iGCSE Biology - The Variety of Living Organisms

The Variety of Living Organisms

 

• Five major groups: plants, animals, fungi, bacteria and viruses.
• Plants:

          Multicellular: their bodies are made up of many cells.

          Their cells contain chloroplast and carry out photosynthesis.

          They have cell walls made of cellulose.

          They produce starch and sucrose, among other things.

              Produce their own food.

• Animals:

          Fish, amphibians, reptiles, birds, and mammals are vertebrates.

              All other animals are invertebrates, i.e. they do not have a                             spine.

              Multicellular: their bodies are made up of many cells.

              Feed on other animals or plants.

              No cell walls, so their cells can change shape in movement.

              Store carbohydrate in their cells as glycogen.

• Fungi:

              Mushrooms, toadstools and moulds are multicellular.

                   Yeasts are unicellular.

              Cell walls made of chitin.

              Multicellular fungi contain hyphae: fine thread-like filaments.

                   A network of hyphae is called a mycelium.

              Hyphae secrete digestive enzymes onto their food, and the                              absorb the soluble substances produced.

                   Feeding on dead organic material: saprotrophic.

                   Enzymes used in this are called extracellular enzymes.

• Bacteria:

              Unicellular organisms.

                   Three basic shapes: spheres, rods and spirals.

              Don’t contain nuclei, instead their entire DNA is stored in a                            single chromosome.

                   Bacteria move through use of flagella.

                   Their cells contain plasmids which carry some of their genes.

                   Some bacteria are pathogens and most are decomposers.

• Viruses:

                   All viruses are parasites.

                   No nucleus or cytoplasm, simply as core of DNA or RNA                      surrounded by a protein coat.

                   A membrane called an envelope may surround a virus but this is                       stolen from the host, and not made.

                   A virus reproduces by taking over its host’s genetic machinery.

                   Influenza virus: circular and flat

                   Tobacco Mosaic virus: cylindrical