Revision: Energy Flow in an Ecosystem Science SSC (English Medium) 9th Standard Maharashtra State Board

A food chain is the sequential sequence of living organisms in an ecosystem where one organism consumes another to transfer food energy.

A network of interconnected food chains showing how various organisms are related through feeding relationships is called a food web.

A predator is any consumer that kills and eats another living organism in order to obtain energy. The organism being eaten is called the prey.

Food chain, in ecology, the sequence of transfers of matter and energy in the form of food from organism to organism. Food chains intertwine locally into a food web because most organisms consume more than one type of animal or plant. Plants, which convert solar energy to food by photosynthesis, are the primary food source. In a predator chain, a plant-eating animal is eaten by a flesh-eating animal. In a parasite chain, a smaller organism consumes part of a larger host and may itself be parasitized by even smaller organisms. In a saprophytic chain, microorganisms live on dead organic matter.

It refers to relationships between organisms of different species that show on intimate association with each other. Symbiotic relationships provide atleast one of the participating species with a nutritional advantage.
Examples of Symbiosis Relationships are of Fungus and photosynthestic alga and Microbes and cattle.

The circular movement of nutrients between living organisms and the physical environment of an ecosystem is called biogeochemical cycle.

Chemoautotrophs are those who make their own food by deriving the energy from chemical reactions. e.g., Nitrifying bacteria, Iron bacteria etc.

• Energy flow in an ecosystem is strictly unidirectional, moving from the sun through producers to consumers and decomposers.
• Unlike physical matter, energy within an ecosystem is never recycled and is progressively lost as heat at every trophic step.
• Less than 50% of incident solar radiation is Photosynthetically Active Radiation (PAR), and plants capture merely 2–10% of this PAR to sustain the living world.
• Grazing Food Chains (GFC) originate with living plants and dominate aquatic ecosystems, while Detritus Food Chains (DFC) originate with dead organic matter and dominate terrestrial ecosystems.
• Interconnected food chains create complex food webs that provide ecosystems with essential stability, resilience, and alternative energy pathways.
• Within a complex food web, a single organism has the ecological flexibility to occupy more than one trophic level simultaneously.
• Organisms are structured into sequential trophic levels, from producers (T1) to apex predators (T4), with available energy drastically decreasing at each successive tier.
• Decomposers act systematically across all trophic levels to break down dead organic matter, forming a critical biological link for continuous nutrient recycling.

• A food chain is a linear sequence of living organisms in an ecosystem where food energy is transferred as one organism consumes another.
• The structural flow typically moves from primary producers to successive levels of consumers, such as from grass to insects, then to a thrush, and finally to a sparrow hawk.
• According to the principle of continuity, a complete food chain requires a continuous external energy supply, always beginning with photosynthesis and ending with decomposition.
• The principle of efficiency dictates that shorter food chains are more stable and energy-efficient because less energy is wasted across fewer transfers.
• Under the 10% law, only about 10% of the available energy is passed to the next trophic level, while the remainder is lost as heat through respiration.

• A food web is a complex network of interconnected food chains that illustrates the broader pattern of energy and nutrient flow within an ecosystem.
• The complexity of a food web increases with the length of its food chains and the availability of alternative dietary choices for consumers.
• Energy flow in an ecosystem is strictly unidirectional, meaning energy captured by autotrophs never reverts back to the solar input or to previous trophic levels.
• As energy transfers progressively through successive trophic levels, the total amount of available energy consistently diminishes due to energy loss at each stage.
• Biological magnification occurs when non-degradable, harmful chemicals enter the food chain and accumulate progressively at each consecutive trophic level.
• Because human beings occupy the apex or top level in food chains, the maximum concentration of these toxic chemicals ultimately accumulates within our bodies.

• Meaning → Movement of nutrient elements through the various components of an ecosystem is called Nutrient Cycling or Biogeochemical Cycles.
• Two Types → Nutrient cycles are of two types — Gaseous and Sedimentary.
• Gaseous Cycle → Reservoir is the atmosphere. Examples: Nitrogen cycle and Carbon cycle.
• Sedimentary Cycle → Reservoir is the Earth's crust. Examples: Sulphur cycle and Phosphorus cycle.
• Importance → Nutrient cycling ensures the continuous recycling of nutrients between the biotic (living) and abiotic (non-living) components of an ecosystem, unlike energy, which flows unidirectionally.

• Carbon is a key element of living organisms and is found in air, water, soil, rocks, and oceans (major reservoirs).
• The carbon cycle is the movement of carbon between the atmosphere, organisms, and the Earth through natural processes.
• Photosynthesis fixes CO₂ into organic matter, which passes through food chains.
• CO₂ returns to the atmosphere through respiration, decomposition, and combustion.
• Carbon sinks & humans: Fossil fuels and rocks store carbon, but human activities increase CO₂, causing global warming.

• Nitrogen Fixation — N₂ → Ammonia/nitrogenous salts. Types: Biological (microbes) and Non-biological (lightning/industrial).
• Nitrification — NH₃ → NO₂⁻ (by Nitrosomonas) → NO₃⁻ (by Nitrobacter). Nitrates absorbed by plants.
• Ammonification — Dead organic matter → NH₃, by ammonifying bacteria in soil.
• Denitrification — NO₃⁻ → N₂ (back to atmosphere), by Bacillus, Paracoccus, Pseudomonas denitrificans.
• Cyclic Flow — Atmosphere → Soil → Plants → Animals → Soil → Atmosphere. Nitrogen is continuously recycled.