Instant Notes: Plant Biology

photorespiratory cyclewhich are located in peroxisomes (Topic B1) and mito-

chondria (Topic B3). The stages are shown in Fig. 5. Overall, the pathway

consumes oxygen and ATP; equal amounts of NADH are used up in the peroxi-

some and produced in the mitochondrion.

The function of photorespiration is unknown; it may be a consequence of the

structure and reaction mechanism of Rubisco; alternatively, it may be a safety

mechanism to protect the photosystems in conditions where light intensity is

high and carbon dioxide low. Some plants show a modified form of photo-

synthesis termed C4 in which photorespiration does not occur (Topic J3).

J2 – Major reactions of photosynthesis 143

CHLOROPLAST

PEROXISOME

Glutamate

α-Ketoglutarate

MITOCHONDRION

Tetrahydrofolate

Methylene

tetrahydrofolate

Ribulose-1,5-bisphosphate

3-Phosphoglycerate

Glycerate

ADP

AT P

NAD+

NADH

NAD+

NADH

Glycerate

Hydroxypyruvate

Serine

2-Phosphoglycolate

Glycolate

Glycolate

Glyoxylate

Glycine

Glycine Serine

H 2 O

H 2 O 2

H 2 O

+

Glycine

Pi

CO 2 +NH 3

O 2

O 2

Fig. 5. The photorespiratory carbon oxidation cycle.