Page 277 - Biology_F5

Biology for Advanced Level Secondary Schools

formed is transported to the chloroplasts, pathway is characterised by four major
and with the help of ATP, it is converted into events: fixation of carbon dioxide in
3-PGA by adding Pi in its carbon number 3. the mesophyll cell, shunting of malate,
regeneration of carbon dioxide acceptor,
and re-fixation of carbon dioxide in the
bundle sheaths.
FOR ONLINE READING ONLY
(a) Fixation of carbon dioxide in the
mesophyll cell
Carbon dioxide fixation occurs in the mesophyll
cells and involves carbondioxide acceptor
known as phosphoenolpyruvate (PEP). The
carbon dioxide is attached to PEP to form
a 4-carbon molecule called oxaloacetate.
This carboxylation reaction is catalysed
by phosphoenolpyruvate carboxylase (PEP
carboxylase) enzyme. In this reaction,
therefore, hydrogen and carbon dioxide
are added to PEP to form oxaloacetate.
The PEP carboxylase has relatively higher
carbon dioxide affinity compared to RuBP
Figure 5.6: Photorespiration pathway carboxylase which is used in carboxylation
in C plants.
C plants 3
4
Oxaloacetate
Photosynthesis in most tropical and PEP + CO PEP carboxylase

2
subtropical plant species, such as maize
and sugarcane yields a 4-carbon compound The oxaloacetate formed is further
called oxaloacetate (oxaloacetic acid) converted into another 4-carbon
after carboxylation, and they are thus compound called malate
referred to as C plants. The C plants NADPH + H + NADP
+
4
4
have 2 major biochemical pathways
(Figure 5.7). The first pathway involves Oxaloacetate Malate
transporting carbon dioxide from the (4c) (4c)
mesophyll to the chloroplast of the bundle (b) Shunting of malate
sheath, a pathway which was described in The shunting commences as malate gets
1966 by Hatch and Slack. This pathway is streamed to chloroplasts of the bundle
therefore called Hatch-Slack pathway in sheath cells through small pores called
honor of these two Australian scientists. plasmodesmata, found in the cell wall. In
The second pathway in C plants is the the chloroplasts, malate dissociates, yielding
4
normal C pathway (Calvin cycle), which hydrogen, carbon dioxide and a 3-carbon
3
commences once the carbon dioxide is acid called pyruvate (pyruvic acid).

released in the bundle sheath cells. C
4

Form Five Student’s Book
270