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An introduction to what cells are and why they are the basic units of all living things.
Imagine you are building a giant LEGO castle. No matter how big the castle is, it is made of thousands of tiny individual bricks. Did you know your own body is built exactly the same way?
A cell is the smallest building block of life. Just like a house is made of bricks, every living thing—from a tiny blade of grass to a giant blue whale—is made of cells. A cell is the smallest unit that can perform all the functions of life, such as taking in energy, growing, and reacting to the world around it. If something is alive, it is made of at least one cell. If it doesn't have cells, it isn't considered a living thing!
Quick Check
What is the smallest unit that can be considered 'alive'?
Answer
The cell.
Not all living things are the same size, and neither are their cell counts! Some organisms are unicellular, which means they are made of only one single cell. Examples include tiny bacteria. Other living things, like humans, dogs, and oak trees, are multicellular. This means they are made of millions or even trillions of cells working together like a giant team to keep the organism healthy and moving.
Let's determine if an object is made of cells: 1. A Kitten: It grows, breathes, and eats. It is alive, so it is made of trillions of cells. 2. A Rock: It does not grow or eat. It is not alive, so it has zero cells. 3. A Leaf: Even though it fell off a tree, it was part of a living thing, so it is made of plant cells.
Cells are incredibly small! Most cells are only about to micrometers wide. To give you an idea of how small that is, you could fit about human cells on the tiny tip of a needle! Because our eyes cannot see things that small, we use a tool called a microscope. A microscope uses glass lenses to bend light and zoom in, making a tiny cell look large enough for us to study.
Imagine a tiny grain of salt is micrometers wide. If a single skin cell is micrometers wide, how many cells could you line up across that one grain of salt?
You could fit cells in a row across just one tiny piece of salt!
Quick Check
Why can't we see cells just by looking at our skin with our eyes?
Answer
Cells are too small for the human eye to see; they require a microscope to be visible.
When a baby elephant grows into a giant adult elephant, do its cells just get bigger and bigger like balloons? 1. No! Cells have a limit on how big they can get. 2. Instead, the elephant's cells divide to create more cells. 3. An adult elephant is bigger because it has more cells, not just larger cells.
Which of these is the best definition of a cell?
If an organism is 'multicellular,' what does that mean?
True or False: All living things, including trees and bacteria, are made of cells.
Review Tomorrow
In 24 hours, try to explain to a friend or family member why you are like a 'LEGO castle' and what your 'bricks' are called.
Practice Activity
Find three things in your house that are made of cells (living or once-living) and three things that are not!