Photosynthesis

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Introduction

Photosynthesis is a process in plants, algae, and some prokaryotes, that coverts solar insolation into chemical energy stored in glucose or other organic compounds. Photosynthesis occurs in slightly different ways in higher plants relative to photosynthetic bacteria.

caption By At09kg (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Photosynthesis (Source: Wikimedia Commons)
 

Photosynthesis in Higher Plants

In higher plants, photosynthesis involves chemical reactions in which the sun's energy is transferred along a series of oxidation and reduction events until it is stabilized in the chemical bonds of glucose. In the broadest sense, light energy converts carbon dioxide (CO2) into chemical energy while water is split to release oxygen.

Most photosynthesis occurs in the leaves of plants, although there may be photosynthetic stems, flowers, and fruits. At the cellular level, photosynthesis occurs inside organelles known as chloroplasts. Plants use photosynthetic pigments (e.g., chlorophyll) to capture the light energy which is ultimately converted into chemical energy in the form of sugars. Photosynthesis involves two stages, the light reactions and calvin cycle reactions.

caption Internal view of a chloroplast. (Source: Carbon Dioxide and the Earth)
Internal view of a chloroplast. (Source: Carbon Dioxide and the Earth)

Light Reactions

Electomagnetic energy from the sun is captured by photosynthetic pigments and is transferred along a series of proteins and iron-sulfur containing compounds along the thylakoid membranes of the chloroplast; the net result being the formation of high-energy compounds such as ATP and NADPH. Water molecules are split during the transfer of light energy along the membranes. Oxgyen is produced as a result of this water-splitting event.

Calvin Cycle Reactions

In the reactions of the Calvin Cycle, chemical energy held within ATP and NADPH are used to convert carbon dioxide into sugars through a series of enzymatic reactions. In the initial step of the Calvin Cycle, carbon dioxide (from the atmosphere) reacts with a five-carbon compound, ribulose bisphosphate (RuBP), in a reaction that is catalyzed by the enzyme RuBP carboxylase/oxygenase ("RuBisco"). The first stable product of this reaction is a three-carbon compound known as phosphoglycerate (PGA). Energy captured in the light reactions in form of ATP and NADPH is used to convert PGA into glyceraldehyde 3-phosphate (G3P) which can be converted to other organic compounds, or using energy from ATP, some is converted into RuBP to continue the cycle. The Calvin cycle reactions occur in the stroma of the chloroplasts.

Alternative Mechanisms of Photosynthesis

caption Sagauaro cacti, a species with CAM photosynthesis. (Source: http://commons.wikimedia.org/wiki/File%3AWhite_Tank_Mountains_Reg_Park_-_Small_cactus_-_60144.jpg)

Sagauaro cacti, a species with CAM photosynthesis.

 

The standard pattern of photosynthesis (described above) is known as C3 photosynthesis because the first stable product of carbon fixation is a three-carbon compound. Approximately 90% of all plants on earth utilize this pathway to convert CO2 into sugars. All trees and many shrub species use this pathway. Two alternative mechanisms of photosynthesis have evolved in plants living in hot, arid environments. Plants using CAM photosynthesis (Crassulacean Acid Metabolism) open their stomata at night in order to take up carbon dioxide when it is cooler so rates of water loss from the plant are lower. Carbon is stored at night as an organic acid. During the daylight hours, the organic acid releases carbon dioxide which then enters the Calvin cycle. Approximately 7% of  all plant species on earth use this photosynthetic strategy to survive. These include the succulent cacti and euphorbias found in the harsh desert areas on earth as well as many tropical orchids that grow as epiphytes on trees.

Many grasses utilize a third photosynthetic biochemical pathway known as [[C4 photosynthesis|RTENOTITLEC4 photosynthesis]], where carbon dioxide is initially incorporated into an organic acid in mesophyll cells which is transported into the bundle sheath cells. The acid is decarboxylated inside the bundle sheath cells and the CO2 is concentrated inside these cells. Rubisco is flooded with CO2 and sugars are made in abundance using the Calvin cycle. Concentrating carbon dioxide in the bundle sheath cells minimizes photorespiration. Only 1% of all plant species on earth utilize this photosynthetic pathway, but these include most tropical and sub-tropical grasses, including crops such as  corn and the millets. Many of these species, such as  switchgrass and Miscanthus  exhibit high yields because photorespiration is reduced.

Factors limiting photosynthesis

The rate of photosynthesis is determined by environmental factors. Factors limiting photosynthetic rates include light intensity, water availability, soil nutrient content, concentration of carbon dioxide and temperature.

When water availability is reduced, photosynthesis is mainly limited by a reduction in the diffusion of CO2 into the leaf through the stomata. Stomata typically close when atmospheric humidity and soil moisture availability decline. Under these conditions, high light conditions can cause thylakoid membranes to become damaged in a process known as photoinhibition. As a result photosynthesis is limited by photoinhibition. A similar decline of photosynthetic efficacy is observed when water availability is inhibited by overgrazing induced root zone reduction combined with excessive leaf destruction.

As internal leaf CO2 concentrations decline due to stomatal closure, the enzyme Rubisco tends to fix more oxygen and liberates CO2 in a process known as photorespiration. The net result is that photosynthesis becomes limited by the process of photorespiration. This process occurs mostly in C3 plants and is enhanced under hot dry conditions.

Importance of Photosynthesis

Photosynthesis is an important process because it harnesses the sun's energy into utilizable forms of energy on earth. Most biological organisms such as animals and fungi are unable to directly use light energy to power biological processes such as active transport, cell division and muscle movement. ATP is used to power these processes. Photosynthesis converts light energy into chemical energy in the form of glucose and then the process of cellular respiration converts energy in glucose to energy in the form of ATP which is ultimately used to power biological processes. The energy produced by photosynthesis forms the basis of virtually all terrestrial and aquatic food chains. As a result, photosynthesis is the ultimate source of carbon in the organic molecules found in most organisms. The high oxygen concentration in the atmosphere is derived directly from the light reactions of photosynthesis. Prior to the evolution of photosynthesis on earth, the atmosphere was anoxic.

The Role of Photosynthesis in Biofuel production

The process of photosynthesisis is of fundamental importance in utilizing the carbon locked up in plant material as a source of biofuel energy. Currently, ethanol is derived from cellulose from corn and sugar cane and biodiesel from soybean and other oil crops. It is estimated that the USA will rely on obtaining approximately 30% of its energy resources from cellulosic ethanol by the year 2030. As a result improving the current yield of fast growing C4 plant species such as corn, switchgrass and Miscanthus under various environmental conditions is of primary interest at many research institutions throughout the world.

Glossary

Citation

Swarthout, D., & McGinley, M. (2013). Photosynthesis. Retrieved from http://www.eoearth.org/view/article/155221

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