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    Cards (173)

    • Photosynthesis is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their activities
    • Photosynthetic organisms use intracellular organic compounds to store the chemical energy they produce in photosynthesis within organic compounds like sugars, glycogen, cellulose and starches
    • Photosynthesis is usually used to refer to oxygenic photosynthesis, a process that produces oxygen
    • Some bacteria also perform anoxygenic photosynthesis, which uses bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water, producing sulfur instead of oxygen
    • Archaea such as Halobacterium also perform a type of non-carbon-fixing anoxygenic photosynthesis, where the simpler photopigment retinal and its microbial rhodopsin derivatives are used to absorb green light and power proton pumps to directly synthesize adenosine triphosphate (ATP)
    • Photosynthesis process
      1. Light energy is absorbed by reaction centers
      2. Energy is used to strip electrons from substances like water, producing oxygen gas
      3. Hydrogen freed from water is used to create NADPH and ATP
      4. In plants, algae, and cyanobacteria, sugars are synthesized by the Calvin cycle using the ATP and NADPH
    • The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons
    • Cyanobacteria appeared later and the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible
    • The average rate of energy captured by photosynthesis globally is approximately 130 terawatts, which is about eight times the current power consumption of human civilization
    • Photosynthetic organisms convert around 100–115 billion tons (91–104 Pg petagrams, or a billion metric tons), of carbon into biomass per year
    • Photosynthesis was first discovered in 1779 by Jan Ingenhousz, who showed that plants need light, not just air, soil, and water
    • Photosynthesis is vital for climate processes, as it captures carbon dioxide from the air and then binds it in plants, harvested products and soil
    • Cereals alone are estimated to bind 3,825 Tg (teragrams) or 3.825 Pg (petagrams) of carbon dioxide every year, i.e. 3.825 billion metric tons
    • Chloroplast ultrastructure
      • Outer membrane
      • Intermembrane space
      • Inner membrane (envelope)
      • Stroma (aqueous fluid)
      • Thylakoid lumen (inside of thylakoid)
      • Thylakoid membrane
      • Granum (stack of thylakoids)
      • Thylakoid (lamella)
      • Starch
      • Ribosome
      • Plastidial DNA
      • Plastoglobule (drop of lipids)
    • Photosynthetic membranes and organelles in photosynthetic bacteria
      • Cell membranes
      • Thylakoids (tightly folded cylindrical sheets)
      • Intracytoplasmic membranes (bunched up round vesicles)
    • Photosynthesis takes place in organelles called chloroplasts in plants and algae
    • A typical plant cell contains about 10 to 100 chloroplasts
    • Chloroplast
      • Enclosed by a membrane composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space
      • Enclosed by the membrane is an aqueous fluid called the stroma
      • Embedded within the stroma are stacks of thylakoids (grana), which are the site of photosynthesis
      • The thylakoids appear as flattened disks
      • The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space
      • Embedded in the thylakoid membrane are integral and peripheral membrane protein complexes of the photosynthetic system
    • Pigments used by plants and algae for photosynthesis
      • Chlorophyll
      • Carotenes
      • Xanthophylls
      • Phycocyanin (in green algae)
      • Phycoerythrin (in red algae)
      • Fucoxanthin (in brown algae and diatoms)
    • Antenna proteins
      Complexes in which pigments are arranged to work together in plants and algae
    • Light-harvesting complex
      Another term for the combination of proteins and pigments that harvest light for photosynthesis
    • The majority of chloroplasts are found in the mesophyll cells of leaves
    • Leaf surface
      • Coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation and decreases absorption of ultraviolet or blue light to minimize heating
      • Transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most photosynthesis takes place
    • Light-dependent reactions
      1. Chlorophyll molecule absorbs a photon and loses an electron
      2. Electron taken up by pheophytin and passed through electron transport chain
      3. Proton gradient created across chloroplast membrane, used by ATP synthase to synthesize ATP
      4. Chlorophyll molecule regains electron when water molecule is split in photolysis, releasing oxygen
    • Non-cyclic electron flow
      The overall equation for the light-dependent reactions in green plants
    • Not all wavelengths of light can support photosynthesis
    • Photosynthetic action spectrum
      Depends on the type of accessory pigments present
    • Photosynthetic action spectrum examples
      • Green plants - resembles absorption spectrum for chlorophylls and carotenoids, with peaks in violet-blue and red light
      • Red algae - blue-green light, allowing them to use the blue end of the spectrum to grow in deeper waters
    • The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color
    • Z scheme
      1. Photons captured in light-harvesting antenna complexes of photosystem II
      2. Electron loosened and taken up by pheophytin
      3. Electron shuttled through electron transport chain, generating chemiosmotic potential
      4. ATP synthase uses chemiosmotic potential to make ATP
      5. NADPH produced as terminal redox reaction product
      6. Electron enters photosystem I, further excited, and used to reduce NADP+ to NADPH
    • Cyclic reaction
      1. Similar to non-cyclic, but generates only ATP, no NADPH
      2. Occurs only at photosystem I, with electron returning to photosystem I after passing through electron acceptors
    • Water photolysis
      1. Two water molecules oxidized by energy of four charge-separation reactions in photosystem II
      2. Yields oxygen molecule and four hydrogen ions
      3. Hydrogen ions contribute to chemiosmotic potential for ATP synthesis
      4. Oxygen is a waste product, but used by most organisms for cellular respiration
    • Calvin cycle
      1. RuBisCO enzyme captures CO2 from atmosphere
      2. CO2 combined with ribulose 1,5-bisphosphate to yield two 3-phosphoglycerate molecules
      3. 3-phosphoglycerate reduced to glyceraldehyde 3-phosphate using ATP and NADPH from light reactions
      4. Most glyceraldehyde 3-phosphate recycled to regenerate ribulose 1,5-bisphosphate
      5. Remaining triose phosphates used to produce sucrose, starch, cellulose, glucose, and fructose
    • C4 carbon fixation, CAM photosynthesis, Alarm photosynthesis
      • Carbon concentrating mechanisms used by plants in hot and dry conditions to reduce photorespiration
    • Hexose phosphates
      Compounds formed from recycled carbon that ultimately yield sucrose, starch, cellulose, glucose, and fructose
    • Carbon metabolism
      Yields carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids
    • C4 carbon fixation
      • Chemically fixes carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenolpyruvate (PEP)
      • Translocates the four-carbon organic acid oxaloacetic acid or malate to specialized bundle sheath cells where RuBisCO and other Calvin cycle enzymes are located
      • CO2 released by decarboxylation of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon 3-phosphoglyceric acids
    • Photorespiration
      Increase in oxygen gas produced by the light reactions of photosynthesis, causing an increase in the oxygenase activity of RuBisCO and decrease in carbon fixation
    • C4 plants
      • Can produce more sugar than C3 plants in conditions of high light and temperature
      • Include important crop plants like maize, sorghum, sugarcane, and millet
    • C3 plants
      Plants that do not use PEP-carboxylase in carbon fixation, where the primary carboxylation reaction catalyzed by RuBisCO produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle