Question

Assertion: There is a decrease in photosynthesis, if the photosynthetic cells are illuminated by light of \(\mathrm{P}_{660} \mathrm{~nm}\) or more wavelength. Reason: In red drop phenomenon the rate of photosynthesis decreases.

Short answer

Yes, the assertion and reason are correct. When photosynthetic cells are illuminated with light of P660 nm or more, there is a decrease in photosynthesis. The reason for this decrease is the red drop phenomenon, where the rate of photosynthesis drops at longer (red) wavelengths due to their lower energy and limited absorption by chlorophyll.

Step-by-step solution

Understanding Photosynthesis

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities. The light-dependent reactions of photosynthesis occur in the thylakoid membranes of the chloroplasts where light energy is converted into chemical energy (ATP and NADPH). However, the efficiency of this process can vary with the wavelength of light.

The Effect of Light Wavelength

Wavelengths ranging from 400 nm to 700 nm provide the energy needed for photosynthesis. This is why this range is referred to as Photosynthetically Active Radiation (PAR). Chlorophyll, the green pigment in most plants, absorbs light most efficiently in the blue (430–450 nm) and red (640–680 nm) regions of the PAR. When plants absorb light of wavelength P660 nm or more, the efficiency of photosynthesis, i.e., the conversion of light energy into chemical energy decreases.

Understanding the Red Drop Phenomenon

The 'red drop' phenomenon is observed in the photosynthetic rate as the wavelength of light approaches the red end of the spectrum (around 680 nm), where the rate of photosynthesis significantly drops. This drop in photosynthesis at longer (red) wavelengths is due to the lower energy of the longer wavelength light and the limited absorption of these wavelengths by chlorophyll. So, the statement in the exercise, 'in red drop phenomenon the rate of photosynthesis decreases' is accurate.

Connecting the Assertion and the Reason

Given the understanding of photosynthesis, the effect of light wavelength, and the red drop phenomenon, it can be said that the assertion and the reason are connected. The decrease in photosynthesis when cells are illuminated by light of P660 nm or above, as mentioned in the assertion, can be explained by the red drop phenomenon – this confirms the reason statement.

Key concepts

Red Drop Phenomenon

The red drop phenomenon is an interesting aspect of photosynthesis where the efficiency of the process is notably affected by the color of light. When photosynthetic cells such as those in plants are exposed to light that falls within a specific range of wavelengths, the rate at which photosynthesis occurs can vary. This phenomenon is observed as a decline in photosynthesis when the light's wavelength extends towards the red end of the spectrum.

Specifically, this occurs beyond about 680 nm, where we see a significant drop in photosynthetic efficiency. This drop is primarily because the energy from longer wavelength light, like red or far-red light, carries less energy. Chlorophyll, the main pigment involved in photosynthesis, does not absorb these longer wavelengths as effectively, contributing to the reduced rate of photosynthesis observed during the red drop.

Light Wavelength

Light wavelength plays a crucial role in the process of photosynthesis. Wavelengths refer to the distance between successive peaks of light waves and determine the color of the light. For photosynthesis, the most crucial range of light lies between 400 nm and 700 nm, known as Photosynthetically Active Radiation (PAR).

Different wavelengths carry different amounts of energy, with shorter wavelengths carrying more energy than longer ones. Within the PAR spectrum, various wavelengths influence photosynthesis efficiency differently. Blue light (around 430–450 nm) and red light (around 640–680 nm) are the most effective for photosynthesis, since these wavelengths are absorbed efficiently by chlorophyll, the pigment responsible for capturing light energy.

Chlorophyll Absorption

Chlorophyll, the green pigment found in plants, plays an essential role in absorbing light energy for photosynthesis. This pigment absorbs light most efficiently at specific wavelengths within the Photosynthetically Active Radiation (PAR) spectrum.

There are two main types of chlorophyll in plants: chlorophyll a and chlorophyll b. Both chlorophylls absorb light exceptionally well in the blue region (430–450 nm) and red region (640–680 nm) of the spectrum. However, they absorb poorly in green wavelengths, which is why plants appear green. The absorption of light by chlorophyll then energizes electrons, initiating the process of converting light energy into chemical energy.

• Chlorophyll a is the primary pigment directly involved in light reactions.
• Chlorophyll b assists in capturing light by expanding the range of wavelengths absorbed.

Photosynthetically Active Radiation

Photosynthetically Active Radiation, commonly abbreviated as PAR, is the range of light wavelengths that plants can use for photosynthesis. This range spans from 400 nm to 700 nm. Within this spectrum, different colors of light contribute to the energy needed for photosynthesis.

PAR includes wavelengths that plants can absorb efficiently to drive the process of converting light energy into chemical energy. Within the PAR range, plants capture a small portion of the light for photosynthesis, effectively utilizing blue and red light wavelengths. Ultraviolet and infrared light fall outside the PAR spectrum. Ultraviolet light carries too much energy and can damage cellular components, while infrared light lacks sufficient energy.

• PAR is essential for understanding how plants harness sunlight.
• The efficiency of photosynthesis can be influenced by how well light is absorbed within this range.