5.6.3 - The Light-Dependent Stage

• The light-dependent stage of photosynthesis occurs in the grana (thylakoids) of chloroplasts and involves the photosystems, it involves the direct use of light energy.
• The light energy absorbed by the photosystems is used for 3 things:

  • ATP Formation - photophosphorylation
  • Making reduced NADP from NADP
  • Splitting water into protons (H+ ions), electrons and oxygen - photolysis
• Photosystem 1 (PSI) - Pigment at the primary reaction centre is a type of chlorophyll A, peak absorption 700nm

  Photosystem 2 (PSII) - Pigment at the primary reaction centre is a type of chlorophyll B, peak absorption 680nm
• In photosystem II, there is an enzyme that, in the presence of light, splits water molecules into protons, electrons and oxygen.

  This splitting of water is called photolysis.
• Some of the oxygen produced during photolysis is used by plant cells in aerobic respiration.

  During high periods of high light intensity, the rate of photosynthesis is greater than the rate of respiration in the plant.

  Much of the oxygen by-product will diffuse out through the stomata in the atmosphere.
• Water:

  • Source of protons (hydrogen ions) to be used in photophosphorylation
  • Donates electrons to chlorophyll to replace those lost when light strikes the chlorophyll.
  • Is the source of the by-product, oxygen.
  • Keeps plants turgid, allowing them to function.
• Photophosphorylation is the generation of ATP from ADP and Inorganic Phosphate in the presence of light.

  • Non-cyclic photophosphorylation involves PSI and PSII and produces ATP, oxygen and reduced NADP.
  • Cyclic photophosphorylation involves PSI and produces ATP in smaller quantities than non-cyclic photophosphorylation
• Both involve iron-containing proteins embedded in the thylakoid membranes that accept and donate electrons, and form an electron transport system.
• Non-Cyclic Photophosphorylation:

  1. The process begins when a photon of light hits photosystem II, and energy is channelled to the primary pigment reaction centre.
  2. The light energy excites a pair of electrons inside the chlorophyll molecule.
  3. The energised electrons escape from the chlorophyll molecule and are captured by an electron carrier, leaving PSII electron deficient.
  4. These electrons are replaced by electrons derive from photolysis
• Non-Cyclic Photophosphorylation involves the transfer of electrons along a chain of electron carriers embodied in the thylakoid membrane, with the energy released from the electrons used to pump protons across the thylakoid membrane in the thylakoid space.
• When an iron ion combines with an electron, it becomes reduced to Fe2+, which can then donate the electron to the next carrier in the chain and become reoxidised to Fe3+.
• As electrons are passed along a chain of electron carriers, some energy associated with the electrons is released, which is used to pump protons across the thylakoid membrane in the thylakoid space.
• Eventually, the electrons are captured by another molecule of chlorophyll in PSI, and these electrons replace those lost from PSI due to excitation by light energy.
• Non-Cyclic Photophosphorylation begins with a protein-iron-sulphur complex called Ferredoxin accepting the electrons from PSI and passing them to NADP in the stroma.
• As protons accumulate in the thylakoid space, a proton gradient forms across the membrane.
• Protons diffuse down their concentration gradient through special channels in the membrane associated with ATP synthase enzymes.
• The flow of protons causes ADP and inorganic phosphate to join and form ATP.
• 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 NADP Reductase.
• The light energy has been converted into chemical energy in the form of ATP by photophosphorylation.
• The products of non-cyclic photophosphorylation are ATP and reduced NADP. The products are now in the stroma ready for the light-independent stage of photosynthesis.
• Cyclic Photophosphorylation:

  • Only uses Photosystem 1

  1. The process begins when light strikes PSI
  2. 2 electrons in the chlorophyll molecule at the reaction centre gain energy and become excited.
  3. The electrons escape from the chlorophyll and pass to an electron carrier system, leaving PSI electron deficient.
• Cyclic Photophosphorylation:

  4. The electrons pass along a chain of electron carriers, losing energy each time they are passed on, but no photolysis of water occurs, so no protons or oxygen are produced and no reduced NADP is generated.

  5. The energy released is used to make ATP from ADP and phosphate. The electrons pass back to photosystem 1.
• Chloroplasts in guard cells contain only PSI.

  They produce only ATP, which actively brings potassium ions to the cells, lowering the water potential so that water follows by osmosis.

  This causes the guard cells to swell and opens the stoma.
• Light energy hits a water molecule and with the aid of enzymes, the water is split into oxygen, hydrogen ions and electrons.

  The hydrogen ions reduce NADP which passes into the light-independent reaction.

  The electrons replace those lost from PSII in non-cyclic photophosphorylation. Oxygen is released as a waste gas.