Plants are said to be autotrophic: their organic matter is synthesized from the substances (water and mineral salts) they extract from the soil or from the aquatic media in which they live.
The energy required to perform this process of synthesis is provided by the sun. This energy is captured by assimilating pigments (chlorophylls) present in the chloroplast of plant cells or in specialised regions of the cell membranes of procaryotic cells (cells with no nucleus). 

Photosynthesis can be described in terms of the following general formula:

1) The principle

The structures responsible for photosynthesis form the photosystem: this system consists of groups of several hundreds of chlorophyll molecules surrounded by the thylakoid (a structural unit composed of sacs and  vesicles), where the photosynthesis takes place. 
In eucaryotes (organisms composed of cells with individual nuclei), there are two kinds of photosystems: I and II (or P700 and P680, respectively). The accessory pigments absorb the light and transport the energy from one molecule to another from the periphery of the system to the reaction centre, consisting of a specialised pair of chlorophyll a molecules. When excited by photons, these molecules are able to produce electron acceptor electrons.

 

Diagram of a photosystem

The electrons excited by light are then accepted by molecules forming an electron transport chain. These reactions occurring within the thylakoid membranes are known as “ photochemical reactions".

2) the light phase in the process of photosynthesis: the two types of photochemical reactions

Cyclic and acyclic photophosphorylation are both  photodependent reactions.

• cyclic photophosphorylation:

This is the simplest pathway taken by excited electrons.

- ATP (the high-energy molecule Adenosine Triphosphate) is produced, but no O₂ or NADPH (the redox potential molecule Nicotinamide adenosine diphosphate).  

- The excited electrons leave the chlorophyll in the reaction centre, travel along a short electron transport chain and return to the reaction centre.

- During a series of oxydoreduction (redox) steps,  the electron is transported from one protein to another.

- All these processes occur within the inner thylakoid membrane.

ATP is produced indirectly by the proton motor force (via an electrochemical gradient) due to the transfer of the protons from the outside to the inside of the thylakoid membrane.

• acyclic photophosphorylation

This reaction involves the two photosystems (I et II) and the reaction  centres (P700 et P680). 
Upon being excited by light energy, an electron leaves the chlorophyll molecule in photosystem II. To compensate for this loss, the molecule in question recovers an electron via the photolysis of the water molecule :

This results in the production of O₂ and ATP (indirectly via the proton motor force) and NADP+ is reduced to NADPH and H+.
The water is therefore the electron donor and NADP+ is the final acceptor;
the O₂, released into the atmosphere is used for cell respiration purposes.

The light phases therefore convert the solar energy captured by the pigments into chemical energy, which is stored in the high-energy ATP molecules and the NADPH (redox potential) molecules.  ATP is therefore synthesized as the result of the proton motor force and ATP synthetase, which triggers the reaction  ADP + Pi ---> ATP.  
The formation of these two molecules favours the binding of CO₂: this is known as the Calvin cycle.  

3) the dark phase in the process of photosynthesis: the Calvin cycle 

The Calvin cycle takes place in the stroma of eucaryotic chloroplasts.
This is the final stage in the process of  photosynthesis in which the ATP and the NADPH produced during the previous photochemical reactions are used.
This cycle consists of a series of biochemical reactions controlled by various enzymes, which result in the reduction and incorporation of atmospheric CO₂ into the organic molecules.

The key enzyme in this cycle is Rubisco, which enables CO₂ to bind to RuBP:  Rubisco, or ribulose-1-5-biphosphate carboxylase, accounts for up to 16 % of all the protein present in the chloroplast; it is one of the most indispensable and abundant proteins on the earth.

This cycle is repeated 6 times (i.e., CO₂ is incorporated 6 times), yielding one glucose molecule, for example. This glucose will subsequently be used to synthesize polysaccharides, fatty acids, amino acids, nucleotides and all the other molecules on which the life of the plant depends.

Other sites:

http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html (nombreux schémas explicatifs)

http://www.lbte.univ-mrs.fr/photweb.html

http://www.brunette.brucity.be/lgmlej/04SetT/04001CoupdSol/13-photosynth.htm

http://mars.rever.fr/Articles/AlguesSymbiotiques.html