Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are the two essential life processes on Earth that help living organisms survive. They are interrelated such that the products of one process are the reactants of the other and thus work in a cycle.

How are Photosynthesis and Cellular Respiration Related

Photosynthesis is a process by which green plants and algae make food in the form of carbohydrates (C₆H₁₂O₆) using water (H₂O), carbon dioxide (CO₂), and chlorophyll in the presence of sunlight as the energy source. Water and oxygen are produced as byproducts. In contrast, cellular respiration breaks down carbohydrates produced through photosynthesis and uses oxygen to produce energy and carbon dioxide. The carbon dioxide is again utilized during photosynthesis to continue the cycle.

The relationship between photosynthesis and cellular respiration can be best understood using the chemical equations given below:

Equations

Photosynthesis: 6CO₂ + 12H₂O + sunlight → C₆H₁₂O₆ + 6O₂ + 6H₂O

Cellular Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (energy)

Thus, photosynthesis is just the opposite process of cellular respiration, and they work in a circle.

Both processes are essential parts of the carbon cycle. While cellular respiration produces carbon dioxide in the environment, photosynthesis removes carbon dioxide from the atmosphere. The exchange of CO₂ and O₂ during photosynthesis and cellular respiration helps recycle carbon dioxide in the biosphere. It keeps oxygen and carbon dioxide levels stable.

Compare and Contrast between Photosynthesis and Cellular Respiration

Although interrelated in nature, photosynthesis and cellular respiration are different in many ways.

How are Photosynthesis and Cellular Respiration Different

Basis	Photosynthesis	Cellular Respiration
1. Location	The process occurs in the chloroplast	The process occurs in mitochondria
2. Inputs (Reactants)	Carbon dioxide (CO₂), water (H₂O), and sunlight	Glucose and oxygen
3. Output (Products)	Oxygen (O₂) and glucose (C₆H₁₂O₆)	Water and carbon dioxide
4. Steps of the Pathway	It occurs in two stages: 1) Light-dependent Step – Chlorophyll converts solar energy to chemical energy stored as NADPH and ATP. It takes place in the thylakoid membrane within the chloroplast 2) Light-independent Step or Calvin Cycle – It 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 finally forms glucose	It occurs in four stages: 1) Glycolysis – Here, glucose gets converted into two pyruvate molecules. In the end, ATP is produced, and NAD⁺ is converted into NADH 2) Pyruvate Oxidation – Each pyruvate from the previous step enters the mitochondrial matrix, which is converted to acetyl CoA. Consequently, carbon dioxide is released, and NADH is generated 3) Citric acid cycle or Krebs cycle – The acetyl CoA combines with a four-carbon molecule. It 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 4) Oxidative phosphorylation – The NADH and FADH₂ formed in the citric acid cycle turn into NAD⁺and FAD⁺ by transferring their electrons to the electron transport chain (ETC). At the end of ETC, the O₂ gets converted into water
5. Net ATP Production	Produce 2 ATP molecules during synthesis of one molecule of glucose	Produce 38 ATP molecules during the oxidation of one molecule of glucose
6. Functions	Provides the source of energy for all organisms on Earth	Releases the stored energy by breaking down the carbohydrate produced through photosynthesis
7. Occurs in	Occurs only in photoautotrophs (all green plants, some photosynthetic bacteria, and algae)	It occurs in every living organism
8. Type of Process	Anabolic process	Catabolic process
9. Energy Required or Released	It is an endothermic reaction. So, it requires solar energy	It is an exothermic reaction. So, it releases energy in the form of ATP
10. Presence or Absence of Sunlight	It occurs only in the presence of sunlight	It does not require sunlight and thus occurs all the time
11. What Powers ATP Synthase	A 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
12. What Pumps Protons Across the Membrane	During 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
13. Electron Source	Photolysis of H₂O	Glucose, NADH⁺, FADH₂
14. Final Electron Acceptor	NADP⁺ (forms NADPH )	O₂(forms H₂O)
15. Coenzymes used	NADP⁺, the oxidized state of nicotinamide adenine dinucleotide phosphate (NADP)	NAD⁺, the oxidized state of nicotinamide adenine dinucleotide (NAD)

How are Photosynthesis and Cellular Respiration Similar

Despite the two processes being opposite, they share some similarities.

They involve multiple complex steps and many same molecules such as oxygen (O₂), carbon dioxide (CO₂), water (H₂O), glucose (C₆H₁₂O₆), and adenosine triphosphate (ATP)
Exchange of gases
Series of redox reactions involving enzymes

Summary

Photosynthesis and cellular respiration are the two basic metabolic processes that work together to drive all cellular activities. All the primary producers in a food chain produce food by photosynthesis. In contrast, cellular respiration occurs in all living organisms on Earth. Food gets oxidized to obtain energy as ATP, which powers almost all the cellular processes. Also, the products of one process are utilized by another, creating a balance in nature.

FAQs

Q1. What role does oxygen play in photosynthesis and cellular respiration?

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

References
- 1. Just Breathe: An Introduction to Photosynthesis and Cellular Respiration – Orise.orau.gov
  2. Difference Between Photosynthesis and Cellular Respiration – Researchgate.net
  3. Steps of cellular respiration – Khanacademy.org
  4. Cellular Respiration and Photosynthesis – Flexbooks.ck12.org