Photosynthesis

• Glucose molecules that are photosynthesized provide the organism with 2 crucial things:

  • energy
  • fixed carbon
• The chemical energy of glucose molecules can be harvested through cellular respiration or fermentation, which releases energy in the form of ATP
• Carbon fixation


  When inorganic carbon (carbon from CO2) is incorporated into an organic system
• Photoautotrophs


  Organisms which feed themselves using photosynthesis
• Heterotrophs


  Organisms which must get fixed carbon in their systems from eating other organisms
• Mesophyll


  Cells in the middle layer of leaf cells that are mainly responsible for photosynthesis
• Stomata


  Small pores on the surface of leaves in most plants which let CO2 diffuse in and O2 diffuse out of the mesophyll
• Chloroplast


  Organelles with the specific function of carrying out photosynthesis that are found in mesophyll cells
• Thylakoid


  Disc like structures within chloroplasts whose membranes are full of chlorophyll that are in stacks called grana
• Grana


  Stacks of thylakoids within chloroplasts
• Stroma


  Fluid filled space in a chloroplast around the grana
• Thylakoid space


  Space within thylakoid discs
• Chlorophyll


  Green-colored pigments that absorb light
• Light - dependent reactions


  Reactions that require a steady stream of light that happens in the chlorophyll within the thylakoid membranes of the chloroplast. Chlorophylls take the light energy and convert it into chemical energy through synthesizing ATP and NADPH
• Calvin cycle


  Light independent reactions
• Calvin cycle


  Takes place in the stroma, using ATP and NADPH from light-dependent reactions to fix carbon and convert it into three-carbon sugars, which join up to form glucose
• Light-dependent reactions temporarily store light energy as chemical energy within ATP and NADPH, which the go into the Calvin cycle where ATP is broken down to release energy and NADPH donates its electrons to convert carbon dioxide molecules into sugars
• Redox reactions


  Reactions involving electron transfers
• Photosynthesis and cellular respiration are essentially perfect opposites in terms of products and reactants, except photosynthesis uses solar energy and cellular respiration uses chemical energy
• Like cellular respiration, photosynthesis uses an electron transport chain to make a H+ concentration gradient , which drives ATP synthesis by chemiosmosis
• Chemiosmosis


  Movement of ions across a semipermeable membrane alone their electrochemical gradient
• Photosystems


  Large complexes of proteins and pigments which are optimized to harvest light and play a crucial role in light reactions
• Both types of photosystems contain pigments to help harvest light energy, as well as a special pair of chlorophyll molecules at the reaction center. PSI and PSII are differentiated by their special chlorophyll molecules (P700 ad P680 , respectively)
• Pigment


  Light absorbing molecule
• Non cyclic photophosphorylation


  Standard form of light dependent reactions, when electrons are removed from water and passes through PSI and PSII before ending up in NADPH. It requires light to be absorbed twice, and ends in ATP
• Steps of photophosphorylation:

  • Light absorption in PSII
  • ATP synthesis
  • Light absorption in PSI
  • NADPH formation
• Light is absorbed by pigments in PSII, then its energy passed inwards until it reaches the reaction core chlorophyll (P680). The energy boots an electron there to a high level, and is then passed on to an acceptor molecule and replaced with a water electron. The water is split because of this, resulting in O2
• ATP synthesis in photosynthesis happens after the high-energy electron is taken into the acceptor molecule, and it gets brought down an electron transport chain, losing energy all the way. Some of this goes towards powering the H+ pumps, which builds a gradient. As they go down this gradient, the H+ ions pass through ATP synthase, driving ATP production through chemiosmosis
• Light absorption in PSI happens after the electron being transported in ATP synthesis arrives in PSI and joins the chlorophyll P700 pair at the center. When light hits it, that electron is boosted again and passed onto an acceptor molecule, and replaced by another electron from PSII
• NADPH is produced when the electron from PSI is passed down a short electron transport chain, then added to NADP+ (along with a 2nd electron)
• In photosynthesis, when light hits PSII, it passes an electron along to PSI and eventually to NADP+ , where it is synthesized into NADPH. Between points, electrons release energy, which is then used to create an H+ gradient, and moves H+ through ATP synthase, which produces and synthesizes ATP
• Cyclic photophosphorylation


  Calvin cycle. When electrons follow a circular path, passing through PSI several times but do not go through PSII at all. Produces ATP but no ADPH
• Endergonic reactions in photosynthesis are still given giant amounts of light energy to get powered
• Photosystem


  Structure made up of protein, 300-400 chlorophylls, and pigments
• When a pigment in a photosystem gets hit by light, it reaches an excited state
• Excited state


  When one of an atom or molecule's electrons gets boosted into a higher energy orbital
• Resonance energy transfer


  When a pigment directly transfers energy to its neighbors through electromagnetic interactions, which can then be passed on to those neighbors. Energy transferred can decrease or stay the same, but not increase (obviously)
• Pigments in a photosystem use resonance energy transfer to get the energy to the reaction center, where it is put into the special pair of chlorophylls. They get excited, and then lose an electron, giving it to the primary acceptor molecule
• Primary acceptor molecule


  Molecule that takes the electron lost from the special pair when it gets excited and brings it to the electron transport chain
• The PSII acceptor molecule is a pheophytin, and the PSI acceptor molecule is a chlorophyll called A0