Biology 12

Chapter 3 Cellular Energy • MHR 65

ATP molecules. During this stage, the six-carbon
glucose is broken down into molecules of three-
carbon pyruvate. Two pyruvate are produced from
each molecule of glucose. The pyruvate can be used
without oxygen in the process of fermentation, but
no further ATP is produced during this process.
You will learn more about fermentation in section
3.2. If oxygen is present, the pyruvate molecules
enter the mitochondria and the process of aerobic
cellular respiration can occur. Aerobic cellular
respiration is a series of redox reactions that
produce water, carbon dioxide, and additional
ATP molecules.

Figure 3.3Steps of cellular respiration

The next stage in the process is the transition
reaction, also called oxidative decarboxylation. In
this reaction each pyruvate loses a carbon atom, or
is decarboxylated, by the oxidative activity of NAD+.
This reaction changes a three-carbon pyruvate to a
two-carbon acetyl group. This smaller molecule

combines with coenzyme A to form acetyl-CoA,

thus connecting glycolysis to the next stage, the

Krebs cycle.

The Krebs cycle, also known as the citric acid

cycle, is a cyclical metabolic pathway located in

the matrix of a mitochondrion. The Krebs cycle

occurs twice (once for each acetyl-CoA molecule)

to oxidize the products of the transition reaction to

carbon dioxide. Only one ATP molecule results

from one cycle of this metabolic pathway.

The final stage, oxidative phosphorylation,

requires oxygen to produce ATP by chemiosmosis,

the movement of concentrated H+ions through a

special protein complex. Oxidative phosphorylation

relies on the electron transport chain. This is a

series of molecules that are embedded on the inner

membrane of the mitochondrion. The molecules

in the electron transport chain are sequentially

reduced and oxidized to move electrons to a final

step where water is produced.

You will now learn about each stage of cellular

respiration. Each stage is complex. The diagrams in

sections 3.1 and 3.2 offer detailed descriptions of

each stage. Study the diagrams as you read through

the descriptions.

Glycolysis:

Reactions in the Cytoplasm

Glucose is the primary reactant for glycolysis. The

source of glucose may be from either carbohydrates

or from glycogen (a molecule made of many

glucose molecules) stored in muscle and liver cells.

Glycolysis occurs in the cytoplasm of all cells, and

it produces two pyruvate molecules and two ATP

molecules. To accomplish this process, 11 different

enzymes are used. Both prokaryotes (cells without

nuclei) and eukaryotes (cells with nuclei) use

glycolysis in some stage of ATP production. A few

eukaryotes (yeast and mature human red blood

cells) and many prokaryotes (some bacteria) can

survive on the energy produced by glycolysis

alone. However, this amount of energy is not

sufficient for most eukaryotes, which use aerobic

respiration in the mitochondria to increase ATP

production. (Aerobic respiration will be covered in

the next section.) Cellular respiration starts with

glycolysis, which has two main phases:

Glycolysis I: the endothermic activation phase,

which uses ATP

Glycolysis II: the exothermic phase, which

produces ATP molecules and pyruvate

acetyl group

pyruvate

Transition

reaction

Glycolysis

Krebs

cycle

Electron

transport

chain

glucose

Fermentation

ATP

ATP

ATP

cytoplasm mitochondrion

Krebs

cycle

Krebs

cycle

C 6

C 3

C 2

CO 2

O 2

CO 2

H 2 O

e−

e−