[Please watch the video attached at the end of this blog for a visual explanation of this topic]

We learnt previously that according to the Linnaean System of Classification, we first divide organisms into Kingdoms, and that is what we will be addressing  in this article.

Before that however, we need to learn the foundation of this lesson: the cell is a basic structural unit of life and every living organism is made up of cells.

There are some simpler organisms that are made up of just one cell (unicellular organisms), but the more complex organisms have many cells (multicellular) in their bodies.

There are some features common to all cells and a typical cell would look like this usually:

1. Cell membrane

Every cell has a cell membrane/ plasma membrane. A cell membrane is a thin membrane that runs around the cell and this separates the contents inside the cell from the outside environment.

2. Cytoplasm

This is the gelatinous fluid that fills up the inside of the cell

3. Genetic material

Every single cell that we come across is going to have genetic material which is usually DNA. However, depending on the Kingdom, the way/place this DNA is found would differ.

The Five Kingdoms

1. Kingdom Animalia (Animal Kingdom)

2. Kingdom Plantae (Plant Kingdom)

3. Kingdom Fungi

4. Kingdom Protista

5. Kingdom Prokaryota/ Kingdom Monera

This five kingdom system of classification was introduced by R. H. Whittaker.

Kingdom Animalia/ The Animal Kingdom

This is a kingdom that all of us are familiar with, and animals are usually very easy to recognise. There are a few main features of living organisms belonging to this kingdom.

1. They are all multicellular – multicellular means that the bodies of animals are made up of many cells.

2. Their cells contain a nucleus but no cell wall or chloroplasts – animals cells have a nucleus that hold the DNA and chromosomes of the animal, and it also consists of a cell membrane and cytoplasm, but there is no cell wall. 

3. They feed on organic substances made by other living things –

4. Some living organisms have an organelle known as chloroplasts which help organisms to photosynthesise and produce their own food. However, animal cells do not have chloroplast, which means that they do not photosynthesise nor do they produce their own food. Therefore, they have to rely on other living things for food. Animals get their nutrition by feeding on organic substances made by other living things.

Kingdom Plantae/ the Plant Kingdom

1. They are multicellular – All plants are multicellular

2. Their cells contain a nucleus, chloroplasts and cellulose cell walls

• We can see that this plant cell has a nucleus, it has a cell membrane, it has cytoplasm, which are the three important features that were shown in animal cells. In addition to those, the plant cell also contains a structure called the cell wall which is made of cellulose, and also little organelles called chloroplasts.

3. They all feed by photosynthesis

Earlier, when discussing chloroplast, it was mentioned that the function of chloroplast is to help an organism photosynthesise and produce its own food. Therefore, since plant cells do have chloroplast, they are able to carry out photosynthesis and produce their own food.

Kingdom Fungi

1. Usually multicellular – in most cases, they are multicellular, like bread moods. Some, such as baker’s yeast, are unicellular.

2. Cells have nuclei and cell walls not made from cellulose

When we observe this fungal cell, we can see that it has a cell membrane, a nucleus, and a cell wall, but this cell wall is made up of a different material to a plant cell. A fungal cell’s cell wall is made up of a material called chitin; it has a chitinous cell wall.

3. Do not photosynthesise, but feed by saprophytic or parasitic nutrition

Chloroplasts are absent in fungal cells and therefore are unable to photosynthesise and produce their own food. They instead get their nutrition by feeding saprophytically (through dead or decaying material) or parasitically (on living material).

Kingdom Protoctista/ Protista

1. Most are unicellular but some are multicellular

2. All have a nucleus, some may have cell walls and chloroplasts

When we talk about protoctists, there are some protists that have chloroplasts in them as well as cell walls (similar to plant cells), and there are some protoctists who do not have chloroplasts within them (which resemble animal cells). However, all of them have a nucleus.

3. Some Protoctisa photosynthesise while some feed on organic substances made by other organisms

The protoctists who do have chloroplasts can carry out photosynthesis and produce their own food, but the protoctists who do not have chloroplast feed on organic substances made by other living things.

Kingdom Prokaryota/ Kingdom Monera/ Prokaryote Kingdom

1. Often unicellular.

2. Have cell walls that are not made up of cellulose, and cytoplasm but no nucleus or mitochondria.

One of the most important points about prokaryotes is that a prokaryotic cell does NOT have a nucleus. Therefore, the DNA material of prokaryotes is just suspended within the cytoplasm. They also do not have other organelles such as mitochondria, but they do have cytoplasm, and some of them have cell walls.

• Not having a nucleus is a unique feature to organisms that belong to this kingdom. 

Viruses

Especially given the past few years, the word “virus” is something that has been heard by every one of us. However, this begets the question as to which kingdom viruses belong to.

Viruses do not belong to any one of these five kingdoms because they are simply not part of the classification system.

As discussed in Features of Living Organisms, there are 8 basic life processes that it must show. When talking about viruses however, viruses show only ONE of these life processes, which is reproduction. Viruses take over a host cell and they make multiple copies of themselves, and thereby reproduce. However, they cannot perform any of the other processes a living organism can. This is why we can’t really consider it as a living thing.

A virus contains the genetic material covered by a nucleocapsid, all enveloped by a protein coat.

Revising the Five Kingdoms

Since this is a subtopic under Characteristics and Classification of Living Organisms, make sure that you know your basics regarding Characteristics of Living Organisms and Classification.

This lesson is quite interesting and questions are of various types. Some questions can be found here as well, and you can time your answers to see if you can stick to the time limit given.

If you are struggling with IGCSE revision or Biology in particular, you can reach out to us at Tutopiya to join revision sessions or find yourself the right tutor for you.

Watch the video below for a visual explanation of the lesson and make sure to attempt the quiz later!

Rizka Naushad

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