5.1 photosynthesis

• what are the stages of photosynthesis

  light dependent reaction - thylakoid membrane of chloroplast

  light independent reaction - stroma of chloroplast
• describe photoionisation in the light-dependent reaction (LDR)

  chlorophyll absorbs light energy which excites it's electrons (higher energy level)

  so electrons are released from chlorophyll (chlorophyll becomes positively charged)
• chemiosmotic theory

  some energy from electrons released in photoionisation is conserved in the production of ATP/reduced NADP
• describe what happens after photoionisation in the LDR

  electrons move along electron transport chain (electron carriers), releasing energy

  this energy is used to actively pump protons from stroma into thylakoid

  protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase

  energy used to join ADP and Pi to form ATP (photophosphorylation)

  NADP accepts a proton and an electron to become reduced NADP
• describe photolysis of water in the LDR

  water splits to produce protons, electrons and oxygen

  electrons replace those lost from cholorphyll
• describe the light-independent reaction of photosynthesis (calvin cycle)

  carbon dioxide reacts with ribulose bisphosphate (RuBP)

  catalysed by the enzyme rubisco

  forming 2 glycerate 3-phosphate molecules (GP)

  GP reduced to triose phosphate (TP) using products from light-dependent reaction, reduced NADP and energy from ATP

  some TP converted to useful organic substances (e.g glucose)

  some TP used to regenerate RuBP in the calvin cycle (using energy from ATP)
• describe and explain how temperature affects rate of photosynthesis

  as temperature increases rate increases, enzymes (rubisco) gain kinetic energy so more enzyme-substrate complexes form

  above an optimum temperature rate decreases, enzymes denature as H bonds in tertiary structure break so fewer enzyme-substrate complexes form
• describe and explain how light intensity affects rate of photosynthesis

  as light intensity increases rate increases, light-dependent reaction increases (e.g more photoionisation of chlorophyll) so more ATP and reduced NADP produced so light-independent reaction increases as more GP reduced to TP and more TP regenerates RubP

  above a certain light intensity rate stops increasing as another factor is limiting e.g temperature/carbon dioxide concentration
• describe and explain how carbon dioxide concentration affects rate of photosynthesis

  as carbon dioxide concentration increases rate increases so light-independent reaction increases as more carbon dioxide combines with RuBP to form GP so more GP reduced to TP so more TP converted to organic substances and more RuBP regenerated

  above a certain carbon dioxide concentration rate stops increasing so another factor is limiting e.g temperature/light intensity
• explain the key consideration when evaluating data relating to agricultural practices used to overcome the effect of limiting factors

  agricultural practice should increase rate of photosynthesis, leading to increase yield as more glucose produced for faster respiration so more ATP to release energy for growth e.g cell division, protein synthesis

  but profit from extra yield should be greater than costs (money & environmental costs)