The Breath of Plants, and the Life of the Planet



The Breath of Plants, and the Life of the Planet

Teacher Background

Does making plant food require sunlight? Light energy is the "force" which holds the raw materials (the atoms of carbon, hydrogen and oxygen) together in the carbohydrate molecules synthesized during the process called photosynthesis. Research on the exact nature of photosynthesis has been challenging and we still don't understand the process completely. We do know that more energy is put into bonding atoms in the carbohydrate molecule than existed in either the carbon dioxide or the water which were the raw materials. Photosynthesis is indeed a remarkable process that is integral to the first link in all food chains and essential to life as we know it on this planet. Food is not sunlight, but food may be said to be solar powered.

Oxygen must be taken in by every living cell in all plants and animals. This life-sustaining process is called respiration. Humans mechanically move air into and out of respiratory organs called lungs during a process called breathing. When we breath, oxygen and carbon dioxide are exchanged between our lungs and blood vessels. The blood transports oxygen to all the living cells of the body and, following cellular respiration, the carbon dioxide produced is carried back to the lungs and exhaled. Plants also respire at the cellular level and like humans exchange gases between their body cells and the external environment, although they lack special breathing organs. Gases diffuse in and out through various tissue surfaces and particularly through the stomata of the leaves. In the well-known equation for photosynthesis shown below, plants take in water and carbon dioxide and produce food (carbohydrates) and oxygen, some of which is released into the environment.

6 CO2 + 12 H2O [pic] C6H12O6 (glucose) + 6 H2O + 6 O2

(Note that according to the equation, for every one molecule of carbon dioxide gas that is used to make carbohydrates, one molecule of oxygen gas is produced.)

Plants photosynthesize only during hours of sunlight but they respire 24 hours a day just as animals do. Photosynthesis produces 5 to 10 times more oxygen than plants use in their own respiration so extra is generated for all other animals to use. The oxygen supply in our current atmosphere comes from the extra oxygen made during photosynthesis by green plants and other non-plant photosynthesizers like algae. If we reduce the number of photosythesizing organisms, we run the risk of depleting the oxygen content of our air. It's estimated that the entire oxygen supply of the world's atmosphere is renewed once every 3000 years by plants. There is no other important source of free oxygen in the world and indeed the Universe, as far as we currently know.

Many students do not understand that plants respire. Most students believe that photosynthesis is the plant's way of "breathing". These Activities should help them realize that plants give off oxygen when photosynthesizing (in sunlight) and carbon dioxide in the dark. It was not until 1779 that Jan Ingen-Housz, an Austrain physician, showed clearly and for the first time that plants not only released oxygen into air, but also take oxygen from it.

Objective

• Students will determine that photosynthesizing plants give off oxygen.

• Students will determine that plants in the dark (not photosynthesizing) give off carbon dioxide.

Materials

(for the class)

Bucket of water, left standing (unstirred) for a day

Baking soda

(for each group of students)

Two identical drinking glasses

Two sprigs of Elodea

Engage

If we provide extra carbon dioxide in an experiment, we could hypothesize that we would see faster photosynthesis. What would tell us that photosynthesis is occurring? (The release of oxygen gas.) Where can we get extra carbon dioxide to test our hypothesis? Have you ever played with baking soda, or made tabletop volcanoes? What kind of gas is given off as baking soda reacts with vinegar? What gas is made in fire extinguishers, and why and how do they work? Mixing baking soda with water produces carbon dioxide, but at a slower rate. What else is needed for photosynthesis in a green plant? (sunlight and water)

Explore

Keep a bucket of water for at least a day so that any excess oxygen dissolved in it is given off. Add a spoonful of baking soda to the water to ensure it has lots of carbon dioxide. Have students pour this water into two matching beakers. Put the two Elodea sprigs in the beakers and cover each with an upside-down glass funnel. Place a test tube filled with water over the shaft of each funnel so the closed end sticks above the water in the glass. Be sure each test tube is entirely full of water. Secure the test tubes so the can be kept overnight. Put one beaker in a light-proof cabinet and put the other beaker under a grow lamp or in sunlight. Keep both beakers under these conditions for 24 hours, measure the height of the gas collected in each test tube.

Explain

Why did the test tube kept in light have gas collected in the closed test tube end? We cannot tell simply by looking if this gas is oxygen, but there are some simple ways to find out. Test this gas by removing the test tube from the water with thumb held tightly over the open end. If we hold the test tube horizontal and insert a wood splint that was lit but then blown out, we should see the splint flame up again if the gas is oxygen. If the gas is carbon dioxide we will see any glowing spark snuff out. Try it—you will see the glowing splint burst into flame. The gas produced by the plant in sunlight is oxygen. You should find that the plant kept in darkness generated much less gas—if any.

Suggested URLs



Arizona State University Photosynthesis Center: information on various research projects and virtual experiments.



From PBS’s “Newton's Apple”: easy to understand explanation of photosynthesis and activities for elementary or middle school.



Science Made Simple: clear description of photosynthesis, why leaves change color in fall, and activities.

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