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• Photosynthesis


  The process by which green plants use light energy to synthesize organic compounds, especially carbohydrates, from carbon dioxide and water
• Photosynthesis


  • It is the primary source of all food on earth
  • It is responsible for the release of oxygen into the atmosphere by green plants
• Chlorophyll, light and CO2 are required for photosynthesis to occur
• Photosynthesis occurs only in the green parts of leaves in the presence of light
• CO2 is required for photosynthesis
• Joseph Priestley performed experiments that revealed the essential role of air in the growth of green plants


  1770
• Jan Ingenhousz showed that sunlight is essential for the plant process that purifies the air


  1799
• Julius von Sachs provided evidence for production of glucose when plants grow


  1854
• The empirical equation representing the total process of photosynthesis for oxygen evolving organisms was understood by the middle of the nineteenth century
• Photosynthesis


  A light-dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates
• The O2 evolved by green plants comes from H2O, not from carbon dioxide
• Photosynthesis takes place in the green leaves and other green parts of plants
• Chloroplast


  • Contains a membranous system responsible for trapping light energy and synthesizing ATP and NADPH
  • Contains the stroma where enzymatic reactions synthesize sugar and form starch
• Light reactions

  The reactions that are directly light driven
• Dark reactions


  The reactions that are not directly light driven but are dependent on the products of light reactions (ATP and NADPH)
• The colour seen in leaves is due to four pigments: chlorophyll a, chlorophyll b, xanthophylls and carotenoids
• Photosynthesis in higher plants involves various pigments that absorb different wavelengths of light
• Chlorophyll a


  • It is the most abundant plant pigment
  • It shows maximum absorption in the blue and red regions of the visible spectrum
  • It is the chief pigment associated with photosynthesis
• Accessory pigments (chlorophyll b, xanthophylls, carotenoids)

  • They also absorb light and transfer energy to chlorophyll a
  • They enable a wider range of wavelengths of light to be utilized for photosynthesis
  • They protect chlorophyll a from photo-oxidation
• Light reaction


  1. Light absorption

  2. Water splitting

  3. Oxygen release

  4. Formation of ATP and NADPH
• Photosystem I (PS I)

  Reaction centre chlorophyll a has an absorption peak at 700 nm (P700)
• Photosystem II (PS II)

  Reaction centre chlorophyll a has an absorption peak at 680 nm (P680)
• Electron transport


  1. Electrons excited in PS II are passed to an electron transport system

  2. Electrons are then passed to PS I

  3. Electrons from PS I are used to reduce NADP+ to NADPH
• Water splitting


  1. Occurs in PS II

  2. Provides electrons to replace those removed from PS I

  3. Produces oxygen as a byproduct
• Cyclic photophosphorylation


  • Occurs when only PS I is functional
  • Electrons are cycled back to PS I
  • Results in ATP synthesis but not NADPH production
• Non-cyclic photophosphorylation


  • Occurs when both PS I and PS II are functional
  • Electrons flow from PS II to PS I
  • Results in both ATP and NADPH production
• Chemiosmotic hypothesis


  1. Protons accumulate in the thylakoid lumen due to water splitting and electron transport

  2. Proton gradient across the thylakoid membrane drives ATP synthesis by ATP synthase
• ATP and NADPH produced in the light reactions are used in the biosynthetic reactions in the stroma to fix CO2 and synthesize sugars
• Chemiosmosis


  1. Requires a membrane

  2. Requires a proton pump

  3. Requires a proton gradient

  4. Requires ATP synthase
• Chemiosmosis


  1. Energy is used to pump protons across a membrane

  2. Creates a gradient or high concentration of protons within the thylakoid lumen

  3. ATP synthase has a channel that allows diffusion of protons back across the membrane

  4. Releases enough energy to activate ATP synthase enzyme that catalyses the formation of ATP
• ATP and NADPH produced by light reaction

  Used in the biosynthetic reaction taking place in the stroma, responsible for fixing CO2, and synthesis of sugars
• Immediately after light becomes unavailable, the biosynthetic process continues for some time, and then stops
• If light is made available, the synthesis starts again
• First product of CO2 fixation in C3 plants

  1. phosphoglyceric acid (PGA)
• First product of CO2 fixation in C4 plants

  Oxaloacetic acid (OAA)
• Primary acceptor of CO2

  Ribulose bisphosphate (RuBP)
• Calvin cycle


  1. Carboxylation

  2. Reduction

  3. Regeneration
• For every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required
• To make one molecule of glucose, 6 turns of the Calvin cycle are required
• In the Calvin cycle, for one molecule of glucose produced, 6 CO2, 18 ATP, 18 ADP, 12 NADPH, and 12 NADP are involved