BIOLOGY U3 AOS2

• Photosynthesis is the biological process where photoautotrophs capture light energy from the sun and convert it into chemical energy
• Photoautotrophs


  • Plants
  • Algae
  • Photosynthetic cyanobacteria
• Producers


  Provide all non-photosynthetic organisms with a source of food (energy)
• Photosynthesis: the complete balanced equation


  1. Water appears on both sides of the equation

  2. Water is split on the left side

  3. New water forms on the right side
• Inputs of photosynthesis


  • Carbon dioxide
  • Water
• Outputs of photosynthesis


  • Glucose
  • Oxygen
  • Water
• Glucose


  The primary product of photosynthesis, used as an immediate source of energy for plants, stored by plants in the form of starch, or as a chemical starting point for synthesis of complex compounds such as cellulose and proteins
• Leaf structure


  • Flat shape provides large surface area
  • Many stomata (pores) allow movement of CO2 and O2 in and out of plant and also control water loss
  • Xylem vessels transport water to photosynthetic cells from roots
• Photosynthesis occurs mainly in the cells located in the leaves of plants
• Mesophyll cells


  Contain large numbers of chloroplasts
• Chloroplasts


  • Enclosed in a double membrane
  • Thylakoid: a membrane-bound disc containing chlorophyll
  • Grana: a stack of thylakoid discs
  • Stroma: the fluid matrix which fills the chloroplast, contains DNA, ribosomes and enzymes
• Light-dependent stage


  • Can only occur in the presence of light
  • Occurs on the chlorophyll-filled thylakoid membranes found in chloroplasts
  • Generates high energy coenzymes like NADPH and ATP to power second stage
• Inputs of light-dependent stage


  • 12 water (H2O) molecules
  • 12 NADP+
  • 18 ADP + Pi
• Outputs of light-dependent stage


  • 6 oxygen (O2) molecules
  • 12 NADPH
  • 18 ATP
• Steps of light-dependent stage


  1. Light energy excites electrons in chlorophyll

  2. Excited electrons move along proteins in thylakoid membrane (Electron Transport Chain)

  3. Energy powers pumping of H+ ions into thylakoid lumen

  4. Water splits to donate electrons to chlorophyll to replace electrons that leave (Photolysis)

  5. Oxygen is released from chloroplasts

  6. H+ ions from water molecules are used to generate the high energy coenzyme NADPH

  7. Movement of H+ ions down concentration gradient generates the high energy coenzyme ATP

  8. ATP and NADPH coenzymes move to light-independent stage
• Enzymes


  • Catalyse most of the reactions in photosynthesis
  • Having enzymes regulate each step ensures reactions are sped up and controlled so that plants can metabolise efficiently
• Coenzymes


  NADPH and ATP cycle through both stages of photosynthesis
• Light-independent stage


  • Does not require light
  • Energised by ATP and NADPH coenzymes produced in light-dependent stage
  • Sometimes referred to as the 'Dark Reaction'
  • Occurs in the stroma of chloroplasts
  • Purpose is to build inorganic carbon dioxide into energy-rich molecules such as glucose (C6H12O6)
• Inputs of light-independent stage

  • 6 carbon-dioxide (CO2) molecules
  • 12 NADPH
  • 18 ATP
• Outputs of light-independent stage


  • Glucose (C6H12O6)
  • 6 water (H2O) molecules
  • 12 NADP+
  • 18 ADP + Pi
• Steps of light-independent stage

  1. Carbon dioxide molecules enter Calvin Cycle

  2. Carbon from CO2 combines with five-carbon molecule (RuBP)

  3. RuBP splits into 2 x three-carbon molecules which continue along cycle (3-PGA)

  4. NADPH molecules donate hydrogen ions and electrons

  5. ATP molecules break into ADP and Pi to release energy to facilitate further changes to carbon molecules

  6. Carbon molecules continue to change and rearrange as they move around cycle

  7. One specific three-carbon molecule (G3P) is created and leaves cycle to contribute to the formation of glucose

  8. Oxygen molecules from CO2 and hydrogen ions from NADPH bind together to form water
• Rubisco


  A crucial enzyme of the light-independent stage of photosynthesis
• Role of Rubisco in Photosynthesis

  1. Rubisco uses 3 x CO2 molecules and 3 x five-carbon molecule (RuBP) to produce 6 x three-carbon molecules (3-PGA)

  2. The 6 3-PGA are then converted by ATP and NADPH (from light-dependent stage) to make different 6 x three-carbon molecule (G3P)

  3. One G3P molecule leaves cycle to undergo further reactions to contribute to making glucose

  4. 3 CO2 molecules must cycle in order for one G3P to be released

  5. To make glucose, 2 cycles must occur

  6. Remaining 5 G3P are recycled with the help of ATP to regenerate 3 RuBP present at start of cycle
• The Calvin Cycle


  1. Carbon fixation - conversion of CO2 and RuBP into 3-PGA

  2. Reduction - NADPH donates electrons to intermediate three-carbon molecule to produce G3P

  3. Regeneration of RuBP - RuBP molecules needed to start cycle again are reproduced
• Photorespiration


  A wasteful and unwanted process in plants where Rubisco binds to O2 instead of CO2, disrupting photosynthesis and negatively impacting a plant's ability to grow, survive and reproduce
• Factors influencing whether Rubisco binds to CO2 or O2

  • Substrate concentration
  • Temperature
• Substrate concentration

  The more substrate (CO2 or O2) present, the greater chance it can bind to Rubisco
• Stomata are open

  More CO2 can enter plant, O2 and water diffuse out, this favours Rubisco binding with CO2
• Temperature is high


  Rubisco is more likely to bind with O2
• Temperature is regular


  Rubisco tends to bind with CO2
• C3 plants


  • Normal plants, make up approximately 85% of plants on Earth, include all trees, cereals, nuts, fruits and vegetables, possess no adaptations to reduce photorespiration
• C4 plants

  • Include corn, sugarcane, switchgrass, initial carbon fixation and Calvin cycle are separated into two different cells, initial carbon fixation occurs in Mesophyll cells, remainder of Calvin cycle occurs in Bundle-sheath cells, this minimises photorespiration but has an energy cost
• CAM plants


  • Include almost all cacti, pineapples, vanilla and orchids, separate the steps of initial carbon fixation and remainder of Calvin cycle over time, at night CO2 is fixed into oxaloacetate by PEP carboxylase, during the day malate is broken down to release CO2 for the Calvin cycle, this minimises photorespiration but has an energy cost
• Leaves appear green because very little light in the green spectrum is absorbed, instead it is reflected
• Factors affecting the rate of Photosynthesis


  • Intensity of radiant light
  • Temperature and pH
  • Concentration of carbon dioxide
  • Water availability
  • Enzyme inhibition
• Light intensity increases


  Rate of photosynthesis increases
• Light intensity reaches saturation point


  Rate of photosynthesis becomes constant (plateaus)
• Light saturation curve

  The plant is saturated with light, enzymes within chloroplasts are operating at full capacity
• After light saturation point


  Another factor (such as temperature or carbon dioxide availability) can become the limiting factor
• Temperature rises

  Rate of photosynthesis increases