So far we have talked in general terms about the breathing process of animals and humans. Now the aim is to talk about the breathing process in plants. We now want to get more specific about what happens during the breathing process of plants. The teaching point here is for students to see the reciprocity between the two life forms. We will begin with animals and then spend more time on what the exchange of gases is like in plants; this should lead into a direct discussion of photosynthesis.
How do plants breathe?
Essentially the leaves on a plant act much like lungs. Thousands of microscopic openings called stomata are located largely, but not exclusively on the underside of leaves. Each stomata is surrounded by special cells, which regulate the size of the opening. Water in the leaf evaporates into the air (transpiration)(see Appendix A, Experiment #6). 10 The stomata play an important part in a plants' survival. In dry spells and at night the stomata can close and keep the plant from losing moisture. Experiment #711 in Appendix A will allow students to see why the stomata are likened to lungs on a plant. In the procedure, a few leaves on a plant are covered with Vaseline. After leaving the plant for a couple of days, the leaves will begin to wilt and die. Oxygen is not being released and carbon dioxide is not entering the problem so the leaves’ system is backing up.
The word photosynthesis means, “putting together with light.” Plants do this when they use carbon dioxide, water, and sunlight to produce the food they need to survive (see Appendix A, Experiment #8). 12 Photosynthesis takes place mainly in a plant’s leaves. Because leaves are so thin no plant cell is very far from the surface. When the sunlight hits the leaves the leaf absorbs the light. Cells in leaves contain chloroplasts that contain a green pigment called chlorophyll. This material gives leaves their green color. When light is absorbed the electrons in the chlorophyll become so filled with energy that they break out of their molecule. Every chlorophyll molecule that loses an electron is incomplete. The only way it can be complete is to take an electron from a water molecule. When an electron is taken away from a water molecule the molecule splits into hydrogen and oxygen. The oxygen returns back into the air where animals and humans breathe it. The hydrogen is used to make glucose a simple form of sugar. The released electron is used to make ATP (adenosine triphosphate). ATP is used by all living things to store, carry, and transfer energy. It has three phosphorus atoms bonded together. When energy is needed the last bond is broken and ATP becomes ADP. In this process, light energy becomes chemical energy that the cells in the plant can use. Carbon dioxide comes into the plant when the stomata are open. It provides the oxygen and hydrogen atoms needed to make glucose. The glucose is made through a series of reactions that are driven by the energy released from the ATP molecule. When the ATP molecules lose their energy they become ADP (adenosine diphosphate). The ATP can be reenergized if the ADP can pick up more energy and reform its bonds. They return to be reenergized. Glucose is used by the plant to make cellulose and fats. The Glucose that is not used is stored as starch. This is the basic food used by plants. Most important to human beings and other life on earth is that in this process of photosynthesis, plants use less than they produce. That left over is returned to the atmosphere. Therefore the plants and trees around us help to balance the gases in our air by using up carbon dioxide and making oxygen. In the case of plants, the opposite of humans and animals is true - carbon dioxide in, oxygen out.13