Photosynthesis

• Light absorption occurs in the thylakoid membranes of chloroplasts.
• Photosynthesis involves the synthesis of large organic molecules from simple inorganic molecules using light from the sun
• Photosynthates are produced at the source (the leaves)
• The main photosynthetic tissue in a leaf is the Palisade mesophyll
• Stomata are essential for gas exchange in a leaf, without which photosynthesis would not occur
• 3 adaptations of the leaf for photosynthesis:

  • Large surface area
  • Thin structure to allow light penetration
  • Densely packed palisade layer with chloroplasts, air spaces for CO2 diffusion
• Photosynthesis involves two stages: Light dependent stage in the Thylakoid membrane and cavity, and Light independent stage in the Stroma of the chloroplast
• Accessory pigments are important in photosynthesis to absorb wavelengths of light not absorbed by primary pigments, increasing efficiency
• Magnesium is needed for the production of chlorophyll a
• Chromatography separates photosynthetic pigments by their solubility in a solvent
• Carotene travels the furthest distance in chromatography due to being the most soluble
• Accessory pigments absorb a wider range of wavelengths, leading to little light absorption between 530nm and 640nm
• Photosystems are found on the thylakoid membrane of a chloroplast
• Photosystems absorb photons of light energy and transfer it to high energy electrons for ATP synthesis
• Light dependent reaction involves Photosystem I (P700) and Photosystem II (P680) in the thylakoid membranes, producing ATP and NADPH
• Non-cyclic photophosphorylation involves electron transfer from PSII to PSI, creating an electrochemical gradient for ATP synthesis
• High concentration of H+ ions in the thylakoid cavity makes the pH more acidic
• Photolysis splits water into oxygen, hydrogen ions, and electrons to replace those lost by PSII
• Cyclic photophosphorylation involves only PSI, producing extra ATP when CO2 is limited
• In non-cyclic photophosphorylation, PSII obtains replacement electrons from water during photolysis
• In non-cyclic photophosphorylation, PSI obtains replacement electrons from PSII
• In cyclic photophosphorylation, electrons excited and released from PSI return to PSI
• In non-cyclic photophosphorylation, oxygen produced from photolysis is released as a byproduct
• Plants shift from non-cyclic to cyclic photophosphorylation when CO2 is limited
• Light Dependent Stage of Photosynthesis:
• Occurs in the thylakoid membranes of chloroplasts
• Requires light to drive the conversion of light energy into chemical energy (ATP and reduced NADP)
• Products used in the light independent stage: ATP and reduced NADP
• Involves the photolysis of water using light energy
• Light intensity affects the rate of cyclic and non-cyclic photophosphorylation
• Factors affecting the rate of photosynthesis include: temperature, carbon dioxide concentration, and light intensity
• Light Independent Stage (Calvin Cycle):
• Uses products from the light dependent stage: ATP and reduced NADP
• Fixes carbon dioxide from the atmosphere to organic molecules
• Carbon dioxide is fixed with RuBP to form Glycerate-3-phosphate
• Triose phosphate is formed from Glycerate-3-phosphate using ATP
• One carbon atom is removed from the Calvin cycle to produce organic molecules like glucose
• Factors Affecting the Rate of Photosynthesis:
• Limiting factors include: temperature, carbon dioxide concentration, and light intensity
• Temperature affects enzyme activity and can speed up the light independent stage more than the light dependent stage