Light-Independent Reactions (Lecture 14)

Question 1

What enzyme is the catalyst in the first step of CO2 reduction?

Answer 1

RuBisCO

Question 2

What energy sources are used from the light dependent reactions?

Answer 2

ATP and NADPH

Question 3

What plant species use this method?

Answer 3

C3 plants

Question 4

Stage 1: carbon fixation

Answer 4

3 CO2 molecules combine with a 5 carbon acceptor molecule, RuBP, catalyzed by RuBisCO

This step makes a 6 carbon compound that splits into two molecules of a 3 carbon compound, 3-PGA

Question 5

Stage 2: reduction

Answer 5

ATP and NADPH are used to convert the 3-PGA molecules into a 3 carbon sugar (G3P–a precursor to glucose)

Called reduction because NADPH donates electrons, or reduces, a three-carbon intermediate to make G3P

Question 6

Stage 3: regeneration

Answer 6

Some G3P molecules go to make glucose, while others are recycled to regenerate the RuBP acceptor

Regeneration requires ATP

Question 7

Why have such an inefficient pathway?

Answer 7

The initial evolution of photosynthesis occurred when atmospheric O2 levels were low. Photorespiration was likely useless. But plants went “all in” on the flawed RuBisCO pathway

Question 8

C4 photosynthesis

Answer 8

This pathway incorporates CO2 into 4-carbon acids using PEP carboxylase

CO2 is then delivered to the bundle sheath cells, creating a high concentration around RuBisCO

Question 9

C4 and photorespiration

Answer 9

CO2 gets concentrated around RuBisCO, maximizing efficiency

C4 plants have little photorespiration resulting in a higher photosynthetic rate…but the steps requires more energy

Question 10

C3 VS C4 PLANTS

Answer 10

95% are C3

C4 plants live in hot-moist, or arid non-saline habitats (grasses, sugarcane, maize, sorghum)

Question 11

When is C4 more efficient than C3?

Answer 11

In high temps
when water is limiting
When nitrogen is limiting

Question 12

When is C3 more efficient than C4?

Answer 12

In low temps
In the shade
When CO2 concentrations are high

Question 13

How did C4 evolve?

Answer 13

arid/semi-arid grasslands
Hot/dry and not optimum for C3 photosynthesis
Evolved in grasses/sedges first; then a small number of broadleaved species (mostly herbs) later

Question 14

CAM photosynthesis

Answer 14

Crassulacean Acid Metabolism
Common in succulents
An adaptation to cope with extreme water limitation
Stomata open at night and absorb CO2. C4 photosynthesis during the day