Photosynthesis is a key part of a cycle that not only maintains life on Earth but keeps Earth's levels of carbon dioxide and oxygen in balance. Noted by Bruno, and Carnajie (2001) that plants convert carbon dioxide into glucose and oxygen, which animals use in a process of combining food with oxygen to release energy, called respiration. Respiration is therefore the reverse of photosynthesis. Oxygen is used up and carbon dioxide and water are given off (for plants to start photosynthesis) in respiration.
Many biological processes are persisted by cycles that continuously consume and renew one or more key intermediates while production some other major product. (Robinson, 2001) Any living organisms require a continuous input of energy. The main source of energy is solar energy on Earth. Solar energy can be transformed by photosynthesis into a form of chemical energy, called ATP which then can be used by any photosynthetic, and nonphotosynthetic organisms alike. Figure 11 (http://www.alienexplorer.com/ecology/topic3.html , 2001) is the generalized process of photosynthesis, a quite generalized, balanced chemical equation for this reaction is:
Photosynthesis consists of two stages: The light reaction, and light independent reaction. The rate of light reaction can be increased by increasing light intensity (within certain limits) but not by increasing temperature. In contradiction, the rate of light independent reaction, can be increased by increasing temperature (within certain limits) but not by increasing light intensity. (Raven, and Crosby, 1996)
The first step in photosynthesis is the absorption of light by pigments. Chlorophyll is the most important of these because it is essential for the process. It captures light energy in the violet and red portions of the spectrum and transforms it into ATP (the chemical energy) through a series of reactions. Photosynthesis takes place in organelles called chloroplasts within cells. Chloroplasts contain the chlorophylls and other chemicals, especially enzymes (proteins that control specific reactions in cells), necessary for the various reactions. The chemicals involved are organized into units of the chloroplasts called thylakoids, and the pigments are embedded in the thylakoids in subunits called photosystems. Light is absorbed by these pigments and energy is transferred to a special form of chlorophyll a called a reaction center. (Raven, and Crosby, 1996)
There are two photosystems, numbered I and II. Light energy is first trapped by photosystem II, and the energized electrons are boosted to an electron receptor, plastoquinone, or PQ. Water molecules are splited into hydrogen ions and oxygen molecules. The energized electrons are passed along the electron transport chain back to photosystem I through CF1 particles, and energy-rich adenosine triphosphate, or ATP, is generated by chemiosmotic phosphorylation during the process. Light absorbed by photosystem I is then passed to its reaction center, and energized electrons are boosted to its electron acceptor. They are passed by means of another transport chain to energize the coenzyme nicotinamide adenine dinucleotide phosphate, or NADP+, resulting in its reduction to NADPH. The electrons lost by photosystem I are replaced by those passed along the electron transport chain from photosystem II. Finally, the light reaction ends with the energy yield stored in the ATP and NADPH2. (Raven, and Crosby, 1996; Robinson, 2001)
Next step, the dark reaction takes place in the stroma of the chloroplasts. The energy stored in the ATP and NADPH2 is used to reduce carbon dioxide to organic carbon in this process through a series of reactions known as the Calvin Benson cycle. One molecule of carbon dioxide enters at each turn of the cycle, and is initially combined with a five-carbon sugar called RuBP (ribulose 1,5-biphosphate) to form two molecules of a three-carbon compound called PGA (3-phosphoglycerate). Three turns of the cycle produce a three-carbon molecule called PGAL (glyceraldehyde 3-phosphate), two molecules of which combine to form a six-carbon simple sugar. Moreover, the RuBP is regenerated with each turn of the cycle. (Raven, and Crosby, 1996)
