Bio IGCSE (1)

1 The nature & variety of living organisms

(a) Characteristics of Living Organisms

Living organisms share the following characteristics:

1.  Movement - An action by an organism or part of an organism that causes a change in position or place
2. Respiration - Chemical reactions that break down nutrient molecules in living cells to release energy
3. Sensitivity - Ability to detect stimuli in their surroundings and to make responses
4. Homeostasis - The ability to control their internal conditions.
5. Growth - Permanent increase in size and dry mass by an increase in the number of cells, the cell size or both
6. Reproduction - Processes that make more of the same kind
7. Excretion - Removal of toxic materials from organisms. Waste products of metabolism and substances in excess
8. Nutrition - The taking in of materials for energy, growth and development

   MRS H. GREN - an acronym for the 8 characteristics of the living organisms

(b) Variety of Living Organisms

Eukaryotic Organisms

• Plants
  • they are multicellular organisms
  • their cells contain chloroplasts to carry out photosynthesis
  • their cells have cellulose cell walls
  • they store carbohydrates as starch or sucrose.
    • Examples include :
    • flowering plants,
    • such as a cereal (for example, maize),
    • and a herbaceous legume (for example, peas or beans).
• Animals
  • these are multicellular organisms
  • their cells do not contain chloroplasts and are not able to carry out photosynthesis
  • they have no cell walls; they usually have nervous co-ordination and are able to move from one place to another
  • they often store carbohydrate as glycogen. 
    • Examples include :
    • mammals (for example, humans) 
    • and insects (for example, housefly and mosquito)
• Fungi
  • these are organisms that are not able to carry out photosynthesis; 
  • their body is usually organised into a mycelium made from thread-like structures called hyphae, which contain many nuclei; 
  • some examples are single-celled; 
  • cells walls are made of chitin; 
  • they feed by extracellular secretion of digestive enzymes onto food material and absorption of the organic products; 
  • this is known as saprotrophic nutrition;
  • they may store carbohydrate as glycogen. 
    • Examples include:
    • Mucor, which has the typical fungal hyphal structure, 
    • Yeast, which is single-celled.
• Protoctists
  • these are microscopic single-celled organisms. 
  • some, like Amoeba, that live in pond water, have features like an animal cell, 
  • while others, like Chlorella, have chloroplasts and are more like plants.
    • Example is
    • Plasmodium, responsible for causing malaria.

Prokaryotic Organisms

• Bacteria
  • these are microscopic single-celled organisms
  • they have a cell wall, cell membrane, cytoplasm and plasmids
  • they lack a nucleus but contain a circular chromosome of DNA; 
  • some bacteria can carry out photosynthesis 
  • but most feed off other living or dead organisms.
    • Examples include 
    • Lactobacillus bulgaricus, a rod-shaped bacterium used in the production of yoghurt from milk,
    • and Pneumococcus, a spherical bacterium that acts as the pathogen causing pneumonia.

Pathogens

Pathogens are disease-causing organisms. Fungi, protoctists, bacteria and viruses can all be classified as pathogens

• Virus
  • these are not living organisms. 
  • they are small particles, smaller than bacteria; 
  • they are parasitic and can reproduce only inside living cells; 
  • they infect every type of living organism. 
  • they have a wide variety of shapes and sizes; 
  • they have no cellular structure but have a protein coat and contain one type of nucleic acid, either DNA or RNA.
    • Examples include 
    • the tobacco mosaic virus that causes discolouring of the leaves of tobacco plants by preventing the formation of chloroplasts, 
    • the influenza virus that causes ‘flu’ 
    • the HIV virus that causes AIDS