Plants do convert carbon dioxide in glucose, by means of what is called Clorophillian Photosynthesis. This essentially means that they breath in carbon dioxide, and use the light energy to run a complicated series of molecular machines, called enzymes, which pick and mix CO2 (and water) molecules, take them apart and end up with glucose.
The waste oxygen freed is a powerful venom, so much so that the first great extinction did happen when the level of this toxyc gas in the atmosphere did raise too much, kiling off most of the life of the time, that was anaerobic – i.e., didn’t use oxygen. Oh no. Not at all.
Luckyly, some bacteria did manage to use this opxygen to perform the opposite reaction, so that they could live off that waste. Probably at the beginning, it was a simple defense mechanism against this toxic gas, exactly as many bacteria can now take apart toxins which would otherwise accumulate within their body and kill them. It did end up however, as the primary energy production mean for these life forms, which now constitute the great majority of life on Earth. Who knows, may be one day some life form will evolve able to thrive on the mountains of toxic junk that we humans are creating. Think about the Toxic Jungle of Nausicaa of the Valley of The wind.
Anyway. I was told in the elementary school that plants only do this magic at day. At night, when there’s no sun, they do consume parto fo that glucose exactly as other animals do. This keep them alive. It makes sense… at night there’s no light, so they can’t do Photosynthesis. To be fair, a certain amount of glucose burning, unless plants have another magic trick to directly use the energy of the sun to power their system, when this is available. I am not aware of this. So, it’s glucose burning. Which produces the same waste as our ordinaryu burning of fossil fuels, i.e. those CO2 and H2O which the plants sequestrated in first place. But there’s a big difference. Plants and animals have a molecular machinery able to perform this burning in small, controllated steps, so that they extract as much energy as they can from the fuel, storing it into molecules of ATP(Adenosin TriPhosphate), the Euro of the body. Any other energy currency has its ATP-equivalent. And any structure, large or small, has a more or less fixed price in ATP. Please pardon my euro-centricity, but i fell the euro better fit the role of universal currency as compared to the US$. The states composing the euro zone are much more different, like liver and heart and lungs. Yet, all these organs are built of the same kind of cells, tweaked to perform better in that environment. Fit to perform a particular role. Boph! Anyway.
I got sidetracked…
My initial curiosity was: light can now be always present in a plant’s environment. How do they cope with it? do they take advantage of it, or they just shut their eyes at night and do without photosynthesis, even if there’s a light bulb half a metre afar? A bit of both i believe. Forcing the cicradian cycle is bound to cause some stress. Some plants will adapt well, some other will not.
I know that Cannabis, for example does actually take full advantage of constant light – don’t ask me how I know. I do. and no, I do not grow cannabis in my cellar. But cannabis plants are usually kept under very bright lights. I guess they would not mind growing on Mercury, if only we could send there a greenhouse with enough soil, water and air. Hell, it would probably manage without too. So, bright lights. I can’t sleep with bright lights pointing just in front of me.
But if t=it’s not too bright, I can sleep just fine. Do plants have a similar request? that is to say, is there any threshold (in lumen) under which a specific plant will not perform photosynthesis, and will instead switch completely to glucose catabolism? I have no freaking clue. Any suggestion is welcome. 
