Knowledge-15 P

• Photosynthesis has two groups of reactions:

  • The light dependent reactions where light is absorbed by chlorophyll and a water molecule is split. This takes place in the thylakoid membrane.
  • The light independent reactions where carbon dioxide is synthesized into useful organic compounds. This takes place in the stroma.
• Light dependent reactions:

  1. chlorophyll in the chloroplasts absorbs light and this leads to photoionization of the chlorophyll.
  2. Electrons are released , and some of the energy is used to produce ATP and reduced NADP.
  3. A water molecule is split into an electron , a proton and oxygen by photolysis.
• Light-dependent reactions:

  Key points:

  • light is absorbed by chlorophyll
  • chlorophyll becomes photoionized, increasing the potential energy of an electron.
  • Photolysis: a water molecule is broken down to produce two protons , two electrons and oxygen,
  • ATP production is catalyzed by ATP synthase within the chloroplasts inner membranes.
  • the resulting ATP and reduced NADP are used in the light independent reaction.
• Triose phosphate is a 3 carbon sugar used to make the end products of photosynthesis including:

  • glucose , used in cellular respiration
  • cellulose , used as a structural component of cell walls
  • Starch , stored as granules as energy reserves
  • disaccharides , including useful sugars
• Light-independent reactions:

  This part of photosynthesis can be done in the absence of light. Carbon dioxide is absorbed and the carbon used to synthesize organic molecules such as simple sugars.

  This is done in the Calvin cycle which uses enzymes to synthesize intermediate organic molecules, using reduced NADP and ATP from light independent reactions.
• Calvin cycle:

  Carbon dioxide reacts with ribulose biphosphate (RuBP) a simple sugar.

  The catalyst for this reaction is rubisco. This forms two molecules of glycerate 3- phosphate.

  GP is reduced to triose phosphate using ATP and reduced NADP from light-dependent reaction.

  Some triose phosphate is used to regenerate RuBP , the rest is used to make useful organic compounds with the plants.
• Chemiosmosis is the diffusion of hydrogen ions through a partially permeable membrane, which is linked to the generation of ATP.
• ATP synthase catalyzes chemiosmotic reactions in the chloroplasts membranes.
• ATP production requires the transfer of electrons down an electron transport chain and the passage of protons across chloroplast membranes.
• Saturation refers to the point when changing a particular factor has no further effect and limiting factor takes over.
• Investigating photosynthetic pigments:

  Photosynthetic pigments are carotenoids and chlorophylls. There are two chlorophyll molecules:

  • chlorophyll a , a yellow-green colour
  • chlorophyll b, a blue-green colour

  Each chlorophyll absorbs a different wavelength of light. Paper or thin layer chromatography can be used to separate the pigments and their Rf values can be calculated.
• Dehydrogenase catalyzes a stage in the light-dependent reaction of chlorophyll in photosynthesis. In this stage , dehydrogenase catalyzes the reaction where electrons are accepted by NADP.
• DCPIP is a redox indicator which accepts electrons. When DCPIP is reduced it turns from blue to colorless.
• Rubp combines with carbon dioxide to form two molecules of glucose phosphate
• Triose phosphate is used to creating useful organic molecules and regenerating rubp