Wk5 Lecture 2 Light-Independent Reactions (Calvin Cycle) Flashcards
Summary of Calvin Cycle
CO2 is incorporated into glyceraldehyde 3 phosphate (G3P)
What are the 3 phases of the Calvin Cycle?
- Fixation
- Reduction
- Regeneration
What is fixation? (description + formula for 3 cycles)
CO2 reacts with RIBULOSE BISPHOSPHATE (RuBP) => 2 molecules of 3-phosphoglycerate (3PGA)
3 RuBp + 3 CO2 -> 6 3PGA
What is reduction? (description + formula for 3 cycles)
3PGA is phosphorylated by ATP and then reduced by accepting electrons from NADPH as phosphate is removed => phosphorylated three-carbon sugar glyceraldehyde-3-phosphate (G3P); some of the G3P synthesized is drawn off to produce other organic molecules, like six-carbon sugar glucose
6 3PGA + 6 ATP + 6 NADPH -> 5 G3P (for regeneration of RuBP) and 1 G3P (for synthesis of other molecules [glucose])
What is regeneration? (description + formula for 3 cycles)
The rest of the G3P keeps cycle going by serving as substrate for 3rd phase in the cycle: reactions that use additional ATP in regeneration of RuBP
5 G3P + 3 ATP -> 3 RuBP
How many molecules of CO2 fixed in one turn of Calvin Cycle?
1 CO2
What do 3 turns of the Calvin Cycle yield?
1 molecule of G3P (later processed into glucose) and 3 regenerated RuBP
What is rubsico?
-A CO2 fixing machine (enzyme) found in all photosynthetic organisms that use Calvin Cycle to fix carbon
-Ribulose 1,5-bisphosphate carboxylase/oxygenase
-Can also add O2 to RuBP
-Most abundant enzyme in the world
Why is rubisco inefficient?
It has multiple active sites, and it can bind both CO2 and O2 which compete for the active sites
What is photosynthesis? (rubisco terms)
RuBP + CO2 –Rubisco–> 2 3-phosphoglycerate (used in Calvin Cycle)
What is photorespiration? (rubisco terms)
RuBP + O2 –Rubsico–> 3-phosphoglycerate (used in Calvin Cycle) + 2-phosphoglycolate (when processed, fixed CO2 released and ATP used)
When is photosynthesis/carbon fixation favored over photorespiration?
High CO2 concentrations/low O2 concentrations
What are stomata?
A pore/opening on the surface of plant leaves/stems where gas exchange occurs; typically surrounded by specialized cells (guard cells) that open the pore
What does the stomata do when a leaf’s CO2 concentration is low during photosynthesis?
Stomata opens to allow atmospheric CO2 to diffuse into the leaf and its cells’ chloroplasts
What maintains a strong concentration gradient favoring entry of CO2? How?
The Calvin cycle; it constantly uses up CO2 to produce sugars
When are stomata normally open? When are they closed?
Open during day; closed at night
What happens to stomata on hot, dry days? What could this lead to?
The stomata close and halt photosynthesis. If oxygen builds up, it could lead to photorespiration.
In which environments does photorespiration usually occur?
Hot and dry environments; low CO2/high O2
What are the 2 mechanisms for increasing CO2 concentration (evolution to maintain high CO2/O2 ratio to circumvent photorespiration wastefulness)?
- The C4 pathway
- CAM (crassulacean acid metabolism) plants
What is the C4 pathway? (general)
-Spatially separates carbon fixation and the Calvin Cycle
-During carbon fixation, incorporate CO2 into 4-carbon (C4) organic acids via PEP carboxylase
-Does not replace Calvin cycle, but serves as an additional fixation step => C4 plants fix CO2 through both pathways
Steps of C4 photosynthesis pathway
- PEP carboxylase fixes CO2 in mesophyll cells
- 4-carbon organic acids are produced and travel to BUNDLE-SHEATH CELLS
- The 4-carbon organic acids release a CO2 which Rubisco uses to form 3PGA => initiate Calvin cycle
What are CAM plants? (describe night->day)
Similar to C4 pathway - temporal separation of fixation and calvin cycle
Night: CAM plants take in CO2 (open stoma), temporarily fixing it into 4 carbon organic acids
Day: Close stoma and CO2 released from stored organic acids which minimizes effects of photorespiration and forms => 3PGA => initiates Calvin cycle
Which is better C3 or C4?
Depends on environment
Warm, dry climates => C4 plants conserve water and prevent photorespiration
Cooler climates => C3 plants use less energy to fix CO2
What percent of plants are C3?
90%
Describe regulation of photosynthesis
Rate of photosynthesis is finely tuned to reflect changes in environmental conditions and use resources efficiently
What process converts G3P to monosaccharide glucose?
Gluceogenesis
What is glucose often combined with?
Fructose (also made from G3P) => forms disaccharide sucrose
When photosynthesis is slow, what happens to the G3P produced? Why?
Almost all the G3P is used to make sucrose b/c it is water soluble and READILY transported to other parts of the plant for cell respiration and growth
When photosynthesis is rapid and sucrose is abundant, what happens to the glucose produced? Why?
Glucose is polymerized to form starch, which is stored in chloroplast. This is because starch is not water soluble => broken down at night and used to make more sucrose
