Chlorophyll is a green photosynthetic pigment found in plants, algae and cyanobacteria. It is responsible for the green color of leaves and plants and plays a critical role in photosynthesis – the process by which plants convert sunlight into chemical energy. There are several different forms of chlorophyll, but the two most common ones found in higher plants are chlorophyll a and chlorophyll b.
Chlorophyll a and Chlorophyll b
Chlorophyll a and chlorophyll b are very similar in structure but have different light absorption spectra. Chlorophyll a has a blue-green color and absorbs wavelengths from 430-662 nm, whereas chlorophyll b is yellow-green and absorbs light from 450-642 nm.
During photosynthesis, chlorophyll a plays a primary role in light absorption and energy transfer. It is found in all photosynthetic organisms. Chlorophyll b assists chlorophyll a by extending the range of wavelengths that can drive photosynthesis. It is found in land plants and green algae.
Chemical Structure
The chemical structure of chlorophyll consists of a chlorin ring surrounded by a long hydrophobic phytol chain. The chlorin ring contains bonded magnesium (Mg) at its center. The different absorption spectra of chlorophyll a and b are due to the subtle differences in the composition of the chlorin ring.
Specifically, chlorophyll a has a methyl group on ring II whereas chlorophyll b has an aldehyde group instead. This aldehyde group causes a shift in the absorption spectrum of chlorophyll b.
Biosynthesis
Chlorophyll a and chlorophyll b are produced from a common precursor called protochlorophyllide. The enzyme protochlorophyllide reductase catalyzes the conversion of protochlorophyllide into chlorophyllide a. This is then converted into chlorophyll a by the addition of phytol chain and magnesium.
Chlorophyll b is formed when the methyl group on ring II of chlorophyll a is oxidized to an aldehyde group by the enzyme chlorophyllide a oxygenase (CAO). Therefore, chlorophyll b is directly synthesized from chlorophyll a and not from protochlorophyllide.
Functions
As mentioned before, the main role of chlorophyll a is to absorb light energy and transfer it during the light-dependent reactions of photosynthesis. The phytol chain of chlorophyll a helps anchors it in the thylakoid membrane in the chloroplast.
Chlorophyll b acts as an accessory pigment, transferring the light it absorbs to chlorophyll a. It increases the range of light wavelengths usable for photosynthesis. This is useful in shaded environments where there is less blue-violet light.
Absorption Spectra
The graph below shows the difference in the absorption spectra of chlorophyll a and chlorophyll b:
| Wavelength (nm) | Chlorophyll a | Chlorophyll b |
|---|---|---|
| 400 | 0.1 | 0.1 |
| 425 | 2 | 1 |
| 450 | 13.5 | 7 |
| 475 | 29 | 24 |
| 500 | 50 | 52 |
| 525 | 71 | 69 |
| 550 | 82.5 | 71.5 |
| 575 | 85 | 65 |
| 600 | 82.5 | 48 |
| 625 | 77.5 | 27.5 |
| 650 | 72.5 | 7.5 |
| 675 | 42.5 | 2 |
As seen from the graph:
- Chlorophyll a has a higher absorption peak at wavelength 425 nm and 675 nm.
- Chlorophyll b has a higher absorption peak at wavelength 450 nm.
- The peak absorption of chlorophyll a is at 675 nm (red light) while chlorophyll b is at 650 nm (orange light).
- Chlorophyll a absorbs blue and red light more strongly than chlorophyll b.
Natural Abundance
In land plants, the ratio of chlorophyll a to chlorophyll b is approximately 3:1. The proportions may vary depending on the species, growth stage and light conditions. Generally, young leaves contain more chlorophyll b compared to mature leaves.
This is because chlorophyll b helps absorb blue-violet light that penetrates young leaves. As leaves mature and thicken, less blue light is absorbed so less chlorophyll b is required.
Importance of Accessory Pigments
Plants have other accessory pigments like carotenoids and anthocyanins that extend the wavelength range over which light can drive photosynthesis. Carotenoids absorb in the 400-500 nm blue-violet region while anthocyanins absorb in the 500-600 nm green region.
So while chlorophyll a is the primary pigment, chlorophyll b and other accessory pigments allow photosynthesis to take place over a larger spectrum of light. This is especially useful in shaded environments.
Color Change in Fall
In temperate regions, the green summer foliage of trees and plants turns into a kaleidoscope of yellow, orange and red hues in autumn. This is caused by the breakdown of chlorophyll as daylight hours decrease.
The decreasing chlorophyll unmasks the other pigments like carotenoids and anthocyanins that produce the warm colors. Leaves with more chlorophyll b tend to turn more yellow compared to leaves with just chlorophyll a.
Conclusion
In summary, chlorophyll a and chlorophyll b are structurally similar green photosynthetic pigments that absorb different wavelengths of light.
Chlorophyll a is the primary pigment that participates directly in the light reactions. It absorbs strongly in the red and blue regions.
Chlorophyll b is an accessory pigment that transfers energy to chlorophyll a. It extends the absorption spectrum by absorbing more blue-violet light.
The different absorption spectra is due to the aldehyde group on ring II of chlorophyll b, compared to the methyl group on chlorophyll a. Chlorophyll b aids photosynthesis in shaded environments with less blue light penetration.
So in essence, the key difference between the pigments of chlorophyll a and chlorophyll b is the presence of the aldehyde group in chlorophyll b, which causes its absorption spectrum to be shifted towards the blue-violet region compared to chlorophyll a. Their differing roles and abundance allow plants to harness a wider range of light wavelengths for photosynthesis.