A2 photosynthesis?

Hi could someone please help me make a distinction between:

The role of the PSII vs the role of the PSI in photosynthesis?

The role of the light dependant stage relative to the role of the light independent stage in photosynthesis?

I'm getting a little mixed because I know the mechanisms but when it comes to questions where, say the light dependant stage is compromised the effects it has on photosynthesis I get suck. So I just need someone to outline for me their roles....

Please

Hi could someone please help me make a distinction between:

The role of the PSII vs the role of the PSI in photosynthesis?

The role of the light dependant stage relative to the role of the light independent stage in photosynthesis?

I'm getting a little mixed because I know the mechanisms but when it comes to questions where, say the light dependant stage is compromised the effects it has on photosynthesis I get suck. So I just need someone to outline for me their roles....

Please

Hey, I hear you, photosynthesis is very confusing if not broken down step by step.

Just to revise, there are 2 types of photosynthetic pigments found in chloroplasts: chlorophyll and carotenoids. Chlorophyll has 2 sub-types called chlorophyll-a and chlorophyll-b. Chlorophyll-b and carotenoids are collectively known as accessory pigments. Chlorophyll-a is considered to be the reaction centre where light energy is converted to chemical energy.

Accessory pigments + reaction centre = photosystem (PS)

There are 2 types of PS: PSI and PSII.
PSI absorbs light of wavelength 700 nanometres. PSII absorbes light of wavelength 680 nanomentres.

Accessory pigments absorb light energy of different wavelengths and transfer it to reaction centres by resonance.

So, we know that photosynthesis consists of 2 main reactions: light-dependent (LD) and light-independent (LI). In LD, light energy is captured by chlorophyll and used to produce ATP and reduced NADP. In LI, the previously formed ATP and NADP are used to convert CO2 to glucose.

The LD reaction takes place in the thylakoid membrane and is the photoactivation of chlorophyll. This is composed of 2 stages: photolysis of water and phosphorylation.
Phosphorylation of ADP to ATP can be cyclic or non-cyclic depending on the pattern of electron flow in one or both photosystems.

Cyclic phosphorylation involves only PSI: Light energy is absorbed by PSI and transferred to the reaction centre where the electrons get excited to a higher energy level and are emitted from the chlorophyll-a molecule. They are captured by electron acceptors and passed back to a chlorophyll-a molecule via a chain of electron carriers. During this process, energy is released to synthesis ATP from ADP and inorganic phosphate by the process of chemiosmosis.

Non-cyclic phosphorylation involves both PSI and PSII: It also involves the photolysis of water which leads to a continuous unidirectional flow of electrons from water to PSII to PSI to NADP. Electrons expelled from a chlorophyll-a molecule never return to the same molecule.
Excited electrons expelled from PSII are transferred to PSI, leaving PSII positively charged. Positively charged PSII acts as a strong oxidising agent and splits water into hydrogen ions, electrons and oxygen. Oxygen is evolved as a by-product. The electrons produced replace the ones lost by PSII. The hydrogen ions combine with electrons from PSI and the carrier molecule NADP to form reduced NADP.

Thus, the ATP and reduced NADP formed in the LD reaction are passed on to the LI reaction stage.

The LI reaction also called the Calvin cycle takes place in the stroma and involves the fixation of CO2 in which CO2 combines with a 5-Carbon sugar called ribulose bisphosphate (RuBP) to give 2 molecules of a 3-Carbon compound gylcerate-3-phosphate (GP). Each GP is then reduced to triose phsophate (TP) which after 6 turns of the Calvin Cycle forms glucose. ATP and reduced NADP from the LD reactions are needed for the reduction reactions of the Calvin cycle.

So, if light energy or water are limited, the LD stage is compromised so ATP and reduced NADP are not generated so the LI stage cannot function so photosynthesis as a whole breaks down. If CO2 is a limiting factor then only LI stage is compromised.

Hope this helps

P.S. Here's a link that might help: http://www.majordifferences.com/2013/02/difference-between-cyclic-and-non.html#.WAiYPdR97ak

Oh my goodness thank you! This was very helpful