Photosynthesis and Cellular Respiration: Physiological and Chemical Cycles

The relationship Between Cellular Respiration and Photosynthesis

Cellular respiration is the reaction that converts biochemical energy into ATP releasing the waste product whereas photosynthesis is the process by which plants make their food taking in carbon dioxide and water. Therefore, these two processes are completely different and work inversely where cellular respiration takes glucose and makes carbon dioxide whereas in case of photosynthesis, carbon dioxide is used to make glucose. Though both this cycles are completely different still, they are dependent upon each other for the continuity of life because the released carbon dioxide in cellular respiration is used for the process of photosynthesis.

If we talk individually, photosynthesis is the process by which plants make food by using carbon dioxide and water along with sunlight and gives out oxygen. The formula here is:

6CO2 + 6H2O +LIGHT = C6H12O6 + 602

From this equation, it would not be difficult to find out the conclusion that carbon dioxide when taken up makes glucose and release oxygen. Likewise, the other reaction that tells about carbon dioxide being produced from cellular respiration is:

C6H12O6 +6O2 = 6CO2 + 6H2O

Both the above reaction shows the important of these processes for continuity of life. Beside this it is important to know that how co2 from cellular respiration is used by plants for the process of photosynthesis. Hence, for this it is important to first understand some of the fact much more clearly in every step.


Here two words are involved i.e. photo called light and synthesis is to make. Hence photosynthesis is the process of making food by plants (also called autotrophs) by using carbon dioxide, water and light from the sun. This may sound simple but the process is little complex that involves mainly two steps: 1 light reaction and 2. Calvin cycle. Light reaction takes place in thylakoid membrane of chloroplast .Water and energy from the sun is converted to ATP and NADPH and oxygen is given out. ATP and NADPH are then used along with carbon dioxide in the Calvin cycle to make sugar. Calvin cycle on the other hand takes place in the stroma region of the chloroplast.


In the thylakoid membrane, the process is little more complex. Here light when comes in and passes through photosystem I and photosystem II. Firstly, light when comes in does two works; one it helps in excitement of chlorophyll and the other that helps in transport of electron down the chain. This electron goes from photo system II and then to photo system I and finally to NADPH. These electrons that are transferred to electron transport chain come for the water that is spitted into hydrogen ion and oxygen.

Chloroplast Stroma

Therefore, oxygen is the product that we get in the light reaction and the released electro travels though the complete system increases the number of proton in the lumen of thylakoid than is used by ATP synthase to form ATP from ADP. This way we get NADPH and ATP from the process of light reaction. In case of Calvin cycle, NADPH and ATP given during light reaction are used along with carbon dioxide to make sugar, particularly glucose.

Coming to cellular respiration, many cycles are involved here that breaks glucose to form carbon dioxide. They are: glycolysis, Krebs’s cycle and finally electron transport system.

Glycolysis: It takes place in cytoplasm where glucose is broken to form two pyruvate. This involves different steps that are well described in the diagram below.


Therefore, in process of glycolysis, finally 2 ATP and 2 NADP are produced. Then is the Krebs cycle also called Calvin cycle to TCA cycle.

Calvin cycle

It takes place in mitochondria that produces NADH and FADH2.

Calvin cycle

Electron transport system

In this the molecules of pyruvate are arranged and rearranged in the Calvin cycle, at the same time carbon dioxide and electrons are released and these electrons are transferred to electron transport system just like that of photosynthesis.

Electron transport system

Therefore, it is noteworthy that during the process of photosynthesis plants use carbon dioxide, water and energy from sun and reversely during respiration plants liberate all these and rather use oxygen and glucose that are product of photosynthetic reaction. From this it can be made clear that photosynthesis and cellular respiration are connected where the product form by one is used in the other process that maintains the existence of life on the planet.

how photosynthesis works

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