Photosynthesis

Photosynthesis

1. Importance

   • converts light energy to chemical used to synthesis large organic molecules - autotrophic nutrition
     • Known as photoautotrophs and producers

2. Respiration

   • Carbon fixation: process of carbon being converted to sugars - this is endothermic (needs energy) and is a reduction reaction (needs electrons)
     • regulates carbon dioxide in atmosphere and oceans
   • Heterotrophs obtain energy from digesting complex organic molecules and using them as respiratory substrates
   • Compensation point: no net loss or gain of carbohydrate
     • time taken to reach this stage is known as compensation period - lower in shade plants as they need a longer light intensity then sun plants

3. Chloroplasts

   • Structure
     • Surrounded by double membrane (the envelope) which is highly permeable
     • 2 distinct regions on an electron micrograph - fluid filled matrix called the stroma and grana made from stacked thylakoid membranes
   • Photosynthetic pigments
     • Inside thylakoid membranes are photosystems that contain photsynthetic pigments (these absorbs light of a particular wavelength and reflect back others)
     • Light funnelled down to the primary pigment reaction centre with a type of chlorophyll at the bottom
     • Chlorophylls are mixtures of pigments with a porphyrin group with a magnesium atom and a large hydrocarbon chain
       • Chlorophyll A absorbs red light, there are 2 kinds: P680 (PS II) and P700 (PS I)
       • Chlorophyll B appears yellow green and absorbs wavelengths of 400-500 and 640
     • Accessory pigments: Carotenoids absorb blue and reflect yellow+orange, Xanthophylls absorb green+blue and reflects yellow
   • Stroma
     • Contains enzymes needed to catalyse reactions of the light-independent stage, as well as starch, oil droplets and small ribosomes
     • Loop of DNA that codes for some of proteins needed for photosynthesis - assembled at chloroplast ribosomes
   • Grana
     • site of light-dependent stage of photosynthesis
     • Thylakoids in a granum may be connected to others in another granum by intergranal lamellae
     • Thylakoids flattened less permeable disc-like sacs (increased surface area) stacked to make granums
     • Proteins in the thylakoid membrane hold the photosystems in place
     • large surface area for distribution of photosystems that contain photosynthetic pigments and electron carriers + ATP synthase enzymes needed to convert light energy into ATP

4. Light-dependent stage

   • Light harvesting, photolysis, photophosphorylation and NADPH formed
   • photolysis
     • in PSII water molecules are split into protons in the presence of light and a certain enzyme
     • 2H2O --> 4H+ + 4e- + O2
     • source of protons, donates electrons to chlorophyll, source of oxygen and keeps cells turgid
   • Photophosphorylation
     • Generates ATP from ADP and P
     • Non-cyclic: in PSI and PSII - makes ATP, oxygen and NADPH
       • Energy channelled to primary pigment reaction centre, used to excite a pair of electrons which are captured by an electron carrier with an iron ion at its centre - electrons replaced by those from photolysis. Iron ion becomes reduced then donates electron - ETC - some energy associated with the electrons is released at each step which is used to pump electrons into the thylakoid space. The electrons are eventually captured by another chlorophyll a molecule in PSI to replace those lost. Ferredoxin (protein-iron-sulfur complex) accepts electron from PSI and passes to NADP in stroma - NADP is reduced by electrons and protons, catalysed by NADP reductase. The protons move down their conc gradient through channels associated with ATP synthase enzymes causing ADP to join with phosphate making ATP
     • Cyclic: in PSI - makes ATP in smaller quanitites
       • Uses PS1 - when light hits PS1 a pair of electrons in the chlorophyll molecule gain energy and become excited - they leave the chlorophyll, pass to an electron carrier system then return to PS1. Small amount of ATP generated along electron carriers.
       • Guard cells have chloroplasts with only PS1 in - make ATP to actively bring K ions into cells, lowering the water potential so water enters and guard cells swell and stoma opens
     • Both have iron-containing proteins in thylakoid membranes that accept and donate electrons
   • in granum (thylakoids)

5. Factors affecting photosynthesis

   • Limiting factors include: CO2, water, light intensity, chlorophyll availability, electron carrier availability, enzyme availability, temperature and cell turgidity
   • Light - provides energy to power first stage and provide ATP and NADPH for second stage + causes stomata to open for transpiration to occur. As light intensity increases so does rate until it s no longer a limiting factor
     • GP cannot be converted to TP so GP levels rise, TP decrease, RuBP decrease as not regenerated
   • CO2 tends not to be a limiting factor but if it is - RuBP cannot accept CO2, GP cannot be made, TP cannot be made
   • Temperature
     • From low to 25-30 degrees as temp incs so does rate, above 30 rate decreases as oxygen competes with CO2 for RuBisCOs active site, above 45 enzymes are denatured and photosynthesis stops
   • Water stress
     • Water has a cooling effect on plants and keeps them turgid so they can function, if there isn't enough then cells become plasmolysed, abscisic acid is produced in the roots and sent to leaves to stop transiration - leaves become flaccid and wilted - rate decreases

6. Light-independent stage

   • CO2
     • Used for the production of all organic molecules which are used for structure (cell membranes, proteins, cell wall etc) or energy stores (starch etc).
     • Enters the leaf through the stomata, diffuses through the spongy mesophyll layer to the palisade layers where it enters the chloroplast. Fixation of carbon dioxide maintains a conc gradient
   • Calvin cycle
     • Reactions where CO2 is converted into organic molecules is called the Calvin cycle
     • 1) CO2 combines with CO2 acceptor (RuBP) - catalysed by RuBisCO 2) RuBP becomes an unstable intermediate 6C compound that breaks down 3) 2 molecules of 3C GP made - CO2 has been fixed 4) GP reduced, using hydrogens from NADPH, to TP using 2 molecules of ATP for every CO2 molecule 5) 10/12 TP molecules used to regenerate 6 molecules of RuBP using phosphate groups - other 2 TP molecules are products
     • Only runs in daylight as ATP and NADPH from light-dependent stage are needed - in the light-dependent stage hydrogen ions are pumped into the thylakoid spaces from the stroma, raising the pH to 8 (optimum for RuBisCO). Also in daylight the conc of Mg ions increases and these attach to active site of RuBisCO, acting as cofactors and also the ferredoxin reduced in PS1 activates enzymes involved in the Calvin cycle
   • Uses of triose phosphate (TP)
     • Some glucose is converted to sucrose, starch and cellulose
     • Some used to synthesis fatty acids, glycerol and amino acids
     • Recycled to regenerate RuBP
   • Occurs in stroma

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