Inner membrane represented as a series of staked membranes = GRANUM (GRANA = plural). Each GRANUM is made up of a number of individual membrane units called a THYLAKOID. The portion of photosynthesis which is called the LIGHT REACTION (LIGHT DEPENDENT Rx--requires light energy in order to occur) takes place in the Grana. (see p. 113, Fig. 7.6 in text)
Photosynthesis is responsive to certain wave lengths of light energy (see Fig. 7.3, 7.4 & 7.5, Pg. 110-111 in text). The visible spectrum, from Red to Violet light is effective in photosynthesis The most effective colors of light energy are in the Red and Blue/Violet range of the color spectrum. These are the wavelengths of light that are absorbed most strongly by plant pigments, particularly the PRIMARY (and dominant) pigment in photosynthesis, CHLOROPHYLL a.
The energy conversion/storage process called photosynthesis occurs in autotrophic plants within organelle called chloroplast. This organelle is double membrane bound structure; necessary pigments are concentrated within inner membrane of chloroplast.
are the tiny structures in plant cells where photosynthesis happens. Chloroplasts contain , a green pigment that absorbs light energy for photosynthesis. However, chloroplasts are not found in all plant cells. For example, they are not found in . These cells absorb the water needed by the plant for photosynthesis. However, since they are usually underground and in the dark, they are unable to photosynthesise anyway.
Chlorophylla-a is the primary for in plants. Its structure is shown at left. It has the composition C55H72O5N4Mg. It exhibits a grass-green visual color and absorption peaks at 430nm and 662nm. It occurs in all photosynthetic organisms except photosynthetic bacteria.
in plants is dependent upon capturing light energy in the pigment , and in particular chlorophyll a. This chlorophyll resides mostly in the and gives leaves their green color. The range of light absorption in leaves is extended by some such as the , but does not cover the entire visible range - that would make the leaves black!
Some plants and plantlike organisms have developed other pigments to compensate for low light or poor use of light. Cyanobacteria and red algae have phycocyanin and allophycocyanin as accessory pigments to absorbe orange light. They also have a red pigment called phycoerythrin that absorbs green light and extends the range of photosynthesis. The red pigment is found in vegetables. Some red algae are in fact nearly black, so that increases their photosynthetic efficiency. Brown algae have the pigment fucoxanthin in addition to chlorophyll to widen their absorption range. These red and brown algae grow to depths around 270 meters where the light is less than 1% of surface light.
In the molecular structure of glucose, C6H12O6, note that the carbon is provided by the CO2 in the Dark Rx. The hydrogen is provided by the NADPH, which is produced in the Light Rx and carried to the Dark Rx; the stored chemical energy as part of the glucose molecule is provided by the ATP (from the Light Rx). The overall photosynthesis process, from start to finish is summarized in the text on page 116 and in Figure 7.10. Be sure to review this material carefully.