law of thermodynamics

Show how the Law of Thermodynamics control the structure and dynamics of an ecosystem

Ecosystem is the "movement of energy and material through organisms and their communities" (Brooker, 2008). The Law of thermodynamics measure the efficiency of energy transfer between organisms in an ecosystem. The first law states that energy cannot be created or destroyed instead it can be changed, moved, controlled, stored, or dissipated. The second law states that the energy conversion is not 100% efficient. Most of the energy is lost as heat in an ecosystem.

The largest source of energy is the sun. Energy flow is the amount of energy that moves through a food chain. Energy and nutrients from one organism is passed onto other organisms through the food chain when one organism feeds on another organism. Much energy from the plants goes unconsumed by herbivores, instead unconsumed plant die and decompose. This material along with dead animals and waste products are consumed by decomposers, they break down dead organic matters from all trophic levels. This supports the first law of thermodynamic which is that energy cannot be destroyed, it changes form. Over time the nutrients are recycled but the energy is lost. Figure 1 shows how the nutrients are recycled.

As shown in figure 1, light energy is converted to chemical energy in autotrophs by photosynthesis. Primary consumers cannot photosynthesise therefore they respire. Respiration converts chemical energy from plants to heat and kinetic energy. This links back to the first law of thermodynamics. Photosynthesis is opposite of respiration. This process is shown below.


  • 6CO2 + 6H2O
  • C6H12O6 + 6O2


  • C6H12O6 + 6O2
  • 6CO2 + 6H2O

There are three ways of representing energy flow:

  • Food chain
  • ,
  • food web
  • and
  • pyramids

Food chains are the simplest representation of energy flow in a community. At the base is energy stored in plants, which are eaten by small organisms, which in turn are eaten by regularly larger organisms. For example, in a terrestrial food chain, plant is eaten by caterpillar, caterpillar is eaten by lizard and snake feed on lizards. In aquatic food chain phytoplankton is eaten by zoo-plankton, fish feed on zoo-plankton and pelican feed on fish. Figure 2 illustrate terrestrial and marine food chains. The food chain is an over simplification in that most animals do not eat only one type of organism.

Food web is a complex network of feeding interrelations among species in a natural ecosystem and it is more accurate and more complex illustration of energy flow than a food chain. In food webs the numbers of links between the trophic levels are short, for example, if a fox feed on a rabbit and a rabbit feed on grass, there would be two chains. This is because as mentioned before the first law of thermodynamics states that energy cannot be created nor destroyed, it only transform. In food webs the trace energy flow from green plants to tertiary consumers, there is not enough energy for the chain to grow because as second law of thermodynamics states that the energy conversion is not 100% efficient. When energy transfer between each trophic levels, some useful energy which can do work is lost. Figure 3 illustrate the food web.

Food pyramids are a way of represent energy flow in an ecosystem; shows producers (mostly plants or other phototrophs) on the first level and consumers on the higher levels. There are three types of pyramids: pyramids of numbers, pyramids of biomass and pyramids of energy.

Pyramid of numbers shows graphically the population of each level in a food chain. This is shown in figure 4.

Pyramid of biomass shows the relationship between biomass and trophic level by measuring the amount of biomass present at each trophic level. Figure 5 shows that the biomass of organisms decreases as the trophic level increases.

There are four important biogeochemical cycles where the nutrients that are essential for the growth of organisms are recycled. There are called water cycle, carbon cycle, nitrogen cycle and phosphorus cycle. These cycles shows how the energy from the sun is transformed into different energy form and how the nutrients are recycled.

In the water cycle, energy is supplied by the sun, which drives evaporation from ocean surface or from treetops. Precipitation occurs when water condenses from gaseous state in the atmosphere and falls to earth as rain. This is shown in figure 7. Animals and plants lose water through evaporation from the body surfaces and through the gas exchange structures, in plants, water is drawn in at the roots and moves to the leaves where it evaporates quickly. This process is called transpiration.

Carbon cycle is the movement of carbon, in its many forms, between the biosphere, atmosphere, oceans, and geosphere. As mentioned above plants absorb CO2 from the atmosphere during photosynthesis and release CO2 back in to the atmosphere during respiration. Another major exchange of CO2 occurs between the oceans and the atmosphere. The dissolved CO2 in the oceans is used by marine biota in photosynthesis. Carbon is also stored in fossil fuels and when the fossil fuels are used as fuels, CO2 returns to the atmosphere. Figure 8 explain the carbon cycle.

Nitrogen is critically important in forming amino acids, proteins and enzymes. A large reservoir (78%) of nitrogen is present in the atmosphere. Nitrogen gas is non-reactive and it a lot of energy to break down nitrogen gas and combine with other elements such as carbon and oxygen. Nitrogen fixing bacteria converts nitrogen gas into nitrates or ammonia. There are three forms of nitrogen fixing bacteria: some are free- living in the soil; some form symbiotic relationship, mutualists association with the root of legumes plants (these bacteria obtain food from root nodules and converts nitrogen to nitrate for plant growth) and the third form is the photosynthetic cyanobacteria which found in water. Nitrates are soluble and transfer to amino acids in plants. Animals gain all of their amino acids when they eat plants or other organisms. Nitrogen in the form of ammonia is released into the soil when plants and animals die. Ammonia is toxic, but nitrite bacteria in the soil and water take up ammonia and change it to nitrite. Nitrate bacteria transfer nitrite to nitrate which can be taken by the plant to continue the cycle. Denitrifying bacteria transfer nitrate to nitrogen gas. Figure 9 explain the nitrogen cycle.

Phosphorus is the key to energy in living organisms; ATP contains 3 phosphates and drives an enzymatic reaction, or cellular transport. Phosphorus binds deoxyribose sugars together, forming the backbone of the DNA molecule. It does the same job in RNA. Phosphorus is also found biological membrane (phospholipids). Plants absorb phosphorous from water and soil into their tissues, and produce organic compounds. Once taken up by the plants, phosphorus is available for animals when they consume. Herbivores and carnivores excrete phosphorus as a waste product in urine and faeces. Phosphorus is released back to the soil when plants or animal matter decomposes after death, by bacterial activity. This is illustrated in figure 10.

The energy flow in an ecosystem and the biogeochemical cycles explains how the sun energy has impact on the organisms in an ecosystem. Sun energy is converted to chemical energy by plants and the chemical energy is converted into different types of energy such as heat and kinetic energy. Once the energy is wasted, it cannot be recycled. To conclude "energy cannot be created or destroyed instead it can be changed, moved, controlled, stored, or dissipated and the energy conversion is not 100% efficient" this tells us that the law of thermodynamics controls the structure and dynamics of an ecosystem.

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