Unit 4 bio photosynthesis

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Not quite... okay, let's look at the entire picture: You place a plant under continuous sunlight. What happens?
1. RuBisCO gets activated:
"The central role of RuBisCO in the fundamental process of photosynthesis means that it must be tightly regulated, to ensure it is active only where and when it should be. One important layer of this regulation is the activation of RuBisCO at the beginning of the day. During the night, the RuBisCO active sites are blocked by inhibitors or misfired reactants. These inhibitors include “daytime” substrates, such as RuBP, as well as specific inhibitors to ensure that RuBisCO is only active when there is a source of free energy from sunlight (otherwise, RuBisCO will use the free energy released by the breakdown of sugars via respiration, which constitutes a futile cycle – RuBisCO uses energy to fix carbon as sugars in the CBB cycle, then breaks these sugars down to re-release the energy so that it can fix more carbon!)." CBB is Calvin cycle.

2. Since RuBisCO has been activated and the leaves are receiving photons, photosynthesis is ready to go. The stomata will open to allow CO2 to enter the leaf for light-independent stage of photosynthesis. The rate of photosynthesis is high.

3. After some time has passed, the continuous sunlight has caused a heating effect in the plant. If allowed to conitnue, it can cause denaturation of certain enzymes (not RuBisCO yet...that starts to denature at temperatures over 50C) and can cause the plant to dry up.

4. Since the stomata are already open, water diffuses out and starts evaporating as part of a cooling mechanism. There is now a danger of the plant losing too much water through evaporation. So the stomata start to close. This prevents CO2 from diffusing in. This leads to a lower rate of photosynthesis as CO2 is essential for the light-independent stage despite the presence of sunlight.

5. Apart from all these factors, the continuous sunlight can also cause photo-oxidation of chlorophyll to make matters worse.

And this is how excess/continuous sunlight can be harmful for a plant. So although the LI stage can occur during the day, the CO2 it needs and the water the LD stage needs may not always be available in the presence of continuous sunlight. There are always some limiting factors that prevent the plant from sustaining a high rate of photosynthesis.

You can find out more on these 2 links: ( http://www.tankonyvtar.hu/en/tartalo...n/ch03s03.html ) and ( https://cambridgecapp.wordpress.com/...hesis/rubisco/ ).

Hope this helps