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Lesson 2 Cell Theory

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Course: Science 10 [5 cr] - AB Ed copy 1
Book: Lesson 2 Cell Theory
Printed by: Guest user
Date: Sunday, 7 September 2025, 6:46 PM

Table of contents

  Introduction

What does the cell theory tell us?




A2.1 A few different human cells
The cell theory is a complex theory that has been developed over hundreds of years. Scientists started working on this theory in the 1600s, and it finally became the popular theory in the 1800s. During this lesson, we will follow the path of these scientists and the development of the cell theory. By the end of this lesson, you will understand what the cell theory is and how it was developed.

  Target

By the end of this lesson, you will be able to
  • explain what the cell theory is and what each point means

  Watch This

The Wacky History of Cell Theory – Lauren Royal-Woods ©YouTube TED-Ed  


This video will provide you with an overview of how the cell theory was developed and what it says.

  Development of the Cell Theory


The cell was not recognized as important to life until the 1830s.




A2.2 The green sections are the cell nuclei
During the 1830s, more scientists started to recognize that the theory of spontaneous generation was wrong and instead started working on studying the cell. Through the experiments of four main scientists, a theory emerged about cells and what their function was. It became clear that cells were very important to life and were, in fact, what all life was made from.

Robert Brown was the first scientist to see the nucleus and determine that it must have some importance to cell structure and function. However, Brown did not know what the function of the nucleus was.

Two scientists named M. J. Schleiden and Theodor Schwann took Brown’s observation and built on it. They discovered that the nucleus was found in both plant and animal cells. Since it was found in both types of cells, they felt it must be the structure that controlled the rest of the cell. It was an amazing breakthrough in the scientific community. Not only did plants and animals both contain cells, but these cells contained similar structures and so must perform similar functions.

A2.3 Animal and plant cell structure


A scientist by the name of Rudolf Virchow supported Schleiden and Schwann’s theory but felt they got one part wrong. Schleiden and Schwann suggested cells formed from non-living materials or by budding from the surfaces of other cells. Virchow felt cells could not come from non-living materials, as this would be spontaneous generation. He made microscopic observations of cells dividing to make more cells and concluded cells must come from other cells.  This led to the final point of the cell theory.

The cell theory:   
  • All living things are made up of one or more cells and the materials produced by these cells.
  • All life functions take place in cells, making them the smallest unit of life.
  • All cells are produced from pre-existing cells through the process of cell division.

A2.4 Cells undergoing cell division

  Did You Know?


A2.5 Repetition is important in science.

Many scientists were amazed by Schleiden and Schwann’s discovery, so they decided to test it for themselves. Experiment after experiment showed the same results and provided supporting evidence for the theory.

Learn More

  Read This

Please read pages 251 and 252 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the scientists and what they discovered about cells. It is also important you know the cell theory. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Write the three points of the cell theory in your own words. This means you cannot copy the points down; you must use words that make sense to you.

    Your answer should be a variation of the following: Cells make up the building blocks that all living things are made of. Everything you need in order to be alive is done in a cell. Only cells can produce other cells.
  1. In point form, outline the development of the cell theory.

    Brown discovered the nucleus and suggested it held an important job within the cell. Schleiden saw the nucleus in plant cells and suggested it was responsible for the development of the cell. Schleiden discussed this with Schwann, and Schwann made similar observations in animal cells. This lead to the first two points of the cell theory. Virchow observed that cells divided to make new cells, which led to the final point in the cell theory.

  Living or Not?

What about cell-like structures?
The cell theory only applies to living organisms and cells. It does not apply to cell-like structures, as these structures do not have all the characteristics of life. There are two such structures we will look at: viruses and prions.


A2.6 Images of different viruses

  Digging Deeper


© Oona RÀisÀnen, via Wikimedia Commons
A2.8 T7 Bacteriophage

Bacteriophages are viruses that infect bacteria. They look similar to alien ships and can be quite deadly to bacteria. Scientists have studied these kinds of viruses in detail as they are quite interesting!

Learn More
A virus is a structure that is made up of a stretch of genetic material (like DNA) within a protein coat. This basically means that it is a piece of DNA inside a room. You probably know about common viruses such as the common cold and flu. Viruses work by getting inside the cells of the organism they infect and taking over the cells’ functions. The virus uses the cell to produce more viruses, which then burst out of the cell to infect other cells. When enough cells are infected, the organism becomes sick. Since viruses do not have the ability to produce new viruses without using cells, they are not considered to be alive.

Prions are another example of a non-cellular structure. A prion is a protein that changes from its normal form into a harmful particle. Just like viruses, prions use the cell to complete life functions and so are not cells.


© Lopez-Garcia, F., Zahn, R., Riek, R., Wuthrich, K. & RCSB, via Wikimedia Commons
A2.9 Structure of a cow prion protein

  Did You Know?


A2.7 Infection and disease control

There are some very interesting viruses out there! They are the stuff of nightmares, and they are often used as the villain in horror movies.

Learn More


  Did You Know?


A2.10 A cow

Mad cow disease is caused by a prion that attacks the brain of infected cows and humans, causing changes in personality and the loss of the ability to walk, as well as other symptoms.

Learn More

  Take Notes

Make sure you take notes on your readings to study from later. You should focus on what prions and viruses are and why they are considered non-living. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What are the similarities and differences between viruses and prions?

    Viruses contain a piece of DNA in a protein coat, while prions are proteins that change into harmful substances. Both need to infect a cell to reproduce and carry out life functions.
  1. Write down one example of a virus and one example of a prion not included in your course content. Note: You will need to do research to find these examples.

    Answers will vary. Some common viruses are HIV, HPS, Rabies, Ebola, measles, polio, and zika. Some common prions are CJD, FFI, GSS, and CWD.

Interactive Activity


This activity shows you how the most common viruses look. Some are very odd looking! It will also give you lots of information on each virus, such as how it is spread and if there is a vaccine for it.

  1. Click on the play icon to open the activity. https://quick.adlc.ca/virus
  2. Click on any of the nine viruses pictured to see more information.
  3. You can move the image of the virus around to see all sides, or you can view the inside of the virus.
  4. When on the main page with all nine viruses, you can sort the viruses based on the characteristics listed across the top of the screen. If you click on hosts, you can choose to see the ones that infect humans specifically.
  5. Explore all the viruses and their characteristics.
  6. Please return to the top of this page and click on analysis to complete the analysis questions.
  1. Can you find the virus mentioned in the Digging Deeper above?

    This is the T7 virus shown on the bottom row in the interactive.
  1. Click on the “Show Relative Sizes” button found at the bottom of the screen. Which virus is the largest? Which is the smallest?

    The Ebola virus is the largest (it is huge in comparison!), and the Zika virus is among the smallest.
  1. Go to the HIV virus. Which continent has the largest amount of adult HIV infections?

    Africa has the largest amount of adult HIV infections. It is much higher than North America. In Africa, 4.5% of the adult population has HIV, while only 0.5% of the adult population in North America does.
  1. Go to the rabies virus. What are the symptoms that occur with this virus?

    This virus has neurological symptoms (the symptoms occur in the brain) that cause aggression. This often leads to biting, which is often how the virus is spread. Once symptoms appear, the infection is fatal. (This sounds like the zombie virus, doesn’t it?)
  1. Go to the influenza A virus. This is what the flu looks like. After reading the information, why do you think we need a new vaccine for the flu each year?

    The virus has its DNA split into many segments so that those segments can interact with other segments of different flu viruses. This allows the viruses to evolve and change very quickly. A new vaccine must be made for each flu season as a specific flu virus never lasts longer than a year. Every year, there is a new version of the flu virus we need to vaccinate against.


Virus Explorer ©2017 Howard Hughes Medical Institute

  Importance of Microscopes

The cell theory is an important theory as it has formed the basis of the study of biology.

A2.11 19th Century Microscope
Without the cell theory, scientists would have no idea how the basic functions of life are performed or what living organisms are made of. It is amazing to think we are all made up of billions of cells.

The cell theory could not have been created without the help of microscopes. The microscopes used by these scientists were not nearly as strong and sophisticated as the ones we have today, but they allowed the scientists to observe cells. In the next lesson, we will take a look at the development of microscopes and the amazing technology we use to look at cells today.

  Digging Deeper


A2.12 Newborn baby with microcephaly caused by Zika
You may have heard about the Zika virus, which is spread by mosquitos, on the news in previous years. There was an outbreak of this virus in the Americas in 2015–2016, which caused pregnant women and women of child bearing age to avoid vacationing in warm places. The virus causes babies with underdeveloped brains to be born. Many of these babies do not survive. It was declared a global health emergency, and scientists are currently working on a vaccine. Go to the following links for more information on the Zika virus.

The Biology of Zika Virus

Zika Outbreak: What You Need To Know

  Problem Solving Activity


How did Schleiden and Schwann follow the process of scientific investigation?



The process of scientific investigation is followed by scientists all over the world. Usually it is followed without intention; instead, it naturally occurs as we investigate a question.

The process of scientific investigation:

  1. Identify the theory or ideas behind the experiment. What do you need to know before starting?
  2. Create a focused question you want to test. If your question is too broad you will not be able to create an experiment to test it properly.
  3. Create the experiment.
  4. Decide how to collect and record the data or evidence from the experiment. For example, is a table going to work or should you take a picture?
  5. Review the evidence and data and determine what it means. Did it answer your question? If so, what is the answer? If not, what can you change to get an answer? 

Reread the information about Schleiden and Schwann. Think about the process outlined above and try to pick out the places where these two followed it. Please click on the analysis tab to complete the analysis questions. You will need these answers for your Assignment A1.
Describe how Schleiden and Schwann followed each of the five points of the process of scientific investigation. You will need some of your answers for Assignment A1: Lesson 2.

Use this answer to help you with question 5 in Assignment A1.

  1. Schleiden and Schwann wanted to see if all living things were made of cells. If this was true, they wanted to see what their function was.
  2. The experiment they created was to observe many different living things to see if they all contained cells.
  3. They collected evidence by observing the different cells and making drawings or writing descriptions of what each looked like under their microscopes
  4. When they reviewed the evidence, they found all living things were indeed made up of cells. They also discovered all life functions were performed within these cells.


1.2 Assignment

Unit 1  Assignment Lessons 2-3


It is now time to complete the Lesson 2 portion of 1.2 Assignment. Click on the button below to go to the assignment page.

1.2 Assignment