Chlorophyll: The Start of It All

In order for light energy to be used by living systems, it must first be absorbed. Photosynthesis occurs in small organelles called chloroplasts, which located inside cells of plants and algae, within the cytoplasm. Chlorophylls are the light-absorbing compounds, or pigments, that are found inside chloroplasts.

Fig. 2 click here if you can't see the above picture.

Each chloroplast has three membrane systems, inner, middle, and outer. (DuTemple, 2000) Chloroplasts are complex (and very useful) organelles, they can be thought of as a factory, providing the plants with food and energy. In Figure 2 (http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html, 2001), it shows a detailed structure of a chloroplast. The inner membrane or thylakoid membrane of chloroplasts, contains most of the proteins required for the light reaction (first stage of photosynthesis). Robinson (2001) noted that the Calvin Benson cycle (second stage of photosynthesis) enzymes that capture CO₂ and produce carbohydrate are located in the water phase (stroma) of the chloroplast outside this photosynthetic membrane.

The thylakoid membrane is a double layer of lipids embedded with proteins, which is called the phospholipid bilayer. A critical feature of the photosynthetic membrane is that it forms a vesicle that defines an inner and an outer water space. (Curtis, and Barnes, 1989) This bilayer is arranged in thick "stacks" called grana. Each granum is composed of thylakoids, they are flattened, membranous sacs surrounded by a protein-rich solution called the stroma. All the thylakoids in a chloroplast are oriented parallel to each other. Therefore, by swing toward the light, the chloroplast can simultaneously aim all of its (no need to be surprised) millions of pigments molecules for optimum reception, since they are miniature electromagentic antennae. Thylakoids contain different types of light-absorbing pigments, of which chloropyll are the most abundant. Red and blue light are mostly absorbed by chlorophyll, and green light is reflect by it. This is why most plants are green. (And my favourite colour too.)

Different groups of plants and algae use various pigments in photosynthesis, and different kinds of chlorophyll vary slightly in their molecular structure. In plants, chlorophyll a is a pigment directly involved in the transformation of light energy to chemical energy. A second type of chlorophyll in plants, is chlorophyll b, which can also be found in most photosynthetic cells. A representative of another group of pigments called the carotenoids, which are red, orange, or yellow pigments. Beta-carotene is the kind found in plants. Examples such as those of a ripe tomato (yum), as well as when leaf cells stop synthesizing chlorophyll in the autumn. (Curtis, and Barnes, 1989; DuTemple, 2000; Robinson, 2001)

The next step of photosynthesis, the light dependent reaction, converts the light energy into chemical energy. Let us find out how it works.

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