Photosynthesis In Higher Plants

• Two perspectives in biology are based on different levels of organization of life forms and phenomena:

  • Organismic and above level of organization resulted in ecology and related disciplines
  • Cellular and molecular level of organization resulted in physiology and biochemistry
• Physiological processes in flowering plants, such as photosynthesis, respiration, and plant growth and development, are described in molecular terms at the cellular and organism levels
• Photosynthesis in higher plants is a physico-chemical process where light energy is used to drive the synthesis of organic compounds
• Green plants carry out photosynthesis, a process that uses light energy to drive the synthesis of organic compounds, making them autotrophs
• Photosynthesis is crucial as it is the primary source of food on earth and responsible for the release of oxygen into the atmosphere by green plants
• Early experiments by Joseph Priestley and Jan Ingenhousz led to the understanding of the essential role of air and sunlight in photosynthesis
• Julius von Sachs provided evidence for the production of glucose in plants and the storage of glucose as starch in the green parts of plants
• T.W Engelmann's experiments with a prism and green alga led to the description of the first action spectrum of photosynthesis, resembling the absorption spectra of chlorophyll a and b
• Cornelius van Niel demonstrated that photosynthesis is a light-dependent reaction where hydrogen from an oxidizable compound reduces carbon dioxide to carbohydrates in green plants
• The overall process of photosynthesis involves the conversion of carbon dioxide and water into glucose and oxygen, with the oxygen released coming from water
• The equation representing photosynthesis is:

  6CO2 + 6H2O + Light energy → C6H12O6 + 6O2
• Twelve molecules of water are used in the equation to provide the necessary substrate for the process of photosynthesis
• Photosynthesis takes place in the green leaves of plants, as well as in other green parts of plants
• Mesophyll cells in leaves contain a large number of chloroplasts, which align themselves along the walls to optimize light absorption
• Chloroplasts have a membranous system consisting of grana, stroma lamellae, and matrix stroma, each with specific functions in photosynthesis
• There are four main pigments involved in photosynthesis: Chlorophyll a, Chlorophyll b, xanthophylls, and carotenoids
• Pigments like chlorophyll a have the ability to absorb light at specific wavelengths, with chlorophyll a being the most abundant plant pigment
• Light reactions in photosynthesis involve light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates ATP and NADPH
• Photosystems I and II contain pigments organized into light-harvesting complexes that make photosynthesis more efficient by absorbing different wavelengths of light
• The Z scheme describes the transfer of electrons in photosynthesis, starting from PS II, moving through the electron transport chain, and ending in the reduction of NADP+ to NADPH + H+
• Water splitting in PS II provides the electrons needed to replace those removed from photosystem I, resulting in the release of oxygen as a byproduct
• The water splitting complex is associated with PS II, located on the inner side of the thylakoid membrane in chloroplasts
• Living organisms can extract energy from oxidizable substances and store it in the form of bond energy, with ATP carrying this energy in its chemical bonds
• ATP carries energy in its chemical bonds
• ATP is synthesised by cells in mitochondria and chloroplasts through phosphorylation
• Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light
• Non-cyclic photophosphorylation occurs when both photosystems work in a series, first PS II and then PS I
• Cyclic photophosphorylation occurs when only PS I is functional, resulting in the synthesis of ATP but not NADPH + H+
• Chemiosmotic hypothesis explains how ATP is synthesised in the chloroplast
• In chemiosmosis, ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane
• The breakdown of the proton gradient leads to the synthesis of ATP through the ATP synthase enzyme
• ATP and NADPH produced in the light reaction are used in the biosynthetic phase to fix CO2 and synthesize sugars
• The Calvin cycle, operating in all photosynthetic plants, involves the regeneration of RuBP and the synthesis of sugars
• The Calvin pathway occurs in all photosynthetic plants, regardless of whether they have C3 or C4 pathways
• The Calvin cycle can be described in three stages: carboxylation, reduction, and regeneration
• Carboxylation is the fixation of CO2 into a stable organic intermediate, catalyzed by RuBP carboxylase, resulting in the formation of two molecules of 3-PGA
• Reduction involves a series of reactions leading to the formation of glucose, requiring 2 molecules of ATP for phosphorylation and 2 of NADPH for reduction per CO2 molecule fixed
• Regeneration of the CO2 acceptor molecule RuBP is crucial for the uninterrupted continuation of the Calvin cycle, requiring one ATP for phosphorylation to form RuBP
• For every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required
• C4 plants have a special leaf anatomy, tolerate higher temperatures, respond to high light intensities, lack photorespiration, and have greater biomass productivity