Multicellular Life — Definition & Overview

Diversity in Organisms

Multicellular organisms are made from cells, tissues, organs, and organ systems. Remember that multicellular means made of two or more cells.

The organism is the highest level of organization. All of the other levels, shown in the diagram below, work together to make up one multicellular living thing.

These organisms have tremendous diversity. This category includes microscopic things like dust mites and some plankton, all the way up to massive organisms like blue whales and redwood trees.

Despite their differences in size and structure, all complex organisms follow the same hierarchy of organization.

The order of this organization goes like this:

Multicellular organisms contain all of the parts and pieces you see above. Just like how many single cells make up tissues, multiple tissues make up organs. Multiple organs make up organ systems, and multiple organ systems make up an organism. (Whew, that was exhausting to even type.)

These organisms are the most complex of the levels of organization.

Multicellular and Unicellular Organisms

Now that you've learned what "multicellular" means, you should be able to understand how organisms with many cells are different from single-cell organisms. Unicellular organisms are rather simple because they are just one single unit.

On the other hand, things like multicellular animals and plants are able to have many parts that work together. They can do this because of cell differentiation. This means using different types of cells to do different tasks. Think of skin cells, liver cells, brain cells... These groups of cells can also work together to complete a task, like helping you move your arms.

None of this is possible in unicellular organisms because they only have one cell to work with. The cell can't possibly do a hundred complex tasks at the same time, so it has to be much more simple.

Evolution of Multicellularity

You may be wondering how these complex organisms even came to be. Well, no one really knows the answer yet, but there are a few theories about their origins, which seem to go back at least 600 million years ago. (Single-cell organisms showed up at least 3.5 billion years ago.) Here are a couple of the more popular ones:

Endosymbiosis Theory: Many tiny organisms of different species worked together to evolve into a single living creature.

Colonial Theory: Large groups from the same species came together and formed a colony. This colony was able to handle many different tasks at the same time. Over time, it evolved into a more complex organism.

It's worth noting that multicellular life has evolved independently many times. Because of this, there may be more than one way that it happens.

Key Vocabulary:

1. Multicellular
2. Organism
3. Organ system
4. Organ
5. Tissue
6. Cell

Individual Living Things

And now, finally, we have a living, breathing organism. An organism is an individual living thing. All of the levels of organization come together with their specializations to create a biological marvel. From the enzyme to the nervous system, every part of the organism is doing its job.

It's pretty important to note that not all multicellular organisms have the same organs, organ systems, or tissues. Sea sponges don't have a nervous system and most insects don't have blood vessels. It's amazing how different organisms function. Also, we tend to only talk about organisms with eukaryotic cells because prokaryotes are always unicellular.

Woohoo, you've finally finished working your way through the organizational pyramid.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Organelle}}$ : Specialized structures within a cell

$\color{#8925AE}{\text{Cell}}$ : The smallest unit of life

$\color{#8925AE}{\text{Tissue}}$ : A group of specialized cells that do specific jobs

$\color{#8925AE}{\text{Organ}}$ : A structure made up of specialized tissues

$\color{#8925AE}{\text{Organ System}}$ : A system made up of organs working together

$\color{#8925AE}{\text{Organism}}$ : An individual living thing

(Video) Biological Levels in Biology: The World Tour

by Amoeba Sisters

Can you name all of the biological levels of organization? If not, don't worry. In this video, the Amoeba Sisters go through each of the levels that make up an organism. The video starts with tiny cells and builds bigger and bigger until we reach the level of a functioning organism.