Introduction

Where does the organic carbon and energy in many biological systems come from? It comes from the following overall reaction:

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CO2 + H2O -----> [CH2O] + O2

which in a more general form may stated as follows:

CO2 + 2 H2A -----> [CH2O] + H2O + 2A

Labeling studies, using 18O-water and carbon dioxide, demonstrated that the oxygen evolved in photosynthesis is derived from water and that the oxygen in carbon dioxide ends up in the carbohydrate and in water (but not oxygen).

CO2 + H2O -----> [CH2O] + H2O + O2

CO2 + H2O -----> [CH2O] + H2O + O2

Structure of the Chloroplast:

Chloroplasts are the organelles of photosynthesis; each leaf cell may have 20-50. Like the mitochondrion, the chloroplast has an outer membrane and an inner membrane. The contents of a chloroplast are called the stroma (similar to the matrix of the mitochondrion); in the stroma are thylakoid disks. which contain chlorophyll-bearing molecules. A stack of these disks is called a granum.

The chlorophylls (a and b):

Before discussing chlorophylls, we should briefly review the nature of light energy:

E = hn

where:

h = Planck's constant = 6.626 x 10-34 Joule-sec

n = the frequency of the light, and

E = the energy of the light in joules

Chlorophyll is contains a substituted tetrapyrrole structure much like heme; it differs from heme in that it has:

a. Fe2+ <--- Mg2+

b. 1 pyrrole ring is partially reduced

c. cyclopentanone ring fused to a pyrrole

d. phytol (C20H39OH) chain attached (4 isoprene units) = 4 x 5

Chlorophyll a contains a methyl group, whereas chlorophyll b contains a formyl group.

This difference in structure results in the two molecules having different absorption spectra. (insert diagram here).

At this point there are two different types of chlorophyll (a and b for each of the two photosystems), in addition, each type exits in two different chemical environments!

The existence of the two environments was recognized when it was calculated that only one oxygen was produced for each 2500 chlorophyll molecules in the leaf. One type of chlorophyll is called antenna chlorophyll; the second is called reaction center chlorophyll. The antenna chlorophyll collect light energy and transfer it (in about 10-12 sec.) to the reaction center.

Photosystems I and II:

The study of photosynthesis is another example of the use of inhibitors.

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2 H2O + chloroplasts + 4 Fe3+CN ----> 4 Fe2+CN + O2 (Hill reaction)

This photosynthetic system gave evolution of oxygen without reduction of CO2 (formation of glucose). By dividing the process of photosynthesis into two halves, biochemists were able to simplify the task of understanding each in detail.

Further study of the light reactions revealed that there are two photosystems. Each one is driven by different wavelengths of light, which represent two different levels of energy.

Upon absorbing 700 nm light, photosystem I produces a strong reductant (NADPH) and a weak oxidant. Upon absorbing 680 nm light, photosystem II produces a weak reductant (xxx) and a strong oxidant (O2).