Extension Questions 28. Plants and animals are part of all of the nutrient cycles through the foods they eat and what eats them (food chains and food webs). Name the four classes of organic compounds (containing carbon) and explain how the carbon cycle and nitrogen cycle contribute to the usable supplies of these macromolecules.

The four classes of organic compounds that contain carbon are: 1. **Carbohydrates**: These are organic compounds made up of carbon, hydrogen, and oxygen, typically in a ratio of 1:2:1. They serve as a primary energy source for living organisms and are found in sugars and starches. 2. **Proteins**: Composed of amino acids, proteins are essential for the structure, function, and regulation of the body’s cells, tissues, and organs. They contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. 3. **Lipids**: These are hydrophobic organic compounds, including fats, oils, and waxes. They are primarily made of carbon and hydrogen and are important for energy storage, insulation, and cellular structure. 4. **Nucleic Acids**: These include DNA and RNA, which are made up of nucleotides containing carbon, hydrogen, oxygen, nitrogen, and phosphorus. They are essential for storing and transmitting genetic information. ### Contribution of the Carbon Cycle The carbon cycle plays a crucial role in the formation of these macromolecules: - **Photosynthesis**: Plants absorb carbon dioxide (CO₂) from the atmosphere and convert it into glucose (a carbohydrate) using sunlight. This process not only produces carbohydrates but also releases oxygen as a byproduct, which is essential for animal life. - **Respiration**: Animals consume plants (and other animals) to obtain carbohydrates, proteins, and lipids. During cellular respiration, they break down these organic compounds to release energy, returning CO₂ to the atmosphere. - **Decomposition**: When plants and animals die, decomposers break down their organic matter, returning carbon to the soil and atmosphere, thus continuing the cycle. ### Contribution of the Nitrogen Cycle The nitrogen cycle is vital for the synthesis of proteins and nucleic acids: - **Nitrogen Fixation**: Certain bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can be used by plants to synthesize amino acids (the building blocks of proteins). - **Nitrification**: Other bacteria convert ammonia into nitrites (NO₂⁻) and then into nitrates (NO₃⁻), which are also usable by plants. This process ensures that plants have access to nitrogen, which is essential for protein and nucleic acid synthesis. - **Decomposition**: When organisms die, decomposers break down their proteins and nucleic acids, releasing nitrogen back into the soil in forms that can be reused by plants. In summary, the carbon cycle provides the carbon necessary for the formation of carbohydrates, proteins, lipids, and nucleic acids, while the nitrogen cycle supplies the nitrogen needed for proteins and nucleic acids. Together, these cycles ensure that essential organic compounds are continuously available to support life.