light-independent reaction

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  • The most outdated name is dark reactions, which can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why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.

• Activation Energy

  • The free energy released from the exergonic reaction is absorbed by the endergonic reaction.
  • However, the measure of the activation energy is independent of the reaction's ΔG.
  • Notice that the activation energy for the reverse reaction is larger than for the forward reaction.
  • A is the frequency factor of the reaction.
  • Activation energy is the energy required for a reaction to proceed; it is lower if the reaction is catalyzed.

• Absorption of Light

  • Light energy initiates the process of photosynthesis when pigments absorb the light.
  • For example, retinal pigments can only "see" (absorb) 700 nm to 400 nm light; this is visible light.
  • When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage.
  • Chlorophyll a absorbs light in the blue-violet region, while chlorophyll b absorbs red-blue light.
  • Other organisms grow in competition for light.

• The First Law of Thermodynamics

  • For instance, light bulbs transform electrical energy into light energy, and gas stoves transform chemical energy from natural gas into heat energy.
  • Conversely, in an exothermic reaction, the heat that is released in the reaction is given off and absorbed by the surroundings.
  • We know that chemical systems can either absorb heat from their surroundings, if the reaction is endothermic, or release heat to their surroundings, if the reaction is exothermic.
  • If you've ever witnessed a video of a space shuttle lifting off, the chemical reaction that occurs also releases tremendous amounts of heat and light.
  • Plants can convert electromagnetic radiation (light energy) from the sun into chemical energy.

• Blue Light Response

  • Phototropism is the directional bending of a plant toward or away from a light source of blue wavelengths of light.
  • Positive phototropism is growth toward a light source , while negative phototropism (also called skototropism) is growth away from light.
  • Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore, which senses blue wavelengths of light.
  • There is some evidence that cryptochromes work by sensing light-dependent redox reactions and that, together with phototropins, they mediate the phototropic response.
  • Phototropism is the growth of plants in response to light.

• The Evolution of Plastids

  • Unlike most prokaryotes, however, they have extensive, internal membrane-bound compartments called thylakoids, which contain chlorophyll and are the site of the light-dependent reactions of photosynthesis .
  • Plastids, like mitochondria, cannot live independently outside the host.
  • (a) Red algae and (b) green algae (visualized by light microscopy) share similar DNA sequences with photosynthetic cyanobacteria.

• The Role of Energy and Metabolism

  • All organisms require energy to complete tasks; metabolism is the set of the chemical reactions that release energy for cellular processes.
  • Plants convert light energy from the sun into chemical energy stored in molecules during the process of photosynthesis.
  • Cellular processes such as the building and breaking down of complex molecules occur through step-by-step chemical reactions.
  • Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed.
  • The hummingbird obtains its energy from taking in food and transforming the nutrients into energy through a series of biochemical reactions.