To know about C3 and C4 plants we should first know about photorespiration.
What is photorespiration?
  • Photorespiration is a metabolic mechanism, occur in photosynthetic organisms, and this reaction happens during the Calvin cycle.
  • During, this reaction the organisms does not produce the chemical energy while releasing carbon dioxide and consuming oxygen.
  • As a result, the growth rate of plants is reduced.
What are C3 plants?
All the plants which do not have a photosynthetic adaptation to photorespiration are known as C3plants. They are identified by Calvin
The reactions of Calvin cycle are observed in these plants. The Calvin cycle begins with an enzyme fixing carbon dioxide, and plants that use this “standard” mode of carbon fixation solely are known as C3 plants.
C3 plants make up around \(95%\) of all plant species on the globe, including rice, wheat, soybeans, and all trees.
c3 plants.jpg
Examples of C3 plants
C3 plants are abundant in temperate conditions.
The photosynthetic efficiency of these plants is comparatively low due to their high rate of photorespiration.
What are C4 plants?
These are the plants which have a photosynthetic adaptation to photorespiration.
These plants use an additional dark reaction pathway called the C4 pathway and were discovered by  M. D. Hatch and C. R. Slack in 1966. Hence, the C4 cycle is also known as Hatch and Slack pathway.
What is Kranz anatomy?
The Calvin cycle and light-dependent reactions are physically segregated in C4 plants.
The Calvin cycle occurs in specific cells around the leaf veins, and the light-dependent responses occur in the mesophyll cells (spongy tissue in the centre of the leaf). These cells are known as the bundle-sheath cells.
This anatomical setup is not observed in theC3 plants. This is known as Kranz anatomy. Chloroplasts are found in their bundle-sheath cells.
Kranz anatomy of C4 plant
Crabgrass, sugarcane, and corn are just a few examples of vascular plants that utilise the C4 pathway.
c3 plants (1).jpg
Examples of C4 plants
C4 grow in hot environments, but they are less prevalent in cooler ones. This is because the benefits of reduced photorespiration in hot conditions are likely to outweigh the ATP cost of transferring CO2 from the mesophyll cell to the bundle-sheath cell.
CAM plants:
Cacti and pineapples, adapted to arid settings, use the crassulacean acid metabolism (CAM) pathway to reduce photorespiration. Thus, these plants are referred to as CAM plants.
The name derives from the Crassulaceae plant family, which is where scientists initially found the pathway.
Crassulaceae plants
Unlike the C3 and C4 plants the CAM plants separate the light-dependent reactions and the use of carbon dioxide in time.
The CAM plants open their stomata during the night allowing CO2 to diffuse into the leaves.
This CO2 is reduced organic acids that are stored in their vacuoles till the next day.
During the day, the CAM plants perform photosynthesis without the opening of their stomata. This is because the organic acids are transported out of the vacuole and broken down to release CO2, which enters the Calvin cycle and gets reduced to glucose.
Difference between C3 and C4 plants:
C3 plants
C4 plants
These plants use Calvin cycle for the dark reactions of photosynthesis.These plants use the Hatch and Slack pathway.
These are cool season plants that are found in the cold and wet areas.These are warm season plants that are commonly seen in the dry areas.
C3 plants make up around \(95%\) of all plant species on the globe.They are about \(5%\) of the plant species.
Their leaves lack Kranz anatomy.Their leaves showKranz anatomy.
Their bundle sheath cells do not contain chloroplasts.Their bundle sheath cells contain chloroplasts.
These plants can perform photosynthesis only when the stomata are open.These plants can perform photosynthesis even when the stomata is closed.
Photorespiration rate is very high in these plants.Photorespiration rate is vey low or mostly absent in these plants.
CO2 fixation is slow in these plants. CO2 fixation is comparatively faster in these plants.
Carbon dioxide fixation occurs only once.Carbon dioxide fixation occurs twice.
C3 photosynthesis is the oldest and the most common.C4 photosynthesis is a recent phenomenon, emerging after C3 photosynthesis.