Explain the Structure of Unicellular Organisms

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! 

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?

Movement

Respiration

Sensitivity

Growth

Reproduction

Excretion

Nutrition

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:

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:

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.

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

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.

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.