Photosynthesis

• Plants adaptations


  • Synthesis part
  • Photo part
• Photosynthesis


  The process that converts light energy (not radiation) into chemical energy
• Photosynthesis is the first step in the flow of energy through most of the living world
• Directly (herbivores) or indirectly (heterotrophs), photosynthesis nourishes almost the entire living world
• Regardless of an ecosystem's size, its dynamics involve two main processes: Energy flow and chemical cycling
• Energy flow


  1. Light energy

  2. Chemical energy

  3. Chemicals pass to organisms that eat the plants
• Chemical cycling


  1. Plants take up chemicals from the soil & air

  2. Photosynthesis
• Life forms depend on energy, and two sources of such energy are light energy (for photosynthesis by plants) and chemical energy (for cellular respiration by animals [and plants themselves])
• Autotrophs


  The producers of the biosphere, producing organic molecules (e.g., carbohydrates, lipids, proteins & nucleic acids) from CO2 and other inorganic molecules (e.g., water)
• Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
• Site of photosynthesis in a plant

  • Organ: Leaf
  • Tissue: Ground
  • Cells: Mesophyll
  • Organelle: Chloroplast
• Thylakoid


  Part of the chloroplast where light reactions occur
• Stroma


  Part of the chloroplast where the Calvin cycle occurs
• Leaf tissues


  • Vascular Tissue: Transport of water & nutrients
  • Dermal Tissue: Structure & protection (also guard cells)
  • Ground Tissue: Photosynthesis & storage
• Not all leaves photosynthesize. Photosynthesis may also occur in algae, some bacteria & protists, and even stems
• Cactus


  • Spines: Modified leaves (for protection & reduction of water loss)
  • Stem: Site for photosynthesis
• Leaf cells


  • Palisade Mesophyll Cells: More chloroplasts here for light to reach them
  • Spongy Mesophyll Cells: The loose arrangement here allows for rapid gas exchange
  • Bundle Sheath Cells: It is called Kranz anatomy in C4 plants
• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf. Each mesophyll cell contains 30 to 40 chloroplasts
• Photosynthesis is a complex series of reactions that can be summarized as the following equation: 6CO2 + 6H2O + Light energy → C6H12O6 + 6O2
• Role of water in photosynthesis

  Chloroplasts split H2O into hydrogen and oxygen. Hydrogen electrons are normally incorporated into sugar molecules, and oxygen is normally released as a by-product
• Role of light in photosynthesis

  The electromagnetic spectrum is the entire range of electromagnetic energy (UV, X-rays, etc.) or radiation. Wavelength is the distance between crests of waves. It determines the type of electromagnetic energy. Visible light consists of wavelengths (including those that drive photosynthesis) that produce colours we can see. Light also behaves as though it consists of discrete particles, that is, photons
• Photosynthetic pigments


  • Pigments are substances that absorb visible light. Different pigments absorb different wavelengths. Wavelengths that are not absorbed are reflected or transmitted. The leaves appear green because chlorophyll reflects and transmits green light
• Engelmann's experiment


  • The oxygen-seeking bacteria showed that oxygen was released around the two wavelengths 420nm & 680nm. Thus, photosynthesis is optimal around the two wavelengths
• Chlorophyll a


  The main photosynthetic pigment. Its absorption spectrum suggests that violet-blue (420nm) and red light (680nm) work best for photosynthesis
• Accessory pigments


  • Chlorophyll b: This pigment broadens the spectrum used for photosynthesis
  • Carotenoids: They absorb excessive light that would damage chlorophyll
• Stages of photosynthesis


  1. Light reactions: These reactions make the photo part. They occur in Thylakoid

  2. Calvin cycle: It is associated with the synthesis part in Stroma
• Light reactions


  Use photons/light energy to generate ATP and NADPH, providing the chemical energy and reducing power needed by the Calvin cycle to make sugar
• Photophosphorylation


  Light energy hence excited electrons, and proton-motive force trigger phosphate group (PO4-) to be added to ADP to produce ATP
• Photolysis


  Light energy hence excited electrons results in a split of water. Hydrogen from water is then attached to NADP to produce NADPH. O2 is also released
• The importance of light reactions is that they convert light energy into chemical energy in the form of ATP and NADPH, which are then used in the Calvin cycle to produce sugar
• Light reactions


  1. NADPH produced

  2. ATP produced
• NADPH and ATP from light reactions

  Used in dark reactions (Calvin cycle) to make glucose
• Dark reactions


  Also known as: Calvin cycle, stroma reactions, C3 pathway, CO2 fixation pathway
• Dark reactions


  1. Carbon fixation or CO2 uptake

  2. Carbon reduction

  3. Regeneration of RuBP
• Carbon fixation


  6 CO2 molecules incorporated into RuBP to produce 1 molecule of 3-phosphoglycerate (PGA)
• Rubisco


  Enzyme that catalyzes carbon fixation
• Carbon reduction


  1. PGA accepts phosphate from ATP

  2. PGA accepts hydrogen and electrons from NADPH

  3. Produces glyceraldehyde 3-phosphate (G3P)
• RuBP regeneration


  10 G3P molecules rearranged into 6 RuBP molecules
• Summary of Calvin cycle
• C3 plants


  • Common in most plants, usually dicots
  • Produce 3-carbon 3-phosphoglycerate after CO2 fixation
  • Use one enzyme Rubisco
  • Lose more water