What is a Cell?

Living cells: are made up of just a few important chemical elements (carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur); are the basic unit of structure and function of all living things; come from other cells after life began; and are different from viruses

Living Cells

Have you ever been asked to sweep or vacuum around your house? If not, have you ever noticed the dust that collects all around a room? Believe it or not, most of this dust comes from you and the other people you live with – as dead skin cells!

Your body is constantly producing more and more new cells while other cells die. In fact, our bodies are only made up of cells and some fluids that are trapped between them. This means that living cells make new cells – but really, what are they?

A cell

A cell is actually very complex. Some scientists spend their entire lives studying just one little part of how just one type of cell works. However, understanding what a cell is and what it does is actually quite simple. Each cell is basically a bag of water, DNA, protein and some things called organelles. The organelles inside of a cell are responsible for making energy, getting rid of waste and making proteins, among other things.

A living thing is made up of a collection of one or more cells. However, one lone cell cannot accomplish very much – take a swim, eat some lunch, find a mate – but a group of cells can get together and form a much more complicated organism. Sure, that group of cells can take swims, find mates and even eat dinner, but it can do more than one thing at one time. A plant, which is a group of cells, can capture the energy of the sun, pick up water with its roots, turn carbon dioxide into sugar and oxygen and open up its flowers all at the same time! And if you as a human are really talented, you can text message, listen to your teacher, and make googly eyes at your boy/girlfriend across the room all at the same time. Just think, if you were a single-celled amoeba, you wouldn’t even have a pocket for your phone. How wonderful it is to have more than one cell!

A virus

We say that cells are the most basic unit of structure. This is true for all living things, whether or not they have bones. For example, a jellyfish has no bones, but cells make up their arms and body. Also, a jellyfish is able to swim and sting their prey because they have special cells that perform certain functions. As humans, we have special cells that beat around 80 times every minute called heart (cardiac) cells. We also have cells that carry oxygen around our bodies called red blood cells. Our bodies need to do thousands of things, or functions, every second. For these reasons, we say that cells are also the basic unit of function in all living things.

A bacterium

You know that if your teacher catches you texting on your phone, that you might get a call home to your grandmother, grandfather, mom or dad. Why are those people most closely related to you, anyway? Because your cells came from their cells! All living cells come from other living cells. A famous experiment was once done on rotting meat. At the time, people believed that flies came from rotting meat – not other flies. So a scientist took two containers of rotting meat; over the first container, he put a screen that prevented flies from landing on it. He left the second container uncovered. Sure enough, flies started to land on top of the second container, and before long, the rotting meat was covered with maggots (fly babies). However, the flies never got on the meat in the first container, no maggots appeared, and no flies appeared inside the container. Flies could only come from other flies!

Nerves connecting the spinal cord to muscles

Since then, scientists have discovered much about cells and also about things that are not considered living cells, such as viruses. Viruses, which lack a nucleus, can infect living cells by using the cell to make more viruses, but without a living cell, a virus cannot make more copies of itself. The virus never gets any bigger or smaller, and doesn’t take any gases or other materials from its environment. Different viruses can survive in different environments, such as HIV which needs to be in body fluids like blood or semen, but no virus can make more copies of itself, by itself. This is why most scientists do not consider viruses to be alive, even though they have organic molecules like proteins.

At this point, you may be asking yourself, what are living cells actually made up of? Truthfully, you’re probably not asking yourself this question, but it would be a good question to ask. All of those organelles in the cell (like the cell membrane, DNA, ribosomes, etc.) are made up of different combinations of just a few chemical elements. It’s thought that these chemical elements were around when the Earth first cooled down 4 billion years ago: carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Believe it or not, almost your entire body is made up of combinations of these elements, elements that originally came from our sun!

1. State how new cells come about.
2. Identify the role of a cell to an organism.
3. In our cells, carbon, oxygen, nitrogen, phosphorus and sulfur. Where did those chemical elements originally come from?
Put it together
4. Differentiate (tell the difference between) a cell and a virus in two ways.
5. Paraphrase (summarize briefly in your own words) the experiment done to prove that cells come from other living cells.
Think about it
6. Argue for the case that a virus is actually a living thing. Use the Sandwich Chart on p.Error: Reference source not found to structure your argument!
7. Which cycle is most important for the health of the Earth? Why?
8. What does it mean that an ecosystem recycles materials?
9. What is the biotic relationship between the butterflies and the birds? Explain.
10. Explain natural selection using terms from this chapter.


  1. The human body is home to hundreds of trillions of bacteria. T / F
  2. Most bacteria are benign (harmless). T / F
  3. The very old, very young (under 6 months) or very ill are most vulnerable to bacteria. T / F
  4. The refrigerator is the best defense against food poisoning. T / F
  5. Viruses are different from bacteria. T / F
  6. Antibacterial cleaning products are just as effective at preventing colds, flu and food poisoning as regular products. T / F

By MARY ROACHNew York Times

I saw a television advertisement recently for a new product called an air sanitizer. A woman stood in her kitchen, spraying the empty space in front of her as though using Mace against an imaginary assailant. She appeared very determined. Where others are satisfied with antibacterial-laced sponges, dish soaps, hand sanitizers and telephone wipes, here was a woman who sought to sterilize the air itself.

As a casual student of microbiology, I find it hard to escape the absurdity here. This woman is, like any human being, home to hundreds of trillions of bacteria. Bacteria make up a solid third, by weight, of the contents of her intestines.

If you were to sneak into her bathroom while she was showering – and based on my general impression of this woman from the advertisement, I don’t recommend this – and secret away a teaspoon of the water at her feet, you would find some 820 billion bacteria. Bacteria are unavoidably, inevitably – and, usually, utterly harmlessly – a part of our world.

The fantasy of a germ-free home is not only absurd, but it is also largely pointless. Unless you share your home with someone very old, very young (under 6 months) or very ill, the few hundred bacteria on a countertop, doorknob or spoon pose no threat. The bacteria that cause food poisoning, the only significant rational bacterial worry in the average home, need to multiply into the thousands or millions before they can overwhelm your immune system and cause symptoms.

The only way common food poisoning bacteria can manage this is to spend four or five hours reproducing at room temperature in something moist that you then eat. If you are worried about food poisoning, the best defense is the refrigerator. If you don’t make a habit of eating perishable food (food that can spoil) that has been left out too long, don’t worry about bacteria.

Viruses are slightly different. You need only pick up a few virus particles to infect yourself with a cold or flu, and virus particles can survive on surfaces for days. So disinfecting the surfaces in the home should, in theory, reduce the chances of picking up a bug. (Some antibacterial products also kill viruses.)

In practice, the issue is less clear. A study by Dr. Elaine Larson at the Columbia School of Nursing called into question the usefulness of antibacterial products for the home. In New York, 224 households, each with at least one preschooler, were randomly assigned to two groups. One group used antibacterial cleaning, laundry and hand-washing products. The other used ordinary products.

For 48 weeks, the groups were monitored for seven symptoms of colds, flu and food poisoning – and found to be essentially the same. According to Dr. Gerba’s research, an active adult touches an average of 300 surfaces every 30 minutes. You cannot win at this. You will become obsessive-compulsive. Just wash your hands with soap and water a few times a day, and leave it at that.

I suspect that a minority of the Americans who buy antigerm wipes and sanitizers are motivated by concerns over food poisoning or colds and the flu. Their behavior is a product not so much of making sure they haven’t missed anything, but of phobia. Phobias are irrational fears, wrought of the union of dread and misunderstanding. People see a headline – an outbreak of E. coli O157:H7 at a hamburger chain, say – and they start to worry.

In the case of bacteria, they cannot see the source of their worry, and they do not know much about it, so they go overboard. They add a few more “wiping events,” as the cleanser market researchers say, to their daily routines. Where there is an irrational fear (one that doesn’t make sense), there is a product-development team to fan it and feed it and exploit it.

According to the research firm Mintel International, 11 new home antibacterial products have appeared on the market this year, more than twice the number in 2003. It is the biggest marketing coup since bottled water.

The makers of antibacterial products are fond of the word “germs.” It is purposefully vague. Do they mean bacteria? Viruses? Both? Neither? Because the idea is simply to mean contamination. These products are as much about cooties as they are about viruses or bacteria.

Contamination is in many ways completely made up. It is the notion that our belongings or our loved ones can become unclean by the mere touch of a stranger. Nothing is actually transferred by the touch. The contamination is symbolic, magical, irrational. It makes sense that the extravagantly rich – Howard Hughes or Donald Trump, for instance – are our most notorious germphobics (people afraid of germs), people made uncomfortable by the thought of shaking a stranger’s hand.

The higher you rise and the better removed you are from the “unwashed masses,” the smaller and dirtier the average Joe must begin to seem. Other human beings become our germs.

A plea, then, for a little calm, a little rationality. Try to look upon bacteria as did their discoverer, Antoni van Leeuwenhoek (LEE-vun-hook), “For me, this was among all the marvels that I discovered in nature the most marvelous of all, and I must say, that for my part, no more pleasant sight has met my eye than this of so many thousand living creatures in one small drop of water.”


  1. Why does the author feel that the air sanitizing depicted in a commercial was an act of “absurdity”?
  2. Why does the writer call the idea of a “germ-free home” a “fantasy”?
  3. How do common food poisoning bacteria reproduce so quickly, and how can you prevent this from occurring?
  4. Describe the study that Dr. Elaine Larson at the Columbia School of Nursing performed using cleaning products. What did her results reveal?
  5. What does it mean for antibacterial products to be “the biggest marketing coup since bottled water”?
  6. To what are all the things that the word “germs” can refer?
  7. Summarize the sentiment expressed in Antoni van Leeuwenhoek’s quote in your own words.

Preparing Slides

You will prepare three slides of your own choosing from substances that you bring in or that are in the classroom. For each slide, do the following:

  1. Clean a slide and slide cover
  2. Place the subject (what you’re looking at) on the slide, then place the slide cover on top of the subject
  3. Describe what you think you will see before you look at it in the scope
  4. At each magnification (4x, 10x, 40x), describe what it is that you see in a few words
  5. Sketch a picture of what you see, as best you can
  6. Wash and dry the slide and slide cover
  1. In your own opinion, are viruses alive? Why or why not?
  2. Talk to three other people who may or may not know about viruses. Explain what a virus is to them and:
    1. Write down their opinion about whether it’s alive or not
    2. Compare this opinion to your own
    3. Write down their name
  3. Has talking to other people changed your opinions at all? Why or why not?
  4. A virus reproduces by using a host cell to clone its DNA (or RNA) for it. Draw or sketch a three-frame comic strip which illustrates how this happens.
  5. Complete a Venn diagram of two similarities and differences for viruses vs. bacteria.
Using a Microscope

Label the microscope below and list the steps to follow when using a microscope to look at a prepared or temporary slide with the parts: Objective Lens, Light Source, Arm, Base, Body Tube, Stage, Stage Clip, Revolving Turret, Fine Focus, Coarse Focus, Diaphragm, Eyepiece

A microscope

What is a Virus?
  1. What is a virus? Complete the table, using the text:
    Function / Structure Present in virus?
  2. With the Play-Doh, create and name your own virus. Sketch, label and explain what your virus infects!
  3. Do you think a virus is a living thing? Explain!
  1. Approximately how many people are living with HIV worldwide?
    39.5 million
    25.8 million
    3.5 million
  2. Approximately how many people are living with HIV in the United States?
    1 million
    500 thousand
    27 thousand
  3. Can you get AIDS from sharing the cup of an infected person?
    Only if you don’t wash the cup
  4. Which protects you most against HIV infection?
    Contraceptive pills
    Spermicide jelly
  5. What are the specific symptoms of AIDS?
    There are no specific symptoms
    A rash from head to toe
    You start to look very tired
  6. Can insects transmit HIV?
    Only mosquitoes
  7. What does STD stand for?
    Sexually Transmitted Disease
    Special Treatment Doctor
    Standard Transmission Deficiency
  8. Is there a cure for AIDS?
    Only available on prescription
  9. Worldwide, HIV is most common in which age range?
    0 – 14 years old
    15 – 24 years old
    25 – 34 years old
  10. Is there a difference between between HIV and AIDS?
    Yes, HIV is the virus that causes AIDS
    No, HIV and AIDS are the same thing
    Yes, AIDS is the virus that causes HIV
  11. What percentage of those infected with HIV are women?
    Nearly 25%
    Nearly 50%
    Nearly 75%
  12. Is it possible to lower the risk of an HIV positive woman infecting her baby?
    Yes, the risk can be made much lower
    No, not at all
    Only very slightly
  13. How many sizes do condoms come in?
    Many different sizes
    Regular and large
    One size fits all
  14. How do most people become infected with HIV?
    Unsafe sex
    Injecting drugs
    Blood transfusions
  15. What is the World AIDS Day international symbol of AIDS awareness?
    A red ribbon
    A white ribbon
    A pink ribbon
    A black ribbon
    A white swan
  16. How can a person become infected with HIV?
    Being sneezed on by an infected person
    Holding hands with an infected person
    Both of these
    Neither of these
  17. How can you tell if someone has HIV?
    They look tired and ill
    They have a bad cough
    There is no easy way to tell
    They are very thin
  18. What is AIDS caused by?
    A virus
    Dirty water
  19. Which people can’t be infected with HIV?
    Gay men
    Married people
How Viruses Spread

By Randall Good

This activity will show you how viruses can spread through a population. You will receive one test tube and one pipette, which may or may not contain a mildly toxic acid, so be careful with it!

  1. Move around the room for five minutes and exchange fluids with at least three other people.
  2. Return the pipettes to the front of the room and then go back to your seat without spilling your test tube.
  3. Put a pipette full of pH indicator in your test tube.

One of the test tubes had originally been infected with the acid; it is now our task to figure out who was the source of the “virus”! This person will be called patient zero.

  1. Write down all of the students that you exchanged fluids with.
  2. Put a star next to each student’s name who tested positive for the virus.
  3. Could you have been patient zero? Why or why not?
Nanotechnology Research

View the Nanotechnology slide show.


  • Scissors
  • A strip of paper (dimensions 150mm x 5mm, or 5.9” x 0.2”)
  • Ruler
  • Pen or pencil*
  • Calculator*
  • Tape*
  1. Guess how many times you would have to cut the paper to make a 10 nanometer long piece.
  2. How many times do you think you can cut the paper before it becomes impossible to cut with the scissors?
  3. Before you begin, make your own measurement table.  You will measure and record the length of each successive piece of paper in millimeters.  After you measure, you can tape the piece of paper into your notebook.  Write the measurement in scientific notation.
  4. Begin cutting the strip of paper crosswise and cutting it in half as many times as you can and record your data.
  5. Were your predictions from #1 and 2 accurate? 
  6. How many times did you cut the paper? 
  7. How close was the smallest piece to the nanoscale? 
  8. Why did you have to stop cutting? 
  9. Can macroscale objects, like scissors, be used on the nanoscale? 
  10. Can you think of any way to cut the paper any smaller? 

– Build graphene

– Build diamond

– Build fullerenes

– Build nanotubes

Skulls and Brains Lab

In this lab, you will understand the relationship between the brain and skull of animals, see how the human skull is both similar and different from the skulls of other animals, and learn about all of the information you can get out of analyzing a skull.


  1. Approximately how long and wide is your skull?
  2. Describe the differences between carnivores, herbivores and omnivores.
  3. What are three different types of teeth?
  4. What are those different types of teeth used for?
  5. Does a bigger brain mean that an animal is smarter? Find examples to support your answer.
  6. For each of the six skeletons, make the observations necessary to fill out the accompanying data sheets.
  7. Ask your instructor for the correct answers after you are all done!
  8. How is the human skull different from the other skulls?  How is it similar?
  9. Write a two-paragraph conclusion by including the following:
    • Whether or not you were correct
    • The relationship between skull size and intelligence
    • The relationship between eye socket size and vision
    • The relationship between nasal passageways and sense of smell
    • The relationship between teeth and diet

Animal Skull #1

Length _______  Width _______       We think this is a _______________ skull.
Describe the size of the eye socket  ___________________________________
Describe the size of the brain cavity ___________________________________
Describe the nasal passageways _____________________________________
Describe the teeth _________________________________________________
If possible, count (or estimate) the number of teeth _______
Circle the correct choice:   Herbivore       Carnivore       Omnivore
In the space below, describe this animal based upon the skull:

Animal Skull #2
Length _______  Width _______       We think this is a _______________ skull.
Describe the size of the eye socket  ___________________________________
Describe the size of the brain cavity ___________________________________
Describe the nasal passageways _____________________________________
Describe the teeth _________________________________________________
If possible, count (or estimate) the number of teeth _______
Circle the correct choice:   Herbivore       Carnivore       Omnivore
In the space below, describe this animal based upon the skull:

Animal Skull #3
Length _______  Width _______       We think this is a _______________ skull.
Describe the size of the eye socket  ___________________________________
Describe the size of the brain cavity ___________________________________
Describe the nasal passageways _____________________________________
Describe the teeth _________________________________________________
If possible, count (or estimate) the number of teeth _______
Circle the correct choice:   Herbivore       Carnivore       Omnivore
In the space below, describe this animal based upon the skull:

Animal Skull #4
Length _______  Width _______       We think this is a _______________ skull.
Describe the size of the eye socket ____________________________________
Describe the size of the brain cavity ___________________________________
Describe the nasal passageways _____________________________________
Describe the teeth _________________________________________________
If possible, count (or estimate) the number of teeth _______
Circle the correct choice:   Herbivore       Carnivore       Omnivore
In the space below, describe this animal based upon the skull:

Animal Skull #5
Length _______  Width _______       We think this is a _______________ skull.
Describe the size of the eye socket  ___________________________________
Describe the size of the brain cavity ___________________________________
Describe the nasal passageways _____________________________________
Describe the teeth _________________________________________________
If possible, count (or estimate) the number of teeth _______
Circle the correct choice:   Herbivore       Carnivore       Omnivore
In the space below, describe this animal based upon the skull:

Animal Skull #6
Length _______  Width _______       We think this is a _______________ skull.
Describe the size of the eye socket  ___________________________________
Describe the size of the brain cavity ___________________________________
Describe the nasal passageways _____________________________________
Describe the teeth _________________________________________________
If possible, count (or estimate) the number of teeth _______
Circle the correct choice:   Herbivore       Carnivore       Omnivore
In the space below, describe this animal based upon the skull:

Making a Model of a Cell

Each part of a plant cell has a specific function, which is often reflected in the size, shape, and location of the part. For example, the nucleus, which controls what happens in a cell, is located in the center of the cell. In this activity, you will work with your classmates to make a model of a plant cell.


  • craft materials
  • scissors
  • index card
  • tape or glue


  1. Your classroom will represent a plant cell. You will make a model of an organelle or other cell part to place in your classroom “cell.”
  2. Use the table on page 175 of your textbook to select a part to model. The drawing on page 174 will give you an idea of the relative sizes of cell parts and their positions in the cell.
  3. Use the materials provided to build a three-dimensional model of your chosen cell part. Refer to other drawings in Lesson 7.2 to make your model as complete and accurate as possible.
  4. Label an index card with the name of your cell part, and list its main features and functions. Attach the card to your model.
  5. Place your model at an appropriate location in your classroom.
  6. As a class, review the completed model. If necessary, relocate some of the cell parts to reflect the spatial and functional relationships between parts of the cell.

Analyze and Conclude

  1. A typical plant cell has a width of 50 micrometers (μm), or 5 × 10–5 m. Calculate the scale of your classroom cell model. Hint: Divide the width of your classroom by the width of a typical plant cell. Use the same unit for both measurements.
  2. How is your model of a cell part similar to the actual cell part? How is it different?
  3. How is the structure of your cell part adapted to its function?

Build Science Skills

If you were starting over, what would you do to improve your model?