Communities and Ecosystems

A series of free High School Biology Video Lessons. Here we discuss some of the basic biological/ecological cycles, such as water cycle, carbon cycle, phosphorous cycle and nitrogen cycle.

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The Water Cycle
The water cycle tracks the movement of water as it progresses through the states of matter and travels through the biosphere. Most of the water on earth is found in its oceans, but when the surface of a body of water gets warm it vaporizes, entering the air as gas and rising to form clouds. As more and more water enters the air it precipitates and falls to earth as rain or snow. Then it either returns to the oceans or falls to earth, sinking into the soil or flowing into rivers.
An overview of the water cycle.

Carbon Cycle
The carbon cycle is a cycle where carbon is exchanged throughout the Earth’s atmosphere, the terrestrial biosphere, the oceans and sediments. Carbon in living organisms turns to soil when they die, entering the sediment. Then, it might return to the biosphere in the form of a plant growing out of that soil which is eaten by an animal. That animal emits carbon in the form of carbon dioxide as a product of respiration.
An overview of the carbon cycle.

Phosphorous Cycle
The phosphorous cycle follows the movement of phosphorous through the biosphere. Phosphorous is found in many forms, both organic and inorganic, but is most commonly found in the phosphate ion. Phosphorous is essential for both plants and animals. In biotic processes, plants remove phosphorous from the soil, animals eat plants and so on. In abiotic processes, phosphate is released by rocks due to weathering, then is deposited in the soil or water through leaching or runoff.
An overview of the phosphorus cycle.

Nitrogen Cycle
The nitrogen cycle follows the movement of nitrogen in its different forms. Nitrogenous gas makes up 80% of the atmosphere and most nitrogen enters into the ecosystem via nitrogen-fixing bacteria, but many organisms still struggle with nitrogen availability because they cannot use it in that form. The process of making nitrogen accessible is called nitrogen fixation. Many forms of bacteria are capable of this. Many of these bacteria are found in the soil but some species have a symbiotic relationship with microbes which are capable of fixation. Other key processes include nitrification, denitrification, and ammonification
An overview of the nitrogen cycle.

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