What Do We Need to Eat?

Living organisms use matter and energy to create a wide variety of organic molecules (e.g., proteins, carbohydrates, lipids and nucleic acids) and to allow life to happen in growth, reacting to the environment, reproduction and movement. Ecosystems also recycle many materials, such as water, carbon and nitrogen.

Organic Molecules

Think about your last meal. What did you eat? Chicken, beef, bacon, beans, fish, pork, eggs, or cheese? Then you had plenty of protein. Did you eat french fries, chips, corn, noodles, pancakes, bagels, or cereal? Then you probably got plenty of carbohydrates. Did you eat cookies, a snack food, donuts, potato chips, or anything that was greasy, oily, or cooked in butter? Then you probably got plenty of fat (lipids).

These three major nutrients are the most important organic molecules, meaning that they all come from living things! Of course, we use all three of these nutrients in order to live. Proteins are used to build muscle, help your immune system, help brain and nerve function, and assist in almost every biological function in your body! The body uses carbohydrates for the energy to do many things, build cell membranes and cell walls, and to store energy as fat. Aside from being stored energy, fats (lipids) are used by the body to make hormones. Lipids contain much more energy than carbohydrates, which are used immediately by your body. Since carbohydrates are broken down into sugars, this is why people sometimes feel a “sugar high” that ends very suddenly!

It’s important to get a balance of all of these nutrients in your body because they are necessary for growth, homeostasis, reproduction and movement: all characteristics necessary for life. Since all living things use these nutrients, over and over again, how is it that we don’t run out? Where does more protein come from? How do we get carbohydrates and lipids, anyway?

The nitrogen cycle

We get proteins from amino acids: remember how the ribosomes put amino acids together to form proteins? Those amino acids, eventually, come from plants. Those plants make amino acids from nitrogen compounds, which they get from bacteria. And the bacteria? The bacteria get nitrogen from – the air! This is the nitrogen cycle.

The main ingredient in urine is urea, which has nitrogen in it. In other words, after we use the nitrogen, we pee it back out! That nitrogen is again used by bacteria and is often put back into the atmosphere. Other times, it is used again by plants, where we eat the nitrogen again and turn it into proteins.

The water cycle

Speaking of urine, it is important to understand how water cycles through the ecosystem. All of us drink water all of the time, but how is it that rivers and lakes continue to flow, even though we are using it? After we use the water, it gets filtered through the ground and returns to rivers, lakes and oceans. We can use the water from Lake Erie (that comes through our pipes), but sometimes that water evaporates – and then falls back down to the ground as precipitation. More importantly, salty water from the ocean evaporates and falls back down to the ground as fresh water that living things can use more easily.

The last major cycle is the carbon cycle. Carbon is the most important part of carbohydrates and lipids, which are critical to your survival. Carbon dioxide is taken up by plants to make stems, roots, fruit, seeds and leaves, which are then eaten by consumers. Consumers, using up those carbohydrates and lipids, give off carbon in the form of carbon dioxide. This cycle works just fine, as long as it’s only between plants and consumers. Unfortunately, humans have started to burn all sorts of fossil fuels which contain fossilized carbon. Once that carbon dioxide is released into the atmosphere, it adds more carbon dioxide than can be taken up by plants! This is how global warming becomes such a big problem and why we need to stop burning so many fossil fuels.

The carbon cycle

1. For each of proteins, carbohydrates and lipids, list their functions in the human body.
2. Which cycle is most important for the health of the Earth? Why?
3. For each of proteins, carbohydrates and lipids, list what foods they can be found in.
Put it together
4. Predict what would specifically happen first if you didn't get enough protein in your diet.
5. What effect would insufficient carbon dioxide in the atmosphere have on humans?
Think about it
6. Combine the three major cycles (carbon, nitrogen, water) into one cycle. Include the ground, a plant, a consumer, a body of water, and the atmosphere. Draw and label arrows showing the path of carbon, nitrogen and water through these five biotic and abiotic factors.
7. Is there just one rock sequence? Why or why not?
8. What is a landfill?
9. If you ate herons in this ecosystem, how many calories would you get per day? Would a human be able to survive (we need about 2000 calories per day)?
10. You have already learned that oxygen was not available on the early Earth. Given that information and the information from this chapter, which of these four energy cycles do you think came before the other ones? Which one was next? Which one was last? Give a reason you came up with this order!
The First Mitochondria

By Cyen Tiss

Shaw High School

After my brush with Robert the ribosome and the other organelles in the eukaroytic cell, I decided to answer a few more of my questions about what’s really going on inside of us. It turns out that there are these organelles called “mitochondria” which are actually the things that take oxygen and make carbon dioxide. They do something called “cellular respiration.” A colleague at the Plain Dealer found a friend with a time machine, so we dialed up the time machine to about 1 billion years ago – the time when the first eukaryotes appeared. Here’s what we found.

All around us, we can see a vast stretch of rock and desert, even though the cool wind reminds us that we are in North America – or at least, what is destined to be Cleveland, Ohio in the United States. But it doesn’t look or feel anything like the conditions in 2007 A.D. – there are no plants or animals to be found anywhere. In fact, it’s a lucky thing that we brought along these oxygen tanks, because the atmosphere is only about 1% oxygen here (in , the atmosphere is 21% oxygen). However, this means that the oxygen must be coming from somewhere, so right now we’re headed to the ocean to figure out what’s going on!

It takes a few hours using our time machine as a dune buggy, but we get over the rugged terrain and to the shoreline. Here, at the ocean, there is a strong, putrid smell and the surface of the ocean is covered in some sort of bluish haze. As we look closer, we find that there is life on this ancient Earth: blue-green algae. My assistant, Alex, reminds me that these were the dominant prokaryotes one billion years ago, taking carbon dioxide from the atmosphere and producing oxygen, much the same way that plants convert carbon dioxide to oxygen in the process of photosynthesis.

Blue-Green Algae

Our time machine, perfectly engineered for a foray into the ocean, takes us through this blue-green muck and kicks up an even worse smell. I smother my face with the oxygen mask and take a deep breath of fresh, clean, “” air. Alex points to a spot in the ocean where there is a break in the algae, much like the sun pokes through rain clouds. As our time machine scoots over to this open pool, I notice that I can breathe without my gas mask. Plants!

Even though algae produce oxygen, it’s not nearly as much as plants can make. In fact, these plants are making so much oxygen in this area, that there seems to be a “dead zone” where no organisms are living at all. We take out the microscope and, scooping out a sample of this “dead zone” water, make a temporary slide. Much to our surprise, it’s not dead – it’s very much alive. Alex recognizes these extremely small cells swimming around as cells called mitochondria. These mitochondria are considered the “power plants” of modern cells, meaning that they produce all of the energy that eukaryotes (plants, animals, fungi, protists) need.

As Alex notes, however, mitochondria don’t exist on their own in . This may be the first time that humans have ever seen a lone mitochondria; a power plant without anything to power but itself! It’s a wonderful and amazing discovery to see this organelle on its own. The mitochondria clearly has a cell membrane and looks a lot like the algae, but it uses oxygen instead of carbon dioxide. As I sit there, pondering this development, Alex tells me to hold on – we’re leaping one million years into the future!

It’s still about a billion years before , but we can see a complete change in the environment. Not only are there algae, but green plants have begun to grow to a size where we can see them. As I take a scoop of the water around us, and prepare a temporary slide, Alex prepares me for what I am going to see. He says that I should see the first animals: microscopic, unicellular organisms, but animals nonetheless!

Early Animal

As I look through the microscope, I see the tiniest of animals, swimming around in this drop of ocean water! As I zoom in, I can see the organelles of this animal, including the mitochondria! It’s amazing to me – and true – that the mitochondria has become a part of the animal’s cells. It still has a cell membrane of its own, but it is completely inside the animal cell, helping the animal cell live.

Alex hands me another slide, this time of the small, green plants. I look closely and find two organelles that look very similar to the mitochondria, but the first one is green. It’s a chloroplast! The chloroplast also has a cell membrane around it, buried inside the plant cell. But the mitochondria is right along side it. I ask Alex to explain how the plant can have mitochondria if plants use carbon dioxide and give off oxygen.


As he dials up on the time machine so we can go back home, he explains that there are thousands of chloroplasts for each mitochondria in the plant cell. This means that there is a lot more photosynthesis going on inside the plant cell than cellular respiration. In other words, a bunch of carbon dioxide is being used up, and a whole lot of oxygen is being produced. Only a small amount of that oxygen needs to be used by the mitochondria in order to release energy for the rest of the cell.

What’s more is that modern scientists believe that early animals took mitochondria in and made them a part of their cell. The same thing happened with plants, except that plants also grabbed chloroplasts in order to help them use the energy of the sun.

Wow, what a trip. To see the first mitochondria, swimming along on their own! Who knows where we’ll end up next …


  1. What were the first organisms to produce oxygen?
  2. What is the process called and the gas exchange that happens in chloroplasts? Mitochondria?
  3. What does the mitochondria do for the cell?
  4. In what types of cells are chloroplasts found? Mitochondria?
  5. How is it believed that mitochondria became a part of the eukaryotic cell?
  6. With the Play-Doh (in pairs), create, label and sketch a plant cell. Make sure to include all of the organelles!
Fat & Protein Test
  1. Form groups according to the instructions.
  2. From home (or school), each group should bring in three foods that they think are high in fat and three foods that they think are high in protein.
  3. Measure out 5 grams of each of the foods.
  4. Test the first three foods for fats, rubbing the food against a blank sheet of paper for 10 seconds. Note how translucent the paper becomes.
  5. Test the other three foods for proteins, using the Biuret reagent according to the instructions.
Microscope: Energy

You will make three slides for the microscope. One will be of pond water, one will be of some mold, and one slide will be of a part of a bean leaf. For each slide, you will make a quick sketch and identify living things that you see by kingdom. Respond:

  1. Find an animal in the pond water and describe how it moves
  2. How is the sun’s energy getting to this animal?
  3. Find evidence of life (movement or cellular organization) in the mold. Where does this life get energy from?
  4. Find the veins (empty lines) in the leaf. what travels through the leaf’s veins?
  5. How does the leaf store energy?
Cycles of Matter

  1. What role do forests play in the cycle of matter?
    1. Identify at least three countries that have lost forests.
    2. What are the effects of losing forests?
  2. Find some worms.
    1. What role do worms play in the cycle of matter?
    2. Why is worm poop used as fertilizer?
    3. How do worms get water, one of their basic needs?
  3. Using a computer, find an animation of the carbon cycle. Describe how carbon gets from the atmosphere to fossil fuels, in detail. Include at least 5 steps!
  4. Find a plant in the room.
    1. What roles do crops (such as beans) play in the cycle of matter?
    2. Identify at least five crops that are used by humans.
    3. What would be immediately affected by the loss of any one crop?
Home Nutrition Survey

For one day, keep track of everything that you and two members of your family (or friends) eat.  For each item, determine:
1.    How many calories it is
2.    Where it comes from (cow, wheat, corn, etc.)

Summarize in one paragraph of 5-7 sentences how healthy you feel that you and your family is eating based upon this information.  What changes do you think you could make in order to improve your and their diet?

Carbon Cycle Game

See the document: Carbon Cycle Game

Objective: Get all of the 7 “Carbon Cycle Role” tokens first!


  • When starting the game, you receive 3 “Carbon Cycle Function” cards.  You may start your playing piece on any Carbon Cycle Role, but you do not get the Carbon Cycle Role token.
  • When it’s your turn, you get asked a question.  If you answer the question correctly, you can either move to a new Carbon Cycle Role along an arrow and receive that role’s token, or you can select a Carbon Cycle Function card.
  • You can only move to a new Carbon Cycle Role if you have the correct Carbon Cycle Function to get you there.

Carbon Cycle Roles (Green):

  • Producer
  • Primary Consumer
  • Secondary Consumer
  • Decomposer
  • Atmosphere
  • Industry
  • Fossil Fuel

Carbon Cycle Functions (Yellow):

  • Respiration
  • Photosynthesis
  • Predation
  • Consumption
  • Combustion
  • Compaction
  • Decomposition

Questions (Peach):

  1. What is photosynthesis?
  2. What is fermentation?
  3. What is chemosynthesis?
  4. What does a producer eat?
  5. What does a primary consumer eat?
  6. What does a secondary consumer eat?
  7. What does a decomposer eat?
  8. What gas does a producer need?
  9. What gas does a consumer need?
  10. What gas does a producer give off?
  11. What gas does a consumer give off?
  12. What is predation?
  13. What is respiration?
  14. What is decomposition?
  15. What is combustion?
  16. What is compaction?
  17. How many carbon atoms does carbon dioxide (CO2) have?
  18. How many carbon atoms does glucose (C6H12O6) have?
  19. How many carbon atoms does methane (CH4) have?
  20. How many protons does a carbon atom have?
  21. What two molecules are given off whenever you burn anything?
  22. How does carbon end up in the atmosphere?
  23. How does carbon end up inside of producers?
  24. How does carbon end up in primary consumers?
  25. How does carbon end up in secondary consumers?
  26. How does carbon end up in decomposers?
  27. How does carbon end up in fossil fuels?
  28. What are two things that store carbon?
  29. What are two things that release carbon?
  30. Why is carbon dioxide bad for the atmosphere?
  31. Why is methane bad for the atmosphere?
  32. What are two greenhouse gases?
  33. What is global warming?
  34. How long does it take for fossil fuels to form?
  35. What are two things fossil fuels are used for?
  36. What is an alternative to the use of fossil fuels?
  37. Does global warming affect land or oceans more?
    Comparing Surface Area and Volume

    In this activity, you will construct a set of paper cubes. The cubes will represent cells at different stages of growth. After you construct your cubes, you will calculate the volume, surface area, and ratio of surface area to volume of each cube.


    • patterns for 6-cm, 5-cm, 4-cm, and 3-cm cubes
    • scissors
    • tape or glue


    1. Cut out the patterns on the following pages and fold them along the dashed lines. Use the tabs to tape or glue the sides together. Do not tape down the top side.
    2. Calculate the surface area of each cube. Find the area of one side of the cube and multiply that area by the number of sides. Record your results in the data table on the next page.
    3. Calculate the volume of each cube. Multiply width times length times height. Record the results in the data table on the next page.
    4. Divide the surface area by the volume to find the ratio of surface area to volume. Record your results in the data table.

    Data Table

    Width of Side

    Surface Area (cm2)

    Volume (cm3)

    Ratio of Surface Area to Volume

    6 cm

    5 cm

    4 cm

    3 cm

    1. Use your data to calculate the number of 3-cm cubes that would fit in the same volume as the 6-cm cube. Also calculate the total surface area for the smaller cubes.

    Analyze and Conclude

    1. Describe the function of a cell membrane and its relationship to what happens inside a cell.
    2. How did the ratio of surface area to volume change as the size of the cubes decreased?
    3. As a cell grows, what happens to the amount of activity in the cell and the need for materials to be exchanged across the cell membrane?

    Build Science Skills

    How could the growth of a cell affect its ability to survive?

    surfacearea2 surfacearea