Factors Affecting Photosynthesis — Revision Notes

⚡ 30-Second Revision

Blackman's Law: — Rate limited by the slowest factor.
Light: — Intensity (saturation point), Quality (red/blue most effective), Duration.
$CO_2$: — Often limiting (0.03-0.04%), saturation point higher for C4.
Temperature: — Affects enzyme activity. Optimal for C3 ($20-25^circ C$), C4 ($30-45^circ C$).
Water: — Indirectly limiting (stomatal closure $ightarrow$ low $CO_2$).
Internal Factors: — Chlorophyll content, leaf age/anatomy, enzyme activity.
C3 vs C4: — C4 plants more efficient at high light, high temp, low $CO_2$ (due to $CO_2$ concentrating mechanism, low photorespiration).

2-Minute Revision

Photosynthesis rate is governed by Blackman's Law of Limiting Factors: the slowest factor dictates the overall speed. Key external factors include light, carbon dioxide, temperature, and water. Light intensity increases the rate up to a saturation point, beyond which other factors like $CO_2$ become limiting.

Light quality (wavelength) is crucial, with red and blue light being most effective. $CO_2$ concentration is often the primary limiting factor in nature due to its low atmospheric levels; C4 plants are more efficient at lower $CO_2$ due to their concentrating mechanism.

Temperature affects enzyme activity, with C3 plants preferring cooler temperatures ($20-25^circ C$) and C4 plants thriving in warmer conditions ($30-45^circ C$). Water is an indirect limiting factor; scarcity causes stomatal closure, reducing $CO_2$ uptake.

Internal factors like chlorophyll content, leaf age, and the efficiency of photosynthetic enzymes also play significant roles. Remember the distinct responses of C3 and C4 plants to these factors, especially regarding photorespiration and optimal conditions.

5-Minute Revision

A comprehensive understanding of factors affecting photosynthesis begins with Blackman's Law of Limiting Factors, which states that the rate of a process is determined by the factor in shortest supply. This principle is vital for interpreting how environmental and internal conditions interact.

External Factors:

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Light: — Both intensity and quality matter. Photosynthesis increases with light intensity up to a 'light saturation point,' beyond which other factors (like $CO_2$) become limiting. C4 plants have higher light saturation points than C3 plants. Red and blue light are most effective. Prolonged high light can cause photoinhibition.
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Carbon Dioxide ($CO_2$): — Often the most limiting factor in natural environments due to its low atmospheric concentration (0.03-0.04%). Increasing $CO_2$ boosts the rate until saturation. C4 plants are more efficient at lower $CO_2$ levels and have a lower $CO_2$ compensation point than C3 plants, which suffer from photorespiration at low $CO_2$/high $O_2$.
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Temperature: — Affects enzyme activity. C3 plants have an optimal range of $20-25^circ C$, while C4 plants are adapted to warmer temperatures, with an optimum of $30-45^circ C$. Extreme temperatures can denature enzymes.
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Water: — Primarily an indirect limiting factor. Water stress causes stomatal closure to conserve water, which in turn restricts $CO_2$ entry, making $CO_2$ the actual limiting factor. It also affects turgor and enzyme function.

Internal Factors:

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Chlorophyll Content: — Directly impacts light absorption. Higher chlorophyll generally means higher rates.
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Leaf Anatomy/Age: — Leaf size, orientation, stomatal density, and age (young and mature leaves are more efficient) influence overall photosynthetic capacity.
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Protoplasmic Factors: — The quantity and activity of enzymes like RuBisCO are crucial. Accumulation of end products can also cause feedback inhibition.

Key Comparison (C3 vs. C4): C4 plants are superior in hot, dry, high-light conditions due to their $CO_2$-concentrating mechanism, which minimizes photorespiration and allows for higher optimal temperatures and $CO_2$ saturation points. C3 plants are more suited to cooler, moderate conditions.

Prelims Revision Notes

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Blackman's Law of Limiting Factors (1905): — The rate of a physiological process is limited by the factor in shortest supply. If light is abundant but $CO_2$ is low, $CO_2$ limits the rate.
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External Factors:

* Light: * Intensity: Rate increases with intensity up to a light saturation point. C3 plants saturate at ~10-20% full sunlight; C4 plants require higher intensity. High light can cause photoinhibition.

* Quality (Wavelength): Red and blue light are most effective (absorbed by chlorophylls). Green light is least effective. * Duration: Longer light periods generally mean more photosynthesis.

* **Carbon Dioxide ($CO_2$):** * Atmospheric $CO_2$ (0.03-0.04%) is often limiting. * Rate increases with $CO_2$ concentration up to a saturation point. * **$CO_2$ Compensation Point:** Rate of $CO_2$ uptake = rate of $CO_2$ release.

C3 plants have a higher compensation point (40-100 ppm) than C4 plants (0-10 ppm). * C3 vs C4: C4 plants are more efficient at lower $CO_2$ concentrations due to PEP carboxylase and $CO_2$ concentrating mechanism; C3 plants suffer from photorespiration at low $CO_2$/high $O_2$.

* Temperature: * Affects enzyme activity. Light reactions are less temperature-sensitive than dark reactions. * Optimal Temperature: C3 plants: $20-25^circ C$. C4 plants: $30-45^circ C$. * High temperatures can denature enzymes.

* Water: * Indirect Limiting Factor: Water stress causes stomatal closure $ightarrow$ reduced $CO_2$ uptake $ightarrow$ $CO_2$ becomes limiting. * Also affects turgor, leaf area, and enzyme hydration.

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Internal Factors:

* Chlorophyll Content: Directly proportional to light absorption and photosynthetic rate. * Leaf Age: Young and mature leaves are most efficient; senescent leaves decline. * Leaf Anatomy: Size, orientation, stomatal density, mesophyll structure. * Protoplasmic Factors: Quantity and activity of photosynthetic enzymes (e.g., RuBisCO, PEP carboxylase). Accumulation of end products can inhibit the process.

Vyyuha Quick Recall

To remember the main external factors, think: Light Can Truly Work.

Light (Intensity, Quality, Duration)
Carbon dioxide ($CO_2$ concentration)
Temperature
Water (indirectly)