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Classifying Living Things - Science Lessons for Grade 4 Students

Due to the great variety of living things around us, scientists classify them into groups. The four main groups of living things are micro-organisms, fungi, plants and animals.


There are many living things around us that are too small to see using only our eyes. These living things are called micro-organisms. The word ‘micro’ means very small, and the word ‘organism’ means life form. To see micro-organisms, we need to use a microscope.

A microscope, as shown below, is a scientific instrument that allows us to observe things closely. When you use a microscope, you need to handle it carefully.


Bacteria (The singular form of bacteria is bacterium) are micro-organisms that can be said to be omnipresent. They can be found in the soil, in water and in the air. Some bacteria are harmful to people. Bacteria in soil, animal waste and on rotten or uncooked food can make you sick. Washing your hands and keeping your body clean are ways of stopping harmful bacteria from entering your body.

Some examples of harmful bacteria are salmonella, E.coli and streptococcus.

Salmonella, as seen in the picture below, can be present in raw or undercooked meat and most salmonella infections are classified as stomach flu (gastroenteritis).

E.coli, as seen in the picture below, is found in the gut of warm blooded animals including humans. Though many strains of this bacteria are harmless, some can cause severe foodborne diseases.

Streptococcus, as seen in the picture below, is found on most tooth surfaces, and hard-to-reach areas like the grooves in your premolars and molars. It promotes decay and the breakdown of tooth enamel.

Some bacteria are helpful to people. The bacteria in your stomach and intestines help you digest food. Bacteria are also used to make some of the foods we eat, such as yoghurt, cheese, sourdough bread and the Korean food kimchi. Lactobacillus Acidophilus are, for example, helpful bacteria to human beings. It is present in our intestine. Exposing milk to lactobacillus acidophilus bacteria, shown below, results in yoghurt.

Bacteria reproduce by dividing into two. This allows them to reproduce very rapidly. Did you know that some bacteria can reproduce as often as once in every 20 minutes? That means billions of bacteria in a day!


Fungi, some of the most fascinating and diverse organisms on Earth, include microorganisms such as molds and yeasts, and mushrooms. They do not move from place to place and are usually fixed in one position. Many fungi such as mushrooms have the same basic parts: a stalk, a cap and gills.

Unlike plants, fungi do not produce seeds. They reproduce from microscopic spores. The spores are let out into the air and carried to new places. Also, unlike plants, fungi cannot make their own food. They feed on dead plants and animals. Fungi can also feed on food that we eat and on our bodies too. Athlete’s foot for example, is caused by fungi feeding on dead skin. It is a fungal skin infection that typically begins between the toes, and commonly occurs in people who wear tight-fitting shoes and hence have very sweaty feet. The signs of athlete’s foot include an itchy, scaly rash.

Fungi are quite important to people. It is used in some of the food we eat and in medicine. Yeast, for example, has been used for millennia to make bread and a variety of similar food items. Mould can be used to add flavor to cheeses. Penicillium is a type of would, shown below, that is used to make the medicine Penicillin.

The cordyceps or caterpillar fungus is used in traditional Chinese and Tibetan medicines. Other than these, numerous fungi have therapeutic effects, including antioxidant and anti-inflammatory properties. Some of the best-known medicinal mushrooms include Reishi (Ganoderma lucidum), Shiitake (Lentinus edodes), Turkey tails (Trametes versicolor) and Lion’s mane (Hericium erinaceus).

Some fungi, on the other hand, can be extremely toxic to human health. Certain mushrooms can be lethal if consumed. Some diseases are also caused by fungi. For example, Sporotrichosis is caused by Sporothrix, a fungus that lives in soil and on plants throughout the world. Another disease caused by certain types of fungi found in water and soil, typically in rural regions of Latin America, Africa and Asia is Mycetoma, which is a chronic inflammation of tissues.

Did you know that the largest living organism in the world is an individual shoestring fungus (Amarilla ostoyae) found in Oregon? It is spread over 2300 acres of land!