Topic 8: Metabolism, cell respiration and photosynthesis (AHL)-8.3 Photosynthesis

Topic 8: Metabolism, cell respiration and photosynthesis (HL)
8.3 Photosynthesis

Structure of chloroplast

  • The chloroplast has an outer membrane and an inner membrane
  • The inner membrane encloses the interconnected membranes called the thylakoid membranes
  • Chlorophyll molecules are grouped together into photosystems contained within the thylakoid membranes.
  • The area within these thylakoid membranes is called the thylakoid space (lumen) and this is where the light-dependent reactions take place Inside the thylakoid, green pigments can be found, which is mainly consisted ofhlorophyll A, chlorophyll B and carotenoids

All those pigments are organized into photosystems

Light-dependent reaction

Photosystem II:

  • Photon of light is absorbed by pigments on photosystem II and the energy is transferred to chlorophyll A and excites one of its electron
  • The electron is captured by an electron carrier to the electron transport chain (ETC)
  • Water is splitted to replace lost electron, and produce H+ ion, oxygen and electrons
  • Water splitting process is called photolysis, which is happened in the enzymes called water splitting enzyme

Electron transport chain:

  • Electrons (from photosystem II)are carried along the electron transport chain, actively transport \(H^+\) ion across the membrane. The de-energised electrons are passed to the photosystem I to replace lost electrons
  • Chemiosmosis: \(H^+\) ions create a concentration difference between the thylakoid space and stroma. Through normal diffusion along the concentration gradient, \(H^+\)ions diffuse back into the thylakoid space (lumen) and then drive ATP synthase to produce ATP

Photosystem I:

  • Photons excite electrons in photosystem I, moved by an electron acceptor to ferredoxin
  • Lost electrons are replaced form electrons travelled from ETC, which is originally excited in photosystem II
  • Electrons are used to reduce \(NADP^+\) to NADPH by NADP reductase

Summative Light-dependent reaction table

Light-independent reaction (Calvin Cycle)

  • One \(CO_2\) molecule enters the Calvin cycle and combines with a 5 carbon molecule called Ribulose bisphosphate (RuBP) to temporarily form a
    6C molecule.
  • This reaction is catalyzed by the enzyme RuBP carboxylase (rubisco).
  • This immediately breaks down into two 3C molecules called glycerate-3-phosphate (G3P).
  • G3P molecules are reduced by adding hydrogen from NADPH using the energy from the breakdown of an ATP molecule, which will be turned into triose phosphate
  • Two TP molecules are used to produce one six carbon glucose phosphate molecule, which can eventually be combined with other glucose phosphate to form starch.
  • The other ten TP (3C) molecules are used to regenerate six RuBP (5C) using 6 ATP molecules for energy.
  • So for every 6 triose phosphate molecules produced, 5 of these triose (3C) sugars are used to reform 3 RuBP (5C) molecules using 3 ATP
    molecules The one remaining triose phosphate forms half a glucose phosphate

Summative Light-independent reaction table

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