   Photosynthesis and the Meaning of Life - August 5, 1998 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County We gardeners compost, cultivate, sow, irrigate, fertilize, hoe, control, and otherwise nurture plants in hope of enjoying some tangible benefit from this investment of energy. The next time you are reclining under the dense shade of a tree, gazing across a well-tended lawn, eating a tree-ripened peach, remember that the real work was not done by humans but by plants themselves. This often forgotten fact is cause for some reflection, and as gardeners, it's time we pay tribute to the process that is the essence of our existence on this planet. Photosynthesis: that nightmare from high school biology class. You probably recall it as a seemingly endless pathway of abstract concepts leading to nowhere. If so, you must give it another chance. All of the food we eat and the air we breathe is produced during photosynthesis. Simply put, photosynthesis uses light energy to convert carbon dioxide and water into carbohydrates (food) and oxygen. It occurs in organisms called producers: green plants, algae and some types of bacteria. Other organisms, including humans, are called consumers and are completely dependent on producers for food and oxygen. This is still a simplified view. To really appreciate the process, let's look at some details. The sun provides the energy for photosynthesis. This light energy is delivered in particles called photons. About one-billionth of sun's total energy is directed at the earth. About half of this is reflected away from earth by clouds, dust, and gas. Of the energy that reaches the earth's surface, about half is in the right spectral range for photosynthesis. When photons enter a leaf, they go through the outer leaf surface and cell membranes until they enter microscopic structures within the cell called chloroplasts. Chloroplasts are the sites where photosynthesis occurs, about 10 microns across (a micron is a millionth of a meter) and made up of complex systems of membranes. The inner membranes contain pigment molecules (chlorophyll). The first step of photosynthesis involves the splitting of a water molecule (H2O) inside the chloroplast. The oxygen from water is released to become the air we breathe. Photons excite the chlorophyll to raise the energy level of the electrons donated by water. This energy plus the hydrogen from the split water is transferred to temporary energy storage compounds. The second step of photosynthesis requires carbon dioxide (CO2) and several enzymes to catalyze a complex series of chemical reactions. The temporary energy storage compounds from the first step provide energy to combine carbon dioxide (CO2) with larger molecules to form glucose: a carbohydrate. Glucose can then leave the chloroplast to be used as energy for the cell. Glucose can also be combined in different ways to make other carbohydrates such as sucrose (table sugar), starch (long-term storage sugar), or cellulose (insoluble fiber). This is truly a simplified overview of photosynthesis. If it still seems confusing, keep the overall picture in mind. From the minuscule amount of the sun's energy that earth receives comes 200 trillion tons of plant biomass each year. Think of the food, fiber, fuel, and other products formed through photosynthesis each year. Fossil fuels also were indirect products of photosynthesis. In fact, the total amount of fossil fuels stored under the earth's surface represent only 60 years of net photosynthesis. So, as you relish that freshly harvested ear of Verde Valley corn and breathe pleasant fresh air, give thanks to plants. They do the real work around here. For more information about plant growth, watering, varieties, and other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113. Or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |