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Biological Cycles


  • Energy, chemical elements and nutrients form cycles in nature;
  • In course of a biological cycle, an element make take various chemical forms.
  • There are several important cycles in nature:
    • Oxygen cycle
    • Water cycle
    • Carbon cycle
    • Nitrogen cycle
  • The biogeochemical cycles make multiple biological processes possible: photosynthesis, protein synthesis, respiration
  • Many stages of these cycles could not be possible without activity of the living beings: bacteria, fungi, plants and animals.

In nature, nothing really stays in one place for eternity. And nothing disappears, either. Most chemicals that living organisms either consume in order to live, or produce in course of their life just change their form. They can do it in various ways:

  • They become part of bigger molecules
  • They change their state (for instance, water can become vapor or ice)
  • They become a part of much simpler molecules (oxides, salts, acids, etc.)

The molecules that these elements form can become:

  • parts of the cells of plants, animals, bacteria, archaea, or fungi
  • form rocks
  • can get dissolved in water
  • portions of the air we breathe

If we use arrows to follow the paths of the elements, we quite likely may draw a circle. Such natural cycles of elements are called biogeochemical cycles.

Let us look at the four elements most important for life: oxygen, nitrogen, hydrogen and carbon.

Oxygen cycle

The flow of oxygen


Oxygen is an element that we find most important for our survival. Yet in our atmosphere there is only 20, 5% of free oxygen. Moreover, a high content of oxygen in the atmosphere is something that came about very slowly, in course of millions of years.

There are several “tanks”, or reservoirs on our planet that contain oxygen. One of the biggest is actually the one we stand on: lithosphere. For example, most minerals we find in the mines are composes of oxides. According to the calculations, the crust and mantle of the Earth underneath us is almost half oxygen in form of various complex inorganic molecules. And this composes around 99, 5% of all oxygen on the planet.

The next significant place of oxygen storage is the hydrosphere – all the water on the planet Remember the chemical formula of water? H20. There is a lot of oxygen tied up there!

But this oxygen is just tied up in compounds. There is a place where free oxygen is actively produced – the biosphere. Main oxygen producers are plants and photosynthetic bacteria. Using photosynthesis, they take up carbon dioxide to make glucose and release oxygen. According to paleontologists, those organisms used to be responsible for two great extinctions in the history of our planet. Many billion years ago, first photosynthetic bacteria began using the light energy to make their sugars. It led to the release of really big amounts of oxygen over time. Other unicellular organisms that functioned without oxygen – anaerobic ones – were killed by such big amounts of new element around them. From this moment on, life has become aerobic. And multiple organisms began to use oxygen for another process – respiration. While the plants released oxygen, animals depending on respiration breathed oxygen and released carbon dioxide, or CO2, instead.

Later on, when plants moved on from water to the land, they began releasing even more oxygen – making the world considerable cooler. This event led to one of the memorable mass extinctions in the Earth history.

As you can see, both plants and animals in the biosphere actively interact with oxygen. Plants release atomic oxygen, O2, while organisms that depend on respiration, including animals, breathe oxygen, use it to gain energy and release CO2 as the result. All of the living organisms also use oxygen as a building block for multiple organic molecules that compose them. But even with such active chemical processes, there is only 0, 01% of oxygen present in this sphere.

And what about the atmosphere? There is much oxygen is in the air, surely? Actually, our air contains only around 20% of oxygen. And on the global scale this is only half of a percent of overall oxygen content! Life on our planet indeed depends on a very small amount of this miraculous element.

There are several ways in which oxygen can be released into the atmosphere:

  • through photosynthesis
  • by weathering of the rocks of the Earth
  • through photolysis – destruction of water molecules in the atmosphere through the energy of sunlight

Oxygen can be “fixed” into molecules through:

  • respiration (and synthesis of CO2 and glucose)
  • burying of the organic matter (the bodies of plants and animals) and formation of fossil fuels and rocks
  • decay

In this list, photosynthesis is the most powerful oxygen producer. And this is why we need as much green plants as possible to be able to breathe.

Remember we mentioned organic molecules? Their synthesis is an important part of another crucial cycle – the cycle of carbon.

Carbon cycle

Carbon cycle

Let us continue from biosphere described in the previous cycle. We were discussing photosynthesis and respiration. Both these processes result in production of organic molecules: sugars, fats, proteins and nucleic acids. All those molecules contain a very high amount of carbon. They compose the bodies of all the living organisms on our planet. When these organisms die, carbon enters the soil. There is both photosynthesis and respiration in our oceans, as well. And bodies of all aquatic plants and animals also fall to the ocean floor and form the sediments. All this organic matter decays and becomes part of the soil.  The soil, in its turn, provides nutrients for the plants. Plants use these nutrients, grow and become food for the animals that release carbon dioxide in the course of their lives. Plants also use respiration to some extent. As the result, the carbon, in form of CO2 returns to the atmosphere.

But human activity also adds to the carbon cycle considerably. The bodies of plants and animals are the reason we have fossil fuels – oil, gas and coal. We actively burn those fuels to produce various things, to make our transport move and also to make our houses warm. Humans also cause accidental forest fires and actively destroy forests for their own use as well. All this additional activity considerably adds to the carbon dioxide content in our atmosphere.

As both carbon and oxygen are necessary for the synthesis of organic molecules, they are interconnected. There is another partner to this trio – cycle of water.

Water cycle

Water cycle

Our Earth is an extremely lucky planet, for it has a lot of liquid water. We have enormous reservoirs with water – rivers, seas, oceans. Yet this water never stays in place. There is a constant movement of water around the Earth, forming a full cycle.

The water cycle is driven by the sun. The sun heats up the water surfaces, and the water turns into water vapor. This vapor is light and warm, and rises up into the atmosphere. The ice and snow in the mountains and at the Poles also gets slowly melted and vaporized as well. As the water vapor gets higher into the atmosphere, the temperatures become much lower. The vapor then condenses into the clouds. Clouds can travel around the globe, and they can collide with each other.

If the conditions are right, the clouds go into the lower layers of the atmosphere, closer to the ground. As a result of a process called precipitation, they can release snow, rain or hail to the ground. This is the water that can form ice caps on the mountains, refill the rivers and seas and also flow underground, forming underground water masses that feed the plants. Then the water evaporates again, returning to the sky.

Remember, that water is also a crucial component in biological processes in plants and animals. Plants use water for making sugars; animals need water for multiple processes as well. Plants also lose water through transpiration, and animals release water through multiple of ways, including sweating and urinating. Then this water also joins the overall cycle.

Nitrogen cycle  

Nitrogen cycle diagram

 The air we breathe contains more nitrogen than oxygen: 78,9% against 20,95%. We need nitrogen, as it is an important component of proteins – the molecules that rule all the biological processes in the body. But what we have in the air is the nitrogen molecule, N2. Most animals or plants cannot use it directly.

Instead, nitrogen goes through several processes, becoming a part of different molecules that can be used by various organisms.

The first stage is nitrogen fixation. How does nitrogen get fixed? Well, by fixation the scientists mean that the atmospheric nitrogen, N2 is used to make ammonia (NH4). When ammonia appears in the soil, it can be more easily taken up by plants. There are several ways in which ammonia can be produced:

  • through lightning strikes
  • through symbiotic bacteria that live in the roots of legumes (Rhizobium)
  • Through free-living bacteria (Azotobacter)
  • Through some species of Archaea

Then the plants take up ammonia – this process is called nitrogen assimilation. Taking up nitrogen from ammonia, plants incorporate this element into the proteins they use for various processes. By eating plants, in their turn, animals eat the newly produced plant proteins and use their building blocks – amino acids – to make proteins of their own.

Then all the living things – both plants and animals die. They form organic matter that fungi and other bacteria can feed on. During this process of decay, a chemical reaction called ammonification takes place. During ammonification, all those big molecules that contain nitrogen are reduced to small ammonia again.

But ammonia can be a gas, too. In areas where decay is very active, we can detect its very specific smell. In big enough quantities, this gas can be toxic to both plants and animals! Thankfully, there are species of bacteria that can take care of that. They put together oxygen and ammonia in a reaction of nitrification. They create nitrites and nitrates – safer compounds that can be used by plants again. Farmers add nitrites and nitrates to the soil where their crops grow to make them rise above the ground faster.

But there are also soil-living bacteria that evade oxygen at all costs. They are called anaerobic bacteria. The typical example is Clostridium – a group of bacteria that can cause severe illnesses, such as botulism or diarrhea in humans. These bacteria use different reactions. They create N2 – atmospheric nitrogen that rises up back into the atmosphere. And with this last stage, denitrification, the cycle is now complete!

There are many other cycles in nature. We have discussed the most important ones. As you can see, there is no minor organisms in the big schemes of scenes – we depend both on huge plants and tiny bacteria to get our food and air.


Links and further reading:

[1.] – simple diagram of oxygen flux;

[2.] – carbon cycle diagram

[3.] – water cycle diagram

[4.] – nitrogen cycle diagram

[5.] Michael Marshall (2 July, 2015). The event that transformed Earth. BBC. Com. – about the Great Oxygenation Event

[6.] Chris Baraniuk (23 June 2015) – The Devonian extinction saw the oceans choke to death. BBC. Com – about the Late Devonian extinction supposedly caused by plants.