HUMAN BIOLOGY

Figure A.8 Summary of the harvest from the energy-releasing pathway of aerobic respiration. Commonly, thirty-six ATP form for
each glucose molecule that enters the pathway. But the net yield varies according to shifting concentrations of reactants, intermediates,
and end products of the reactions. It also varies among different types of cells.

Cells differ in how they use the NADH from glycolysis, which cannot enter mitochondria. At the outer mitochondrial membrane, these
NADH give up electrons and hydrogen to transport proteins, which shuttle the electrons and hydrogen across the membrane. NAD^1 or
FAD already inside the mitochondrion accept them, thus forming NADH or FADH 2.

Any NADH inside the mitochondrion delivers electrons to the highest possible entry point into a transport system. When it does, enough
H^1 is pumped across the inner membrane to make three ATP. By contrast, any FADH 2 delivers them to a lower entry point. Fewer hydrogen
ions can be pumped, so only two ATP can form.

In liver, heart, and kidney cells, for example, electrons and hydrogen from glycolysis enter the highest entry point of transport systems,
so the energy harvest is thirty-eight ATP. More commonly, as in skeletal muscle and brain cells, they are transferred to FAD—so the
harvest is thirty-six ATP. (© Cengage Learning)

Electron Transfer

Phosphorylation

glucose

2 pyruvate

2 acetyl–CoA

2 NADH

6 NADH

2 FADH 2

4 CO 2

oxygen

2 CO 2

(2 net)

cytoplasm

outer membrane

intermembrane space

inner membrane

matrix

2

32

4

2

2 NADH

2 NADH

2 NAD+

e–

e–

Glycolysis

Krebs

Cycle

ATP

ATP

ATP

ATP

ATP

ATP

ATP

H+H+ H+H+H+H+

A First stage: Glucose is
converted to two
pyruvate; two NADH and
four ATP form. An energy
investment of two ATP
began the reactions, so
the net yield is two ATP.

B Second stage:

ten more

coenzymes

accept electrons

and hydrogen ions

during the

second-stage

reactions. All six

carbons of

glucose leave the

cell (as six O 2 ),

and two ATP form.

C Third stage:
Coenzymes that
were reduced in the
first two stages give
up electrons and
hydrogen ions to
electron transfer
chains. Energy lost
by the electrons as
they flow through
the chains is used
to move H+ across
the membrane. The
resulting gradient
causes H+ to flow through
ATP synthases, driving
synthesis of ATP.

Summary of glycolysis and aerobic

cellular respiration

Appendix i A-7

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