Photosynthesis

[lightenings]

I am having problems understanding this subject and i wanted to know why plants need to be able to produce ATP by respiration AS well as by photosynthesis. I know its got something to do with using the co2 produced in light reaction ie photosynthesis to create Atp in dark reactions respiration....... i think. Help!!

[jeskill]

As far as I remember (I don't have a textbook handy) one of the end products of photosynthesis is glucose, is it not? Glucose needs to be broken down to make energy -- this can be done very efficiently by mitochondria, which use oxidative phosphorylation to get 30 ATP from one molecule of glucose.

[lightenings]

well a college of mine mentioned something about seeds not being able to photosynthesize and so require the atp made during respiration. She also mentioned winter months where there is a lack of sunlight which would = less photosynthesis......if anyone can elaborate on this would be appreciated.

[Yggdrasil]

The light reaction of photosynthesis produces ATP which is used to make the biosynthestic reactions of the Calvin-Benson cycle (Dark Reaction)thermodynamically favorable.

 

So, one may ask, why do plants use the ATP to produce sugars, when the energy from sugars is just going to be used to make ATP anyway? Well, the answer is that 1) sugars are a more stable form of stored energy and plants can build up larger stores of sugars than of ATP, since ATP/ADP ratios are strictly regulated within the cell. Furthermore 2) sugars play roles other than in energy metabolism. For example, the main structural component of plants, cellulose, is a polymer of glucose molecules. Sugars play other roles including acting as signaling molecules on the cell surface (in the form of glycoproteins and glycolipids) and they also aid in the reproduction of many angiosperms since the sugars in the fruit attract animals which then help disperse the seed in the fruit.

[jeskill]

One other example of that is the sugar in maple tree sap. In the spring, sugars get shuttled into the xylem to increase osmotic potential. Water moves along the gradient into the xylem, the air bubbles that were in the xylem get pulled into solution, then the sugars are used for energy (via respiration) to aid in new leaf growth. This is an example of sugars being used as a) osmotic regulators and then b) energy. Respiration has to occur in this situation because there are no photosynthesizing leaves at this point in time.

[Skye]

In addition, not all of a plants cells photosynthesise or have chloroplasts, but these cells still need to produce ATP to carry out their normal functions. Much of a plant is under the ground, constantly growing, storing nutrients, synthesising protective chemicals, etc. Plastids that become chloroplasts in photsynthetic cells are used for many of these tasks.