Re: Do photosynthesis and respiration occur at the same time

Plants evolve oxygen during photosynthesis in what are called the "light reactions". As the name implies, this only occurs when the plants are exposed to light. Plants use energy captured from light to split water and remove the electrons from it. These electrons are excited by light energy and used to drive the construction of sugars, and other energy rich molecules. Oxygen is given off as a byproduct of the light reactions. This is where the entire supply of oxygen in the atmosphere comes from. The sugars are actually constructed in a separate set of reactions called the dark reactions. They were called the dark reactions because they were found to be separate from the light reactions, and to be carried out even when there was no influx of light energy. However, it is now know that they are regulated by the light reactions and their activity also drops during periods of darkness. During the dark reactions, plants consume carbon dioxide from the atmosphere to construct the sugars from. There are some excellent resources on photosynthesis on the internet at:

http://www.li fe.uiuc.edu/bio100/Link_page/photosyn.htm

Beginning of a particularly informative hypertext series on photosynthesis:

http://esg-www.mit. edu:8001/esgbio/ps/psdir.html

Excellent site with lots of great pictures of photosynthetic process diagrams and molecules involved:

http:/ /gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html

The photosynthesis center at Arizona State University:

http://photosci ence.la.asu.edu/photosyn/default.html

Carbon dioxide (CO2) is produced by organisms as they break down sugars, extract the energy from them, and use it to do work. This process is called respiration. Plants also carry out respiration and give off CO2. This can occur either in the dark or in the light. However, it is down-regulated in the light to allow the plant to accumulate the products of photosynthesis, instead of breaking them down as soon as they are made.

Thanks for the question,
Todd Holland
Graduate Student and Mad Scientist