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Mechanism of C4 Photosynthesis:
A Threshold Concept in Plant Sciences
Lucy Taylor and Keith Macaulay
The Mechanism of C4 Photosynthesis
RuBisCO is inefficient in
the presence of oxygen,
which
reduces
the
efficiency of C3 plants.
C4 plants overcome this by
concentrating CO2 in the
bundle sheath cells around
the leaf veins.
This
involves
spatial
separation of enzymes to
allow a cycle to occur
between the two cell types.
Teacher Perspectives

CO2
Mesophyll cell
OAA
PEP
O2 high
No RuBisCO
Malate
Pyruvate


Malate
Pyruvate
CO2
Calvin Cycle
O2 low
CO2 concentrated
around RuBisCO
Bundle sheath cell
Learner Perspectives
How did you learn about C4?

Why a Threshold Concept?

Transformative
Highlighting
similarities
and
differences between C3, C4 and
CAM photosynthesis helps.
This can be done by filling in a
table
of
similarities
and
differences in supervisions
It is important for students to
learn the biochemical pathway in
order to understand the overall
process

Subject
is
broken
into
biochemical
and
anatomical
adaptations
Links
to
other
types
of
photosynthesis
discussed
in
supervisions
Plants have evolved a complex system to overcome the effects of oxygen
on RuBisCO. Understanding this system and its evolution leads to the
appreciation that plants need to be adaptable to cope with different Is it a threshold concept?
environmental conditions.
 Integrative both from biochemical
 Irreversible
and ecological sides
Although the details of the metabolic pathway are unlikely to be  Probably transformative since an
understanding changes how you
memorised, once the general principles have been appreciated they are
think about photosynthesis in
not difficult to remember.
general
 Integrative
 Probably not irreversible: quite
easy to forget details of the
The evolution of C4 photosynthesis involves changes in biochemistry and
biochemistry and anatomy
leaf anatomy, underpinned by genetic adaptations, which affect the
physiology of the plant. These changes affect how C4 plants survive in
Implications
different ecological niches.
 Supervisions
should integrate
 Troublesome
biochemical
and
anatomical
alterations
The carbon concentrating mechanism involves spatial separation of
 Links
to
the
ecological
biochemical processes, which can be hard to visualise. It may be difficult
implications should be covered in
to understand that the mechanism itself comes at a metabolic cost and
lectures
therefore C4 plants only show increased efficiency at higher light levels.
 These links could be highlighted
using
additional
electronic
 Bounded
resources to link lecture notes
C4 photosynthesis is only found in plants and some algae.
from different lecture sets
“Transforming Perspectives: Technology to Support Teaching and Learning of Threshold Concepts” is an ESRC/EPSRC
Technology Enhanced Learning Project and is part of the Teaching and Learning Research Programme.
It is directed from the Centre for Applied Research into Educational Technologies by Dr. Patrick Carmichael.
Project Contact: Dr. Naomi Irvine, CARET, 16 Mill Lane, Cambridge, CB2 1SB.
Email: 01223 765362 Project Website: http://www.caret.cam.ac.uk/tel