Why is the photosynthetic reaction kept unbalanced?
To perform the process of photosynthesis, a plant requires the energy provided by sunlight to be able to carry out this cellular work. The organic molecules and oxygen they generate can be used by eukaryotes as fuel for the process of cellular respiration. Prokaryotes can also use glucose as fuel during fermentation.
The set of chemical reactions performed by living things is called metabolism. The reaction of photosynthesis is an anabolic pathway where energy must be used by the organism in order to make complex molecules from simple less complicated ones.
Energy is required by living organisms for all metabolic processes to continue because energy allows an organism to do cellular work. If the amount of energy was balanced, no amount of work could occur. Energy can exist in many forms and can be transformed from one form to another.
Organisms are not closed systems and matter as well as energy is transferred between a living thing and its environment. For photosynthetic organisms, light energy enters the leaf by absorption and carbon dioxide enters through pores called stomates in the leaf. Water is transported up to the leaves via conducting tubes called xylem within veins. With these reactants, photosynthesis will then produce sugar, oxygen and water. The sugar is a stored energy reserve that can be used as a fuel for cellular process. Some energy exits the plant as heat.
The energy transfer from light to heat within a plant illustrates that plants don't produce energy however, they can transform light to chemical energy and heat.
In terms of photosynthesis, if the organism and its environment were at equilibrium, it would indicate that the system could do no work. However, when photosynthesis occurs, the organism will absorb energy from its environment which is an endergonic reaction. Sunlight is required to drive this process--photosynthesis needs an investment of energy to proceed. To convert carbon dioxide and water to sugar, +686 kcal/mol of energy is needed. This represents change in free energy (G) which is required to power the process of photosynthesis. A cell at equilibrium has a change of free energy of zero and is unable to do any cellular work--it would be dead.
In terms of photosynthesis, the sun provides a source of free energy each day to carry out this endergonic process. The chemical energy stored in the bonds of glucose can be used as a fuel by living organisms to allow them to continue to carry out important cellular work and as long as there is a supply of fuel, the cell will not be at an equilibrium so they can continue to remain alive and perform necessary life functions.
From a chemical reaction standpoint, the process of photosynthesis does result in a balanced chemical equation: 6CO2 + 6H2O --> C6H12O6 +6O2. However, in order for this process to continue, there must be an input of carbon dioxide and water. Something else must happen to provide that carbon dioxide and water. This is where cellular respiration comes in, with its equation of C6H12O6 + 6O2 --> 6CO2 + 6H2O which is chemically the reverse of photosynthesis. (I use the term 'chemically' here because there are some profound differences between the processes of cellular respiration and photosynthesis).
Plants and animals (and yes, bacteria, too) both use the process of cellular respiration to break down glucose (sugar) and create ATP which the cells can directly use as energy for all life processes. So by using the glucose during cellular respiration and thus releasing carbon dioxide and water, organisms are replenishing the ingredients for photosynthesis so that plants can make more glucose. (Keep in mind, thought, that plants aren't picky. They will take any carbon dioxide and water generated by any means, not just cellular respiration)
So, I wouldn't say that the reaction is kept unbalanced. Instead, there is a continual cycle happening between photosynthesis and cellular respiration that keeps both of them going.