Class 12 Biology - PUNJAB
Ecosystem
The chapter 'Ecosystem' in Class 12 Biology explores the structural and functional units of nature where living organisms interact with each other and their physical environment. Punjab (PSEB) students will learn about ecosystem productivity, decomposition, energy flow through trophic levels, ecological pyramids, and major biogeochemical cycles like carbon and phosphorus. Additionally, the chapter covers ecological succession and ecosystem services. This is a high-scoring unit in the board exams, frequently featuring direct theoretical questions, diagram-based questions on ecological pyramids, and numerical problems related to energy transfer and 10% law.
Start Learning FreeKey Concepts
Productivity
The rate of biomass production is called productivity. Gross Primary Productivity (GPP) is the total rate of photosynthesis, while Net Primary Productivity (NPP) is the available biomass for consumption by heterotrophs after respiratory loss.
Decomposition
The breakdown of complex organic matter into inorganic substances like carbon dioxide, water, and nutrients by decomposers. Key steps include fragmentation, leaching, catabolism, humification, and mineralization.
Energy Flow
Energy flow in an ecosystem is always unidirectional and follows the First and Second Laws of Thermodynamics. According to Lindeman's 10% law, only 10% of energy is transferred from one trophic level to the next.
Ecological Pyramids
Graphical representations of trophic structure and function at successive trophic levels. Pyramids can be of number, biomass, or energy, and may be upright or inverted depending on the ecosystem.
Ecological Succession
The predictable and progressive change in the species composition of an area over time, leading to a climax community. It starts with pioneer species on bare rocks (primary) or disturbed soils (secondary).
Important Formulas
Board Exam Info
In the Punjab (PSEB) Class 12 Biology board exam, the Ecosystem chapter is part of the Ecology unit, which carries a significant weightage of around 10-14 marks. Students can expect 1-mark objective questions, 2 or 3-mark short answer questions regarding ecological pyramids or decomposition steps, and 5-mark long answer questions explaining energy flow or ecological succession.
Frequently Asked Questions
Why is the pyramid of energy always upright?
The pyramid of energy is always upright because energy is lost as heat at each trophic level during transfer, according to the second law of thermodynamics, leaving less energy available for higher trophic levels.
What is the difference between primary and secondary succession?
Primary succession starts in areas where no living organisms ever existed (like bare rock), while secondary succession begins in areas where natural life was destroyed (like an abandoned farm or burnt forest).
What are the limitations of ecological pyramids?
Ecological pyramids do not take into account the same species belonging to two or more trophic levels at the same time, and they do not accommodate food webs, assuming a simple food chain.
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