Cells and organisms

##Seeing what was always there

For thousands of years, people studied living things by observing what they could see. They described the shapes of leaves, the structure of bones, and the way bodies grew and healed. But even the most careful observers were only seeing part of the picture.

Hidden from the human eye was another world entirely—one that all living things share. Without the right tools, no one even knew it was there.

That changed around the year 1600, when scientists began using microscopes. Microscopes are instruments that make objects too small for the human eye appear larger and clearer.

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When scientists looked through microscopes for the first time, they were surprised by what they observed. Materials that looked solid (such as wood) were filled with tiny structures. Clear water that seemed empty was full of moving organisms.

These discoveries raised new questions:

• Are all living things built from tiny parts?

• Are those parts alive?

• Where do new living parts come from?

Answering these questions took many years, better tools, and careful use of evidence. Over hundreds of years, the ideas that grew from this work became known as cell theory. Cell theory explains what all living organisms are made of.

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##What are living things made of?

Throughout the 1600s, many scientists explored the natural world through microscopes. One scientist—Robert Hooke—made an observation that would change how people understood living things. In 1665, Hooke examined a thin slice of cork, which comes from the bark of a tree.

Hooke's microscope made the cork appear about $50$ times larger than it looks to the unaided eye. At that magnification, he observed something surprising. The cork was made of many tiny, empty, box-like spaces arranged in a pattern. Hooke called these spaces cells because they reminded him of small rooms.

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In the years that followed, a scientist named Antonie van Leeuwenhoek built microscopes that could magnify objects even more. When he examined drops of pond water and other samples, he observed tiny organisms moving on their own. For the first time, scientists had evidence that living organisms could be too small to see with the unaided eye.

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The discoveries made by Hooke and van Leeuwenhoek led to more questions:

• Were cells found in all living organisms?

• Were cells alive?

• Where did cells come from?

Scientists did not yet have the tools or evidence to answer these questions. However, Hooke and van Leeuwenhoek had given them a place to start.

##What is the smallest unit of life?

Following the work of Hooke and van Leeuwenhoek, scientists continued to observe many different organisms under microscopes for over a hundred years. Yet, many questions remained.

Hooke had found tiny cell-like structures in cork, but cork comes from a tree. So, did cells make up other types of organisms? Van Leeuwenhoek saw microscopic organisms in pond water. Were those organisms made of cells, too?

In the early 1800s, two scientists made discoveries that helped answer these questions. Matthias Schleiden studied plant [[☃ definition 1]] and Theodor Schwann studied animal tissues. Even though they were studying very different organisms, both scientists noticed the same pattern: all of the tissues they examined were made of cells.

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When Schleiden, Schwann, and other scientists looked more closely at cells in living tissues, they noticed something important. Unlike the empty cells Hooke had observed in cork, the cells in living tissues were not empty at all. They contained material that moved and changed over time. Scientists also observed that cells were doing the work of keeping organisms alive—carrying out processes needed for growth and survival.

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##Thinking like a scientist

Making observations is an important part of science. Scientists then use those observations as evidence to make claims and explain their ideas. One way to learn how scientists do this is through an approach called Claim–Evidence–Reasoning (CER).

• A claim is a statement that answers a question.

• Evidence includes observations or data that support the claim.

• Reasoning explains how the evidence supports the claim.

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##Where do new cells come from?

As microscopes continued to improve, scientists were able to watch living cells over time. In the mid-1800s, scientists were still unsure of where new cells came from. Some thought that new cells could appear on their own.

However, through careful observation, scientists such as Robert Remak and Rudolf Virchow saw something different. They observed cells grow and then split into two new cells. Each new cell formed from a cell that already existed.

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The same pattern was observed again and again, across many different tissues and organisms. These observations led Remak and Virchow to an important conclusion: all cells come from other, pre-existing cells.

This idea helped explain how organisms grow, repair damaged tissues, and replace old cells.

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##Cell theory

Cell theory did not come from a single scientist or a single discovery. It was built over hundreds of years, through the work of many scientists around the world.

Hooke and van Leeuwenhoek revealed a microscopic world that no one had seen before. Schleiden and Schwann found that all living tissues (whether from plants or animals) are made of cells. Remak and Virchow observed that new cells only come from other cells.

Together, their work—and the work of countless other scientists—led to one of the most important explanations in all of biology: cell theory.

Cell theory explains what scientists have observed over time to be true for all cells:

• All organisms are made of one or more cells.

• The cell is the basic unit of structure and function in living organisms.

• All cells come from other, pre-existing cells.

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##Summary

• Microscopes were the key tool that allowed scientists to discover and study cells, revealing a microscopic world that could not be seen with the unaided eye.

• Over hundreds of years, many scientists built on each other's work to develop cell theory.

• Cell theory explains what scientists have observed over time to be true for all cells:

• All organisms are made of one or more cells.

• The cell is the basic unit of structure and function in living organisms.

• All cells come from other, pre-existing cells.

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