Cam Plants & Photorespiration

Question 1

What is Photorespiration?

Answer 1

Photorespiration
• As its name suggests, RUBISCO catalyses two different reactions: Ribulose bisphosphate carboxylase oxygenase
 adding CO2 to ribulose bisphosphate - the carboxylase activity (‘fixing’ atmospheric carbon dioxide into sugars)
 adding O2 to ribulose bisphosphate - the oxygenase activity.
• Which one operates depends on the relative concentrations of O2
and CO2 :with
 high CO2; low O2 favouring the carboxylase action (Making sugars – cool!)
 high O2; low CO2 favouring the oxygenase action (which is a wasteful reaction –poop!)
1. Recall what causes photorespiration in C3 plants
Photorespiration
• Stomata usually close on hot, dry days.
 Closed stomata prevent carbon dioxide from entering the leaf.
- When carbon dioxide levels drop below about 50 parts per million, photorespiration is initiated.
 RUBISCO fixes oxygen instead of carbon dioxide.
 So RUBISCO is like Jekyll and Hyde – a split personality!!
 Examples of C3 plants: wheat, barley, oats, potatoes, lettuce, tomatoes

Question 2

Photorespiration is a wasteful process

Answer 2

• The light reactions of photosynthesis liberate oxygen and more oxygen dissolves in the cytosol of the cell at higher temperatures. Therefore,
 highlightintensitiesand
 hightemperatures(above~30°C)
• favour the oxygenase (second) reaction of RUBISCO.
Photorespiration

Question 3

C4 Plants and Photorespiration?

Answer 3

• They all use a supplementary method of CO2 uptake which initially forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle (Light- independent reactions). Hence these plants are called C4 plants. (Plants that have only the Calvin cycle are thus C3 plants. Plants with C4 pathway and the Calvin cycle - C4 plants
2. Compare C3 and C4 plants
C4 plants have a way to reduce photorespiration
• These C4 plants are well adapted to (and likely to be found in) habitats with
 high daytime temperatures
 intense sunlight.
• Some examples of C4 plants:
 kikuyu
 crabgrass
 corn (maize)
 sugarcane
 sorghum

Question 4

What Are Light Independent Reactions?

Answer 4

Light Independent Reactions - In Depth
• C4 plants & The 4-Carbon Pathway
 Plants have Kranz Anatomy
- Largechloroplastwithfewtonogranainthe
bundle sheath cells surrounding the veins. - Smallerchloroplastswithwell-developed
grana in the mesophyll cells.
- KranzisGermanfor“wreath”whichis what the bundle sheath cell arrangement looks like!

Question 5

What is the 4- Carbon Pathway?

Answer 5

4-Carbon Pathway
• Plants with Kranz Anatomy produce oxaloacetic acid (4-carbon compound).
 Phosphoenolpyruvate (PEP) and carbon dioxide combined in mesophyll cells with the aid of the enzyme PEP carboxylase.
- TheC4pathwaygreatlyreduces photorespiration.

Question 6

CAM Vs C4 Photosynthesis

Answer 6

CAM V’s C4 Photosynthesis
• Similar to C4 photosynthesis in that 4-carbon compounds are produced during the light-independent reactions.
 However, in CAM, the 4- carbon organic acids accumulate at night (taste bitter) and break down during the day, releasing carbon dioxide inside the plant which is used in photosynthesis (sugars – sweet during the day).
- Allows plants to function well under limited water supplies, as well as high temperatures and light intensity.

Question 7

High Water Conservation - Crassulacean Acid Metabolism (CAM)

Answer 7

1. In desert conditions many plant close stomata during the day to conserve water.
2. At night stomata are open.
3. At night CO2 is fixed into malate (malic acid) in the
   mesophyll and actively transported into the vacuole to
   store.
4. During the day when stomata close. Malate is brought
   out of the vacuole and CO2 is released and fixed by the C3 pathway.

Question 8

Review

Answer 8

C = RUBISCO is inefficient because of 3
photorespiration
C plants move where Carbon fixation occurs to avoid 4
RUBPOxygenase activity. Once away from O2, carbon enters the C3 cycle.
CAM plants change the times when fixation occurs by storing Carbon at night (stomata open) then use this stored Carbon during the day (stomata closed) to fix in the C3 cycle.