Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Still inside the mesophyll cells, the oxaloacetate is
converted to malate, and this is passed on to the bundle
sheath cells. Now the carbon dioxide is removed from the
malate molecules and delivered to RuBP by rubisco in the
normal way. The light independent reaction then proceeds
as usual.
Enzymes in C4 plants generally have higher optimum
temperatures than those in C3 plants. This is an adaptation
to growing in hot climates. For example, in one study
it was found that in amaranth, which is a C4 plant, the
optimum temperature for the activity of PEP carboxylase is
around 45 °C. If the temperature drops to 15 °C, the enzyme
loses around 70% of its activity. By contrast, the same
enzyme in peas, which are C3 plants, was found to have an
optimum temperature of around 30 °C and could continue
to work at much lower temperatures than in amaranth.
light
carbon dioxide
photosynthesis in ring
of mesophyll cells
Figure 13.11 Photomicrograph of a section through a leaf of
maize (× 125).
PEP C
light
3
dependent
reactions
oxaloacetate
C 4
malate C 4
294
pyruvate C 3
Calvin
cycle
carbon
dioxide
photosynthesis in
bundle sheath cells
sugars
Figure 13.12 Photomicrograph of a section through a leaf of
sugar cane (× 120).
Figure 13.14 C4 photosynthesis.
upper epidermis
mesophyll
ring of mesophyll cells
This tight ring of specialised mesophyll
cells excludes air from the cells inside
the ring. The cytoplasm fixes carbon
dioxide. The chloroplasts capture light
and carry out the light dependent
reactions but not the Calvin cycle.
lower epidermis
bundle sheath cells
The bundle sheath cells carry out the Calvin
cycle but not the light dependent reactions.
No air gets to these cells, and they get
carbon dioxide from the mesophyll cells.
Figure 13.13 Tissues surrounding a vascular bundle of a C4 leaf.