Temp

• 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 different types of pigments
• Pigments

  • Substances that have an ability to absorb light at specific wavelengths
  • Chlorophyll a is the most abundant plant pigment in the world
• Pigments found in leaves

  • Chlorophyll a (bright or blue green)
  • Chlorophyll b (yellow green)
  • Xanthophylls (yellow)
  • Carotenoids (yellow to yellow-orange)
• Chlorophyll a is the chief pigment associated with photosynthesis
• Wavelengths at which chlorophyll a shows maximum absorption


  Also show higher rate of photosynthesis
• There is not a complete one-to-one overlap between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis
• Light reaction

  Light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH
• Photosystems

  • Photosystem I (PS I) and Photosystem II (PS II) are discrete photochemical light harvesting complexes
  • Each photosystem has a light harvesting system called antennae made up of pigment molecules bound to proteins
  • The reaction centre chlorophyll a in PS I has an absorption peak at 700 nm (P700)
  • The reaction centre chlorophyll a in PS II has an absorption peak at 680 nm (P680)
• Electron transport

  1. Electrons excited in PS II are transferred to an electron acceptor, then to an electron transport system, and finally to PS I

  2. Electrons excited in PS I are transferred to another acceptor molecule with greater redox potential, and then to NADP+ reducing it to NADPH + H+

  3. This whole scheme of electron transfer is called the Z scheme
• Water splitting

  Water is split into 2H+, [O] and electrons in PS II, providing electrons to replace those removed from PS I
• Cyclic photophosphorylation


  Occurs when only PS I is functional, with the excited electron being cycled back to PS I, resulting in ATP synthesis but not NADPH production
• Non-cyclic photophosphorylation

  Occurs when both PS I and PS II work in series, resulting in the synthesis of both ATP and NADPH
• Chemiosmotic hypothesis

  • ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane, with protons accumulating in the lumen
  • The proton gradient is broken down by the movement of protons through the ATP synthase enzyme, providing energy to synthesize ATP
• The ATP and NADPH produced in the light reaction are used in the biosynthetic reactions in the stroma, responsible for fixing CO2 and synthesizing 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