Oxygen-Carbon-Dioxide Cycle

•	Around 2.5 billion years ago, some cells evolved a method of capturing _________ from the sun and converting it to sugar that could be used as a food source.  This process, called ____________________, also produced ______________ as a byproduct.

•	Because these organisms needed only ____________, water, and readily available carbon dioxide for their survival, they thrived and multiplied in the environment.

•	Gradually, the atmosphere filled with ____________, about 350 million years ago, the concentration of oxygen reached a level similar to what it is today.

•	As _____________, oxygen-breathing organisms evolved, they joined plants in a balance that led to our present atmosphere.  

•	Oxygen and ___________ are necessary to sustain life. These elements are the source of all energy on a planet with living things. Oxygen alone is responsible for our being able to breathe and obtain energy from the food we eat. Can these elements be used up and disappear from a planet? Or will they be regenerated in some way? 

•	Carbon dioxide (________) is essential for the survival of life on this planet. Together with the energy from the sun, carbon dioxide is used by plants to produce the energy necessary for the survival of all living things.

•	Carbon dioxide is a _________. It is released from the ocean, produced by combustion of fossil fuels (wood, natural gas, coal and oil), eruption of volcanoes and by respiration (breathing) of all living things. There are about 23 million tons of carbon dioxide in the atmosphere at any one time. 

•	Oceans store 50 times more carbon dioxide than the atmosphere as a gas and in the form of carbonate compounds (carbonates are polyatomic ions -CO3). The carbon dioxide may be dissolved near the surface of the ocean. 

•	The ocean absorbs more carbon dioxide when it is cold. The one celled organisms that live near the surface of the oceans (near coasts and around the south pole) can change the carbon dioxide into oxygen and food molecules (energy) by ___________________. These one celled organisms are called phytoplankton or just plankton. Algae, such as seaweed also undergo photosynthesis. THESE SMALL ORGANISMS PRODUCE THE MAJOR PORTION OF THE ____________ FOR BREATHING ON THIS PLANET BY PHOTOSYNTHESIS . 

•	Photosynthesis is the chemical ____________ on Earth which allows the amount of carbon dioxide in the atmosphere to remain constant. Photosynthesis removes 60 million tons of carbon dioxide from the air each year. 

•	It occurs in all green __________ on the surface of the Earth and also in the algae (seaweed) and in plankton (one-celled organisms) living near the surface of bodies of water (such as the ocean).

•	We know plants need ____________  ____________ for photosynthesis and ________ production.  Oxygen is then released as _________ product of photosynthesis.  Animals breathe oxygen during _____________ and release carbon dioxide as waste.  Plants and other organisms then use the carbon dioxide they exhale, and the process is _____________.