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Mechanism of Photosynthesis - Light Reaction

The mechanism of photosynthesis involves two important reactions- the Light/Hill/Photochemical Reaction and the Dark/Biosynthetic Reaction. This topic explains the photolysis of water, photophosphorylation, Calvin Cycle and Hatch & Slack Cycle.

Light Reactions Of Photosynthesis :

In addition to mitochondrial ATP synthesis, plants can also make ATP by a similar process during the light reactions of photosynthesis within their chloroplasts. Electrons flow through a cytochrome transport system on thylakoid membranes in a region of the chloroplast called the grana; except that the electrons come from excited (light activated) chlorophyll molecules rather than the break down of glucose. This is an especially vital source of ATP for plants because ATP is also needed for them to synthesize glucose in the first place. Without a photosynthetic source of ATP, plants would be using up their ATP to make glucose, and then using up glucose to make ATP, a "catch-22" situation.


A transparent-green solution of chlorophyll is made by grinding up spinach or grass leaves in acetone (in a mortar and pestle), and then filtering it through cheesecloth and course filter paper. If a bright beam of light is directed at this chlorophyll solution, a deep red glow is emitted from the test tube. The chlorophyll electrons become excited by the light energy, but have no cytochrome transport system to flow along because the chloroplast thylakoid membranes have been dissolved away. Therefore, the chlorophyll electrons give up their excited energy state by releasing energy in the form of a reddish glow. This phenomenon is known as fluorescence, and is essentially the same principle as a neon light. In a neon light, the electrons of neon gas become excited and then release their energy of activation as a white glow inside the glass tube. In an intact chloroplast with thylakoid membranes, ATP is generated by an electron flow along the cytochrome transport system. Since the electrons are being transported to other "carrier" molecules, their energy is used to generate ATP and no reddish glow is emitted. Leaves generally appear green because wavelengths of light from the red and blue regions of the visible spectrum are necessary to excite the chloroplast electrons, and unused green light is reflected. Thus, we perceive trees, shrubs and grasses as green. During the fall months when chlorophyll production ceases in deciduous trees and shrubs, the leaves turn golden yellow or red due to the presence of other pigments, such as yellow and orange carotenoids and bright red anthocyanins.


Another important ingredient for photosynthesis is also produced during the light reactions. During these light-dependent reactions of photosynthesis, a chemical called NADP picks up two hydrogen atoms from water molecules forming NADPH2, a powerful reducing agent that is used to convert carbon dioxide into glucose during the dark reactions of photosynthesis (also called the Calvin Cycle). When the two atoms of hydrogen join with NADP, oxygen is liberated, and this is the source of oxygen gas in our atmosphere. ATP and NADPH2from the light reactions are used in the dark reactions of photosynthesis that take place in the stroma region of the chloroplast.


Similar electron transport systems occur in the membranes of prokaryotic bacteria. Methanogenic bacteria live in marshes, swamps and your gastrointestinal tract. In fact, they are responsible for some intestinal gas, particularly the combustible component of flatulence. They produce methane gas anaerobically (without oxygen) by removing the electrons from hydrogen gas. The electrons and H+ ions from hydrogen gas are used to reduce carbon dioxide to methane. In the reaction, the H+ ions combine with the oxygen from carbon dioxide to form water. During this process, the electrons are shuttled through an anaerobic electron transport system within the bacterial membrane which results in the phosphorylation of ADP (adenosine diphosphate) to form ATP (adenosine triphosphate). This process is much less efficient than aerobic respiration, so only two molecules of ATP (rather than 38) are formed. Desert varnish bacteria make their ATP in a similar fashion, only the electrons are coming from the aerobic oxidation of iron and manganese. A thin coating of iron or manganese oxide is deposited on the surfaces of desert boulders and rocky slopes. During the oxidation process, the electrons are shuttled through an iron-containing cytochrome enzyme system on the inner bacterial membrane. One has only to gaze at the spectacular panoramas of varnish-coated, granitic boulders throughout desert areas of the American southwest to appreciate the magnitude of this bacterial ATP production. The mechanism of ATP synthesis in prokaryotic bacteria is remarkably similar to eukaryotic cells. In addition, the circular DNA molecules of these bacteria are similar to the DNA molecules within some organelles of eukaryotic cells. In fact, some biologists believe that mitochondria (and chloroplasts) within eukaryotic animal and plant cells may have originated from ancient symbiotic bacteria that were once captured by other cells in the distant geologic past. This fascinating idea is called the "Endosymbiont Theory" (or "Endosymbiont Hypothesis" for those who are more skeptical).


Trace amounts of DNA can be amplified (cloned) into millions of copies using the PCR technique (Polymerase Chain Reaction) discovered by Kary Mullis of UCSD. This technique provides investigators with sufficient DNA to use in sequencing gels in which the banding patterns represent different base pair sequences. DNA sequencing is widely used in modern research, including crime scene investigations to determine genetic "fingerprints" (e.g. the O.J. Simpson Trial). [And remember the DNA evidence on Monica Lewinski's dress that almost led to the removal of President Clinton from office!] DNA is also used in phylogenetic studies (cladistics) to show evolutionary trends and relationships among plant and animal species. Depending on the level of study, certain types of genes are preferred. For larger taxonomic groups at the plant family level, chloroplast DNA is particularly useful. For phylogenetic studies at the species level, mitochondrial DNA and small subunit ribosomal DNA is commonly used. Researchers can compare their results with others and download gene sequences from the GenBank Data Base at the National Center For Biotechnology Information. Please consult your textbook for more information and illustrations of this remarkable class of compounds which truly are the chemicals of life.


  1. rachellee saidMon, 25 May 2009 11:20:41 -0000 ( Link )

    Also visit NeoK12.com for educational videos on photosynthesis and other topics to learn science online.

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