Complete the crossword, then click on "Check" to check your answer. If you are stuck, you can click on "Hint" to get a free letter. Click on a number in the grid to see the clue or clues for that number.

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Across: 1. A chemical element required by an organism in relatively large amounts. 2. An organism that breaks down dead tissues into simple chemical components, thereby returning nutrients to the physical environment. 3. An organism that uses energy from an external source such as the sun to produce its own food without having to eat other organisms or their remains. 4. An oxygen-binding pigment that is used to carry oxygen from the gas exchange surfaces to the tissues. 5. The gas exchange structure of terrestrial animals, formed by an infolding of epidermal tissue. 7. The enzyme that catalyzes the first reaction of carbon fixation in photosynthesis. 8. The openings to the intercellular air spaces in plant leaves that open and close to regulate gas exchange and evaporative water loss. 9. A consumer that relies on living plant tissues for nutrients. 10. An organism that obtains its energy by eating other organisms or their remains. ____ include herbivores, carnivores, and decomposers. 12. The binding of the enzyme rubisco to oxygen, rather than carbon dioxide, when O2 concentrations are high relative to CO2 concentrations. 13. The chemical breakdown of food. 14. The gas exchange surface of aquatic animals, typically consisting of finely folded sheets of thin epithelial tissue. 15. The waterproof outer layer of a multicellular organism.

Down: 1. A chemical element required by an organism in tiny amounts. 3. (____ parenchyma) The tissue in plant leaves that is specialized for photosynthesis. 6. An organic compound required by consumers in small amounts. 10. A consumer that relies on living animal tissues for nutrients. 11. (____ parenchyma) The tissue in plant leaves that is specialized for gas exchange and the evaporation of water that drives the movement of xylem sap.