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Photosynthesis
PHOTOSYNTHESIS—a function or activity of green plants—is the production of sugar and pure oxygen from light, water, and air. Although only plants can photosynthesize, all animals are dependent on sugars and starches created by plants: all animals eat plants. . .or they eat other animals that eat plants. . .or they eat other animals that eat other animals that eat plants. Simple Summary Plants capture a part of the air and a part of water. They combine these into a new material: sugar! This sugar is the plants' own food. They eat it and grow. Human beings and other animals also eat sugars made by plants. For example, the next time you eat a carrot, notice how sweet it tastes.We human beings are dependent on photosynthesis for every morsel of food we consume, from salad and bread to roast chicken and chocolate milk. Photosynthesis is actually a series of chemical reactions, which take place in plant leaves, in cells called chloroplasts. Chloroplasts contain a green pigment called chlorophyll. 1. In the LIGHT REACTION, chloropyll interacts with light to split water into its basic parts or elements—hydrogen and oxygen. This split takes place in a fraction of a second. 2. The slower DARK REACTION happens during and after the light reaction. Leaves absorb carbon dioxide through tiny pores called stomata. This carbon dioxide reacts with sugars that are stored in the chloroplasts , splitting it into carbon and oxygen. 3. Next, Chloroplasts combine the hydrogen produced from water with carbon and oxygen produced from carbon dioxide to make sugar. The extra oxygen produced from water is released through the stomata and back into air in the form of pure oxygen gas. Plants are often called "the lungs of the Earth" because they absorb carbon dioxide and emit oxygen—in somewhat the same way that animals inhale oxygen and exhale carbon dioxide. Animals depend on plants to fill the Earth's air and water with pure oxygen, ready to be breathed through lungs and gills or absorbed directly through membranes. Chemical Equation 6H2O + 6CO2 > light&plant > C6H12O6 + 6O2 Energized by light, a chloroplast breaks six molecules of water into 12 hydrogen atoms and 6 oxygen atoms. At the same time, nearby sugars break six carbon dioxide molecules into six carbon atoms and twelve oxygen atoms. The chloroplast recombines loose atoms into one glucose (sugar) molecule containing six carbon, twelve hydrogen, and six oxygen atoms. Six leftover oxygen atoms are strung together in groups of two and released into the air as molecules of oxygen gas. Key to the equation: H2O is a water molecule; CO2 is a carbon dioxide molecule; C6H12O6 is a sugar molecule; O2 is an oxygen molecule. PLANT FOODS—following photosynthesis, plants combine tiny bits of sugar to make starch, and they combine sugar and starch with nutrients they absorb from the soil—for example nitrogen, potassium, calcium, sulfur, and iron—to make protein, enzymes, and vitamins. These foods are stored in roots, stems, leaves, flowers, fruits, and seeds. Examples of starchy roots or tubers include beets, carrots, potatoes, yams, taro, and ginger root. Starchy or sugary stems include asparagus and celery stalks, sugar cane, and sorghum. Of the thousands of vitamin-packed, edible leaves, human beings most prefer lettuce, cabbage, spinach, herbs, and some kinds of seaweed—while our dairy and beef cattle mainly like grasses. Fruits and seeds, the most compact or efficient sources of plant sugar and protein, include corn, wheat, rice, peanuts, beans, citrus fruit, apples, bananas, mangos, and tomatoes. Human beings drink teas infused from chamomile, jasmine, and hibiscus flowers and eat cauli-flowers and broccoli buds. Flower pollen and nectar are foods for bees, wasps, beetles, butterflies, spear-nosed bats, humming birds and thousands of other creatures. AUTUMN LEAVES—in addition to cholorphyll, plants may contain three other pigments: carotene (yellow-orange), xanthophyll (pale yellow or yellow), and anthocolorin (red and purple). (Carotene and xanthophyll may participate in photosynthesis; anthocolorin never does.) When leaves lose their chlorophyll, these other pigments are revealed—leaves turn gold, bronze, red, or buff. Loss of cholorphyll occurs in some plants at the end of summer, as days grow shorter and nights cooler. When photosynthetic pigment levels drop, photosynthesis ceases and leaves die and drop from their stems and branches. Dead plants and plant detritus—fallen leaves, broken branches, dropped fruit—decompose on the ground and release carbon, hydrogen, and oxygen back into the air. These elements are then ready again for photosynthesis by other plants. SCIENTISTS WHO STUDY PHOTOSYNTHESIS are called plant physiologists. They are biologists or botanists who specialize in how plants do what they do—how they grow, move, produce food, take up nutrients from soil, air, and water, adapt to their environments, and reproduce. Students who use scientific investigation to learn about photosynthesis and other plant functions are doing science. Transpiration Sources Twin Cities Public Television (KTCA), "Photosynthesis: How Do Plants Make Food," on Newton's Apple, the family science program on PBS:, Show Number 907, http://www.newtonsapple.tv/ TeacherGuides_animalsPlants.php?page=7 Brian Capon, Botany for Gardeners: An Introduction and Guide. Timber Press, Inc: Portland, Oregon, 1990 |
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