Science Experiment: Watching Plants Breathe

This science project is great to help seventh graders understand how plants breathe!

Setting the Scene: Photosynthesis and Respiration

Photosynthesis. pH. Respiration. Acidity in water. You may have learned about these concepts in science. Respiration is easy to see and understand – you automatically breathe in and out every few seconds. When you inhale, you are breathing in air that is full of oxygen. Your lungs extract some of that oxygen from the air, and your blood carries it to your cells to keep them healthy. As it releases oxygen to the cells, your blood also picks up the waste product from the cells, including carbon dioxide, or CO2. When you exhale, your breath contains CO2.

Where does the oxygen in the air come from? Do other organisms breathe in oxygen and exhale carbon dioxide, like humans?

The oxygen comes from trees and plants through the process of photosynthesis. Photosynthesis takes place when sunlight strikes chlorophyll, the substance in plants that makes them green. The biggest single contributor to the earth's oxygen supply is the phytoplankton in the world's oceans. Almost every other organism on earth, plants and animals alike, breathes in oxygen and exhales carbon dioxide. Animals and plants have one giant difference, though. Animals always use oxygen and release carbon dioxide; they never produce any new oxygen. Plants, on the other hand, “respire” when the sun is not shining, which means that they use oxygen and release carbon dioxide, just like animals. Because they photosynthesize much more than they respire – meaning that they make more oxygen than they use – plants really are the world’s most important oxygen factories.

The Basic Science: The Plant's Night and Day Cycle

During the day, when the sun is shining, plants photosynthesize and produce oxygen. In the nighttime darkness, they use oxygen and release carbon dioxide. If you could see the gas around a plant, you would see that it is rich in oxygen during the day and rich in carbon dioxide at night.

It just so happens that you can “see” the gas released by plants. Although you can't really see it with your eyes, you can observe and measure its effects by studying the chemistry of water that has aquatic plants living in it.

Remember, when plants photosynthesize in the presence of sunlight, they produce oxygen. Then, in the absence of sunlight, they "respire," or use oxygen and release carbon dioxide, or CO2. Carbon dioxide reacts instantly with water (H20) to form a mild acid called "carbonic acid." Its chemical formula is H2CO3. As a result, when aquatic plants are respiring, or releasing CO2, the water around them is slightly acidic, and when they are photosynthesizing, the acidity decreases.

The standard measure of acidity is pH. When water is acidic, the pH measure is less than 7. The opposite of pH is called "alkalinity.” When water is alkaline, the pH measure is more than 7. When water is perfectly neutral – neither acidic nor alkaline – the pH measure is exactly 7.

As plants release carbon dioxide, the water they live in becomes acidic. As they photosynthesize, the water becomes neutral or alkaline. What's more, this natural change in water chemistry happens every single day, as the sun rises and sets.

Watching Plants Breathe

You can easily build an ecosystem that will enable you to observe and experiment with an aquatic plant’s day and night cycle of photosynthesizing and respiring. Specifically, you can follow the pattern of water turning more and less acidic throughout the night and day cycle. You will need just a few easy-to-obtain materials.

Materials List

  • A one or two-quart clear glass jar, such as a Mason jar or a clean mayonnaise jar
  • Distilled water (from a supermarket)
  • A few aquatic plants, such as elodea, available from an aquarium store
  • Mid-range pH indicator (roughly pH-5 – pH-9), which is also available from aquarium stores, or you may be able to borrow equipment from your science teacher
  • A very sunny window and a dark drawer or closet (to mimic bright sun and deep darkness)

Procedure for making the model

  1. Fill the jar with distilled water and put in the aquatic plants. You will have to replenish the water once in a while regardless of whether you keep the jar open or covered. If you keep it covered, though, you will have to replenish the water less often. (You may also want to experiment to see if being open to the air instead of closed results in differences in the chemistry.)
    NOTE: It is important that you use distilled water for this procedure. Tap water and other types of bottled water contain minerals. Those minerals "buffer" the acidity, meaning that they absorb acid and keep the pH from changing as much.
  2. During the day, keep the plants in bright sunshine so they photosynthesize and produce oxygen. When the sun sets, move the jar into a very dark location, such as a drawer, so there is no oxygen production whatsoever.
  3. At regular times during the day, take a small sample of water from the jar and test its pH.
  4. Keep a careful log of your results that include notations on amount of sunlight, time of day, unusual conditions, and perhaps temperature.

Suggested Projects

  1. Measure the pH twice each day, once in mid-afternoon when then sun is at its highest, and once in early morning before the plant has started to photosynthesize. (If you keep the plant in a dark place at night, you won't have to worry about waking up before dawn to take your prephotosynthesis measurement!) Record your measurements and plot them on a graph.
  2. Change the experimental conditions. You could keep the plants in darkness for longer periods or put them under a bright light so they photosynthesize more. You could vary the number of plants in the water. You can experiment in any way that seems interesting to you. Just be sure you carefully measure the changes that take place.
  3. Research the "diurnal" cycle and write a report on other diurnal phenomena in nature. ("Diurnal" means happening on a daily cycle. One of the diurnal phenomena you witness every day in northeast Florida is the tides.)
  4. With adult supervision, find a safe place to grab water from a natural water body so you can test it in the same way you are testing your experimental model. Ideally, you will find a pond or canal containing a lot of algae or plant growth. Test the pH each day early in the morning and again late in the afternoon. Observe the conditions that seem to create changes in the outcomes. If the results are different from your controlled experiment, try to find reasons for the differences. Perhaps, for example, the water you are testing contains a high mineral content, so the pH changes more slowly and less dramatically.
  5. Research the role of ocean-borne phytoplankton in the production of the earth's oxygen and its role in combating the dangers associated with global warming. Write a research paper on it.
  6. Use the Internet to learn more about photosynthesis, plankton, pH, and diurnal cycle.

Putting it Together

Display your model in your classroom. Create charts and diagrams showing your findings. Explain why the acidity/alkalinity of water can change through the course of the day. Prepare and deliver a written or oral report about the importance of pH and oxygen level in water. Include observations and recommendations about bodies of water in your community.

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