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• 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), the "energy currency" of cells
• The process of photosynthesis always begins when light energy is absorbed by the reaction centers, proteins that contain photosynthetic pigments or chromophores
• In plants, these proteins are chlorophylls (a porphyrin derivative that absorbs the red and blue spectrums of light, thus reflecting a green color) held inside chloroplasts, abundant in leaf cells
• In the light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas
• The hydrogen freed by the splitting of water is used in the creation of two important molecules that participate in energetic processes: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and ATP
• In plants, algae, and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle
• In this process, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP)
• Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose
• 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; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible
• Today, 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 also convert around 100–115 billion tons (91–104 Pg petagrams, or a billion metric tons), of carbon into biomass per year
• Photosyntesis was first discovered in 1779 by Jan Ingenhousz; he 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


  Organelle in plants and algae where photosynthesis takes place
• 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)
• Thylakoid


  Flattened disks within the chloroplast where photosynthesis takes place
• Photosynthetic bacteria


  • Proteins that gather light for photosynthesis are embedded in cell membranes
  • Membrane may be tightly folded into cylindrical sheets called thylakoids
  • Membrane may be bunched up into round vesicles called intracytoplasmic membranes
• Plant cell


  Contains 10 to 100 chloroplasts
• Chloroplast membrane


  Composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space
• Stroma


  Aqueous fluid enclosed by the chloroplast membrane
• Thylakoid membrane


  Embedded with integral and peripheral membrane protein complexes of the photosynthetic system
• Antenna proteins


  Complexes in which pigments are arranged to work together
• Light-harvesting complex


  Combination of proteins and pigments
• Leaves


  • Contain the majority of chloroplasts in a plant
• Mesophyll


  Interior tissues of a leaf containing 450,000 to 800,000 chloroplasts per square millimeter
• Cuticle


  Water-resistant waxy coating on the leaf surface
• Epidermis


  Transparent layer allowing light to pass through to the palisade mesophyll cells
• Light-dependent reactions


  1. Chlorophyll absorbs photon, electron is taken up by pheophytin

  2. Electron passed through electron transport chain

  3. Proton gradient created across chloroplast membrane

  4. ATP synthase uses proton gradient to synthesize ATP

  5. Chlorophyll regains electron from water splitting (photolysis), releasing oxygen
• Overall equation for light-dependent reactions: 2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2
• Photosynthetic action spectrum


  Depends on accessory pigments present
• In green plants, action spectrum resembles absorption spectrum for chlorophylls and carotenoids with peaks in violet-blue and red light
• In red algae, action spectrum is blue-green light, allowing growth in deeper waters
• Z scheme


  Diagram showing the light-dependent reactions in the thylakoid membrane
• Non-cyclic light-dependent reactions


  1. Photons captured in antenna complexes of photosystem II

  2. Electron from chlorophyll taken up by pheophytin

  3. Electron transported through Z-scheme

  4. Proton gradient generated

  5. ATP synthase uses proton gradient to make ATP

  6. NADPH produced at end of Z-scheme