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Cells: Units of Life · All living things are made of cells, which carry out the functions that keep an organism alive · MS-LS1-1

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Cells: Units of Life

with Atlas

Atlas the explorer leans over a glowing microscope in a sunlit lab, pointing at a magnified cell projected on the wall beside a leaf, an ant, and a drop of pond water.

Explain that every living thing is made of one or more cells.
Identify the cell as the smallest unit that is alive.
Recognize examples of life functions that cells carry out to keep an organism alive.
Distinguish living things built from cells from nonliving objects that are not.

Key terms

Cell: The smallest unit that is alive and the building block of every living thing.
Basic unit of life: The phrase describing the cell because nothing smaller can carry out life on its own.
Life function: An activity such as gaining energy, removing waste, growing, or responding that cells perform to stay alive.
Nonliving thing: An object such as a rock or raindrop that is not built from cells and performs no life functions.

One Cell or Many

Living things come in two sizes of construction. A bacterium or an amoeba is unicellular, meaning a single cell handles everything it needs to survive — feeding, growing, and reproducing all in one tiny package. A tree, an ant, or a person is multicellular, built from trillions of cells that divide the work among muscle cells, nerve cells, skin cells, and more. Whether an organism has one cell or trillions, the cell remains the smallest level at which we can say life is happening.

Cells Are Workers, Not Just Bricks

It is tempting to picture cells as tiny bricks that only give an organism shape, but they are constantly busy. A cell pulls in nutrients and converts them into usable energy, expels waste so it does not poison itself, senses changes around it, and copies itself to make new cells. Because nonliving objects like a spoon or a raindrop contain no cells, they can do none of these jobs. This is why the presence of working cells is the surest test of whether something is truly alive.

Worked examples

Determine whether a growing crystal is a living thing.

Recall the test: a living thing must be made of cells that carry out life functions.
Inspect the crystal: it gets larger by adding mineral material on its surface, not by cells dividing.
Check for cells: a crystal contains no cells, so it cannot take in nutrients, remove waste, or respond.
Conclude: growing in size is not the same as being alive.

Answer: A growing crystal is nonliving because it has no cells and performs no life functions.

Hi, I'm Atlas, and today we're shrinking down to explore the smallest piece of life that can still count as alive: the cell. Here's the big idea. Every living thing on Earth is built from cells. Some living things, like bacteria or a tiny amoeba in pond water, are made of just one cell. Others, like a tree, an ant, or you, are made of trillions of cells working together. Because the cell is the smallest thing that is alive, we call it the basic unit of life. But cells are not just building blocks sitting still. Cells are where the work of staying alive actually happens. A cell takes in nutrients and turns them into energy, gets rid of waste, grows, senses and responds to its surroundings, and can make new cells. When your muscle cells pull, your stomach cells break down food, and your nerve cells carry messages, that teamwork between cells is what keeps the whole organism alive. Here's a useful test. Nonliving things, like a rock, a raindrop, or a plastic spoon, are NOT made of cells, so they can't carry out these life functions. If you ever feel stuck deciding whether something is alive, slow down and ask one question: is it built from cells that do the jobs of living? If yes, it's a living thing.

Activity

Sort each item into 'Made of cells (living)' or 'Not made of cells (nonliving)'.

Practice

Decide whether a flickering candle flame is living or nonliving and explain using cells.

Name three life functions that a single bacterium can carry out on its own.

Common mistakes to avoid

Living things must be made of many cellsA single cell, such as a bacterium, is enough to be a complete living organism.
Anything that grows must be aliveNonliving things like rocks or crystals grow by physical processes without any cells or life functions.

Check your understanding

What is the smallest unit that is alive and is the building block of every living thing?

A single bacterium is made of just one cell. Is it considered a living thing?

Why are cells so important to keeping an organism alive?

A friend says, 'A rock can get bigger over time, so it must be made of cells like living things.' Why is this wrong?

Recap

The cell is the smallest unit that is alive, so every living thing is built from one or more cells that carry out life functions, while nonliving objects have no cells and cannot perform those jobs.

Reflect

Think of one object you saw today and ask yourself: is it built from cells doing the jobs of living?