Biology 12

90 MHR • Unit 1 Metabolic Processes

molecules formed then leave the thylakoid

membrane and enter the stroma, where a series of

enzymes perform synthesis reactions in the Calvin

cycle. The Calvin cycle is named after biochemist

Melvin Calvin. In the late 1940s, Calvin led a

team of researchers to determine the steps of this

synthesis reaction.

Every photosynthetic plant uses the Calvin cycle

to form PGAL. PGAL is then used to synthesize

many different molecules. Using PGAL as the

building block, plants can synthesize amino acids

and fatty acids. Other molecules that can be formed

from PGAL include fructose phosphate, glucose,

sucrose, starch, and cellulose. Although plants

synthesize these molecules, not every plant uses

the same metabolic pathway.

The Calvin cycle has three distinct stages, as

shown in Figure 3.27, on the previous page:

1.Stage 1: carbon fixation

2.Stage 2: reduction

3.Stage 3: re-formation of RuBP (ribulose 1,5

bisphosphate)

These three stages will now be discussed.

Stage 1: Carbon Fixation

Carbon fixationis the initial incorporation of

carbon into organic molecules. To eventually build

complex molecules, such as glucose, plants must

first attach carbon to smaller carbon-containing

molecules. They do this by taking carbon dioxide

from the atmosphere and attaching it to RuBP,

ribulose bisphosphate, as shown in Figure 3.28. A

six-carbon molecule is the product of this reaction,

but this molecule is extremely unstable and

immediately splits into two molecules of three-

carbon PGA (phosphoglycerate). The enzyme RuBP

Figure 3.28In the Calvin cycle, C 3 fixation produces two

three-carbon PGA. The reaction is catalyzed by the enzyme

RuBP carboxylase.

carboxylasecatalyzes this reaction, as shown in

Figure 3.28. This reaction is called C 3 fixation

because it produces two three-carbon molecules of

PGA. This molecule then passes into the next stage

of the Calvin cycle. C 3 fixation is used by plants,

such as rice, wheat, and oats, which occur mainly

in temperate regions.

To form a molecule of glucose (C 6 H 12 O 6 ), six

carbon atoms must be fixed. Figure 3.27 shows

that nine molecules of ATP are required to fix the

three carbon atoms in the PGAL that is available

to be used for glucose production. Therefore,

18 molecules of ATP are needed to fix the six

carbon atoms required to form a glucose molecule.

In addition to carbon fixation, RuBP carboxylase

oxidizes RuBP with O 2 to form CO 2 by a process

called photorespiration. Photorespiration creates

an inefficiency in the carbon fixation process, since

both the oxidation of RuBP and carbon fixation are

catalyzed by the same enzyme — RuBP carboxylase.

Both oxygen and carbon dioxide compete to bind

with RuBP. The Calvin cycle is an ancient process

that developed in an atmosphere with little

free oxygen.

The rate of reactions in the Calvin cycle

increases with temperature to about 25°C. Reaction

rate levels out and declines when temperatures

approach or exceed 37°C. At warmer temperatures,

RuBP carboxylase is mainly involved in oxidizing

RuBP, and very little carbon fixation occurs. Thus,

plants that live in warmer climates have developed

a different approach to fixing carbon. For example,

C 4 fixation is used by plants, such as sugarcane and

corn. In these plants, the Calvin cycle takes place

in bundle-sheath cells, as shown in Figure 3.29.

Plants that use C 4 fixation form the four-carbon

oxaloacetate and malate in parenchyma cells. The

malate moves into the bundle-sheath cells and a

carbon is removed as CO 2. Inside the bundle-

sheath cells, there is a greater concentration of CO 2

and a lower concentration of oxygen than in

parenchyma cells at the surface of the leaf. This

difference in concentration allows CO 2 to have a

greater opportunity to bind with RuBP carboxylase.

As a result, the plant can fix sufficient amounts of

carbon to produce glucose in the Calvin cycle.

In tropical climates, where the temperature often

exceeds 28°C, food crops such as corn and sugarcane

are commonly grown. Crops that use C 3 fixation,

however, do not survive well in tropical climates

because they fix relatively less carbon and form

fewer glucose molecules. Thus, the types of crops

HC

C

O

OH

HC OH

O

PGA

(phosphoglycerate)

RuBP

(Ribulose

1,5 bisphosphate)

RuBP

carboxylase

H 2 C P

H 2 C O P

HC

C

O

OH

O

H 2 C

O−

P

HC

C

O

OH

O

H 2 C

O−

P

CO 2 +H 2 O

+H+

+H+