photosynthesis

• Joseph Priestley (1733-1804) in 1770 performed experiments showing the essential role of air in the growth of green plants
• Priestley hypothesized that plants restore to the air what breathing animals and burning candles remove
• Jan Ingenhousz (1730-1799) demonstrated that sunlight is essential for plants to purify air fouled by burning candles or breathing animals
• Ingenhousz showed that only the green part of plants can release oxygen
• Julius von Sachs (1832-1897) provided evidence in 1854 for the production of glucose in plants, which is usually stored as starch
• Sachs found that chlorophyll (later known as chloroplasts) in plant cells is where glucose is made and stored as starch
• T.W Engelmann (1843-1909) used a prism to split light into spectral components and illuminated a green alga, Cladophora, in a suspension of aerobic bacteria
• Engelmann observed that bacteria accumulated mainly in the region of blue and red light in the split spectrum, describing the first action spectrum of photosynthesis
• Membranous system within the chloroplast consists of grana, stroma lamellae, and matrix stroma
• Membrane system in chloroplast is responsible for trapping light energy and synthesizing ATP and NADPH
• Enzymatic reactions in stroma synthesize sugar, which forms starch
• Light reactions (photochemical reactions) are directly light-driven
• Dark reactions (carbon reactions) are not directly light-driven but dependent on products of light reactions (ATP and NADPH)
• Chlorophyll a is bright or blue-green in the chromatogram
• Chlorophyll b is yellow-green
• Xanthophylls are yellow
• Carotenoids are yellow to yellow-orange
• Pigments in photosynthesis absorb light at specific wavelengths
• Light reactions or the ‘Photochemical’ phase include:

  • Light absorption
  • Water splitting
  • Oxygen release
  • Formation of high-energy chemical intermediates, ATP and NADPH
• Pigments are organised into two discrete photochemical light harvesting complexes within Photosystem I and Photosystem II
• The pigments are named in the sequence of their discovery, not in the sequence in which they function during the light reaction
• Light Harvesting Complexes (LHC) are made up of hundreds of pigment molecules bound to proteins
• Each photosystem has all the pigments except one molecule of chlorophyll a forming a light harvesting system, also called antennae
• Pigments help make photosynthesis more efficient by absorbing different wavelengths of light
• The single chlorophyll a molecule forms the reaction centre in each photosystem
• The reaction centre chlorophyll a in Photosystem I has an absorption peak at 700 nm, called P700

  The reaction centre chlorophyll a in Photosystem II has an absorption maxima at 680 nm, called P680
• In photosystem II, the reaction center chlorophyll a absorbs 680 nm wavelength of red light, causing electrons to become excited and jump into an orbit farther from the atomic nucleus
• These excited electrons are picked up by an electron acceptor and passed to an electron transport system consisting of cytochromes
• The movement of electrons in the electron transport chain is downhill in terms of an oxidation-reduction or redox potential scale
• The electrons from photosystem II are passed on to the pigments of photosystem I without being used up
• Electrons in the reaction center of photosystem I are excited by receiving red light of wavelength 700 nm and transferred to another acceptor molecule with a greater redox potential
• The excited electrons from photosystem I are then moved downhill to NADP+ molecule, reducing it to NADPH + H+
• The transfer of electrons from photosystem II to photosystem I, excitation of electrons, transfer to another acceptor, and reduction of NADP+ to NADPH + H+ is known as the Z scheme, named after its characteristic shape formed when all carriers are placed in a sequence on a redox potential scale
• Electrons that were moved from photosystem II must be replaced
• This is achieved by electrons available due to splitting of water
• Water is split into 2H+, [O] and electrons, associated with PS II
• Splitting of water creates oxygen, a net product of photosynthesis
• Electrons needed to replace those removed from photosystem I are provided by photosystem II
• ATP is synthesised by cells in mitochondria and chloroplasts, named phosphorylation
• Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light