Chemical Cycles - Biology

Nitrogen fixation is mostly done by bacteria living in the soil. Plants need nitrogen to grow, but they cannot use it straight from the atmosphere or as ammonia from the soil.

Humans and animals largely obtain their necessary nitrogen by consuming plants, and do not fix nitrogen or rely directly on bacteria for the process.

Condensation is not part of the nitrogen cycle. It is part of the water cycle, during which water molecules condense together in the atmosphere to form clouds.

Nitrification is the process by which nitrite (NO2-) is converted to nitrate (NO3-). This is the final step required in the processes used to oxidize nitrogen wastes (ammonia) to usable nitrate ions.

The conversion of gaseous nitrogen to ammonia (N2 to NH3) describes nitrogen fixation, and is usually done by nitrogen-fixing bacteria.

The conversion of nitrate to plant matter (NO3- to plants) describes the process of assimilation.

The conversion of nitrate to gaseous nitrogen (NO3- to N2) describes denitrification, and is performed by denitrification bacteria.

The conversion of animal waste to NH3 describes ammonification, and is accomplished by saprobiotic (decomposing) bacteria.

The nitrogen in plants comes from the soil. Bacteria in the soil take nitrogenous wastes and convert it into forms of nitrogen that plants can use. Plants then take up nitrogen through their roots.

The atmosphere is composed of 78% nitrogen gas and while by mass the atmosphere is less massive than the all the other choices the other choices are not primarily composed of nitrogen and contain relatively little compared to the nitrogen in the atmosphere.

Since the nitrogen gas that composes 78% of the atmosphere is not immediately usable to all organisms except for nitrogen-fixing organisms the nitrogen that composes the Earth's major nitrogen reserve is not immediately usable to most organisms.

Volatilization is a process by which fixed nitrogen is released back into the atmosphere as gas.

Only two options here actually lead to the conversion of fixed nitrogen to atmospheric nitrogen, volatilization and denitrification of which denitrification is a relatively rapid process carried out by numerous denitrifying microbes thus making it the greater contributor to the return of nitrogen to the atmosphere from fixed nitrogen.

The water cycle is an important aspect of all ecosystems. It involves biotic and abiotic factors, which play a role in the pathways that move water in the water cycle. The water cycle is a recycling process that has no starting point, nor ending point. It can be divided into three stages: gas, liquid, and solid. The evaporation of water from bodies of water, such as an ocean, returns it to the atmosphere. This water, now in the clouds as vapor, can be converted back to liquid, which is known as condensation. The cooling of vapor in the clouds makes precipitation that is returned to the ground as rain, sleet, hail, and snow.

The next step of the water cycle after evaporation is condensation, causing the water to form clouds after it leaves the ocean. The clouds are then able to transport the water over land and return it to the soil via precipitation.

Most of the planet's water is located in the oceans as salt water. The earth's surface is about 71% covered by the oceans, and those oceans hold about 96% of all the water on earth.

Infiltration is the process by which water can seep or be absorbed into the soil, which makes the soil wet or turns the water into groundwater. If there is something covering the soil, like a road or a sidewalk, then the water cannot be absorbed and instead sits on the hard surface. Parks and gardens allow the soil to be exposed and the water cycle to continue.

When plants lose water through its leaves, it is called transpiration. This happens when it gets too hot and the water they have stored evaporates and escapes from the stomata of their leaves. Respiration refers to the cellular process by which energy is produced. Perspiration is also known as sweating, and is used to prevent overheating. Precipitation is the falling of water (in one of several forms) from the atmosphere.

Evaporation is the process of water moving from the liquid phase to the gaseous phase and rising into the atmosphere. Once water has evaporated, it rises up into the atmosphere where it cools and forms clouds. During precipitation, this water may turn into river, lake, or ground water, which plants may have access to through their roots.

Evaporation, transpiration and sublimation are the three processes by which water is vaporized and allowed to enter into the atmosphere. Transpiration is the process by which water is lost from plants during photosynthesis and respiration. Evaporation is the process by which water is vaporized by absorbing enough solar energy to break away from the rest of the water molecules, note the body of water does not come to a boil during evaporation. Sublimation is the vaporization of ice into water vapor in much the same way that evaporation occurs.

Only precipitation and runoff are the only two hydrologic cycle processes from the potential answers that result in major erosion.

The Earth's freshwater composes 2.5% of the total water on Earth of which only 1.2% is available as surface water.

The glacial ice of the Arctic and Antarctic along with mountain glacial peaks compose 68.7% of all the freshwater on Earth.

The carbon cycle is one of the most important cycles to living organisms. Carbon is one the most abundant elements on earth, helping to form molecules such as sugars, lipids, and proteins. There is a constant exchange of carbon from the abiotic and biotic environmental elements to the atmosphere. The breakdown of glucose is known as cellular respiration, and creates the byproduct carbon dioxide. This exhaled carbon dioxide is the method by which humans return carbon to the carbon cycle. Plants capture this carbon dioxide and use it to make sugars in a process called photosynthesis. As organisms die, they decompose and get compressed by soil, sand, or ice. These remains result in stored fossil fuels, which will be utilized by plants and extracted by humans for energy. Their extraction can cause a buildup of carbon dioxide in the atmosphere, leading to global warming.