Energy Transformation 2

ENERGY TRANSFORMATION 2
At the end of the lesson, you shall be able to:

• discuss the role of pigments in photosynthesis
• illustrate how chlorophyll absorbs and transforms light energy
• define and describe a photosystem
Pigments
Pigments are substances that absorb visible light. Different pigments absorb light of different
wavelengths.
Light, as it encounters an object, is either reflected, transmitted, or absorbed. Visible light, with a
wavelength of 380–750nm, is the segment in the entire range of electromagnetic spectrum that is most important
to life on earth. It is detected as various colors by the human eye. The color that is not absorbed by pigments of
objects is transmitted or reflected and that is the color of the object that we see.
The Electromagnetic Spectrum
Pigments are the means by which plants capture sun’s energy to be used in photosynthesis. However,
since each pigment absorbs only a narrow range of wavelength, there is usually a need to produce
several kinds of pigments of different colors to capture more of sun’s energy.
Chlorophyll
Chlorophyll is the greenish pigment found in the thylakoid membrane inside the chloroplast of a plant
cell. The figure below shows the location and structure of a chloroplast.
Chlorophyll absorbs blue and red light while it transmits and reflects green light. This is why leaves
appear green. There are several kinds of chlorophyll. Among these, chlorophyll a plays the most important role in
photosynthesis. It directly participates in converting solar energy to chemical energy.
Other pigments in the chloroplast play the part of accessory pigments. These pigments can absorb
light and transfer the energy to chlorophyll a. One of these accessory pigments is chlorophyll b. Some
carotenoids also contribute energy to chlorophyll a. Other carotenoids, however, serve as protection for
chlorophyll by dissipating excessive energy that will otherwise be destructive to chlorophyll.
Structure of Chlorophyll
• Head—a flat hydrophilic head called porphyrin ring. It has a magnesium atom at its center. Different
chlorophylls differ on the side groups attached to the porphyrin.
• Tail—a lipid-soluble hydrocarbon tail.
How does photoexcitation of chlorophyll happen?

1. A chlorophyll molecule absorbs photon or light energy.
2. An electron of the molecule in its normal orbital, said to be in its ground state, will be elevated to
an orbital of a higher energy. The molecule is now in an excited state. The molecule only absorbs
photon that has the energy that is equal to the energy needed for it to be able to elevate from the
ground state to the excited state.
3. The excited state is unstable. Hence, excited electrons drop back down to the ground state
immediately after, releasing energy in the form of heat and photon. This happens in isolated
chlorophyll molecules. However, chlorophyll molecule that is found in its natural environment in the
thylakoid membrane forms a photosystem together with proteins and other organic molecules to
prevent the loss of energy from the electrons.
Photoexcitation of Chlorophyll
Photosystem
A photosystem is an aggregate of pigments and proteins in the thylakoid membrane responsible for
the absorption of photons and the transfer of energy and electrons. It is composed of:
• Light-harvesting complex— is also called the ‘antenna’ complex and is consisted of several different
pigments (chlorophyll a, chlorophyll b, and carotenoids) bounded with proteins. When a pigment
molecule absorbs a photon, energy is passed on from one pigment molecule to another pigment
molecule until the energy reaches the reaction center.
• Reaction-center complex—is composed of a pair of chlorophyll a and a primary electron acceptor.
The primary electron acceptor is a specialized molecule that is able to accept electrons from the
pair of chlorophyll a. The pair of chlorophyll a in the reaction-center is also specialized because they
are capable of transferring an electron to the primary electron acceptor and not just boosting the
electron to a higher energy level.
There are two types of photosystem:
• Photosystem II—was discovered later after the discovery of Photosystem I, but functions first in the light
reaction of photosynthesis. The chlorophyll a in the reaction-center of Photosystem ll effectively absorbs light
with a wavelength of 680nm and thus called P680.
• Photosystem I—was discovered first. Its reaction-center has a chlorophyll a called P700 because it is
effective in absorbing light with a wavelength of 700nm.

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ENERGY TRANSFORMATION 2

At the end of the lesson, you shall be able to:

The Electromagnetic Spectrum

How does photoexcitation of chlorophyll happen?

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