C4 Pathway and Photorespiration
Hatch–Slack pathway, Kranz anatomy and why C4 plants avoid photorespiratory loss. (Biology › Photosynthesis in Higher Plants, NEET UG syllabus.)
What is C4 Pathway and Photorespiration?
A photosynthetic pathway in certain plants that minimizes photorespiration by spatially separating initial CO2 fixation (in mesophyll cells) from the Calvin cycle (in bundle sheath cells).
Key formula / rule: Initial CO2 fixation in C4 pathway
Key points
- Describe the steps of the C4 pathway (Hatch-Slack pathway).
- Explain the significance of Kranz anatomy in C4 plants.
- Identify the key enzymes (PEPcase, RuBisCO) and their locations in C4 plants.
- Compare and contrast C3 and C4 pathways, highlighting their adaptations.
Common exam trap
Believing C4 plants do not have RuBisCO (they do, but only in bundle sheath cells).
Definitions
- Term
C4 Pathway
- Meaning
A photosynthetic pathway in certain plants that minimizes photorespiration by spatially separating initial CO2 fixation (in mesophyll cells) from the Calvin cycle (in bundle sheath cells).
- Term
Photorespiration
- Meaning
A wasteful process in C3 plants where RuBisCO binds with oxygen instead of carbon dioxide, leading to the loss of fixed carbon as CO2 without ATP or NADPH production.
- Term
Kranz Anatomy
- Meaning
A specialized leaf anatomy characteristic of C4 plants, featuring a ring of large bundle sheath cells surrounding the vascular bundles, which are themselves encircled by mesophyll cells.
- Term
PEPcase (Phosphoenolpyruvate Carboxylase)
- Meaning
An enzyme found in the mesophyll cells of C4 plants that catalyzes the initial fixation of CO2 into a 4-carbon compound, oxaloacetate. It has a high affinity for CO2 and no affinity for O2.
- Term
RuBisCO (Ribulose-1,5-bisphosphate Carboxylase/Oxygenase)
- Meaning
The primary enzyme of the Calvin cycle, responsible for fixing CO2 to RuBP. In C4 plants, it is located in the bundle sheath cells, where high CO2 concentration ensures its carboxylase activity.
Learning objectives
Describe the steps of the C4 pathway (Hatch-Slack pathway).
Explain the significance of Kranz anatomy in C4 plants.
Identify the key enzymes (PEPcase, RuBisCO) and their locations in C4 plants.
Compare and contrast C3 and C4 pathways, highlighting their adaptations.
Explain how C4 plants minimize photorespiration and its advantages.
Formulae
- Name
Initial CO2 fixation in C4 pathway
- Note
Occurs in mesophyll cells.
- Expression
PEP + CO2 + H2O → OAA + Π (catalyzed by PEPcase)
- Name
Decarboxylation in C4 pathway
- Note
Occurs in bundle sheath cells, releasing CO2 for Calvin cycle.
- Expression
Malate (or Aspartate) → Pyruvate + CO2 (catalyzed by malic enzyme or aspartate aminotransferase)
- Name
Regeneration of PEP
- Note
Occurs in mesophyll cells.
- Expression
Pyruvate + ATP → PEP + AMP + Π (catalyzed by pyruvate, orthophosphate dikinase)
Prerequisites
Basic understanding of photosynthesis (light and dark reactions).
Knowledge of C3 cycle (Calvin cycle) and the role of RuBisCO.
Familiarity with plant cell structure and enzyme functions.
Common mistakes
Believing C4 plants do not have RuBisCO (they do, but only in bundle sheath cells).
Confusing the location of initial CO2 fixation (mesophyll) with the Calvin cycle (bundle sheath).
Assuming C4 plants are universally more efficient than C3 plants (only in hot, dry conditions due to photorespiration avoidance).
Not understanding that photorespiration is a wasteful process.
Incorrectly identifying the first stable product of the C4 cycle.
Keywords
C4 pathway
Hatch-Slack pathway
Photorespiration
Kranz anatomy
Mesophyll cells
Bundle sheath cells
PEPcase
RuBisCO
Oxaloacetate
Malate
Pyruvate
Calvin cycle
CO2 fixation
Maize
Sugarcane
Practice preview
C4 plants are able to effectively minimize photorespiration primarily due to:…
hard
The primary advantage of the C4 pathway over the C3 pathway in hot and dry environments is that C4 plants:…
medium
The enzyme responsible for the initial carbon dioxide fixation in the mesophyll cells of C4 plants is:…
medium