Units of Life

So we know what qualifies something as alive, but what exactly is life made of?

Whether you’re a redwood tree or a tardigrade, all living things share one fact: they’re made of cells. Cells are the smallest units of life, and they’re the foundation of everything biology studies.

Let’s break down what “units of life” actually means.

1. The Cell: Life’s Smallest Unit

A cell is the basic structural and functional unit of all living organisms. It’s like a mini factory: it’s constantly taking in materials, creating energy, responding to messages, and reproducing itself. All living things are made up of one or more cells.

There are two main categories of organisms:

• Unicellular: These are one-celled organisms like bacteria and many protists. One cell does everything.
• Multicellular: These organisms, like plants, animals, and humans, are made of many cells that specialize and work together (like muscles, nerves, skin, etc.).

Whether it’s a single bacterium or your entire body, the cell is the basis of all life.

2. Levels of Biological Organization

1. Cells – The first level of life. They perform all the functions of a living thing.
2. Tissues – Groups of similar cells working together (like muscle tissue).
3. Organs – Tissues molded and grouped together to perform a function (like the heart or lungs).
4. Organ Systems – Organs working together (like the circulatory system).
5. Organism – A complete, living individual.

3. All Cells Aren’t Created Equal

Cells differ depending on what organism they come from, but there are two types of cells:

Prokaryotic Cells

• No nucleus
• Simple structure
• Examples: Bacteria and Archaea

Hint: Prokaryotic cells are “pros” at working without a nucleus.

Eukaryotic Cells

• Have a nucleus
• More complex and contain organelles
• Examples: Plants, animals, fungi, and protists

Hint: Eukaryotic sounds like you-karyotic, and we have nuclei in our cells!

Fun Fact:

Your body has about 37 trillion cells. For reference, 37 trillion seconds is 1.1 million years!

Why This Matters

When scientists study life, they start with the cell. Want to cure cancer? Study how cells divide. Want to fight infection? Understand how bacterial cells work. Every major discovery in biology comes back to the basic units of life.

And remember: even the most complex life started as a single cell. You did too.

The next time you look in the mirror, don’t just see a face. See trillions of microscopic units, each one humming with activity, each one doing its part to keep you alive. You are, quite literally, a universe of life.