C4 Plants

C4 plants are those that use the C4 pathway for photosynthesis. Usually, they are monocots. They are only found in angiosperms. Carbon dioxide (CO₂) is initially integrated into a 4-carbon compound that’s the reason why it is called C4. Photosynthesis takes place both in bundle sheath cells and mesophyll cells. The carbon dioxide acceptor is PEP carboxylase. The compound which is stable and initially formed is oxaloacetic acid, a four-carbon compound. Photorespiration does not occur.

Characteristics of C4 Plants

The following are some of the characteristics of C4 plants:

• C4 plants contribute 3% of the plant kingdom.
• C4 photosynthesis takes place in them.
• Photorespiration is not measurable in C4 plants.
• The optimum temperature of C4 plants is 30-40 degree centigrade.
• CO₂ fixation happens from the C3 pathway in bundle sheath cells and the C4 pathway in mesophyll cells.
• Their leaves are thin, closer to the arrangement of the vascular bundle.
• C4 plants have smaller air spaces than C3 plants and the veins are surrounded by thick membrane BSC and remaining by mesophyll cells.
• Both BSC and mesophyll hold chloroplast but the maximum amount is contained in BSC.
• The transpiration of C4 plants is low.
• The light intensity of C4 plants is 6000+ fc.
• Stomata are open during the day and close at night.
• The Environmental adaptation is Tropical or semi-tropical areas, areas of high light intensity, areas of high temperature, and drought conditions.

Examples of C4 Plants

• Grasses
• Millets
• Sorghum
• Maize
• Sugar cane

Anatomy of C4 Plants

The essential anatomy of C4 plants is Kranz’s anatomy. Its significant highlights of Kranz’s anatomy are –

1. Apart from the mesophyll cells, a few particular cells exist these cells are known to be bundle sheath cells.
2. Across the vascular bundle, bundle sheath cells are located in a ring structure. Hence, they structure a sheath (covering) across the vascular bundles.
3. The bundle sheath cells chloroplast is not the same as compared to the mesophyll cells. In Bundle sheath cells the chloroplasts and the thylakoids are not organized in that manner of stacks or grana. Such a chloroplast is known to be an agranular chloroplast, Then mesophyll cells’ chloroplasts are granular. Subsequently, C4 plants hold dimorphic chloroplast.

Leaves of C4 Plant

• Leaves of C4 plants represent Kranz’s anatomy.
• Mesophyll and BSC form 2 concentric layers across a vascular bundle.
• Bundle sheaths are close together

Evolution of C4 Plants

C4 photosynthesis has developed in excess of multiple times which is greater than 60 times as a carbon-concentrating component to expand the C3 photosynthetic pathway. The rate and the effectiveness of photosynthesis are more prominent in the C4 than C3 type under climatic CO₂ exhaustion, high light, and temperature, recommending these elements as significant particular agents. This speculation is reliable with similar investigations of grasses, which demonstrate rehashed evolutionary transitions from concealed forests to open territories. Hence, such natural transitions likewise influence emphatically on plant-water relations. The top interest for water transport related to low CO₂, high light, and temperature would have been chosen for C4 photosynthesis not exclusively to build the proficiency and pace of photosynthesis, yet additionally as a water-saving system. The C4 pathway permits high rates of photosynthesis at low stomatal conductance, even given low climatic CO₂. The hydraulic system is protected by decreasing in transportation and permitting stomata to stay open and photosynthesis to be supported for longer under drying climatic and soil conditions. The development of C4 photosynthesis subsequently will develop plant carbon and water relations, giving solid advantages as atmospheric CO₂ declines and a natural interest for water.

Efficiency of C4 Plants

• During high temperatures, high light, and the ongoing CO₂ concentration in the air, the C4 pathway is more productive than C3 photosynthesis since it builds the CO₂ concentration around the significant CO₂ fixation enzyme, named Rubisco. The oxygenase response and, likewise, photorespiration is generally suppressed.
• Because of the high rate of photosynthesis and the reduced rate of photorespiration the C4 plants are more efficient than C3 plants.
• The photosynthetic efficiency is estimated to be less than 4.6% in C3 plants and in C4 plants the efficiency can be estimated to be 6%. Therefore, there is a great ability to increase photosynthetic efficiency and hence productivity also increases.
• In C4 plants, the Carbon-dioxide concentration is high at the RuBisCO area and therefore the photorespiration is highly reduced.
  C4 plants consist of unique leaf anatomy called ‘Kranz anatomy.
  • There is a conversion of oxaloacetic acid to malic acid which is then transferred to bundle sheath cells. In the bundle sheath cells, Carbon dioxide is released and this gets involved in the Calvin cycle.

C4 Cycle

1. It is another way of the C3 cycle to fix carbon dioxide.
2. The first stable compound in this cycle is a 4-carbon compound called oxaloacetate. Therefore it is known as the C4 cycle.
3. This pathway is also known as the Slack and Hatch pathway as they worked out this pathway in the year 1966.
4. These pathways are generally seen in grasses, sugarcane, and sorghum.
5. C4 pathways involve two carboxylation reactions in which one exists in the chloroplast of bundle sheath cells and another in the chloroplast of mesophyll cells.

Plants that use C4 Carbon Fixation

• C3 and C4 pathways vary in the initial product of carbon fixation.
• In C4 plants the C3 pathway is also utilized in the formation of a glucose molecule.
• In C4 plants there will be Kranz anatomy in leaves to permit high temperatures. Large bundle sheath cells are located across vascular bundles of leaves.
• Bundle sheath cells have thick surfaces and there are no intercellular spaces and large chloroplasts will exist.
• Carbon fixation happens in mesophyll cells. Carbon dioxide acceptor is a 3-carbon compound phosphoenolpyruvate (PEP).
• The enzyme named phosphoenolpyruvate carboxylase (PEPcase) catalyzes the reaction. The initial product of CO₂ fixation is 4 carbon compound Oxaloacetic acid.
• The oxaloacetic acid is transformed into other 4C acids such as malic acid and aspartic acid. They are transferred to bundle sheath cells
  By the process of decarboxylation in bundle sheath cells, Carbon dioxide is released and involved in the Calvin cycle.
• The 3-carbon acid is transferred to the mesophyll cells.
• RuBisCO exists in the bundle sheath cells but  PEPcase is absent.

Steps in C4 Cycle

The C4 cycle consists of two steps they are :

• The reaction occurring in mesophyll cells:
  • Formation of oxaloacetate.
  • Formation of malic acid and aspartic acid.
• The reaction occurring in bundle sheath cells:
  • Formation of pyruvic acid.
  • Formation of phosphoenol pyruvic acid.

The Reaction Occurring in Mesophyll Cells

Formation of oxaloacetate:

• The primary acceptor of co2 in this cycle is the 3-carbon compound-phosphoenol pyruvic acid.
• The PEP combines with co2 and forms 4-carbon acid oxaloacetic acid in the presence of PEP Carboxylases. The enzyme remains present in large amounts in mesophyll cells.

Formation of malic acid and aspartic acid:

• Oxaloacetic acid is quite unstable and is converted either into aspartic acid or malic acid.
• The oxaloacetic acid is reduced to malic acid by utilizing the light-generated NADPH+. This reaction is catalyzed by using the enzyme malic dehydrogenase.
• This oxaloacetic acid can also be transformed into aspartic acid in the enzyme aspartic transaminase existence.
• The C4 acids named malic acid and aspartic acid are then transferred to the chloroplast of the bundle sheath.

The Reaction Occurring in Bundle Sheath Cell Chloroplast

Formation of Pyruvic Acid:

• In bundle sheath chloroplast, the malic acid undergoes oxidative decarboxylation to generate pyruvic acid and carbon dioxide in which malic enzymes exist.
• The carbon dioxide and NADPH+ are produced by oxidative decarboxylation of malic acid which is entered in the Calvin cycle.
• Carbon dioxide combines with ribulose diphosphate (RuDP) to yield 2 molecules of phosphoglyceric acid(PGA).
• CO2 + RuDP ⁠→ 2 molecules of PGA

Formation of Phosphoenolpyruvate:

• The pyruvic acid which is produced by oxidative decarboxylation is transferred back to the mesophyll cells where it is phosphorylated to phosphoenolpyruvate with the help of pyruvate dikinase.
• This enzyme is also used in the breakdown of ATP in light reactions.

Need for C4 Pathway

• The C4 pathway is intended to effectively fix CO2 at low concentrations and the plants that utilize this pathway are known as C4 plants. These plants connect Carbon dioxide into a four-carbon compound (C4) known as oxaloacetate. This happens in cells that are known to be mesophyll cells.
• The initial advantage is that the system doesn’t go through photorespiration, an interaction that opposes photosynthesis. The subsequent one is that plants allow their ores to close for longer periods, accordingly staying away from water loss.

Difference Between C3 cycle and C4 cycle:

C3 cycle	C4 cycle
The C3 cycle is generally found almost in all plants.	The C4 cycle can be seen only in C4 plants such as maize sorghum etc.
The The The The C3 cycle involves granular Granular chloroplasts.	The C4 cycle involves both Granular and Agranular chloroplasts.
The C3 cycle is carried out by only mesophyll cells.	The C4 cycle is carried out by both mesophyll cells and bundle sheath cells.
The first stable product in the C3 cycle is a 3-carbon compound, which is phosphoglyceric acid(PGA).	The first stable product in C4 cycle is 4- the carbon compound, which is oxaloacetic acid(OAA).
3ATP and 2NADPH are required for the fixation of one molecule of CO2.	5ATP and 2NADPH are required for the fixation of one molecule of CO2.
The optimum temperature required for the C3 cycle is 10-25 degree Celsius.	The optimum temperature required for the C4 cycle is 30-45 degree Celsius.
The C3 cycle is inhibited by oxygen.	The C4 cycle does not require inhibition.
The C3 cycle can be saturated with sunlight.	The C4 cycle does not saturate with sunlight.
In the C3 cycle, only a single carbon fixation happens.	In the C4 cycle double carbon fixation happens.
Carbon fixation of the C3 cycle is slow and less efficient.	Carbon fixation of the C4 cycle is fast and more efficient.

Significance of C4 Cycle

• The C4 cycle increases the photosynthetic yield two to three times more than C3 plants.
• C4 plants perform a high rate of photosynthesis even when stomata are nearly closed.
• The C4 cycle increases the adaptability of C4 plants to high temperatures and light intensities.
• They can grow very well in saline soil because of C4 organic acid.
• CO₂ from the atmosphere is fixed by the enzyme PEP carboxylase, which results in the formation of the 4-carbon compound oxaloacetate and hence called the C4 cycle.
• Plants having the C4 cycle are mainly from tropical and subtropical regions and are able to survive in environments with low CO₂ concentrations.

Converting C3 plants to C4

As the population in the world increases, the production of food started to decrease rapidly. To raise food production, mainly rice plants, the development of crops is very crucial and the development in the photosynthetic capacity of rice is also very essential for implanting the C4 photosynthetic pathway in C3 plants for crop yield expansion. The photosynthetic mechanisms which are enhanced by the crops will utilize solar radiation and this solar energy is transformed into biological energy which directly rises the crop yield. The types of pathways include the C4 pathway of photosynthesis, C3 photosynthesis, and CAM pathway of photosynthesis.

C3 photosynthesis manifests in the rice plants but because of less yield, the rice plant is transformed into a C4-type plant. There are also various types and methods of genetic tools, sources, genes, etc. Which are utilized to transform C3 plants into C4 plants. As the genetic composition of C3 plants is similar to that of C4 plants. Therefore, it is possible to transform it.

Advantages of C4 plants

• Photosynthesis is more efficient than C3 plants under high light intensity and high temperatures because carbon dioxide concentration is high, which doesn’t allow it to grab oxygen and undergo photorespiration.
• It has better water-utilizing efficiency because PEP Carboxylase brings in CO₂ faster and hence it does not need to keep stomata open as much (less water lost by transpiration) for the same amount of carbon dioxide gain for photosynthesis.

FAQs on C4 Plant

Question 1: For what reason are C4 plants so unique?

Answer:

C4 plants are unique. They have extraordinary kinds of leaf life structures (Kranz life structures), they endure high temperatures, they show a reaction to high light powers, they lack in a process called photorespiration and have more efficiency of biomass.

Question 2: Define dark reaction.

Answer:

A dark stage of photosynthesis also happens in plants known as a dark reaction. It is a sort of cycle where photosynthesis is performed without daylight. The creation of carbohydrates from carbon dioxide happens during this stage. This sort of response is worked with by three cycles. They are:

• C3 pathway
• C4 pathway
• CAM pathway.

Question 3: What separates the C4 and C3 pathways of CAM plants?

Answer:

Time requirements change the C4 and C3 pathways of CAM plants.

Question 4: In which plants does the Calvin pathway occur?

Answer:

Calvin pathway occurs in all photosynthesis plants. Example: C3 plants, C4 plants.

Question 5: Define some of the similarities between C3 and C4 plants.

Answer:

C3 and C4 plants are examples of dark reactions to photosynthesis. For the process of photosynthesis, the C3 and C4 plants require chloroplast. Both plants’ energy is stored in the sunlight. The light reactions in both C3 and C4 plants are the same.