Difference Between Photosynthesis and Cellular Respiration

Respiration is an essential process in all organisms, whereas photosynthesis occurs in green plants, algae, and a few types of bacteria only. The process of respiration takes place in plants and animals at alternative times. Animals breathe during the day. But, plants photosynthesize during the day and respire at night. Cellular respiration is one of the types of respiratory processes which occurs at the cellular level. It is a vital process to obtain energy for cellular functions. Both are essential for the survival of life. Here are all the differences between photosynthesis and cellular respiration along with their definitions.

Comparison Table

Characteristic	Photosynthesis	Cellular Respiration
Definition	Food manufacturing process	Energy liberation process
Chemical Equation	CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂	C₆H₁₂O₆ + O₂ → CO₂ + H₂O + ATP
Occurrence	Green plants, algae, and cyanobacteria	All prokaryotic & eukaryotic organisms
Location	Chloroplasts	Cytoplasm, mitochondria
Time	Daytime	All day & night
Steps	Light-dependent & Light-independent Reactions	Glycolysis, Pyruvate Oxidation, Kreb’s cycle, Oxidative Phosphorylation
Process Type	Anabolic	Catabolic
Reactants	CO2, water, sunlight	Oxygen & glucose
Products	Oxygen, water, glucose	Water, ATP, CO2
Coenzymes	NADP	NAD + FAD
Pigments	Chlorophyll	No pigments
Reaction Type	Endothermic	Exothermic
Phosphorylation	Photophosphorylation	Oxidative phosphorylation
Carbohydrates	Produced	Consumed
Energy Conversion	Light energy to potential energy	Potential energy to kinetic energy
Dry Weight	Increased	Decreased

What is Photosynthesis?

Photosynthesis is an energy-producing process carried out by plants in the presence of chlorophyll to produce sugar molecules (energy). It uses sunlight, carbon dioxide, and water to produce oxygen and sugar. These sugar molecules act as an energy source in various molecular and cellular functions. Plants, like mosses, and other organisms, such as algae, and some cyanobacteria also perform photosynthesis to obtain energy. Herbivores utilize the energy stored in different leaves of green plants to fulfill their nutrient demands.

Photosynthesis is either oxygenic or anoxygenic, depending on the organism’s requirements. The process only takes place in green plants because of chlorophyll a and b in chloroplasts. It consists of two stages; light-dependent and light-independent. The light-dependent reactions occur in the thylakoid membrane in the presence of sunlight. The energy absorbed from the sunlight converts to NADPH and ATP molecules. The other part of the process is glucose production in the stroma without sunlight. It uses the energy from NADPH and ATP. This stage is also called the Calvin Cycle. The equation for the process is

6CO2 + 6H2O + Light energy → C6H12O6 (sugar) + 6O2

While plants use photosynthesis to manufacture sugar, algae convert solar energy into chemical energy. The process also utilizes light and liberates oxygen as a by-product.

Cellular Respiration

Cellular respiration works conversely to fermentation and photosynthesis and breaks down glucose to produce energy for body functions. Organic substances break down to provide biochemical energy. Each glucose molecule liberates between 30 and 32 ATP molecules. Cellular respiration is vital for an organism as many functions cannot take place without usable energy.

Cellular respiration occurs in both prokaryotic and eukaryotic organisms. The site of cellular respiration in Prokaryotic cells in the cytoplasm. On the contrary, it takes place in cytosol and mitochondria in plant and animal cells. The process is divided into four fundamental stages in eukaryotes – glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation.

Glycolysis

Glycolysis is the first step of cellular respiration that produces two pyruvic acid molecules. It comprises ten steps and is also called the Embden-Meyerhof-Parnas pathway. The obtained energy is stored in ATP while NAD+ converts to NADH. This stage of the cycle holds significant importance as it gives the initial ATP molecules and facilities the process further.

Pyruvate Oxidation

The pyruvate molecules then move to mitochondria, transforming into the Acetyl coenzyme used in Krebs’ cycle. Sometimes you may see only three steps of Cellular Respiration in many places. Many researchers believe that it is not a distinct step of the process while others think otherwise.

Kreb’s Cycle

The third step of Cellular respiration is Kreb’s cycle. It is also known as the citric acid cycle or TCA (Tricarboxylic acid) cycle. Kreb’s cycle is a detailed process in itself with eight steps. Each of these steps uses eight specific enzymes to produce energy. FAD and NAD+ contribute majorly to the energy in the cycle, which later converts into ATP. One Kreb cycle produces three NAD+ molecules and one FAD molecule. These products are reduced to NADH and FADH2. Krebs’ cycle gives carbon dioxide as a by-product of respiration.

Oxidative Phosphorylation

It is the last step of the process in which each pair of hydrogen atoms from FADH₂ and NADH provides an electron pair. They reduce oxygen atoms to produce water. The transport of electrons is also termed the electron transport chain. It enables a gradual reduction in the energy of the electrons. Every pair of electrons involved in the process gives three ATP molecules as the product. The chemical reaction of cellular respiration is

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

The Bottom Line

Photosynthesis and cellular respiration are important processes for the sustenance of life. Cellular respiration occurs in all prokaryotic and eukaryotic organisms. In contrast, photosynthesis occurs in green plants, algae, and a few bacteria. The main difference between photosynthesis and cellular respiration is their metabolic contribution. Photosynthesis uses energy to produce glucose. On the contrary, cellular respiration utilizes energy by breaking down molecules. While they are distinct in their mechanism and products, both are essential to life.

Chelsea Rogers

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