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Photosynthesis in Higher Plants - Factors affecting photosynthesis

Grade 11CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Blackman’s Law of Limiting Factors: Proposed in 1905, it states that if a chemical process is affected by more than one factor, its rate is determined by the factor which is nearest to its minimal value (the factor that directly affects the process if its quantity is changed).

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Light Intensity: At low light intensities, there is a linear relationship between incident light and CO2CO_2 fixation. At higher light intensities, the rate gradually stabilizes as other factors become limiting. Light saturation usually occurs at 10%10\% of the total sunlight.

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Light Quality: Photosynthesis occurs maximum in the blue and red regions of the visible spectrum. Green light is the least effective as it is mostly reflected by chlorophyll.

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Carbon Dioxide (CO2CO_2) Concentration: It is the major limiting factor in nature. Atmospheric concentration is approx 0.03%0.03\% to 0.04%0.04\%. C3C_3 plants show saturation only beyond 450μlL−1450 \mu l L^{-1}, while C4C_4 plants show saturation at about 360μlL−1360 \mu l L^{-1}.

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Temperature: The dark reactions (enzymatic) are highly temperature-sensitive. C4C_4 plants have a higher temperature optimum (30∘C30^{\circ}C to 40∘C40^{\circ}C) compared to C3C_3 plants (20∘C20^{\circ}C to 25∘C25^{\circ}C).

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Water: Water stress causes stomatal closure, reducing CO2CO_2 availability. It also leads to leaf wilting, which reduces the surface area available for light absorption and metabolic activity.

📐Formulae

6CO2+12H2O→Light, ChlorophyllC6H12O6+6H2O+6O2↑6CO_2 + 12H_2O \xrightarrow{\text{Light, Chlorophyll}} C_6H_{12}O_6 + 6H_2O + 6O_2\uparrow

Q10=Rate at (t+10)∘CRate at t∘C≈2 (for enzymatic dark reactions)Q_{10} = \frac{\text{Rate at } (t + 10)^{\circ}C}{\text{Rate at } t^{\circ}C} \approx 2 \text{ (for enzymatic dark reactions)}

Light Compensation Point:Rate of PS=Rate of Respiration\text{Light Compensation Point}: \text{Rate of } PS = \text{Rate of Respiration}

💡Examples

Problem 1:

Explain why C3C_3 plants like tomatoes and bell peppers are often grown in CO2CO_2 enriched greenhouses.

Solution:

C3C_3 plants respond to higher CO2CO_2 concentrations (up to 450μlL−1450 \mu l L^{-1}) by showing increased rates of photosynthesis.

Explanation:

Since the current atmospheric CO2CO_2 levels (around 400400 ppm) are limiting for C3C_3 plants, providing an environment with higher CO2CO_2 concentrations leads to higher biomass and productivity, a phenomenon known as the CO2CO_2 fertilization effect.

Problem 2:

What happens to the rate of photosynthesis when a plant is moved from 10%10\% sunlight to 50%50\% sunlight, assuming CO2CO_2 is abundant?

Solution:

The rate will likely remain constant or show very little increase.

Explanation:

Light saturation for most plants occurs at about 10%10\% of full sunlight. Beyond this point, light is no longer the limiting factor; instead, factors like CO2CO_2 concentration or temperature limit the rate according to Blackman's Law.

Factors affecting photosynthesis - Revision Notes & Key Diagrams | CBSE Class 11 Biology