Rather than worrying so much about why we exist, we should celebrate what keeps us existing. In which case, long live photosynthesis! The process plants and bacteria use to produce sugar from water, carbon dioxide, and sunlight. Like any talented performer, plants make the complicated appear simple and effortless. Beneath leaves’ innocuous green exterior, a complex series of reactions dependent on light (Photosystems I/II), as well as independent of light (Calvin Cycle) do what few other organisms can, transform the inorganic into organic. While the products of photosynthesis are the same across all plants, the process is different, adapted to fit environmental conditions. These changes can be categorized into three different photosynthetic strategies: C3, C4, or CAM.

C3 Pathway

The advantages of these strategies become clear after taking a look first at the ubiquitous C3 pathway. Fueled by the energy harvested by the sun in the light dependent reactions and CO₂, the Calvin Cycle is the process a plant uses to produces sugar. The C3 pathway, used by around 85% of plants, is described below.

Carbon Fixation

• CO₂  first enters the leaf through the stomata (leaf pores) and diffuses into the chloroplast, where the carbon atoms from  CO₂ are fixed by ribulose-1,5-biphospahte carboxylase/oxygenase (RuBisCo) to the five-carbon ribulose bisphosphate RuBP.
•  The resulting six-carbon compound splits into two three-carbon compounds, 3-phosphoglyceric acid (3-PGA). 

Reduction

• ATP and NADPH convert 3-PGA into a 3-Carbon sugar, glyceraldehyde-3-phosphate (G3P). Reduction is the addition of electrons, in this case form NADPH to 3-PGA.

Regeneration

•  One of the G3Ps is turned into glucose, while the rest are converted to RuBP by ATP.

The Calvin cycle in concrete numbers: Three  CO₂ molecules need to enter the cycle (one turn of the cycle per CO₂ molecule) to produce six G3P molecules, and only then can one G3P leave to produce glucose. Glucose requires two G3Ps, therefore six turns of the cycle are required to make one glucose molecule. 

Therefore, a considerable amount of carbon is required, and the most abundant source is atmospheric CO₂. The enzyme RuBisCo by fixing CO₂ bridges atmosphere and biosphere, but is not a fast worker, with a rate of three CO₂ molecules per second. To compensate, plants pack chloroplasts with RuBisCo, making it the most plentiful enzyme on Earth. It has another disadvantage, it has difficulty discerning between O₂ and  CO₂. When it accidentally adds O₂ to RuBP, the mistake can only be undone through photorespiration, an energy and carbon intensive process. To avoid this inefficient step, the stomata are opened to increase gas circulation, preventing O₂ buildup. This strategy is not effective at high temperatures or low humidity as described below. 

High Temperatures: 

• Higher temperatures correlate with increased radiation, increasing photosynthesis and therefore O₂ levels inside the leaf. Even with open stomata, the ratio is shifted towards O₂.
• RuBisCo bonds more easily with O₂ than CO₂ at higher temperatures

 Low Humidity (High VPD):

• Low humidity increases transpiration, which if the stomata are not closed, leads to significant water loss.

C4 and CAM pathways are adaptations that minimize photorespiration in hot and dry conditions by ensuring a high concentration of CO₂ in the leaf.  

Examples of C3 Plants: rice, cassava, cotton, wheat, barley, rye, oat, soybean

C4 Pathway

C4 plants physically separate light-dependent reactions and the Calvin cycle. The former occurs in the spongy mesophyll cells, and the latter in specially devised bundle-sheath cells surrounding the vascular tissues (Krantz anatomy). Two locations are needed to prevent the exposure of RuBisCo to O₂. The sequence is described below.

• Atmospheric  CO₂ is fixed to 3-carbon phosphoenolpyruvate (PEP) in the spongy mesophyll by PEP carboxylase forming oxaloacetate, a 4-carbon organic acid (hence C4). 
• Oxaloacetate is converted to malate and transported to the bundle sheath cells.
• In the bundle sheath cells, malate breaks down into  CO₂  and pyruvate. The one molecule of  CO₂ is fixed by RuBisCo and enters the Calvin cycle as described in the C3 pathway.
• ATP and a phosphorous group fixed to pyruvate, regenerate a PEP molecule that returns to the mesophyll cell. 

The enzyme PEP carboxylase is more discriminatory than RuBisCo, as CO₂ is its only substrate. Consequently, the efficiency of RuBisCo is increased by preventing interaction with O₂  sheltered in the bundle sheath cells. Using two locations allows the plant to maintain high concentrations of CO₂, significantly reducing photorespiration. As a result, photosynthesis remains efficient even in hot, sunny conditions, when the photosynthesis rate and therefore O₂ concentration are highest. The higher selectivity of PEP also reduces the time the stomata are open for gas diffusion, reducing water loss. The energy used in converting and transferring  CO₂ between cells is compensated for by the  C4 plant’s greater ability to capitalize on intense radiation and high temperatures, uninhibited by photorespiration. 

Examples of C4 Plants: sugar cane, sorghum, maize, millet, switchgrass

CAM Pathway

While the C4 pathway derives benefit from division of space, crassulacean acid metabolism (CAM) divides time. CO₂ fixation by PEP carboxylase and the Calvin cycle run separately over day and night.

Night

• The stomata open at night, allowing CO₂ to diffuse into the leaves.
• The CO₂ is fixed to 3-carbon phosphoenolpyruvate (PEP) by PEP carboxylase, converting it to 4-carbon oxaloacetate, which is then converted to malate (or another organic acid) and stored outside of the chloroplast in the cell vacuole.

Day

• In daylight, the stomata don’t open, instead the stored CO₂ is transferred from the vacuoles to the chloroplast, and enters the Calvin cycle, powered by the energy generated from the light-dependent reactions.
• The malate, or other organic acid, releases CO₂ into the Calvin cycle. 

CAM plants are best suited for extremely hot and dry conditions. Without the need to open stomata during the day when photosynthesis and transpiration are highest, water loss is kept to the bare minimum. In fact, the stomata could remain closed both day and night in a state referred to as ‘CAM-idle’: the O₂ produced from photosynthesis is reused in respiration, whose byproduct is CO₂  which can be used for photosynthesis. A perpetual motion machine, except ‘CAM-idle’ is energetically expensive. Similar to the C4 pathway, the CAM pathway uses energy due to the extra steps required, though in harsh desert environments the reduction in water loss is worth it. CAM plants have been found to able to switch between C3 and CAM pathways if conditions are sufficiently moist and cool.

Examples of CAM Plants: pineapple, sisal, cacti, orchids, agave

C3, C4, and CAM-Who cares!?

Understanding the photosynthetic strategies used by different plant species improves understanding of their environmental limitations. For example, cropping C4 and CAM plants in temperate climates is inefficient, and will lead to lower yields. Lower temperatures and radiation intensity reduce the advantage gained by the energy intensive steps that reduce photorespiration. Whereas C3 plants are less water efficient and tolerant to heat and aridity. However, they are the most energy efficient within their optimal temperature ranges. Other than more effective matching between growing environment and photosynthetic pathway, there are efforts to genetically transfer C4 characteristics to C3 plants. This is particularly urgent due to projected climate changes, such as higher temperatures and erratic rainfall patterns, that will pose the greatest challenges for C3 crops.