What Are the Roles of Chlorophyll A & B?

Ever wonder why plants are green? The color is due to a specialized organic molecule found within plant cells called chlorophyll. Chlorophyll absorbs certain wavelengths of light and reflects green light. When that reflected light enters your eyes, you perceive plants as green.

You may be wondering, why does chlorophyll absorb and reflect light?

TL;DR (Too Long; Didn't Read)

Chlorophyll's role is to absorb light for photosynthesis. There are two main types of chlorophyll: A and B. Chlorophyll A's central role is as an electron donor in the electron transport chain. Chlorophyll B's role is to give organisms the ability to absorb higher frequency blue light for use in photosynthesis.

What Is Chlorophyll?

Chlorophyll is a pigment or a chemical compound that absorbs and reflects specific wavelengths of light. Chlorophyll is found within cells in the thylakoid membrane of an organelle called the chloroplast.

Pigments such as chlorophyll are useful for plants and other autotrophs, which are organisms that create their energy by converting light energy from the sun into chemical energy. The primary role of chlorophyll is to absorb light energy for use in a process called photosynthesis — the process by which plants, algae and some bacteria convert light energy from the sun into chemical energy.

Light is made up of bundles of energy called photons. Pigments like chlorophyll, through a complex process, pass photons from pigment to pigment until it reaches an area called the reaction center. After photons reach the reaction center, the energy is converted into chemical energy to be used by the cell.

The main pigment used by organisms for photosynthesis is chlorophyll. There are six distinct types of chlorophyll, but the main types are chlorophyll A and chlorophyll B.

Role of Chlorophyll A

The primary pigment of photosynthesis is chlorophyll A. Chlorophyll B is an accessory pigment because it is not necessary for photosynthesis to occur. All organisms that perform photosynthesis have chlorophyll A, but not all organisms contain chlorophyll B.

Chlorophyll A absorbs light from the orange-red and violet-blue areas of the electromagnetic spectrum. Chlorophyll A transfers energy to the reaction center and donates two excited electrons to the electron transport chain.

The central role of chlorophyll A is as a primary electron donor in the electron transport chain. From there on, the energy from the sun will ultimately become chemical energy that can be used by the organism for cellular processes.

Role of Chlorophyll B

One of the main distinctions between Chlorophyll A and B is in the color of the light that they absorb. Chlorophyll B absorbs blue light. Chlorophyll B’s central role is to expand the absorption spectrum of organisms.

That way, organisms can absorb more energy from the higher frequency blue light part of the spectrum. The presence of chlorophyll B in cells helps organisms convert a wider range of the energy from the sun into chemical energy.

Having more chlorophyll B in chloroplasts of cells is adaptive. Plants that receive less sunlight have more chlorophyll B in their chloroplasts. An increase in chlorophyll B is an adaption to the shade, as it allows the plant to absorb a broader range of wavelengths of light. Chlorophyll B transfers the extra energy it absorbs to chlorophyll A.

Structural Differences Between Chlorophyll A and B

Both Chlorophyll A and B have very similar structures. Both are “tadpole” shaped due to a hydrophobic tail and hydrophilic head. The head consists of a porphyrin ring, with magnesium in the center. The porphyrin ring of chlorophyll is where light energy is absorbed.

Chlorophyll A and B differ in only one atom in a side-chain on the third carbon. In A, the third carbon is attached to a methyl group whereas, in B, the third carbon is attached to an aldehyde group.

Outline of Differences Between Chlorophyll A and B

Chlorophyll A:

  • The primary pigment of photosynthesis
  • Absorbs violet-blue and orange-red light
  • Blueish green in color
  • Methyl group(-CH3) at the third carbon

Chlorophyll B:

  • An accessory pigment of photosynthesis
  • Absorbs blue light
  • Olive green in color
  • Aldehyde group (-CHO) at the third carbon

References

About the Author

Lorin Martin is a writer, editor and science communicator. She holds a Bachelor's degree in Neuroscience from Transylvania University.

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