NCERT grounding
Section 12.2 of the Class 12 Biology chapter Ecosystem opens the functional study of ecosystems with productivity. The text states that a constant input of solar energy is the basic requirement for any ecosystem to function and sustain, and defines primary production as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. Productivity is the rate of that production, split into gross primary productivity (GPP) and net primary productivity (NPP) and tied together by the equation GPP − R = NPP. The NIOS module Principles of Ecology reinforces the same relationship, writing GPP = NPP + R.
“Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).”
NCERT Class 12 Biology — Ecosystem, Section 12.2
Primary production and productivity
The distinction NEET tests most often is between production and productivity. Primary production is a quantity: the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis, expressed in weight (g m−2) or energy (kcal m−2). The rate of biomass production is called productivity. Because it is a rate, productivity carries a time unit and is expressed in g m−2 yr−1 or (kcal m−2) yr−1 — units that let ecologists compare different ecosystems on a common scale.
The word “primary” signals that the organic matter is created for the first time, by autotrophs — chiefly green plants — fixing inorganic carbon into organic molecules during photosynthesis. Producers occupy the first trophic level, so primary production is the energy gateway of the whole ecosystem: every joule that later moves through consumers must first enter as primary production.
Production vs Productivity
Production is a standing quantity (weight or energy per unit area). Productivity is a rate — the same quantity divided by time. The presence of yr−1 is the visible marker of productivity.
Primary production has two layers. The total organic matter fixed during photosynthesis is the gross figure; the part that survives the plant’s own respiration is the net figure. NCERT therefore defines gross primary productivity and net primary productivity as the two divisions of primary productivity.
GPP, NPP and the equation
Gross primary productivity (GPP) of an ecosystem is the rate of production of organic matter during photosynthesis. The NIOS text calls it the total rate at which radiant energy is stored by photosynthesis in green plants, and notes it is also known as total photosynthesis — GPP measures everything the producers fix, before any of it is spent.
A considerable amount of GPP is utilised by plants in respiration. Respiration powers the plant’s own metabolism, maintenance and reproduction; the energy for all these functions comes from cellular respiration. Whatever remains after these respiratory losses are paid is the net figure. Net primary productivity (NPP) is gross primary productivity minus respiration losses (R):
Figure 1. GPP is the total organic matter fixed during photosynthesis. Respiration losses (R) are subtracted from it; the remainder is NPP, the biomass available for consumption to heterotrophs — herbivores and decomposers.
In symbols the relationship is written GPP − R = NPP, or equivalently GPP = NPP + R. Here R is respiratory loss — not respiratory quotient, not radiant energy, not reproductive allocation. NEET 2021 and NEET 2023 both tested exactly this identification of R, so the safest mental label is “R = respiration.”
Gross Primary Productivity
GPP
total photosynthesis
- Rate of production of organic matter during photosynthesis
- Includes the fraction later lost in respiration
- Also called total photosynthesis (NIOS)
- Always the larger of the two values
Net Primary Productivity
NPP
GPP minus respiration
- GPP minus respiration losses (R)
- The biomass actually stored and left over
- Available for consumption to heterotrophs
- Always smaller than GPP (since R is positive)
The biological meaning of NPP is what makes it central to the rest of the chapter. Net primary productivity is the available biomass for the consumption to heterotrophs — herbivores and decomposers. It is not GPP that herbivores feed on; the producers have already burned part of GPP to stay alive. What is passed up the food chain, and what fuels the pyramids of biomass and energy, is NPP. The chapter summary phrases this as NPP being the remaining biomass or energy left after utilisation by producers.
From sunlight to consumable biomass
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Step 1
Solar input
Constant input of solar energy reaches the producers.
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Step 2
GPP fixed
Photosynthesis fixes inorganic carbon into organic matter — this rate is GPP.
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Step 3
Respiration (R)
A considerable part of GPP is used up by plants in respiration.
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Step 4
NPP remains
GPP − R = NPP — the biomass available to heterotrophs.
Secondary productivity
Producers are not the only organisms that build organic matter. Consumers also assemble new tissue from the food they assimilate. Secondary productivity is defined as the rate of formation of new organic matter by consumers. The chapter summary restates it as the rate of assimilation of food energy by the consumers. Herbivores, carnivores and decomposers are all heterotrophs, so secondary productivity is a property of every trophic level above the producers.
Secondary productivity is built on net primary productivity, not gross. Because NPP is the biomass available for consumption to heterotrophs, the energy entering any consumer level is capped by the NPP of the producers beneath it. At each transfer a large share of energy is lost as heat and in respiration, so secondary productivity is always far smaller than the primary productivity that supports it.
| Term | Definition | Performed by |
|---|---|---|
| Gross primary productivity | Rate of production of organic matter during photosynthesis | Producers (autotrophs) |
| Net primary productivity | GPP minus respiration losses (R); biomass available to heterotrophs | Producers (autotrophs) |
| Secondary productivity | Rate of formation of new organic matter by consumers | Consumers (heterotrophs) |
Factors affecting productivity
Primary productivity is not a fixed number. NCERT lists four influences: the plant species inhabiting a particular area, a variety of environmental factors, the availability of nutrients, and the photosynthetic capacity of plants. Because these vary from place to place, productivity varies in different types of ecosystems — a tropical rain forest is far more productive than a desert or the open ocean.
NCERT line to memorise: primary productivity depends on the plant species, on environmental factors, on the availability of nutrients, and on the photosynthetic capacity of plants — therefore it varies in different types of ecosystems.
Plant species
The species inhabiting an area set the upper limit. A dense forest canopy fixes far more carbon than sparse grassland or tundra vegetation.
Environmental factors
Light, temperature, water and the length of the growing season govern how much photosynthesis is possible across the year.
Nutrient availability
Access to nutrients limits growth. In aquatic systems scarce surface nutrients are a key reason ocean productivity stays low.
Photosynthetic capacity
The intrinsic efficiency of the plants' photosynthetic machinery determines how much of the available light is converted to biomass.
Scaling these local figures up to the planet gives one of the most quoted statistics in the chapter. The annual net primary productivity of the whole biosphere is approximately 170 billion tons of dry weight of organic matter. Of this total, despite the oceans occupying about 70 per cent of the Earth’s surface, the productivity of the oceans is only about 55 billion tons. The rest — the larger share — is produced on land.
Figure 2. The biosphere fixes about 170 billion tons of organic matter a year. Oceans, despite covering roughly 70 per cent of the surface, contribute only about 55 billion tons; the remaining bulk is produced on land.
The contrast is striking: the larger surface produces the smaller share. Ocean productivity is held down by limited nutrient availability in surface waters and by light reaching only a thin upper layer, so most of the ocean volume cannot photosynthesise. Productivity is governed by limiting conditions, not by sheer area — a point NCERT invites students to discuss with their teacher.
Worked examples
An ecosystem has a gross primary productivity of 1,000 kcal m−2 yr−1. Plants spend 400 kcal m−2 yr−1 in respiration. What is the net primary productivity, and which value is available to heterotrophs?
Apply NPP = GPP − R. NPP = 1,000 − 400 = 600 kcal m−2 yr−1. NPP is the available biomass for consumption to heterotrophs, so 600 kcal m−2 yr−1 is the figure passed to herbivores and decomposers. GPP (1,000) is never the value available to consumers, because respiration losses are deducted first.
Which of these is a valid unit of productivity: g m−2, kcal m−2, kcal m−3, or kcal m−2 yr−1? Justify the choice.
Only kcal m−2 yr−1 is a unit of productivity. Productivity is a rate, so it must include a time unit (yr−1). The units g m−2 and kcal m−2 express production or standing biomass, not productivity, because they carry no time. kcal m−3 uses volume instead of area and is not used here. This is the exact logic tested in NEET 2025 Q.91.
A leaf-eating insect assimilates 50 kcal m−2 yr−1 of food energy and converts part of it into new body tissue. Is this primary or secondary productivity?
It is secondary productivity — the rate of formation of new organic matter by consumers. The insect is a heterotroph, so any organic matter it builds counts as secondary, not primary, productivity. Primary productivity is restricted to producers fixing carbon during photosynthesis.
Common confusion & NEET traps
The errors NEET exploits in this subtopic are almost always vocabulary slips: confusing production with productivity, mislabelling R, or forgetting which value heterotrophs actually receive. The callouts below collect the high-yield traps.