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__**Energy for Life**__: In an indirect way, the fuel molecules in food represent solar energy. We can trace the energy stored in all our food to the sun. Humans and other animals depend on plants to convert the energy of sunlight to the chemical energy of sugars and other organic molecules.

Producers and Consumers: The star we call the sun is at the center of the solar system, almost 100 million miles away. A tiny fraction of its light powers life. The process that makes this possible is called photosynthesis. Photo means "light," and synthesize means to build, so photosynthesis means to build with light. In other words, photosynthesis uses light energy to power a chemical process that makes organic molecules.

In plants, photosynthesis occurs mainly in green cells within their leaves. Leaves are green due to chlorophyll, the pigment contained in chloroplasts. Chloroplasts are the organelles that house the equipment for photosynthesis. Chloroplasts trap light energy and use it to produce sugars and other energy-rich organic molecules.

Most ecosystems depend entirely on photosynthesis for food. That is why biologists refer to plants and other photosynthetic organisms as producers. The organic matter they produce not only nourishes the plants, but also supplies food for other organisms. Consumers, including humans and other animals, obtain their food by eating plants or by eating animals that have eaten plants. Animals depend on food not only for fuel, but also for the raw organic materials needed to build cells and tissues.

Chemical and Energy Cycling: Both photosynthesis and cellular respiration occurs in plants. There is a cycling of ingredients and products between these two processes. The ingredients for photosynthesis are carbon dioxide (CO2) and water H20). The carbon dioxide, a gas in the surrounding air, enters the plant through tiny pores in the surface of the leaves. The water is absorbed from the damp soil by the plant's roots. Water moves up the plant's veins from the roots to the leaves. Chloroplasts rearrange the atoms of these inorganic ingredients to produce sugars and other organic molecules. One key product of photosynthesis is the sugar glucose (C6,H12,06). A byproduct of this food production is oxygen (02).

Just as animals rely on plants for fuel, plants themselves use some of the organic molecules they make for their own fuel. A chemical process called cellular respiration harvests energy that is stored in sugars and other organic molecules. Cellular respiration uses oxygen to help convert energy extracted from organic fuel to another form of chemical energy called ATP. Cells spend the ATP for almost all their work. Production of ATP during cellular respiration occurs mainly in the organelles called mitochondria. Animals lack chloroplasts and are not capable of photosynthesis, but animals and plants both have mitochondria that perform respiration.

Notice that the waste products of cellular respiration are carbon dioxide (CO2) and water (H20), which are the chemical ingredients for photosynthesis. Plants recycle chemicals as they store chemical energy by photosynthesis and harvest chemical energy by cellular respiration. However, plants usually make more organic molecules than they need for fuel.

The photosynthetic surplus of plants provides the organic material for it to grow. This surplus is also the source of food for humans and other consumers. Analyze any, food chain, and you can trace the energy and raw materials for growth back to photosynthesis. The biosphere is solar powered.

__Reading Guide:__ Copy each item and fill in the blanks: 1. ............... uses light energy to power a chemical process that makes organic molecules. 2. ............... are the organelles that house the equipment for doing photosynthesis. 3. ............... build and feed themselves using the process and products of photosynthesis. 4. ............... obtain their food by eating plants or by eating animals that have eaten plants. 5. ............... and ............... are taken in by plants for use in photosynthesis. 6. ............... and ............... are products of photosynthesis. 7. The chemical process of ............... harvests energy that is stored in sugars to make ATP. 8. Cells spend the energy in ............... to run almost all their cellular processes. 9. Production of ATP during cellular respiration occurs mainly in the organelles called ............... . 10. The products waste products of cellular respiration are ............... and ............... . 11. The ingredients of photosynthesis are .............. and ............... . 12. The ............... products of cellular respiration are the ............... of photosynthesis. Review: 13. The main purpose of photosynthesis is to produce ............... . 14. The main purpose of cellular respiration is to produce ............... .