Biochemistry - Dark Reactions

Question 1

What is the first step of DR?

Answer 1

CO2 combines with ribulose 1,5-biphosphate, catalysed by Rubisco

Question 2

Rubisco

Answer 2

Ribyulose-1,5-biphosphate carboxylase/oxygenase

Question 3

What does CO2-R15BP form?

Answer 3

A 6C intermediate with splits intwo two 3PG, where 3PGK coupled with ATP forms 1,3-BPG

Question 4

What happens to 1,3BPG?

Answer 4

It is reduced by GHAPD using NADPH, forming GAH3P.

Question 5

What are the reactants and products in Rubisco?

Answer 5

3 CO2 molecules combine with 3 Ribulose 1,5-BP, forming 6 GAP.

Question 6

What are the fates of the 6 GAP?

Answer 6

5 for regeneration of R15BP, whilst one is used for amino acids/lipids or glucose sugars

Question 7

What are the Dark reactions regulated by?

Answer 7

Thioredoxin, pH and Mg

Question 8

How do light reactions regulate dark?

Answer 8

Proton gradient formation increases stroma magnesiums and pH concentrations, where PSI changes sensed by thioredoxin.

Question 9

What is the sole function of rubisco?

Answer 9

CO2 incorporation into carbohydrate?

Question 10

What is the structure of rubisco?

Answer 10

Eight large subunits with catalytic and regulatory activity, with eight small subunits assisting in catalytic.

Question 11

First step in rubisco CO2 incorporation

Answer 11

R15BP protonated forming enediolate intermediate

Question 12

Second step in rubisco CO2 concentration

Answer 12

Enediol combined with CO2, forming 6C intermediate, hydrolysed forming two 3PG

Question 13

Why is Mg important in rubisco catalysis?

Answer 13

Lysine in active site reacts with CO2, forming carmate ion, important in catalysis and binding of R15BP

Question 14

How do light reactions regulate rubisco?

Answer 14

Carabamte ions require Mg concentration and alkaline pH levels, altered by light reactions.

Question 15

Photorespiration

Answer 15

The process where O2 reacts with RUBISCO instead of CO2.

Question 16

What does photorespiration form instead?

Answer 16

3PG and phosphoglycolate.

Question 17

Why does photorespiration have a net loss of energy?

Answer 17

PG conversion to 3P6, which requires ATP.

Question 18

What is the carboxylation/oxygenation ratio?

Answer 18

3:1, due to high O2 concentrations despite its less favourabiltiy.

Question 19

CO2 concentration mechanisms

Answer 19

This is the balacning of CO2 surrounding RUBISCO relative to O2

Question 20

Where does CO2-CM get the extra CO2 fROM?

Answer 20

Mesophyll cell PEP carboxylase fixses CO2 into oxaloacetate.

Question 21

What happens to Oxaloacetate after when required?

Answer 21

Reduced to malate using NADPH.

Question 22

What happens to malate?

Answer 22

Exported to bundle sheath cells, decarboxylated to ptruvate, to generate NADPH and CO2

Question 23

What is the CO2 generated by pyruvate used for?

Answer 23

By rubisco in calvin cycle

Question 24

How is PEPE recycled?

Answer 24

Pyruvate returns to mesophyll, phosphortlated using ATP

Question 25

Why are stomata closed in hot, dry and bright conditiosn?

Answer 25

Avoid excessive water loss and withering, internal CO2 is lower so rubisco synthesis harder.