1. How are the carbon and oxygen cycles related?
Carbon and oxygen are necessary for life. Carbon combines with oxygen and hydrogen to form the compounds that are the building blocks of living things.
All of the organic matter in an area is known as biomass. Carbon in biomass is returned to the soil by decomposers. People also aid in the cycling of carbon through composting. Compost is partly decomposed organic matter used to enrich soil.
1. Explain Why are carbon and oxygen necessary for life?
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2. The carbon cycle is the continuous movement of carbon in different forms through an ecosystem. The processes that recycle carbon and oxygen are linked.
Producers use CO2 from the air during photosynthesis. They use carbon to make sugars and starches.
As consumers eat producers, they take in carbon-containing molecules. As food is broken down, both producers and consumers release CO2 and water.
When organisms die, decomposers break down their remains and return carbon molecules to the soil and air.
Carbon is released into the atmosphere when fossil fuels and wood are burned.
Producers release oxygen as waste during photosynthesis. Most organisms take in the O2 to carry out life processes.

3. How does nitrogen cycle through ecosystems?
Like carbon, nitrogen is a building block of living things. Nitrogen cycles through the ecosystem in the nitrogen cycle.
Plants use simple nitrogen compounds to make proteins and other complex molecules.
Consumers eat nitrogen compounds in plants.
When organisms die, decomposers return simple nitrogen compounds to the soil.
Soil bacteria release some free nitrogen into the air.
Bacteria in root nodules fix free nitrogen into simple compounds that plants can use.
2. Explain How does free nitrogen in the atmosphere become a part of living organisms?

4. What processes are involved in the water cycle?
The water cycle is the continuous process by which water moves from Earth’s surface to the atmosphere and back.
Evaporation Heat from the Sun causes water to evaporate and form water vapor.
Condensation Rising water vapor cools and condenses into tiny drops of liquid water. The droplets collect around dust particles to form clouds.
Precipitation As more water vapor condenses, the drops grow. The heavy drops fall to Earth as precipitation.