The Difference between Chlorophyll A & B and Photosynthesis Overview



Photosynthesis is a naturally occurring process that takes place in plants, algae and even some forms of bacteria.  They utlize the sun’s energy to convert CO2 into carbohydrates. Photosynthesis is an essential part of life on Earth.  During photosynthesis CO2 expelled by animals, humans and burning hydrocarbons is absorbed and oxygen is released.  There are some forms of bacteria that perform anoxygenic photosynthesis and no oxygen is released.  Photosynthesis the counterpart of cellular respiration.


Chlorophyll is essential for photosynthesis.  It serves two primary functions in the photo systems. The major function of chlorophyll is to absorb light and transfer that light energy into chemical energy.  There is more than one type of chlorophyll, each type plays a different role and has a unique chemical structure.  Note that providing soluble silicon to plants results in increased concentration of chlorophyll in leaf tissues, resulting in enhanced metabolism resulting from increased use of available light energy.

Let’s take a look at the difference between Chlorophyll a & b.

Chlorophyll a                                                                   Chlorophyll b

The principal photosynthetic pigment                              Accessory photosynthetic pigment

Formula is C55H77O5N4                                                     Formula is C55H70O6N4Mg

Molecular weight of 873                                                       Molecular weight of 907

a & b

Chlorophyll a or Chl-a is a large molecule that has a porphyrin ring with a magnesium atom at its center.  Attached to the porphyrin is a long, insoluble carbon-hydrogen chain which interacts with the proteins of the thylakoids and serves to anchor the molecule in the internal membranes of the choloroplast.

Chl-a is the pigment that interacts directly in the light requiring reactions of photosynthesis.  Chlorophyll b or Chl-b is different from Chl-a in only one of the functional groups bonded to the porphyrin. It is an accessory pigment and acts indirectly in photosynthesis by transferring light it absorbs to Chl-a.  Alternating single and double bonds or conjugated bonds, like the porphyrin ring of chlorophylls, are common among pigments and are responsible for the absorption of visible light by these substances.

Both Chl-a and Chl-b primarily absorb red and blue light, the most effective colors in photosynthesis.  They reflect or transmit green light, which is why leaves appear green. The ratio of Chl-a to Chl-b in the chloroplast is 3:1.

Chlorophyll a

The principal photosynthetic pigment

Exists in al phototrophs with bacteria being the exception

Formula is C55H77O5N4Mg

Molecular weight of 873

Chlorophyll b

Accessory photosynthetic pigment

Exists in all phototrophs other than diatoms, cyanobacteria and algaes

Formula is C55H70O6N4Mg

Molecular weight of 907

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Light Absorption for Photosynthesis

Photosynthesis depends upon the absorption of light by pigments in the leaves of plants.  The most important of these is chlorophyll-a, chlorophyll-b also plays a role.  The measured rate of photosynthesis as a function of absorbed wavelength correlates well with the absorption frequencies of chlorophyll a, but makes it evident that there are some other contributors to the absorption.

The plot of the absorption spectra of the chlorophylls plus beta carotene correlates well with the observed photosynthetic output.  The measure of photochemical efficiency is made by measuring the amount of oxygen produced by leaves following exposure to various wavelengths.  It is evident that only the red and blue ends of the visible part of the electromagnetic spectrum are used by plants in photosynthesis.  The reflection and transmission of the middle of the spectrum gives the leaves their green visual color.