Why are C4 plants mainly tropical plants?

Why are C4 plants mainly tropical plants?

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Why is it that most C4 plants are tropical plants?

As far as I am aware the C4 cycle does not prevent water loss; so having it exist in tropical areas does not serve these plants any good other than preventing photo-respiration. Hence I do not understand why it is solely found in tropical plants and not plants from other areas as photo-respiration is a problem not limited to tropical areas


This is not my field, so the following is based solely on a brief reading of Internet sources.

The Wikipedia entry on C4 carbon fixation states:

When grown in the same environment, at 30°C, C3 grasses lose approximately 833 molecules of water per CO2 molecule that is fixed, whereas C4 grasses lose only 277.

However it does not explain why this is so.

In contrast, the section in Berg et al. focuses on the energy losses caused by photorespiration and makes the following initial point:

… the oxygenase activity of rubisco increases more rapidly with temperature than does its carboxylase activity. How then do plants, such as sugar cane, that grow in hot climates prevent very high rates of wasteful photorespiration?

The answer to this (didactic) question is the C4 pathway (as described in full in that text), but the answer to the question posted here is that “tropical plants employ the C4 pathway because of this increased wastage of energy through photorespiration at the higher temperatures in which they live.”

C4 carbon fixation

C4 carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the discovery by Marshall Davidson Hatch and Charles Roger Slack [1] that some plants, when supplied with 14 CO
2 , incorporate the 14 C label into four-carbon molecules first.

C4 fixation is an addition to the ancestral and more common C3 carbon fixation. The main carboxylating enzyme in C3 photosynthesis is called RuBisCO, and catalyses two distinct reactions, with CO
2 (carboxylation), and with oxygen (oxygenation), which gives rise to the wasteful process of photorespiration. C4 photosynthesis reduces photorespiration by concentrating CO
2 around RuBisCO. To ensure that RuBisCO works in an environment where there is a lot of carbon dioxide and very little oxygen, C4 leaves generally differentiate two partially isolated compartments called mesophyll cells and bundle-sheath cells. CO
2 is initially fixed in the mesophyll cells by the enzyme PEP carboxylase which reacts the three carbon phosphoenolpyruvate (PEP) with CO
2 to form the four carbon oxaloacetic acid (OAA). OAA can be chemically reduced to malate or transaminated to aspartate. These diffuse to the bundle sheath cells, where they are decarboxylated, creating a CO
2 -rich environment around RuBisCO and thereby suppressing photorespiration. The resulting pyruvate (PYR) together with about half of the phosphoglycerate (PGA) produced by Rubisco diffuse back to the mesophyll. PGA is then chemically reduced and diffuses back to the bundle sheath to complete the reductive pentose phosphate cycle (RPP). This exchange of metabolites is essential for C4 photosynthesis to work.

On the one hand, these additional steps require more energy in the form of ATP used to regenerate PEP. On the other, concentrating CO
2 allows to overcome the reduction of gas solubility with temperatures (Henry's law) allowing high rates of photosynthesis at high temperatures. The CO
2 concentrating mechanism also allows to maintain high gradients of CO
2 concentration across the stomatal pores. This means that C4 plants have generally lower stomatal conductance, reduce water losses and have generally higher water use efficiency. [2] C4 plants are also more efficient in using nitrogen, since PEP carboxylase is much cheaper to make than RuBisCO. [3] However, since the C3 pathway does not require extra energy for the regeneration of PEP, it is more efficient in conditions where photorespiration is limited, like, typically, at low temperatures and in the shade. [4]


The C4 photosynthetic carbon cycle is an elaborated addition to the C3 photosynthetic pathway. It evolved as an adaptation to high light intensities, high temperatures, and dryness. Therefore, C4 plants dominate grassland floras and biomass production in the warmer climates of the tropical and subtropical regions ( Edwards et al., 2010).

In all plants CO2 is fixed by the enzyme Rubisco. It catalyzes the carboxylation of ribulose-1,5-bisphosphate, leading to two molecules of 3-phosphoglycerate. Instead of CO2, Rubisco can also add oxygen to ribulose-1,5-bisphosphate, resulting in one molecule each of 3-phosphoglycerate and 2-phosphoglycolate. Phosphoglycolate has no known metabolic purpose and in higher concentrations it is toxic for the plant ( Anderson, 1971). It therefore has to be processed in a metabolic pathway called photorespiration. Photorespiration is not only energy demanding, but furthermore leads to a net loss of CO2. Thus the efficiency of photosynthesis can be decreased by 40% under unfavorable conditions including high temperatures and dryness ( Ehleringer et al., 1991). The unfavorable oxygenase reaction of Rubisco can be explained as a relict of the evolutionary history of this enzyme, which evolved more than 3 billion years ago when atmospheric CO2 concentrations were high and oxygen concentrations low. Apparently, later on, it was impossible to alter the enzyme’s properties or to exchange Rubisco by another carboxylase. Nevertheless, plants developed different ways to cope with this problem. Perhaps the most successful solution was C4 photosynthesis.

The establishment of C4 photosynthesis includes several biochemical and anatomical modifications that allow plants with this photosynthetic pathway to concentrate CO2 at the site of Rubisco. Thereby its oxygenase reaction and the following photorespiratory pathway are largely repressed in C4 plants. In most C4 plants the CO2 concentration mechanism is achieved by a division of labor between two distinct, specialized leaf cell types, the mesophyll and the bundle sheath cells, although in some species C4 photosynthesis functions within individual cells ( Edwards et al., 2004). Since Rubisco can operate under high CO2 concentrations in the bundle sheath cells, it works more efficiently than in C3 plants. Consequently C4 plants need less of this enzyme, which is by far the most abundant protein in leaves of C3 plants. This leads to a better nitrogen-use efficiency of C4 compared to C3 plants, since the rate of photosynthesis per unit nitrogen in the leaf is increased ( Oaks, 1994). Additionally C4 plants exhibit better water-use efficiency than C3 plants. Because of the CO2 concentration mechanism they can acquire enough CO2 even when keeping their stomata more closed. Thus water loss by transpiration is reduced ( Long, 1999).

In the mesophyll cells of C4 plants CO2 is converted to bicarbonate by carbonic anhydrase and initially fixed by phosphoenolpyruvate (PEP) carboxylase (PEPC) using PEP as CO2 acceptor. The resulting oxaloacetate is composed of four carbon atoms, which is the basis for the name of this metabolic pathway. Oxaloacetate is rapidly converted to the more stable C4 acids malate or Asp that diffuse to the bundle sheath cells. Here, CO2 is released by one of three different decarboxylating enzymes, which define the three basic biochemical subtypes of C4 photosynthesis, NADP-dependent malic enzyme (NADP-ME), NAD-dependent ME (NAD-ME), and PEP carboxykinase (PEPCK). The released CO2 is refixed by Rubisco, which exclusively operates in the bundle sheath cells in C4 plants. The three-carbon compound resulting from CO2 release diffuses back to the mesophyll cells where the primary CO2 acceptor PEP is regenerated by pyruvate orthophosphate dikinase by the consumption of, at the end, two molecules of ATP ( Hatch, 1987).

Figure 1 shows a scheme of the NADP-ME subtype of C4 photosynthesis. Here malate is the dominant transport metabolite while Asp can be used in parallel. The synthesis of malate occurs in the mesophyll chloroplasts, the decarboxylation by NADP-ME in the bundle sheath chloroplasts.

NADP-ME type of C4 photosynthesis. 3-PGA, 3-Phosphoglyceric acid AspAT, Asp aminotransferase AlaAT, Ala aminotransferase CA, carbonic anhydrase MDH, malate dehydrogenase OAA, oxaloacetate PPDK, pyruvate orthophosphate dikinase TP, triosephosphate.

NADP-ME type of C4 photosynthesis. 3-PGA, 3-Phosphoglyceric acid AspAT, Asp aminotransferase AlaAT, Ala aminotransferase CA, carbonic anhydrase MDH, malate dehydrogenase OAA, oxaloacetate PPDK, pyruvate orthophosphate dikinase TP, triosephosphate.

The two other biochemical subtypes differ from the NADP-ME type by the transport metabolites used and the subcellular localization of the decarboxylation reaction. In NAD-ME plants Asp, which is synthesized in the mesophyll cytosol, is used as transport metabolite. After deamination and reduction, the resulting malate is decarboxylated by NAD-ME in the bundle sheath mitochondria. Plants of the PEPCK type use Asp as well as malate as transport metabolites. Asp is synthesized in the cytosol of mesophyll cells and decarboxylated in the cytosol of bundle sheath cells by the combined action of Asp amino transferase and PEPCK. As in NADP-ME-type C4 species, malate is synthesized in the mesophyll chloroplasts but decarboxylated by NAD-ME in the mitochondria of bundle sheath cells. This reaction produces NADH that is used in the mitochondria to produce the ATP needed to drive the PEPCK reaction ( Hatch, 1987). If Asp is used as transport metabolite, usually the three-carbon decarboxylation product, pyruvate, is partially transported back to the mesophyll cells in the form of Ala to maintain the ammonia balance between the two cell types ( Hatch, 1987).

Compared to C3 plants the bundle sheath cells of C4 plants have expanded physiological functions. This is reflected by the enlargement and a higher organelle content of these cells in most C4 species. For the efficient function of the C4 pathway a close contact between mesophyll and bundle sheath cells is indispensable and they are tightly interconnected to each other by high numbers of plasmodesmata ( Dengler and Nelson, 1999). To ensure a direct contact between bundle sheath and mesophyll cells, C4 plants possess a characteristic leaf anatomy. The bundle sheath cells enclose the vascular bundles and are themselves surrounded by the mesophyll cells. The high vein density in the leaves of C4 plants leads to a nearly one-to-one ratio of the volumes of mesophyll and bundle sheath tissues. The internal anatomy of a C4 leaf is often composed of a repeating pattern of vein-bundle sheath-mesophyll-mesophyll-bundle sheath-vein. Because of its wreath-like structure this type of leaf anatomy was termed Kranz anatomy by the German botanist G. Haberlandt (1904). Kranz anatomy is found with more or less considerable variations in nearly all monocotyledonous and dicotyledonous lineages that use the two-cell mode of C4 photosynthesis.

While the above differences are directly related to the CO2 concentration mechanism, there are many further modifications known that evolved to integrate the C4 pathway optimally into the plant’s metabolism. For instance, C4 species of the NADP-ME subtype are depleted in PSII in their bundle sheath cells to lower oxygen production in these cells. Accordingly, the production of reduction equivalents in the bundle sheath cells is reduced and the reduction phase of the Calvin-Benson cycle, i.e. the conversion of 3-phosphoglycerate to triose phosphate, has been at least partially shifted to the mesophyll cells ( Fig. 1). There is another adaptation in C4 plants that affects the light reactions of photosynthesis. Compared to C3 photosynthesis the C4 pathway consumes one (PEPCK type) or two (NADP-ME and NAD-ME type) additional molecules of ATP per fixed CO2 without the need of additional reduction equivalents. This increase in ATP-to-NADPH ratio is compensated for in some C4 plants by enhancing cyclic electron flow around PSI, which provides additional ATP without concomitantly producing NADPH. Large-scale trancriptomic and proteomic approaches also revealed that other metabolic pathways such as amino acid synthesis, nitrogen or sulfur assimilation, and lipid metabolism are compartmentalized between mesophyll and bundle sheath cells in at least some C4 plants ( Majeran and van Wijk, 2009).

C4 and CAM Plants

C4 and CAM plants are plants that use certain special compounds to gather carbon dioxide (CO 2 ) during photosynthesis. Using these compounds allows these plants to extract more CO 2 from a given amount of air, helping them prevent water loss in dry climates.

All photosynthetic plants need carbon to build sugars, and all get their carbon from CO 2 in the air. CO 2 must first be bound, or ȯixed," to another molecule inside the plant cell in order to begin its transformation into sugar. In most plants, carbon fixation occurs when CO 2 reacts with a five-carbon compound called RuBP (ribulose 1,5-bisphosphate). The product splits immediately to form a pair of three-carbon compounds, and therefore this pathway is called the C3 pathway. Further reaction leads to the creation of a sugar (glyceraldehyde-3-phosphate) and the regeneration of RuBP. This series of reactions is known as the Calvin-Benson cycle after the two scientists who elucidated it.

The enzyme that catalyzes the joining of RuBP and CO 2 is known as RuBP carboxylase, also called Rubisco. Rubisco is believed to be the most abundant protein in the world. However, Rubisco is not very efficient at grabbing CO 2 , and it has an even worse problem. When the concentration of CO 2 in the air inside the leaf falls too low, Rubisco starts grabbing oxygen instead. The ultimate result of this process, called photorespiration, is that sugar is burned up instead of being created. Photorespiration becomes a significant problem for plants during hot, dry days, when they must keep their stomates (leaf pores) closed to prevent water loss.

Diverse groups of plants have evolved different systems for coping with the problem of photorespiration. These plants, called C4 plants and CAM plants, initially bind carbon dioxide using a much more efficient enzyme. This allows a more efficient harvest of CO 2 , allowing the plant to trap sufficient CO 2 without opening its stomates too often. Each then uses the CO 2 in the Calvin-Benson cycle.

C4 (ȯour-carbon") plants initially attach CO 2 to PEP (phosphoenolpyruvate) to form the four-carbon compound OAA (oxaloacetate) using the enzyme PEP carboxylase. This takes place in the loosely packed cells called mesophyll cells. OAA is then pumped to another set of cells, the bundle sheath cells, which surround the leaf vein. There, it releases the CO 2 for use by Rubisco. By concentrating CO 2 in the bundle sheath cells, C4 plants promote the efficient operation of the Calvin-Benson cycle and minimize photorespiration. C4 plants include corn, sugar cane, and many other tropical grasses.

CAM (Ȭrassulacean acid metabolism") plants also initially attach CO 2 to PEP and form OAA. However, instead of fixing carbon during the day and pumping the OAA to other cells, CAM plants fix carbon at night and store the OAA in large vacuoles within the cell. This allows them to have their stomates open in the cool of the evening, avoiding water loss, and to use the CO 2 for the Calvin-Benson cycle during the day, when it can be driven by the sun's energy. CAM plants are more common than C4 plants and include cacti and a wide variety of other succulent plants.

Plant Life

Alternative forms of photosynthesis are used by specific types of plants, called C4 and CAM plants, to alleviate problems of photorespiration and excess water loss.

Photosynthesis is the physiological process whereby plants use the sun’s radiant energy to produce organic molecules. The backbone of all such organic compounds is a skeleton composed of carbon atoms. Plants use carbon dioxide from the atmosphere as their carbon source.

The overwhelming majority of plants use a single chemical reaction to attach carbon dioxide from the atmosphere onto an organic compound, a process referred to as carbon fixation. This process takes place inside specialized structures within the cells of green plants known as chloroplasts.

The enzyme that catalyzes this fixation is ribulose bisphosphate carboxylase (Rubisco), and the first stable organic product is a three-carbon molecule. This three-carbon compound is involved in the biochemical pathway known as the Calvin cycle. Plants using carbon fixation are referred to as C3 plants because the first product made with carbon dioxide is a three-carbon molecule.

For many years scientists thought that the only way photosynthesis occurred was through C3 photosynthesis. In the early 1960’s, however, researchers studying the sugarcane plant discovered a biochemical pathway that involved incorporation of carbon dioxide into organic products at two different stages.

First, carbon dioxide from the atmosphere enters the sugarcane leaf, and fixation is accomplished by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase). This step takes place within the cytoplasm, not inside the chloroplasts. The first stable product is a four-carbon organic compound that is an acid, usually malate. Sugarcane and other plants with this photosynthetic pathway are known as C plants.

In C4 plants, this photosynthetic pathway is tied to a unique leaf anatomy known as Kranz anatomy. This term refers to the fact that in C4 plants the cells that surround the water- and carbohydrate conducting system (known as the vascular system) are packed very tightly together and are called bundle sheath cells.

Surrounding the bundle sheath is a densely packed layer of mesophyll cells. The densely packed mesophyll cells are in contact with air spaces in the leaf, and because of their dense packing they keep the bundle sheath cells from contact with air. This Kranz anatomy plays a major role in C4 photosynthesis.

In C4 plants the initial fixation of carbon dioxide from the atmosphere takes place in the densely packed mesophyll cells. After the carbon dioxide is fixed into a four-carbon organic acid, the malate is transferred through tiny tubes from these cells to the specialized bundle sheath cells.

Inside the bundle sheath cells, the malate is chemically broken down into a smaller organic molecule, and carbon dioxide is released. This carbon dioxide then enters the chloroplast of the bundle sheath cell and is fixed a second timewith the enzyme Rubisco and continues through the C3 pathway.

Advantages of Double-Carbon Fixation

The double-carbon fixation pathway confers a greater photosynthetic efficiency on C4 plants over C3 plants, because the C3 enzyme Rubisco is highly inefficient in the presence of elevated levels of oxygen. In order for the enzyme to operate, carbon dioxide must first attach to the enzyme at a particular location known as the active site.

However, oxygen is also able to attach to this active site and prevent carbon dioxide from attaching, a process known as photorespiration. As a consequence, there is an ongoing competition between these two gases for attachment at the active site of the Rubisco enzyme. Not only does the oxygen outcompete carbon dioxide when oxygen binds to Rubisco, it also destroys some of the molecules in the Calvin

At any given time, the winner of this competition is largely dictated by the relative concentrations of these two gases. When a plant opens its stomata (the pores in its leaves), the air that diffuses in will be at equilibrium with the atmosphere, which is 21 percent oxygen and 0.04 percent carbon dioxide.

During hot, dry weather, excess water vapor diffuses out, and under these conditions plants face certain desiccation if the stomata are left open continuously.When these pores are closed, the concentration of gases will change. As photosynthesis proceeds, carbon dioxide will be consumed and oxygen generated.

When the concentration of carbon dioxide drops below 0.01 percent, oxygen will outcompete carbon dioxide at the active site, and no net photosynthesis occurs. C4 plants, however, are able to prevent photorespiration, because the PEP carboxylase enzyme is not inhibited by oxygen.

Thus, when the stomata are closed, this enzyme continues to fix carbon inside the leaf until it is consumed. Because the bundle sheath is isolated from the leaf’s air spaces, it is not affected by the rising oxygen levels, and the C3 cycle functions without interference. C4 photosynthesis is found in at least nineteen families of flowering plants.

No family is exclusively composed of C4 plants. Because C4 photosynthesis is an adaptation to hot, dry environments, especially climates found in tropical regions, C4 plants are often able to out compete C3 plants in those areas. In more temperate regions, they have less of an advantage and are therefore less common.

A second alternative photosynthetic pathway, known as crassulacean acid metabolism (CAM), exists in succulents such as cacti and other desert plants. These plants have the same two carbon-fixing steps as are present in C4 plants, but rather than being spatially separated between the mesophyll and bundle sheath cells, CAM plants have both carbon dioxide-fixing enzymes within the same cell.

These enzymes are active at different times, PEP carboxylase during the day and Rubisco at night. Just as Kranz anatomy is unique to C4 plants, CAM plants are unique in that the stomata are open at night and largely closed during the day.

The biochemical pathway of photosynthesis in CAM plants begins at night. With the stomata open, carbon dioxide diffuses into the leaf and into mesophyll cells, where it is fixed by the C4 enzyme PEP carboxylase. The product is malate, as in C4 photosynthesis, but it is transformed into malic acid (a nonionic form of malate) and is stored in the cell’s vacuoles (cavities within the cytoplasm) until the next day.

Although the malic acid will be used as a carbon dioxide source for the C3 cycle, just as in C4 photosynthesis, it is stored until daylight because the C3 cycle requires light as an energy source. The vacuoles will accumulate malic acid through most of the night.

A few hours before daylight, the vacuole will fill up, and malic acid will begin to accumulate in the cytoplasm outside the vacuole. As it does, the pH of the cytoplasm will become acidic, causing the enzyme to stop functioning for the rest of the night.

When the sun rises the stomata will close, and photosynthesis by the C3 cycle will quickly deplete the atmosphere within the leaf of all carbon dioxide. At this time, the malic acid will be transported out of the vacuole to the cytoplasmof the cell. There it will be broken down, and the carbon dioxide will enter the chloroplast and be used by the C3 cycle thus, photosynthesis is able to continue with closed stomata.

Crassulacean acid metabolism derives its name from the fact that it involves a daily fluctuation in the level of acid within the plant and that it was first discovered to be common in species within the stonecrop family, Crassulaceae.

The discovery of this photosynthetic pathway dates back to the 1960’s. The observation that succulent plants become very acidic at night, however, dates back to at least the seventeenth century, when it was noted that cactus tastes sour in the morning and bitter in the afternoon.

There are two distinctly different ecological environments where CAM plants may be found. Most are terrestrial plants typical of deserts or other harsh, dry sites.

In these environments, the pattern of stomatal opening and closing provides an important advantage for surviving arid conditions: When the stomata are open, water is lost however, the rate of loss decreases as the air temperature decreases. By restricting the time period of stomatal opening to the nighttime, CAM plants are extremely good at conserving water.

The other ecological setting where CAM plants are found is in certain aquatic habitats. When this environment was first discovered, it seemed quite odd, because in these environments conserving water would be of little value to a plant. It was found, however, that there are aspects of the aquatic environment which make CAM photosynthesis advantageous.

In shallow bodies of water, the photosynthetic consumption of carbon dioxide may proceed at a rate in excess of the rate of diffusion of carbon dioxide from the atmosphere into the water, largely because gases diffuse several times more slowly in water than in air.

Consequently, pools of water may be completely without carbon dioxide for large parts of the day. Overnight, carbon dioxide is replenished, and aquatic CAM plants take advantage of this condition to fix the plentiful supply of carbon dioxide available at night and store it as malic acid.

Hence, during the day, when the ambient carbon dioxide concentration is zero, these plants have their own internal supply of carbon dioxide for photosynthesis. Thus, two very different ecological conditions have selected for the identical biochemical pathway.

These two modified photosynthetic pathways adequately describe what happens in most terrestrial plants, although there is much variation. For example, there are species that appear in many respects to have photosynthetic characteristics intermediate to C3 and C4 plants.

Other plants are capable of switching from exclusively C3 photosynthesis to CAM photosynthesis at different times of the year. Photosynthesis by aquatic plants appears to present even more variation. C3-C4 intermediate plants seem to be relatively common compared to the terrestrial flora, and several species have C4 photosynthesis but lack Kranz anatomy.

National Science Foundation - Where Discoveries Begin

Mandibles of A. anamensis (left) from Kenya and A. afarensis from Ethiopia.

June 3, 2013

This material is available primarily for archival purposes. Telephone numbers or other contact information may be out of date please see current contact information at media contacts.

Most apes eat leaves and fruits from trees and shrubs.

But new studies show that human ancestors expanded their menu 3.5 million years ago, adding tropical grasses and sedges to an ape-like diet. The change set the stage for consuming more modern fare: grains, grasses, and meat and dairy from grazing animals.

In four studies of carbon isotopes in fossilized tooth enamel from scores of human ancestors and baboons in Africa from 4 million to 10,000 years ago, researchers found a surprise increase in the consumption of grasses and sedges--plants that resemble grasses and rushes but have stems with triangular cross sections.

"At last, we have a look at 4 million years of the dietary evolution of humans and their ancestors," says University of Utah geochemist Thure Cerling, lead author of two of four papers published online today in the journal Proceedings of the National Academy of Sciences (PNAS).

Funding was primarily from the National Science Foundation's (NSF) Divisions of Behavioral and Cognitive Sciences, Earth Sciences and Integrative Organismal Systems.

"For a long time, primates stuck by the old restaurants--leaves and fruits--but by 3.5 million years ago, they started exploring new diet possibilities--tropical grasses and sedges--that grazing animals discovered a long time before, about 10 million years ago," Cerling says, when African savanna began expanding.

"Tropical grasses provided a new set of restaurants. We see an increasing reliance on this resource by human ancestors, one that most primates still don't use today."

Grassy savannas and grassy woodlands in East Africa were widespread by 6 million to 7 million years ago. A major question is why human ancestors didn't start exploiting savanna grasses until less than 4 million years ago.

The isotope method cannot distinguish what parts of grasses and sedges human ancestors ate--leaves, stems, seeds and/or underground storage organs such as roots or rhizomes.

The method also can't help with determining when human ancestors began getting much of their grass through eating grass-eating insects or meat from grazing animals.

Direct evidence of human ancestors scavenging meat doesn't appear until 2.5 million years ago, and definitive evidence of hunting dates to only about 500,000 years ago.

With the new findings, "we know much better what they were eating, but mystery does remain," says Cerling.

"We don't know if they were pure herbivores or carnivores, if they were eating fish [which leave a tooth signal that looks like grass-eating], if they were eating insects, or if they were eating mixes of all these."

Why our ancestors' diets matter

The earliest human ancestor to consume substantial amounts of grassy foods from dry, more open savannas "may signal a major and ecological and adaptive divergence from the last common ancestor we shared with African great apes, which occupy closed, wooded habitats," writes geologist Jonathan Wynn, lead author of one of the papers.

Wynn is currently a program director in NSF's Division of Earth Sciences, on leave from the University of South Florida.

"Diet has long been implicated as a driving force in human evolution," says Matt Sponheimer, an anthropologist at the University of Colorado, Boulder.

He notes that changes in diet have been linked to larger brain size and the advent of upright walking in human ancestors roughly 4 million years ago.

Human brains were larger than those of other primates by the time our genus, Homo, evolved 2 million years ago. (Our species, Homo sapiens, arose 200,000 years ago.)

"If diet has anything to do with the evolution of larger brain size and intelligence, then we are considering a diet that is very different than we were thinking about 15 years ago," says Cerling. At the time, it was believed that human ancestors ate mostly leaves and fruits.

How the studies were performed: you are what you eat

The new studies analyze carbon isotopes in 173 teeth in 11 species of hominins.

Hominins are humans, our ancestors and extinct relatives that split from other apes roughly 6 million years ago.

Some of the analyses were done in previous research, but the new studies include new carbon-isotope results for 104 teeth from 91 individuals of eight hominin species.

Those teeth are in African museums and were studied by two groups of scientists working at separate early human sites in East Africa.

Wynn wrote the paper about teeth from Ethiopia's Hadar-Dikika area, where research is led by Arizona State University's William Kimbel and California Academy of Sciences scientist Zeresenay Alemseged.

Cerling wrote the paper about teeth from the Turkana Basin in Kenya, where the research team is led by Turkana Basin Institute paleoanthropologist Meave Leakey, Cerling and geologist Frank Brown of the University of Utah. Cerling also wrote a paper about baboon diets. Sponheimer wrote a fourth paper, summarizing the other three.

The method of determining ancient creatures' diets from carbon isotope data is less than 20 years old, and is based on the idea that "you are what you eat," Sponheimer says.

Tiny amounts of tooth enamel were drilled from already broken fossil teeth of museum specimens of human ancestors and relatives.

The powder was placed in a mass spectrometer to learn ratios of carbon isotopes incorporated into tooth enamel via diet.

Ratios of rare carbon-13 to common carbon-12 reveal whether an animal ate plants that used so-called C3, C4 or CAM photosynthesis to convert sunlight to energy.

Animals eating C4 and CAM plants have enriched amounts of carbon-13.

C3 plants include trees, bushes and shrubs and their leaves and fruits most vegetables cool-season grasses and grains such as timothy, alfalfa, wheat, oats, barley and rice soybeans non-grassy herbs and forbs.

C4 plants are warm-season or tropical grasses and sedges and their seeds, leaves or storage organs like roots and tubers. C4 plants are common in African savannas and deserts.

C4 grasses include Bermuda grass and sorghum. C4 grains include corn and millet.

CAM plants include tropical succulent plants such as cactus, salt bush and agave.

Today, North Americans eat about half C3 plants, including vegetables, fruits and grains such as wheat, oats, rye and barley, and about half C4 plants, which largely come from corn, sorghum and meat animals fed on C4 grasses and grains.

The highest human C3 diets are in northern Europe, where only C3 cool-season grasses grow, so meat animals there graze on them rather than on C4 tropical grasses.

The highest C4 diets likely are in Central America because of the heavily corn-based diet.

If early humans ate grass-eating insects or large grazing animals like zebras, wildebeest and buffalo, it also would appear they ate C4 grasses.

If they ate fish that ate algae, it would give a false appearance of grass-eating because of the way algae takes up carbonate from water, Cerling says.

If they ate small antelope and rhinos that browsed on C3 leaves, it would appear they ate C3 trees-shrubs.

Small mammals such as hyrax, rabbits and rodents would have added C3 and C4 signals to the teeth of human ancestors.

The findings: a dietary history of human ancestors and relatives

  • Previous research showed that 4.4 million years ago in Ethiopia, early human relative Ardipithecus ramidus ("Ardi") ate mostly C3 leaves and fruits.
  • About 4.2 million to 4 million years ago on the Kenyan side of the Turkana Basin, Cerling's results show that human ancestor Australopithecus anamensis ate at least 90 percent leaves and fruits--the same diet as modern chimps.
  • By 3.4 million years ago in northeast Ethiopia's Awash Basin, according to Wynn, Australopithecus afarensis was eating significant amounts of C4 grasses and sedges: 22 percent on average, but with a wide range among individuals of anywhere from 0 percent to 69 percent grasses and sedges. The species also ate some succulent plants.
    Wynn says that the switch "documents a transformational stage in our ecological history." Many scientists previously believed A. afarensis had an ape-like C3 diet. It remains a mystery why A. afarensis expanded its menu to C4 grasses when its likely ancestor, A. anamensis, did not, although both inhabited savanna habitats, Wynn says.
  • Also by 3.4 million years ago in Turkana, human relative Kenyanthropus platyops had switched to a highly varied diet of both C3 trees and shrubs and C4 grasses and sedges. The average was 40 percent grasses and sedges, but individuals varied widely, eating anywhere from 5 percent to 65 percent, Cerling says.
  • About 2.7 million to 2.1 million years ago in southern Africa, hominins Australopithecus africanus and Paranthropus robustus ate tree and shrub foods, but also ate grasses and sedges and perhaps grazing animals.
    A africanus averaged 50 percent C4 grass-sedge-based foods, but individuals ranged from none to 80 percent. P. robustus averaged 30 percent grasses-sedges, but ranged from 20 percent to 50 percent.
  • By 2 million to 1.7 million years ago in Turkana, early humans, Homo, ate a 35 percent grass-and-sedge diet - some possibly from the meat of grazing animals - while another hominin, Paranthropus boisei, was eating 75 percent grass - more than any hominin, according to a 2011 study by Cerling.
    Paranthropus likely was vegetarian. Homo had a mixed diet that likely included meat or insects that had eaten grasses. Wynn says that a drier climate may have made Homo and Paranthropus more reliant on C4 grasses.
  • By 1.4 million years ago in Turkana, Homo had increased the proportion of grass-based food to 55 percent.
  • Some 10,000 years ago in Turkana, Homo sapiens' teeth reveal a diet split 50-50 between C3 trees and shrubs and C4 plants and likely meat - almost identical to the ratio in modern North Americans, Cerling says.

Humans: the only surviving primates with a C4 grass diet

Cerling's results show that while human ancestors ate more grasses and other apes stuck with trees and shrubs, two extinct Kenyan baboons represent the only primate genus that ate primarily grasses and perhaps sedges throughout its history.

Theropithecus brumpti ate a 65 percent tropical grass-and-sedge diet when the baboons lived between four million and 2.5 million years ago, contradicting previous claims that they ate forest foods.

Theropithecus oswaldi ate a 75 percent grass diet by two million years ago and a 100 percent grass diet by one million years ago. Both species went extinct, perhaps due to competition from hooved grazing animals.

Modern Theropithecus gelada baboons live in Ethiopia's highlands, where they eat only C3 cool-season grasses.

Cerling notes that primate tropical grass-eaters--Theropithecus baboonsand Paranthropus human relatives--went extinct while human ancestors ate an increasingly grass-based diet.

Additional support for the research came from many other organizations, including the National Geographic Society and the Leakey Foundation.

Geologist Thure Cerling of the University of Utah in the Turkana Basin in Kenya.
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Image showing multiple skulls of hominins from the Turkana Basin
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Field crew sieves for fossils with geologist Jonathan Wynn.
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The fossil-bearing sediments of Ethiopia's Hadar Formation at Dikika and Hadar.
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"Commuting" to the field research site through the Awash Valley in Ethiopia.
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Media Contacts
Cheryl Dybas, NSF, (703) 292-7734, email: [email protected]
Lee Siegel, University of Utah, (801) 581-6773, email: [email protected]
Jim Scott, University of Colorado, (303) 492-3114, email: [email protected]

Related Websites
NSF News Release: Six Million Years of African Savanna:

The U.S. National Science Foundation propels the nation forward by advancing fundamental research in all fields of science and engineering. NSF supports research and people by providing facilities, instruments and funding to support their ingenuity and sustain the U.S. as a global leader in research and innovation. With a fiscal year 2021 budget of $8.5 billion, NSF funds reach all 50 states through grants to nearly 2,000 colleges, universities and institutions. Each year, NSF receives more than 40,000 competitive proposals and makes about 11,000 new awards. Those awards include support for cooperative research with industry, Arctic and Antarctic research and operations, and U.S. participation in international scientific efforts.

Geologist Thure Cerling of the University of Utah in the Turkana Basin in Kenya.
Credit and Larger Version

Image showing multiple skulls of hominins from the Turkana Basin
Credit and Larger Version

Field crew sieves for fossils with geologist Jonathan Wynn.
Credit and Larger Version

The fossil-bearing sediments of Ethiopia's Hadar Formation at Dikika and Hadar.
Credit and Larger Version

"Commuting" to the field research site through the Awash Valley in Ethiopia.
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The diffusion of CO2 into the leaf and chloroplast is directly dependent on temperature via diffusivity effects, stomatal control, solubilization and membrane permeability. Stomatal responses to temperature will not be covered in detail here, as they are highly variable and would require a lengthy review to provide a cogent synthesis. Depending upon species and growth conditions, stomata can open with rising temperature (a common response when vapor pressure deficit is low), close (often in response to increasing vapor pressure deficit with rising temperature) or remain unaffected ( Kemp & Williams 1980 Monson et al. 1982 Tenhunen et al. 1984 Sage & Sharkey 1987 Santrucek & Sage 1996 Cowling & Sage 1998 Yamori et al. 2006a ). It is worth noting that the sensitivity of A to variation in stomatal conductance generally increases at warmer temperatures, because the biochemical controls over A at high temperature are more sensitive to changes in Ci. If A is Pi-regeneration limited at cooler temperatures, large changes in stomatal conductance have little affect on A by contrast, a Rubisco limitation on A at elevated temperature creates a steep A/Ci response such that substantial changes in A would result from small changes in stomatal conductance. For this reason, stomatal limitations are generally greater at elevated temperature, regardless of the stomatal response ( Sage & Sharkey 1987 Hendrickson et al. 2004a ).

Mesophyll conductance (the conductance of CO2 from the stomata to the chloroplast stroma) affects stromal CO2 levels and as such, potentially contributes to the thermal response of A. Mesophyll conductance is highly sensitive to temperature, showing a Q10 near 2 below the thermal optimum, and a thermal optimum that is similar to the thermal optimum of A ( Bernacchi et al. 2002 Warren & Dreyer 2006 Yamori et al. 2006a ). This high Q10 indicates that the temperature response of mesophyll conductance is largely controlled by proteins ( Yamori et al. 2006a ). Both aquaporins and carbonic anhydrase are known to be important facilitators of CO2 entry into the cell and chloroplast ( Evans & Loreto 2000 Terashima et al. 2006 Yamori et al. 2006a ). Unlike the response of mesophyll conductance below the thermal optimum of A, above the thermal optimum, there is no clear pattern. Substantial reductions in mesophyll conductance at elevated temperature are reported for tobacco and cool-grown spinach ( Bernacchi et al. 2002 Warren and Dreyer 2006 Yamori et al. 2006a ), but little if any reductions occur in oak and warm-grown spinach between 25 and 35 °C ( Warren & Dreyer 2006 ). No study has examined the response of mesophyll conductance above 40 °C, so its contribution in the thermal range where electron transport or Rubisco activase may assert control is unknown. If mesophyll conductance substantially declines above 40 °C, then it could be a major limitation on A at elevated temperatures.

Mesophyll conductance also acclimates to variation in growth temperature. In rice, plants had a threefold higher mesophyll conductance when grown at 32 °C relative to 25 °C ( Makino et al. 1994 ). In spinach, the mesophyll conductance increased in warm-grown plants (30/25 °C) relative to cool grown (15/10 °C), but only at measurement temperatures above 20 °C plants from both growth conditions had the same thermal response of mesophyll conductance below 20 °C ( Yamori et al. 2006a ).

Why are C4 plants mainly tropical plants? - Biology

Why is Carbon Dioxide Important for Photosynthesis

Most of us know that humans and animals inhale oxygen and exhale carbon dioxide, which is absorbed by plants, but do we know why do plants need carbon dioxide?

What is the food source for plants?

There do they get the energy to grow?

There is a lot of talk on why carbon emissions are dangerous for the climate, but do we why it is dangerous for the environment?

Carbon dioxide is an important element in photosynthesis, which is a process that converts energy from sunlight to chemical energy stored in glucose.

The by-product of photosynthesis is oxygen, which is an essential element of life.

Carbon dioxide is important for photosynthesis because it provides the carbon required for the plant to produce glucose, which is used to complete cellular processes in the plant.

These cellular processes enable the plants to develop seeds, grow, make fruit, and form flowers.

What is photosynthesis?

During photosynthesis, plants gather carbon dioxide and water from air and soil. The water goes through oxidation, while the carbon dioxide goes through reduction.

This process converts carbon dioxide into glucose and the water into oxygen. Inside in the plant cell, there is a light-absorbing pigment called chlorophyll, which has the role of absorbing blue and red light waves from sunlight and reflect green light waves.

This gives the plants their green color. Chlorophyll allows us to absorb the energy needed to complete the photosynthesis process.

The process of photosynthesis can be divided into two main types of reactions. There are light-dependent reactions and light-independent reactions.

The light-dependent reactions convert chemical energy in the form of NADPH and ATP. The light-dependent reactions include the Calvin Cycle, during which energy is used to gather glucose from carbon dioxide.

Carbon molecules are converted from carbon dioxide to stored fuel in the form of carbohydrates. These carbohydrates are used as a source of food or energy for the plant.

The process of photosynthesis is often written as the following chemical formula:

This means that six molecules of carbon dioxide (CO2) react with six molecules of water (H2O) to form glucose molecules and oxygen.

It is not just plants that can photosynthesize, but other eukaryotic and prokaryotic organisms also able to harvest their energy from photosynthesis.

The importance of carbon dioxide for plants

Photosynthesis is critical for the existence of life on earth. In a way, how the energy in the biosphere reaches living things on earth. The organisms that use photosynthesis form the primary producers of oxygen in the world.

Almost all of the oxygen on earth comes from photosynthesis. If this process was to stop, the world is left with no oxygen. The importance of photosynthesis extends to many life forms on earth, including plants.

Only organisms that could exist without oxygen are certain bacteria. Everything else is dependent on photosynthesis to produce oxygen.

It has been established that the process of photosynthesis cannot be completed without carbon dioxide. People often used they are feeding the plant by watering them or providing fertilizers, but the needs of the plant are not complete without carbon dioxide and sun.

Photosynthesis is one of the most important processes on earth. Not only is it used by plants but also other microorganisms and algae. Just as humans and animals need respiration to stay alive, plants need photosynthesis.

The entire process of photosynthesis can be summarized as a way to transfer energy from the sun to the plant.

Different types of plants have evolved to require a different amount of water, sunlight, and carbon dioxide. Plants in the desert, such as a cactus plant, are naturally designed to require less water, whereas plants in a pond as an abundance of water. Similarly, different plants require different levels of carbon dioxide.

What effect does increasing levels of carbon dioxide have on photosynthesis?

The amount of carbon dioxide in the world is increasing. According to, carbon dioxide levels in the world are its highest level since 800,000 years.

There are several reasons for the rise in carbon dioxide levels. The primary reason is the increase in the burning of fossil fuels such as oil and coal. These fossil fuels contain carbon, which has been absorbed through hundreds of years of photosynthesis.

You might think that if carbon dioxide is so important to plants, an increasing level of carbon dioxide should be immensely useful to plants, including food crops.

The level of food production in the world should increase, and the entire talk of fossil fuels being bad for the environment should be rubbished. It is true that in complete isolation, increasing levels of carbon dioxide will increase photosynthesis.

However, it is important to note that plants require more than just carbon dioxide to function. They need water, nitrogen, and other nutrients to function.

Any limitations of these essential elements will not allow the plants to grow. This is why plants need fertilizers that contain nitrogen. Any positive effect of increasing carbon dioxide in the world is negated by the increase in global temperature caused by global warming.

The increase in temperature has a devastating effect on plant life.

Future of photosynthesis

The increasing world population and the use of fossil fuels are putting an intense strain on the natural resources of the world. Food security is a critical issue for the survival of life on earth.

If the productivity of crops can be increased, it will have a significant positive impact on the sustainability of life.

Photosynthesis is a magnificent process of nature. However, there is a flaw in the process, or maybe nature intended it to be that way.

The oxygen and carbon dioxide molecules are similar in size and shape. An enzyme by the name of RuBisCO in plants is used to harvest carbon dioxide.

At times, this enzyme harvests an oxygen molecule mistaking it for a carbon dioxide molecule. The harvesting of oxygen molecules instead of carbon dioxide is putting a strain on the energy and resources of plants.

With global warming, the temperature of the earth is increasing. With the increase in temperature, the RubisCO is getting more prone to errors. Water also evaporates faster in increased temperatures.

This is straining the ecosystem of plants around the globe. As the RubisCO enzyme gets limited carbon dioxide, it depletes the energy of the plant is trying to harvest oxygen.

Some plants have evolved to handle this issue by pushing extra carbon dioxide to the RubisCO enzyme. This is like a turbocharged version of photosynthesis.

Plants that can do this are categorized as C4 plants. These plants can be highly effective in hot and dry weather, but as the global climate gets hotter, more of such plants will be seen everywhere.

At the moment, only 3% of the world’s flowering plants take the C4 route to photosynthesis. However, this 3% of the plants account for 24% of the world’s plant primary productivity in the world.

The type of plants mostly using the C4 pathway includes corn and sorghum. If somehow, other productive crops such as rice use the C4 pathway to photosynthesis, it can have an immense impact on the economics and food security of the world.

Content: C3, C4, and CAM pathways

Comparison Chart

Basis for ComparisonC3 pathway C4 pathway CAM
DefinitionSuch plants whose first product after the carbon assimilation from sunlight is 3-carbon molecule or 3-phosphoglyceric acid for the
production of energy is called C3 plants, and the pathway is called as the C3 pathway. It is most commonly used by plants.
Plants in the tropical area, convert the sunlight energy into C4 carbon molecule or oxaloacetice acid, which takes place before the C3 cycle
and then it further convert into the energy, is called C4 plants and pathway is called as the C4 pathway. This is more efficient than the C3 pathway.
The plants which store the energy from the sun and then convert it into energy during night follows the CAM or crassulacean acid
Cells involvedMesophyll cells.Mesophyll cell, bundle sheath cells.Both C3 and C4 in same mesophyll cells.
ExampleSunflower, Spinach, Beans, Rice, Cotton. Sugarcane, Sorghum and Maize. Cacti, orchids.
Can be seen in All photosynthetic plants.In tropical plantsSemi-arid condition.
Types of plants using this cycleMesophytic, hydrophytic, xerophytic.Mesophytic.Xerophytic.
PhotorespirationPresent in high rate.Not easily detectable.Detectable in the afternoon.
For the production of glucose12 NADPH and 18 ATPs are required.12 NADPH and 30 ATPs are required. 12 NADPH and 39 ATPs are required.
First stable product3-phosphoglycerate (3-PGA).Oxaloacetate (OAA).Oxaloacetate (OAA) at night, 3 PGA at daytime.
Calvin cycle operativeAlone.Along with the Hatch and Slack cycle.C3 and Hatch and Slack cycle.
Optimum temperature for photosynthesis15-25 °C30-40 °C> 40 degrees °C
Carboxylating EnzymeRuBP carboxylase.In mesophyll: PEP carboxylase.
In bundle sheath: RuBP carboxylase.
In the dark: PEP carboxylase.
In light: RUBP carboxylase.
CO2: ATP: NADPH2 ratio1:3:21:5:21:6.5:2
Initial CO2 acceptorRibulose-1,5-biphophate(RuBP).Phosphoenolpyruvate (PEP).Phosphoenolpyruvate (PEP).
Kranz Anatomy Absent.Present.Absent.
CO2 compensation point (ppm) 30-70.6-10.0-5 in dark.

Definition of a C3 pathway or Calvin cycle.

C3 plants are known as cool-season or temperate plants. They grow best at an optimum temperature between 65 to 75°F with the soil temperature suited at 40- 45°F. These types of plants show less efficiency at high temperature.

The primary product of C3 plants is 3-carbon acid or 3-phosphoglyceric acid (PGA). This is considered as the first product during carbon dioxide fixation. The C3 pathway completes in three steps: carboxylation, reduction, and regeneration.

C3 plants reduce into the CO2 directly in the chloroplast. With the help of ribulose biphosphate carboxylase (RuBPcase), the two molecules of 3-carbon acid or 3-phosphoglyceric acid are produced. This 3- phosphoglyceric justifies the name of the pathway as C3.

In another step, NADPH and ATP phosphorylate to give 3-PGA and glucose. And then the cycle again starts by regenerating the RuBP.

The C3 pathway is the single step process, takes place in the chloroplast. This organelle act as the storage of sunlight energy. Of the total plant present on earth, 85 percent uses this pathway for the production of energy.

The C3 plants can be perennial or annual. They are highly proteinaceous than the C4 plants. The examples of annual C3 plants are wheat, oats, and rye and the perennial plants include fescues, ryegrass, and orchardgrass. C3 plants provide a higher amount of protein than the C4 plants.

Definition of C4 pathway or Hatch and Slack pathway.

Plants, especially in the tropical region, follow this pathway. Before Calvin or C3 cycle, some plants follow the C4 or Hatch and Slack pathway. It is a two step process where Oxaloacetic acid (OAA) which is a 4-carbon compound is produced. It occurs in mesophyll and bundle sheath cell present in a chloroplast.

When the 4-carbon compound is produced, it is sent to the bundle sheath cell, here the 4-carbon molecule further get splits into a carbon dioxide and the 3-cabon compound. Eventually, the C3 pathway starts to produce energy, where the 3-carbon compound act as the precursor.

C4 plants are also known as warm-season or tropical plants. These can be perennial or annual.The perfect temperature to grow for these plants is 90-95°F. The C4 plants are much more efficient in utilizing nitrogen and gathering carbon dioxide from the soil and atmosphere. The protein content is low as compared to C3 plants.

These plants got their name from the product called as oxaloacetate which is 4 carbon acid. The examples of perennial C4 plants are Indian grass, Bermudagrass, switchgrass, big bluestem and that of annual C4 plants are sudangrasses, corn, pearl millet.

Definition of CAM plants

The noteworthy remark which distinguishes this process from the above two is that in this type of photosynthesis the organism absorbs the energy from the sunlight at the day time and uses this energy at the night time for the assimilation of carbon dioxide.

It is a kind of adaptation at the time of periodic drought. This process permits an exchange of gases at the night time when the air temperature is cooler, and there is the loss of water vapor.

Around 10% of the vascular plants have adapted the CAM photosynthesis but mainly found in plants grown in the arid region. The plants like cactus and euphorbias are the examples. Even the orchids and bromeliads, adapted this pathway due to an irregular water supply.

In the day time, malate gets decarboxylated to provide CO2 for the fixation of the Benson-Calvin cycle in closed stomata. The main feature of CAM plants is an assimilation of CO2 at night into malic acid, stored in the vacuole. PEP carboxylase plays the main role in the production of malate.

Photorespiration and the Evolution of C4 Photosynthesis

C4 photosynthesis is one of the most convergent evolutionary phenomena in the biological world, with at least 66 independent origins. Evidence from these lineages consistently indicates that the C4 pathway is the end result of a series of evolutionary modifications to recover photorespired CO2 in environments where RuBisCO oxygenation is high. Phylogenetically informed research indicates that the repositioning of mitochondria in the bundle sheath is one of the earliest steps in C4 evolution, as it may establish a single-celled mechanism to scavenge photorespired CO2 produced in the bundle sheath cells. Elaboration of this mechanism leads to the two-celled photorespiratory concentration mechanism known as C2 photosynthesis (commonly observed in C3–C4 intermediate species) and then to C4 photosynthesis following the upregulation of a C4 metabolic cycle.


This review resulted from a working group meeting funded by the ARC-NZ Research Network for Vegetation Function, headquartered in the Department of Biological Sciences, Macquarie University, Sydney, Australia. We thank the Network staff for their expert assistance in attending to organisational details. We thank Nerea Ubierna Lopez for comments on the manuscript. Lucas A. Cernusak gratefully acknowledges support from the Australian Research Council in the form of an APD Fellowship.

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Watch the video: Photosynthesis: Comparing C3, C4 and CAM (August 2022).