Wk5 Lecture 2 Light-Independent Reactions (Calvin Cycle)

Question 1

Summary of Calvin Cycle

Answer 1

CO2 is incorporated into glyceraldehyde 3 phosphate (G3P)

Question 2

What are the 3 phases of the Calvin Cycle?

Answer 2

1. Fixation
2. Reduction
3. Regeneration

Question 3

What is fixation? (description + formula for 3 cycles)

Answer 3

CO2 reacts with RIBULOSE BISPHOSPHATE (RuBP) => 2 molecules of 3-phosphoglycerate (3PGA)

3 RuBp + 3 CO2 -> 6 3PGA

Question 4

What is reduction? (description + formula for 3 cycles)

Answer 4

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])

Question 5

What is regeneration? (description + formula for 3 cycles)

Answer 5

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

Question 6

How many molecules of CO2 fixed in one turn of Calvin Cycle?

Answer 6

1 CO2

Question 7

What do 3 turns of the Calvin Cycle yield?

Answer 7

1 molecule of G3P (later processed into glucose) and 3 regenerated RuBP

Question 8

What is rubsico?

Answer 8

-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

Question 9

Why is rubisco inefficient?

Answer 9

It has multiple active sites, and it can bind both CO2 and O2 which compete for the active sites

Question 10

What is photosynthesis? (rubisco terms)

Answer 10

RuBP + CO2 –Rubisco–> 2 3-phosphoglycerate (used in Calvin Cycle)

Question 11

What is photorespiration? (rubisco terms)

Answer 11

RuBP + O2 –Rubsico–> 3-phosphoglycerate (used in Calvin Cycle) + 2-phosphoglycolate (when processed, fixed CO2 released and ATP used)

Question 12

When is photosynthesis/carbon fixation favored over photorespiration?

Answer 12

High CO2 concentrations/low O2 concentrations

Question 13

What are stomata?

Answer 13

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

Question 14

What does the stomata do when a leaf’s CO2 concentration is low during photosynthesis?

Answer 14

Stomata opens to allow atmospheric CO2 to diffuse into the leaf and its cells’ chloroplasts

Question 15

What maintains a strong concentration gradient favoring entry of CO2? How?

Answer 15

The Calvin cycle; it constantly uses up CO2 to produce sugars

Question 16

When are stomata normally open? When are they closed?

Answer 16

Open during day; closed at night

Question 17

What happens to stomata on hot, dry days? What could this lead to?

Answer 17

The stomata close and halt photosynthesis. If oxygen builds up, it could lead to photorespiration.

Question 18

In which environments does photorespiration usually occur?

Answer 18

Hot and dry environments; low CO2/high O2

Question 19

What are the 2 mechanisms for increasing CO2 concentration (evolution to maintain high CO2/O2 ratio to circumvent photorespiration wastefulness)?

Answer 19

1. The C4 pathway
2. CAM (crassulacean acid metabolism) plants

Question 20

What is the C4 pathway? (general)

Answer 20

-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

Question 21

Steps of C4 photosynthesis pathway

Answer 21

1. PEP carboxylase fixes CO2 in mesophyll cells
2. 4-carbon organic acids are produced and travel to BUNDLE-SHEATH CELLS
3. The 4-carbon organic acids release a CO2 which Rubisco uses to form 3PGA => initiate Calvin cycle

Question 22

What are CAM plants? (describe night->day)

Answer 22

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

Question 23

Which is better C3 or C4?

Answer 23

Depends on environment

Warm, dry climates => C4 plants conserve water and prevent photorespiration

Cooler climates => C3 plants use less energy to fix CO2

Question 24

What percent of plants are C3?

Answer 24

90%

Question 25

Describe regulation of photosynthesis

Answer 25

Rate of photosynthesis is finely tuned to reflect changes in environmental conditions and use resources efficiently

Question 26

What process converts G3P to monosaccharide glucose?

Answer 26

Gluceogenesis

Question 27

What is glucose often combined with?

Answer 27

Fructose (also made from G3P) => forms disaccharide sucrose

Question 28

When photosynthesis is slow, what happens to the G3P produced? Why?

Answer 28

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

Question 29

When photosynthesis is rapid and sucrose is abundant, what happens to the glucose produced? Why?

Answer 29

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