KK5 - C3, C4 and CAM plants

C3 plants

• Considered "normal" plants as they make approx. 85% of plants on Earth

• Undertake the normal photosynthesis

• Possess no adaptations to reduce photorespiration

• Rubisco is responsible for fixing CO2 into three-carbon compounds (3-PGA)

  • Cycles through the pathway, all occurring within a single mesophyll cell

• Name from the three-carbon 3-PGA that the initial carbon fixation produces

C4 plants

• Light-dependent stage of photosythesis in C4 plants are the same as C3 plants

• The initial carbon fixation and the remainder of the Calvin cycle are separated into different cells instead of one cell

• Initial carbon fixation occurs in a mesophyll cell, but the remaining Calvin cycle occurs in specialised cells called bundle-sheath cells

  • C4 photosynthesis has an additional biochemical pathway compared to C3 photosynthesis

  • C4 plants are uniquely specialised to allow for this pathway

• Gets its name from the first four carbon molecule produced in the initial carbon fixation

Step 1 - C4 plants

• Atmospheric CO2 enters mesophyll cell

• Fixed by the enzyme PEP carboxylase

  • Added the carbon from CO2 to a three carbon molecule (PEP) to create four-carbon molecule (oxaloacetate)

  • Responsible for the initial carbon fixation in C4 plants

• PEP carboxylase has no affinity to bind to O2

Step 2 - C4 plants

• 4 carbon molecule (oxaloacetate) converted into a different four-carbon molecule (malate)

• Capable of being transported to bundle-sheath cells

Step 3 - C4 plants

• Inside bindle-sheath cell, malate breaks down and releases CO2

• Enters the Calvin cycle the same way as C3 photosynthesis

  • Leads to glucose production

Step 4 - C4 plants

• Pyruvate formed from the breakdown of malate is transported back to the mesophyll cell

• Converted to another molecule, PEP, with the help of ATP

Step 5 - C4 plants

• PEP ready to contribute to the fixation of CO2 and production of oxaloacetate

• Cycle continues all over again

advantages

• When the mesophyll cells constantly pump a source of CO₂  (form of malate) into the bundle-sheath cells, there is always a higher concentration of CO₂  present

  • Photorespiration is minimised and photosynthesis is maximised 

• More advantageous in hot environments as C3 plants suffer from increase photorespiration

  • Benefits of reduced photorespiration outweigh the cost of using ATP in C4 photosynthesis

disadvantages

• Cost to undertaking C4 photosynthesis over C3 photosynthesis

  • ATP needed to convert pyruvate to PEP for initial carbon fixation

  • C4 plants use more energy to undertake photosynthesis

CAM plants

• Light-dependent stage of photosynthesis in CAM is identical to C3 and C4 plants

• Light-independent stage differs

  • Instead of separating the initial carbon fixation and the remainder of the Calvin cycle spatially over two cells like in C4 plants

  • CAM plants separate the steps over time

CAM plants at night

• CAM plants open their stomata to bring CO2

  • CO2 is fixed into a four-carbon molecule (oxaloacetate) by the enzyme PEP carboxylase (like C4 plants)

• Oxaloacetate converted into different four-carbon molecules (can be malate or other organic molecule)

  • Stored inside vacuoles within the mesophyll cells until daytime

CAM plants in the day

• CAM plants don't open their stomata to prevent water loss

  • Makes them very resistant to water loss

• Can still photosynthesis during the day

  • malate (or other) molecule is transported out of the vacuole and broken down to release CO2

• CO2 fee to enter Calvin cycle in the same fashion as in C4 and C3 plants, leading to glucose production

C4 and CAM

Controlled releases type of molecules out of vacuole ensure a high concentration of CO2 is maintained near Rubisco

• Maximises photosynthesis and minimising photorespiration

advantages

• Controlled releases type of molecules out of vacuole ensure a high concentration of CO₂ is maintained near Rubisco

  • Maximises photosynthesis and minimises photorespiration

• Water is conserved in CAM plants as their stomata only open up at night when it is cooler and more humid

  • CAM plants prominent in dry hot areas like deserts

disadvantages

Requires more ATP than C3 photosynthesis to cycle PEP