Year 7 & 8 TOC
Unicellular & Multicellular
Identifying structures within cells and describing their function
There are over 8.7 million organisms on earth! Many of them are made up of only 1 cell, these organisms are called unicellular. Unicellular organisms are too small to be seen with the naked eye, but multicellular organisms, like plants & animals, can be seen.
Multicellular organisms are made up of different types of cells. Each type of cell in a multicellular organism has a specialised job.
Unicellular organisms are living things that are made up of just one cell. Bacteria, protozoa and unicellular fungi are all examples of unicellular organisms.
Now you might think, that since they are only one cell they’re really simple but unicellular organisms can be very complex. Since they are completely exposed to the external environment they have adaptations which help them survive.
A typical bacteria cell is only a few micrometers across. While they are microscopic they play a very big part in our lives.
You’ve probably heard of bacteria causing infections and disease. Tuberculosis, Leprosy, Anthrax and Cholera are all examples of diseases caused by bacteria. Different bacteria tend to affect different areas of the body. Tuberculosis, for example, affects the lungs, while Leprosy affects nerve cells. However! Bacteria aren’t all bad! We have 100s of bacteria in our gut and on our body which keep us healthy and prevents bacteria causing us infections. We also use bacteria to make cheese and yoghurt.
The structure of a bacterial cell is different to animals, fungi & plant cells. They are are unicellular and lack organelles or other internal membrane-bound structures. This makes them prokaryotes.
Select the information icons on the bacterial diagram below.
You may have heard of the black plague or black death in history class, but did you know it was caused by a bacteria! click the button to learn more.
Protozoa live in water or damp places. While these protozoa are just a single cell, they have adaptations that let them behave like an animal. They can move, engulf food and remove waist from their cell.
The video below shows an Amoeba moving around a paramecium and engulfing it.
From the past section you’ve probably learnt that some fungi are multicellular, while of others, like yeast, are unicellular. Yeast is common in the production of beer, wine and bread. Yeast have cell walls like plant cells but no chloroplast, this means they need to absorb sugars for their nutrition.
Yeast reproduce by creating a bud. The bud grows until it is large enough to split from the parent cell as a new yeast cell.
Multicellular organisms are things like humans, fish, plants etc. They are made of millions of cells! All the cells work together and perform a specific function to keep the organism alive.
Learn more about multicellular cells on the next page.
Something that all organisms have in common, is their need to replicate or reproduce.
Unicellular organisms will reproduce by dividing them selves into 2 identical new cells. For bacteria this is called, binary fission.
Multicellular organisms have cells which divide by a process called Mitosis. This process also produces 2 identical cells. At the beginning the nucleus divides into 2, then the cytoplasm and by the end we have 2 identical cells. Each time you cut yourself or damage your body, your cells will replicate to fix you. In fact, you started life off as just a single cell and now you are around 1 billion cells!