Biology 12

Chapter 3 Cellular Energy • MHR 81

Other Metabolic Pathways

Glycolysis, the transition reaction, the Krebs cycle,
and oxidative phosphorylation are the key steps in
aerobic cellular respiration. However, it is important
to consider that intermediate compounds produced
in aerobic cellular respiration may be shunted
sideways into other reactions that are not part
of aerobic cellular respiration. For instance,
carbohydrate metabolism in the mitochondrion,
including the oxidation of glucose by aerobic

cellular respiration, produces many molecules

important in cellular processes (see Figure 3.17).

Anabolic reactions take the carbon-based molecules

that result from cellular metabolism and build

them into such complex macromolecules as steroid

hormones, lipids, porphyrins (e.g. hemoglobin),

and proteins.

Although glucose molecules are the main

substrate in cellular respiration, ATP can be

manufactured through the breakdown of proteins

and fats as well. The breakdown of proteins in this

type of reaction is called deaminationwhich

involves the initial removal of an −NH 2 group from

the protein. In fats, β-oxidationremoves two-

carbon acetate units from the carboxyl end of long-

chain fatty acids. This reaction occurs in the outer

mitochondrial membrane where enzymes catalyze

the Coenzyme A reaction to produce acetyl-CoA.

This is the same acetyl-CoA that is produced by the

transition reaction of pyruvate. The acetyl-CoA

enters the Krebs cycle in exactly the same manner.

Some organisms, such as anaerobic bacteria,

make enough ATP using fermentation to survive.

Eukaryotes, however, tend to rely on mitochondria

to produce energy. The mitochondria are more

efficient, using the product of glycolysis —

Athletes train to improve the ability of their muscle cells

to accommodate lactic acid buildup. They do this by using

breathing techniques and “sprinting” exercises that serve

to saturate muscle cells with lactic acid. Such exercises

involve working muscles at a maximum heart rate for about

20 minutes, then resting until the body has recovered and

removed the lactic acid from the blood.

BIO FACT

Try the investigation: Aerobic or Anaerobic: Can You Tell

When the Switch Occurs?

Probeware

P

carbohydrate

cholesterol

nucleic acids

steroid

hormones

porphyrins

(e.g. hemoglobin)

protein

protein

lipid

glucose-6-

GAP

pyruvate

acetyl-CoA

Krebs

cycle

citrate

α-ketoglutarate

succinate

oxaloacetate

alanine

aspartate

pyrimidine

nucleotides

glutamate

glycerol

fatty acids

Figure 3.17Molecules formed in

conjunction with aerobic cellular

respiration