bio ldr

• Photosynthesis


  The biological process by which green plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy stored in glucose. This process occurs within the chloroplasts of plant cells.

  
• Reactants and Products of Photosynthesis


  • Sunlight
  • Carbon dioxide (CO2)
  • Water (H2O)
  • Glucose
  • Oxygen (O2)

  
• Chloroplasts


  Specialized organelles found in plant cells and algae where photosynthesis takes place. Chloroplasts contain various pigments, including chlorophyll, which absorb light energy necessary for photosynthesis. They are responsible for the conversion of light energy into chemical energy in the form of glucose during photosynthesis.

  
• Structure of Chloroplasts


  • Outer Membrane
  • Inner Membrane
  • Intermembrane Space
  • Stroma
  • Thylakoid Membranes
  • Thylakoid Lumen
• Stroma


  • The fluid-filled matrix inside the chloroplast, containing enzymes, ribosomes, DNA, and other molecules necessary for photosynthetic reactions. The site where light independent reaction (Calvin Cycle) takes place.
• Thylakoid Membranes


  • Interconnected membrane sacs arranged in stacks called grana. These membranes house the chlorophyll molecules and other pigments. The site involved in the light-dependent reaction takes place.
• Functions of Chloroplasts in Photosynthesis


  • Light Absorption
  • Light-Dependent Reactions
  • ATP and NADPH Production
  • Calvin Cycle (Light-Independent Reactions)

  
• Light-dependent reactions are the initial phase of photosynthesis, occurring in the thylakoid membranes of chloroplasts.

  
• These reactions harness light energy to produce energy-rich molecules ATP and NADPH, which are essential for the Calvin cycle.
• Understanding the mechanisms of light-dependent reactions is crucial for comprehending the overall process of photosynthesis.

  
• Light Absorption


  Light energy is absorbed by chlorophyll and other pigments embedded in the thylakoid membranes. Chlorophyll molecules are specialized pigments capable of capturing photons of specific wavelengths, mainly in the red and blue regions of the spectrum.
• Photosystem II (PSII)


  1. Light energy absorbed by chlorophyll in PSII excites electrons, causing them to move to a higher energy state

  2. Excited electrons are transferred along the electron transport chain (ETC) within the thylakoid membrane

  3. As electrons move through the ETC, they release energy, which is utilized to pump protons (H⁺) from the stroma into the thylakoid lumen, creating a proton gradient.
• Water Splitting and Oxygen Evolution


  1. Water molecules are split by the light energy absorbed by Photosystem II into oxygen (O₂), protons (H⁺), and electrons (e⁻)

  2. This process, known as photolysis, replenishes electrons lost from chlorophyll in PSII and releases oxygen as a byproduct, which is vital for aerobic respiration and atmospheric oxygen levels.
• Photosystem I (PSI) and Electron Transfer


  1. Excited electrons from PSII are eventually transferred to photosystem I (PSI) through the electron transport chain (ETC)

  2. Light energy absorbed by PSI boosts electrons to even higher energy levels

  3. Electrons from PSI are then transferred to another electron carrier, ferredoxin, and subsequently to NADP⁺, reducing it to NADPH

  4. The reduction of NADP⁺ to NADPH stores energy in the form of chemical bonds, providing reducing power for the Calvin cycle.
• Chemiosmosis and ATP Synthesis


  1. Protons pumped into the thylakoid lumen during electron transport create a proton gradient across the thylakoid membrane

  2. Protons flow back into the stroma through ATP synthase, a protein complex embedded in the membrane, driving the synthesis of ATP from ADP and inorganic phosphate (Pi)

  3. This process, known as chemiosmosis, generates ATP, providing energy for the Calvin cycle and other cellular processes.
• Organisms that perform photosynthesis


  • Plants
  • Algae
  • Some bacteria

  
• Purpose of Photosynthesis


  To convert light energy into chemical energy stored in glucose

  
• Parts of Chloroplast Diagram


  • Location of light reactions
  • Location of Calvin cycle
  • Thylakoid
  • Stroma
• Function of Photosystems


  To harness light energy and convert it into chemical energy in the form of ATP and NADPH

  
• Where light energy absorbed and converted by chlorophyll is stored


  • ATP
  • NADPH
• Photosynthesis


  A set of chemical reactions in which light energy is converted to chemical energy

  
• Sequence of Light-Dependent Reactions


  1. Photosystem II absorbs light and increases the electrons' energy level

  2. Electrons are passed to the electron transport chain

  3. Energy from the electrons is used by proteins in the chain to pump H+ ions from the stroma into the thylakoid space

  4. Two carrier molecules absorb more energy: ATP and NADPH

  5. Electrons will be accepted by another pigment molecule in photosystem I, causing them to break away from the atom of the chlorophyll molecule
• Light Reaction Process Diagram Labels


  • Electron Transport Chain
  • ATP synthase
  • H+
  • Photosystem I
  • Light (680 nm)
  • ATP
  • Thylakoid membrane
  • Oxygen
  • Light (700 nm)
  • Cytochrome b6f complex
  • Photosystem II
  • NADPH