Photosynthesis

Photosynthesis

1. Interrelationship with Respiration

   • Water and CO2 = products of respiration and reactants of photosynthesis
   • Glucose and Oxgen = products of P/s and reactants of respiration
   • releasing energy= purpose of respiration
     • Trapping energy = purpose of photosynthesis

2. Structure of a Chloroplast

   • Granum
     • a stack of several thylakoids, contains chlorophyll pigments, where light reactions occur during photosynthesis
   • DNA
     • circular loop of DNA - with ribosomes - can synthesise their own proteins
   • Stroma
     • Fluid filled matrix containing enzymes for the second stage to p/s
     • location of light indeed. stage
   • Thylakoids
     • a series of membranous compartments that contain chlorophyll and molecules needed for light-dependent reaction
     • location of light dep. stage
   • Chloroplast envelope
     • Double plasma membrane that surrounds the organelle, highly selective in allowing substances through
   • Lamellae
     • membranous channels which join grana together in a chloroplast

3. Importance of photosynthetic pigments

   • Light harvesting systems
   • Photosystems
   • Photosynthetic Pigment= a coloured biological compound present in chloroplasts and photosynthetic bacteria and which captures light energy for photosythesis
   • 2 types of pigments in plants
     • Cartenoids (oranges+yellows)
     • Chlorophylls
       • Chlorophyll A
         • Light Dep. reaction - PSI +PSII
       • Chlorphyll B

4. Thin Layer Chromatography (TLC)

   • method used to separate photosynthetic pigments

5. Light Dependent Stage

   • Location: Thylakoid membrane of the Chloroplasts
     • Photosystems I and II (light harvesting systems) containing Chlorphyll
       • PSI peak absorption = 700nm
       • PSII peak absorption = 680nm
   • Cyclic photophosphorylation
     • Sun--> PSI -excited electrons released from the reaction chain of PSI passed to an electron transport chain
       • Then returned to PSI
         • instead of being used to form reduced NADP
     • reduced NADP not formed
   • Non-cyclic photophosphorylation
     • Absorbed light excites electrons at the reaction centres of the photosystems
       • The excited electrons are released from the reaction centre of PSII and are passed to an electron transport chain
         • ATP is produced by the process of chemiosmosis
           • The electrons lost from the reaction centre at PSII are replaced from water molecules broken down using energy from the sun
             • Excited electrons are released from the reaction centre at PSI
               • These are passed on to another electron transport chain and ATP is again produced by chemiosmosis
                 • The electrons lost from this reaction centre are replaced by electrons that have just travelled along the first electron transport chain after being released from PSII
                   • The electrons leaving the electron transport chain following PSI are accepted along with a hydrogen ion by the coenzyme NADP --> forming reduced NADP
                     • Reduced NADP provides the hydrogen or reducing power in the production of organic molecules, such as glucose, in the light independent stage which follows
             • Photolysis
               • The process by which water molecules are split into hydrogen ions, electrons, and oxygen molecules using energy from the Sun
                 • The oxygen evolving complex which forms part of PSII is an enzyme that catalyses the breakdown of water
                   • the H+ (protons) are released into the lumen of the thykaloids, increasing the H+ conc across the membrane
                     • As the move back through the membrane down a concentration and electrochemical gradient, they drive the formation of more ATP
                       • once the hydrogen ions are returned to the storm they combine with NADP and an electron from PSI to form reduced NADP
                         • This is used in the light independent stage of p/s
                         • this process removes hydrogen ions from the storm so it helps to maintain the proton gradient across the thylakoid membranes
                 • why H20 is a raw material needed for photosynthesis
                 • & why O2 gas is a byproduct
           • Chemiosmosis
         • Electron Transport Chain
   • Energy from Sunlight is absorbed and used to form ATP. Hydrogen from water is used to reduce coenzyme NADP to reduced NADP

6. Light Independent Stage (Calvin Cycle)

   • Hydrogen from reduced NADP and CO2 is used to build organic molecules such as glucose. ATP supplies the required energy
   • Fixation - CO2 is fixed (incorporated into an organic molecule)
     • Reduction- GP is reduced to TP by the addition of hydrogen from reduced NADP using energy supplied by ATP
       • Regeneration- RuBP is regenerated from the recycled TP
         • For one glucose molecule to be produced, six- carbon dioxide molecules have to enter the Calvin cycle, resulting in six full turns of the cycle
           • This produces :
             • 12 TP molecules (2 of which will be removed to make the glucose molecule)
               • 10 TP molecules are recycled to regenerate 6 RuBP molecues (used in 6 turns of the cycle)
                 • 30 carbons total = (10 x 3-carbon TP's ) gives --> (6x 5-carbon RuBP)
         • Energy supplied by ATP
       • Each GP molecule is converted to another 3 carbon molecule TP, using a hydrogen atom from reduced NADP and energy supplied by ATP (both from the Light Dep. phase of p/s
         • TP = triose phosphate
           • Uses of Triose Phosphate (TP)
           • The majority of TP is recycled to regenerate RuBP so that the Calvin cycle can continue
             • It is the starting point for the synthesis of many complex biological molecules, including other carbohydrates, lipids, proteins, and nucleic acids
     • CO2 enters the intercellular spaces within the spongy mesophyll of leaves by diffusion from the atmosphere through the stomata
       • It diffuses into cells and into the storm of chlorplasts where it combines with RuBP (5 carbon molecule) - this reaction is catalysed by RuBIsCo
         • RuBp = ribulose bisphosphate
         • RuBisCO = Ribulose bisphosphate carboxylase
           • Key enzyme in photosynthesis - very inefficient as it is competitively inhibited by Oxygen
         • An unstable 6 carbon intermediate is formed
           • This immediately breaks down forming two 3 carbon GP molecules
             • GP = glycerate - 3 - phosphate

7. Factors affecting p/s

   • practical investigations into these factors
   • Concentration of CO2
   • Light Intensity
   • Temperature
   • Degree to which stomata are open or closed
   • Water supply
   • Availability of chlorophyll, carrier molecules, and enzymes

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