
  • Nutrition: Some bacteria can carry out photosynthesis, but most feed on other living or dead organisms (saprotrophic decomposers or heterotrophic consumers).
  • Examples:
    • Lactobacillus bulgaricus: A rod-shaped bacterium used in the production of yoghurt from milk.
    • Pneumococcus: A spherical bacterium that acts as the pathogen causing pneumonia.

4. Pathogens and Viruses

4.1 What is a Pathogen?

A pathogen is an organism (or microscopic agent) that causes disease. Pathogens may include:

  • Fungi: (e.g. fungi causing Athlete's foot).
  • Bacteria: (e.g. Pneumococcus causing pneumonia).
  • Protoctists: (e.g. Plasmodium causing malaria).
  • Viruses: (e.g. Influenza virus causing flu).

4.2 Features of Viruses

Viruses are not considered true living organisms because they do not carry out all eight life processes independently.

  • Size: Very small, microscopic particles that are significantly smaller than bacteria.
  • Structure: They have no cellular structure (no cytoplasm, membrane, or organelles). A virus consists of a protein coat surrounding genetic material, which is a single type of nucleic acid — either DNA or RNA.
  • Parasitic Nature: Viruses are strictly parasitic. They can only reproduce inside living host cells by taking over the host cell's machinery. They infect every type of living organism.
  • Examples:
    • Tobacco mosaic virus (TMV): Infects plants (such as tobacco and tomato), causing discolouration of leaves by preventing the formation of chloroplasts.
    • Influenza virus: Causes the 'flu' in humans and other animals.
    • HIV (Human Immunodeficiency Virus): The virus that causes AIDS in humans.