Biochemical Cycling Summarized Biology Notes

Learning objectives
At the end of this unit, you should be able to:
- describe the nitrogen cycle
- state the roles of microorganisms in the nitrogen cycle limited to decomposition, nitrification, nitrogen fixation and denitrification
- describe the carbon dioxide cycle limited to photosynthesis, respiration, feeding, decomposition, fossilisation and combustion
- discuss the effects of the combustion of fossil fuels and the cutting down of forests on the balance between oxygen and carbon dioxide
- discuss the possible effects of a long-term increase in the percentage of carbon dioxide in the atmosphere and on the carbon cycle.

 Introduction
Have you established a waste management club at your school? It is a club that organises recycling projects, recycling materials, such as newspapers, glass bottles or plastic, or perhaps there are different colour bins for collecting different types of waste to recycle. The remains of dead plants and animals are also recycled, but they are broken down by bacteria and fungi. The nutrients in their bodies are returned to the soil and reused by other organisms for growth. In this post, we will focus mostly on the nitrogen cycle and the carbon cycle.

 The nitrogen cycle
Nitrogen is an essential element of all protein molecules, the building blocks of all living organisms. Figure below shows how nitrogen is recycled.

Look at the nitrogen cycle in figure below and the label 'nitrogen in air'. About 80% of the air is nitrogen gas. Nitrogen gas is very unreactive and will not easily react with other substances. So, although the air is full of nitrogen, plants and animals cannot use it. The nitrogen must be changed into a more reactive form, such as ammonia or nitrates. This process is called nitrogen fixation. There are several ways that this can happen:

Lightning:
causes some of the nitrogen gas in the air to combine with oxygen, forming nitrogen oxides; the oxides dissolve in rain, and are washed into the soil, where they form nitrates.

 Nitrogen -fixing bacteria (Rhizobium):
live in the soil, or in root nodules of leguminous plants like peas, beans and clover; these bacteria combine nitrogen gas from the air spaces in the soil with other substances to make nitrates.

 Nitrifying bacteria:
these are any bacteria in the soil that can change or convert ammonia into nitrite or change nitrite into nitrate.

Once the nitrogen has been fixed, it can be absorbed by the roots of p lants, and used to
make proteins. Animals eat the plants, So animals get their nitrogen in the form of proteins.
When an animal or plant dies, bacteria and fungi decompose the bodies. The protein, which contains nitrogen, is broken down to ammonia and this Is released. Another group of bacteria, called nitrifying bacteria, turn the ammonia into nitrates, which plants can use again. Nitrifying bacteria are bacteria living in the soil that use the ammonia from excretory products and decaying organisms as a source of energy. In the process of getting energy from ammonia, the bacteria produce nitrates.

The nitrite bacteria oxidise ammonium compounds to nitrites:
NH₄ -------------> NO₂
Nitrate bacteria oxidise nitrites to nitrates
NO₂ -------------> NO₃

Although plant roots can take up ammonia in the form of its compounds, they take up nitrates more readily, so the nitrifying bacteria increase the fertility of the soil by making nitrates available to the plants. Therefore nitrification involves the conversion of ammonia into nitrite by the Nitrosomonas bacteria. Simultaneously, denitrification occurs where in nitrate is converted to nitrite and then to ammonia, which when acted upon by Bacillus, Pseudomonas and Clostridium, are converted into non-toxic nitrogen gas, nitrous oxide and nitric oxide, and are released in the process. Nitrogen is also returned to the soil when animals excrete nitrogenous waste material. It may be in the form of ammonia, urea or uric acid. Again, nitrifying bacteria will convert it to nitrates. Denitrifying bacteria turn nitrates and ammonia in the soil into nitrogen gas, which goes into the atmosphere. Look at figure above to see where these bacteria are shown.

 Processes that remove nitrates from the soil Uptake by plants
Plant roots absorb nitrates from the soil and combine them with carbohydrates to make proteins.

 Leaching
Nitrates are very soluble and as rain water passes through the soil, it dissolves the nitrates and carries them away in the run-off, or to deeper layers of the soil. This is called leaching.

 Denitrifying bacteria
These are bacteria that obtain their energy by breaking down nitrates to nitrogen gas, which then escapes from the soil into the atmosphere. This is called denitrification.

 The carbon cycle
Carbon is an essential element of carbohydrates, fats and proteins. Carbon, in the for of carbon dioxide, is in the air breathed out by all animals and plants during respiration. Carbon dioxide is also formed when fossil fuels, such as coal, oil and gas, are burnt. Figure below shows the carbon cycle.

Let us look at the label 'carbon dioxide in the air' in Figure above. Air contains about 0.04% carbon dioxide. When plants photosynthesise, carbon atoms from carbon dioxide become part of glucose and starch molecules in the plant. Some of the glucose will be broken down by the plant in respiration. The carbon in the glucose becomes part of a carbon dioxide molecule again, and is released back into the air. Animals cannot use carbon from the carbon dioxide in the atmosphere. They get their carbon in the form of carbohydrates, proteins and fats, when they eat plants or other animals. The animals respire, releasing some of the carbon back into the air as carbon dioxide. When plants and animals die, they are broken down by decomposers, such as bacteria and fungi. The carbon becomes part of the decomposers' bodies. When they respire, they release carbon dioxide into the air again.

How humans affect the carbon cycle
We eat plants or animals, such as mealies or chicken, to get our supply of carbon. Through the process of respiration, we break down these carbon-containing substances and release carbon dioxide into the air. Figure below shows how humans fit into the carbon cycle.

 Combustion of fossil fuels
Humans use a lot of energy each day, for example, for street lights, lights in homes, machines, vehicles, factories and electricity power stations. To get this energy, we use oil, and products that we make from it, such as petrol, diesel and paraffin. We also use coal. We burn these substances to release energy from them. We may use the energy straight away for heating things or making a car engine work. Or we may use the energy to produce electricity. Coal and oil are called fossil fuels. They were made millions of years ago, when plants and other living organisms died and sank into swamps. They are descrbed as non-renewable resources because they cannot be replaced. When fossil fuels are burnt, the carbon in them combines with oxygen in the air, and forms carbon dioxide. This process is called combustion. Combustion of fossil fuels releases carbon dioxide into the atmosphere.

 Cutting down of forests
In many parts of the world, the dominant form of natural vegetation is trees. In order to create space for building and for agriculture, large areas of forests have been removed. The trees are then burnt and this produces large amounts of carbon dioxide. It is the high levels of carbon dioxide released from this burning that is causing a major concern to biologists at this time. Figure below shows how much the level of carbon dioxide has risen in the past 250 years.

As you can see in Figure above, the level of carbon dioxide in the atmosphere is steadily rising. The carbon dioxide released when trees are burnt is an important reason for this increase. Have you ever stood inside a greenhouse and felt how hot it seems, even on a cold day? Read the next section to find out why!

 The Greenhouse Effect
The carbon dioxide in the atmosphere acts like a blanket around the Earth. It helps to keep in the warmth from the sun. Without any carbon dioxide, the Earth would be much too cold to live on. Carbon dioxide is sometimes described as a 'greenhouse gas'. Figure below shows a simplified version of how the greenhouse effect works. Other greenhouse gases are CFCs, methane and nitrogen oxides.

You can see in Figure above that heat from the sun passes through the Earth's atmosphere to the Earth. Only some of this heat escapes. Some of the heat is reflected back to Earth by the 'greenhouse gases' in the atmosphere. Many scientists think that there is too much carbon dioxide in the atmosphere and that the Earth may get warmer than it is now. This is called globalwarming. The effects of global warming could have serious implications for life. To begin with, ice caps would begin to melt and this would  raise the sea level to a point that many cities would be flooded. Global warming has been blamed for the changing weather patterns observed over the past 20 years. Many parts of the world are seeing extensive droughts, while others are suffering from severe flooding.

Summary
- Nitrogen in the air cannot be used by plants or animals. Nitrogen-fixing bacteria that live in the root nodules of leguminous plants fix the nitrogen by converting it into nitrates. Plants then absort the nitrates through their roots and use the nitrogen to make proteins. Animals obtain their nitrogen by eating plants or other animals.
- The bodies of dead animals and plants are decomposed by microorganisms and the nitrates are released into the soil. Denitrifying bacteria change the nitrates to nitrogen gas, which goes into the air, so completing the cycle.
- Carbon dioxide is taken from the air when plants photosynthesise, and becomes part of the plants.
- Animals obtain their carbon when they eat plants or other animals. Carbon dioxide returns to the air when living organisms respire.
- Humans release carbon dioxide into the air when they burn fossil fuels; this may increase the concentration of carbon dioxide in the air and may cause global warming.
- Burning fossil fuels, such as coal and oil, produces large quantities of carbon dioxide. When forests are cut down, the trees cannot remove carbon dioxide by photosynthesis and so levels of carbon dioxide rise and levels of oxygen fall.
- The effects of global warming could have serious implications for life. Ice caps would begin to melt and this would raise the sea level to a point that many cities would be flooded. A change in weather patterns has been observed over the past 20 years. Many parts of the world are seeing extensive droughts, while others are suffering from severe flooding.

 The end, posted by Miss Elisabeth N.