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In this video lesson, we’ll take a look at how elements large and small use other elements to grow. This is achieved through cycles. We’ll look at macronutrients, trace elements and the biogeochemical and phosphorous cycles.

Composition of Living Organisms

All organisms are composed of matter, and although all organisms grow and reproduce, they cannot create new matter.

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In order to grow and accumulate biomass, they must use the elements that are already around them in the air, soil, water and other organisms. All living organisms are mainly composed of the same four elements. By weight, oxygen is the most abundant and accounts for about 65% of all living matter. Carbon comes in second at 18-19%, followed by hydrogen at 9-10%, and nitrogen, which accounts for about 3.5% of all living matter. After nitrogen, the exact order of composition varies among organisms but there are a number of macronutrients, or elements required by all organisms in relatively large amounts.

Living organisms are mainly composed of these four elements
Composition of living organisms

In addition to oxygen, carbon, hydrogen and nitrogen, the other macronutrients are phosphorus, calcium, potassium, sulfur, sodium, chlorine and magnesium. In addition, there are several necessary elements, found in very small amounts in organisms, called trace elements. Trace elements are elements that are needed by organisms, but only in extremely small amounts.

The most abundant trace element is iron, which some may consider to be a macronutrient. But it makes up less than a tenth of a percent of living matter, and that’s where we’re going to draw the line separating macronutrients and trace elements. Other trace elements include fluorine, zinc, silicon, rubidium, copper, iodine, boron, selenium, manganese and cobalt.

Biogeochemical Cycles

Organisms are constantly consuming molecules from their environment, incorporating elements that they need and eliminating the byproducts of biochemical processes. These byproducts are generally composed of all the same elements that the organisms contain themselves. The result is that all elements that living things use are constantly cycling through organisms and the environment they live in.

How an element cycles through organisms and the environment is called a biogeochemical cycle.There are many possible routes that elements can take in a biogeochemical cycle, but most of them can be categorized into one of the routes in this diagram. Producers are the key to the biogeochemical cycling of pretty much all necessary elements in living organisms. They are the only organisms that can incorporate carbon into organic molecules from carbon dioxide. More than that though, they usually get all of their other essential elements from either the air, soil or water around them.

Compare that to most consumers that get almost all of their essential elements from other organisms, except for the hydrogen and oxygen in the water they drink and the oxygen they breathe.

This chart shows the routes elements can take when cycling through organisms and the environment
Biogeochemical cycling

All of these elements are then cycled between the various trophic levels until they are either disposed of as waste or wind up as detritus. At this point, they can either be recycled into other living organisms by detritivores or enter the pool of unavailable organic compounds, where they may stay out of the biological part of the cycle for millions of years and be transformed into crude oil, coal or natural gas. Over the last few centuries, humans have been exploiting these deposits, burning the fossil fuels to produce energy and releasing the elements back into the pool of available inorganic materials. From that pool, they can either be incorporated into sedimentary rock and enter the pool of unavailable inorganic materials, or they can be incorporated back into living organisms by producers.

Phosphorus Cycle

Not all elements take the same route through the biogeochemical cycle. Each element is different and is available to organisms in different ways and in different concentrations. So let’s take a closer look at how phosphorus is cycled. The phosphorus cycle is the biogeochemical cycle that phosphorus follows through organisms and the environment.

As far as biogeochemical cycles go, the phosphorus cycle is fairly simple. Phosphorus is found in many types of rock as phosphate ions. Weathering of the rocks releases phosphate into the soil, where it can be absorbed by plants. Once in plants, the phosphate becomes available to consumers and eventually detritivores, which can recycle it back into the soil or back into consumers.

Phosphorous leaks from rocks into the soil, is absorbed by plants, and recycled by detritus eaters
Phosphorous cycle

Phosphate is bound by soil, and it doesn’t enter the atmosphere as a gas, so it tends to stay in the local biological cycle for long periods of time. However, it can be removed from an area by harvesting of crops, and some of it does leach through the soil and into the local water table. This phosphate eventually becomes part of new sedimentary rocks and is unavailable for biological use until the rock is brought to the surface by geological forces and weathering releases phosphate into the soil. Not surprisingly, people have found a way to replace phosphate removed by the harvest of crops by mining for phosphate-containing rocks and turning them into fertilizer. This is essentially speeding up the natural weathering process, at least regarding phosphate distribution.

Lesson Summary

Let’s review.

All living organisms are mainly composed of the same four elements: oxygen, carbon, hydrogen and nitrogen. Together, these four elements make up about 96% of the total weight of all living organisms. In addition, living things require significant amounts of several other elements. These other elements are called macronutrients, or elements required by all organisms in relatively large amounts. In addition to oxygen, carbon, hydrogen and nitrogen, the macronutrients are phosphorus, calcium, potassium, sulfur, sodium, chlorine and magnesium.

Organisms also require trace elements, or elements that are needed by organisms, but only in extremely small amounts. Some of the more significant trace elements include iron, fluorine, zinc, silicon, rubidium, copper, iodine, boron, selenium, manganese and cobalt.How an element cycles through organisms and the environment is called a biogeochemical cycle.

There are many possible routes that elements can take in a biogeochemical cycle, but they all cycle through the trophic levels of their ecosystem as herbivores eat producers, carnivores eat herbivores and detritivores recycle dead organic material. The phosphorus cycle is the biogeochemical cycle that phosphorus follows through organisms and the environment. Phosphorus is found in many types of rock as phosphate ions. Weathering of the rocks releases phosphate into the soil, where it can be absorbed by plants. Once in plants, the phosphate becomes available to consumers and eventually detritivores, which can recycle it back into the soil or back into consumers. Phosphate can be removed from an area by the harvesting of crops. Some of it does leach through the soil and into the local water table, eventually becoming part of new sedimentary rocks and unavailable for biological use until the rock is brought back to the surface by geological forces and weather releases phosphate into the soil.

However, phosphate removed by the harvesting of crops can be replaced by using mined phosphate as fertilizer.

Lesson Objectives

Once you complete this lesson you’ll be able to:

  • List the four main elements of living organisms
  • Recall the macronutrients
  • Give examples of trace elements
  • Define biogeochemical cycles
  • Explain the phosphorus cycle

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