photosynthesis in plants

• Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds
• Green plants synthesise their food through photosynthesis and are called autotrophs
• All living forms on earth depend on sunlight for energy
• Photosynthesis is the primary source of all food on earth and responsible for the release of oxygen into the atmosphere by green plants
• Chlorophyll, light, and CO2 are required for photosynthesis to occur
• Photosynthesis takes place in the green leaves of plants and other green parts of plants
• Chloroplasts in mesophyll cells align themselves along the walls to get the optimum quantity of incident light
• Within the chloroplast, there is a membranous system consisting of grana, stroma lamellae, and matrix stroma
• The membrane system traps light energy and synthesises ATP and NADPH, while enzymatic reactions in the stroma synthesise sugar and starch
• Light reactions are directly light-driven, while dark reactions are dependent on the products of light reactions (ATP and NADPH)
• There are four pigments involved in photosynthesis:

  • Chlorophyll a (bright or blue green)
  • Chlorophyll b (yellow green)
  • Xanthophylls (yellow)
  • Carotenoids (yellow to yellow-orange)
• Pigments have the ability to absorb light at specific wavelengths
• Chlorophyll a is the most abundant plant pigment in the world
• Chlorophyll a shows maximum absorption at the blue and red regions of the spectrum
• Chlorophyll a is the chief pigment associated with photosynthesis
• Accessory pigments like chlorophyll b, xanthophylls, and carotenoids absorb light and transfer energy to chlorophyll a
• Light reactions in photosynthesis include light absorption, water splitting, oxygen release, and the formation of ATP and NADPH
• Photosystem I (PS I) and Photosystem II (PS II) are two discrete photochemical light harvesting complexes
• Photosystem I has a reaction centre chlorophyll a with an absorption peak at 700 nm (P700), while Photosystem II has an absorption peak at 680 nm (P680)
• Electrons in Photosystem II are excited by 680 nm wavelength of red light, causing them to move through an electron transport system
• The Z scheme describes the transfer of electrons from PS II to PS I, leading to the reduction of NADP+ to NADPH+H+
• Splitting of water in PS II provides electrons to replace those removed from Photosystem I
• Non-cyclic photophosphorylation occurs when both PS II and PS I work in series, synthesizing ATP and NADPH+H+
• Cyclic photophosphorylation occurs when only PS I is functional, resulting in the synthesis of ATP but not NADPH+H+
• ATP synthesis in chloroplasts 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
• Chemiosmosis requires:

  • A membrane
  • A proton pump
  • A proton gradient
  • ATP synthase
• Energy is used to pump protons across a membrane to create a gradient or a high concentration of protons within the thylakoid lumen
• ATP synthase has a channel that allows diffusion of protons back across the membrane, releasing enough energy to activate the ATP synthase enzyme that catalyses the formation of ATP
• ATP and NADPH produced by the movement of electrons are used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO2 and synthesis of sugars
• The first product of CO2 fixation in the Calvin cycle is a 3-carbon organic acid called 3-phosphoglyceric acid (PGA)
• Plants have two main types of CO2 assimilation during photosynthesis:

  • C3 pathway: first product of CO2 fixation is a C3 acid (PGA)
  • C4 pathway: first product of CO2 fixation is a C4 acid (oxaloacetic acid or OAA)
• The primary acceptor of CO2 in the Calvin cycle is a 5-carbon ketose sugar called ribulose bisphosphate (RuBP)
• The Calvin cycle can be described under three stages:

  • Carboxylation: fixation of CO2 into a stable organic intermediate
  • Reduction: series of reactions leading to the formation of glucose
  • Regeneration: crucial for the cycle to continue uninterrupted
• For every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required

  • 6 turns of the cycle are required to form one molecule of glucose
• C4 plants have a special type of leaf anatomy, tolerate higher temperatures, show a response to high light intensities, lack photorespiration, and have greater biomass productivity
• The Hatch and Slack Pathway in C4 plants involves:

  • Primary CO2 acceptor: phosphoenol pyruvate (PEP)
  • Formation of C4 acids in mesophyll cells, transported to bundle sheath cells for breakdown to release CO2 and a 3-carbon molecule
  • CO2 released in bundle sheath cells enters the Calvin pathway common to all plants
• Photorespiration is a process that creates a difference between C3 and C4 plants

  • RuBisCO enzyme can bind to both CO2 and O2, affecting CO2 fixation in C3 plants
• C3 Plants:

  • Calvin cycle takes place in mesophyll cells
  • Initial carboxylation reaction occurs in mesophyll cells
  • Two cell types fix CO2: mesophyll
  • Primary CO2 acceptor: RuBP
  • Number of carbons in primary CO2 acceptor: 5
  • Primary CO2 fixation product: PGA
  • Number of carbons in primary CO2 fixation product: 3
  • Plant has RuBisCO
  • Plant has PEP Case
  • Rubisco in mesophyll cells
  • Low CO2 fixation rate under high light conditions
  • Photorespiration is high at low and high light intensities
  • Photorespiration is high at low and high CO2 concentrations
  • Temperature optimum: 20-25°C
• • Examples: Kranz anatomy not present