Biology 12

THINKING LAB

Metabolic Rate and

the Structure of Molecules

Background

The molecule used by plants, such as corn and potatoes,

to store energy is called starch. Starch is a large polymer

composed of about 1000 glucose molecules. The starch is

formed through condensation reactions, which link together

individual glucose molecules. The starch molecule may

contain side-branches, as shown in the illustration. Before

a plant can use starch in aerobic cellular respiration, the

starch must be broken down into individual glucose

molecules. Recall that glucose is the molecule that first

enters glycolysis. The rate at which glucose is available to

be used in respiration can affect how quickly the cell will

carry out metabolic processes. In other words, the availability

of glucose can determine a plant’s metabolic rate.

You Try It

1.What reaction is needed to break down a starch

molecule into individual glucose molecules?

A starch molecule

2.If sweet corn contains mainly glucose molecules and

starchy corn contains mainly starch molecules, in which

type of corn would you expect cells to have a slower

metabolic rate? Explain briefly.

3.Discuss how the structure of molecules might affect

metabolic rate.

4.How could you determine if corn is sweet or starchy

without tasting it?

5.What is the energy storage molecule in animal cells?

O O O

O O O

O

O

O

O

O

O

O

O

O

O O

O

O

CH 2

92 MHR • Unit 1 Metabolic Processes

Stage 3: Re-formation of RuBP

Recall from Figure 3.27 and Figure 3.28 that RuBP,

ribulose bisphosphate, is required in the carbon

fixation stage of the Calvin cycle. RuBP is used to

produce PGA, needed for the reduction stage of the

cycle. Because PGAL is needed to reform RuBP, the

majority of PGAL molecules, do not contribute to

glucose production. The Calvin cycle reactions

must occur twice to create one molecule of glucose.

This is because for every three times that the

Calvin cycle reactions occur, five PGAL are used to

re-form three RuBP, ribulose bisphosphate, as

shown in Figure 3.32. Notice from Figure 3.27 that

5 three-carbon PGALs contain the same number of

carbon atoms as 3 five-carbon RuBPs.

Figure 3.32Re-formation of RuBP. As five molecules of

PGAL become three molecules of RuBP, three molecules of

ATP become three molecules of ADP+Pi.

To summarize the synthesis reactions of the

Calvin cycle:

Glucose: The Ultimate Food Source

After glucose is produced in the synthesisreactions,

plant cells can use glucose for glycolysis, followed

by aerobic respiration in the mitochondria. The

products and intermediary molecules of aerobic

respiration provide the carbon-based molecules

necessary to build amino acids, as well as the

precursors to nucleic acids and lipids. However,

there are many other ways that plants use glucose,

for example,

the conversion of glucose to starch,

the formation of cellulose from glucose, and

the conversion of glucose to sucrose.

Stage 1: Carbon fixation, which takes carbon

atoms from atmospheric carbon dioxide

molecules and incorporates these atoms

into organic molecules.

Stage 2: Reduction, which involves the

formation of PGAP and its reduction to PGAL.

Stage 3: Re-formation of RuBP, which uses

most of the PGAL molecules formed in the

reduction stage to produce RuBP. This is then

used to form more PGA in the Calvin cycle.

5 PGAL

3 ATP 3 ADP 3

3 RuBP

+ Pi