What Is Photosynthesis and Light Reaction (Calvin Cycle)

The sun is the main source of energy for all animate things on earth. The energy from the sun is received in form of light and heat energy. The heat energy cannot be directly used by plants or animals but warms up the non living environment. The light energy, on the other hand, is directly captured and utilized by plants through the process of photosynthesis after which it will convert it into stored energy. Photosynthesis is the process by which light energy is converted by organisms into chemical energy and stored in the form of sugar (Deborah 23). It is the physicochemical process through which plants, alga and photosynthetic bacteria utilize light energy to synthesize organic compounds. Chlorophyll, a substance contained in plants that captures energy, makes the process of photosynthesis possible. Additionally, the plants contain an organelle in their cells known as a chloroplast. This chloroplast permits the plant to absorb energy from the sun, and its specialized pigments together with the chlorophyll utilize this energy to complete a chemical reaction (Scott 23).

→ 6 CO2 + 6 H2O + energy C6H12O6 + 6 O2 (Koning)

The produced glucose acts as food for the plant and is used to prepare other compounds. The entire process of photosynthesis depends on two distinct reactions; the dark reaction also known as the Calvin cycle and the light reaction. In the light reaction, light is required for it to take place while in the dark reaction, light is not a necessity (Koning). This paper aims to look at the Calvin cycle and the light reaction stages of the process of photosynthesis.

Generally, the light reaction and the Calvin cycle are mutually dependent in that the Calvin cycle requires NADPH and ATP produced from the light reaction and which can only take place in the presence of sunlight. Likewise, the light reaction needs a provision of ADP, Pi, and NADP+ which directly comes from the Calvin cycle. Chlorophyll and other molecules that cause the light reaction occur in the thylakoid membrane whereas the enzymes responsible for catalyzing the Calvin cycle are situated in the stroma”. First, the chlorophyll absorbs light energy from the sun which is then converted by the light reactions into chemical energy in the forms of ATP and NADPH. The ATP is the supplier of energy, while the NADPH delivers electrons for the cycle, which changes carbon dioxide into glucose. The ADP and NADP+ that comes from the Calvin cycle and eventually go back to the light reactions, which renew ATP and NADPH (Koning 56).

The light reaction involves the electron transfer of excited electrons from the sunlight. In brief, “light reactions split water molecules to produce electrons and oxygen gas. Light energy energizes the electrons to higher energy levels within the chlorophyll molecule. They then pass through an electron transfer system within the thylakoid membrane” (Golberg 466). The electrons are at last used to make NADPH. The energy stored in NADPH is used in carbon reduction. Likewise, the light reactions also bring about huge amounts of ATP molecules which are shaped through attaching a phosphate group to an adenosine tri-phosphate (ADP).

The Calvin cycle is a metabolic pathway that happens in the stroma and takes place in a three-step process. It begins with carbon dioxide fixation; a process during which the carbon dioxide from the atmosphere is reacted with a five carbon compound known as ribulose bi-phosphate (RuBP). This enzyme RuBP carboxylase oxygenase RuBisco is used as a catalyst. During this reaction the six-carbon molecule is split into three carbon molecules. This is an all-important step for the process and should take place in order that the process proceeds in a proper way (Golberg 63).

Following this is the carbon dioxide reduction step. This is the process where R-CO2 is turned into R-CH20 and G3P is changed into glucose or other organic molecules. The NADPH and ATP of the light reaction stage must be present during this process as it energizes the 3-PGA thus enabling the process to take place.

Lastly, the final process of the Calvin cycle is called the regeneration stage. The unexpended PGAL is converted by the enzymes from a three-carbon sugar phosphate into five-carbon molecules of bi-phosphate. Again, this process requires a lot of energy and hence ATP must be present in order to add a second phosphate. “All the three steps of the Calvin cycle; carboxylation, reduction and regeneration, involves soluble enzymes” (Golberg 57).

The 3-phosphoglycerate makes an addition of a phosphate from adenosine tri-phosphate to the 3-PGA to form 1, 3-bisphosphoglycerate. The glyceraldehyde-3-phosphate dehydrogenase then takes out hydrogen from NADPH and puts it into 1, 3-bisphosphoglycerate to form glyceraldehyde-3-phosphate (PGAL) (Scott 34). This reduction sees the removal of a phosphate also. The ensuing NADP+ and ADP can be reused in the light reaction stage. These steps clearly establish the reason why the Calvin cycle and the light reaction are mutually dependent upon one another. Without one, the other cannot function (Solomon, Berg and Martin 518).

The 6 PGAL from the Calvin cycle can be used in a number of other different ways such as making sugars, starch and cellulose to be used by the plant and may include being nutrients for an animal. The sugars formed can be made into many other molecules and different chemicals for plants. Other uses include making of flavors, fragrance, medicinal substances, and lignin and so on. Amino acids and proteins originate from the PGAL too. Therefore, although the original product of photosynthesis is sugars, one has to bear in mind that sugars are materials with great diversities regarding the biochemical contexts of plant cells (Koning).

Most plants use the Calvin cycle to fix carbon. Those plants that utilize only the Calvin cycle to fix carbon are referred to as the c3 plants. In these plants, all the three processes, photosynthesis, carbon fixation and the Calvin cycle appear at the same time in one chloroplast. In c4 plants, photosynthesis happens in chloroplasts of thin walled mesophyll cells and a four-carbon acid transmitted over to a thick walled bundle sheath cells where the Calvin cycle occurs in that second-cell chloroplast (Scott 173-176). This shields the Calvin cycle from the impacts of photorespiration. In CAM plants, the first carbon dioxide fixation and photosynthesis take place during the night. The Calvin cycle functions during the day in the same chloroplast as photosynthesis and carbon dioxide fixation. The initial carbon fixation step is therefore different in c4 plans and CAM plants (Scott 173-176).

The Calvin cycle enzymes serve three major functions; they make up the dominant carbon dioxide fixation pathway, achieve the reduction of 3-phosphoglycerate into glyceraaldehyde-3-phosphatemay and finally, they catalyze reactions that transubstantiate three carbon compounds into four, five, six or seven carbon compounds. Moreover, the enzymes of the Calvin cycle can be regulated by other factors such as the light dependent ion movements, rubisco activase and phosphate availability. In the light dependent ion movements, rubisco together with other enzymes are activated by light induced increase in stomal ph. Rubisco works best at a ph of 8 but is slightly inactive at a lower ph. Rubisco is separately regulated by an enzyme called rubisco activase which binds to an inactive rubisco and on ATP hydrolysis shifts the conformation of rubisco into an extremely active form. In phosphate availability, tri-phosphate moves from the chloroplast to the cytoplasm for production of sucrose or respiration. It is important to reuse the phosphate in the chloroplast so that continuous production of ATP is insured (Solomon, Berg and Martin 628-635).

The function of the Calvin cycle is not to solely regenerate its intermediates and ADP and NADP. Each intermediate of the cycle has a phosphate bound to its molecules, for example, 3PGA, triose-phosphate and so forth. The Calvin cycle is normally described as part of the dark reaction of photosynthesis as its activities do not directly require light energy. However, as noticed from the discussion, several enzymes are indirectly changed through the photosynthesis activity of the light reaction. The major regulatory steps of the Calvin cycle are thus inactive in the absence of sunlight and therefore during the night, the photosynthetic fluxes of the cycle close down. In conclusion, both the light reaction and the Calvin cycle are important in the process of photosynthesis and one cannot function without the other.

Works cited

Golberg, Deborah. Barron’s AP Biology. Hauppauge, NY: Barron’s Educational Series, 2010. Print.

Koning, Ross. “Calvin Cycle.” Plant Physiology Information Website. 1994.Web.

Scott, Peter. Physiology and Behaviour of Plants. New York: John Wiley & Sons, 2008. Print.

Solomon, Eldra Pearl, Linda Berg and Diana Martin. Biology. London: Cengage Learning, 2004. Print.

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