Mechanism of Photosynthesis
Plant physiology deals with the studies of life processes that are similar in many organisms. Thousands of chemical reactions are underway in every living cell transferring water, mineral salts and gases from the environment into organized plant tissue. Plants are also endowed with the property of photosynthesizing, a unique feature that is inevitable for life on earth. Plant function can ultimately be understood on the basis of the principles of physics and chemistry. Studies in plant physiology depend strongly upon plant anatomy, cell biology and structural & functional chemistry. At the same time, the structural sciences of plant anatomy and cell biology become more meaningful because of plant physiology. It deals with the various processes seen in plants throughout its life from the moment of conception when it begins as a zygote until death.[userpro_private]
It is the only mechanism of energy input in the living world. Like energy-yielding oxidation reactions upon which all life depends, photosynthesis involves oxidation and reduction. The overall process is an oxidation of water (removal of electrons with the release of oxygen as a by-product) and a reduction of CO2 to form organic compounds such as carbohydrates.
Photosynthesis is a two component reaction system:
Photosynthesis consists of two types of reactions: a light dependent one and a light independent one. The light dependent reaction is a photochemical reaction, culminating in the generation of NADPH + H+, ATP are energy rich and form the assimilatory power, the source of energy utilized for CO2 fixation can go apace irrespective of whether light is present or absent &therefore are collectively designated, dark reactions. But the driving force for the events of dark reaction is provided by the products of light reaction. It was Blackman, who first recognized that photosynthesis is a two component reaction system. Here are some evidences, experimental and otherwise, that confirms this.
Photosynthetic rate in continuous and intermittent light:
Warburg (1919) found that Chlorella registered a higher rate of photosynthesis, when exposed to intermittent light than when exposed to continuous light. The explanation for this observation is that during the short intervals of dark period, the products of light reaction could be utilized. But, when the algal cells were exposed to continuous light, the products of the light reaction accumulated and not enough of them could be utilized since, no dark reaction followed. This accounts for the shortfall in the products of photosynthesis
1) Employment of inhibitors: Warburg (1920) showed that cyanide added to a photosynthetic system reduced the rate of photosynthesis. This depression in the rat of photosynthesis was much greater under intense light than in weak light, indicating that it is the dark reaction which is affected.
2) Temperature coefficient: The temperature coefficient of any chemical reaction is the ratio between its rates at two temperatures, 10o C apart from one another (Q10). Almost all chemical reactions have a Q10 value of two, i.e., the rate of chemical reaction doubles up, if the temperature is raised by 10oC. Within a certain range of temperature, this is true of photosynthesis also. This means that part of the photosynthesis reactions are regulated wholly by temperature, unaffected by light.
3) Tracer Technique: Green plants kept in light for some time and shifted to a dark chamber thereafter, continued to produce the end product of photosynthesis. Radioactive isotopes supplied under such conditions always showed out in an array of products, wholly considered photosynthesis. These experiments provide direct proof of theory that photosynthesis consists of a light reaction and dark reaction.
4) Work of Arnon’s group: This group of workers successfully separated the lamellar and stroma parts of chloroplasts. The lamellar fragments, on exposure to light produced NADPH2, ATP and molecular oxygen, the last of which escaped from the system. The assimilatory power, consisting of NADPH2 and ATP, when fed to the stroma fragment of the chloroplast yielded products of photosynthesis is made up of two sets of reactions, one light dependent and another light-independent, but they also identified the physical locations where those reactions occour.[/userpro_private]