Biochem Ch 23 - The Calvin Cycle

The Calvin cycle synthesizes hexoses from what two molecules?

CO2, H2O

What are the three stages of the Calvin cycle?

1. fixation of CO2 by ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate

2. reduction of 3-phosphoglycerate to form hexose sugars

3. regeneration of ribulose 1,5-bisphosphate

What happens in the first stage of the Calvin cycle?

CO2 reacts with ribulose 1,5-bisphosphate to form 2 molecules of 3-phosphoglycerate

What enzyme catalyzed the first stage of the Calvin cycle?

rubisco

What is considered the rate limiting step in hexoses synthesis?

the fixation of CO2 (stage 1)

What is the general structure of rubisco?

comprised of 8 large subunits and 8 small subunits

active sites lie in the large subunits

What does rubisco require as a cofactor for catalytic activity?

Mg2+

Where does CO2 bind to rubisco? What attaches to the CO2 once it it attached?

CO2 binds to lysine 201 on the large subunit of rubisco

Mg2+ attaches to the carbamate ion formed

What happens to rubisco in the absence of CO2?

rubisco binds ribulose 1,5-bisphosphate so tightly that the enzyme is inhibited

What enzyme activates rubisco? How?

rubisco activatase; by using ATP to remove ribulose 1,5-bisphosphate

What happens in the second stage of the Calvin cycle?

3-phosphoglycerate is converted into hexoses monophosphates using ATP and NADPH synthesized during the light reactions

The pathway from 3-phosphoglycerate to hexoses is very similar to what pathway?

gluconeogenic pathway

What two molecules are required for stage 2 of the Calvin cycle?

ATP, NADPH

What are the steps of Hexose Phosphate Formation?

• 2 3-phosphoglycerate → 2 1,3-BPG (uses 2 ATP)

• 2 1,3-BPG → 1 GAP + 1 DHAP (uses 2 NADPH)

• 1 GAP + 1 DHAP → 1 fructose 1,6-bisphosphate

• fructose 1,6-bisphosphate → fructose 6-phosphate

• fructose 6-phosphate ←→ glucose 6-phosphate

• glucose 6-phosphate ←→ glucose 1-phosphate

What happens in the third stage of the Calvin cycle?

the regeneration of ribulose 1,5-bisphosphate

What is the net reaction of stage 3, the regeneration of ribulose 1,5-bisphosphate, in the Calvin cycle?

fructose 6-phosphate + 2 GAP + DHAP + 3 ATP →

3 ribulose 1,5-bisphosphate + 3 ADP

What 2 enzymes generate 5-carbon sugars from 6-carbon and 3-carbon sugars?

transketolase, aldolase

What two 5-carbon sugars are created to be converted into ribulose 5-phosphate?

ribose 5-phosphate, xyulose 5-phosphate

How many CO2 are needed to create 1 molecule of DHAP?

3 CO2

How many molecules of DHAP are required to be converted into fructose 6-phosphate?

2 DHAP

The Calvin cycle entails many reactions that lead ultimately to the synthesis of what?

glucose

How many molecules of ATP and NADPH are required for the incorporation of each CO2 into a Hexose?

3 ATP, 2 NADPH

What is the balanced equeation for the net reaction of the Calvin cycle?

6CO2 + 18 ATP + 12 NADPH + 12 H2O →

C6H12O6 + 18 ADP + 18 Pi + 12 NADP + 6H

What are the two principle fates of the fixed carbons from photosynthesis?

starch, sucrose

Where is starch stored in plants?

in the chloroplast

What is sucrose synthesized from? Where?

from 3-phosphoglycerate in the cytoplasm

Is starch or sucrose the transport form of carbohydrates in many plants?

sucrose

Explain the synthesis of sucrose.

• 2 3-phosphoglycerate molecules are used to synthesize fructose 6-phosphate.

• fructose 6-phosphate reacts with the activated intermediate UDP-glucose

• glucose attaches to fructose 6-phosphate to form sucrose 6-phosphate leaving UDP alone

Why is the Calvin cycle crucial to the functioning of all life forms?

the Calvin cycle is the primary means of converting CO2 into organic matter-biomolecules. Every carbon atom in your body passed through rubisco and the Calvin cycle at some point in the past.

How do light reactions regulate the Calvin cycle?

by altering the environment of the stroma

How do light reactions alter the environment of the stroma? (3)

• increase in stromal pH

• the formation of a proton gradient

• the movement of Mg2+ into the stroma

Compare the environment of light reactions and the Calvin cycle.

light reactions cause:

• more Mg2+ in stroma

• more H+ thylakoid

• reduced Ferredoxin in stroma

• NADPH in stroma

Calvin cycle causes:

• more Mg2+ in thylakoid

• more H+ in stroma

• oxidized Ferredoxin in stroma

• NADP+ in stroma

What two molecules from photosynthesis regulate enzymes of the Calvin cycle?

reduced Ferredoxin, NADPH

How does thioredoxin regulate enzymes of the Clavin cycle?

• thioredoxin is reduced by reduced Ferredoxin (causes ferredoxin to become oxidated)

• reduced thioredoxin is used to activate enzymes of the Calvin cycle by reducing disulfide bridges

  • this reaction reoxidizes thioredoxin

Why is rubisco commonly known as an inefficient enzyme?

it can react with O2 instead of CO2 to generate 3-phosphoglycerate and phosphoglycolate

What happens to phosphoglycolate when it is formed by rubisco reacting with O2?

phosphoglycolate is metabolized to yield a molecule of CO2

What is the process of photorespiration? Why is it considered negative?

Photorespiration is a light-dependent metabolic pathway in plants where the enzyme Rubisco acts on oxygen instead of carbon dioxide. This "mistake" by Rubisco occurs primarily under hot, dry conditions when stomata close, reducing availability inside the leaf.

Instead of producing two molecules of 3-PGA to create sugar, the reaction produces one 3-PGA and one 2-phosphoglycolate (2PG), a two-carbon compound that is toxic to the cell. The process is termed "photorespiration" because it consumes oxygen and releases CO2

How do some plants reduce the effects of photorespiration? What is this pathway called?

some plants concentrate CO2 in photosynthetic cells; C4 pathway

The C4 pathway causes CO2 to react with what molecule to form the four-carbon molecule, oxaloacetate?

phosphoenolpyruvate

What enzyme catalyzes the reaction between CO2 and PEP?

PEP carboxylase

What is the fate of the four-carbon molecule formed int he C4 pathway?

oxaloacetate is converted to malate, which is then converted back into pyruvate to yield a CO2 that can be used in the Calvin cycle.

Why is the C4 pathway beneficial to tropical plants?

CO2 is concentrated in bundle-sheath cells

rubisco is supplied with enough CO2, the stomata do not have to open completely in the heat of the tropical day, preventing water loss

What is the overall reaction of the C4 pathway?

CO2 (mesophyll cells) + ATP + H2O →

CO2 (bundle sheath cells) + AMP + 2 Pi + H

What is the net reaction of the C4 pathway and the Calvin cycle?

6 CO2 + 30ATP + 12 NADPH + 12 H2O →

C6H12O6 + 30 ADP + 30 Pi + 12 NADP + 18H

Q1: Why is the Calvin cycle crucial to the functioning of all life forms?

the Calvin cycle is crucial to life because it is the primary mechanism by which inorganic carbon dioxide is converted into organic sugars, forming the basis of the global food web

Q4: The Calvin cycle can be though of as taking place in three stages. Describe the stages.

1. fixation of CO2 by carboxylation of ribulose 1,5 bisphosphate to form 2 molecules of 3-phosphoglycerate

2. reduction of the fixed carbon to begin the synthesis of hexose

3. regeneration of ribulose 1,5-bisphosphate

Q6: Suggest a reason why rubisco might be the most abundant enzyme in the world.

rubisco is pretty slow and wasteful, and so we need enough of it for sugars to be formed

Q10: Rubisco requires a molecule of CO2 covalently bound to lysine 201 for catalytic activity. The carboxylation of rubisco is favored by high pH and high Mg2+ concentration in the stroma. Why does it make good physiological sense for these conditions to favor rubisco carboxylation?

High pH and high Mg2+ concentrations signify active light-dependent reactions. Activating Rubisco only under these conditions ensures that carbon fixation (Calvin cycle) occurs only when sufficient energy (ATP/NADPH) is available, preventing wasteful energy consumption in the dark and allowing the enzyme to function optimally when the chloroplast is ready to produce sugar

Q11: What are the light-dependent changes in the stroma that regulate the Calvin cycle?

the increase of pH, decrease of H+, increase of Mg2+, reduced ferredoxin, NADPH

Q19: Identify the similarities and differences between the Krebs cycle and the Calvin cycle.

The Krebs cycle (citric acid cycle) and Calvin cycle are both regenerative, cyclical metabolic pathways, but they serve opposite purposes: the Krebs cycle breaks down carbon compounds (catabolic) to produce ATP/energy, while the Calvin cycle uses ATP/energy to synthesize carbon compounds (anabolic). Both are critical for energy metabolism but occur in different locations using different enzymes

Q26: Explain the differences between the light reactions and the dark reactions of photosynthesis and describe the relationship between the two sets of reactions.

Light reactions (light-dependent) convert light energy into chemical energy (ATP, NADPH) within the thylakoid membranes, releasing oxygen. Dark reactions (Calvin Cycle/light-independent) occur in the stroma, using that ATP/NADPH to fix into glucose. They are interdependent: light reactions supply energy for carbon fixation, while dark reactions return ADP and to fuel light reactions.