The best‐understood reaction for the synthesis of glucose, and probably the most important quantitatively, is photosynthesis. Photosynthesis converts carbon from carbon dioxide to glucose with reducing equivalents supplied from water and energy supplied from light.
The energy in light is dependent on its wavelength, and is given by the following relationship.
The Greek letter nu, ν, stands for the frequency of the light, h is a constant called Planck’s constant, c is the speed of light, and λ is the wavelength. In other words, the energy of light is inversely proportional to its wavelength. The longer the wavelength, the less energy it contains. In the visible spectrum, the highest‐energy light is toward the blue or violet end, while the lowest‐energy is to the red.
Two photosynthetic reactions
Photosynthesis involves two sets of chemical events, termed the light and dark reactions. This terminology is somewhat misleading, because the entire process of photosynthesis is regulated to take place when an organism absorbs visible light. The light reactions refer to the set of reactions in which the energy of absorbed light is used to generate ATP and reducing power (NADPH). The dark reactions use this reducing power and energy to fix carbon, that is, to convert carbon dioxide to glucose. Biochemically, converting CO 2 to glucose without light is possible if a supply of reducing equivalents and ATP are available.
In higher plants, both the light and the dark reactions take place in the chloroplast, with each reaction set occurring in a different substructure. In electron micrographs, the chloroplast is seen as a series of membranes that come together to form grana, or grains, set in the stroma, or spread‐out region as seen in Figure . Within the grana, the membranes stack upon each other in a disk‐like arrangement called the thylakoid. Each region of the chloroplast is specialized to carry out a specific set of reactions. The light reactions occur in the grana and the dark reactions occur in the stroma.
Chlorophyll and the action spectrum of photosynthesis
The green color of the chloroplast (and therefore plants) comes from the chlorophyll that is stored in them. Chlorophyll is a tetrapyrrole ring system with a Mg2+ ion in the center, coordinated to the nitrogen of each pyrrole ring. The tetrapyrrole ring system is found as a bound cofactor (a prosthetic group) in many electron‐carrying proteins, enzymes, and oxygen transporters. For example, tetrapyrroles are essential to the functioning of cytochrome c, various mixed function oxidases, and hemoglobin. Chlorophylls differ from other tetrapyrroles by possessing a long, branched phytol joined to the tetrapyrrole in an ether linkage. The phytol is an “anchor” to keep the chlorophyll inside the thylakoid membrane.
- Photosynthesis begins with the absorption of light in the thylakoid membrane. The energy of the light makes a difference in its effect on photosynthesis. The following considerations can help you understand this concept.
- The energy of a single photon of light is inversely proportional to its wavelength, with the visible region of the spectrum having less energy per photon than the ultraviolet region, and more than the infrared region. The energy of the visible spectrum increases from the red wavelengths through the blue and violet, according to the mnemonic ROY G. BIV (red, orange, yellow, green, blue, indigo, violet).
- Ultraviolet light, which has more energy than blue light, does not support photosynthesis. If it did reach the earth’s surface, ultraviolet light would be energetic enough to break carbon‐carbon bonds. The bond‐breaking process would lead to a net loss of fixed carbon as biomolecules were broken apart. Fortunately, the ozone layer in the atmosphere absorbs enough UV radiation to prevent this from occurring.
- Chlorophyll comes in two varieties, chlorophyll a and chlorophyll b. Although the wavelengths at which they absorb light differ slightly, both absorb red and blue light. The chlorophyll reflects the other colors of light; the human eye sees these colors as green, the color of plants.
- Other pigments, called antenna pigments, or accessory pigments, absorb light at other wavelengths. The accessory pigments are responsible for the brilliant colors of plants in the autumn (in the Northern Hemisphere). The breakdown of chlorophyll allows us to see the colors of the accessory pigments.
- The antenna pigments and most chlorophyll molecules don’t participate in the direct light reactions of photosynthesis. Instead they are part of the light‐harvesting complex, which “funnels” the photons they capture to a reaction center, where the actual reactions of photosynthesis occur. All together, the light‐harvesting complex is over 90 percent efficient—almost all the photons that fall on the chloroplast are absorbed and can provide energy for synthesis.
- Chlorophyll a and chlorophyll b participate in aspects of the light reaction; each must absorb a photon for the reaction to occur.