Standard 9 Science and Technology - 7. Energy Flow in an Ecosystem

Question 1:

Complete the following table (Carefully study the carbon, oxygen and nitrogen cycles).

 

Bio-geo-chemical cycles	Biotic processes	Abiotic processes
1. Carbon cycle
2. Oxygen cycle
3. Nitrogen cycle

Answer 1:

Bio-geo-chemical cycles	Biotic processes	Abiotic processes
1. Carbon cycle	Release of oxygen and CO2 through photosynthesis and respiration respectively.	Release of CO2 in the atmosphere by burning of fossil fuels, wood etc.
2. Oxygen cycle	Release of O2 through photosynthesis and its consumption in processes like respiration etc.	Photo dissociation of water vapour in the upper atmosphere, as a result of which oxygen is released when water molecules are split by light
3. Nitrogen cycle	Nitrogen fixation, nitrification, amoonification	Nitrogen fixation in atmosphere and denitrification

Question 2:

Correct and rewrite the following statements and justify your corrections .

a.   Carnivores occupy the second trophic level in the food chain.

b.   The flow of nutrients in an ecosystem is considered to be a ‘one way’ transport.

c.  Plants in an ecosystem are called primary consumers.

 

Answer 2:

a.   Carnivores occupy the third trophic level in the food chain because the second level is occupied by herbivores. Herbivores are organisms who feed on plants. Carnivores occupy the next level after herbivores.

b.   The flow of nutrients in an ecosystem occurs in a cyclic manner. The continuous transfer of nutrients which are necessary for the growth of organisms occurs from abiotic to biotic factors and biotic to abiotic factors in an ecosystem. This nutrient cycle operates continuously through the biosphere formed by lithosphere, atmosphere and hydrosphere. 

c.  Plants in an ecosystem are called primary producers. Plants are autotrophs who can produce their own food and other organisms are dependant on them to meet their nutritional requirements.
 

 

Question 3:

Give reasons.

 

a.   Energy flow through an ecosystem is ‘one way’.

b. Equilibrium is necessary in the various bio-geo-chemical cycles.

c.   Flow of nutrients through an ecosystem is cyclic.

Answer 3:

 a. The producers convert solar energy into chemical energy in the form of organic compounds. The primary consumers (herbivores) derive their nutrition from the producers. According to the energy transfer law, only 10% of energy is transferred from one trophic level to the other. So, the energy that is captured by the producers does not revert back to the Sun and the energy transferred to the herbivores does not come back to the producers. It just keeps on moving to the next trophic level in a unidirectional way. That is why the flow of energy in the food chain is always unidirectional/one way.


b. The cyclic flow of nutrients within an ecosystem is called as the biogeochemical cycle. A continuous transfer of nutrients which are necessary for the growth of organisms occurs from abiotic to biotic factors and biotic to abiotic factors in an ecosystem. This nutrient cycle operates continuously through the biosphere formed by lithosphere, atmosphere and hydrosphere. The recycling of these biogeochemicals depends upon the level of energy transfer in the ecosystem. Any type of disturbance at any level of these cycles affects the ecosystems and nature's balance. Hence, there should be equilibrium in the various biogeochemical cycles.


c. Flow of nutrients through an ecosystem is cyclic as the nutrients move from one trophic level to another trophic level all the way up and then back down. The cyclic flow of nutrients occur in the following manner:

• Producers/green plants which are present at the lowest trophic level manufacture the nutrients by the help of the sun's energy through the process of photosynthesis.
• These nutrients manufactured by the producers are passed on to the primary consumers/herbivores.
• The secondary consumers feed on primary consumers resulting in the passage of nutrients from the primary consumers to the secondary consumers.   
• The tertiary and the quaternary consumers also obtain nutrients from feeding one or all of the lower trophic levels. 

Finally, these nutrients are transferred back to the soil from the higher trophic levels through decomposition. The process of decomposition is carried out by organisms such as bacteria and fungi. 

 

Question 4:

Explain the following cycles in your own words with suitable diagrams.

 

a.   Carbon cycle.

b.   Nitrogen cycle.

c.   Oxygen cycle.

Answer 4:

a. Carbon cycle

 

 

 

• Carbon enters life forms through the process of photosynthesis. During photosynthesis, carbon dioxide and water combine to produce glucose and oxygen. This changes the atmospheric carbon into glucose molecules.
• Glucose, which is a source of food, is utilized by organisms to produce energy during respiration. During this process, glucose is broken down in the presence/absence of oxygen to produce carbon dioxide.
• Another process that releases carbon dioxide is combustion or burning. Many substances release carbon dioxide on burning. Vehicular emissions, industrial fumes and the gases released during the process of cooking are some instances of release of carbon dioxide through combustion.
• Thus, carbon is cycled repeatedly through different forms by the various physical and biological activities.

b. Nitrogen cycle

The nitrogen cycle involves conversion of nitrogen from its elemental form in the atmosphere into simpler molecules in the soil and water. These further get converted into more complex molecules in living beings. Finally, the complex molecules are reduced and released back into the atmosphere as simple nitrogen molecules.

Fixing of atmospheric nitrogen involve the following steps:

1. Nitrogen fixation: It is the process wherein atmospheric nitrogen is converted into water-soluble nitrates. This step is performed by organisms like Rhizobium, Azotobacter and blue-green algae.

 

N2  NO3− or NH4

2. Nitrification: It is the process in which ammonia is first converted into nitrites and then into nitrates. This step is performed by nitrifying bacteria like Nitrosomonas and Nitrobacter.

NH3  NO2−  NO3−

3. Assimilation: In this step, NH3 and NO3 are incorporated into the biological tissues.

4. Ammonification: It is the process in which complex, dead organic matter is decomposed into ammonia. This step is performed by organisms like decay bacteria and fungi.             Organic nitrogen compounds  NH4+ 

5. Denitrification: It is the process wherein the nitrates present in the soil are reduced to release nitrogen back into the atmosphere. 
                 
       NO3−  N2

c. Oxygen cycle

 

Oxygen is an important component of life. We cannot survive without oxygen. It comprises about 21% of atmospheric air. It is also present in dissolved form in water bodies and helps in the survival of aquatic life. In combined form, it is found both in Earth’s crust and in the air. In the air, it occurs as carbon dioxide. In the crust, it is present as oxides of most metals and silicon and also as carbonate, sulphate, nitrate and other minerals.

Oxygen is also a part of several essential bio-molecules such as carbohydrates, proteins, nucleic acids and fats.
There are processes that utilize atmospheric oxygen. At the same time, the balance of oxygen in the atmosphere is also maintained. 

• Oxygen is utilized during respiration, combustion and formation of oxides of nitrogen.
• Oxygen is returned to the atmosphere in only one major way, and that is photosynthesis.

Question 5:

What would you do to help maintain the equilibrium in the various bio-geo-chemical cycles? Explain in brief.

Answer 5:

Biogeochemical cycles are the basis of life and play an important role in maintenance of life on earth. Any kind of disturbance to these cycles can result in the destruction of life. We have seen from all the biological cycles that plants play a crucial role in these cycles. Plants are required for the existence of life on earth and any kind of harm to them can directly affect the life on earth. Deforestation due to industrialisation and increased population has resulted in the lost of forest cover on earth. In order to maintain the equilibrium in biological cycles following steps should be undertaken:

• reduce deforestation
• reduce the carbon imprint
• minimise the use of fossil fuels and encourage the use of renewable sources of energy
• conserve water
• conserve biodiversity as both plants and animal are a part of these biological cycles

Question 6:

Explain in detail the inter-relationship between the food chain and food web.

Answer 6:

The energy flow among the various constituent animals is known as the food chain and the length of a food chain depends upon the number of organisms. A food chain represents the relationship between organisms.



Food web or food cycle is the connection between different food chains and represents what species eats what in an ecological system.  A food web starts with the producers in ecosystem and then branches off into interconnected food chains that show who eats whom in an ecosystem.

Question 7:

State the different types of bio-geo-chemical cycles and explain the importance of those cycles.

Answer 7:

To maintain the concentrations of different naturally occurring substances in the environment, there are mechanisms that constantly recycle these substances. These recycling mechanisms are called biogeochemical cycles. So, biogeochemical cycles are the cyclic flow of nutrients between non-living environment and living organisms.

Types of Biogeochemical Cycles

There are two major types of biogeochemical cycles

1. Gaseous cycle: These are the biogeochemical cycles in which the reservoir pool is the atmosphere or hydrosphere. For example, carbon cycle, nitrogen cycle etc.

2. Sedimentary cycles: These are the biogeochemical cycles in which the reservoir pool is generally lithosphere. For example, Phosphorous cycle, Sulphur cycle etc.

Nitrogen Cycle 
The concentration of nitrogen in the atmosphere is about 78%. Nitrogen is essential for all life forms. It is a structural component of many important bio-molecules such as DNA, RNA and vitamins. 

The nitrogen cycle involves conversion of nitrogen from its elemental form in the atmosphere into simpler molecules in the soil and water. These further get converted into more complex molecules in living beings. Finally, the complex molecules are reduced and released back into the atmosphere as simple nitrogen molecules.

Fixing of atmospheric nitrogen involve the following steps:

1. Nitrogen fixation: It is the process wherein atmospheric nitrogen is converted into water-soluble nitrates. This step is performed by organisms like Rhizobium, Azotobacter and blue-green algae.

 

N2  NO3− or NH4

2. Nitrification: It is the process in which ammonia is first converted into nitrites and then into nitrates. This step is performed by nitrifying bacteria like Nitrosomonas and Nitrobacter.

NH3  NO2−  NO3−

3. Assimilation: In this step, NH3 and NO3 are incorporated into the biological tissues.

4. Ammonification: It is the process in which complex, dead organic matter is decomposed into ammonia. This step is performed by organisms like decay bacteria and fungi.
            Organic nitrogen compounds  NH4+ 

 5. Denitrification: It is the process wherein the nitrates present in the soil are reduced to release nitrogen back into the atmosphere. 
                 
       NO3−  N2

Oxygen cycle

Oxygen is an important component of life. We cannot survive without oxygen. It comprises about 21% of atmospheric air. It is also present in dissolved form in water bodies and helps in the survival of aquatic life. In combined form, it is found both in Earth’s crust and in the air. In the air, it occurs as carbon dioxide. In the crust, it is present as oxides of most metals and silicon and also as carbonate, sulphate, nitrate and other minerals.

 

Oxygen is also a part of several essential bio-molecules such as carbohydrates, proteins, nucleic acids and fats. There are processes that utilize atmospheric oxygen. At the same time, the balance of oxygen in the atmosphere is also maintained. 

• Oxygen is utilized during respiration, combustion and formation of oxides of nitrogen.
• Oxygen is returned to the atmosphere in only one major way, and that is photosynthesis.

Carbon cycle:

All organic substances are made up of carbon. Fats, vitamins, nucleic acids, carbohydrates and proteins contain carbon as a structural component. Carbonate salts form endoskeletons and exoskeletons of many animals.

 

Carbon enters life forms through the process of photosynthesis. During photosynthesis, carbon dioxide and water combine to produce glucose and oxygen. This changes the atmospheric carbon into glucose molecules.

• Glucose, which is a source of food, is utilized by organisms to produce energy during respiration. During this process, glucose is broken down in the presence/absence of oxygen to produce carbon dioxide.
• Another process that releases carbon dioxide is combustion or burning. Many substances release carbon dioxide on burning. Vehicular emissions, industrial fumes and the gases released during the process of cooking are some instances of release of carbon dioxide through combustion.
• Thus, carbon is cycled repeatedly through different forms by the various physical and biological activities.

Question 8:

Explain the following with suitable examples.

a.  What type of changes occur in the amount of energy during its transfer from plants to apex consumers?

b.  What are the differences between flow of matter and of energy in an ecosystem? Why?

Answer 8:

a. The amount of energy reduces as we move from lower trophic levels to higher ones. Energy is lost as it gets transferred from one organism to another because energy is lost as metabolic heat when the organism of one trophic level consumes the organisms of another trophic level. This can also be explained on the basis of 10% rule which states that when energy is passed in an ecosystem from one trophic level to the next, only ten percent of the energy will be passed onto the next trophic level.

This can be explained with the help of energy pyramid in case of an aquatic ecosystem.



As, we move from lower trophic level to the higher trophic level, there is a decrease in the amount of energy which is transferred.


b. The major difference between flow of matter and of energy is that the flow of matter occurs in a cyclic manner i.e. it is recycled, however, the flow of energy in an ecosystem is unidirectional which means, it is not recycled. The flow of matter follows the following path- 

• Decomposers release nutrients when they break down dead organisms.
• The nutrients are taken up by plants through their roots.
• The nutrients pass to primary consumers when they eat the plants.
• The nutrients pass to higher level consumers when they eat lower level consumers.
• When living things die, the cycle repeats.

In case of flow of energy, the energy flows through an ecosystem in one direction, from photosynthetic organisms to herbivores to omnivores and carnivores and decomposers, less and less energy becomes available to support life. This loss of useable energy occurs because each energy transfer results in the dissipation of some energy into the environment as heat.