Class 9 MAHARASHTRA STATE TEXTBOOK BUREAU Solutions Science Chapter 7 - Energy Flow in an Ecosystem

1. Carnivores occupy the third trophic level in the food chain because second trophic level is occupied by the herbivores. Carnivores feed on herbivores which in turn feed on plants.
2. The flow of energy in an ecosystem is considered to be a 'one way' transport. The energy from the Sun is used by the plants to prepare food through photosynthesis. The energy from the plants is then passed on from one organism to another in a food chain and it never returns to the sun.
3. Plants in an ecosystem are called producers. Green plants prepare food using sunlight. Other organisms cannot make their food and depend on green plants for food. Therefore, green plants are the producers of food.

• The Sun is the most important source of energy in an ecosystem.
• Green plants store some amount of solar energy in the form of food.
• This energy passes from one trophic level to another and finally reaches the decomposers.
• Decomposers dissipate some amount of energy in the form of heat.
• However, no part of the energy ever returns to the Sun.
• Hence, energy flow through an ecosystem is 'one way'.

• Nutrients necessary for the growth of organisms are continuously transferred from abiotic to biotic factors and biotic to abiotic factors within an ecosystem.
• The cyclic flow of nutrients within an ecosystem is called biogeochemical cycles.
• Any imbalance in the cycles will break the link between the biotic and abiotic factors.
• Therefore, equilibrium is necessary between biogeochemical cycles.

• All organisms need nutrients for their growth.
• Nutrients like carbon, oxygen, nitrogen, iron, calcium etc. are circulated and recycled from the biosphere to the living organisms and after their death back to the biosphere.
• Nutrients are taken up by the plants and then passed on to the consumers from one trophic level to another.
• Eventually, after death, all types of consumers are decomposed by decomposers like bacteria and fungi. The nutrients are again released into the biosphere and are again made available to living organisms by the plants.
• Therefore, the flow of nutrients through an ecosystem is cyclic.

Carbon Cycle

• Circulation and recycling of carbon from the atmosphere to living organisms and after their death back to the atmosphere is called carbon cycle.
• Abiotic carbon atoms are circulated and recycled into biotic form mainly through photosynthesis and respiration.
• Hence, the carbon cycle is one of the important biogeochemical cycles.
• Plants convert carbon dioxide into carbohydrates by the process of photosynthesis. Similarly, they produce carbon compounds like proteins and fats.
• Carnivores feed on herbivores. In this way, biotic carbon is transported from plants to herbivores, from herbivores to carnivores and from carnivores to apex consumers.
• Main processes in the carbon cycle:
  
• Eventually, after death, all types of consumers, are decomposed by decomposers like bacteria and fungi and carbon dioxide is released again into the atmosphere and is used again by the living organisms.
• In this way, carbon is continuously passed on from one living organism to another.
• After the death of living organisms, carbon enters the atmosphere and is again taken up by the living organisms for their growth and development.

Nitrogen cycle

• Nitrogen gas accounts for 78% of the atmosphere. It is necessary for the maintenance of the cycle of nature.
• Circulation and recycling of nitrogen in the form of different compounds through various biotic and abiotic processes in nature is called the nitrogen cycle. All organisms participate in the nitrogen cycle.
• Nitrogen is an important component of proteins and nucleic acids. As compared to other elements, it is inactive and does not easily combine with other elements. Most organisms cannot use free form of nitrogen.
• Important processes of nitrogen cycle:

1. Nitrogen fixation: Conversion of nitrogen into nitrates and nitrites through atmospheric, industrial and biological processes.
2. Ammonification: Release of ammonia through the decomposition of dead bodies and excretory wastes of organisms.
3. Nitrification: Conversion of ammonia into nitrites and then nitrates.
4. Denitrification: Conversion of nitrogen compounds into gaseous nitrogen.

Oxygen cycle

• Oxygen accounts for 21% of the atmosphere. It is also present in the hydrosphere and lithosphere.
• The circulation and recycling of oxygen within the biosphere is called the oxygen cycle.
• This cycle includes both biotic as well as abiotic components. Oxygen is continuously produced as well as used up in the atmosphere.
• Oxygen is highly reactive and readily reacts with other elements and compounds.
• As oxygen is found in various forms like molecular oxygen (O₂), water (H₂O), carbon dioxide (CO₂) and inorganic compounds, the oxygen cycle of the biosphere is extremely complex.
• Oxygen is released during the process of photosynthesis, whereas it is used up in processes like respiration, combustion, decomposition, corrosion and rusting.

Strategies to maintain equilibrium in various biogeochemical cycles:

• Biogeochemical cycles always involve the achievement of equilibrium, i.e., a balance in the cycling of nutrients between the spheres.
• Human activities that are known to be environmentally unfriendly can disrupt this balance.
• We should avoid deforestation as trees play an important role in maintaining the balance.
• We should avoid overuse of fertilizers. The fertilizers get washed away in the nearby water bodies disrupting the balance.
• Avoid burning of fossil fuels as these fuels release a large amount of carbon in the form of carbon dioxide, thus disrupting the balance.
• Vehicular emissions is another contributor to the disruption in balance of the various cycles. These emissions release oxides of nitrogen and carbon and other hazardous air pollutants.
• It is important that we switch to better quality fuel like CNG or ethanol to reduce vehicular emissions.

Inter-relationship between food chain and food web:

• Continuous interaction occurs between the producers, consumers and decomposers.
• There is a definite sequence in these interactions which constitutes the food chain.
• Each food chain consists of four to five links.
• An ecosystem consists of several food chains that are interconnected at various levels. Thus, a food web is formed.
• An organism may be the prey for many other organisms.
• For example, an insect feeds upon the leaves of various plants but the same insect is the prey for different animals like frog, wall lizard and birds.
• Thus, many food chains interconnected together form an intricate web called the food web.

Types of bio-geochemical cycles:

• Nitrogen cycle
• Oxygen cycle
• Carbon cycle
• Water vapour cycle
• Iron cycle
• Calcium cycle
• Phosphorus cycle

Importance of bio-geochemical cycles:

• Nutrients necessary for the growth of the organisms are continuously transferred from the abiotic to the biotic factors and from biotic to the abiotic factors within an ecosystem.
• The biogeochemical cycles operate continuously through the medium of biosphere formed by the lithosphere, atmosphere and hydrosphere.
• Recycling of biological, geological and chemical sources of nutrients takes place through these cycles.
• Nutrients from the biosphere enter the bodies of plants and animals. Eventually, after death, all types of consumers are decomposed by decomposers like bacteria and fungi. The nutrients are again released into the biosphere and are again made available to living organisms by the plants.
• Therefore, the biogeochemical cycles help in maintaining the flow of nutrients and energy through the ecosystem and the equilibrium in the ecosystem.

• In an ecosystem, plants store some amount of solar energy in the form of food.
• This energy is then passed on to the consumers from one trophic level to another before reaching the decomposers.
• At every trophic level, some amount of energy is used by the organism for its own life processes and some amount of energy is lost to the surroundings.
• Decomposers dissipate some amount of energy in the form of heat.
• However, no part of the energy ever returns to the Sun. Hence, such transfer of energy is referred to as 'one way' transport.
• Therefore, energy is maximum at the base of the pyramid and least at the apex. This means that lowest trophic level, the producers have the maximum energy while the apex or top consumers have the least amount of energy.
• Example: Phytoplanktons which form the base of the pyramid have 10,000 kcal of energy while humans at the apex have 10 kcal of energy.

Differences between flow of matter and flow of energy in an ecosystem: