C h a p t e r 1
Cell Structure and Function
11
Outer membrane
Inner membrane
Electron-transport chain
ATP synthesis
Matrix
ADP ATP
H +
H +
O
2
2 H
2
O
H + H +
H + H + H +
e -
+
Pi
NADH
H +
INTACT MITOCHONDRIUM
Matrix
Outer membrane
Inner membrane
Intermembrane
space
Electron Transport Chain.
At
the completion of the citric acid
cycle, each glucose molecule has
yielded only four new molecules of
ATP (two from glycolysis and two
from the citric acid cycle). In fact,
the principal function of these ear-
lier stages is to make the electrons
(e
–
) from glucose and other food
substrates available for oxidation.
Oxidation of the electrons carried
by NADH and FADH
2
is accom-
plished through a series of enzy-
matically catalyzed reactions in the
mitochondrial electron transport
chain. During these reactions, pro-
tons (H
+
) combine with O
2
to form
water (H
2
O), and large amounts of
energy are released and used to add
a high-energy phosphate bond to
adenosine diphosphate (ADP), con-
verting it to ATP. There is a net yield
of 36 molecules of ATP from 1 mol-
ecule of glucose (2 from glycolysis,
2 from the citric acid cycle, and 32
from the electron transport chain).
In general, the net amount of ATP
formed from each gram of protein
that is metabolized is less than for
glucose, whereas that obtained from
fat is greater (e.g., each 16-carbon
fatty acid molecule yields about 129
molecules of ATP).
3
the cytoplasm and the aerobic pathways in
the mitochondria. The most efficient of these
pathways is the aerobic citric acid cycle and
electron transport chain in the mitochondria.
This pathway, which requires oxygen, produces
carbon dioxide and water as end products
and results in the release of large amounts
of energy that is used to convert adenosine
diphosphate (ADP) to ATP. The glycolytic
pathway in the cytoplasm involves the
breakdown of glucose to form ATP. This pathway
can function without oxygen by producing lactic
acid.
SUMMARY CONCEPTS
■■
Metabolism is the process whereby the
carbohydrates, fats, and proteins from the foods
we eat are broken down and subsequently
converted into the energy needed for cell
function. Energy is stored in the high-energy
phosphate bonds of adenosine triphosphate
(ATP), which serves as the energy currency for
the cell.
■■
Two sites of energy conversion are present
in cells: the anaerobic glycolytic pathway in
