Cell Theory

Essential Question

What do all living cells have in common?

Your body, a tree, and the bacteria on your skin look nothing alike, but they are all built from cells. In this lesson, you’ll learn the three parts of cell theory, see how scientists built it from evidence, and name the parts that cells share. By the end, you should be able to use evidence to explain why the cell is called the basic unit of life.

The Three Parts of Cell Theory

• All living things are made of one or more cells
• The cell is the basic unit of structure and function
• Today, new cells come only from existing cells

Cell theory is one of the central ideas of biology. It says that all living things are made of one or more cells, and that the cell is the basic unit of structure and function, meaning the smallest unit that can carry out the functions of life. A cell can take in materials, make proteins, and release energy, but the molecules inside it cannot do those things alone. The third part says that new cells come from existing cells, by cell division, which you will study in Unit 7. Viruses are not made of cells, and most biologists do not count them as living things.

How Scientists Built Cell Theory

• 1665: Hooke names cells
• 1670s: Leeuwenhoek sees living microbes
• 1838 and 1839: plants and animals are made of cells
• 1855: Virchow: cells come from cells

No single scientist discovered cell theory. It was built from evidence over about 200 years, and each step depended on better microscopes. Robert Hooke looked at thin slices of cork and saw rows of small, box-like chambers, which he called cells. Antonie van Leeuwenhoek used single-lens microscopes to see living microorganisms in pond water. In 1838 and 1839, Matthias Schleiden and Theodor Schwann concluded that plants and animals are made of cells. In 1855, Rudolf Virchow proposed that new cells come from existing cells.

Parts Cells Share

• A cell membrane around the outside
• Cytoplasm inside the membrane
• DNA that holds genetic instructions
• Ribosomes that build proteins

Cells come in many shapes and sizes, but they share four basic parts. The cell membrane is a boundary of phospholipids and proteins that controls what enters and leaves. The cytoplasm is the fluid inside the membrane, where many chemical reactions happen. DNA holds the instructions for making proteins. Ribosomes read those instructions and build proteins. A few specialized cells lose a part as they develop. For example, mature red blood cells in humans have no nucleus and no DNA.

One Cell or Many Cells

• Unicellular: one cell is the whole organism
• Multicellular: many cells that share the jobs
• A single cell must carry out every life function

Some organisms are unicellular. Bacteria, yeast, and Paramecium are each a single cell that must do everything itself: take in food, get rid of wastes, respond to its surroundings, and reproduce. Other organisms are multicellular, such as plants, animals, and mushrooms. Their bodies have many cells, and different kinds of cells do different jobs. You will see how cells become specialized in a later lesson.

Evidence That Cells Come From Cells

• Many people once thought life could arise from non-living matter
• Pasteur boiled broth in swan-neck flasks
• Broth stayed sterile until microbes could reach it

For a long time, many people believed that living things could arise from non-living matter, such as microbes appearing in spoiled broth. In about 1860, Louis Pasteur tested this idea. He boiled broth in flasks with long, curved necks. Air could enter, but the curve trapped dust and microbes. The broth stayed clear until the flask was tilted so the broth touched the trapped dust, and then it quickly turned cloudy. The results supported the idea that microbes come from existing microbes. The figure is a model of this experiment with sample results.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Pond Water Organism

A student looks at a drop of pond water under a microscope. She sees one organism that swims, takes in food particles, and later splits into two organisms. Use the three parts of cell theory to explain what this observation shows.

Show the solution

1. Part 1: the organism is a single cell, so it is a unicellular organism. All living things are made of one or more cells.
2. Part 2: this one cell takes in food and moves on its own, so the cell is the basic unit of structure and function.
3. Part 3: the cell splits into two cells, which shows that new cells come from existing cells.
4. Answer: It is a unicellular organism, and its cell does every life function and divides to make new cells.

Example 2: Swan-Neck Flasks

Use the flask results to decide whether the microbes in the broth came from the broth itself or from existing microbes in the air. Write a claim, evidence, and reasoning.

Flask	Setup	Result
A	Boiled broth, straight neck, open to air	Cloudy after 2 days
B	Boiled broth, curved neck, open to air	Clear after 30 days
C	Same as B, then tilted so the broth touched the curve	Cloudy after 2 days

Show the solution

1. Claim: the microbes came from existing microbes in the air, not from the broth itself.
2. Evidence: the same boiled broth stayed clear for 30 days in Flask B but turned cloudy in 2 days in Flask A. Flask C turned cloudy in 2 days after it was tilted.
3. Reasoning: the only difference was whether microbes could reach the broth. The curve in Flask B trapped them. This supports the third part of cell theory, that new cells come from existing cells.
4. Answer: The microbes came from existing microbes that reached the broth, not from the broth itself.

Example 3: Four Parts Every Cell Needs

A bacterium and a human cheek cell both have the four shared parts shown in the model. Name the four parts and give the job of each.

Show the solution

1. Cell membrane: a boundary that controls what enters and leaves the cell.
2. Cytoplasm: the fluid inside the membrane, where many chemical reactions happen.
3. DNA: holds the genetic instructions for making proteins.
4. Ribosomes: read the instructions and build proteins.
5. Answer: Cell membrane (boundary), cytoplasm (reactions), DNA (instructions), ribosomes (build proteins).

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Practice

Match observations to the parts of cell theory, label a cell, and use flask data as evidence that cells come from cells. Try it on your own first, then check your work with the answer key.

• Cell Theory Practice Worksheet (PDF)
• Cell Theory Practice Answer Key (PDF)

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Lesson Slides

• 6.1 Cell Theory Slides (PDF)

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Big Ideas

• Cell theory: all living things are made of cells, the cell is the basic unit of life, and new cells come from existing cells
• Cell theory was built from evidence over about 200 years
• Cells share a membrane, cytoplasm, DNA, and ribosomes
• Unicellular organisms do every life function in one cell

Back to the essential question: cells share a membrane, cytoplasm, DNA, and ribosomes. These parts let a cell hold its own chemistry, store instructions, and build proteins. Cell theory says every living thing is made of one or more cells and that new cells come from existing cells. Next, you’ll compare the two main kinds of cells, prokaryotic and eukaryotic.

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