Biology 12

Chapter 3 Cellular Energy • MHR 91

phosphoenol-

pyruvate (PEP)

CO 2

CO 2

CCC

CCC

CCC

oxaloacetate

CCCC

CCCC

CCCC

malate

malate

Calvin

cycle

sugar

pyruvate

pyruvate

AMP

AT P

parenchyma cell

bundle-

sheath

cell

Figure 3.29Carbon fixation in C 4 plants

Figure 3.30Wheat (A), and corn plants (B) have

adapted in different ways to their climates to

circumvent the problem posed by active-site

competition in the fixation of carbon dioxide.

A

B

that can be grown in warmer climates are limited
mainly to plants that use C 4 carbon fixation.

Stage 2: Reduction

In the second stage of the Calvin cycle, the stroma
performs the necessary enzymatic reactions that
reduce PGA to form PGAL. This happens in two
stages. First, ATP molecules donate phosphate

groups to the PGA molecules, converting them to

bisphosphoglycerate, or PGAPmolecules (see

Figure 3.31). Secondly, an NADPH molecule, which

was produced during the photoreactions, donates

a hydrogen ion and two electrons to PGAP. This

reduces PGAP to glyceraldehyde phosphate, or

PGAL — the building block for anabolic processes

including the synthesis of glucose. The oxidized

NADP+can return to the thylakoid membrane to

be reduced again.

Figure 3.31During the reduction stage, PGAP is reduced to

become PGAL.

ATP ADP+

PGA PGAP PGAL

NADPH NADP++H+

Pi

http://www.mcgrawhill.ca/links/biology12
Deserts plants, such as cacti and aloe vera, use a third method
of carbon fixation called CAM (crassulacean-acid metabolism)
fixation. Why do desert plants require a different method of
carbon fixation? What processes are involved in CAM fixation?
To find out more about CAM fixation, go to the web site above,
and click on Web Links. Prepare an illustrated information
handout that explains the process of carbon fixation in desert-
dwelling species of plants.

WEB LINK