Comparative Table: Photosynthesis in C3 and C4 Plants
All plants use light energy from the sun as their ultimate source of energy for survival. This energy is captured by the process known as “Photosynthesis” (“photo” meaning light). Photosynthesis is a complex pathway which is used by plants to fix carbon, present in the atmosphere, into sugar (carbohydrate) molecules. All plant species rely on these products to produce their source of energy.
What are C3 Plants
Plants have evolved to capture the ultimate source of energy in different ways. From an evolutionary point of view, plants first developed the “C3 pathway”. It is the simplest form of photosynthesis, also known as the Calvin cycle. A typical plant on the earth that uses photosynthesis is a C3 plant. In this process carbon dioxide enters a plant through its stomata, and the enzyme Rubisco fixes carbon into sugar using the Calvin cycle. It fuels plant growth. This fixation of carbon dioxide by rubisco is the first step of the Calvin cycle. The plants that use this mechanism of carbon fixation are called C3 plants. Approximately 95% of plants on the earth are C3 plants. They are also known as temperate plants.
The photosynthesis process can take place only when the micropores (stomata) on leaves are open. The leaves of C3 plants do not show kranz anatomy. C3 plants exhibit the C3 pathway. It is the three-carbon compound (3-PGA). Here the first carbon compound produced has three carbon atoms hence the name “C3 pathway”.
The Calvin cycle is useful to convert CO2 into carbon. It eliminates greenhouse gas (CO2) from the atmosphere efficiently. The Calvin cycle helps plants to store energy for a more extended period.
C3 plants are highly rich in proteins. They can be annual perennial. Some of the C3 plant examples are wheat, rye, oats, and orchard grass.
Since C3 pathway is a more primitive pathway than C4, they have no known adaptive features to combat photorespiration. In the Calvin cycle, approximately 25% of the RuBP is oxygenated (addition of O2 instead of CO2) by the enzyme RUBISCO, an undesirable feature, causing wastage of energy, as it cannot further undergo the Calvin cycle.
What are the C4 Plants?
C4 plants are known to have evolved from the C3 plants. Various evolutionary trends suggest that the development of the C4 pathway was in response to the low carbon dioxide levels in the atmosphere. Thus, the C4 pathway confers an evolutionary advantage to these plants. They possess a particular type of leaf anatomy and use Phosphoenolpyruvate carboxylase (PEP enzyme) instead of photorespiration to enter the Calvin cycle. Enzymes of C4 metabolism are regulated by light. PEP enzyme is more attracted to CO2 molecules and reacts less with O2 molecules. PEP carboxylase does not tend to bind oxygen.
This process takes place in the mesophyll cells (spongy cells in the middle of the leaf) instead of the stomata where CO2 and O2 enter the plant. The light-dependent reaction occurs in mesophyll cells, and the Calvin cycle occurs in bundle-sheath cells around the leaf veins. Carbon dioxide present in the atmosphere is fixed in the mesophyll cells to form a pure 4-carbon organic acid (oxaloacetate) by the non-rubisco enzyme.
The 4-carbon organic acid is then converted to a similar molecule, called malate, that can be transported into the bundle-sheath cells. Inside the bundle-sheath cells, malate breaks down and releases a molecule of CO2.
Enzymes of C4 metabolism - PEP enzyme
(Image will be uploaded soon)
Then the rubisco fixes the carbon through the Calvin cycle, the same as by C3 plants in photosynthesis.
C4 plants exhibit the C4 pathway. Examples are maize, sorghum, and sugarcane. The leaves possess kranz anatomy. Approx 5% of plants on earth are C4 plants. C4 plants examples are pineapple, corn, sugar cane, etc.
C4 photosynthesis is capable of increasing crop yields. Researchers are focusing on understanding the evolution of the C4 plant’s metabolism better, in an attempt to engineer important crops with more energy and water efficiency because they use less water and can grow in conditions of drought too.
A Diagram showing C3 and C4 photosynthesis
(Image will be uploaded soon)
Let’s explain more to understand the similarities and differences between C3 and C4 plants.
Comparison Between C3 and C4 Plants
C4 plants have 50% higher photosynthesis efficiency than C3 plants.
Unlike C4 plants, C3 plants consist of 3-phosphoglycerate with three carbon atoms.
C4 plants have better robustness no matter if the objective function is biomass synthesis or CO2 fixation.
C4 plants are more productive in hot and dry climates than C3 products because they use 3-fold less water and can grow in conditions of drought or high temperature.
Unlike C4 plants, C3 plants reduce carbon dioxide directly in the chloroplast.
C3 plants have a denser topology than C4 plants.
C3 Plants have less modularity than C4 plants.
C4 plants have more carbon dioxide than C3 plants.
C4 has higher radiation use efficiency than C3 plants
C3 photosynthesis uses the Calvin cycle only for carbon fixation catalyzed by Rubisco, inside the chloroplast in mesophyll cells. While C4 plants’ photosynthesis activities are divided between mesophyll and bundle sheath cells where carbon fixation is catalyzed by phosphoenolpyruvate carboxylase (PEPC).
The Systematic Comparison of C3 and C4 Plants can be made through Metabolic networks.
Difference Between C3 and C4 Plants
FAQs on C3 vs C4 Plants: Detailed Differences for Biology Students
1. What is the fundamental difference between C3 and C4 plants?
The fundamental difference lies in the first stable product formed after carbon dioxide fixation. In C3 plants, the first product is a 3-carbon compound called 3-phosphoglycerate (3-PGA). In C4 plants, the first product is a 4-carbon compound, typically oxaloacetate, which is then converted to malate or aspartate. This initial difference is due to the primary CO₂-fixing enzyme used: RuBisCO in C3 plants and PEP carboxylase in C4 plants.
2. What is Kranz anatomy and why is it significant for C4 plants?
Kranz anatomy is a specialised leaf structure found in C4 plants, such as maize and sugarcane. It is characterised by two types of photosynthetic cells: the mesophyll cells and the large bundle sheath cells arranged in a wreath-like (Kranz) pattern around the vascular bundles. This anatomy is significant because it spatially separates the two key steps of C4 photosynthesis. Initial CO₂ fixation occurs in the mesophyll, and the Calvin cycle occurs in the bundle sheath cells, which protects the RuBisCO enzyme from oxygen and prevents photorespiration.
3. Why are C4 plants considered more efficient at photosynthesis in hot and dry climates?
C4 plants are more efficient in hot and dry conditions for two main reasons:
Suppression of Photorespiration: The C4 pathway acts as a CO₂-concentrating mechanism. It pumps CO₂ into the bundle sheath cells, creating a high CO₂ concentration around RuBisCO. This minimises the oxygenase activity of RuBisCO, virtually eliminating the wasteful process of photorespiration, which is prevalent in C3 plants at high temperatures.
High CO₂ Affinity: The enzyme PEP carboxylase, which performs the initial carbon fixation in C4 plants, has a very high affinity for CO₂ and is not inhibited by oxygen. This allows C4 plants to fix carbon effectively even when stomata are partially closed to conserve water.
4. What are the primary CO₂ acceptor molecules in C3 and C4 plants?
The primary CO₂ acceptor molecules are different in each pathway. In C3 plants, the primary CO₂ acceptor is a 5-carbon molecule called Ribulose-1,5-bisphosphate (RuBP). In C4 plants, the primary CO₂ acceptor in the mesophyll cells is a 3-carbon molecule called Phosphoenolpyruvate (PEP).
5. What are some common examples of C3 and C4 plants?
Most plants are C3, while C4 plants are common in tropical and subtropical regions. Here are some examples:
C3 Plants: Rice, wheat, barley, soybeans, potatoes, and spinach.
C4 Plants: Maize (corn), sugarcane, sorghum, millets, and amaranth.
6. How do the carbon fixation cycles differ in their location within the leaf?
In C3 plants, the entire process of carbon fixation via the Calvin cycle occurs within the mesophyll cells. In C4 plants, the process is spatially separated: initial carbon fixation into a 4-carbon acid happens in the mesophyll cells, and this acid is then transported to the bundle sheath cells where it releases CO₂ for the Calvin cycle.
7. If C4 plants are more efficient, why haven't they replaced all C3 plants?
This is because the C4 pathway has a higher energy cost. Regenerating PEP from pyruvate requires additional ATP compared to the C3 pathway. This extra energy expense is only beneficial in conditions where photorespiration is a major problem (i.e., hot, dry, and sunny climates). In cooler, temperate, and moist environments, the C3 pathway is actually more energy-efficient, allowing C3 plants to thrive and dominate these ecosystems.
8. How do CAM plants represent a different adaptation for photosynthesis compared to C3 and C4 plants?
CAM (Crassulacean Acid Metabolism) plants, like succulents and pineapples, also use the C4 pathway to fix CO₂. However, instead of a spatial separation like in C4 plants, they use a temporal separation. To minimise water loss in arid environments, CAM plants open their stomata only at night to fix CO₂ into organic acids. During the day, they close their stomata and release the stored CO₂ to be used in the Calvin cycle, using the light energy captured at the same time.
