light-independent reaction

(noun)

chemical reactions during photosynthesis that convert carbon dioxide and other compounds into glucose, taking place in the stroma

Related Terms

Examples of light-independent reaction in the following topics:

  • The Two Parts of Photosynthesis

    • Light-dependent and light-independent reactions are two successive reactions that occur during photosynthesis.
    • In the light-independent reactions or Calvin cycle, the energized electrons from the light-dependent reactions provide the energy to form carbohydrates from carbon dioxide molecules.
    • Although the light-independent reactions do not use light as a reactant (and as a result can take place at day or night), they require the products of the light-dependent reactions to function.
    • In addition, several enzymes of the light-independent reactions are activated by light.
    • Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle (light-independent reactions).

  • The Calvin Cycle

    • Once in the mesophyll cells, CO2 diffuses into the stroma of the chloroplast, the site of light-independent reactions of photosynthesis.
    • Other names for light-independent reactions include the Calvin cycle, the Calvin-Benson cycle, and dark reactions.
    • The most outdated name is dark reactions, which can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it.
    • The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.
    • The Calvin cycle is not totally independent of light since it relies on ATP and NADH, which are products of the light-dependent reactions.

  • Processes of the Light-Dependent Reactions

    • The overall function of light-dependent reactions, the first stage of photosynthesis, is to convert solar energy into chemical energy in the form of NADPH and ATP, which are used in light-independent reactions and fuel the assembly of sugar molecules.
    • The light-dependent reactions begin in photosystem II .
    • Cyclic phosphorylation is important to maintain the right proportions of NADPH and ATP, which will carry out light-independent reactions later on.
    • A photosystem consists of a light-harvesting complex and a reaction center.
    • Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center.

  • CAM and C4 Photosynthesis

    • CAM concentrates it temporally, providing CO2 during the day and not at night, when respiration is the dominant reaction.
    • C4 plants, in contrast, concentrate CO2 spatially, with a RuBisCO reaction centre in a "bundle sheath cell" that is inundated with CO2.
    • C4 plants can produce more sugar than C3 plants in conditions of high light and temperature.
    • Plants that do not use PEP-carboxylase in carbon fixation are called C3 plants because the primary carboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle.
    • The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-independent reactions of photosynthesis.

  • Energy Changes in Chemical Reactions

    • By the Law of Conservation of Energy, however, we know that the total energy of a system must remain unchanged, and that oftentimes a chemical reaction will absorb or release energy in the form of heat, light, or both.
    • Exothermic reactions release heat and light into their surroundings.
    • Excess energy from the reaction is released as heat and light.
    • Endothermic reactions, on the other hand, absorb heat and/or light from their surroundings.
    • Thus, an endothermic reaction is said to have a positive enthalpy of reaction.

  • Photochemistry

    • The study of chemical reactions, isomerizations and physical behavior that may occur under the influence of visible and/or ultraviolet light is called Photochemistry.
    • The first law of photochemistry, the Grotthuss-Draper law, states that light must be absorbed by a compound in order for a photochemical reaction to take place.
    • The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for subsequent reaction.
    • The biacetyl product, formed in the third reaction, may itself be excited by light or accept excitation energy from an excited acetone molecule, further complicating this process.
    • By comparison, the light induced chlorination of methane, or other alkanes, has a large quantum yield, often near 106, because of the secondary chain reactions that follow the primary cleavage of the Cl-Cl bond.

  • Exothermic and Endothermic Processes

    • Endothermic reactions absorb energy from the environment, while exothermic reactions release energy to the environment.
    • Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light.
    • Endothermic reactions are reactions that require external energy, usually in the form of heat, for the reaction to proceed.
    • As such, the change in enthalpy for an endothermic reaction is always positive.
    • In an endothermic reaction, the products are higher in energy than the reactants.

  • Theoretical Models for Pericyclic Reactions

    • The greater the degree of bonding found in the transition state for the reaction, the lower will be its activation energy and the greater will be the reaction rate.
    • The π-orbital on the left has one nodal plane (colored light blue), and the π*-orbital on the right has a second nodal plane (colored yellow).
    • These localized orbitals may be classified by two independent symmetry operations ; a mirror plane perpendicular to the functional plane and bisecting the the molecule (colored yellow above), and a two-fold axis of rotation (C2) created by the intersection of this mirror plane with the common nodal plane (colored light blue).
    • The original approach of Woodward and Hoffmann involved construction of an "orbital correlation diagram" for each type of pericyclic reaction.
    • If a symmetry barrier was present, the reaction was designated symmetry-forbidden.

  • Zero-Order Reactions

    • A zero-order reaction has a constant rate that is independent of the concentration of the reactant(s); the rate law is simply $rate=k$ .
    • Unlike the other orders of reaction, a zero-order reaction has a rate that is independent of the concentration of the reactant(s).
    • This is the integrated rate law for a zero-order reaction.
    • For a zero-order reaction, the half-life is given by:
    • The reverse Haber process is an example of a zero-order reaction because its rate is independent of the concentration of ammonia.

  • The Aldol Reaction

    • Addition reactions of enolate species to aldehydes and ketones, known as the aldol reaction, are structurally analogous to the allyl and crotyl addition reaction described in the previous concept.
    • It is appropriate and instructive, therefore, to examine how far the previous analyses and interpretations may be extended and applied to the aldol reaction.
    • Conversely, similar reactions of Z-enolates occur by preferential bonding of the re (or si) face of the aldehyde to the si (or re) face of the enolate.
    • Enolborinates were among the most reliable and selective reagents revealed by a host of aldol studies, a quality reflected in reactions of the crotylboronates.
    • In light of previous discussions, it may be anticipated that the course of aldol reactions will be further influenced by the presence of a nearby chiral center in the carbonyl acceptor or the enolate donor.