Photosynthesis

• Joseph Priestly discovered Oxygen in?


  1774
• What experiment did Joseph Priestly use to discover Oxygen?

  Bell Jar and mouse experiment
• Priestly's hypothesis says?


  Plants restore to the air whatever breathing animals and burning candles remove.
• Jan Ingenhousz showed that _ is essential for photosynthesis

  sunlight
• Ingenhousz used what plant in what kind of experiment?

  aquatic plant in an elegant experiment
• Ingenhousz's contribution?


  green parts of plant released Oxygen
• It was not until about 1854 that Julius von Sachs provided evidence for production of glucose when plants grow. Glucose is usually stored as starch. His later studies showed that the green substance in plants (chlorophyll as we know it now) is located in special bodies (later called chloroplasts) within plant cells. He found that the green parts in plants is where glucose is made, and that the glucose is usually stored as starch.
• T.W Engelmann used a prism he split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of O2 evolution. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. A first action spectrum of photosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll a and b.
• By the middle of the nineteenth century the key features of plant photosynthesis were known, namely, that plants could use light energy to make carbohydrates from CO2 and water. The empirical equation representing the total process of photosynthesis for oxygen evolving organisms was then understood as:

  
• A milestone contribution to the understanding of photosynthesis was that made by a microbiologist, Cornelius van Niel, who, based on his studies of purple and green bacteria, demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates. This can be expressed by:

  
• In green plants H2O is the hydrogen donor and is oxidised to O2 . Some organisms do not release O2 during photosynthesis. When H2 S, instead is the hydrogen donor for purple and green sulphur bacteria, the ‘oxidation’ product is sulphur or sulphate depending on the organism and not O2 . Hence, he inferred that the O2 evolved by the green plant comes from H2O, not from carbon dioxide. This was later proved by using radio isotopic techniques.
• Photosynthesis is a multistep process. Not a single step.
• Within the chloroplast there is membranous system consisting of grana, the stroma lamellae, and the matrix stroma. There is a clear division of labour within the chloroplast.
• The membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH. This is called light reaction.
• light reaction is also known as photochemical reaction
• In stroma, enzymatic reactions synthesize sugar, which in turn forms starch. This is the dark reaction. They are not directly light driven but are dependent on the products of light reactions (ATP and NADPH).
• dark reactions are also known as carbon reactions.
• Chlorophyll a (bright or blue green in the chromatogram), chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange).
• Absorption spectra

  
• Action spectra

  
• Several protein complexes are involved in the process of light reaction.
• The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II). These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction.
• The 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. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680.
• H2O -> O2 is oxidation
• CO2 -> C6H12O6 is reduction
• In photosystem II the reaction centre chlorophyll a absorbs 680 nm wavelength of red light.
• These electrons are picked up by an electron acceptor which passes them to an electrons transport system consisting of cytochromes. This movement of electrons is downhill, in terms of an oxidation-reduction or redox potential scale. The electrons are not used up as they pass through the electron transport chain, but are passed on to the pigments of photosystem PS I. Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm and are transferred to another accepter molecule that has a greater redox potential.
• The addition of these electrons reduces NADP+ to NADPH + H+
• Z scheme's 'Z shape' is formed when all the carriers are placed in a sequence on a redox potential scale.
• Living organisms have the capability of extracting energy from oxidisable substances and store this in the form of bond energy.
• The process through which ATP is synthesised by cells (in mitochondria and chloroplasts) is named phosphorylation.
• When the two photosystems work in a series, first PS II and then the PS I, a process called non-cyclic photo-phosphorylation occurs.
• Both ATP and NADPH + H+ are synthesised by non cyclic photophosphorylation.