The C4 Pathway: Kranz Anatomy and the Hatch and Slack Pathway
Plants of dry tropical regions face intense light, high leaf temperatures, and frequent stomatal closure. The C4 pathway is their biochemical answer: it does not replace the Calvin cycle but protects and feeds it by concentrating carbon dioxide around RuBisCO.
This quiz tests your grasp of the key ideas from Lesson 13:
- Why C4 plants are adapted to dry tropical regions and how they differ from C3 plants
- The meaning and structure of Kranz anatomy (wreath-like bundle sheath cells)
- The Hatch and Slack Pathway: PEP as primary acceptor, PEPcase in mesophyll, OAA formation, and the return of a 3-carbon molecule
- Why C4 plants avoid photorespiration and achieve greater biomass productivity
Work carefully through each question and read the explanations — they reinforce the logic that links anatomy to enzyme distribution and environmental response.
Q1. Plants possessing the C4 pathway are typically adapted to which type of environment?
The lesson states that plants adapted to dry tropical regions have the C4 pathway, where intense light, high temperatures and water conservation shape their carbon fixation strategy.
Q2. In C4 plants, what is the first ##\ce{CO2}## fixation product?
C4 plants have the C4 acid oxaloacetic acid (OAA) as the first ##\ce{CO2}## fixation product, formed in the mesophyll cells.
Q3. Although C4 plants form OAA first, which pathway remains their main biosynthetic route?
The lesson emphasises that C4 plants use the C3 pathway or Calvin cycle as the main biosynthetic pathway; the C4 route only feeds ##\ce{CO2}## into it.
Q4. The term 'Kranz' in Kranz anatomy means:
'Kranz' means 'wreath', reflecting the wreath-like arrangement of bundle sheath cells around the vascular bundles in C4 leaves.
Q5. Which structural feature is characteristic of bundle sheath cells in C4 plants?
Bundle sheath cells of C4 plants have a large number of chloroplasts, thick walls impervious to gaseous exchange, and no intercellular spaces.
Q6. Which pair of plants is suggested in the lesson for observing Kranz anatomy in leaf sections?
The lesson suggests cutting sections of leaves of C4 plants such as maize or sorghum to observe Kranz anatomy and mesophyll distribution.
Q7. What is the primary ##\ce{CO2}## acceptor in the Hatch and Slack Pathway?
The primary ##\ce{CO2}## acceptor is the 3-carbon molecule phosphoenol pyruvate (PEP), present in the mesophyll cells.
Q8. Which enzyme is responsible for the initial ##\ce{CO2}## fixation in mesophyll cells of C4 plants?
The enzyme responsible for this fixation is PEP carboxylase or PEPcase, which fixes ##\ce{CO2}## onto PEP to form OAA in the mesophyll cells.
Q9. Which enzyme is notably absent from the mesophyll cells of C4 plants?
The lesson stresses that mesophyll cells of C4 plants lack the RuBisCO enzyme; RuBisCO is confined to the bundle sheath cells.
Q10. In the Hatch and Slack Pathway, where is OAA formed?
The C4 acid OAA is formed in the mesophyll cells, where PEPcase fixes ##\ce{CO2}## onto PEP.
Q11. After OAA is formed, what happens next in the Hatch and Slack Pathway?
OAA forms other 4-carbon compounds such as malic acid or aspartic acid in the mesophyll, which are then transported to the bundle sheath cells.
Q12. What happens to the C4 acids inside the bundle sheath cells?
In the bundle sheath cells the C4 acids are broken down to release ##\ce{CO2}## and a 3-carbon molecule; the ##\ce{CO2}## then enters the Calvin cycle.
Q13. Why do C4 plants lack photorespiration?
The breakdown of C4 acids in bundle sheath cells raises the intracellular ##\ce{CO2}## concentration, ensuring RuBisCO acts as a carboxylase and minimising its oxygenase activity.
Q14. Which set of features correctly describes C4 plants as listed in the lesson?
The lesson lists these as the distinguishing features of C4 plants: special leaf anatomy, tolerance to higher temperatures, response to high light intensities, lack of photorespiration and greater productivity of biomass.
Q15. In C4 plants, where does the Calvin cycle take place?
Unlike C3 plants, where the Calvin pathway occurs in all mesophyll cells, in C4 plants it takes place only in the bundle sheath cells.