Difference Between Photosynthesis and Cellular Respiration

Photosynthesis vs Cellular Respiration

Photosynthesis and cellular respiration are both cellular processes organisms use to obtain energy. Photosynthesis converts the light energy obtained from the sun into chemical energy of sugar and oxygen, whereas cellular respiration is a biochemical process by which cells obtain energy from the chemical bonds of food molecules. Photosynthesis and cellular respiration are both key processes for life. Oxygen is a by-product of Photosynthesis and is important to all living beings. Sugar and carbohydrate produced by photosynthesis are the simplest form of food that could be processed by living organisms to obtain energy.


Photosynthesis occurs in photoautotrophs, which refers to their ability to produce their own food. Photoautotrophs include plant, algae and some forms of bacteria excluding archaea. Photosynthesis involves a set of chemical reactions, where Carbon dioxide and water are converted and released as glucose and oxygen to the atmosphere with the help of light energy.

The following equation is the most common form of summarizing photosynthesis:

Carbon Dioxide(6 CO2) + Water (6 H2O) —-converted with help of light energy -→ Sugar (6(CH2O) + Oxygen (6 O2sugar)

Even though the process of photosynthesis may vary in different forms of life, there are a few key features that are similar. Photosynthesis happens when light energy is absorbed by proteins (photosynthetic reaction center). These proteins contain chlorophylls. Chloroplast contains chlorophylls in plants and in bacteria. They are held in the plasma membrane. Chlorophylls sort light energy in the form of adenosine tri phosphate (ATP), while some of the energy is used to extract electrons from water, and the extracted electrons help convert Carbone dioxide in to organic compounds.

Photosynthesis can be divided in to two states; Light dependent (Light reaction) and Light independent (Dark reaction). Light dependent photosynthesis needs actual sunlight to react, whereas dark reaction photosynthesis only needs the byproducts of sunlight to proceed. Light reaction needs the photons and water to give out oxygen leading to production of ATP and NADPH. NADPH is a reducing agent that can reduce its hydrogen molecule. Dark reaction photosynthesis, also known as the Calvin cycle, use the Carbone dioxide and the newly formed NADPH to produce phosphogylcerides; the three carbon sugar could combine later to form sugar and starch. The produced sugar and starch is stored in the plant for future use in form of fruits, yams, etc.

Photosynthesis is a very crucial process which allows life to continue on earth. Oxygen is released in to the atmosphere as a byproduct of the process and it is essential for the process of respiration of most living creatures.

The sugar and starch which are produced in excess amounts are stored in the plants as fruits and yams, etc, and they provide food for animals which can’t produce food ontheir own.

Cellular Respiration

Cellular respiration is the method of obtaining energy, which is found in organisms that can not photosynthesise. All forms of animals and most forms of organisms use this method to obtain energy. Cellular respiration is the process used by animals to obtain energy from organic compounds they eat.

Cellular respiration is a chemical process by which the cells derive the chemical energy of “food” molecules (Glucose) into energy in the form of ATP. Cellular respiration uses carbohydrates, fats, and proteins as fuels. Glucose is most commonly used to explain cellular respiration. All living organisms carry out respiration; aerobic respiration in the presences of oxygen, and anaerobic respiration when oxygen is not present. Prokaryotic cells carry out cellular respiration in the cytoplasm or on the inner surface of the cell, whereas eukaryotic cells carry out cellular respiration mostly in the mitochondria of the cell. In aerobic respiration, one glucose molecule can generate 36-38 molecules of ATP, but in anaerobic respiration, (through glycolysis and fermentation) only 2 ATP molecules could be derived.

In simple, cellular respiration can be divided in to three metabolic processes:

1. Glycolysis- which occurs in the cytosol.

2. The Krebs cycle-that occurs in mitochondrial matrix.

3. Oxidative phosphorylation via the electron transport chain which occurs at inner mitochondrial membrane.

If oxygen is not present the respiration would happen by two metabolic pathways in the cytosol glycolysis and fermentation.

Aerobic Respiration

In aerobic respiration, the form of cell respiration that happens in the presence of oxygen uses glucose as its initial energy source. Glucose goes through glycolysis and end product of glycolysis goes through tricarboxylic acid cycle (TCA cycle) to form more energy in the form of ATP than that could be obtained in the anaerobic pathway.

The end product ATP is formed by substrate-level phosphorylation, NADH and FADH.

The following is the simplified reaction of Aerobic respiration

C6H12O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + Energy

Anaerobic Respiration

Anaerobic respiration is the soul form of respiration of most anaerobic bacteria; even eukaryotes subsidies anaerobic respiration when oxygen supply is low. When oxygen is not present, pyruvate is metabolised in a process called fermentation. The first step of anaerobic respiration is glycolysis and kerb cycle, which yields two ATP molecules that could be used in fermentation by oxidising the NADH produce, by glycolysis to lactate and NAD+ which can then be used again in glycolysis to produce more ATP.

The animals are dependant on external sources such as plants to obtain organic compound as they can’t produce food of their own. Cellular respiration is the process used by animals to obtain energy from organic compounds they eat.

Difference between Photosynthesis and Cellular Respiration

Both Photosynthesis and Cellular Respiration are processes, which organisms use to obtain energy to sustain life. Both the processes have two stages; photosynthesis occurs in the presence and the absence of light. And cellular respiration occurs in the presence and the absence of oxygen. However, that is where the similarities end.

While photosynthesis converts light energy in to chemical energy, cellular respiration breaks down chemical bonds of food molecules to obtain energy. Also in photosynthesis the converted energy is stored in the form of organic compounds (Sugar, starch). On the other hand, cellular respiration breaks down organic materials to obtain energy.

While the process of cellular respiration can occur regardless of the time, some parts of photosynthesis can only occur in daylight


In conclusion, we can summarize that both photosynthesis and cellular respiration are processes, which organisms use to obtain energy. However, while photosynthesis stores the energy in form of sugar and starch, cellular respiration breaks down organic compounds such as sugar and starch to obtain energy.

Both these processes are dependant on each other; animals need the fruits of plants to obtain organic compounds for cellular respiration to function and the oxygen released in to the air to breath and plants in turn need animals for the exclude carbon dioxide in to the air which plants need for the process of photosynthesis. In the absence of one, the chances of survival for the other drastically degrade.

  • gjk

    thanks a lot!!!