Lecture 5 - Photorespiration | Plant Metabolism - Botany PDF Download

 Page 1


Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
                                                                                              
 
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Physiology 
Lesson: Photorespiration 
Lesson Developer: Dhara Arora 
Department/College: Department of Botany, 
 University of Delhi 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 2


Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
                                                                                              
 
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Physiology 
Lesson: Photorespiration 
Lesson Developer: Dhara Arora 
Department/College: Department of Botany, 
 University of Delhi 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
 
Learning outcomes: 
The students will be able to 
? Discuss the observations that led to the discovery of photorespiration 
? Understand the need of photorespiration in plants 
? Comprehend the dual nature of rubisco 
? Describe terms like compensation point, sink capacity, CO
2
 fertilization effect 
? Understand the regulation of pathways at genetic level 
? Describe the energy cost involved in photorespiration 
? Understand the concept of compartmentalization of the photorespiratory 
pathway 
? Describe the assimilation of ammonia produced by C
2
 cycle 
? Understand the light and the dark side of photorespiration 
? Understand the relationship between photorespiration and changing climatic 
conditions 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 3


Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
                                                                                              
 
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Physiology 
Lesson: Photorespiration 
Lesson Developer: Dhara Arora 
Department/College: Department of Botany, 
 University of Delhi 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
 
Learning outcomes: 
The students will be able to 
? Discuss the observations that led to the discovery of photorespiration 
? Understand the need of photorespiration in plants 
? Comprehend the dual nature of rubisco 
? Describe terms like compensation point, sink capacity, CO
2
 fertilization effect 
? Understand the regulation of pathways at genetic level 
? Describe the energy cost involved in photorespiration 
? Understand the concept of compartmentalization of the photorespiratory 
pathway 
? Describe the assimilation of ammonia produced by C
2
 cycle 
? Understand the light and the dark side of photorespiration 
? Understand the relationship between photorespiration and changing climatic 
conditions 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
Table of Contents   
Chapter: Photorespiration  
? Introduction 
? Discovery of Photorespiration 
? Dual nature of rubisco 
? Photorespiratory pathway 
? Assimilation of ammonia produced in C
2
 cycle 
? Energy cost 
? Photorespiration and climatic conditions 
? The light side and the dark side of photorespiration 
? Removal of toxic intermediates 
? Protects from photoinhibition 
? Assists plant defense reactions 
? Integrates into primary metabolism 
? Summary 
? Exercise/ Practice 
? Glossary 
? References/ Bibliography/ Further Reading 
 
 
 
 
 
 
 
 
 
 
 
 
Page 4


Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
                                                                                              
 
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Physiology 
Lesson: Photorespiration 
Lesson Developer: Dhara Arora 
Department/College: Department of Botany, 
 University of Delhi 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
 
Learning outcomes: 
The students will be able to 
? Discuss the observations that led to the discovery of photorespiration 
? Understand the need of photorespiration in plants 
? Comprehend the dual nature of rubisco 
? Describe terms like compensation point, sink capacity, CO
2
 fertilization effect 
? Understand the regulation of pathways at genetic level 
? Describe the energy cost involved in photorespiration 
? Understand the concept of compartmentalization of the photorespiratory 
pathway 
? Describe the assimilation of ammonia produced by C
2
 cycle 
? Understand the light and the dark side of photorespiration 
? Understand the relationship between photorespiration and changing climatic 
conditions 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
Table of Contents   
Chapter: Photorespiration  
? Introduction 
? Discovery of Photorespiration 
? Dual nature of rubisco 
? Photorespiratory pathway 
? Assimilation of ammonia produced in C
2
 cycle 
? Energy cost 
? Photorespiration and climatic conditions 
? The light side and the dark side of photorespiration 
? Removal of toxic intermediates 
? Protects from photoinhibition 
? Assists plant defense reactions 
? Integrates into primary metabolism 
? Summary 
? Exercise/ Practice 
? Glossary 
? References/ Bibliography/ Further Reading 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
Introduction 
Photorespiration is a process in plant metabolism where light-dependent uptake of 
molecular oxygen (O
2
) takes place along with release of carbon dioxide (CO
2
) from 
organic compounds. This process is also termed as the oxidative photosynthetic 
carbon cycle or C
2
 photosynthesis. Though the gas exchange during 
photorespiration resembles cellular respiration, but it consumes ATP rather than 
producing it.  
Photorespiration is an exceptional biochemical pathway. In this pathway the enzyme 
ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) fixes molecular oxygen 
instead of executing its intrinsic function of fixing carbon dioxide during 
photosynthesis.  
  
Figure : Oxygenase activity of rubisco leads to loss of CO
2
, which is majorly (75%) 
recovered by photorespiration.  
Source:Author 
Discovery of Photorespiration 
 "Warburg effect", was discovered as early as 1920s by  biochemist Otto Warburg 
(Nobel laureate, 1931). It was observed that increasing the external oxygen 
concentration inhibited photosynthesis in Chlorella. When the oxygen concentration 
was doubled (from atmospheric 21%), photosynthetic rate declined as much as 50% 
and on decreasing oxygen concentration to 2%, photosynthetic rate doubled.  CO
2
 
compensation point increased with increasing oxygen concentration, suggesting a 
competition between the two gases during photosynthesis.  When photosynthesizing 
tissues were transferred to darkness, a burst of CO
2
 is released immediately. This 
 
O
2
 
+  
RuBP 
RUBISCO 
3-PGA  
+  
2-PG 
 
CO
2
 
Page 5


Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
                                                                                              
 
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Physiology 
Lesson: Photorespiration 
Lesson Developer: Dhara Arora 
Department/College: Department of Botany, 
 University of Delhi 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
 
Learning outcomes: 
The students will be able to 
? Discuss the observations that led to the discovery of photorespiration 
? Understand the need of photorespiration in plants 
? Comprehend the dual nature of rubisco 
? Describe terms like compensation point, sink capacity, CO
2
 fertilization effect 
? Understand the regulation of pathways at genetic level 
? Describe the energy cost involved in photorespiration 
? Understand the concept of compartmentalization of the photorespiratory 
pathway 
? Describe the assimilation of ammonia produced by C
2
 cycle 
? Understand the light and the dark side of photorespiration 
? Understand the relationship between photorespiration and changing climatic 
conditions 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
Table of Contents   
Chapter: Photorespiration  
? Introduction 
? Discovery of Photorespiration 
? Dual nature of rubisco 
? Photorespiratory pathway 
? Assimilation of ammonia produced in C
2
 cycle 
? Energy cost 
? Photorespiration and climatic conditions 
? The light side and the dark side of photorespiration 
? Removal of toxic intermediates 
? Protects from photoinhibition 
? Assists plant defense reactions 
? Integrates into primary metabolism 
? Summary 
? Exercise/ Practice 
? Glossary 
? References/ Bibliography/ Further Reading 
 
 
 
 
 
 
 
 
 
 
 
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
Introduction 
Photorespiration is a process in plant metabolism where light-dependent uptake of 
molecular oxygen (O
2
) takes place along with release of carbon dioxide (CO
2
) from 
organic compounds. This process is also termed as the oxidative photosynthetic 
carbon cycle or C
2
 photosynthesis. Though the gas exchange during 
photorespiration resembles cellular respiration, but it consumes ATP rather than 
producing it.  
Photorespiration is an exceptional biochemical pathway. In this pathway the enzyme 
ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) fixes molecular oxygen 
instead of executing its intrinsic function of fixing carbon dioxide during 
photosynthesis.  
  
Figure : Oxygenase activity of rubisco leads to loss of CO
2
, which is majorly (75%) 
recovered by photorespiration.  
Source:Author 
Discovery of Photorespiration 
 "Warburg effect", was discovered as early as 1920s by  biochemist Otto Warburg 
(Nobel laureate, 1931). It was observed that increasing the external oxygen 
concentration inhibited photosynthesis in Chlorella. When the oxygen concentration 
was doubled (from atmospheric 21%), photosynthetic rate declined as much as 50% 
and on decreasing oxygen concentration to 2%, photosynthetic rate doubled.  CO
2
 
compensation point increased with increasing oxygen concentration, suggesting a 
competition between the two gases during photosynthesis.  When photosynthesizing 
tissues were transferred to darkness, a burst of CO
2
 is released immediately. This 
 
O
2
 
+  
RuBP 
RUBISCO 
3-PGA  
+  
2-PG 
 
CO
2
 
Photorespiration 
 
Institute of Lifelong Learning, University of Delhi 
 
post-illumination burst of CO
2
 is directly proportional to the external CO
2
 
concentration. All these observations ultimately led to the discovery of novel process 
of photorespiration.  
 
Figure: A post-illumination burst of CO
2
 is shown by photosynthesizing leaves, which 
varies in strength according to ambient O
2
 concentration. This positive response to 
O
2
 was observed at 105 µmol m
-2
 s
-1
 and is functionally linked to oxygen effects on 
the CO
2
 compensation point as measured under steady—state conditions. (Based on 
Krotkov 1963) 
Source: http://plantsinaction.science.uq.edu.au/book/export/html/47  
Dual nature of Rubisco 
Ribulose-1,5-bisphosphate-carboxylase/oxygenase (RUBISCO) is the enzyme 
which catalyzes the entrance reactions to both photosynthesis and photorespiration. 
Rubisco is a dual-function enzyme which results in oxygenation and carboxylation of 
the same substrate, ribulose-1,5-bisphosphate (RuBP).  
? Carboxylation of RuBP results in the formation of two molecules of 3-
phosphoglycerate (PGA). 
? Oxygenation of RuBP results in the formation of one molecule of each, 3-
phosphoglycerate (PGA) and 2-phosphoglycolate (PG.)  
PGA is again recycled back to RuBP via the Calvin cycle but PG cannot be used for 
biosynthetic reactions and inhibits the chloroplastic function as well.