Carbon Fixation

Question 1

Give an overview of photosynthesis ?

Answer 1

1. Light absorption, generation of high-energy electrons, and O2 formation from H2O
2. Electron transport leading to reduction of NADP+ to NADPH and proton motive force generation
3. Synthesis of ATP
4. Conversion of CO2 into carbohydrates (carbon fixation)

Question 2

Animal cells: use 3C intermediates for ?

Answer 2

Synthesis of biomolecules, but the 3C intermediates must be made from degradation of a larger molecule

Question 3

Plant cells: can also make?

Answer 3

3C intermediates for further synthesis

Question 4

What is Carbon fixation/assimilation ?

Answer 4

Making CO2 into intermediates

Question 5

Where does Carbon fixation occur ?

Answer 5

Occurs in the stroma of chloroplasts via a cyclic process known as the Calvin cycle

Question 6

The light-driven synthesis of ATP and NADPH provides ?

Answer 6

Energy and reducing power for the fixation of CO2 into trioses, which are used to produce hexose phosphates to make cellular carbohydrates

Question 7

What is a key intermediate of carbon fixation ?

Answer 7

Key intermediate ribulose 1,5-bisphosphate is constantly regenerated using energy of ATP

Question 8

What is the net result of carbon fixation ?

Answer 8

The net result is the reduction of CO2 with NADPH that was generated in the light reactions of photosynthesis

Question 9

The carbon-fixation cycle consumes ?

Answer 9

9 ATP and 6 NADPH to form one glyceraldehyde 3-phosphate from 3 CO2 and 5 H2O

Question 10

Briefly explain the 3 stages of the Calvin cycle?

Answer 10

1) CO2 fixation: 3 ribulose 1,5-bisphosphate + 3 CO2 → 3-phosphoglycerate, catalyzed by Rubisco
2) Reduction: 3-phosphoglycerate → glyceraldehyde 3-phosphate using NADPH and ATP from photosynthesis
3) Regeneration of ribulose 1,5-bisphosphate

Question 11

What is Carbon fixation catalysed by ?

Answer 11

The enzyme Rubisco

Question 12

Role of Rubisco ?

Answer 12

• Responsible for the fixation of 100000000000 tonnes of carbon per year
• Rate-limiting step of CF with very small turnover rate
• Catalyzes the reaction: ribulose 1,5-bisphosphate + CO2 → 2 x 3-phosphoglycerate
• Large Mg2+ dependent enzyme with 8 large and 8 small subunits

Question 13

Explain in detail Stage 1: Fixation ?

Answer 13

• Carboxylation reaction by Rubisco: the most abundant enzyme in the biosphere
• Note that these steps take place on the magnesium ion
• Overall pathway for the conversion of ribulose 1,5-bisphosphate and CO2 into two molecules of 3-phosphoglycerate

Question 14

Explain in detail Stage 2: Redction ?

Answer 14

• Next, the 3-phosphoglycerate generated by rubisco is reduced to glyceraldehyde 3-phosphate using NADPH and ATP from photosynthesis
• Catalyzed by chloroplast isoforms of 3-phosphoglycerate kinase and glyceraldehyde 3-phosphate dehydrogenase from glycolysis
• G3P can then be converted to hexose monophosphates if required

Question 15

G3P and DHAP are building blocks for ?

Answer 15

Hexose sugars in the stroma and cytosol

Question 16

Explain Glyceraldehyde-3-phosphate (from stage 2) in carbohydrate synthesis ?

Answer 16

• Synthesise starch for energy storage in the stroma
• Exported to the cytosol and used to synthesize sucrose for export to plant tissues
• Catabolized by glycolysis for energy conversion in cytosol

Question 17

Explain in detail Stage 3: Regeneration of RuBP ?

Answer 17

• The challenge of the third stage of the Calvin cycle is the formation of a five-carbon sugar from six-carbon and three-carbon sugars.
• First, transketolase converts a 6C sugar and a 3C sugar into a 4C & 5C sugar. Then, aldolase combines the 4C product and a 3C sugar to form a 7C sugar. Finally, this 7C sugar reacts with another 3C sugar to form two more 5C sugars

Question 18

The light reactions regulate the Calvin cycle by altering ?

Answer 18

The environment of the stroma

Question 19

Explain Light regulation of the Calvin Cycle ?

Answer 19

• Electrons are transferred out of the thylakoid lumen into the stroma and protons transferred the other way
• Leads to an increase in stromal pH and an increase in the stromal concentrations of Mg2+, NADPH, and reduced ferredoxin, all of which activate the enzymes of the Calvin cycle.

Question 20

Explain the Rubisco oxygenase reaction ?

Answer 20

• Rubisco can react with O2 instead of CO2 to generate 3-phosphoglycerate and phosphoglycolate (catalytic imperfection)
• No carbons are fixed: it is a wasteful side reaction
• A complex ATP-consuming process, the C2 pathway, recovers C2 fragments from photorespiration: reactions across the chloroplast, peroxisome and mitochondria ultimately make 3-phosphoglycerate, but with the loss of CO2
• Because O2 is consumed and CO2 is generated, the process is called photorespiration

Question 21

What are the two major disadvantages to C3 photosynthesis:

Answer 21

1) Water loss due to stomatal opening to admit CO2 - the ratio [grams water lost : grams CO2 fixed] is ~ 400: 900
2) Photorespiration - arises from the dual activity of Rubisco

Photorespiration results in O2 uptake with CO2 and NH3 release in the light and may reach rates of ~ 40 - 50 % of the net photosynthesis rate

Question 22

Superimposed on C3 photosynthesis there are two strategies aimed at overcoming these two major drawbacks. What are these ?

Answer 22

CAM and C4 photosynthesis

Question 23

Explain CAM ?

Answer 23

• In desert succulent plants

- Captures CO2 at night and stores it in 4C compound then fixes CO2 in the day

Question 24

Explain C4 ?

Answer 24

• Found in tropical plants
• One cell type is required for CO2 uptake and another for rubisco-mediated carboxylation
• Captures CO2 in the form of oxaloacetate in mesophyll cells and then delivers it to bundle sheath cells as malate, another C4 intermediate

Question 25

CAMP plants with time separate CO2 trapping and fixation ?

Answer 25

• First discovered in Crassulacae, so called Crassulacean Acid Metabolism
• Found in plants for which water loss is a big problem
• Only open stomata at night when it is cooler to avoid water loss
• CO2 absorbed at night is fixed to make oxaloacetate (4C) via PEPCK
• Oxaloacetate is reduced to malate and stored in vacuoles
• In daytime, malate releases CO2 via NADP-linked malic enzyme, and CO2 fixation proceeds: since stomata are closed, [O2] is low

Question 26

How does CAM solve the two problems of conventional photosynthesis ?

Answer 26

1) Water conservation
Only opening stomata at night reduces water loss. Thick leaf cuticles and other adaptations also curtail water loss from leaves
2) Photorespiration
This is greatly reduced by the raised internal CO2 concentrations in the light (brought about by the decarboxylation of malate) competitively inhibiting Rubisco’s oxygenase reaction

Question 27

C4 plants structurally separate CO2 trapping and fixation ?

Answer 27

Tend to grow in hotter, sunnier climates: have high rates of growth, photosynthesis, low water loss & special leaf structure

Question 28

C4 plants have an earlier step (before rubisco):

Answer 28

Bypass the C-3 fixation step by fixing CO2 into a 4-C compound (oxaloacetate from acetyl CoA) then delivers to bundle sheath cells in form of malate

Question 29

The C4 pathway has a higher energy cost, but ?

Answer 29

Also has increased efficiency in heat: as temperature increases, rubisco affinity for CO2 increases

Question 30

There are several variants of the C4 pathway of CO2 fixation:

Answer 30

• The 4C-acid transported from mesophyll cells may be malate or aspartate
• The 3C-acid transported back from bundle-sheath cells may be pyruvate or alanine
• The enzyme that catalyses decarboxylation in the bundle-sheath cells varies. In maize and sugarcane, it NADP-malic enzyme; in millet, it is NAD-malic enzyme; in Guinea grass it is PEPCK

Question 31

How does C4 solve the two problems of conventional photosynthesis ?

Answer 31

1) Water conservation
C4 plants have fewer stomata per unit leaf area and do not open them as widely as C3 plants.
2) Photorespiration
Photorespiration is virtually absent as Rubisco is present in the bundle sheath chloroplasts: here internal CO2 concentrations are elevated by the decarboxylation
of malate
- The high CO2 competitively inhibits the oxygenase activity of Rubisco

Question 32

C3 vs C4

Answer 32

• C4 plants have a growth advantage over C3 plants in hot climates because of reduced rates of photorespiration
• C3 plants have a growth advantage over C4 plants at cooler temperatures due to lower energy requirements
  E.g.
• Crabgrass is a C4 plant that outgrows turfgrass in the summer
• Turfgrass grows faster than crabgrass in cooler climates because of the reduced energy requirements of the normal Calvin cycle pathway

Question 33

Photosynthetic electron transport leads to ?

Answer 33

Proton translocation into the thylakoid lumen and the production of NADPH in the stroma

Question 34

The ATP synthase complex is oriented outward from the lumen, so that ?

Answer 34

ATP synthesis occurs in the stromal compartment

Question 35

Calvin cycle enzymes located in the stroma use?to produce ?

Answer 35

Calvin cycle enzymes located in the stroma use NADPH and ATP to produce G3P from CO2

Question 36

This triose phosphate can either be ?

Answer 36

Used in the stroma or transported to the cytosol to make hexoses/go into glycolysis

Question 37

Oxidative phosphorylation versus photophosphorylation

Answer 37

1) In a sense these are “reverse” reactions, but both generate high energy electrons
- Many of the components of the electron transport chains are very similar:
- Plastoquinone (Q), closely resembles the ubiquinone of mitochondria.
- The cytochrome b6-f complex resembles the cytochrome c reductase complex of mitochondria

2) Both mitochondria and chloroplasts use an ATP synthase embedded in the membrane that uses the energy of the electrochemical proton gradient to produce ATP