Theres an exam question asking how light is a limiting factor, heres my answer (please can you check if its correct/ what more needs to be added?)
How light effects the LIS:
Lack of light will limit photo-phosphorylationLess light means less photons hitting chlorophyll in PSII, this means less excited electrons, which means less moving a long electron carriers and less energy to pump protons into thylakoid space. This means that chemiosmosis is reduced as the amount protons which diffuse involved in chemisomosis are reduced. This means less ATP being made. Also less NADPH. As well as this a lack of light limits photosynthesis as that needs light to hydrolyses water to oxygen, electrons and protons. So again this will limit photophosphorylation.
How light effects the LDS:
Light causes the stomata to open, which allows carbon dioxide to enter. If there is a lack of light, less co2 comes in which limits the amount of RuBP which can be carboxylated, which means less GP, thus less TP- thus less organic molecules produced.Also if lack of light limits LDS, where the amount of ATP and NADPH is limited this limit LIS which needs products of LDS to occur. With a limited amount of ATP and NADPH this means less GP is converted to TP, and less TP is recycled to RuBP the process is also slower. The amount of GP will continue to build up because just CO2 is needed to make it, but the amount of TP and RuBP will reduce as they will be used up to make GP but are not being made as quickly. With the lack of TP this means the amount of organic molecules like hexose sugars being produced is reduced.
How does lack of light effect photosynthesis? watch
- Thread Starter
- 16-02-2016 17:38
- 17-02-2016 22:13
To go into so much depth about light being a limiting factor is definitely quite interesting!
Since I don't know the exact question, I'll try my best and just focus on correcting your response. As a quick disclaimer though I'm in the middle of doing my A levels and so technically many of this is beyond the scope of my 'A level specification knowledge', so feel free to disagree with what I write.
And final point before I begin (and I'll be taking a stab in the dark) is that if LIS = light-independent stage and LDS = light-dependent stage, you have got them the wrong way around.
Light dependent stage (or your LIS):
When you refer to 'light', perhaps the more correct term is light intensity. The reason I would specify this is because it is really the light intensity that you're referring to i.e. the number of photons per unit area per unit time (or for non-physicists = more photons).
So I would rephrase 'less light' as 'lower light intensity'
The next point is the word 'hitting', which although technically true, it isn't the fact that the photons are hitting the chlorophyll to cause excitation of electrons, but rather the absorption of photons that causes this.
Continuing on, I would preferably change 'amount' to 'rate of' (a completely minor point but its good practice anyway)
Where you wrote 'Also less NADPH' didn't really link in with the sentence before, but perhaps more importantly I think (again, don't know the actual question) the main point of photosynthesis is to synthesise the organic compound G3P/TP -> sugars, the synthesis of ATP (photophosphorylation) in my opinion is a much less significant part of it.
Nice point on the splitting of water - I'm sure you know it, it's called 'Photolysis' and you could just add in that the purpose of this is to reduce the chlorophyll so that it has electrons available to be excited.
Light-independent reaction (or your LDR):
Light does indeed cause stomata to open (you definitely don't need to recall the details although it is interesting!)
and carbon dioxide hence is able to diffuse through from the atmosphere.
Again, instead of amount, it's more correct to say 'rate at which'
Ok, I can see why you emphasised ATP production, since yes it is required to reduce GP->TP (the overall reaction anyway). But still I'd like to argue that it is the NADPH that is more likely to be limiting, since ATP can be generated (and in larger quantities) from process such as respiration, so yes although significant, you may want to edge the emphasis more to NADPH.
Beyond that, everything else is very logical (well actually your answer was on the whole completely logical and easy to follow) and correct... so overall, well done!