Question

Wavelengths The rate of photosynthesis in a green plant, measured by ${\mathrm{O}}_2$ production, is higher when illuminated with light of wavelength $680\mathrm{nm}$ than with light of wavelength 700 $\mathrm{nm}$. However, illumination by a combination of light of 680 $\mathrm{nm}$ and $700\mathrm{nm}$ gives a higher rate of photosynthesis than light of either wavelength alone. Explain.

Short answer

Using both 680 nm and 700 nm light enhances photosynthesis due to synergistic effects in the chlorophyll absorption spectrum.

Step-by-step solution

Understanding Wavelengths and Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. Chlorophyll, the primary pigment in plants, absorbs light primarily in the red and blue wavelengths.

Clarifying the Role of 680 nm and 700 nm Light

The light with a 680 nm wavelength corresponds to red light, which is absorbed efficiently by chlorophyll in plants, particularly chlorophyll a at its peak absorption. Light with a 700 nm wavelength is slightly less efficiently absorbed but still within the red range.

Reason for High Photosynthesis Rate with 680 nm Light

When illuminated by 680 nm light, the rate of photosynthesis is high because this wavelength matches the peak absorbance of chlorophyll a, allowing for efficient light absorption and conversion into chemical energy.

Interpreting 700 nm Light Illumination

Although 700 nm light is absorbed less efficiently than 680 nm light, it still contributes to photosynthesis, just at a lower rate. This is because the absorption spectrum of chlorophyll declines slightly as you approach 700 nm.

Synergistic Effect of Combined Wavelengths

When both 680 nm and 700 nm lights are used together, they illuminate different but overlapping parts of the chlorophyll absorption spectrum, compensating one another's weaknesses. This combination allows more electrons to be excited and transferred through the electron transport chain, increasing overall photosynthesis efficiency compared to using either wavelength alone.

Key concepts

Wavelengths

Wavelengths are a fundamental concept in understanding how light interacts with photosynthetic organisms. They define the distance between consecutive peaks of a wave of light, measured in nanometers (nm). In the context of photosynthesis, different wavelengths correspond to different colors of visible light.

Photosynthesis is most efficient when light energy is absorbed by pigments within the chloroplasts of plant cells. These pigments, primarily chlorophyll a and b, have specific preferences for certain wavelengths, which can influence the rate of photosynthesis. Light with a wavelength of 680 nm, which falls within the red region of the visible light spectrum, is particularly effective at driving photosynthesis due to its alignment with the absorption peak of chlorophyll a.

Chlorophyll Absorption

Chlorophyll absorption is crucial for the photosynthetic process. Chlorophyll a and b are the main pigments in plant cells that absorb light energy. These pigments are responsible for capturing light and initiating the electron transport chain within the chloroplasts.

Chlorophyll a absorbs light most efficiently at two primary peaks: around 430 nm in the blue region and 662 nm in the red region of the light spectrum. Chlorophyll b complements this absorption by capturing light around 453 nm and 642 nm. Thus, chlorophyll a is most efficient at utilizing red light, like that at 680 nm, which explains its vital role in photosynthesis. Despite 700 nm being absorbed less efficiently, it still contributes to overall light absorption, complementing the energy collected by chlorophyll.

Light Spectrum

The light spectrum encompasses all the wavelengths of light, including those visible to the human eye and beyond. It ranges from shorter wavelengths, like blue and violet light, to longer wavelengths, such as red and infrared light. Each wavelength of light carries a different amount of energy, influencing how pigments like chlorophyll absorb and utilize them for photosynthesis.

In photosynthesis, the visible spectrum—ranging approximately from 380 nm to 750 nm—is most relevant. While green plants appear green because they reflect rather than absorb green light, it's the red and blue wavelengths that are efficiently absorbed by chlorophyll. The utilization of specific wavelengths like 680 nm and 700 nm illustrates how plants have evolved to maximize energy capture from various parts of the light spectrum, leading to higher photosynthesis rates when these wavelengths are combined.

Electron Transport Chain

The electron transport chain (ETC) is a critical component of photosynthesis. It refers to a series of protein complexes and molecules within the chloroplasts that transfer excited electrons, resulting from light absorption, through various reactions.

When chlorophyll absorbs the photons of light, particularly at 680 nm, electrons are elevated to a higher energy state. These high-energy electrons are then transported through the ETC, leading to the generation of ATP and NADPH, which are essential energy carriers for the synthesis of glucose during photosynthesis.

• Each wavelength contributes differently to this process: while 680 nm light efficiently supplies high-energy electrons, 700 nm light supports continuity when combined with other wavelengths.
• This synergistic effect of different wavelengths engaging the ETC is why a mixture of 680 nm and 700 nm light results in a higher photosynthesis rate than either alone.

By optimizing light absorption through varied wavelengths, plants enhance their energy production efficiency, ensuring survival and growth.