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Explain the Structure of Unicellular Organisms

In this worksheet, students will explain the structure of some unicellular organisms.

'Explain the Structure of Unicellular Organisms ' worksheet

Key stage:  KS 3

Year:  Year 8 Science worksheets

Curriculum topic:   Biology: Structure and Function of Living Organisms

Curriculum subtopic:   Cells and Organisation

Difficulty level:  

Worksheet Overview

Did you know that there are about 37 trillion cells in the human body?! Even then, that's an estimate, so it could be more! 


boy pulling silly face


So we know that living organisms are made up of cells, and all living organisms carry out the seven life processes. Do you remember what these are?










Organisms usually use different organ systems to carry out the life processes, for example, humans have a digestive system to help with nutrition and a circulatory system to help with respiration. These organ systems are made up of groups of tissues and organs that work together. 


However, not all organisms have organ systems or even tissues. Some are only made up of one cell! We call these unicellular organisms. These unicellular organisms can still carry out the seven life processes but only have one cell with which to do so.


As they are only made up of one cell, unicellular organisms are extremely small and need a microscope to be seen. 


An example of a unicellular cell is bacteria - you can see what a typical bacterial cell looks like below:


Image of bacterial cell


Did you notice that a bacterial cell doesn't have a nucleus? Instead, it has strands of DNA that are found in the cytoplasm. The cytoplasm is where the cell's chemical reactions happen.


It also has a cell membrane (in this diagram it's called the plasma membrane). The cell membrane controls which substances can travel into and out of the cell, just like in an animal or plant cell.


A bacterial cell, like a plant cell, also has a cell wall. The cell wall provides support. 


Interestingly, this bacterial cell has something called flagella (flagellum if it's just one; not all bacterial cells have these). These flagella help the bacteria to move -they look a bit like a tail!   


Other examples include yeast, which is a unicellular fungus:


Image of yeast cells


Fungi have similar structures to both animal and plant cells. They have a nucleus, cytoplasm and cell membrane. Unlike animal cells however, fungi have a cell wall like plant cells. Similar to plant and bacterial cells, the cell wall provides it with support.


Image of yeast cell structure


Another example of a unicellular organism is algae, which belongs to a group called the protozoa:  


Image of algae


Algae have a lot in common with plants - their cells have cell walls, vacuoles and chloroplasts. Chloroplasts are used to trap the sun's light energy to produce glucose in a process called photosynthesis. 


Vacuoles usually store water (or sap) in plants, whereas in algae it's often used to store food.


Image of algal cell structure


Protists (organisms that belong to the protozoa group) generally have a nucleus, cytoplasm and cell membrane (like plant and animal cells). Some have structures called cilia which are like tiny hairs. These move and wave food towards itself to absorb. 


Some protists also have flagella like bacteria. This helps them to respond to the environment that they're in and to move.


Bacteria are the smallest unicellular organism followed by unicellular fungi. Protists are often the larger unicellular organism - they come in many different shapes and sizes.


In this activity, we're going to explain the structure of unicellular organisms.


girl using a microscope

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