Common Features: Eukaryotic Organisms (Edexcel IGCSE Biology)

• All living organisms can be grouped or 'classified' using a classification system that consists of five kingdoms
• These five kingdoms are:
  • Animals
  • Plants
  • Fungi
  • Protoctists
  • Prokaryotes
• The first four kingdoms in this list (the animals, plants, fungi and protoctists) can actually be grouped together, as they are all eukaryotic organisms (also known as eukaryotes)

Animals, plants, fungi and protoctists are all eukaryotes.

• Eukaryotic organisms can be multicellular or single-celled and are made up of cells that contain a nucleus with a distinct membrane

An animal cell (left) and plant cell (right) as seen under a light microscope. They are both eukaryotic cells as they both have a distinct membrane-bound nucleus.

• Prokaryotic organisms (also known as prokaryotes) are in a separate kingdom and are different from eukaryotes as they are always single-celled and do not contain a nucleus (instead, the nuclear material of prokaryotic cells is found in the cytoplasm)
  • Bacteria are prokaryotic organisms
• Prokaryotic cells are substantially smaller than eukaryotic cells

• The main features of animals:
  • They are multicellular
  • Their cells contain a nucleus with a distinct membrane
  • Their cells do not have cellulose cell walls
  • Their cells do not contain chloroplasts (so they are unable to carry out photosynthesis)
  • They feed on organic substances made by other living things
  • They often store carbohydrates as glycogen
  • They usually have nervous coordination
  • They are able to move from place to place

A typical animal cell

Cell structures found in both animal and plant cells table

• The main features of plants:
  • They are multicellular
  • Their cells contain a nucleus with a distinct membrane
  • Their cells have cell walls made out of cellulose
  • Their cells contain chloroplasts
  • They feed by photosynthesis
  • They store carbohydrates as starch or sucrose
  • They do not have nervous coordination

A typical plant cell

Cell structures found only in plant cells table

• Main features of fungi:
  • They are usually multicellular but some are single-celled (e.g. yeast)
  • Multicellular fungi are mainly made up of thread-like structures known as hyphae that contain many nuclei and are organised into a network known as a mycelium
  • Their cells contain a nucleus with a distinct membrane
  • Their cells have cell walls made of chitin (chitinous cell walls)
  • Their cells do not contain chloroplasts (so they cannot carry out photosynthesis)
  • They feed by secreting extracellular digestive enzymes (outside the mycelium) onto the food (usually decaying organic matter) and then absorbing the digested molecules. This method of feeding is known as saprotrophic nutrition
  • Some fungi are parasitic and feed on living material
  • Some fungi store carbohydrates as glycogen
  • They do not have nervous coordination
  • Examples of fungi include: moulds, mushrooms, yeasts

A typical fungal cell

The typical structure of a multicellular fungus e.g. Mucor (bread mould)

• Main features of protoctists:
  • The protoctists are a very diverse kingdom of organisms that don't really belong in any of the other eukaryotic kingdoms (animals, plants and fungi)
  • They are mainly microscopic and single-celled but some aggregate (group together) into larger forms, such as colonies or chains of cells that form filaments
  • Their cells contain a nucleus with a distinct membrane
  • Some have features making them more like animal cells e.g. Plasmodium (the protoctist that causes malaria)
  • Some have features, such as cell walls and chloroplasts, making them more like plant cells e.g. green algae, such as Chlorella
  • This means some protoctists photosynthesise and some feed on organic substances made by other living things
  • They do not have nervous coordination
  • Examples of protoctists include: amoeba, Paramecium, Plasmodium, Chlorella

Two examples of protoctist cells