Conduct investigations to collect evidence that living...

SECTION 1

How Did We Discover Cells?

Imagine looking at a drop of pond water and seeing tiny creatures swimming around. For most of human history, nobody knew these creatures existed. People had no idea that their own bodies were made of trillions of tiny parts. The invention of the microscope (a tool that makes small things look bigger) changed everything.

Our anchoring phenomenon for this lesson is simple but amazing: when you place any living thing under a microscope, you see it is made of smaller units called cells. Why does every living thing share this feature? Scientists spent hundreds of years collecting evidence to answer that question.

1665

Robert Hooke Names "Cells"

Robert Hooke used a microscope to look at thin slices of cork. He saw tiny box-like spaces and called them cells because they reminded him of small rooms in a monastery.

1674

Leeuwenhoek Sees Living Cells

Anton van Leeuwenhoek built powerful lenses and observed living single-celled organisms in pond water. He called them "animalcules" — tiny animals.

1838

Schleiden Studies Plants

Matthias Schleiden examined many plants under a microscope. He concluded that all plants are made of cells.

1839

Schwann Studies Animals

Theodor Schwann studied animal tissues. He found that animals are also made of cells. Together with Schleiden, they formed the first parts of cell theory.

1855

Virchow Completes Cell Theory

Rudolf Virchow stated that all cells come from other cells. This completed the three main ideas of the cell theory.

Each of these scientists collected evidence by making careful observations. They used the science practice of planning and carrying out investigations. They looked at many different living things and found the same pattern: cells everywhere. This pattern across organisms is a crosscutting concept called Patterns. When you see the same thing again and again in nature, it usually points to a big idea.

SECTION 2

Core Principles: Cell Theory and Types of Cells

Centuries of investigation led to three big ideas about cells. Together these ideas form the cell theory. Cell theory is one of the most important explanations in all of biology. It tells us that cells are the basic unit of all life.

All living things are made of cells

Every organism — from bacteria to blue whales — is built from one or more cells. A cell is the smallest unit that can carry out the functions of life.

Cells are the basic unit of life

Cells are the smallest structures that are considered alive. They can take in energy, grow, respond to their environment, and reproduce.

All cells come from existing cells

New cells are made when existing cells divide. Cells do not appear from nowhere. This means all life is connected back through generations of cell division.

Unicellular vs. Multicellular

Unicellular organisms (like bacteria) are made of just one cell. Multicellular organisms (like you!) are made of many cells working together.

Cells Have Common Structures

Most cells share structures like a cell membrane (outer boundary), cytoplasm (jelly-like inside), and genetic material (DNA).

✦ KEY TAKEAWAY

Think of cells like bricks in a building. A small shed might be made of just one or two bricks (unicellular). A skyscraper needs millions of bricks organized into floors and rooms (multicellular). No matter the size of the building, bricks are always the basic building unit — just like cells are always the basic unit of life.

SECTION 3

Seeing Cells: Animal vs. Plant Cells Under the Microscope

When you look at living things under a microscope, you notice something important. Both animal and plant cells share some structures, but they also look different. The diagram below shows the key parts of a typical animal cell and a typical plant cell. Notice the crosscutting concept of Structure and Function — each cell part has a shape that matches its job.

Both cells share a nucleus, cell membrane, mitochondria, ribosomes, and cytoplasm. The plant cell has extra structures: a rigid cell wall, green chloroplasts for photosynthesis, and a large central vacuole for storing water.

Notice the pattern: both cells have a membrane, a nucleus, and cytoplasm. This is powerful evidence that all living things share a common organization. When scientists observe this same pattern in bacteria, fungi, protists, plants, and animals, it supports cell theory. The crosscutting concept of Patterns helps scientists recognize that these shared structures point to a deep connection between all living things.

SECTION 4

How to Investigate: Planning a Cell Study

Scientists don't just look through a microscope randomly. They use a science and engineering practice called Planning and Carrying Out Investigations. This means deciding what question to ask, what samples to look at, and how to record observations. Let's walk through how a cell investigation works.

Steps for a Microscope Investigation

Ask a question: "Are both plant and animal tissues made of cells?"
Gather materials: Microscope, glass slides, cover slips, water dropper, stain (like iodine or methylene blue), and samples (onion skin, cheek cells, pond water).
Prepare a wet mount: Place a thin sample on a slide, add a drop of water or stain, and lower a cover slip gently.
Observe and record: Start at the lowest magnification. Focus carefully, then switch to higher power. Draw what you see and label the structures.
Analyze evidence: Compare your drawings across samples. Look for patterns — do all samples show cells?

Understanding Magnification

A microscope uses two lenses to make things look bigger. Magnification (how many times bigger something appears) is calculated by multiplying the power of the two lenses together.

TOTAL MAGNIFICATION

Total Magnification = Eyepiece Power × Objective Power

The eyepiece is the lens you look through (usually 10×). The objective is the lens near the slide (common powers: 4×, 10×, or 40×).

For example, if your eyepiece is 10× and your objective is 40×, the total magnification is 10 × 40 = 400×. That means the image looks 400 times bigger than what your eye alone can see! Most cells need at least 100× to be visible.

🔬 Why Use Stains?

Many cells are nearly see-through. A stain is a colored dye that sticks to certain cell parts. Iodine turns starch dark, which helps you see plant cell walls. Methylene blue colors the nucleus in animal cells. Stains make invisible structures visible — they are an important tool in any cell investigation.

SECTION 5

Collecting and Comparing Evidence from Different Organisms

To build a strong argument that all living things are made of cells, you need to look at many different types of organisms. Scientists call this collecting multiple lines of evidence. If you only looked at onion skin, you could only say onions are made of cells. By examining many organisms, the evidence gets stronger.

Five different organisms from five kingdoms of life all show cells when viewed under a microscope. This repeated pattern across diverse organisms is strong evidence for cell theory.

Comparison of five organisms examined under a microscope

Organism	Kingdom	Cell Type	Key Feature Observed
Onion skin	Plant	Eukaryotic	Rectangular cells with visible cell walls
Human cheek	Animal	Eukaryotic	Flat, round cells with nucleus visible after staining
Paramecium	Protist	Eukaryotic	Single cell that moves on its own using tiny hairs (cilia)
Mushroom tissue	Fungus	Eukaryotic	Thread-like cells (hyphae) with cell walls
Yogurt bacteria	Bacteria	Prokaryotic	Very tiny cells with no visible nucleus

Notice two types of cells in the table: eukaryotic (cells with a nucleus surrounded by a membrane) and prokaryotic (cells without a membrane-bound nucleus, like bacteria). Even though these cells look different, they are all still cells. This is strong evidence for cell theory.

SECTION 6

Worked Example: Investigating Elodea and Cheek Cells

Let's walk through a real investigation. You want to answer this question: "Do both a plant (Elodea, an aquatic plant) and an animal (your cheek cells) show evidence of being made of cells?" Here is how you would collect evidence step by step.

Investigation: Comparing Elodea Leaf and Human Cheek Cells

Step 1 — State Your QuestionAre both Elodea (a plant) and human cheek tissue composed of cells? What structures can we observe in each?

Step 2 — Prepare the Elodea SlidePluck a small leaf from the Elodea plant. Place it flat on a glass slide. Add one drop of water and carefully lower a cover slip. The leaf is thin enough to see through.

Step 3 — Observe Elodea at 400×Use the 10× eyepiece and the 40× objective (10 × 40 = 400× total magnification). You should see rectangular, brick-like cells arranged in neat rows. Inside each cell, small green dots called chloroplasts may be moving — this movement is called cytoplasmic streaming.

Evidence: Elodea is made of cells with cell walls, cell membranes, and chloroplasts.

Step 4 — Prepare the Cheek Cell SlideGently scrape the inside of your cheek with a clean toothpick. Smear the sample on a slide. Add one drop of methylene blue stain and a cover slip. Wait about 30 seconds for the stain to soak in.

Step 5 — Observe Cheek Cells at 400×At 400× magnification, you should see flat, roundish cells scattered on the slide. The methylene blue makes the nucleus appear as a dark blue circle inside each cell. You will not see cell walls or chloroplasts — those are plant-only structures.

Evidence: Cheek tissue is made of cells with a cell membrane, cytoplasm, and a nucleus.

Step 6 — Analyze and Construct an ExplanationBoth samples showed cells. The Elodea cells had rectangular shapes, cell walls, and chloroplasts. The cheek cells were round, lacked cell walls, and had no chloroplasts. Despite these differences, both had a cell membrane, cytoplasm, and a nucleus.

Conclusion: Both the plant and the animal are composed of cells. This supports the cell theory.

🧪 NGSS Connection

In this worked example, you used the SEP of Planning and Carrying Out Investigations and the SEP of Constructing Explanations. You also applied the CCC of Structure and Function by connecting what you saw (structure) to what cells do (function).

SECTION 7

Strengths and Limitations of Microscope Investigations

Microscope investigations are powerful, but they also have some limits. Understanding these helps you design better experiments and interpret your results honestly. Scientists always think carefully about what their tools can and cannot show them.

Strengths and limitations of light microscope investigations

Strengths	Limitations
You can directly observe cells in living and non-living samples.	Light microscopes can only magnify up to about 1,000× — too weak to see molecules.
Stains help reveal structures that are normally invisible.	Stains can kill living cells, so you may not see natural behavior.
The investigation is repeatable — anyone with a microscope can verify your results.	Thin slices are needed; you only see a flat 2D view of a 3D structure.
Comparing multiple organisms gives strong evidence of patterns.	Some organisms (like viruses) are too small for light microscopes.

✦ KEY TAKEAWAY

Think of a microscope investigation like using binoculars at a sports game. Binoculars help you see players' faces, but they can't tell you what a player is thinking. Similarly, a microscope shows you cell structures, but it cannot show you how molecules work inside those structures. To learn more, scientists use electron microscopes or chemical tests.

SECTION 8

Connecting to Advanced Cell Biology

In this lesson, you used a light microscope to see cells. But science doesn't stop there. As you move into high school and beyond, you will learn about more powerful tools and deeper questions about cells.

How today's lesson connects to future biology topics

What You Learn Now	What Comes Next
All living things are made of cells (cell theory).	Cells contain organelles that each perform specific chemical reactions.
Light microscopes magnify cells up to ~1,000×.	Electron microscopes magnify up to 2,000,000× and reveal molecules inside cells.
Cells have a nucleus that holds genetic material.	DNA inside the nucleus stores coded instructions that control cell functions.
New cells come from existing cells.	Cells divide through mitosis and meiosis, processes with specific stages.
Cells take in energy from food.	Cellular respiration converts glucose and oxygen into ATP energy inside mitochondria.

The crosscutting concept of Scale, Proportion, and Quantity becomes very important as you zoom in further. Cells are measured in micrometers (μm), and the molecules inside them are measured in nanometers (nm). One micrometer is one-thousandth of a millimeter — that's incredibly small! Understanding scale helps you appreciate why special tools are needed.

🤯 Fun Fact: How Many Cells Are You?

The average human body contains about 37 trillion cells. That's 37,000,000,000,000! Each one is alive, carrying out its own functions. And amazingly, they all started from just one single fertilized egg cell.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

Which of the following is one of the three main ideas of cell theory? A) All cells contain chloroplasts. B) All living things are made of cells. C) Cells can only be seen with electron microscopes. D) Animal cells have cell walls.

PROBLEM 2 — BASIC CALCULATION

A student uses a microscope with a 10× eyepiece and a 40× objective lens. What is the total magnification? A) 50× B) 400× C) 4,000× D) 30×

PROBLEM 3 — INTERMEDIATE

A student observes onion skin cells under a microscope and sees rectangular shapes with visible boundaries. She then observes cheek cells and sees round shapes with no rigid boundaries. What can she conclude? A) Only onion skin is made of cells. B) Both samples are made of cells, but plant cells have cell walls that animal cells lack. C) The cheek cells are not real cells because they are round. D) The onion cells are not alive because they don't move.

PROBLEM 4 — APPLIED

A scientist discovers a new organism living in a deep-sea vent. She wonders if it is truly alive. She takes a sample and prepares a microscope slide. She observes small structures with membranes, cytoplasm, and DNA but no nucleus. What should she conclude? A) The organism is not alive because it lacks a nucleus. B) The organism is alive and made of prokaryotic cells. C) The organism is a plant because it has a membrane. D) The organism is not made of cells because the cells are too small.

PROBLEM 5 — CRITICAL THINKING

A classmate argues: "I looked at a rock under a microscope and it didn't have cells. That proves rocks are not alive." Is this a valid scientific argument? Explain your reasoning. A) Yes — seeing no cells proves rocks are not alive, which is a complete scientific argument. B) No — you cannot prove something is not alive just by looking at one rock. C) Partly — the observation is correct evidence, but a stronger argument would examine multiple non-living things AND compare them with living things to show the pattern. D) No — rocks might actually have cells that are too small to see.

SUMMARY

Lesson Summary

In this lesson, you learned that the cell theory states three things: all living things are made of cells, cells are the basic unit of life, and all cells come from existing cells. Scientists like Hooke, Leeuwenhoek, Schleiden, Schwann, and Virchow built this theory over hundreds of years by planning and carrying out investigations with microscopes. You can calculate total magnification by multiplying eyepiece power by objective power.

By examining organisms from all five kingdoms — plants, animals, protists, fungi, and bacteria — you can observe the pattern that every living thing contains cells. Eukaryotic cells have a nucleus, while prokaryotic cells do not. Despite these differences, all cells share a cell membrane, cytoplasm, and genetic material. The crosscutting concepts of Patterns and Structure and Function help us recognize that shared cell structures across all organisms point to a fundamental feature of life.