Photosynthesis

• Photosynthesis Chemical Equation:

  6CO2 + 12H2O --> C6H12O6 + 6O2 +6H2O
• What is photosynthesis?

  Photosynthesis is the process of converting light energy to chemical energy stored in glucose.
• 1. Leaves: Main site of photosynthesis
  2. Mesophyll cells: Plant cell type that contains a lot of chloroplast
  3. Stomata: Pore on the leaf’s surface that opens and closes to control gas exchange
  4. Xylem: Responsible for transporting water and minerals from the roots to the leaves.
• Grana are stacks of thylakoid disks.

  Contains chlorophyll, which is a green pigment responsible for absorbing light energy in photosynthesis.
• The stroma is a gel-like fluid between the grana.
• Light Dependent Stage:

  Light energy is absorbed by the chlorophyll to split water into hydrogen ions, oxygen, and electrons. The oxygen is released as a product, and the electrons and hydrogen ions are picked up by coenzymes ADP and NADP+ to form ATP and NADPH.

  
  

  -       Location: Thylakoid membrane/ Grana of the chloroplast

  -       Inputs: Water (H2O) NADP+, ADP+Pi

  -       Outputs: Oxygen (O2), NADPH, ATP
• Light Independent Stage:

  Products of the LDS diffuses into the stroma, which contains all necessary enzymes. The Calvin Cycle takes place during this stage, Energy for this reaction to occur is supplied by ATP from the first stage, where carbon dioxide is modified by a series of chemical reactions to form carbs, which are then converted into glucose and stored as starch until use.

   

  -       Location: Stroma of the chloroplast

  -       Inputs: NADPH, ATP, Carbon Dioxide (CO2 )

  Outputs: NADP+, ADP+Pi, Glucose, and water
• What is Rubisco?

  Rubisco is an enzyme involved in the light independent stage (Calvin cycle) of photosynthesis.

   

  Rubisco’s Role:

  The role of rubisco is to bind CO2 and ‘fix’ the carbon into organic molecules to produce glucose.
• What is Photorespiration?

  • Photorespiration in plants occurs when RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted. This process occurs in hot and dry environments, resulting in a net loss of carbon and energy.
  • Rubisco sometimes binds O2 rather than CO2, a process known as photorespiration.

  
  

  -The binding of O2 by rubisco wastes energy and reduces the rate of photosynthesis, therefore decreasing the efficiency of the plant.
• C3 Plants:   

  1. Regular photosynthesis process and most common
  2. Considered the normal photosynthesis

  Both the Light Dependent and Light Independent Stages occurs in the same cell at the same time
• C4 Plants

  ·       Light Dependent Stage occurs the same as normal

  ·       Light Independent Stage occurs in two separate cells to avoid photorespiration

  ·       Carbon fixation occurs in the mesophyll cell

  ·       Calvin cycle occurs in the bundle sheath cell

   

   

  1. Mesophyll cell (Carbon fixation occurs there/ Light Dependent stage)
  2. Bundle Sheath cell (Calvin cycle/ Light independent stage)

   

  -       Process uses ATP and is therefore more energy expensive than C3 plants.
• CAM Plants

  ·       Light Dependent Stage occurs the same as normal

  ·       Light Independent Stage occurs in the same cell but at different times

  ·       CO2 intake occurs at night and stored in vacuoles

  ·       During the day the stored CO2 is used to complete the Calvin cycle

  ·       The intake of CO2 at night is done as to reduce the loss of water when the stomata open
• Factors affecting the rate of Photosynthesis:

  1. Concentration of CO2
  2. Light intensity
  3. Water availability
  4. Temperature
• Limiting factor:

  1. CO2 Concentration:

  Carbon dioxide is one of the reactants in photosynthesis. If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase. At some point, another factor may become limiting, and this is shown by the plateau (flattened section) of the graph.
• Limiting factor:

  2. Light Intensity:

  Rate increases up until the

  optimum point, where it levels

  off due to lack of thylakoid to

  photosynthesise.
• Limiting factor:

  3. Water availability:

  Limited water availability causes plants to become dehydrated, losing turgor and causing stomata to close, preventing gas exchange.
• Limiting factor:

  4. Temperature:

  At low temperatures, the rate of photosynthesis is limited by the number of collisions between enzymes and substrate. As temperature increases the number of collisions increases, therefore the rate of photosynthesis increases. However, at high temperatures, enzymes are denatured.

   and this will decrease the rate of photosynthesis.