Solution-Class 9-Science-Chapter-7-Energy Flow In An Ecosystem-Maharashtra Board

Correction-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. Carnivorous feed upon these Herbivorous animals therefore carnivores occupy the next level after herbivores.

Correction-The flow of nutrients in an ecosystem is considered to be cyclic. 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.

By photosynthesis green plants convert minerals and inorganic compounds into glucose. This food is transferred along with food chain from producers to primary consumers to secondary consumers. The decomposers of the ecosystem decompose all the dead and decaying organisms and return these nutrients back to the ecosystem. Thus, the flow of nutrients is cyclic and not one way transport.

Correction-Plants in an ecosystem are called producers. Plants are autotrophs, they carried out photosynthesis with the help of solar energy and produce food. This food is utilized by other organisms. Other herbivorous organisms are dependent on the food produce by plant to meet their nutritional requirements. Therefore herbivorous are primary consumer and plants are producer

• Sun is ultimate source of energy.
• The producers convert solar energy into chemical energy in the form of organic compounds.
• The primary consumers (herbivores) derive their nutrition from the producers.
• 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.

• The cyclic flow of nutrients within an ecosystem is called as the bio-geochemical 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.
• Soil bacteria and other decomposers play vital role in these cycles.
• The recycling of these bio-geo-chemicals depends upon the level of energy transfer in the ecosystem.
• Any type of disturbance at any level of these cycles affects the ecosystems and destroy nature's balance.
• Hence, there should be equilibrium in the various bio-geochemical cycles.

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 obtain nutrients from the soil, and carbon dioxide from the atmosphere and synthesise food from them with the help of photosynthesis.
• During this process, the inorganic substances are converted into organic substances.
• Consumers consume the food produced by producers. The energy in the form of food is thus passed on from one trophic level to the other trophic level.
• Both producers and consumers after their death undergo decomposition and after decomposition the nutrients released from them mix with the soil. In the same way, the excreta of animals undergo decomposition and release nutrients to the environments.

Thus, we see that the flow of nutrients in the ecosystem follows a cyclic path.

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.

The atmospheric nitrogen moves from atmosphere to the soil, from the soil to the plants, from plants to the animals and from all the living beings to the soil and back to atmosphere from the soil. This cyclic journey of the nitrogen is called nitrogen cycle.

(1) Source of nitrogen : Plants cannot take the atmospheric nitrogen as it is. Therefore they take up various compounds of the nitrogen that are available in the soil. For the plants nitrogen compounds in the soil is the only source of nitrogen.

(2) Nitrogen fixation : Nitrogen fixation is the process in which nitrogen is converted into compound such as nitrates and nitrites. There are two processes of nitrogen fixation. viz., physical fixation and biological fixation.

Physical fixation Further divided into atmospheric fixation and industrial fixation

(a) Physical fixation : The lightning in the sky brings about oxidation of atmospheric nitrogen and converts it into nitrogen oxide. Nitrogen oxide (NO₂) dissolves in rain water and forms nitrous and nitric acid. These acids seep in the soil with the rain water. They undergo chemical reactions with other elements in soil and form different compounds of nitrogen.

(i) Atmospheric nitrogen Lightening> Nitrogen oxide

(ii) Nitrogen oxide + Rain water — Nitrous acid and nitric acid.

(b) Biological fixation :

It is the process wherein atmospheric nitrogen is converted into water-soluble nitrates. This step is performed by organisms like Rhizobium Azotobactea and blue-green algae, living in the root nodules of the leguminous plants carry out the function of biological nitrogen fixation.

(3) Utilization of nitrogen compounds by the living organisms :

• Plants take up nitrates and nitrites from the soil. They synthesise proteins and nitrogenous compounds from the absorbed nitrates and nitrites.
• Herbivorous animals consume plants and fulfil their protein requirement.
• Proteins are broken down to amino acids during the metabolism. In the bodies of animals the amino acids are once again assembled to form required proteins.

In addition to nitrogen fixation there are other three processes which are important in nitrogen cycle. They are as follows :

1. 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.
2. 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.
3. Denitrification: It is the process wherein the nitrates present in the soil are reduced to release nitrogen back into the atmosphere.

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 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 by one major way, and that is photosynthesis.

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.

For maintaining the balance in the bio-geochemical cycles, following measures have to be taken :

(1) We have to conserve plants like trees, creepers and the entire available flora. For this, we have to at once stop felling of the trees. If plants are destroyed, the productivity of the ecosystem is hampered. Therefore the flora has to be well maintained.

(2) We should not kill and poach carnivorous wild animals. This results in disturbance in the ecological balance.

(3) Under any condition, apex consumer should not be killed. The hunting makes the valuable species to become extinct. By exterminating the apex consumers, the interrelationships between food chains and food web will suffer.

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

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.

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.

e.g. Insects (Primary consumer) nibble grass (Producer). Frogs (Secondary consumer) feed on insects. This is a straight food chain. Insects are also eaten by sparrow. Sparrow becomes food forowl. Owl also kills snakes. Snakes (Tertiary consumer) feed on frogs. Apex carnivore, hawk, feeds on sparrow, snake or frog too. In this way, smaller food chains are interconnected in a web form constituting the food web.

Thus, there is an interrelationship between the food chain and food web.

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 bio-geochemical cycles. So, bio-geochemical 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 viz. gaseous cycle and sedimentary cycle.

1. Gaseous cycle:The gaseous cycle includes cyclic transformation of carbon dioxide, oxygen, nitrogen, water vapour, etc. 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: Sedimentary cycle includes cyclic movement of calcium, phosphorus, iron, etc. that are present in soil. These are the biogeochemical cycles in which the reservoir pool is generally lithosphere. For example, Phosphorous cycle, Sulphur cycle etc.

• In an ecosystem, producers produce food by making use of solar energy. While doing so, they absorb nutrients from soil, air and water. In gaseous bio-geochemical cycle, the nutrients are in air while in sedimentary cycles they are in soil.
• Consumers that feed on producers also pass nutrients to their bodies. :

Importance of bio-geochemical cycle :

• Plants require water, CO₂, phosphorus, potassium, nitrogen, etc. as nutrients while animals require carbohydrates, proteins, fats, etc. as nutrients.
• Producers and consumers after their death undergo decomposition and release elements which mix with the soil.
• The cyclic movement of nutrients is possible through bio-geochemical cycle.
• The cyclic movement of elements like carbon, Nitrogen, phosphorus, etc. continuously occur on the earth.

(1) Green plants of the ecosystem store large amount of energy in the form of carbohydrates and proteins.

(2) Before reaching the decomposers, this energy is passed through different trophic levels like primary consumer, secondary consumer, tertiary consumer, etc.

(3)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.

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.

Flow of matter:

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.
• Nutrients are not destroyed into the ecosystem.

Energy Flow :

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.