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The purpose of photosynthesis is to capture the energy from the sun and to store the energy in a carbon compound.  In photosynthesis, light energy is converted by green plants to chemical energy stored in the bonds of glucose.  Glucose can be broken down to respiration, or it can be stored as starch or as a disaccharide.   Photosynthesis needs water.  Water and nutrients are taken in and transported via the xylem.  

Water moves against the flow of gravity.  There are three ways it travels.  The first way is transpiration.  This is also known as the cohesion theory.  This is controlled by the stomata.  It is the attraction between molecules of water.  The second way water can travel against the flow of gravity is by root pressure.  This occurs when the stomata is closed, usually at night.  Both water and pressure build up.  The third way is capillary action.  This is also called adhesion.  It is the ability of water to cling to other things.  The water travels up a very thin tube.  The thinner the tube, the higher the water goes.  

The Chloroplast

The chloroplast was believed to be photosynthetic bacteria that was engulfed by a host and then entered a symbiotic relationship with the host.  It has its own genetic material, its own ribosomes, and it has an inner and outer membrane.  Each of these membranes is a phospholipid bilayer.  It has a complex system of inner membranes.  

Capturing Light 

Pigments and photocenters are used to capture light.  


One of the pigments is chlorophyll.  There is chlorophyll a (methyl) and chlorophyll b (formyl).  These two have a slight difference in structure, they have a different absorb spectra.  They are complementary, very efficient, and their energy capture is based on photoelectric effect.  This is a good pigment because it has alternating double and single bonds.  Another pigment is carotenoids.  They absorb all wavelengths.  It is these that give the plants the yellow and orange colours.  They are accessory pigments that aren't as efficient.  The third kind of pigment is flavoriods.  This is another accessory pigment.  It is a red colour.    


Most plants and algae use two photosystems to generate ATP.  

Photosystem II:

This photosystem works first.  First the light hits antenna pigments of photocenter.  Electrons of chloporophyll are promoted and energy of excited chlorophyll is transferred to P680 by resonance transfer.   Excited electrons of P680 are actually ejected from the molecule and are transferred to substance Q.  Because of this, P680 is oxidized, and Q is reduced.  Electrons travel from Q to plastoquirrone (PQ) and PQ carries two hydrogens from the stroma to the thylakoid space.  The hydrogens go through the ATPase and thus produce ATP by chemiosmosis.  Electrons are then transferred to cyt F.  By now, they have lost all their energy that they had gained from the photon.  Cyt F transfers the electrons to P700picture of photosystem II

Photosystem I:

The photons hit the photocenter and excite the electrons.  Energy is transferred to P700 by resonance transfer.  P700 raised electrons gained from photosystem II to higher energy level and eventually ejects it.  This is called the photoelectric effect.  Because of this, P700  is oxidized.  The electron moves to FD (ferredoxin).  FD donates a high energy electron to NADP to NADPH, therefore the NADP is reduced.  

P680 lost an electron when it was donated to substance Q.  To replenish the lost electron water is split apart.  This is called Photolysis

Calvin Cycle

The calvin cycle occurs in the stroma.  It isn't directly dependent on light, but it is independently dependent.  There are three phases.  The first phase is carbon fixation.  Here, carbon dioxide is combined with a five carbon sugar called ribulose diphosphate, with the help of enzymes.  The second phase is reduction.  ATP and NADPH from the light reaction, are consumed to form a three carbon molecule of high energy.  The third phase is regennation.  This is where ribulose diphosphate is regenerated to allow another reaction to cycle.  To get one PGAL for use in making carbohydrates you need six of each molecule of carbon dioxide.