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Environmental Science43Updated Sep 14, 20265 pages

Understanding Biochemical Cycles: Water, Carbon, Oxygen, Nitrogen, and Phosphorus

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Savannah Gurley@savannahgurley_qvfi
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The water, carbon, nitrogen, and phosphorous cycles are natural processes that keep Earth's essential elements moving through living organisms and the environment. These cycles are powered by the sun and are critical for all life on Earth. Understanding how these elements cycle through our world helps explain many natural processes you see every day.
Biochemical cycles – page 1

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The Water Cycle

Ever wonder where rain comes from? The water cycle is a continuous process driven by the sun that moves water through the Earth and atmosphere. Water never actually leaves Earth—it just changes form and location!

When the sun heats water in oceans, lakes, and ponds, it transforms into a gas through evaporation. Plants also release water vapor through a process called transpiration. This water vapor rises, cools, and forms clouds through condensation.

Eventually, water returns to Earth as precipitation (rain, snow, or hail). Some water is absorbed into the ground becoming groundwater, while other water flows across land as runoff into streams, rivers, and eventually back to the oceans.

Did you know? The water you drink today might have once been part of a dinosaur's body millions of years ago!

Biochemical cycles – page 2

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The Water Cycle Steps & Phosphorous Cycle

The water cycle follows a simple pattern: the sun causes evaporation and transpiration, increasing water vapor in the air. This warming triggers more evaporation and transpiration, creating a continuous cycle that keeps water moving through our environment.

The phosphorous cycle involves the movement of phosphorous between soil, water, and living organisms. Unlike other elements, phosphorous cannot be found in air—it's always in soil, rocks, or water. Plants absorb dissolved phosphorous through their roots.

Phosphorous is vital for life as a key component of cell membranes and molecules essential to all living things. However, too much phosphorous in water bodies causes eutrophication, leading to harmful algae overgrowth. This typically happens when rain and wind erode phosphorous-rich rocks.

Think about it: Phosphorous is in your DNA and the bones in your body—it's literally part of who you are!

Biochemical cycles – page 3

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The Carbon Cycle

Carbon is everywhere—it makes up all living things including fats, carbohydrates, proteins, and nucleic acids. The carbon cycle describes how carbon moves between living organisms, the atmosphere, oceans, and Earth.

Plants capture carbon dioxide through photosynthesis, combining CO₂ and water with sunlight energy to produce glucose and oxygen (CO₂ + H₂O + sunlight → C₆H₁₂O₆ + O₂). The glucose stores energy for the plant, while oxygen is released into the air. Chlorophyll in plant cells helps capture the necessary sunlight.

When plants and animals die, they become detritus (waste and remains). As these materials decompose, carbon returns to the soil or atmosphere. Carbon dioxide is a greenhouse gas that absorbs heat, affecting Earth's temperature.

Cool fact: Every time you exhale, you're releasing carbon dioxide that becomes part of the carbon cycle!

Biochemical cycles – page 4

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The Carbon and Nitrogen Cycles

The carbon cycle continues as carbon moves through the environment. Carbon dioxide is absorbed during photosynthesis, transformed into organic carbon in plants, and eventually released back into the atmosphere through respiration, decomposition, and human activities like burning fossil fuels.

The nitrogen cycle is equally important, though more complex. Although nitrogen makes up 79% of our atmosphere, most organisms can't use it directly. Nitrogen must be converted to forms like ammonia (NH₃) or nitrates (NO₃) to be useful.

Nitrogen fixers (special bacteria) convert atmospheric nitrogen into ammonia through nitrogen fixation. Nitrification occurs when this ammonia is converted to nitrates. Plants absorb these compounds through assimilation. Denitrification returns nitrogen to the atmosphere, completing the cycle.

Warning: When nitrogen gets into groundwater through nitrification, it can create dangerous drinking water conditions.

Biochemical cycles – page 5

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The Nitrogen Cycle In Detail

Nitrogen may be abundant (making up 79% of our atmosphere), but it's tricky—most organisms can't use nitrogen in its atmospheric form (N₂). It only becomes useful when converted to reactive forms like ammonia (NH₃) or nitrates (NO₃).

Nitrogen is crucial as the major component of proteins and DNA. It's often the most limiting nutrient for plant growth, meaning plants can't grow without enough usable nitrogen. When nitrogen cycles get out of balance, they can harm the environment.

The full nitrogen cycle involves multiple steps: atmospheric nitrogen is converted to ammonia by nitrogen-fixing bacteria (nitrogen fixation), then to ammonium (NH₄⁺), then to nitrites (NO₂) and nitrates (NO₃) by nitrifying bacteria. Plants use these compounds, animals eat the plants, and when organisms die, decomposers release nitrogen back to the soil. Some bacteria convert nitrates back to atmospheric nitrogen (denitrification).

Mind-blowing fact: The nitrogen in your body has been cycled countless times through other organisms, perhaps even through dinosaurs millions of years ago!

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