Essential Cell Biology 5th edition

The Use of Energy by Cells
87
H 2 O
PHOTOSYNTHESIS
CO 2 + H 2 O O 2 + SUGARS
O 2
PLANTS
ALGAE
SOME BACTERIA
ENERGY
OF
SUNLIGHT
CO 2
SUGARS AND
OTHER ORGANIC
MOLECULES
CELL RESPIRATION
SUGARS + O 2 H 2 O + CO 2
MOST
LIVING
ORGANISMS
planet for so long that they have become an essential part of each other’s
environments. The oxygen released by photosynthesis is consumed by
nearly all organisms for the oxidative breakdown of organic molecules.
And some of the CO 2 molecules that today are incorporated into organic
molecules by photosynthesis in a green leaf were released yesterday into
the atmosphere by the respiration of an animal, a fungus, or the plant
itself—or by the burning ECB5 of fossil e3.09/3.09 fuels. Carbon atoms therefore pass
through a huge cycle that involves the entire biosphere—the collection
of living things on Earth—as they move between individual organisms
(Figure 3–10).
Oxidation and Reduction Involve Electron Transfers
The cell does not oxidize organic molecules in one step, as occurs when
organic material is burned in a fire. Through the use of enzyme catalysts,
metabolism directs the molecules through a series of chemical reactions,
few of which actually involve the direct addition of oxygen. Before we
consider these reactions, we need to explain what is meant by oxidation.
Although the term oxidation literally means the addition of oxygen
atoms to a molecule, oxidation is said to occur in any reaction in which
electrons are transferred between atoms. Oxidation, in this sense,
involves the removal of electrons from an atom. Thus, Fe 2+ is oxidized
when it loses an electron to become Fe 3+ . The converse reaction, called
reduction, involves the addition of electrons to an atom. Fe 3+ is reduced
when it gains an electron to become Fe 2+ , and a chlorine atom is
reduced when it gains an electron to become Cl –.
Because the number of electrons is conserved in a chemical reaction
(there is no net loss or gain), oxidation and reduction always occur
simultaneously: that is, if one molecule gains an electron in a reaction
(reduction), a second molecule must lose the electron (oxidation).
O 2
CO 2
USEFUL
CHEMICAL-
BOND
ENERGY
H 2 O
Figure 3–9 Photosynthesis and cell
respiration are complementary processes
in the living world. The left side of the
diagram shows how photosynthesis—
carried out by plants and photosynthetic
microorganisms—uses the energy of
sunlight to produce sugars and other
organic molecules from the carbon
atoms in CO 2 in the atmosphere. In turn,
these molecules serve as food for other
organisms. The right side of the diagram
shows how cell respiration in most
organisms—including plants and other
photosynthetic organisms—uses O 2 to
oxidize food molecules, releasing the same
carbon atoms in the form of CO 2 back to the
atmosphere. In the process, the organisms
obtain the useful chemical-bond energy that
they need to survive.
The first cells on Earth are thought to have
been capable of neither photosynthesis
nor cell respiration (discussed in Chapter
14). However, photosynthesis must have
preceded cell respiration on the Earth,
because there is strong evidence that
billions of years of photosynthesis were
required to release enough O 2 to create an
atmosphere that could support respiration.
CELL RESPIRATION
animals
humus and dissolved
organic matter
CO 2 in atmosphere and water
FOOD
CHAIN
PHOTOSYNTHESIS
plants, algae,
bacteria
sediments and
fossil fuels
Figure 3–10 Carbon atoms cycle
continuously through the biosphere.
Individual carbon atoms are incorporated
into organic molecules of the living world by
the photosynthetic activity of plants, algae,
and bacteria. They then pass to animals
and microorganisms—as well as into
organic material in soil and oceans—and
are ultimately restored to the atmosphere
in the form of CO 2 when organic molecules
are oxidized by cells during respiration
or burned by humans as fossil fuels. In
this diagram, the green arrow denotes an
uptake of CO 2 , whereas the red arrows
indicate CO 2 release.