Mechanisms of Photosynthesis

• Arrange the following steps in photosynthesis. Number the steps from 1 to 5 with 1 being the first.

  • Glucose leaves the leaf. Oxygen also leaves through the stomata. → 5
  • Carbon dioxide also enters the leaf through the stomata. → 3
  • Light energy is captured by chloroplasts found in the mesophyll cells. → 1
  • Water enters the leaf. → 2
  • Glucose and oxygen are produced through a series of chemical reactions inside the chloroplasts. → 4
• Light-Dependent Reactions

  • The light-dependent reactions (or simply light reactions) occur in the thylakoids, a part of the chloroplast.
  • During the reaction, sun's light splits water and releases oxygen into the atmosphere. Light energy is converted to chemical energy in the form of ATP and NADPH.
  • The light-dependent reactions occur in the thylakoid. Sun's light splits water and releases oxygen. Light energy is converted to chemical energy to produce ATP and NADPH.
• Light-independent Reactions

  • The light-independent reaction (also known as dark reaction or Calvin cycle) occurs in the stroma.
  • ATP produced from the light-dependent reaction is used as a source of energy and NADPH as a reducing agent to form sugar from carbon dioxide.
• Steps of the Light-Dependent Reaction

  1. The chloroplast absorbs and traps light from the sun. 
  2. Trapped light is transferred by the chlorophyll to the photosystems.
  3. The photosystems use light energy to split water into hydrogen ions (H+), and oxide ions (O2–), and an electron. Two oxide ions combine to form a molecule of oxygen.
• Steps of the Light-Dependent Reaction (4-5)

  1. The electrons are passed through several electron carriers in an electron transport chain. During the movement of electrons, energy is gained and lost. The net energy causes the hydrogen ions to be actively pumped across the thylakoid membrane against the concentration gradient.
  2. Once protons are on the other side of the membrane in an area called the lumen, they diffuse back across to the stroma through a membrane-bound protein called ATP synthase.
• Steps of the Light-Dependent Reaction (6-7)

  1. Energy is released due to the backflow of H+ down their concentration gradient. It is used to create a molecule of ATP from ADP and inorganic phosphate.
  2. The electron is used to reduce NADP to NADPH along with hydrogen from photosynthesis. NADPH and ATP will serve as electron carriers in the next stage of photosynthesis.
• Calvin Cycle

  1. Carbon Fixation
  2. Reduction
  3. Regeneration of carbon dioxide acceptor
• Carbon Fixation

  • CO_2 is attached to a five-carbon sugar (ribulose bisphosphate, RuBP).
  • The process is catalyzed by RuBisCo, an enzyme.
  • It forms a new six-carbon molecule which splits in half to form two molecules with three carbons each called 3-phosphoglycerate.
• Reduction

  • ATP is used to convert some of the 3-carbon molecules into a sugar precursor molecule (glyceraldehyde-3-phosphate, G3P).
• Regeneration of RuBP

  • The remaining 3-carbon molecule is converted again to the five-carbon sugar RuBP using ATP so the cycle can continue.
• In total, the Calvin Cycle spends three molecules of carbon dioxide and nine molecules of ATP to make one molecule of glyceraldehyde-3-phosphate.
• Difference

  • The light-dependent reactions (or simply light reactions) occur in the thylakoids. Light energy is converted to chemical energy in the form of ATP and NADPH.
  • The light-independent reaction (also known as dark reaction or Calvin cycle) occurs in the stroma. Sugar is formed from carbon dioxide.
• Identify the term(s) being described in each of the following statements.

  1. It is the site of the light-dependent reaction. thylakoids
  2. It is the site of the light-independent reaction. stroma
  3. These are responsible for absorbing the sun's light in the chlorophyll. photosystems
  4. This organelle absorbs and traps light from the SUN. chloroplast
• Identify the term(s) being described in each of the following statements.

  1. It is an enzyme that catalyzes the carbon fixation process of the light-independent reaction. RuBisCo
  2. This product is the result of combining two oxide ions. O_2
  3. This molecule is the source of energy in the light-independent reaction. ATP
  4. This is the 3-carbon molecule formed from the splitting of the 6-carbon molecule during carbon fixation. 3-phosphoglycerate