Essential Cell Biology 5th edition

Essential Concepts
491
(A)
1 µm
(B)
Figure 14–47 Methanococcus represents
life-forms that might have existed
early in Earth’s history. (A) Scanning
electron micrograph showing individual
Methanococcus cells. These deep-sea
archaea use the hydrogen gas (H 2 ) that
bubbles from deep-sea vents (B) as the
source of reducing power to generate
energy via chemiosmotic coupling.
(A, from C.B. Park & D.S. Clark, Appl.
Environ. Microbiol. 68:1458–1463, 2002.
With permission from the American Society
for Microbiology; B, National Oceanic and
Atmospheric Administration’s Pacific Marine
Environmental Laboratory Vents Program.)
of the energy that Methanococcus requires for both processes is derived
from the transfer of electrons from H 2 to CO 2 , with the release of large
amounts of methane (CH 4 ) as ECB4 a waste e14.46/14.47 product (thus producing natural
gas and giving the organism its name). Part of this electron transfer
occurs in the plasma membrane and results in the pumping of protons
(H + ) across it. The resulting electrochemical proton gradient drives an
ATP synthase in the same membrane to make ATP.
The fact that such chemiosmotic coupling exists in an organism like
Methanococcus suggests that the storage of energy in a proton gradient
derived from electron transport is an extremely ancient process. Thus,
chemiosmotic coupling may have fueled the evolution of nearly all lifeforms
on Earth.
ESSENTIAL CONCEPTS
• Mitochondria, chloroplasts, and many prokaryotes generate energy
by a membrane-based mechanism known as chemiosmotic coupling,
which involves using an electrochemical proton gradient to drive the
synthesis of ATP.
• In animal cells, mitochondria produce most of the ATP, using energy
derived from the oxidation of sugars and fatty acids.
• Mitochondria have an inner and an outer membrane. The inner membrane
encloses the mitochondrial matrix; there, the citric acid cycle
produces large amounts of NADH and FADH 2 from the oxidation of
acetyl CoA derived from sugars and fats.
• In the inner mitochondrial membrane, high-energy electrons donated
by NADH and FADH 2 move along an electron-transport chain and
eventually combine with molecular oxygen (O 2 ) to form water.
• Much of the energy released by electron transfers along the electrontransport
chain is harnessed to pump protons (H + ) out of the matrix,
creating an electrochemical proton gradient. The proton pumping is
carried out by three large respiratory enzyme complexes embedded
in the inner membrane.
• The electrochemical proton gradient across the inner mitochondrial
membrane is harnessed to make ATP when protons move back into
the matrix through an ATP synthase located in the inner membrane.
• The electrochemical proton gradient also drives the active transport
of selected metabolites into and out of the mitochondrial matrix.
• During photosynthesis in chloroplasts and photosynthetic bacteria,
the energy of sunlight is captured by chlorophyll molecules embedded
in large protein complexes known as photosystems; in plants,
these photosystems are located in the thylakoid membranes of chloroplasts
in leaf cells.