Photosynthesis in Higher Plants

C3 and C4 pathways

  • C3 or Calvin cycle was first observed in chlorella and Scenedesmus with radioactive 14C to trace the path of CO2.
  • The dark reaction that happens through the C3 cycle to fix CO2 into starch, happens with 3 steps – (i) Carboxylation (ii) reduction (iii) Regeneration.
  • CO2 acceptor → RUBP (ribulose – 1,5-bisphosphate).
  • Enzyme → RuBisCO (Previously called as Carboxydismutase)
  • The first stable product of C3 cycle → 3-phosphoglyceric acid (PGA).
  • These three steps come under Carboxylation.
  • RuBisCO is the most abundant protein (enzyme) of the biological world (plants).
  • The reduction is otherwise termed as a glycolytic reversal as PGA formed is reduced to DHAP (Dihydroxy acetone -3 Phosphate) which further combines with G-3-P to form glucose → starch.
  • The regeneration step requires one ATP to convert glyceraldehyde 3-phosphate to RuBP.
  • As the first stable product of the cycle is phosphoglyceric acid, a 3-carbon compound, the cycle is named as the C3 cycle.
  • The net reaction of C3 fixation of CO2 is as follows.
    6 RuBP + 6CO2 + 18 ATP + 12 NADPH → 6 RuBP + C6H12O6 + 12NADP+ + 18 ADP + 18 Pi
  • C4 cycle also known as Hatch and Slack’s cycle, reported that some plants like sugar cane, produce the first stable product as Oxaloaacetic acid, a 4-carbon compound.
  • C4 plants have a special leaf anatomy, called Kranz anatomy, where the vascular bundles are surrounded by bundle sheaths, which are again surrounded by mesophyll cells.
  • 2 types of chloroplast found in kranz anatomy are bundle sheath chloroplast (larger in size without grana with starch grains) and mesophyll chloroplast (smaller size with grana lacking starch grains) and hence 2 different carboxylation occur.
  • CO2 acceptor → PEP [Phospho Enol Pyruvate]
  • PEP is later converted into oxalic acid → Malic acid → Aspartic acid
  • Malic acid is then transported into bundle sheath cells and undergoes decarboxylation reaction to form CO2 and Pyruvate, where the CO2 enters C3 cycle (2nd carboxylation) & puruvate regenerates PEP
  • C4 cycle requires 30 ATP and 12 NADPH2 to form one molecule of glucose.
  • Examples of C3 palnts: beans, rice, wheat, potatoes, barley, spinach etc.
  • Examples of C4 palnts: Sugarcane, amaranth, corn etc.
  • Crassulacean acid metabolism (CAM) was first observed in Bryophyllus, a member of Crassulaceae and hence it is named so.
  • Eg: Agave, opuntia, Aloe, Kalanchoe.
  • The stomatal pore of these plants open during the night time and close during the day time.
  • CAM is an adaptation is succulents to carry out photosynthesis without much loss of water.
  • During the night time, Malic acid a formed as dark acidification occur, which then breaks up into CO2 and Pyruvic acid in the day time and the CO2 enters C3 cycle.

Part-1: View this video for the topic from 0:07 to 10:27

Part-2: View this video for the topic from 0:07 to 17:24

Part-3: View this video for the topic from 0:07 to 8:28

Part-4: View this video for the topic from 0:07 to 13:15

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