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Lesson 1: What Is Life?
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Chapter 3
Organization of Living Things
Your cells are the smallest living part of you. Your cells attach to each other to form tissues, which gather together to form organs, which are grouped into the organ systems that make up you, the entire organism! Let's take a closer look at what each of these terms means.
Cells are the smallest living part of the body. For example, a muscle cell is a type of cell in your body. Muscle cells are specialized to perform contractions.
Tissues form when cells that have similar structures and functions attach. For example, muscle cells attach to each other to form muscle tissue. One special type of muscle tissue in your body is cardiac muscle tissue.
Organs are made of groups of tissues. Each organ does a specific task or group of tasks for the body. For example, cardiac muscle tissue combines with other tissues to form the organ we know as the heart.
Organ systems are groups of organs that work together to perform a function for the body. For example, the heart is part of the organ system called the circulatory system, which carries important materials, such as oxygen, all around your body.
An organism is any individual living thing. An organism can be as simple as a single cell or as complex as a human with trillions of cells and multiple organ systems—for example, you!
Now let's put your new vocabulary to work: In the following illustration, you see cells that make up lung tissue. Lungs are organs that are part of the respiratory organ system, which controls breathing. A human is the organism that contains the organ system. Are you starting to get the picture?

Organization of the human body
What Makes Up Cells?
Although the cell is the smallest living part of your body, it's not the smallest part of all. Cells themselves consist of smaller structures called molecules, which are in turn made up of atoms, which are also made of smaller particles such as protons, neutrons, electrons, and even more tiny particles that physicists are busy discovering.
Take a look at the illustration below to see the smaller structures that make up cells. This diagram is just to give you an idea of their relative sizes—it's not to scale.

Cells contain smaller structures
This diagram shows you that cells are bigger than molecules, which are bigger than atoms, which are bigger than protons, neutrons, and electrons. (You'll soon see that this makes sense because cells are made of molecules, which are made of atoms, which are made of protons, neutrons, and electrons!) In this kind of diagram, however, it's almost impossible to show you how the sizes of these things compare. We can compare sizes much better in a diagram like this one:

Relative sizes of common objects
This diagram shows what we can see with our eyes, versus what we can see with light microscopes (the kind of microscope you'd find in a typical science class), versus what we can see with electron microscopes (very specialized microscopes found in large research facilities like universities). This should give you a better perspective on the comparative sizes of things. As you can see, cells are so small that we need a light microscope to see them. Molecules are so small that you can barely see them with the most powerful microscopes ever made, and atoms and subatomic particles are much, much smaller than that!
Atoms might be tiny, but they're incredibly important. They are the basic units of matter, the stuff that our world is made of. Everything, living or not, is made of matter. Dogs, cats, bushes, sand, rocks, computers, airplanes, oceans, your morning cup of coffee—you name it, it's made of matter (which