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Cellular Respiration
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Photosynthesis is a series of interrelated reactions, known as a biochemical pathway.  It is made up of two stages: the light reactions and the Calvin cycle.

         Light Reactions
Chlorophyll transfers its excited state into useful biosynthetic energy. A system of membrane-bound enzymes transfers the electrons taken by chlorophyll from water down an energy pathway. When it does that it pumps hydrogen ions across the thylakoid membrane. It generates a charge separation across the membrane, due to the concentration of a positive charge on the side to which the hydrogen has been transferred. This charge separation is alleviated by allowing the hydrogen ions to pass back through the membrane through an ATP-synthase pump that generates ATP with the electric potential produced by the charge separation. The electrons are finally transferred to NADP, producing NADPH which can be used as a source of reductive power for biosyntheses.
Pigments form aggregates on the thlakoid membrane called photosystems. The purpose of the photo systems is to collect energy over a broad range of wavelengths and concentrate it to one molecule called a reaction center which uses the energy to pass one of its electrons on to a series of enzymes.
In photosystem I energy is absorbed by a pair of 700 chlorophyll a molecules raising electrons to an excited energy level. Photosystem II works together with photosystem I to absorb enough energy to separate the oxygen of a water molecule from its electrons. Photosystem II is the second photosystem to develop in most higher autotrophs. This is the first half of the photosynthesis half reaction:
Photosystem II also contains chlorophyll a, as well as up to 50% chlorophyll b. Photosystem I works independently of photosystem II to produce ATP through a process called cyclic photophosphorylation. In this process, if too little ATP is produced relative to a NADP, then an alternative route is used to take the electron from ferrodoxin of photosystem I and move it to the plastoquinone of photosystem II instead of being used to make more of NADPH.
The products of the light reactions (ATP and NADPH), are then used in the Calvin Cycle to produce organic compounds.

The Calvin Cycle

The Calvin Cycle is the second stage of photosynthesis. This stage is also referred to as the dark reactions. The Calvin Cycle is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar. The carbon atoms from carbon dioxide in the atmosphere are fixed into organic compounds. The Calvin Cycle only occurs in the stroma of the chloroplast. The Calvin Cycle produces a compound called G3P. Most of the G3P molecules are then converted into RuBP to keep the Calvin Cycle working in an orderly fashion. One out of six G3P molecules is used to make organic compounds, including amino acids, lipids, and carbohydrates. C3 plants are fix carbon using only the Calvin Cycle. 

C4 and CAM pathways fix carbon through alternative processes.  C4 plants fix carbon into a four carbon molecule before using the carbon from this molecule to begin the Calvin Cycle. This is done to prevent excessive water loss in the plant. Most C4 plants are tropical. CAM plants take carbon dioxide in at night and fix the carbon into organic compounds. Then, in the day, the compounds enter the Calvin cycle CAM plants include cacti and other desert plants. 

The rate of photosynthesis is affected by light intensity, carbon dioxide levels, and temperature. The rate of photosynthesis increases and then reaches a plateau as light intensity or carbon dioxide concentration increases. The rate of of photosynthesis increases as temperature increases until it reaches a peak. If temperature continues to rise, the photosynthesis rate will decrease.

(More on the Calvin cycle)


Cactus plants use the CAM pathway.

The next stage of energy transfer is cellular respiration.  Heterotrophs receive stored energy from autotrophs and other heterotrophs by eating them.  However, this stored energy can not be used by the heterotroph until it has been broken down by the processes of cellular respiration.

To find out how these processes work, click the picture below.


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