Photosynthesis
Photosynthesis
Photosynthesis is the biological conversion of light energy into chemical energy. It occurs in green plants and photosynthetic bacteria through a series of many biochemical reactions.
In plants and algae, light absorption by chlorophyll catalyzes the synthesis of carbohydrate (C6H12O6) and oxygen gas (O2) from carbon dioxide gas (CO2) and water (H2O). The process in bacteria is similar but not identical.
Up until the early decades of the twentieth century, the prevailing scientific opinion was that photo-synthesis produced oxygen by splitting carbon dioxide. But this dogma was refuted by the Dutchborn American resident microbiologist Cornelius van Niel (1897-1985). Van Niel studied photosynthesis in anaerobic bacteria, rather than in higher plants. Like higher plants, these bacteria make carbohydrates during photosynthesis. Unlike plants, however, they do not produce oxygen during photosynthesis; they use bacteriochlorophyll rather than chlorophyll as a photosynthetic pigment. Van Niel established that photo-synthetic bacteria like purple sulfur bacteria require a compound that can accept electrons (in other words, the compound becomes oxidized).
Van Niel’s proposal that the oxygen which plants produce during photosynthesis is derived from water, not from carbon dioxide, was proven true. This brilliant insight was a major contribution to our modern understanding of photosynthesis.
Modern plant physiologists commonly think of photosynthesis as consisting of two separate series of interconnected biochemical reactions, the light reactions and the dark reactions. The light reactions use the light energy absorbed by chlorophyll to synthesize high-energy molecules that break down rapidly. The dark reactions use these molecules to synthesize carbohydrates, a more stable form of chemical energy that can be stored by plants. Although the dark reactions do not require light, hence their name, they often occur in the light because they are dependent upon the light reactions. In higher plants and algae, the light and dark reactions of photosynthesis occur in chloroplasts, specialized chlorophyll-containing intracellular structures which are enclosed by double membranes.
In the light reactions of photosynthesis, light energy excites photosynthetic pigments to higher energy levels. When liberated, the energy is used to make two other high energy compounds, ATP (adeno-sine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
In higher plants and algae, the light reactions occur on the thylakoid membranes of the chloroplasts. The thylakoid membranes are inner membranes of the chloroplasts, which are arranged like flattened sacs. The thylakoids are often stacked on top of one another, like a roll of coins. A stack of thylakoids is referred to as a granum.
The light reactions of higher plants require two photosynthetic compounds that contain chlorophyll and other compounds that are called carotenoids. These pigments are associated with special proteins that are embedded in the thylakoid membranes. The chlorophyll compounds strongly absorb light in the red and blue regions of the spectrum. Most carotenoids strongly absorb blue light. Thus, plant leaves are green simply because their photosynthetic pigments absorb blue and red light but not green light. The absorbed light is utilized in a series of chemical reactions that synthesize the high-energy ATP and NADPH compounds.
There are many different groups of photosynthetic algae. Like higher plants, they all have a chlorophyll compound as a photosynthetic pigment. They differ from higher plants in having different complements of additional chlorophylls. The different chlorophylls and other photosynthetic pigments allow algae to utilize different regions of the solar spectrum to drive photosynthesis.
Another photosynthetic microorganism are the cyanobacteria. This group was formerly called the blue-green algae and these organisms were once considered members of the plant kingdom. However, unlike the true algae, cyanobacteria are prokaryotes, in that their DNA is not sequestered within a nucleus. Their photosynthetic mechanism is similar in some respects to that of higher plants. But, cyanobacteria differ from higher plants in that they have additional photosynthetic pigments, referred to as phycobilins. Phycobilins absorb different wavelengths of light than chlorophyll and thus increase the wavelength range, which can drive photosynthesis.
Anaerobic photosynthetic bacteria, which cannot grow in the presence of oxygen do not produce oxygen during photosynthesis and only photosynthesize in environments which are lack oxygen (anaerobic). These bacteria use carbon dioxide and another oxidizable substrate such as hydrogen sulfide to make carbohydrates, and have bacteriochlorophylls and other photosynthetic pigments which are similar to the chlorophylls used by higher plants.
Another type of photosynthetic bacteria are two species in the genus Halobacterium. Most biologists now place this genus with methanogenic (methane-producing) bacteria in the Archaebacteria, a separate kingdom of organisms. Halobacteria thrive in very salty environments, such as the Dead Sea and the Great Salt Lake.
Halobacteria are unique in that they perform photosynthesis without chlorophyll. Instead, their photosynthetic pigments are bacteriorhodopsin and halorhodopsin. These pigments are similar to sensory rhodopsin, the pigment that humans and other animals use for vision. Bacteriorhodopsin and halorhodopsin are embedded in the cell membranes of halobacteria and each pigment consists of retinal, a vitamin-A derivative, bound to a protein. Irradiation of these pigments causes a structural change in their retinal, referred to as photoisomerization. Retinal
KEY TERMS
Calvin cycle— A series of energy-requiring photo-synthesis reactions used synthesize carbohydrates.
Chloroplast— Green organelle in higher plants and algae in which photosynthesis occurs.
Cyanobacteria (singular, cyanobacterium)— Photo-synthetic bacteria, commonly known as blue-green alga.
Enzyme— Biological molecule, usually a protein, which promotes a biochemical reaction but is not consumed by the reaction.
Eukaryote— A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.
Organelle— Membrane enclosed structure within a eukaryotic cell which is specialized for specific cellular functions.
Prokaryote— Cell without a nucleus, considered more primitive than a eukaryote.
Stomata— Pores in plant leaves which function in exchange of carbon dioxide, oxygen, and water during photosynthesis.
Stroma— The material that bathes the interior of chloroplasts in plant cells.
Thylakoid— A membranous structure that bisects the interior of a chloroplast.
photoisomerization leads to the synthesis of ATP, the same high-energy compound synthesized during the light reactions of higher plants.
Resources
BOOKS
Blankenship, Robert E. Molecular Mechanisms of Photosynthesis. Boston: Blackwell Publishing, 2002.
Collings, Anthony F. and Christa Critchley. Artificial Photosynthesis: From Basic Biology to Industrial Application. New York: John Wiley & Sons, 2005.
Govindjee, J.T. Beatty, H. Gest, and J.F. Allen, eds. Discoveries in Photosynthesis (Advances in Photosynthesis and Respiration). New York: Springer, 2006.
Hopkins, William G. Photosynthesis And Respiration (The Green World). New York: Chelsea House Publications, 2006.
Peter A. Ensminger