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(Photosynthesis )is a process in which light energy is trapped and converted into chemical energy by colored compounds (pigments) such as chlorophylls, carotenoid, and xanthophylls. (more on Photosynthesis Pigments .)The chemical energy is ultimately stored in sugars and other organic compounds. This process of photosynthesis is carried out by photosynthetic bacteria, such as cyanobacteria, eukaryotic algae and by chloroplasts in higher plants.
The Summary Equation for Photosynthesis is:
6 Carbon dioxides + 12 water = Glucose + 6 oxygen molecules + 6 water.
This equation only gives the reactants (left side of the equation) and the products (right side of the equation). The photosynthesis equation is exactly opposite the Summary Equation for Respiration.
Glucose + 6 oxygen molecules + 6 water = 6 Carbon dioxides + 12 water.
Therefore, the products of photosynthesis are the reactants for respiration and the products of respiration of reactants of photosynthesis. These are equal and opposite reactions. This relationship or balance between photosynthesis and relationship is important in both individual plants and in ecosystems. Photosynthesis storing energy and respiration releasing energy.
Photosynthesis consists of :
two main sets of chemical reactions
known as:
Phostosynthesis in higher plants will be our main focus of study and it occurs in the organelle known as the chloroplast, an ovoid structure that consists of outer and inner membranes. In higher plants,
The pigments that trap light energy are clustered into groups known as photosystems which are embedded in the thylakoid membranes. Photosystems have action center of chlorophyll "a" which is surrounded by other chlorophyll a's chlorophyll b's and carotenoids and xanthophylls known as the antennae pigments. Antennae pigment help absorb other wavelengths of light and funnels the energy into the action center which becomes highly energized so that the chlorophyll a's lose electrons. The chemical energy that is stored by the light reactions are stored temporarily in the organic compounds known as NADPH and/or ATP. Photosynthetic pigments absorb a range of wavelengths of light (an absorption spectrum) and affect the rate of photosynthesis. (Action Spectrum
Two basic types of photosystems have been recognized because they maximally absorb slightly different wavelengths of light and store the light energy they trap in different chemical forms. The two different photosystems are referred to as photosystem I and II. Photosystem I's action center maximally absorbs 680 nanometer wavelengths and Phostosystem II s action center absorbs 700 nanometer wavelengths maximally.
CHLOROPHYLL
Photosystem I may work cooperatively with photosystem II known as the Non-cyclic electron flow the light energy trapped will then be stored in the form both NADPH and ATP.
ATP
Electrons leaving photosystem I will go to an oxidized NADP and reduce it. The electrons lost from photosystem I will be replaced by electrons from photosystem II via an electron transport chain between the two photosystems. Photosystem II will replace its lost electrons by splitting water and stealing the electrons from the Hydrogen that was in the water. The oxygen in water will be released as a by-product of light reactions. When the electron is taken from the Hydrogen, its proton will be pumped into the thylakoid compartment making an acid environment which diffuses through ATP synthase complex port favoring the formation of ATP from ADP and P. The proton once escaped from the thylakoid compartment will join the NADP reduced molecule to make NADPH. When the electrons flow from Photosystem II to I - it is called non-cyclic electron flow.
If photosystem I works alone or independ of Photosystem II, the electrons from energized Photosystem I do not go to reduce NADP, they will return to its own action center by way of an electron transport chain using this energy to favor making only ATP. This is called cyclic electron flow when only ATP is produced from photosystem I.
The Calvin Cycle
Once ATP and NADPH is produced by the light reaction in the grana, sugar can be synthesized in the stroma. The simplest protocol necessary to form a glucose molecule is each of six pre-existing 5-C sugars known as 1 -5 Ribulose bisphosphate (RuBP) in the stroma have a Carbon dioxide molecule joined to it by an enzyme called Rubisco. This forms six unstable 6-C compounds each of which breaks down in two 3 phosphoglyceric acid (3 - PGA) forming a total of 12. 12 ATP's then energizes each of the 3 - PGA forming 12 1-3 diphosphoglyceric acids (1-3 PGA). 12 NADPHs are used to reduce each of these 1-3 PGA by adding Hydrogens to form 12 glyceradehyde-3-phosphates (GAP, also known as PGAL ) ( a removal of an inorganic phosphate is also required). Two of the 12 glyceradehyde-3-phosphates can be used to form a glucose, while the other 10 are used to reconstitute the six 1 -5 Ribulose bisphosphate (RuBP) to make one complete turn around the Calvin Cycle.
