3.5.1 PHOTOSYNTHESIS

• Photosynthesis = energy from light used to make glucose from water and carbon dioxide. Light energy converted to chemical energy in form of glucose — C6H12O6. 

  6CO2 + 6H2O + Energy --> C6H12O6 + 6O2

  Occurs in series of small enzyme-controlled reactions.
• PHS takes place in the chloroplasts of plant cells.

  • Contain photosynthetic pigments (e.g. chlorophyll a, chlorophyll b and carotene) - coloured substances that absorb the light energy needed for PHS. 

  Pigments found in the thylakoid membranes — they’re attached to proteins.

  Protein + pigment = a photosystem. 2 photosystems used by plants to capture light energy.
• Carbohydrates produced by photosynthesis but not used straight away are stored as starch grains in the stroma.
• Coenzyme in photosynthesis is NADP - transfers hydrogen from one molecule to another — this means it can reduce (give hydrogen to) or oxidise (take hydrogen from) a molecule.
• 2 Stages of PHS:

  • Light-dependent reaction
  • Light-independent reaction.
• Stage 1. The Light-Dependent Reaction

  Needs light energy.

  Takes place in the thylakoid membranes of the chloroplasts. 

  Produces ATP, reduced NADP and oxygen (O2). 
• Energy resulting from the photoionisation of chlorophyll used for:

  1. Photophosphorylation (adding phosphate to a molecule using light) - a phosphate group is added to ADP to form ATP 
  2. To reduce NADP to form reduced NADP

  ATP transfers energy and reduced NADP transfers hydrogen to the light-independent reaction. 
• Photosystems (in thylakoid membranes) linked by electron carriers. Form an electron transport chain — chain of proteins through which excited electrons flow. 

  Electron carriers = proteins that transfer electrons. 
• LDR 1 - Photoionisation.

  • Chlorophyll absorbs light.
  • Excites electrons in chlorophyll (gives them more energy).
  • Electrons lost - move down e.t.c. Chlorophyll becomes positively charged.
  • To replace electrons, photolysis occurs - light energy splits water into protons, electrons + oxygen.
• LDR 2

  • Excited electrons lose energy as they move down the e.t.c. 
  • Energy used to transport protons into thylakoid so has higher proton conc than stroma. Forms a proton gradient across thylakoid membrane. 
  • Protons move down gradient into stroma, via ATP synthase (embedded in thylakoid membrane). 
  • Energy from this movement combines ADP and Pi to form ATP.
• The process of electrons flowing down the electron transport chain and creating a proton gradient across the membrane to drive ATP synthesis is called chemiosmosis. It’s described by the chemiosmotic theory.
• LDR 3 - Energy from excited electrons generates reduced NADP.

  • Light energy absorbed by PSI - excites electrons again to even higher energy level.
  • Finally, electrons transferred to coenzyme NADP, along with a proton (H+ ion) from stroma, to form reduced NADP.
• Stage 2. The Light-Independent Reaction (the Calvin Cycle)

  • Doesn’t directly use light energy. Uses ATP and reduced NADP from LDR - supply energy & hydrogen.
  • In stroma.
  • Makes triose phosphate from CO2 and ribulose bisphosphate
  • Starting compound RuBP regenerated.
• Calvin Cycle (LIR)

  • CO2 reacts with ribulose bisphosphate (RuBP)
  • Produces 2 glycerate 3-phosphate (GP). Catalysed by rubisco.
  • GP reduced to triose phosphate (TP)
  • Using reduced NADP and energy from (hydrolysis of) ATP (both from LDR)
  • Some TP converted to useful organic compounds (eg. glucose), but 5 of 6 TP molecules used to regenerate RuBP, using rest of ATP from LDR.
• 2 molecules of TP (2 x 3C) join to make 1 hexose sugar (6C), which can be used to make larger carbohydrates.

  The Calvin cycle needs 6 turns to produce 2 TP molecules:

  • 1 turn produces 2 TP (2 for each CO2 used)
  • 5 out of 6 TP are used to regenerate RuBP (so you need 12 TP) 

  6 turns of the cycle need 18 ATP and 12 reduced NADP from the LDR.

  This keeps the cycle going and makes sure there’s always enough RuBP ready to combine with CO2 taken in from the atmosphere.
• TP and GP molecules used to make carbohydrates, lipids and amino acids:

  • Carbohydrates — hexose sugars made from 2 TP molecules. Join hexose sugars in different ways to make larger carbohydrates.
  • Lipids — Glycerol synthesised from TP, fatty acids from GP.
  • Amino acids — some amino acids made from GP.
• Limiting Factors of Photosynthesis

  • Light - provides energy for LDR
  • Temperature - impacts enzymes in PHS (inactive/denatured)
  • Carbon Dioxide - used in LIR to produce GP

  All 3 must be at right level for max rate of photosynthesis. 

  If 1 factor not optimal, PHS limited (slower), regardless of other factor's levels.
• Create an environment where no factor is limiting - increase plant growth and yield (farming). 

  Optimum conditions created in greenhouses: 

  • CO2 conc - CO2 added to air eg. by burning propane in a CO2 generator.
  • Light - Light through glass. Lamps at night.
  • Temperature - Greenhouses trap heat energy from sunlight (warms air), heaters and cooling systems maintain optimum temp, air circulation systems to ensure even temp.