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OIL RIG, a cute little mnemonic device to remember that oxidation is loss and reduction is gain. Perhaps better put, oxidation results in the net loss of an electron or electrons, while reduction results in a net gain of an electron or electrons. The electrons come from compounds within the process or donated in from previous processes. These types of chemical reactions are found scattered throughout the processes within photosynthesis and respiration.
Einstein called the discrete particles of light photons. Particles (photons) and waves are both currently accepted aspects of light. The quantum (energy) of photons is different depending on what kind of light they are in. Longer wavelength light has lower photon energies, while light with shorter wavelengths have higher photon energies.
As mentioned earlier, every pigment color has a different distinctive pattern of light absorption—called the pigment’s absorption spectrum. The energy levels of some of the pigment’s electrons are raised when the pigment absorbs light. If energy is emitted immediately upon absorption, the effect is called fluorescence. The red part of light does this characteristically, as demonstrated when chlorophyll is placed in light it will appear red. If the absorbed energy is emitted as light after a delay, then the effect is called phosphorescence. The energy may be converted to heat or stored, as in photosynthesis within chemical bonds.
The carbon-fixing reactions used to be called dark reactions because light does not play a direct role in their functioning. The reactions take place in series outside of the grana in the stroma of the chloroplast. These reactions only occur if the end products of the light reactions are available for use. Depending on the plant involved, the carbon-fixing reactions may develop or progress in different ways. The most common type of carbon-fixing reactions in plants is the process called the Calvin cycle.
In the Calvin cycle, carbon dioxide from the atmosphere is combined with a 5-carbon sugar—RuBP, or ribulose bisphosphate. The combined molecules are converted via several steps into a 6-carbon sugar, such as glucose. The ATP and NADPH molecules from the light reactions provide the energy and resources for the reactions. Some of the sugars produced are further combined into polysaccharides (strings of simple sugars) or are stored as starch within the plant. There are other variations, including the 4-carbon pathway which is usually found in desert plants (C4 plants).
Before getting into respiration let’s take a closer look at what happens in both the light reactions and the carbon-fixing reactions.
The light reactions involve light striking the chlorophyll molecules embedded in the thylakoids of chloroplasts. The subsequent reaction results in the conversion of some light energy to chemical energy. In the light reactions, water molecules are split apart into hydrogen ions and electrons and oxygen gas is released. In addition, ATP (adenosine triphosphate) molecules are created and the hydrogen ions derived from the water molecules are involved in “loading” NADP which carries the hydrogen as NADPH. NADPH is integral in providing the hydrogen ions used in the second series of major photosynthetic reactions: the carbon-fixing reactions.
A few things to know about chlorophyll before we get into the nitty-gritty of photosynthesis and respiration. There is more than one type of chlorophyll, however, they all have one atom of magnesium in the center. In some ways, the chlorophyll is quite analogous to the heme structure in hemoglobin (the iron-containing pigment that carries oxygen in the blood). Chlorophyll has a long lipid tail that anchors the molecule in the lipid layers of the thylakoid membranes—recall that thylakoids are coin-like discs in stacks within the stroma of the chloroplasts.
The chloroplasts of most plants contain two types of chlorophyll embedded in the thylakoid membranes. The formula for bluish-green chlorophyll a is C55H72O5N4Mg and the formula for yellow-green chlorophyll b is C55H70O6N4Mg. In general, most chloroplast has about three times as much chlorophyll a than b. The main role of chlorophyll b is to broaden the spectrum of light available for photosynthesis: chlorophyll b absorbs light energy and transfers the energy to a chlorophyll a molecule. Other pigments are contained in chlorophyll c, d, and e and take the place for chlorophyll b in some cases. Note that all the chlorophyll molecules are related to each other and differ only slightly in molecular structure. Light-harvesting complexes contain 250 to 400 pigment molecules and are referred to as a photosynthetic unit. There are countless numbers of these units spread throughout the grana of a chloroplast. In the chloroplasts of green plants, two types of these harvesting units operate together in order to bring about the first phase of photosynthesis.
The photosynthetic process occurs in two successive processes: the light reactions and the carbon-fixing reactions.
Recommended reading: Müller‐Schüssele, S. J., et al. (2020). Chloroplasts require glutathione reductase to balance reactive oxygen species and maintain efficient photosynthesis. The Plant Journal. https://doi.org/10.1111/tpj.14791.
Read more articles like this here: The Plant Journal – provides a dynamic forum for the ever-growing international plant sciences research community and publishes in all key areas of plant biology.
Light has a dual nature, in that it exhibits properties of both waves and particles. The energy from the sun comes to earth in various wavelengths, the longest being radio waves and the shortest are gamma rays. Approximately 40% of the radiant energy the earth receives from the sun is visible light. Visible light ranges from red, 780 nanometers to violet, 390 nanometers. The violet to blue and red to orange ranges are the most often used in photosynthesis. Most light in the green range is reflected. Of the visible light that reaches a leaf, approximately 80% is absorbed. Light intensity varies widely.
Time of day, temperature, season of year, altitude, latitude, and other atmospheric conditions all play roles in the intensity of the radiant energy that will reach the earth and its organisms. High-intensity light isn’t necessarily a beneficial thing for plants. In high-intensity light, photorespiration may occur, which is a type of respiration that uses oxygen and releases carbon dioxide but differs from standard aerobic respiration in the pathways that it utilizes.
Water is plentiful on earth, however, it may or may not be plentiful at the location of each individual plant. Therefore, plants will close their stomata, if need be, which reduces the CO2 supply to the mesophyll. Not even 1% of the water that is absorbed by plants is used in photosynthesis, the remainder is either transpired or incorporated into protoplasm, vacuoles or other cell materials. The water utilized in photosynthesis is the source of oxygen released as a photosynthetic byproduct.
The earth’s atmosphere contains approximately 79% nitrogen, 20% oxygen, and the remaining 1% is a mixture of less common gases—including 0.039% carbon dioxide. Carbon dioxide in the atmosphere reaches plant mesophyll via the stomata. The carbon dioxide dissolves on the thin film of water that covers the outside of cells. The carbon dioxide then diffuses through the cell wall into the cytoplasm in order to reach the chloroplasts. The oceans hold a large reservoir of carbon dioxide, which keeps the atmospheric levels essentially constant. Although there are some indicators that the atmospheric levels of CO2 are rising and adding to the global warming issue. That is a whole other topic though.
Photosynthesis is the process by which light energy is captured, converted and stored in a simple sugar molecule. This process occurs in chloroplasts and other parts of green organisms. It is a backbone process, in the sense that all life on earth depends on it’s functioning. The following equation sums up the process:
6CO2 (carbon dioxide) + 12 H2O (water) + light energy -> C6H12O6 (glucose) + 6O2 (oxygen) +6H2O (water)
As you see from the equation, this process is vital to us as humans, because it transforms carbon dioxide into oxygen—which we enjoy with every breath!