Biology Now, 2e

Rock Eaters ■ 89

e– e–

e– e– e–

Glucose

Pyruvate

To Krebs cycle

To oxidative phosphorylation

NADH

NAD+

NADH

NAD+

ADP+

ATP

ATP

ATP

1

1

Glycolysis occurs in

cell cytoplasm (outside

mitochondrion).

3 Oxidative

phosphorylation occurs

in inner mitochondrial

membrane.

2 Krebs cycle, or citric

acid cycle, occurs in

mitochondrial matrix.

H+ H

+ H+ H+

H+

H+

H 2 O

O 2

H+

H+

H+

H+

Glucose in

Carbon

dioxide out

Mitochondrial

matrix

Inner

mitochondrial

membrane

Cytoplasm

Energy

(ATP, etc)

Water out

Oxygen in

P

ADP

P

3

H+

ATP

synthase

FADH 2

FAD

NADH

NAD+

ATP

ADP+P

2 Pyruvate

CO 2

Hydrogen ions build

up and move through

the ATP synthase,

fueling ATP synthesis.

Protein complexes serve

as proton (H+) pumps

and electron carriers.

aElectrons attain a high

energy state as they move

along the electron transport

chain (blue arrows).

b c

Figure 5.11

Cellular respiration is highly efficient in eukaryotes

In glycolysis, each six-carbon glucose molecule is converted into two molecules of pyruvate. The next two stages of cellular

respiration require oxygen. The Krebs cycle (bottom left) releases carbon dioxide and generates high-energy molecules. Oxidative

phosphorylation (bottom right), the last stage in cellular respiration, produces more ATP than any other metabolic pathway. M

Q1: What are the products of cellular respiration?

Q2: Considering the inputs and products of each process, why is cellular respiration considered the reciprocal process to

photosynthesis?

Q3: Which of the three stages of cellular respiration—glycolysis, the Krebs cycle, or oxidative phosphorylation—could

organisms have used 4 billion years ago, before photosynthesis by cyanobacteria released oxygen into the atmosphere?