the light-dependent stage

• Light dependent stage of photosynthesis consists of :

  1. light harvesting at the photosystems
  2. photolysis of water
  3. photophosphorylation - the production of ATP in the presence of light
  4. the formation of reduced NADP

  oxygen, the by-product of photosynthesis, also produced in light dependent
• Two types of photosystem

  • in photosystem I (PSI), the pigment at the primary reaction centre iis a type of chlorophyll a, which has a peak absorption of red light of wavelength 700 nm (P700)
  • In photosystem II (PSII), the pigment at the primary reaction centre is also a type of chlorophyll a, but this has a peak absorption of red light of wavelength 680 nm (P680)
• The role of water

  In PSII, there is an enzyme that, in the presence of light, splits water molecules into protons (hydrogen ions), electrons + oxygen

  →photolysis

  2H2O → 4H+ + 4e- + O2

  some of the oxygen produced during photolysis is used by plant cells for aerobic respiration, but during periods of high light intensity the rate of photosynthesis is greater than rate of respiration in plant, so much of the oxygen by-product will diffuse out of the leaves, through stomata, into surrounding atmosphere
• The role of water 2

  Water:

  • source of protons that will be used in photophosphorylation
  • donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
  • source of by-product, oxygen
  • keeps plant cells turgid, enabling them to function
• Photophosphorylation

  generation of ATP from ADP and inorganic phosphate, in the presence of light

  2 types of photophosphorylation :

  • non-cyclic photophosphorylation involves PSI and PSII producing ATP, oxygen + reduced NADP
  • cyclic photophosphorylation involves only PSI producing ATP but in smaller quantities than are made by non-cyclic photophosphorylation
• Non-cyclic photophosphorylation

  1. When a photon of light strikes PSII (P680), its energy is channelled to the primary pigment reaction centre
  2. light energy excites a pair of electrons inside the chlorophyll molecule
  3. the energised electrons escape from the chlorophyll molecule + are captured by an electron carrier, which is a protein with iron at its centre, embedded in the thylakoid membrane
  4. these electrons are replaced by electrons derived from photolysis

  
• Non-cyclic photophosphorylation 2

  5. when this iron ion combines with an electron it becomes reduced (Fe 2+)

  → it can then donate the electron, becoming reoxidised (Fe 3+), to the next electron carrier in the chain

  6. as electrons are passed along a chain of electron carriers embedded in the thylakoid membranes at each step some energy associated with the electrons is released

  7. this energy is used to pump protons across the thylakoid membrane into the thylakoid space
• Non-cyclic photophosphorylation 3

  8. eventually the electrons are captured by another molecule of chlorophyll a in PSI

  → these electrons replace those lost from PSI due to excitation by light energy

  9. a protein-iron-sulphur complex called ferredoxin accepts the electrons from PSI + passes them to NADP in the stroma

  10. as protons accumulate in the thylakoid space, a proton gradient forms across the membrane
• Non-cyclic photophosphorylation 4

  11. proton diffuse down concentration gradient though special channels in the membrane associate with ATP synthase enzymes + so the flow of protons causes ADP + inorganic phosphate to join, forming ATP

  12. as the protons pass through the channel they are accepted, along with electrons, by NADP which becomes reduced

  → the reduction of NADP is catalysed by enzyme NADP reductase

  • the light energy has been converted into chemical energy in the form of ATP by photophosphorylation

  →ATP + reduced NADP are now in the stroma ready for the light-independent stage
• Cyclic Photophosphorylation

  this uses only PSI (P700)

  • as light strikes PSI, a pair of electrons in the chlorophyll molecule at the reaction centre gain energy + become excited

  → they escape from the chlorophyll + pass to an electron carrier system + then pass back to PSI

  • during the passage of electrons along the electron carriers, a small amount of ATP is generated

  → however no photolysis of water occurs, so no protons or oxygen are produced + no reduced NADP is generated
• Cyclic photophosphorylation

  • chloroplasts in guard cells contain only PSI, they produce only ATP which actively brings potassium ions into the cells, lowering the water potential so that water follows by osmosis

  →this causes the guard cells to swell + opens the stoma

  figure 2 is summarising main events of cyclic + non-cyclic photophosphorylation, known as Z scheme