Photosynthesis (6): More efficient C4 Cycle, Carbon Fixation

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Disadvantages of C3 Plants in the Calvin Cycle

Wheat, rice, potatoes, and fruit trees are common C3 plants whose initial photosynthetic product is a three-carbon saccharide, glyceraldehyde-3-phosphate. C3 plants can directly absorb carbon dioxide from atmosphere to make carbohydrates. Carbon dioxide enters mesophyll cells through the stomata on leaves and participates in dark reaction for biological carbon fixation.

A fatal weakness is that their stomata will close to prevent transpiration in hot and arid regions. Although this reduces water loss temporarily, it also prevents carbon dioxide from entering mesophyll cells for photosynthesis. At this time, the light reaction fills the chloroplasts of C3 plants with oxygen. Rubisco can catalyze both gases to combine with pentose in dark reaction, but in the oxygen rich conditions, it is more frequent that oxygen is fixed in pentose to form a two-carbon compound. Therefore, in hot and dry climates, C3 plants often trigger photorespiration whose rate increases with hot temperature often accompanied by water limitation. The carbohydrate production will reduce, or even the growing will stop at this time. Despite the fact that stomata remain open in suitable weather, the photosynthesis efficiency in C3 plants isn’t high. About 30% of saccharide is lost in photorespiration.

C4 Carbon Fixation, Hatch-Slack Pathway

In 1966, Hatch and Slack discovered C4 photosynthesis in sugarcane and maize, hence it’s also known as Hatch-Slack pathway. These plants make organic matters efficiently even at low CO₂ concentrations. The main reason is their unique structure and two different chloroplasts. In the bundle sheath cells, carbon dioxide is released and dark reactions take place in the chloroplasts with degenerative thylakoids. The chloroplasts with well-developed thylakoid perform light reactions and use PEP carboxylase to fix carbon element in mesophyll cells that encircle bundle sheath cells to create a flower-like structure. These structures aren’t present in C3 plants whose chloroplasts can complete all steps of photosynthesis.

In the C4 cycle, carbon dioxide is transported by a four-carbon compound. The light and dark reactions are allocated to different types of cells. Oxaloacetate is converted from bicarbonate and PEP with help of PEP carboxylase in mesophyll cells. It’s then reduced to malate by NADPH, and leaves for bundle sheath cells where carbon dioxide released from malate was used by Calvin cycle. Meanwhile the pyruvate returns to mesophyll cells to resynthesize PEP.

Carbon dioxide levels around Rubisco increase dozens of times, and oxygen concentrates only in mesophyll cells without Rubisco. Hence, the photorespiration (glycolate cycle) is negligible in C4 cycle, but the cost is energy consumption. In terms of carbon fixation, PEP carboxylase is 60 times more capable than rubisco. Bicarbonate is more soluble in water than carbon dioxide. Therefore, nutrients are made from carbon dioxide diffusing via cell gaps in C4 plants despite the closed stomata. Their photosynthetic efficiency is twice more than C3 plants.

Another notable feature of C4 pathway is water conservation. Only half water was consumed to fix one carbon. Their stomata don’t need to open widely because the 0.3% concentration is sufficient.

There’re only a few thousand species of C4 plants. All of them are angiosperms. They’re undoubtedly successful species. The 20-25% of Earth's total primary production comes from them, despite their limited variety. Many crops, such as maize, sorghum, millet and sugarcane synthesize saccharide via this pathway.

Frequently Asked Questions

Why did C4 plants appear?

Abundant resources in ancient times allowed plants to waste some through photorespiration. However, the life prosperity resulted in a brutal competition. Especially the rise of angiosperms, CO₂ was decreased more quickly. The collision of Indian and Eurasian plates uplifted mountains to intensify rock weathering since the Eocene. As carbonic acid was constrained in rocks as carbonates, CO₂ was reduced further. The oceanic moisture was also blocked from reaching inland areas by high mountains. Large arid zones appeared. C4 plant evolved to overcome these adverse conditions. The emergence of tropical savannas prompted humans to leave forest and become upright beings.

Why are C4 plants less widespread despite their strong photosynthetic ability?

Although C4 plants have high photosynthetic efficiency, it’s based on large ATP consumption. Fixing one carbon requires 5 ATP and 2 NADPH. Once temperature drops below 15°C, or light is insufficient, their production decreases. C4 plants are more common in dry hot tropical regions. However, they only account for about 1% in tropical rainforests. It may be that C3 plants aren’t disadvantaged in water-rich rainforests. Though some saccharide is lost in photorespiration, the simpler structure and metabolic pathway allow C3 plants to use carbon dioxide directly. Only 3 ATP and 2 NADPH are used to capture one CO₂ molecule. They may struggle in high-temperature environments but thrive in subtropical, temperate, frigid and high-altitude zones.


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