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Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are the two most important biochemical processes of life.

Photosynthesis is the process used by autotrophs to produce food (glucose) utilizing water, carbon dioxide, and chlorophyll in the presence of sunlight, releasing oxygen as a by-product. On the other hand, cellular respiration is a set of metabolic reactions that occurs in all living cells converting glucose into carbon dioxide and liberating energy as ATP.

During photosynthesis, plants trap the radiant solar energy into glucose, whereas, in cellular respiration, this stored energy gets transformed into a usable form. Also, carbon dioxide released as the end product of respiration is harmful to all living beings. Plants and other autotrophs utilize this CO2 and transform it into essential lifesaving oxygen, acting as nature’s air filter.

Compare and Contrast

Photosynthesis and cellular respiration are the two sides of the same coin. They share some features and are also distinct in various ways.

Let us explore some of their differences:

Photosynthesis vs Cellular Respiration

BasisPhotosynthesisCellular Respiration
1. Occurs inOccurs only in photoautotrophs (all green plants, some photosynthetic  bacteria, and algae)Occurs in every living organism
2. LocationThe process takes place in the chloroplast.The process takes place in mitochondria.
3. Type of ProcessAnabolic processCatabolic process
4. Energy Required or ReleasedIt is an endothermic reaction. So, it requires solar energy.It is an exothermic reaction. So, it releases energy in the form of ATP.
5. Requirement of SunlightOccurs only in the presence of sunlight, i.e., daytimeDoes not require sunlight as it occurs all the time
6. ATP ProductionProduce 2ATPs for the synthesis of one molecule of glucoseduring the light reaction by photophosphorylationProduce 38 ATPs by the oxidation of one molecule of glucose (theoretically), but the actual yield is 30-32
7. Function/PurposeIt converts solar energy into chemical energy and then stores that chemical energy in glucose for future use.It releases stored energy by breaking down the glucose molecules and converting them into a usable form.
8. Inputs/ReactantsCarbon dioxide (CO2), water (H2O), and sunlightGlucose and oxygen
9. Output/ProductsOxygen (O2) and glucose (C6H12O6)Water and carbon dioxide
10. Chemical Equation6CO2 + 12H2O + light → C6H12O6 + 6O2 + 6H2OC6H12O6 + 6O2 → 6CO2 + 6H2O + ATP (energy)
11. Stages of the PathwayPhotosynthesis takes place in two stages: Light-dependent stage – In this stage, chlorophyll absorbs energy from sunlight and converts it into chemical energy, storing it in the form of the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy currency molecule ATP. The reactions in this stage take place in the thylakoid membrane within the chloroplast. Light-independent stage or Calvin Cycle – This stage takes place in the stroma of the chloroplast. Here, the CO2 uses the chemical energy harvested during the light-dependent stage to produce GA3P (glyceraldehyde-3-phosphate), which in turn forms glucose.Cellular respiration happens in four stages: Glycolysis – In this stage, glucose gets converted into two molecules of pyruvate. In the end, ATP is produced, and NAD+ is converted into NADH. Pyruvate oxidation – Each pyruvate from the previous step enters the mitochondrial matrix, where they get converted into acetyl CoA. Consequently, carbondioxide is released, and NADH is generated. Citric acid cycle or Krebs cycle – The acetyl CoA combines with a four-carbon molecule and undergoes a cycle of reactions, regenerating the four-carbon starting molecule. In the end, ATP, NADH, and 2FADH2 are produced, and carbon dioxide is released. Oxidative phosphorylation – The NADH and FADH2 formed in the previous steps turn into NAD+ and FAD by depositing their electrons in the electron transport chain (ETC). At the end of ETC, the O2 gets converted into water.
12. What Powers ATP SynthaseA gradient caused due to high H+ concentration in the thylakoid lumen powers ATP synthase.A gradient caused due to high H+ concentration in the intermembrane space of mitochondria powers ATP synthase.
13. What Pumps Protons Across the MembraneDuring cyclic phosphorylation, the flow of electrons through cytochrome bf complex pumps protons into the thylakoid lumen.Complexes I, III, and IV of the electron transport chain act as proton pumps. These complexes capture the energy released by electrons moving downhill and pump out H+ ions from the inner mitochondrial matrix to the intermembrane space.
14. Electron SourcePhotolysis of H2OGlucose, NADH + , FADH2
15. Final Electron AcceptorNADP+ (forms NADPH )O2 (forms H2O)
16. Coenzymes usedNADP+, the oxidized state of nicotinamide adenine dinucleotide phosphate (NADP)NAD+, the oxidized state of nicotinamide adenine dinucleotide (NAD)

How are Photosynthesis and Cellular Respiration Related To Each Other

Despite the differences mentioned above, photosynthesis and cellular respiration share some similarities.

  • Both convert energy into its different forms through a series of redox reactions. All these biochemical reactions are controlled by enzymes, utilizing and producing energy (ATP).
  • Both involve exchange of gases.

How do Photosynthesis and Cellular Respiration Work Together

Though the reactions of photosynthesis and cellular respiration are opposite, they are complementary to each other.

Glucose, the end product of photosynthesis, is used as a reactant in cellular respiration. During photosynthesis, plants take in CO2 and water to produce glucose and oxygen, whereas, in cellular respiration, this glucose gets oxidized to liberate CO2. On the other hand, water gets broken down to form oxygen during photosynthesis, but in cellular respiration, oxygen gets combined with hydrogen to form water. As a result, photosynthesis requires carbon dioxide and releases oxygen, while cellular respiration requires oxygen and releases carbon dioxide. Living cells breathe oxygen, which gets carried through the blood cells, allowing cellular respiration to proceed.

While cellular respiration liberates carbon dioxide into the environment, photosynthesis pulls carbon dioxide out of the atmosphere. In this way, these two processes maintain a stable level of atmospheric O2 and CO2.

This mutually beneficial interrelationship between photosynthesis and cellular respiration enables the survival of living beings.


Photosynthesis and cellular respiration are the two fundamental metabolic processes that help to drive all the cellular activities in the body. All the primary producers in food chains produce food by photosynthesis. In contrast, cellular respiration occurs in all living forms on earth, by which food gets oxidized to obtain the stored potential energy, ATP, that powers almost all the cellular processes. Also, the waste products of one are utilized by another, creating a balance in nature.


Q1. What product of photosynthesis is used in cellular respiration?

Ans. Glucose, the end product of photosynthesis, is used in cellular respiration.

Q2. What role does oxygen play in photosynthesis and cellular respiration?

Ans. Oxygen is released as a by-product in photosynthesis, whereas, in cellular respiration, oxygen helps to oxidize glucose to liberate energy.

Article was last reviewed on Monday, July 5, 2021

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