chapter 9

• Chloroplasts


  A type of membrane-bound plastids that contain a network of membranes embedded into a liquid matrix and harbor the photosynthetic pigment called chlorophyll
• Chloroplasts


  • Found in the cells of the mesophyll in plant leaves
  • There are usually 30-40 per mesophyll cells
• Structure of chloroplasts

  • Biconvex or planoconvex shaped
  • May have different shapes like spheroid, filamentous, saucer-shaped, discoid or ovoid-shaped
  • Vesicular with a colorless center
  • Average size of 4-6 μ in diameter and 1-3 μ in thickness
  • Have an inner and outer membrane with an empty intermediate space in between
• Parts of a chloroplast


  • Envelope (Outer membrane)
  • Intermembrane Space
  • Inner membrane
  • Stroma
  • Thylakoid System
  • Peripheral Reticulum
• Envelope (Outer membrane)

  Semi-porous membrane, permeable to small molecules and ions, not permeable to larger proteins
• Intermembrane Space


  Thin space of about 10-20 nanometers between the outer and inner membrane
• Inner membrane


  Forms a border to the stroma, regulates passage of materials in and out, site of fatty acid, lipid and carotenoid synthesis
• Stroma

  Alkaline, aqueous, protein-rich fluid within the inner membrane, contains chloroplast DNA, chloroplast ribosomes, thylakoid system, starch granules and many proteins
• Thylakoid System


  Collection of membranous sacs called thylakoids, where chlorophyll is found and light reactions of photosynthesis occur, arranged in stacks called grana
• Peripheral Reticulum

  Additional set of membranous tubules that originate from the inner membrane of the envelope
• Functions of chloroplasts


  • Site of photosynthesis, including light-dependent and light-independent reactions
  • Participate in regulatory functions and photorespiration
  • Provide diverse metabolic activities like synthesis of fatty acids, membrane lipids, isoprenoids, tetrapyrroles, starch, and hormones
  • Key organelles for pathogen defense
  • Can serve as cellular sensors
• Non-cyclic photophosphorylation

  1. Electrons removed from water and passed through PSII and PSI before ending up in NADPH

  2. Requires light to be absorbed twice, once in each photosystem

  3. Produces ATP
• Photosystems


  Large complexes of proteins and pigments (light-absorbing molecules) that harvest light energy, including photosystem I (PSI) and photosystem II (PSII)
• Special pairs


  Unique pair of chlorophyll a molecules at the core (reaction center) of each photosystem, P680 in PSII and P700 in PSI
• Light absorption in PSII


  Energy passed from pigments to P680, boosting an electron to a high energy level, which is then passed to an acceptor molecule, splitting water and releasing O2
• ATP synthesis


  High-energy electron travels down electron transport chain, releasing energy that drives pumping of H+ ions into thylakoid interior, creating a gradient that flows through ATP synthase to produce ATP
• Light absorption in PSI

  Energy absorbed by pigments and passed to P700, boosting electron to very high energy level and transferring it to an acceptor molecule, with electron from PSII replacing the missing electron in P700
• NADPH formation

  High-energy electron from PSI travels down a short second leg of electron transport chain, passed to NADP+ to make NADPH
• Cyclic photophosphorylation

  Electrons follow a different, circular path and only ATP (no NADPH) is produced
• Photosystem


  Complexes of photosynthetic pigments and proteins that harvest light energy, with light-harvesting complexes and a reaction center containing a special pair of chlorophyll a molecules
• Key differences between photosystem I and II

  • Special pairs absorb different wavelengths (P680 vs P700)
  • Primary electron acceptors are different (pheophytin vs chlorophyll A0)
  • Source of electrons is different (water vs electron transport chain from PSII)