Photosynthesis

Every year, the visible portion ofm the suns energy, what we call sunlight, converts about 200 billion tons of carbon dioxide into more complex and useful organic molecules. Most of this occurs in the ocean. A by-product of this reaction is oxygen, and it is from this reaction that the earths oxygen is supplied. Photosynthesis is the means by which life on earth is possible today. The atmosphere of the earth prior to life was anaerobic, containing no oxygen. All of our atmostpheric oxygen comes from the biological process of photosynthesis.

The essence of this process is that energy from the sun is used to split water into it's constituents of Hydrogen and Oxygen. The hydrogen is used to convert carbon dioxide into more useful hydrocarbons (organic molecules). The oxygen is released into the atmosphere.

The family of molcules responsible for photosynthesis are collectively known as photosynthetic pigments. Of these, the chlorophylls are the major contributors and of this group, chlorophyll a, is the principal pigment found in algae and higher plants.

What else does chlorophyll do.

Absorbs light energy

Can pass this energy to other chlorophyll molecules

Can use this energy to split water.

What makes a pigment

Chlorophyll, like all photosynthetic pigments is a hydrocarbon, a complex structure composed primarily of carbon and hydrogen atoms with other elements such as oxygen, nitrogen and even metal atoms mixed in. Carbon has four sites where it can bond to other atoms. These bonds can form singly with another atom, or a double or even a triple bond can form. The type of bond has a direct effect on the shape of the resultant molecule.

All photosynthetic pigments possess a long, regular chain of carbon atoms which are bonded by alternating single and double bonds, an arrangment called a conjugated system. The reason this arrangement creates color in these molecules is because of groups of highly mobile electrons that result from conjugated systems. These are known as pi electrons. Electrons are generally associated with individual molecules in a chemical bond but pi electrons can move freely throughout the conjugated system. This arrangement also allows them to absorb light energy more readily than more rigidly bound electrons and the actual amount of energy they are able to absorb is quite literally reflected in the wavelength of light they react with. Thus the pi electrons in chlorophyll require 41-42 kilocalories of energy to excite its pi electrons to a energized state which corresponds to a light wavelength of 680 millimicrons or what we would call red light. When full sunlight illuminates a chlorophyll molecule and the pi electrons absorb the red light what we see is what is reflected back or what is left over. We see green. This is where plants get their green color.

Passing it on

The conjugated system of alternating single and double bonds is what gives pigments their ability to absorb light. In the chlorophylls, this chain of bonds is turned back on itself to form a ring known as a porphyrin. When a conjugated chain absorbs light energy the pi electrons oscillate back and forth along the chain. In a porphyrin the activated electrons can circulate around the ring. This resonance has significance in photosynthesis. This property allows chlorophyll to retain and transfer energy from one molecule to another. Thus, if a chlorohyll has absorbed a quanta of light and then receives another, it can pass some of this energy to a non-energized neighbor. This dramatically increases the efficiency of photosynthesis as light energy that might otherwise be lost is utilized.

Phototropism is the tendency to be attracted to light and in the context of this site refers to this tendency in plants. In plants, the wavelengths of light that are employed for phototropism are different than those used in photosynthesis so it seems logical that a different photoactive pigment is responsible. The red wavelengths of sunlight are used in photosynthesis whereas phototropism appears to use the wavelengths found in the blue, violet and green ends of the spectrum. Thus, the pigments used for phototropism must be a yellow color as this pigment would be absorbing that end of the spectrum from sunlight. All plants that exhibit phototropism have these pigments, known as carotenes.

It is also known that some invertebrates exhibit phototropism. Those that do appear to be sensitive to the same wavelengths of light as plants and thus are believed to use carotenes as well to provide the organism with the ability to detect light and grow differentially in response. This is an important clue in understanding the basis of vision as we shall soon see.