Photosynthesis

Photosynthesis is the process by which plants convert sunlight into sugar. Chemically, photosynthesis is the reverse reaction of respiration.

While respiration is the complete oxidation of glucose to H₂O and CO₂, photosynthesis is the reduction of CO₂ using electrons from H₂O. Photosynthesis is thus an endergonic reaction. During photosynthesis, sunlight (specifically visible light), fuels the reduction of CO₂.

Plants use the principal photosynthetic enzyme Rubisco to convert light, carbon dioxide and water into sugars that drive plant development.

Photosynthesis began in the absence of oxygen; it came before oxygenic respiration on earth. Increasing oxygen in the atmosphere led to the development of oxygenic respiratory pathways (the Krebs cycle, electron transport and oxidative phosphorylation).

Photosynthesis and respiration have electron transport-ATP synthesizing systems with similar features. This suggests that they share a common evolutionary ancestry

Pathways

Two biochemical pathways make up photosynthesis:

Light-dependent reactions that use visible light energy to remove electrons from water, reduce electron carriers, pump protons and make ATP; and
Light-independent reactions that use ATP to transfer electrons from the reduced electron carriers to CO2 to synthesize glucose.

Photorespiration

Plants convert sunlight into energy through photosynthesis; however, most crops on the planet are plagued by a photosynthetic glitch, and to deal with it, evolved an energy-expensive process called photorespiration that drastically suppresses their yield potential.

Over millennia, Rubisco has become a victim of its own success, creating an oxygen-rich atmosphere. Unable to reliably distinguish between the two molecules, Rubisco grabs oxygen instead of carbon dioxide about 20 percent of the time, resulting in a plant-toxic compound that must be recycled through the process of photorespiration.

Photorespiration is anti-photosynthesis. It costs the plant precious energy and resources that it could have invested in photosynthesis to produce more growth and yield.

(By Paul South, a research molecular biologist with the Agricultural Research Service, who works on the RIPE project at Illinois)

Rubisco has even more trouble picking out carbon dioxide from oxygen as it gets hotter, causing more photorespiration. Our goal is to build better plants that can take the heat today and in the future, to help equip farmers with the technology they need to feed the world.

(By Amanda Cavanagh, an Illinois postdoctoral researcher working on the RIPE project)

C3 photosynthesis

C3 photosynthesis, in which the first carbon compound created contains three carbon atoms. In this process, carbon dioxide enters a plant through its stomata (microscopic pores on plant leaves), where amidst a series of complex reactions, the enzyme Rubisco fixes carbon into sugar through the Calvin-Benson cycle. However, two key restrictions slow down photosynthesis.

Rubisco aims to fix carbon dioxide, but can also fix oxygen molecules, which creates a toxic two-carbon compound. Rubisco fixes oxygen about 20 percent of the time, initiating a process called photorespiration that recycles the toxic compound. Photorespiration costs the plant energy it could have used to photosynthesize.
When stomata are open to let carbon dioxide in, they also let water vapor out, leaving C3 plants at a disadvantage in drought and high-temperature environments.

Alternatives to C3 Photosynthesis

Photosynthesis occurs in the vast majority of plants in a form known as C3. C3 works best under conditions that are easy for plants—moderate temperatures and sunlight, and ample water. Because C3 photosynthesis requires that the pores on the leaves—the stomata, which allow intake of carbon dioxide—be open at the same time that sunlight is fueling photosynthesis, C3 photosynthesis comes at the cost of substantial water loss through the stomata. C3 plants, therefore, don't do well where water is limited.

Crassulacean Acid Metabolism (CAM)

As plants began moving into more marginal environments, such as deserts, C3 photosynthesis posed a particular problem, and two new forms of photosynthesis evolved. One of them is CAM photosynthesis, which allows plants to separate in time when they open their stomata to take in carbon dioxide from when sunlight is fueling their photosynthesis. Having their stomata open at night, when temperatures and therefore evaporative loss are lower, allows CAM plants, like cacti and orchids, to conserve water.

CAM comes with a cost, however, and is more metabolically expensive to accomplish than C3 photosynthesis is. But, in environments where sunlight is plentiful but water is not, CAM wins hands down against C3.

Crassulacean acid metabolism (CAM) was discovered in the Crassulaceae. These are succulents like sedum (a common ground cover), cactuses and jade plants, and some orchids. Stomata in chlorenchymal (mesophyll) leaf cells close during the day to minimize water loss by transpiration. The stomata open at night, allowing plant tissues to take up CO₂.

CAM plants fix CO₂ by combining it with PEP (phosphoenol pyruvate). This eventually produces malic acid that is stored in plant cell vacuoles. By day, stored malic acid retrieved from the vacuoles splits into pyruvate and CO₂. The CO₂ then enters chloroplasts and joins the Calvin Cycle to make glucose and the starches.

In sum, CAM plant mesophyll cells

open stomata to collect, fix and store CO₂ as an organic acid at night.
close stomata to conserve water in the daytime.
re-fix the stored CO₂ as carbohydrate using the NADPH and ATP from the light reaction the next the day.

The C4 Photosynthetic Pathway

Unique leaf structures permit carbon dioxide to be concentrated in 'bundle sheath' cells and near Rubisco during C4 photosynthesis, which produces four-carbon molecules. This arrangement successfully eliminates Rubisco's interface with the oxygen and the necessity for photorespiration by delivering carbon dioxide directly. Furthermore, this adaptation permits plants to retain water by allowing them to fix carbon even while their stomata are closed.

C4 plants, such as maize, sugarcane, sorghum, and silvergrass (miscanthus), as well as supergrains amaranth and quinoa, prevent photorespiration by utilising PEP, an enzyme, during the initial step of carbon fixation. This process occurs in the mesophyll cells, which are located close to the stomata where carbon dioxide and oxygen enter the plant. PEP is more attracted to carbon dioxide molecules and is, therefore, much less likely to react with oxygen molecules. PEP fixes carbon dioxide into a four-carbon molecule, called malate, that is transported to the deeper bundle sheath cells that contain Rubisco. The malate is then broken down into a compound that is recycled back into PEP and carbon dioxide that Rubisco fixes into sugars—without having to deal with the oxygen molecules that are abundant in the mesophyll cells.

C4 refers to malic acid, the 4-carbon end product of CO₂ fixation. In this regard, the C4 pathway is the same as in CAM metabolism! In both pathways, PEP carboxylase is the catalyst of carbon fixation, converting phosphoenol pyruvate (PEP) to oxaloacetate (OAA). The OAA is then reduced to malic acid.

C4 metabolism diverges from CAM pathway after malic acid formation. PEP carboxylase catalysis is rapid in C4 plants, in part because malic acid does not accumulate in the mesophyll cells. Instead, it is rapidly transferred from mesophyll to adjacent bundle sheath cells, where it enters chloroplasts. The result is that C4 plants can keep stomata open for CO₂ capture (unlike CAM plants), but closed at least part of the day to conserve water. The 4-carbon malic acid is oxidized to pyruvate (three carbons) in the bundle sheath cell chloroplasts. The CO₂ released enters the Calvin cycle to be rapidly fixed by Rubisco. Of course, this system allows more efficient water use and faster carbon fixation under high heat, dry conditions than does C3 photosynthesis.

C4 plants are mainly found in tropical and warm-temperate regions, predominantly in open grasslands where they are often dominant. While most are graminoids, other growth forms such as forbs, vines, shrubs, and even some trees and aquatic plants are also known among C4 plants.

Monocots – mainly grasses (Poaceae) and sedges (Cyperaceae) – account for around 80% of C4 species, but they are also found in the eudicots. Moreover, almost all C4 plants are herbaceus, with the notable exception of some woody species from the Euphorbia genus, such as the tree Euphorbia olowaluana. The reason behind C4 metabolism extreme rarity in trees is debated: hypotheses vary from a possible reduction in photosynthetic quantum yield under dense canopy conditions, coupled with an increased metabolic energy consumption (inherent to C4 metabolism itself), to less efficient sunflecks utilization.

Some well-known crops using the C4 photosynthesis path are:

maize
sugarcane
sorghum
pearl millet

There are also quite a few C4 weeds and invasive plants.

Although only 3% of flowering plant species use C4 carbon fixation, they account for 23% of global primary production.

The repeated, convergent C4 evolution from C3 ancestors has spurred hopes to bio-engineer the C4 pathway into C3 crops such as rice.

Comparison between

C3	C4	CAM
The most common kind of The Calvin cycle creates three carbon compounds during photosynthesis.	Photosynthesis yields an intermediate four-carbon molecule that is divided into three carbon compounds for the Calvin cycle.	Photosynthesis that collects sunlight during the day and fixes carbon dioxide at night.
The first stable product is 3-phosphoglycerate	The first stable product is oxaloacetate (4-carbon)	The first stable product is mallic acid, stored as a four carbon compound
only happens in mesophyll cells	occurs in mesophyll and bundle sheath cells	occurs in mesophylls
photorespiration is high	photorespiration is low	photorespiration is very low
water-use efficiency is low	water-use efficiency is moderate	water-use efficiency is high
CO₂ compensation point is 50-100 ppm	CO₂ compensation point is 10-20 ppm	CO₂ compensation point is very low
optimal temperature for photosynthesis is 15-25C	optimal temperature for photosynthesis is 30-40C	optimal temperature for photosynthesis is 35-45C
C3	C4	CAM
in most plants	in around 3% of vascular plants (maize, sugarcane, sorghum, millet)	in plants that are suited to arid settings (many succulents such as Cactaceae, Agavacea, Crassulaceae, Euphorbiaceae, Liliaceae, Vitaceae (grapes), Orchidaceae, bromeliads, cycads, peperomias, also pineapple, aloe, agave, moringa)
no specific features to counteract photorespiration	reduces photorespiration by carrying out carbon dioxide fixation and the Calvin cycle in separate cells	reduces photorespiration by executing carbon dioxide fixation and the Calvin cycle at different periods

Non-Chemical Solutions

Another solution to the problem of water loss is not so much biochemical as morphological. As an organism decreases its surface area to volume ratio, becoming ever more sphere-like, the amount of water lost from its surface is reduced. There is no negotiating with the math: more spherical cacti lose less water than long and lean cacti because they have less surface area relative to their volume from which to lose water. Many plants employ multiple strategies, of course—alternative metabolic pathways, in the form of CAM, and shape changes to reduce water loss.