| MadSci Network: Biochemistry |
hi narjis,
Thanks for your question. It's great that you're interested in respiration and photosynthesis - they are surely two of the most fundamental biochemical process on earth.
I think by your question that you are a little confused about the concepts of reversibility, equilibrium and thermodynamics, so let's look at these difficult ideas. I guess when you say "reversible", you mean that respiration generates carbon dioxide and water from glucose (sugar), whereas photosynthesis does the opposite. Sometimes you see a simple chemical equation which summarised each process like this:
photosynthesis 6CO2 + 12H2O => C6H12O6 + 6O2 + 6H2O respiration C6H12O6 + 6O2 => 6CO2 + 6H2O
Those equations certainly summarise the reduction of CO2 to glucose and the oxidation of glucose to CO2 and they look like the reverse of each other. However, you need to remember that these equations are just simplified summaries for a much more complex process.
You might like to have a look at the KEGG pathway database, which is a database of biochemical reactions and the enzymes that catalyze them. Don't be worried if it look complex and hard to follow - it is! Here, for instance is a map showing all of the reactions in a process called glycolysis, which is just the first stage of respiration:
GlycolysisHere is the next stage which uses the product from glycolysis and is where the carbon dioxide is generated - it's called the TCA cycle:
TCA cycleAnd here is the last step of respiration, where products of the TCA cycle are oxidised to generate ATP (energy):
ATP generationAll of those reactions and enzymes are summarised by that simple equation that we saw before. So in one sense, respiration and photosynthesis are the reverse of each other, but that's not the same as saying that they are reversible. Parts of each pathway can certainly go in either direction though, as you'll see if you look at the arrows on those maps. Just to complete this discussion, here's the photosynthesis pathway:
Photosynthesisand the products from that go on to another pathway named CO2 fixation, which makes the sugar:
CO2 fixation
Phew, enough pathways! Next question - do these processes represent equilibrium? No, they don't. In equilibrium, we would have a situation where the overall concentrations of reactants and products stayed constant. In other words, you can imagine a plant cell which is generating sugar by photosynthesis, but also oxidising sugar by respiration and the overall amount of sugar, water, carbon dioxide and oxygen is staying the same. This is unlikely ever to be true in a plant cell, or a whole plant or on the whole earth, because the concept of equilibrium really only applies to what's called a closed system. You can imagine a pot of boiling water with a sealed lid - water is constantly turning to steam and condensing on the lid back to water, with the amounts of steam and water staying the same. But living organisms are not closed like this - they exchange materials and energy with their environment. You might like to look at what Wikipedia has to say about equilibrium.
Thermodynamics is a large and complex subject, especially when applied to biochemistry. I'm going to cheat here and summarise by saying that basically, things happen in cells because processes drive them to happen. If you have one chemical process generating an excess of a molecule (like ATP), then that molecule is available to do work and drive other processes and so you have a complex, interacting system of reactions. Again, I recommmend Wikipedia for more about this subject.
I hope that helps with your questions,
Neil
Try the links in the MadSci Library for more information on Biochemistry.