Unit A-Chapter 4-Lesson 2

How Are Materials Recycled?

Oxygen-Carbon Dioxide Cycle

• Water is one of the most important materials required by an organism.

Photosynthesis, green plants use energy from the sun to convert carbon dioxide and water into sugar and oxygen.

• Photosynthesis = Carbon dioxide + Water + Energy ->Sugar + Oxygen

Respiration is an energy-producing process in which a cell combines oxygen with sugars and gives off carbon dioxide and water.

• Respiration = Sugar + Oxygen -> Carbon Dioxide + Water + Energy

• Plants carry out both photosynthesis and respiration.

The Nitrogen Cycle

• All living things need nitrogen to make proteins. 
• Although 80% of the air consists of nitrogen gas, few living things can use nitrogen in this form.
• Nitrogen gas must first be changed into one of a variety of nitrogen compounds and cycled through the environment.

Nitrogen gas can be converted into forms usable by living things in two ways:

1. Nitrogen-fixing bacteria that grow in the root nodules of legume plants such as peas, soybeans, and clover combine nitrogen in the air with hydrogen to form nitrogen compounds that plants can use to make proteins.
2. Lightning can cause nitrogen gas to combine with oxygen in the air to form nitrogen compounds that return to the earth in rain or snow.

When the compounds of protein are broken down during decomposition, nitrogen is returned to the air as nitrogen gas.

Pollution Affects the Cycles

• Human activities can disturb the natural recycling of oxygen and carbon dioxide. 
• The burning fossil fuels, for example, results in increased levels of carbon dioxide in the air.
• Excess carbon dioxide and other "greenhouse" gases absorb infrared radiation coming from the  earth's surface and trap this heat in the atmosphere, which could result in an eventual overall increase in the earth's average temperature.