Our earth has a long history. To understand the processes occurring on and in our earth now we must think back to the earth’s origins. In this lesson, students will be asked about the formation of oxygen in the atmosphere. As plants began to grow and carry out photosynthesis, our atmosphere received oxygen as a waste from the plants. The production of oxygen by plants during photosynthesis is still happening today. The cycling of oxygen and carbon dioxide through our atmosphere are examples of biogeochemical cycles. It is through these and other cycles that the earth supplies the necessary chemicals for life to exist. Biogeochemical cycles will be an important part of upcoming lessons. It is important to understand that many things in science occur in cycles. By studying these cycles, scientists can better understand the world around us. Although these lessons will involve many different biogeochemical cycles, the iron cycle will be particularly important to your work. Iron can exist in many states. In particular, iron can be in the 2+ or 3+ states. This means that there are more protons (2 more or 3 more) in the iron atom than there are electrons. When iron (symbol Fe) is changed from Fe2+ to Fe3+, the atom is losing an electron. When iron is changing from Fe3+ to Fe2+, it is gaining an electron. Bacteria help to make these changes by removing or adding electrons to the iron. Aerobic bacteria (bacteria that need oxygen to survive) oxidize iron which changes the Fe2+ into Fe3+. Some anaerobic bacteria (those that do not need oxygen or can not survive with oxygen) reduce the bacteria from Fe3+ into Fe2+. In addition there are other non-living things, such as chemicals, which can also help to change the iron