Approximately how much of the solar energy that falls on the leaves of a plant is converted to chemical energy

NCERT Exemplar · Q8

Q. Approximately how much of the solar energy that falls on the leaves of a plant is converted to chemical energy by photosynthesis?

(a) Less than 1%

(b) 2-10%

(d) 50%

Chandigarh CbseMCQ· 1mImportance★★★★★est

29% · 19/65 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Photosynthetically Active Radiation

Imagine you are standing in a sunlit garden. You feel the warmth of sunlight on your skin, and you can see the bright green of the leaves. But here is the key: the plant does not use all that sunlight for its food-making process. It is picky. It only uses a specific slice of the sunlight — the part that is "photosynthetically active."

That specific slice is called Photosynthetically Active Radiation, or PAR for short.

The Precise Meaning

Sunlight is a mixture of different colours (wavelengths), from violet and blue to green, yellow, orange, and red. Plants have a pigment called chlorophyll that captures light energy. But chlorophyll does not absorb every colour equally. It absorbs light most strongly in the blue and red regions of the spectrum. It reflects green light — that is why leaves look green to us.

Photosynthetically Active Radiation is simply the portion of the light spectrum that plants can actually use for photosynthesis. In scientific terms, it covers the wavelength range from about 400 to 700 nanometres. This range includes blue light, red light, and everything in between — but it excludes ultraviolet (shorter than 400 nm) and far-red/infrared (longer than 700 nm).

Note

PAR is not a measure of how much light there is in total. It is a measure of how much usable light is available for photosynthesis. A dim, red-lit room might have high PAR, while a bright, green-lit room might have low PAR — because plants cannot use green light well.

Why It Matters (Even for a Humanities Student)

You might wonder: why should a commerce or humanities student care about a technical term from plant biology? Here is why.

Agriculture and food security: Farmers and agronomists measure PAR to know if crops are getting enough usable light. If a crop is shaded by a building or a taller plant, the PAR drops, and yield falls. This directly affects food prices and supply chains — something a commerce student studies.
Climate and environment: PAR is a key input in models that predict how much carbon dioxide forests and oceans absorb. This links to climate change, which affects everything from insurance premiums to migration patterns — topics a humanities student encounters.
Urban planning and architecture: When designing green buildings or vertical gardens, architects must ensure enough PAR reaches the plants. A poorly lit indoor garden will fail, no matter how beautiful the design.

Important

PAR is not the same as total sunlight. A cloudy day may still have high PAR if the clouds are thin, while a bright sunny day may have low PAR if the sun is low in the sky (more red/infrared, less blue). Always think: usable light, not visible light.

What the NCERT Textbook Says

The NCERT Class 11 Biology textbook (Chapter 13: Photosynthesis in Higher Plants) states clearly: …

Start your 14-day free trial to unlock the full solution →

Only a tiny fraction — less than 1% — of the sunlight that strikes a leaf is actually captured and stored as chemical energy through photosynthesis.

Photosynthesis is often described as the most important biochemical process on Earth, and for good reason. It is the primary way that energy from the sun enters the living world. But when you look at a sunlit leaf, it is easy to assume that most of that bright sunlight is being put to work. The reality, as your NCERT textbook explains, is far more modest.

The sunlight that falls on a leaf is called Photosynthetically Active Radiation, or PAR. This is the portion of the solar spectrum (roughly the visible light between 400 and 700 nanometres) that plants can actually use. Even so, not all of that PAR gets converted. A significant amount is reflected off the leaf surface, some passes right through the leaf without being absorbed, and some of the energy that is absorbed is lost as heat. Only the energy that is trapped by chlorophyll molecules and used to drive the light reactions of photosynthesis counts as converted chemical energy.

Note

The term PAR is important in ecology and agriculture because it helps scientists measure exactly how much usable light a plant canopy is receiving, rather than just total sunlight.

So what is the final figure? The NCERT textbook states clearly that less than 1% of the solar energy that falls on a leaf is converted into chemical energy (the energy stored in glucose and other organic compounds). This might seem shockingly low, but it makes sense when you consider all the inefficiencies involved: the limited wavelengths that chlorophyll can absorb, the energy lost as heat during electron transport, and the fact that the Calvin cycle itself uses a lot of the captured energy just to fix carbon dioxide. …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ7 — Pyramid of numbers is: (a) Always upright (b) Always inverted (c) Eith…→Next questionQ9 — Among the following, where do you think the process of decomposition w…

Q.Approximately how much of the solar energy that falls on the leaves of a plant is converted to chemical energy by photosynthesis?

🎓Unlock everything free for 14 days

Unlock everything free for 14 days