Photosynthesis Mechanisms in Pyrenoids

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Questions and Answers

In the C4 cycle as proposed by Hatch and Slack, which compound is labeled first during the pulse-chase study?

Hexose phosphates

Malate (correct)

C3 acids

3-PGA

What does the novel carboxylation reaction in the C4 cycle initially combine with CO2?

3-PGA

Hexose phosphates

Malate

C3 compounds (correct)

What is produced after the decarboxylation step in the C4 cycle before it undergoes carboxylation again?

3-PGA (correct)

Ribulose bisphosphate

Oxaloacetate

Glucose

What role does Rubisco play in the C4 pathway as indicated in the process?

Catalyzing carboxylation reactions Signup and view all the answers

Which of the following is NOT an intermediate in the C4 pathway proposed by Hatch and Slack?

Pyruvate Signup and view all the answers

What is the primary function of Rubisco in cyanobacterial carbon fixation?

To convert carbon dioxide into organic compounds Signup and view all the answers

In what structure are pyrenoids primarily located within chloroplasts?

Thylakoid membranes Signup and view all the answers

Which method was employed to reveal the native architecture of the Chlamydomonas chloroplast?

In situ cryo-electron tomography Signup and view all the answers

What structural feature intersperses the Rubisco found in pyrenoids?

Thylakoid membranes Signup and view all the answers

Which of the following components is not part of the chloroplast structure mentioned in the context?

Flagella Signup and view all the answers

What is the main role of carboxysomes in cyanobacteria?

Concentration of CO2 for photosynthesis Signup and view all the answers

Which enzyme is primarily associated with the carboxylation reaction in C4 plants?

Phosphoenolpyruvate carboxylase (PEPC) Signup and view all the answers

In which structures do many algae concentrate CO2 for photosynthesis?

Pyrenoids Signup and view all the answers

Which process is related to the endosymbiotic origin of chloroplasts?

Photosynthetic carbon fixation Signup and view all the answers

What is the function of carbonic anhydrase (CA) in photosynthetic organisms?

To catalyze the interconversion of CO2 and bicarbonate Signup and view all the answers

Which type of plants primarily use the C4 pathway for photosynthesis?

Certain crops like corn and sugarcane Signup and view all the answers

Which component is involved in the downstream fixation of CO2 in C4 plants?

C4 acids Signup and view all the answers

What evolutionary development is indicated to have occurred long after plastid endosymbiosis?

Formation of pyrenoids and carboxysomes Signup and view all the answers

What structural feature is labeled with GFP in the study of cyanobacterial carboxysomes?

Protein structures Signup and view all the answers

What advantage do C4 plants have in photosynthesis over C3 plants?

Improved efficiency in low CO2 conditions Signup and view all the answers

What does the process of bicarbonate transport refer to in the context of cyanobacterial photosynthesis?

Movement of HCO3- molecules into the chloroplast Signup and view all the answers

What role does carbonic anhydrase (CA) play in the chloroplasts?

It converts bicarbonate (HCO3-) into CO2. Signup and view all the answers

Which statement best describes the function of transporters in the context of CO2 and HCO3- in chloroplasts?

Transporters specifically move HCO3- into the chloroplasts. Signup and view all the answers

What characteristic of the pyrenoid's starch layer affects CO2 back-diffusion?

It slows back-diffusion of CO2 effectively. Signup and view all the answers

How does HCO3- permeability compare between chloroplast membranes and pyrenoids?

Chloroplast membranes are more permeable to HCO3- than pyrenoids. Signup and view all the answers

Which of the following statements about CO2 concentration in pyrenoids is true?

CO2 is concentrated through specialized transporters and CA. Signup and view all the answers

What is the primary function of the bicarbonate transporter in chloroplasts?

To transport HCO3- into chloroplasts for conversion to CO2. Signup and view all the answers

Why is the incorporation of CO2 into pyrenoids important for photosynthesis?

It enhances CO2 availability for Rubisco to facilitate carbon fixation. Signup and view all the answers

What limits the movement of CO2 from thylakoids to the stroma?

The starch layer of the pyrenoid acting as a barrier. Signup and view all the answers

Which of the following correctly describes the carbon fixation process by Rubisco?

It operates efficiently only under high CO2 concentrations. Signup and view all the answers

What is the significance of using bicarbonate as a carbon source instead of CO2?

Bicarbonate serves as a more stable form of carbon in photosynthesis. Signup and view all the answers

What is the primary advantage of C4 plants in relation to environmental conditions?

Higher productivity in dry, hot, sunny regions Signup and view all the answers

What is the role of glycine decarboxylase (GDC) in C3 photosynthesis?

Decarboxylation of glycine in the mesophyll Signup and view all the answers

Which of the following strategies is proposed to engineer more efficient C3 plants?

Introducing pyrenoids and carboxysomes Signup and view all the answers

What is the function of the centripetal microanatomy within bundle sheath cells?

To facilitate reassimilation of carbon dioxide Signup and view all the answers

What limitation is associated with the glycine decarboxylase (GDC) enzyme in the mesophyll?

It lacks a critical P subunit necessary for its activity Signup and view all the answers

Which environmental factor primarily benefits C4 plants over C3 plants?

High irradiance conditions Signup and view all the answers

What is one proposed method to increase Rubisco selectivity?

Introducing new synthetic pathways Signup and view all the answers

What is the expected outcome of raising the concentration of CO2 at Rubisco?

Increased efficiency of carbon fixation Signup and view all the answers

Study Notes

Pyrenoids

Rubisco is highly concentrated in a structure called a pyrenoid
Pyrenoids are interspersed with thylakoid membranes
Pyrenoids are located within the chloroplast
Starch granules surround the pyrenoid

Carbon Dioxide Concentration

Carbon dioxide (CO2) is concentrated in pyrenoids in order to improve the efficiency of photosynthesis
Carbonic anhydrase (CA) within the chloroplast converts bicarbonate (HCO3-) to CO2
CO2 diffusion is slowed by the pyrenoid’s starch layer, which prevents back-diffusion

Evolution of Pyrenoids and Carboxysomes

Pyrenoids and carboxysomes evolved long after plastid endosymbiosis

Carbon Concentrating Mechanisms

Cyanobacteria concentrate CO2 in protein structures called carboxysomes
Many algae concentrate CO2 in pyrenoids
C4 plants concentrate and store CO2 using phosphoenolpyruvate carboxylase (PEPC) upstream of Rubisco
C4 plants have an advantage at higher temperatures and in full sunlight
C3 plants are more efficient in cool, temperate climates
C3-C4 intermediates rely on metabolite transfer to specialized bundle sheath cells
Bundle sheath cells contain an inactive version of glycine decarboxylase (GDC) which is unable to process glycine
Glycine is transferred from mesophyll cells to bundle sheath cells and decarboxylated by GDC in mitochondria
Bundle sheath chloroplasts overlay mitochondria, facilitating reassimilation of CO2

Engineering C3 Plants

Increasing CO2 concentration at Rubisco could be achieved by introducing:
- Bicarbonate transporters
- Pyrenoids
- Carboxysomes
Other strategies for improving C3 photosynthesis include:
- Increasing Rubisco selectivity
- Engineering new carbon fixation pathways
- Engineering more efficient photorespiration pathways

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Description

This quiz explores the role of pyrenoids in photosynthesis, focusing on their structure, function, and the evolution of carbon concentrating mechanisms. It covers critical aspects such as Rubisco concentration, CO2 dynamics, and differences between C3 and C4 plants.