HUMAN BIOLOGY

Figure A.5 Glycolysis, first stage of the main energy-releasing pathways. The reaction steps proceed inside the cytoplasm of every
living prokaryotic and eukaryotic cell. In this example, glucose is the starting material. By the time the reactions end, two pyruvate, two
NADH, and four ATP have been produced. Cells invest two ATP to start glycolysis, however, so the net energy yield of glycolysis is two ATP.

Depending on the type of cell and on environmental conditions, the pyruvate may enter the second set of reactions of the aerobic pathway,
which includes the Krebs cycle. Or it may be used in other reactions, such as a fermentation pathway. (© Cengage Learning)

2 ATP produced

by substrate-level

phosphorylation

2 ADP

2 NAD+ + 2 Pi

2 reduced coenzymes

NADH

ADP

ADP

glucose

glucose-6-phosphate

fructose-1,6-bisphosphate

2 PEP

2 pyruvate

to second stage

2 PGA

2 PGAL

2 ADP

2 ATP produced

by substrate-level

phosphorylation

Net 2 ATP + 2 NADH

Glycolysis

ATP

ATP

ATP

ATP

P

P P

P

P

P P

PP

P

P

A An enzyme transfers a phosphate

group from ATP to glucose, forming

glucose-6-phosphate.

b A phosphate group from a second

ATP is transferred to the glucose-

6-phosphate. The resulting molecule is

unstable, and it splits into two three-

carbon molecules. The molecules are

interconvertible, so we will call them

both PGAL (phosphoglyceraldehyde).

So far, two ATP have been invested in

the reactions.

E Enzymes transfer a phosphate

group from each of two intermediates

to ADP. Two more ATP have formed by

substrate-level phosphorylation.

Two molecules of pyruvate form at this

last reaction step.

F Summing up, glycolysis yields two

NADH, two ATP (net), and two pyruvate

for each glucose molecule.

Depending on the type of cell and envi-

ronmental conditions, the pyruvate may

enter the second stage of aerobic res-

piration or it may be used in other ways,

such as in fermentation.

ATP-Requiring Steps

ATP-Generating Steps

c Enzymes attach a phosphate to the

two PGAL, and transfer two electrons

and a hydrogen ion from each PGAL

to NAD^1. Two PGA (phosphoglycerate)

and two NADH are the result.

d Enzymes transfer a phosphate

group from each PGA to ADP. Thus,

two ATP have formed by substrate-

level phosphorylation.

The original energy investment of

two ATP has now been recovered.

A-4 Appendix i

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