Approximately what percentage of the sunlight falling on their leaves is captured by green plants in a terrestrial ecosystem for photosynthesis?

The Sun is the ultimate source of energy for almost all ecosystems on Earth. Green plants, the producers, capture a part of the solar energy and convert it into chemical energy stored in food through photosynthesis. This chemical energy then flows through the food chain to herbivores, carnivores and decomposers.

However, plants capture only a very small part of the sunlight that reaches them. Of the total solar radiation reaching the Earth, only about half is photosynthetically active radiation (PAR), the part of visible light (wavelengths of about 400-700 nanometres) that plants can use. Much of the light is reflected, transmitted through leaves, or lost as heat. As a result, green plants in terrestrial ecosystems capture only about 1-5% of the photosynthetically active radiation, and only about 1% of the total sunlight falling on them is converted into chemical energy in food. This is often stated in NCERT textbooks.

The rate at which plants produce organic matter by photosynthesis is called productivity:

Gross Primary Productivity (GPP): the total rate of photosynthesis, or the total organic matter produced.

Net Primary Productivity (NPP): GPP minus the energy used by plants for their own respiration. NPP is the energy available to consumers.

After plants capture energy, only a small part of it is passed on at each step of the food chain. According to the ten per cent law, proposed by Raymond Lindeman (1942), only about 10% of the energy at one trophic level is transferred to the next. The rest is used for life processes or lost as heat. This is why food chains are usually short.

Tropical rainforests have the highest net primary productivity among terrestrial ecosystems, while deserts have the lowest. Among aquatic ecosystems, coral reefs and estuaries are highly productive. Oceans, though vast, have low productivity per unit area, but they contribute a large share of the world's total productivity because of their size.

Why not the other options?

• ✗About 10% is the energy transferred between trophic levels (ten per cent law).
• ✗About 25% and about 50% are far too high; about 50% is the share of sunlight that is photosynthetically active.