Problem 134 - Objective Biology for NEET

Question

Select the incorrect statement from the following: (a) Dark reaction is more sensitive to temperature. (b) Light reaction is less sensitive to temperature. (c) \(\mathrm{C}_{4}\) plant responds to higher temperature and \(\mathrm{C}_{3}\) plant have lower optimum temperature. (d) Tropical plant have lower optimum temperature for photosynthesis.

Step-by-Step Solution

Answer

The incorrect statement is (d) 'Tropical plant has a lower optimum temperature for photosynthesis.'

Step 1: Analyze each option

Begin by considering the first statement, (a): 'Dark reaction is more sensitive to temperature.' Dark reaction, also known as the Calvin cycle, indeed occurs in the stroma of the chloroplast and is sensitive to temperature changes, and is less efficient in high temperatures.

Step 2: Examine the second statement

Now look at (b): 'Light reaction is less sensitive to temperature.' Light reaction, also known as the light-dependent reaction, indeed has less sensitivity to temperature because it relies upon light, not heat.

Step 3: Investigate the third statement

Next, consider (c): 'C4 plant responds to higher temperature and C3 plant have lower optimum temperature.' This is also true, as C4 plants have adaptations that make photosynthesis more efficient in high temperatures and light intensities, while C3 plants do better in cooler temperatures.

Step 4: Evaluate the last option

Finally, look at (d): 'Tropical plant have lower optimum temperature for photosynthesis.' This is false, as tropical plants are typically adapted for higher temperatures, so they will have higher optimum temperatures for photosynthesis.

Key Concepts

Dark Reaction

The dark reaction, often referred to as the Calvin cycle, is a part of photosynthesis that does not require light to occur. This process takes place in the stroma of the chloroplasts.
While the light reaction harnesses the sun's energy, the dark reaction uses ATP and NADPH generated by the light reactions to convert carbon dioxide into glucose.
It's also known as the light-independent reaction.

• Temperature Sensitivity: The dark reactions are temperature-sensitive because they are controlled by enzymes that have an optimal temperature range.
  If the temperature is too low, the reactions slow down; too high, and the reactions can be disrupted.
• Carbon Fixation: During the Calvin cycle, an enzyme called RuBisCO initiates the fixation of carbon dioxide, creating a three-carbon compound.

Thus, the correct statement is that dark reactions are very sensitive to changes in temperature, impacting the efficiency of photosynthesis in plants during fluctuating environmental conditions.

Light Reaction

Light reaction, or light-dependent reaction, is the initial phase of photosynthesis where the light energy is converted into chemical energy. It takes place in the thylakoid membranes of the chloroplasts.
This part of photosynthesis relies on sunlight, which energizes electrons, leading to the creation of ATP and NADPH.

• Lower Temperature Sensitivity: Unlike the dark reaction, the light reaction isn't significantly affected by temperature changes.
  Its primary influencer is light intensity, as the reaction ceases in the absence of sunlight.
• Photolysis of Water: During the light reaction, water molecules are split, releasing oxygen as a byproduct.

This means the process is less sensitive to temperature compared to the dark reaction, aligning with the need for continuous light presence for efficient photosynthesis.

C4 Plants

C4 plants, including species like corn and sugarcane, have a unique adaptation that allows them to efficiently conduct photosynthesis in hot and dry environments.
They do this by minimizing water loss and maximizing carbon fixation, which is effective at high temperatures.

• High Temperature Adaptation: C4 plants thrive in environments with high light intensity and temperatures.
  They have a specialized anatomy - the Kranz anatomy - that helps in concentrating carbon dioxide around RuBisCO, minimizing photorespiration.
• Two-Step Carbon Fixation: Carbon fixation occurs in two stages - once in the mesophyll cells and then in the bundle-sheath cells, effectively concentrating carbon dioxide in specialized cells.

This process enables C4 plants to efficiently photosynthesize in higher temperatures, contrary to C3 plants, which experience reduced efficiency under such conditions.

C3 Plants

C3 plants, such as wheat and rice, are adapted to environments that experience cooler temperatures and moderate sunlight conditions.
Their photosynthetic process is known as the Calvin cycle or C3 cycle, where carbon is fixed directly into a three-carbon compound.

• Optimum Temperature Range: C3 plants have a lower optimum temperature for photosynthesis. They perform best in cooler climates.
  In high temperatures, they are prone to photorespiration, which reduces the overall photosynthetic efficiency.
• Photorespiration Challenge: In warmer conditions, RuBisCO enzyme starts binding with oxygen instead of carbon dioxide, leading to photorespiration.
  This effect is a significant challenge for C3 plants in adapting to high-temperature environments.

Overall, C3 plants struggle with high temperatures due to increased photorespiration, making them less effective during such periods apart from C4 plants which are more adept.