Re: Disucss the chloroplast structure and light reactions of photosynthesis.

Dear Ranaee,

Here's an answer to your question that should point you in the right 
direction but definitely does NOT contain ALL I have on the subject.

Chloroplast structure:
Chloroplasts are organelles which are surrounded by an outer and an inner 
membrane (the envelope), with the inner membrane being much more 
selectively permeable (i.e. controlling which solutes enter and exit the 
organelle) than the outer one. The aqueous inside compartment is called the 
stroma. It contains a system of stacked, disc-like membrane vesicles 
(grana) that are connected by unstacked vesicles (stroma lamellae). 
Together, these membrane "sacks" are called thylakoids. The stroma contains 
the biological catalysts (enzymes) which speed up the chemical reactions 
involved in the fixation of carbon dioxide and its reduction to 
carbohydrates. The thylakoid membranes are the sites of the photosynthetic 
light reactions: the harvesting of sunlight, transport of electrons and 
synthesis of ATP (the energy currency of living organisms).

Energy transfer:
Light is absorbed by antenna pigments, chlorophylls (absorb red and blue 
visible light) and carotenoids (e.g. beta-carotene; absorb light of 
wavelengths not absorbed by chlorophylls and are called accessory 
pigments). Together, they make use of a broader range of wavelengths in 
visible light than either pigment type by itself would be able to absorb. 
The pigments are bound to proteins that are anchored in the thylakoid 
membranes. The pigment/proteins are arranged in light harvesting complexes 
that absorb light and pass the light energy from pigment to pigment 
molecule to special chlorophyll molecules that are known as the 
photosynthetic reaction centers. This happens in less than 0.0000000001 sec 
and is over 90% efficient. There are two types of photosynthetic reaction 
centers: photosystem I and photosystem II.

Light reactions:
The energy transferred to the reaction centers excite (activate) the 
chlorophylls and cause them to give up electrons and pass them to other 
electron carriers (i.e. reduce those carriers). The lost electrons are 
replaced with electrons from water in the case of photosystem II. When 
water gives up electrons to photosystem II, its oxygen is oxidized to 
molecular oxygen (O2) and 4 H+ are released. Photosystem I replenishes its 
lost electrons from photosystem II (through a chain of electron carriers). 
The electrons lost from photosystem I are passed to a soluble electron 
carrier named nicotinamide adenine dinucleotide phosphate (NADP+, a 
positively charged molecule). The NADP+ is reduced to NADPH + H+ when it 
accepts the electrons. It, in turn, can provide its electrons for the 
reduction of carbon dioxide to carbohydrate.

In the course of passing electrons from carrier to carrier between the two 
photosystems, the protons (H+) released from water upon oxidation are 
pumped from the stroma to the interior of the thylakoid membrane sacks. 
This difference in proton concentration is maintained because protons 
cannot leak through the thylakoid membranes back into the stroma by 
themselves. An enzyme protein located in the thylakoid membrane, however, 
allows the protons to return into the stroma while at the same time 
synthesizing ATP, the energy currency of the cell. It is necessary to go 
though this rather complicated series of steps since it costs energy to 
make ATP. Letting the protons back into the stroma is like opening 
floodgates of a dammed-up river in a hydroelectric power plant: energy is 
released and can be made use of to make ATP (or drive a turbine in case of 
the power plant). Why does the chloroplast need to make ATP? It costs 
energy to make carbohydrate from carbon dioxide. 

Ultimately, absorbing energy from sunlight by the light harvesting complex 
in chloroplasts leads to the splitting of water and the generation of 
oxygen (O2). The electrons lost from the water in this process ultimately 
are passed on to carbon dioxide, which is converted to carbohydrate in a 
series of steps known as the Calvin-Benson cycle (also known as dark 
reactions; take place in stroma).

I hope I have not confused you with a lot of technical terms, but since you 
asked for a lot of information, I am giving you a lot.  If you need even 
more details, please take a look at the following web site: 
http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html. For a 
collection of botany-related sites on the web, go to: 
http://www.ou.edu/cas/botany-micro/bot-linx/subject.shtml.