Chloroplasts, the Calvin Cycle, and plant adaptations

Created March 2020, Offline version here
Video by Paul Anderson, also on his website Bozeman Science.

    Photosynthesis is a process that occurs in many taxonomically diverse organisms, such as plants, algae, protists, and bacteria. Photosynthesis allows an organism to autotrophically produce food from light, , and water.
    This process occurs in a plant’s , which is the organelle within a plant cell that contains where the light reactions take place.
    A stack of these photosynthetic structures is known as a .
    The stroma is an aqueous fluid that fills plant cells, this is where the Calvin Cycle takes place.
    Therefore, the reactions take place inside the chloroplasts but the production of sugars takes place outside the chloroplasts in this liquid medium.
    Chlorophyll A is one of several that work together in photosynthesis.
    The pigments have different absorption spectrums; since they absorb a lot of the blue and red ends of the visual spectrum but much of the mid-range of the visual spectrum, thus photosynthesizing organisms predominately appear to be green.
    As mentioned, photosynthesis can be separated into two reactions, the light-dependent process inside the chloroplast and the light-independent process that takes place in the stroma. In the light-dependent reactions light provides energy and water contributes electrons that pass through .
    This produce the energetic molecules .
    These energy molecules then go on to fuel the Calvin Cycle. During the Calvin Cycle ATP is used as energy and converted to ADP and NADPH is used as energy and converted to NADP+. These two molecules are the energy currencies of photosynthesis, created from light and water! The cycling of this energy produces even more energy-rich molecules (sugars) that plants (and other photosynthesizing organisms) use as food. The Calvin Cycle begins with the five carbon molecule ribulose, it is then joined with the one carbon molecule carbon dioxide via the enzyme .
    The newly built six carbon molecule immediately breaks into a pair of three carbon molecules. These molecules are further assembled, with the energetic input of , into the molecule G3P.
    This molecule is a building block that can be further assembled into glucose, sucrose, maltose, and other sugars and it can also be changed into to act as the building molecule at the start of the Calvin Cycle.
    The majority of plants on Earth are known as C3 plants, relying on the production of the previously mentioned three carbon molecules produced in the Calvin Cycle to build other carbon based molecules that the plant uses. However, if a plant is limited in its intake of carbon dioxide the step in the cycle that involves the enzyme RuBisCo may be forced to react with oxygen instead. When this occurs the process is known as and the products are useless molecules that the plant has to invest even more energy into breaking down.
    Why would a plant not be able to acquire enough carbon dioxide for its Calvin Cycle reactions? The answer lies in the evolutionarily adaptive CAM plants. Unlike the C3 plants, CAM plants are more adapted to extreme environments allowing them to thrive in extremely where the loss of water would normally kills or greatly reduces the fitness of a C3 type plant.
    CAM plants, like cactus and pineapples for example, have evolved a system to reduce water loss by keeping their stomata, tiny pores on the plant’s surface, during the heat of the day to prevent transpiration —the evaporation of water from the plant through these openings.
    When night falls, CAM plants open their pores when the threat of water loss is greatly reduced. The problem of preventing water loss caused a new problem, it reduces the plants ability to intake carbon dioxide needed for photosynthesis. CAM plants have another adaptive solution; the carbon dioxide that is taken inside the plant through the open pores at night and is converted into the molecule , which stores the carbon until it can be used in the daytime when light is available.
    There is another type known as C4 plants. Instead of using the day and night strategy of the CAM plants, C4 plants produce a four carbon molecule which is then taken in by specially adapted bundle sheath cells inside the leaf, this adapted molecule when incorporated into the Calvin Cycle of a C4 plant can replace carbon dioxide when it is limited.