Green plants are the only organisms that fix light energy from the sun into food or chemical energy. The life of all heterotrophic plants and animals depends on the energy trapped by green plants in their food molecules. This is why green plants are called primary producers.
Describe the energy flow of ecosystem
The energy flow of ecosystem can be described as the movement of energy from organisms in one trophic level in the food chain to another level, in the form of chemical energy. This energy is not created but converted from light energy into chemical energy by the green plants and phytoplankton.
Laws of Thermodynamics
The fixation of solar energy by photosynthetic plants and its utilization in the form of food by living organisms obey the two fundamental laws of thermodynamics.
- According to the first law of thermodynamics, energy can neither be created nor be destroyed; it can only be transformed from one form to another. Eg. Solar energy is converted into food energy but it cannot be created nor destroyed.
- As per the second law of thermodynamics, every energy transformation is accompanied by the simultaneous degradation of energy from a concentrated form to a dispersed form. In other words, some energy is lost as heat energy during energy transformation.
Green plants absorb only 65% of the total sunlight that falls on Earth. Out of this, only less than half is absorbed by the chloroplast, and the rest is reflected from the leaf surface. Photosynthesis fixes only 1% of solar energy that falls on them into food energy. This energy is used to fix atmospheric carbon dioxide to form organic compounds.
About 90% of the world’s photosynthesis is done by aquatic plants, especially diatoms and algae present in ocean and freshwater sources. The remaining energy is fixed by green plants living on the land.
Among the land plants, the largest contribution towards energy conservation is made by forests, followed by cultivated plants and grasses. The desert areas contribute the least amount of energy fixation.
Law of 10%
In each step of the food chain, when the food energy is transformed from one trophic level to the next higher level, or from one organism to another, a major part of the energy is lost as heat. According to Russian ecologist Kozolovsky, approximately 10% of energy moves up from one food level to the next which is known as the law of 10%. The remaining 90% is used by the organism for its respiration and other biological activities. The remaining amount of energy stays unused or is lost.
Phytoplankton —> Zooplankton —> Fish —> Man
Let us presume that the total energy that falls on the green plants (L) per day is 3000 Kcalories. Out of this energy, the amount absorbed (LA) by the green plants will be 1500 cal (50% of the incident light).
Out of this the net amount of energy fixed in the photosynthesis process will be 15 cal (1% of absorbed light energy). The rest will be lost as heat during the transformation of light energy into potential energy in the form of food. This is per the second law of thermodynamics.
15 cal represents the net primary production (PN) in the first trophic level. A part of this energy is passed on to the herbivores in the form of food. The rest is retained by the primary producers for their use. Only about 1.5 cal will be incorporated as net food energy by the second trophic level (P2).
A similar degradation of energy takes place when food energy is passed on to the primary carnivores which form the third trophic level (P3) and so on. The quantitative rate of energy right from the sunlight through the various trophic levels may be represented below.
L(3000 cal) —> LA (1500 cal)—> PN (15 cal) —> P2 (1.5 cal) —>P3 (0.15 cal) and so on.
Assimilated and Respiration Energy
Assimilated energy is the energy equivalent to the food that is dejected and absorbed by the organism. This energy is stored and used later for performing various metabolic activities inside the organisms. The respiration energy is the energy that is lost as heat and unused by these organisms while performing their metabolic activities.
Herbivores receive only a fraction of the energy from autotrophs since more amount of energy from the producers is lost as heat. This energy that herbivores receive will be converted into kinetic energy. The stored energy is used to generate energy molecules or ATPs.
This stays true to the first law of thermodynamics. Energy is not created here. Instead, energy in one form is converted into another form.
Once again, a good portion of this energy is lost as heat, and a smaller amount of energy is transferred to the primary carnivores. Generation and utilization of ATPs is through which the energy is lost as heat. Thus the amount of energy is degraded with each trophic level.
This proves the second law of thermodynamics where a good amount of energy is lost as heat energy and there is a degradation of energy from one level to the next.
Unidirectional Energy Flow
The flow of energy in an ecosystem is unidirectional. It can only be converted from one specific form into another specific form. For example, light energy is converted into chemical energy which is impossible the other way. Similarly, chemical energy can only be converted into heat energy and not vice versa. In addition, the amount of energy that can be transferred from one trophic level to the higher one decreases gradually as you go up.
Trophic Levels
Even though the direction of energy flow is unidirectional, there are multiple levels through which the energy passes through. Each level through which the energy pass through is called a trophic level. Specific types of organisms occupy each trophic level and they have similar nutritional habits. The trophic levels are differentiated into these types.
- Primary trophic level: Producers occupy this level, where energy from the sun is assimilated and stored. Eg., All green plants and phytoplankton.
- Secondary trophic level: Herbivores that feed on producers occupy this level. Eg., Deer, rabbits, birds, etc.
- The tertiary trophic level has carnivores that feed on herbivores. Eg. Lions, tigers, etc.
- The quarternary trophic level is occupied by omnivores that feed on carnivores and herbivores. This level also includes carnivores that feed on carnivores from the previous trophic level. Eg. Dogs, foxes, bears, etc.
- Decomposers are the final trophic level that feeds on the dead and decaying matter of organisms from all the trophic levels mentioned above. Eg., Bacteria, fungi, mushrooms, etc.
Conclusion
Thus, energy flow In the ecosystem starts from the green plants or producers that capture light energy from the sun and convert it into chemical energy. This chemical energy is passed on through various trophic levels and food chains until it reaches the decomposers. Decomposers, break this chemical energy into inorganic compounds. The green plants use these inorganic compounds once again to produce chemical energy with the help of sunlight. This way, energy flow in an ecosystem could be described as a cyclic process. It ensures a feedback mechanism and the energy flow constantly from one form to another.
References
- Shukla, R.S. and Chandel, P.S. (2001) Plant Ecology. S. Chand and Company Ltd., New Delhi.
- Verma, P.S., Agarwal, V.K. (1999). Cell biology genetics molecular biology evolution and ecology. New Delhi: S.Chand Co.(Pvt) Ltd.