Biology (Holt)

Section 3 Cellular Respiration

104 CHAPTER 5Photosynthesis and Cellular Respiration

Cellular Energy

Most of the foods we eat contain usable energy. Much of the energy

in a hamburger, for example, is stored in proteins, carbohydrates,

and fats. But before you can use that energy, it is transferred to ATP.

Like in most organisms, your cells transfer the energy in organic

compounds, especially glucose, to ATP through a process called

cellular respiration. Oxygen in the air you breathe makes the pro-

duction of ATP more efficient, although some ATP is made without

oxygen. Metabolic processes that require oxygen are called

(ehr OH bihk). Metabolic processes that do not require oxygen are

called (AN ehr oh bihk),meaning “without air.”

The Stages of Cellular Respiration

Cellular respiration is the process cells use to harvest the energy in

organic compounds, particularly glucose. The breakdown of glucose

during cellular respiration can be summarized by the following

equation:

C 6 H 12 O 6 6O 2 → 6CO 2  6H 2 O energy

As Figure 10shows, cellular respiration occurs in two stages:

Stage 1 Glucose is converted to pyruvate (PIE roo vayt),producing

a small amount of ATP and NADH.

Stage 2 When oxygen is present, pyruvate and NADH are used

to make a large amount of ATP. This process is called aero-

bic respiration. Aerobic respiration

occurs in the mitochondria of eukary-

otic cells and in the cell membrane of

prokaryotic cells. When oxygen is not

present, pyruvate is converted to either

lactate (LAK tayt)or ethanol (ethyl alco-

hol) and carbon dioxide.

The equation above does not show

howcellular respiration occurs. It simply

shows that the complete enzyme-assisted

breakdown of a glucose molecule uses

six oxygen molecules and forms six

carbon dioxide molecules, six water

molecules, and ATP. Aerobic respiration

produces most of the ATP made by cells.

Intermediate products of aerobic respi-

ration form the organic compounds that

help build and maintain cells.

anaerobic

aerobic

Objectives

Summarizehow glucose is

broken down in the first

stage of cellular respiration.

Describehow ATP is made

in the second stage of cellu-

lar respiration.

Identifythe role of fermen-

tation in the second stage of

cellular respiration.

Evaluatethe importance

of oxygen in aerobic

respiration.

Key Terms

aerobic

anaerobic

glycolysis

NADH

Krebs cycle

FADH 2

fermentation

enzymes

oxygen

gas

glucose carbon

dioxide

water ATP

Glucose

Stage 1

Stage 2

Aerobic

(with O 2 )

Anaerobic

(without O 2 )

Pyruvate

Ethanol

and CO 2 , or

lactate

AT P

AT P

ADP

Mitochondrion

NAD+

NADH

1. First, glucose is
   broken down to
   pyruvate.


2. Then, either aerobic
   respiration or anaerobic
   processes occur.

Cellular respiration occurs

in two stages.

Figure 10

Cellular respiration

4B

4B

4B