EBIO 4140 Plant Ecology: Photosynthesis, C3/C4 Pathways

This essay examines plant ecology, focusing on biomes, photosynthesis, and the impact of climate change. It begins by defining biomes and their characteristics, with a specific emphasis on the desert biome and its challenges. The essay then delves into the process of photosynthesis, including the light and carbon fixation cycles, and the role of water and nutrients in plant growth. Part 2 of the essay explores the differences between C3 and C4 photosynthetic pathways, their evolutionary origins, and their responses to changing climate conditions. It highlights the advantages of C4 plants in warmer, drier environments and discusses the implications of global warming on plant survival. The essay also includes references to support the discussed concepts.

Running head: PLANT ECOLOGY
EBIO 4140
Name of the Student
Name of the University
Author Note

PLANT ECOLOGY
Part 1
A biome can be classified as a community with large ecological areas on a global
scale. They are defined by abiotic factors such as vegetation, soils, geology, relief, climate
and temperature. The major types of biomes are forest, desert, aquatic and grasslands. Land-
based biomes are known as terrestrial biomes and water-based biomes are known as aquatic
biomes. Biomes contain several ecosystems within the same ecological area distribution.
Biomes are distributed globally based on the average annual precipitation and average annual
temperature. Trends of temperature are determined by the proximity of the ecological area to
the equator, greater the distance, lower will be the temperature. Areas with more precipitation
tend to have greater vegetation. The biome that receives the lowest precipitation is the desert
biome, a terrestrial biome. The primary characteristics of the desert biome are extreme
temperatures, low humidity, low precipitation, aridity, and drought. Few of the threats
associated with this biome are intense solar radiation, highest potential evaporation, absence
of biological soil, and the soil is sandy, loose and devoid of nitrogen and organic carbon.
Photosynthesis is a critical process that helps the plant to produce organic molecules
and support the functioning of its system. It is defined by the process by which the plants
harness the solar energy from the sun and utilize it to turn into chemical energy, which can be
later exploited by the plant systems for their own activities. The electromagnetic radiation
emitted by the sun is absorbed by the pigments, of which, chlorophyll a and chlorophyll b are
the primary absorbing pigments. Pigment molecules grouped with proteins form a
photosystem, which then absorbs one photon. This photon reaches a single molecule of
chlorophyll a in the reaction center, which gets excited to donate an electron to a primary
electron acceptor. To replace this electron, water molecules get broken to form hydrogen ions
[H+] and oxygen molecule (O2). The donated electron from chlorophyll participates in the
electron chain transport system and increasing concentration of hydrogen ions are allowed to

PLANT ECOLOGY
pass through ATP synthase, a protein complex embedded in the thylakoid. ATP is formed
with the process of photophosphorylation. NADPH energy-carrier molecule is generated and
stored in energy carriers and this stored chemical energy will further used in the Calvin cycle
for assembly of sugar molecules. The carbon fixation process during the Calvin cycle is
started with hydrolyzing of unstable 6 carbon molecule to 3-phosphoglycerate. The reaction
is catalyzed by the enzyme RuBisCO.
The carbon fixation process in the Calvin cycle is followed by reduction and
regeneration to form the final product of glyceraldehyde-3-phosphate (G3P). Glyceraldehyde-
3-phosphate is a three-carbon molecule, which is synthesized into organic molecules, starch
and sugar after leaving the Calvin Cycle. The synthesis of the G3P is important for cellular
metabolism. However, an important thing to note that is only one molecule of G3P is
generated per three turns of the Calvin cycle. According to Kirschbaum (2011), an increase of
30 per cent in the net photosynthetic rate has reported showing an increase of only 10 per
cent of the overall plant growth, thus underlying the insignificant correlation of the net
photosynthetic rate and plant growth rate. According to the authors, the realized efficiency of
the photosynthetic process in converting available solar energy into chemical energy is 2 to
4%, which indicates significant inefficiency of the process. Water and nutrient limitation can
be the primary background cause of these statistics.
Water in the plants acts as a medium for the biochemical reactants to dissolve and is
crucial for plant’s metabolism and growth. During the photosynthetic process, water along
with carbon dioxide helps in the synthesis of carbohydrates. More importantly, it acts as a
transport medium for the nutrient movement in the plant. Through the process of
transpiration, passive transportation of water absorbed from roots takes place in the xylem,
which is then released into the air from leaf surfaces by diffusion through stomata (Huang, Li
& Li, 2018). Transpiration can be deemed as a process of evaporation of water in plants. The

PLANT ECOLOGY
situation of water deficit in cold deserts leads to plant stress, due to a decrease in the
photosynthetic activity, which ultimately limits the growth of the plant. To ensure the
presence of adequate balance between the characteristics of water loss and mesophyll
photosynthetic CO2 demands, the stomatal pore aperture is adjusted. Due to reduction of
water loss to cope up with water deficit, the plant may undergo a reduction in metabolic
activities, due to restrained carbon dioxide uptake and reduced photosynthetic activity
(Lawson & McElwain, 2016).
For proper functioning of the plant activities, vital elements of minerals are essentially
required. Soil serves as the primary source of nutrients to the plants. There are several
inorganic nutrient elements that the plant absorbs from the soil to perform its biochemical and
physiological functions properly. Macro-nutrients include nitrogen, phosphorous, potassium,
calcium, magnesium, sulfur and iron. Micro-nutrients include manganese, copper, boron,
molybdenum, chlorine and zinc (Nature.com., 2020). The nutrient acquisition is primarily
through the roots of the plants; however, the composition and chemistry of the soil influence
the efficiency of nutrient absorption. The three ways in which the plant may acquire nutrient
are direct uptake from soil (important for the acquisition of potassium and iron), symbioses
with soil-based microorganisms (for nitrogen and phosphorous) and mycorrhizal interactions
with plants (Pandey, 2015). For symbioses with soil-based microorganisms, the plant forms a
symbiotic relationship with Rhizobia, a nitrogen-fixing bacteria, which converts atmospheric
nitrogen to a usable form of ammonia.
In water-stressed condition, that is drought conditions in the cold deserts, and it is
hypothesized that significant reduction in the plant growth will be observed, caused by the
reduction in the relative water content of the cells which limits the water availability for cell
expansion (Tátrai et al., 2016). Moreover, by further addition of nitrogen in cold desert
system, it is expected that the net photosynthetic rate of the plants will increase and a positive

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correlation with the greater concentrations of chlorophyll will be observed (Cui et al., 2019).
To perform the experiment and determine the impact of water stress and nitrogen addition on
the growth and physiological responses, the plants will be kept in the desired conditions and
the morphological characteristics, such as total biomass, average leaf area, shoot-to-root ratio,
leaf number and root weight will be measured (Zhou et al., 2011).
Part 2
Global warming is a major climate change issue presently and it is predicted that an
increase of over 1.5-4.5°C of the average global surface temperature will be recorded over
the next century. Photorespiration in plants and the photosynthetic efficiency in plants is
effectively compromised by C3 photosynthesis. In comparison to plants pertaining to C3
photosynthesis, C4 photosynthesis is more suitable for plants living in conditions of drier
environments with higher temperatures.
It is now evident that plants with C3 as the preferred pathway for photosynthesis were
evolved during the time when the carbon dioxide concentration in the atmosphere was high
and the concentration of oxygen was low. Photorespiration is a significant disadvantage of
the C3 pathway and areas with hot, humid weather and high oxygen concentration will have a
number of C4 plants. Considering the timeline of the evolution of the plants with these two
pathways, C3 is more ancient and it speciated before the incidence of C4 plants. The evolution
of C4 plants is more advanced to fit in a natural environment which is not suitable for C3
plants. Thus, it can be concluded that in an environment of low carbon dioxide, high light
intensity, increased temperature and excessive demand for water transport, the favored is a C4
plant, which has significantly low stomatal conductance, allowing efficient water
conservation mechanism and higher rates of photosynthesis.

PLANT ECOLOGY
The C4 pathway of photosynthesis has been observed to obtain higher photosynthetic
rates in comparison to the C3 pathway in the condition of low carbon dioxide availability.
Molecular oxygen is outcompeted as carbon dioxide is concentrated at the site of the enzyme
RuBisCO, which helps to achieve a drastic reduction in the photorespiration rates. Increased
efficiency of carbon fixation with reduction of photorespiration can be observed in the C4
pathway. In C3 plants, a 3-carbon sugar (3-Phospho Glyceric Acid) is the first product after
carbon fixation performed with the use of enzyme Rubisco. In C4 plants phosphoenolpyruvate
(PEP) carboxylase is the enzyme that catalyzes the formation of the first product, which is a
4-Carbon sugar (Oxalo Acetic Acid). This 4-Carbon compound is transported to bundle
sheath cells, where decarboxylation occurs to form 3-carbon sugar and carbon dioxide, of
which, the sugar is transported back to the mesophyll cells. The carbon dioxide in the bundle
sheath cells is then fixed by the Rubisco carboxylase to be reduced to sugars via the Calvin
cycle. C4 plants thus can perform photosynthesis with their stomata closed, significantly
reducing the incidence of water loss in plants, which is crucial for cold desert biomes having
water scarcity. Thus, the two advantage C4 plants have over C3 plants are greater water use
efficiency and CO2 fixation, which is resistant to drought and heat with no photorespiration
(Srinivasan & Pignon, 2017).
Globally, the majority of the plants undergo the C3 pathway. It is the oldest pathway
of carbon fixation in plants and all taxonomies have plants that follow the C3 pathway. Some
of the common species of plants that follow this photosynthetic pathway are peach, apple,
yams, tomatoes, spinach, potatoes, cassava, barley, rice, soybeans, and wheat. The biomass
rates of this plant type are typically between -22% to -35%. In comparison to the vast global
distribution of C3 plants, C4 plants constitute only 3 per cent of the total count of plant species
in the land. Moreover, this distribution is highly limited to warm temperate zones, subtropics,
and tropical grasslands. These crops are not usually deemed suitable for human consumption.

PLANT ECOLOGY
The only marked exceptions of C4 plants are maize, sugar cane and sorghum. The average
biomass rates of such type of photosynthetic plants are between -9 to -16 per cent. With the
increasing global surface temperature, it can be predicted that the C3 plants will struggle to
survive. Stress conditions such as high light and drought will increase the extent of
suppression to dramatic levels, effective the humankind as we are so greatly dependent on the
byproducts of C3 plants (Kumar et al., 2017). However, the predictability of which kind of
photosynthetic plant will struggle to survive is difficult to be foretold, as the current trends
also observe a global increase of carbon dioxide, which doe not favour the growth of C4
plants (Upasani & Barla, 2018). Moreover, according to Luo et al. (2018), C3 plants have a
greater capability of using nitrogen along the aridity gradient.

PLANT ECOLOGY
References
Cui, X., Yue, P., Wu, W., Gong, Y., Li, K., Misselbrook, T., ... & Liu, X. (2019). The growth
and N retention of two annual desert plants varied under different nitrogen deposition
rates. Frontiers in plant science, 10, 356.
Huang, G., Li, C. H., & Li, Y. (2018). Phenological responses to nitrogen and water addition
are linked to plant growth patterns in a desert herbaceous community. Ecology and
evolution, 8(10), 5139-5152.
Kirschbaum, M. U. (2011). Does enhanced photosynthesis enhance growth? Lessons learned
from CO2 enrichment studies. Plant physiology, 155(1), 117-124.
Kumar, V., Sharma, A., Soni, J. K., & Pawar, N. (2017). Physiological response of C3, C4
and CAM plants in changeable climate. The Pharma Innovation, 6(9, Part B), 70.
Lawson, T., & McElwain, J. C. (2016). Evolutionary trade-offs in stomatal spacing. New
Phytol., 210(4), 1149-1151.
Luo, W., Wang, X., Sardans, J., Wang, Z., Dijkstra, F. A., Lü, X. T., ... & Han, X. (2018).
Higher capability of C3 than C4 plants to use nitrogen inferred from nitrogen stable
isotopes along an aridity gradient. Plant and soil, 428(1-2), 93-103.
Nature.com. (2020). Plant-Soil Interactions: Nutrient Uptake | Learn Science at Scitable.
Retrieved 20 February 2020, from
https://www.nature.com/scitable/knowledge/library/plant-soil-interactions-nutrient-
uptake-105289112/
Pandey, R. (2015). Mineral nutrition of plants. In Plant biology and biotechnology (pp. 499-
538). Springer, New Delhi.
Quigley, M., Kravchenko, A., Negassa, W., Guber, A., & Rivers, M. (2018, April).
Relationship of pores to the fate and distribution of newly added carbon. In EGU
General Assembly Conference Abstracts (Vol. 20, p. 9173).

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Srinivasan, V., & Pignon, C. (2017, December). Making C4 crops more water efficient under
current and future climate: Tradeoffs between carbon gain and water loss. In AGU
Fall Meeting Abstracts.
Tátrai, Z. A., Sanoubar, R., Pluhár, Z., Mancarella, S., Orsini, F., & Gianquinto, G. (2016).
Morphological and physiological plant responses to drought stress in Thymus
citriodorus. International Journal of Agronomy, 2016.
Upasani, R. R., & Barla, S. (2018). Weed Dynamics in Changing Climate. Journal of
Experimental Botany, 66(12), 3435-50.
Zhou, X., Zhang, Y., Ji, X., Downing, A., & Serpe, M. (2011). Combined effects of nitrogen
deposition and water stress on growth and physiological responses of two annual
desert plants in northwestern China. Environmental and Experimental Botany, 74, 1-
8.

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