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How are respiration and photosynthesis linked?

With each breath you exchange the gases oxygen (O_2) and carbon dioxide (CO_2). Why is this exchange important? Why does your body need oxygen? What happens to the CO_2? Where do we get O_2? These are some of the questions that this section will help you answer.

Every cell in your body needs oxygen to function. You get the oxygen your cells need from the air you breathe. The air you breathe is made up of 20 percent oxygen. The rest of the air is mostly nitrogen (79%). Your body cells use the oxygen you breathe to get energy from the food you eat. This process is called cellular respiration. During cellular respiration the cell uses oxygen to break down sugar. Breaking down sugar produces the energy your body needs. This is very similar to wood burning in a fire. As the wood burns, it combines with oxygen and releases heat energy and carbon dioxide. When the cell uses oxygen to break down sugar, oxygen is used, carbon dioxide is produced, and energy is released. But instead of heat energy, much of the energy produced in cellular respiration is stored chemically for the cell to use later. Carbon dioxide is the waste product of cellular respiration that you breathe out each time you breathe. Blood picks up oxygen and releases carbon dioxide in the lungs. The opposite takes place in the cells where the blood releases oxygen and picks up carbon dioxide.

The Breath of Life

Think about where the O_2 you inhale comes from and where the CO_2 you exhale goes. The exchange of gases doesn't only take place in the cells in your body. Actually, gas exchange is taking place all around you. In fact, oxygen and carbon dioxide are involved in the most important relationship that exists between plants and animals.

Figure 3.1 The blood in your lungs picks up oxygen and releases carbon dioxide. In your cells blood picks up carbon dioxide and releases oxygen.

Do you know where the oxygen your body needs comes from? Almost all the oxygen you breathe comes from green plants. They produce oxygen during a process called photosynthesis. During photosynthesis green plants manufacture the sugar molecules fructose and glucose. Green plants use energy from sunlight to build sugar molecules from carbon dioxide and water. Oxygen is produced when the plant combines the carbon dioxide and the water by using the Sun's energy. Plants use the sugar they produce to make plant structure and to provide the energy they need to live. Green plants use some of the oxygen they produce for their own life processes. But they release most of the oxygen produced during photosynthesis into the air as a waste product.

Unlike plants, which can get energy directly from sunlight, animals (including humans) must get energy from the food they eat. That food can be plants and/or other animals. Now think about the process of cellular respiration again. During cellular respiration animal cells combine oxygen with food molecules to release energy to live and function. Remember that cellular respiration produces carbon dioxide as a waste product. Animals use energy to grow, reproduce, and to function. They release the carbon dioxide into the air as a waste product. Plants help animals and animals help plants.

Did You Know?

It wasn't until the 1850s that scientists realized the Earth is a closed system-nothing comes in or goes out. Elements in our environment have cycles. They are used again and again. Recycling is a natural part of our existence. That's one reason we need to be careful and keep our environment clean and healthy.

Figure 3.2 Plants use carbon dioxide during photosynthesis to produce sugars and oxygen. Animals and plants use oxygen in respiration to produce carbon dioxide.


All living organisms-plants and animals-carry out cellular respiration 24 hours a day. Cellular respiration takes place every minute of every day of every month of every year, and so on. The process of respiration can be shown with words or in a chemical equation. Both are written below.

&\text{Oxygen} + \text{Glucose} \ \longrightarrow \text{Energy} \ + \ \text{Carbon dioxide} + \text{water}\\\\&\quad 6O_2 \ \ + C_6H_{12}O_6 \longrightarrow 36 \ \text{ATP} + \qquad 6CO_2 \quad + \quad 6H_2O

Let's read the chemical equation for cellular respiration. The equation shows that cells use six molecules of oxygen (6O_2) to break down one molecule of glucose (C_6H_{12}O_6). The arrow shows that when the sugar is broken down something else is produced. Energy, water, and carbon dioxide are produced. When the chemical bonds of the glucose molecule are broken, energy is released. The cell is able to store that energy in the chemical bonds of a special molecule called adenosine triphosphate or ATP. The energy stored in ATP can be used by the cell to do various kinds of work. For each molecule of glucose broken down, 36 molecules of ATP can be produced. Finally when the glucose molecule is broken down, the waste products are six molecules of carbon dioxide (6CO_2) and six molecules of water (6H_2O).


The waste product of cellular respiration, carbon dioxide, is just what green plants need for photosynthesis. But green plants need more than just carbon dioxide to carry out the process of photosynthesis. They use a colored pigment called chlorophyll to collect energy from the Sun. During photosynthesis the plant uses the energy from the Sun to combine carbon dioxide and water to make the sugar called glucose. Like cellular respiration, the process of photosynthesis can be explained with words or in a chemical equation.

&\text{Light} + \text{Carbon Dioxide} + \text{water} \longrightarrow \text{Oxygen} + \text{Glucose}\\&\text{Light} + \qquad 6CO_2 \qquad \ + 6H_2O \longrightarrow \ \ 6O_2 \quad + C_6H_{12}O_6

Now let's read the chemical equation of photosynthesis. Here's what this chemical equation says. Green plant cells use the energy collected from sunlight to combine six carbon dioxide molecules (6CO_2) and 6 water molecules (6H_2O) to make one glucose molecule (C_6H_{12}O_6) and 6 molecules of oxygen (6O_2).

Did You Know?

One acre of trees produces enough oxygen to keep 18 people alive for one year. One tree absorbs about 13 pounds of carbon dioxide each year. One acre of trees absorbs 2.6 tons of CO_2, That's enough to offset the CO_2 produced by driving a car 26,000 miles.

Miles = Trees Estimate the number of miles that your family drives in one year. How many acres of trees would your family have to plant to offset the amount of carbon dioxide produced by the number of miles you estimated? What other ways, besides driving, cause CO_2 to be produced?

Now let's stop and think about the cycle we just investigated. Photosynthesis is the process by which plants use the energy of sunlight to make sugar (glucose) from carbon dioxide (CO_2) and water. Respiration is the process by which animals use O_2 to get energy from food. So plants take in CO_2 from the air and produce O_2, And animals take in O_2 from the air and produce CO_2.

Notice that these two chemical equations a related. Now we can do some activities based on the photosynthesis/respiration cycle. Photosynthesis and respiration form an extremely important cycle in nature.

Assume that all the oxygen you breathe in comes from plants and all the carbon dioxide plants use comes from animals. Why is the destruction of the tropical rain forest of concern to us in the United States?

Figure 3.3 Plants and animals produce energy through reverse or opposite processes.

Activity 3-1: It's a Gas-Carbon Dioxide and Oxygen

Part A: The Gas-Carbon Dioxide


When the cells of your body burn fuel (sugar), they produce a gas-carbon dioxide. When the cells of a green plant make sugar, they need a gas-carbon dioxide. In this activity you can see how carbon dioxide gets recycled. When you exhale, some of the carbon dioxide in your breath will dissolve in the water and turn the indicator bromthymol blue from blue to green to yellow. The indicator will change back to the original blue if carbon dioxide is not longer present.


  • Bromthymol blue indicator solution.
  • Plastic wrap or two test tube stoppers
  • Masking or labeling tape
  • Straws
  • 2 Test tubes
  • Beaker or similar container
  • Elodea or some other water plant
  • Pond water
  • Activity Report


Day 1:

Step 1 Write 1 on a small piece of masking tape. Write 2 on another small piece of masking tape. Place one of the pieces of labeled masking tape on one test tube and the other piece of labeled masking tape on a second test tube. Pour bromthymol blue solution into both of the test tubes labeled 1 and 2.

Step 2 Slowly bubble the air you exhale through a straw into each test tube.

Step 3 Observe and record your results on your Activity Report.

Step 4 Add sprigs of Elodea to test tube labeled 1 only.

Step 5 Cover the mouth of the test tubes with plastic wrap, wax paper, or a stopper.

Step 6 Place both test tubes in the light for 24 hours.

Day 2

Step 7 After 24 hours observe and record the results observed in the two test tubes.

Part B: The Gas-Oxygen

When the cells of a plant make sugar (fuel), they produce a gas-oxygen. In Part B of this activity you can see evidence of oxygen. By providing a strong light source for water plants, you can see the bubbles of gas the plant produces. This gas is oxygen-the gas you breathe and which our cells use to produce energy during cellular respiration.

Step 1 Place several healthy branches of Elodea in a large beaker of water.

Step 2 Place the beaker with the Elodea in a strong light source. Observe the Elodea and the water in the beaker over a 20-minute time period. Record your observations.

Figure 3.4 Elodea is shown in a beaker of water.

So you discovered that a green plant such as Elodea uses carbon dioxide and releases oxygen. That is opposite of what we and other animals do. Remember that we use oxygen and release carbon dioxide. As a matter of fact, your cells behave like candles in some ways. Both your cells and candles use oxygen to produce energy from fuel. Your cells use sugar as fuel. A candle burns the wax as fuel. Let's try another activity to see if candles use oxygen and release carbon dioxide like our cells do.

Activity 3-2: Cell Candles


Do you know how candles behave like cells in your body? Both use oxygen to release energy from fuel. The candle burns wax. Your cells “burn” nutrients from food. The nutrient the cell uses for energy is glucose, a sugar, which is a carbohydrate. Both the cell and the candle produce carbon dioxide in the burning process.

\text{fuel} + 2O_2 \longrightarrow 2H_2O + CO_2 + \text{energy}

Let's read this equation. You know the word fuel. In your cells the fuel is glucose or sugar. In the candle the fuel is wax. So fuel combines with oxygen (O_2). The arrow means to become or produce. When fuel combines with oxygen, the combination produces water (H_2O), carbon dioxide (CO_2), and energy. Now try this activity to see if this equation really does happen when a candle burns.


  • 1 Tall glass, glass jar, or beaker
  • 1 Bowl larger than the glass, glass jar, or beaker
  • 1 Candle
  • Matches
  • Water
  • Data Table
  • Activity Report

BE CAREFUL WITH MATCHES! Here's an important safety tip before you begin this activity. Make sure you are wearing safety goggles if you are using a match. Matches can be very dangerous, so be extremely careful. Make sure you douse the match and the candles in water before you discard them.


Step 1 Carefully use a lit match to melt the bottom of a candle and stick the candle firmly in the center of the bowl.

Step 2 Fill the bowl with 3-cm of water.

Step 3 Carefully use a match to light the candle. Place a glass over the candle. Observe what happens to the candle, the water level, and the inside of the jar. Record your observations on your Activity Report.

Step 4 Repeat your experiment two more times. Record your data in your data table.

Figure 3.5 The bowl contains water with one candle covered.

Explain how burning wood in a fireplace is like consuming glucose in your cells. How is it different? What is the chemical make up of wood?

Cellular Respiration and Burning Candles

Until the American Revolution people thought that air was just a single substance. A British scientist named Joseph Priestly showed that this couldn't be true. He showed that a candle in a sealed volume of air went out even though only a portion of the air was used up. He experimented with both a living animal, a mouse, and with a candle. He showed that introducing a growing, green plant could renew the oxygen in the air. His experiment showed that the mouse used up the oxygen in the air but not the nitrogen and other gases. And he showed that the green plant replaced the oxygen that was used up.

Let's look closer at the exchange of the two gases-oxygen and carbon dioxide. Remember that you can think of cellular respiration as being similar to a fire such as the burning candle. A fire uses oxygen to burn and gives off the waste product carbon dioxide (CO_2). Without oxygen a fire dies. When a fire doesn't have fuel it dies. When a fire burns fuel such as the wax in a candle, the molecules in the fuel combine with molecules of oxygen. When the molecules of oxygen and the molecules of fuel combine, the reaction releases energy as heat energy. Your body uses food as fuel to produce energy to keep you running all day. In a similar way, a candle burns wax, a car uses gas, or a train burns diesel fluid to keep going.

Did You Know?

Joseph Priestly's experiments were the first to demonstrate the cycle of respiration and photosynthesis. That's pretty interesting science information about Dr. Priestly. But here's a little history information about him. Joseph Priestly was an Englishman, but he supported America during the American Revolution. As a result, he became very unpopular in England and had to flee. He moved to Philadelphia where he became a chemistry professor at the University of Pennsylvania.

Figure 3.6 Cells are like candles. What did the candle under the glass do? It combined wax and oxygen, burned, and released carbon dioxide and water into the air. Wax is the fuel for the candle. Glucose (a sugar) is the fuel for your cells.

Just like a burning candle, your cells actually produce heat. How do you know your cells make heat? When you huddle under a blanket, you get warm. Why do you think you warm up under a blanket? Your body produces a lot of heat. Actually your body produces almost as much heat as a 50-watt light bulb does. But since the body is always making heat, you might ask this question. What does the blanket have to do with getting warmer? It's true that the body is constantly producing heat. However, a lot of the heat escapes through your skin. The more layers you cover yourself with, such as the blanket, the more heat is trapped. The blanket holds the heat close to you and keeps you warm. If you get too warm you remove the blanket to let the extra heat escape.

So animal and plant cells carry on respiration and green plant cells carry on photosynthesis. Now let's explore some other important topics about your breathing machine. For example, what happens to your breathing when you exercise?

Exercise and Breathing

Do your cells burn more fuel or glucose when you're resting or when you're playing soccer? Do you need more oxygen when you exercise? If you need more oxygen, then do you make more carbon dioxide? Let's find out.

What Do You Think?

Why do you think a person's breathing rate is one of the factors measured in a lie detector test?

Your heart pumps faster and your breathing rate increases when you exercise. Why do you think that statement is true? First imagine that cellular respiration is similar to a tiny candle burning in each cell. When you exercise, the tiny candles burn brighter. They burn their fuel faster and they use up the O_2 faster. As they burn the glucose fuel, they produce water and carbon dioxide. Your heart beats faster pumping more fuel and oxygen to the cells and taking carbon dioxide away from the cells. Your lungs inhale more and faster trying to take in enough O_2 and give off the excess carbon dioxide.

Did You Know?

Carbon dioxide is the gas that makes champagne fizz. When yeast cells use sugar for energy, they produce carbon dioxide. Sometimes bottles of wine are sealed before all of the yeast cells finish using up all the sugar in the wine. But the yeast continue using the sugar and releasing the carbon dioxide in the corked bottle. Carbon dioxide makes sodas fizz, too. But there's no yeast in soda. The carbon dioxide is forced into soda bottles and cans.

Figure 3.7 shows, in a simple way, how your heart and blood vessels connect to your cells and lungs. The heart pumps blood through the lungs. In the lungs the blood receives oxygen and releases carbon dioxide. Then the heart pumps this oxygen-rich blood from the lungs to your cells. When the oxygen-rich blood gets to the cells, the cells receive the oxygen and release the carbon dioxide. The blood with less oxygen and a lot of carbon dioxide returns to the heart. Then the heart returns this blood to the lungs where carbon dioxide is released and oxygen is received. And the Cycle starts all over again.

What happens when you exercise? The harder you exercise the more fuel or glucose your cells must burn. The more glucose your cells burn the more oxygen they use. So it's obvious that the cells need to get more fuel or glucose and oxygen to continue exercising. At the same time, the cells must get rid of all the carbon dioxide they are producing as you continue to exercise. That's the reason your heart beats faster and your lungs move more air in and out when you exercise. Your heart and lungs are keeping up with the demands of your cells. Engineers admit that they couldn't design a more efficient system than the heart and lungs. Our hearts and lungs, like many other animals' hearts and lungs, work to match the flow of blood and gas to the needs of the cells. When the cells need more fuel and oxygen and need excess CO_2 removed, the heart and lungs work harder than when the cells need less.

Write a letter to your town council in support of, or objecting to, a paper production company locating to your town. The company will bring lots of jobs to the area. But it will begin the cutting of the forest around your town, too. The people in your town need the jobs. But are the new jobs worth the cost of cutting down trees? Take into account the health effects of cutting the trees. Give the letter to your teacher to mail.

Figure 3.7 The heart is two pumps in one. One pump pushes oxygen-rich blood from the lungs to the cells of the body. The other pump pushes oxygen-poor blood back to the lungs.

Back to Normal How long does it take for your heart and lungs to return to their normally functioning rate after you exercise? Find out. Find your heart rate while sitting quietly. Now run in place for 3 minutes. Take your heart rate again. Wait 1 minute and take your heart rate a third time. Wait another minute and take your heart rate again the fourth time. Record your heart rate each time. Continue to find your heart rate until it is back to your resting rate. How long did it take? Do you think this time period would get longer or shorter if you were an Olympic athlete who's been training for years? Now do the same experiment for your breathing rate. Try not to intentionally alter your breathing rate. It is best to have a partner measure your breathing rate while you think of other things.

Review Questions

  1. Explain how photosynthesis and respiration are part of the same cycle. Draw a picture and describe the cycle.
  2. Explain how your cells are like burning candles.
  3. How do the cells in your body respond when you exercise?
  4. How do your heart and lungs respond to the increased demand for oxygen when you exercise?

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Date Created:

Feb 23, 2012

Last Modified:

Dec 11, 2013
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