Variety of Living Organisms

All living things can be grouped in one of five kingdoms: animals, plants, fungi, protoctists and bacteria. There’s also viruses, but these aren’t technically living organisms.

Types of living organism

Animals

• Multicellular organisms

• Do not photosynthesise — their cells contain no chloroplasts.

• Obtain food by eating and breaking down large molecules into smaller ones — these can now be absorbed by cells.

• Cells do not contain cell walls.

• Usually have a nervous system and can move from place to place.

• Store carbohydrates as glycogen.

Plants

• Multicellular organisms.

• They use photosynthesis to obtain glucose — their cells contain chloroplasts.

• Their cells also have cell walls made of cellulose.

• Store carbohydrates as starch.

Fungi

• Usually multicellular but some exist as single cells.

• Their body is organised into a mycelium made of thread-like structures called hyphae, which contain many nuclei.

• Their cell walls are made of chitin.

• They do not carry out photosynthesis but feed by secreting digestive enzymes which break down food molecules in their environment. The fungi then absorb these broken down molecules. This type of extracellular digestion is called saprotrophic nutrition.

• Fungi store carbohydrates as glycogen.

Mushrooms and moulds such as Mucor (found on mouldy cheese) are examples of multicellular fungi. Yeast is an example of a single-celled fungus.

Protoctists

• Microscopic unicellular organisms with a range of features.

• Contain organisms which do not fit into the other categories — it’s sometimes called the 'dustbin kingdom'.

• Some have chloroplasts and look a bit like plant cells e.g. Chlorella, whilst others more closely resemble animal cells e.g. Amoeba.

• A pathogenic example is Plasmodium which is carried by mosquitos and causes malaria.

Bacteria

• Microscopic unicellular organisms.

• Cells contain a cell wall but no nucleus.

• DNA exists freely in the cytoplasm in the form of a large circular chromosome and many smaller plasmids.

• Some bacteria can photosynthesise but the majority feel on other living or dead organisms. 

Lactobacillus is a bacteria involved in the production of yoghurt. It digests lactose in yoghurt into lactate (aka lactic acid). This gives yoghurt its thicker texture and makes yoghurt more acidic which helps to preserve yoghurt by killing microorganisms.

Viruses

Don't mistake viruses as living organisms! They are not classified as one of the five kingdoms as they do not carry out basic life processes independently.

Viruses are not living organisms as they can only reproduce when they have invaded the cells of another organism. They consist of a protein coat surrounding nucleic acids (DNA or RNA but not both). An example of a virus is the HIV or influenza virus.

Pathogens

A pathogen is any organism which causes disease. Pathogens can include: fungi, protoctists, bacteria and viruses.

• Athlete's foot is a fungal infection caused by the tinea fungus.

• The protoctist Plasmodium infects and destroys red blood cells, causing malaria.

• Bacteria can cause a range of illnesses, including meningitis and cholera. The Salmonella bacteria causes food poisoning.

• Viruses are responsible for the common cold, flu and Ebola. 

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