Cycling of Matter 7.5B
The total amount of matter and energy available of a closed system is a fixed quantity. Matter is not created or destroyed, but rearranged and changed into different types of particles. For any chemical reaction in a closed system, the mass of the reactants must equal the mass of the products. In ecosystems, matter is cycled through various chemical processes as living organisms grow and reproduce. As organisms die, they are decomposed cycling matter throughout the various cycles again. The cycles in which matter and energy interact in living systems are the water, carbon, oxygen and nitrogen cycles.
In living systems, plants take in carbon dioxide gas during photosynthesis and use the carbon to produce other carbon-containing molecules such as sugars and starches. Consumers obtain energy from these molecules (from producers) and break them down into simpler molecules through respiration. Carbon dioxide gas releases during respiration. Oxygen also cycles through living systems. Producers release oxygen during photosynthesis and most organisms take in oxygen and use it during respiration.
In the nitrogen cycle, nitrogen moves from the air to the soil into living things and back into the air. Free nitrogen in the atmosphere is combined with other elements through nitrogen fixation and this process is done by nitrogen-fixing bacteria. Nitrogen is fixed into compounds used by other organisms. Consumers eat nitrogen compounds in plants, for example, to build proteins and other substances. Decomposers break down wastes and the remains of organisms and return nitrogen compounds to the soil. Certain bacterium releases some free nitrogen back to the air.
Biomass is organic material from plants and animals. When living organisms die, bacteria and other decomposers break down the material in biomass breaking larger molecules into smaller ones. The decay of biomass produces matter in the form of small molecules and releases energy stored in the chemical compounds in the biomass. This energy is released in the form of heat. Compost is matter being decomposed and recycled as a fertilizer or soil supplement for plants. A compost bin is designed to allow air to circulate through the compost to aid in the breaking down of the biomass. Aerobic bacteria aid in the process by converting some biomass into substances such as ammonia and ammonium compounds, which in turn converts into nitrites and nitrates through nitrification. Composting decreases some biomass that would, otherwise, be sent to landfills. At the same time, it returns some of the energy and nutrients of the biomass to the soil. It is an excellent way of recycling matter that returns some of the energy stored in biomass to the soil for use, while at the same time reducing the amount of waste sent to landfills. The nutrients produced during composting is used in gardens or landscaping as a natural fertilizer.
Student Expectation
The student is expected to demonstrate and explain the cycling of matter within living systems such as in the decay of biomass in a compost bin.
- Key Concept 1: Soil contains decomposers, such as bacteria and fungus, which break down decayed biomass. This process produces thermal energy, gases (such as carbon dioxide and nitrogen), and simple molecules (such as water). This matter is released back into the soil and atmosphere to be reused by producers to make food and to grow.
- Key Concept 2: Carbon is essential to life and cycles in many forms within living systems.
Carbon dioxide in the atmosphere enters the leaves of plants and is used in the process of photosynthesis to make glucose. Some animals eat plants and use the carbon of the glucose for energy and daily functions. When animals breathe, carbon dioxide is released into the air. When animals die, organic compounds which contain carbon in their body are broken down by decomposers and returned to the soil to be used by other plants and microorganisms. - Key Concept 3: Nitrogen is another element essential to life. It is cycled from the atmosphere to living things and back into the atmosphere. Nitrogen in the atmosphere enters soil and is broken down by bacteria into a usable form that is absorbed by the roots of plants. Plants use nitrogen for growth and productivity. Animals eat plants and also use nitrogen to grow and function. Decomposers break down nitrogen compounds within decaying plants and animals, which returns to the soil and atmosphere.
- Key Concept 4: Water is a simple molecule that is cycled throughout living systems and the atmosphere. Plants absorb water from soil through their roots and use it for photosynthesis. During this chemical reaction, water molecules are broken down into oxygen atoms that are released into the atmosphere and hydrogen atoms that are used to form glucose, the energy stored in plants.
Fundamental Questions
- What processes are involved in the cycling of matter within living systems?
- How does the essential element carbon cycle through living systems?
- How does the essential element nitrogen cycle through living systems?
- How does water cycle through living systems?
- What organisms, found in the soil, aid in the cycling of matter?