bio topic 4

• Stages of photosynthesis


  1. Light-dependent stage

  2. Light-independent stage
• Photosynthesis

  • Series of biochemical reactions that utilises energy from sunlight, captured by chlorophyll, to produce glucose
  • Glucose can be used as an immediate energy source for aerobic respiration, stored as starch, or as a starting molecule to synthesise other organic molecules
  • Occurs in terrestrial and aquatic plants, algae and photosynthetic bacteria (cyanobacteria), known as autotrophs
• Chloroplast


  Site where all stages of photosynthesis take place in plants and algae
• Components of chloroplast


  • Outer and inner phospholipid membrane
  • Flattened stacks of thylakoid membranes called grana containing chlorophyll
  • Stroma - gel-like substance containing chloroplast DNA, enzymes, ribosomes and lipids
• Light-dependent stage


  1. Occurs on the thylakoid membranes

  2. Chlorophyll absorbs sunlight and gains energy

  3. Energy used to split water molecules into hydrogen ions and oxygen, where oxygen is released

  4. Hydrogen ions joined with NADP+ to make NADPH

  5. Free floating phosphates joined with ADP to make ATP

  6. ATP and NADPH move into the stroma
• Light-independent stage


  1. Occurs within the stroma

  2. Carbon dioxide absorbed from atmosphere and binds with RuBP to the enzyme Rubisco

  3. Series of reactions where energy from ATP and electrons from NADPH assist enzymatic reactions

  4. Glucose produced as a result

  5. ADP, Pi and NADP+ move back to thylakoid membranes
• Inputs and outputs of photosynthesis stages

  • Light-dependent: 6 H2O, 12 NADP+, 18 ADP + Pi -> 6 O2, 12 NADPH, 18 ATP
  • Light-independent: 6 CO2, 12 NADPH, 18 ATP -> C6H12O6, 12 NADP+, 18 ADP + Pi
• Rubisco


  Enzyme involved in the first major step of the Calvin Cycle
• Rubisco is thought to be the most abundant protein on Earth
• Photorespiration


  Pathway initiated when Rubisco binds to oxygen rather than carbon dioxide, wasting energy and producing no glucose
• C3 plants


  • Do not have adaptations to reduce photorespiration, best suited to cool, wet environments
  • problem photorespiration
• C4 plants


  • stomata open during the day
  • carbon dioxide binds to PEP carboxylase (1st) converted to malate in mesophyll cell
  • malate is transported into bundle sheath cells where the carbon is releases
  • Light-dependent and light-independent reactions physically separated in different cells
  • Carbon dioxide fixed in mesophyll cells then transported to bundle sheath cells where Rubisco is located
• CAM plants


  • Separate light-dependent and light-independent reactions in time rather than space
  • Open stomata at night to take in carbon dioxide, store it as malate, then release it during the day when stomata are closed
• Comparison of C3, C4 and CAM plants

  • C3: No separation of CO2 fixation and Calvin Cycle, stomata open during day, best for cool wet environments
  • C4: Separation of CO2 fixation and Calvin Cycle between mesophyll and bundle sheath cells, stomata open during day, best for hot sunny environments
  • CAM: Separation of CO2 fixation and Calvin Cycle between night and day, stomata open at night, best for very hot dry environments
• Factors affecting rate of photosynthesis

  • Light availability
  • Water availability
  • Temperature
  • Carbon dioxide concentration
• C3, C4 and CAM plants

  All use Rubisco and the Calvin Cycle to make glucose from CO2, but the pathway used for carbon fixation differs between them
• C3 plants


  • Work well for plants in cooler, wet environments
• C4 and CAM plants

  • Work well for plants in hotter and drier environments
• Separation of CO2 fixation from Calvin Cycle

  1. C3: No separation

  2. C4: Between mesophyll and bundle sheath cells

  3. CAM: Between night and day
• C3 plants


  • Stomata open during the day, best adapted to cool, wet environments
• Rubisco

  • the enzyme involved in the first major step of the calvin cycle
  • involved in carbon fixation to the molecule RuBP to form a C6 molecule
• CAM plants


  • Stomata open at night, best adapted to very hot, dry environments.
  • carbon dioxide binds to PEP carboxylase ( 1st carbon fixation ) and converted into malate stored in vacuole.
  • during the day when stomata are closed , malate releases carbon and binds to Rubisco ( 2nd)
• Photorespiration reduces the growth and glucose yield of C3 plants (only around 75% efficient)
• Approaches to manipulate photosynthesis

  • Enhancing a plant's ability to capture sunlight
  • Reducing the occurrence of photorespiration
  • Altering electron transport, producing more ATP and NADPH
  • Increasing the flow of carbon dioxide through the Calvin Cycle
• Light-dependent stage of photosynthesis

  Requires sunlight to create ATP and NADPH required for the Calvin cycle
• If light availability is low

  This will directly affect the Calvin Cycle and ultimately result in lower quantities of glucose produced
• As light increases


  The rate of photosynthesis increases
• Until another limiting factor prevents the rate from increasing further

  Such as temperature or carbon dioxide concentration
• Plateau point


  The point at which the rate of photosynthesis will not increase any further
• Wavelength of light


  • The rate of photosynthesis is greatest under violet, blue and red, and relatively low under green light
• light availability

  • it is required for light dependent stage
  • light excites the chlorophyl to build NADPH,ATP
  • light increase as the rate increase
  • another limiting factor may prevent rates from increasing, causing the rate to plateau
• photorespiration

  • when [CO2] are low, Rubisco will tend to bind with oxygen instead, which does not result in the production of glucose.
  • energy is wasted
• Rubisco

  involved in carbon fixation to the molecule RuBP to form a C6 molecule
• C4 plants


  • Stomata open during day
  • Carbon dioxide binds to PEP carboxylase (first carbon fixation) and converted to malate in mesophyll cells
  • Malate is tansported into bunde sheath cells where the carbon is released and binds to Rubisco (second carbon focadon)
  • Reduces photorespiration as malate is more stable
  • Produces higher quantites of glucose compared to C3 plants
• C3, C4 and CAM plants are best suited for different environments
• C3 plants are best suited for cool environments
• CAM plants are best suited for very hot, dry environments
• C4 plants are best suited for hot, dry environments
• Photorespiration is an energy wasting process in C3 plants
• Light spectrum


  • Blue and red end
  • Work under these