Where Does Energy Go?

Cells and organisms get and give off energy in a variety of ways: photosynthesis, chemosynthesis, cellular respiration and fermentation.

Cycles of Energy

Energy is needed for all living things to survive. We need energy to get nutrients, move nutrients around our bodies, get rid of wastes, and maintain a balance between our bodies and the environment. But where does all of that energy come from?

Photosynthesis happens in the chloroplasts of plants

Ultimately, we all “eat the sun.” If you are a plant, you use the sun directly. If you are a primary consumer, then you use the energy that the plants get from the sun. If you eat a primary consumer, then you are still eating energy that’s essentially coming from the sun! Without photosynthesis, there would be very little usable energy on Earth. So, here’s what happens in the chloroplasts of plants:

Photosynthesis: 6CO2 + 6H2O + sunlight → 6O2 + C6H12O6

Tubeworms that perform chemosynthesis

This means that carbon dioxide, water and sunlight are all combined to make oxygen and glucose (a type of sugar). The energy from the sun is stored in the chemical bonds of glucose, which can then be used by the plant or by anything else that eats it.

There are some producers that cannot get sunlight because they are deep in the ocean, where the sun does not reach. However, like many forms of life, they have found a way to survive. These organisms gather around vents in the sea floor that spit out hydrogen sulfide. They then combine the hydrogen sulfide with carbon dioxide and oxygen to make formaldehyde, sulfur and water:

Chemosynthesis: CO2 + O2 + 4H2S → CH2O + 4S + 3H2O

A mitochondrion, where respiration happens in eukaryotes

The hydrogen sulfide that comes from the vents in the sea floor contains energy; this energy comes directly from inside the Earth. Since the Earth came from the sun, this energy, too, is ultimately from the sun! In the case of chemosynthesis, it gets stored as formaldehyde, which humans use to preserve dead organisms.

Once either photosynthesis or chemosynthesis has trapped energy from the sun, the sugar (or formaldehyde) that gets produced can then be moved around. Just like we have power plants to produce electricity and electrical lines to move that electricity around, producers use sugars to move energy around. All living things then need to free up that energy and use it, to do things like move around, eat, drink, and reproduce. If there’s oxygen available, then a living thing will do respiration:

Respiration: 6O2 + C6H12O6 → 6CO2 + 6H2O + energy

Some products of fermentation

In respiration, oxygen is combined with glucose to make carbon dioxide, water and energy. Inside of us, our cells are constantly doing respiration, which is why we need to breathe in oxygen and we then give off carbon dioxide. This is what happens most of the time in organisms’ mitochondria.

Some of the time, however, when oxygen is unavailable, fermentation happens. Most famously, this happens in yeast, bacteria and in our muscle cells. When our muscle cells cannot get oxygen because we are working very hard, then they do fermentation instead. In our muscles, this produces lactic acid which causes a burning sensation: if you “feel the burn”, then your muscles are not getting enough oxygen:

Fermentation: C6H12O6 → C2H5OH + CO2 + energy

In yeast and some bacteria, fermentation takes glucose and produces ethanol (a type of alcohol), carbon dioxide and energy. Different types of fermentation produce bread, beer, cheese, yogurt, sour cream, and wine!

1. In your own words, define photosynthesis, chemosynthesis, respiration and fermentation.
2. List at least three organisms that do not perform photosynthesis.
3. 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!
Put it together
4. In a Venn diagram, compare/contrast grid, or paragraph, differentiate photosynthesis and chemosynthesis in two ways.
5. In a Venn diagram, compare/contrast grid, or paragraph, differentiate respiration and fermentation in two ways.
Think about it
6. Use the Four Door for the four energy cycles mentioned in this chapter. For each one, write the formula inside with an explanation of the reactants and products. Also, write one interesting fact inside each door.
7. List the order of biological classifications.
8. Find the nutrition labels for at least two foods or beverages. What are at least three corn products that you've used or eaten? In what form was each one in?
9. In what ways is Earth like a living thing?
10. What are the four major biotic roles in an energy pyramid?

You can complete this homework in pairs or groups of up to three people. You will need the following materials: 1 package of yeast, warm water, 1 teaspoon sugar, spoons, and a large bowl. Yeast are alive, but inactive unless they have food (sugar) and warmth. When they eat the sugar, they will give off a gas.

  1. Pour in a large bowl and add 1/4 cup of warm water and 1 teaspoon sugar.
  2. Now wait about 10 minutes. When you check it, you should see bubbles.
  3. The bubbles you see are the gas. Using the chemical formula for fermentation, determine what gas this is. What beverages do you know that have this gas? How do you think they get this gas in those beverages?
  4. What foods are produced by the process of fermentation (hint: think about what yeast is used for)?
  5. Our bodies sometimes use fermentation in order to produce energy, but only when we can’t oxygen. Do the yeast need oxygen in order to survive? How could you test your hypothesis?
Photosynthesis and Respiration
  1. Demonstrate the photosynthesis and respiration equations.
    1. Compare the reactants of the photosynthesis equation to the products of the equation for respiration. What do you notice?
    2. Compare the reactants of the respiration equation to the products of the photosynthesis equation. What do you notice?
    3. How many molecules of carbon dioxide and how many molecules of water are needed for green plants to synthesize one molecule of glucose and six molecules of oxygen?
  2. Name three ways that animals lose water.
  3. What type of nutrient is glucose (carbohydrate, protein, nucleic acid or lipid)?
  4. What are other sources of carbon dioxide (besides animals exhaling)?
  5. Get the following materials for the experiment:
    1. red cabbage solution (about 10 mL)
    2. one sprig of Elodea (or other aquatic plant that does not have needle-like leaves)
    3. two 50-ml beakers or clear cups
    4. 20 ml of water
    5. carbonated water
    6. one straw
  6. Respond:
    1. What color does the red cabbage solution turn in the presence of carbonated water?
    2. What color does the red cabbage solution turn after you exhale into it for approximately two minutes?
    3. What do you think would happen if you put the aquatic plant into the red cabbage solution?
    4. What gas (or gases) can red cabbage solution serve as an indicator for?
    5. What gas do you exhale?
    6. What gas do plants give off?
    7. How long (in seconds) did you have to exhale into the red cabbage solution to elicit a color change?
    8. How many breaths did it take?
  7. Produce a graph that illustrates a relationship between number of breaths exhaled into the red cabbage solution and the time it took for the solution to change color
Products of Energy at Home

From around your house, find and describe at least one example of a product of each of the following:

  1. Photosynthesis
  2. Respiration
  3. Fermentation [Hint: Fermentation happens in yeast]
Fuel and Organic Compounds

Americans love their cars. Most Americans use gasoline-powered cars to commute, run errands, take family vacations, and get places they want to go. Americans consume 25 percent of the world’s oil each year, but the country only provides 2-3 percent of the world’s oil resources, according to the U.S. Department of Energy. As demand for oil grows, car manufacturers and scientists have been looking for alternatives fuels to reduce cost, dependence on international sources of oil, and the amount of greenhouse gases that contribute to global warming.

Today’s typical car releases “greenhouse gases.” Ozone, Nitrogen Oxides, and carbon monoxide are pollutants that come from motorized vehicles when fuel is burned up in internal combustion engines to produce energy to move the car forward. People have been using this type of engine for over 100 years.

Gasoline is an aliphatic hydrocarbon, which means it is made up of molecules composed of hydrogen and carbon arranged in chains. Gasoline is made from crude oil. The crude oil pumped out of the ground is called petroleum.

Many new cars have been designed to use alternative fuels to run the engine. Alternative fuels for vehicles are any materials or substances that can be used as a fuel, other than conventional fossil fuels (oil and natural gas). The alternative fuels discussed here today include Ethanol (E85), natural gas (CNG), and biodiesel.


The main fuel we use today is gasoline, and is composed of several different hydrocarbons, including octane and benzene. Octane has eight carbons. Benzene has six carbons and is arranged in a ring.


Ethanol is an alcohol produced from feed corn that is used to fuel internal combustion engines, either alone or in combination with other fuels. When alcohol fuel (ethanol) is mixed into gasoline, the result is labeled with an ‘E’ followed by the percentage of Ethanol. E10 is commonly found throughout the southern United States and E85 refers to an 85 percent ethanol fuel. To be considered an alternative fuel vehicle (for tax incentives), the car or truck must be able to operate on up to 85 percent ethanol.

Ethanol has two carbons and an alcohol group.


Compressed Natural Gas (CNG) is high-pressure compressed natural gas, mainly composed of methane that is used to fuel normal combustion engines instead of gasoline. Gasoline cars can be retrofitted to compressed natural gas and become natural gas vehicles (NGVs) that use both gasoline and compressed natural gas.

Methane, the main component of CNG, is made up of one carbon.


Biodiesel is a processed fuel derived from biological sources (such as vegetable oils), which can be used in diesel-engine vehicles. Biodiesel is biodegradable and largely non-toxic. Most cars need to be modified to run on 100 percent biodiesel, but nearly all diesel engine cars can run on a blend of biodiesel without modifications.

A common component of biodiesel is methanol, which is made up one carbon and one alcohol group.


  1. You will use an organic molecule kit to complete this assignment. You will need three types of atoms: carbon, oxygen and hydrogen. Note that carbon makes four bonds, oxygen makes two bonds, and hydrogen makes one bond. Most of the time, one atom will be bonded to another with a single bond, but sometimes you will find that two of those bonding sites are used at the same time in a double bond. An alcohol group is made up of an oxygen bonded to a hydrogen.
  2. When making organic molecules, you will fill in any of the empty bonds with hydrogens. You will start with methane. Build methane, filling in any empty bonds with hydrogen atoms. Make a sketch of what you just built, labeling each atom.
  3. Each bond with a hydrogen atom has energy in it. How many energetic bonds does this methane have?
  4. When methane combines with oxygen and heat, it is burned to form two compounds. One of those compounds contains the carbon atom bonded with oxygen, and the other contains the hydrogen atoms bonded with oxygen. What are these two compounds?
  5. Write out the balanced reaction of the combustion of methane.
  6. For every molecule of natural gas that is burned, what is the contribution of carbon dioxide to the atmosphere?
  7. Repeat steps #2 – 6 for octane, assuming that the carbons are arranged in a chain.
  8. Change octane so that two more carbons are attached to the second carbon in the chain for a total of four carbons attached to the second carbon. Then take the fourth carbon in the chain and attach an extra carbon. Make sure that it still only has eight carbons and repeat steps #2 – 6 for this molecule, isooctane.
  9. Let’s move on to biodiesel. Methanol has an alcohol group, so remember this as you repeat steps #2 – 6.
  10. Moving on to ethanol (another biodiesel), repeat steps #2 – 6 for this fuel.
  11. Finally, consider benzene. Make a chain of six carbons, but then join the last one to the first one. In benzene, each carbon makes a double bond with one other carbon, and makes a single bond to the other carbon. Repeat steps #2 – 6 for this molecule.
  12. Considering the above molecules, which molecule would be the best fuel? Why?
  13. Which molecule would be the worst fuel? Why?
  14. You may have noticed a pattern to the names. Here is the key:
    • Meth- = 1
    • Eth- = 2
    • Prop- = 3
    • But- = 4
    • Pent- = 5
    • Hex- = 6
    • Hept- = 7
    • Oct- = 8
    • Non- = 9
    • Dec- = 10
  15. Write out the chemical structures and formulas for the following compounds:
    1. Propane (used in grills)
    2. Butane (used in lighters)
    3. Hexane (used in laboratories for storage)
    4. Butanol (used as a solvent)
    5. Heptanol (used in perfumes)
The American Diet

Researchers who study eating habits are concerned about the number of fast-food meals Americans eat each week. For example, a researcher in the Midwest studied the eating habits of 2500 teens. She reported that 27 percent of high-school girls and 30 percent of high-school boys eat fast food more than three times a week.

The circle graphs compare a typical American diet to a recommended diet. The graphs show the percent of energy from different classes of nutrients.


Analyze and Conclude

  1. Convert the percentages in the recommended diet into Calories. Assume 2000 Calories are needed per day.
  2. One gram of protein or carbohydrate has about 4 Calories. One gram of fat has about 9 Calories. Calculate the total number of Caloried from carbohydrate, unsaturated fat, saturated fat, and protein in the fast-food meal described on the next page.

The table shows nutritional information from a typical fast-food meal. The fries and the soda are both large. Use the table to answer Questions 2–5.

Typical Fast-Food Meal (% daily value based on 2000 C)


Nutrition Facts




% Daily






% Daily






% Daily



Total Fat

29.0 g


25.0 g


Saturated Fat

10.0 g


3.5 g



75 mg



1040 mg


350 mg


20 mg


Total Carbohydrates

45.0 g


63.0 g


86.0 g


Dietary Fiber

3.0 g


6.0 g



9.0 g

86.0 g

25.0 g

6.0 g

Vitamin A






Vitamin C









  1. How many Calories are in the fast-food meal? Compare this number to the daily requirement of 2000 Calories.
  2. What portion of the daily requirements for total fat and total carbohydrates does the fast-food meal meet? Does this result reflect a healthy diet? Explain. Hint: Consider other daily requirements and what happens during the rest of the day.
  3. The sum of the Calories in Question 2 is slightly larger than the total in Question 3. Propose a possible explanation for this difference.

Build Science Skills

Make a circle graph in the space below showing the percentage of energy from carbohydrate, unsaturated fat, saturated fat, and protein in the fast-food meal described in the table. You will need to use information from Question 2. Hint: The total number of Calories is not 2000. When you are done, compare your graph to the graphs of the typical American diet and recommended diet.