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Energy snd Life
Sourcesond Uses of EnergYin
Living Orgonisms
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energy.
Liferequires
702
Chapterlntroduction
CHAPTER OUTLINE
CnserruPotNr:A mysterious
fatigue
23.1 Metabolism
23.2 Photosynthesis
A Closrn Loor<:Phototherapies
23.3 The energyand carbon cycle
2 3 . 4 A d e n o s i n et r i p h o s p h a t e
2 3 . 5 C e l l u l a re n e r g e t i c s
23.6 Oxidativephosphorylation
A ClosenLoor<:Mitochondria
A ClosenLoor: Oxygen,Disease,
and Aging
Follow-upro rHr Cnsr rruPorlr:
A mysteriousfatigue
23.7 Cellulaw
r ork
70:'
pnergy and life-the two terms are practically synonl.rnous. We
r ,may speak of someone who has "boundless energy" or who is
"glowing with energy."And we are not far wrong, because all living
organismsneed ample energyto maintain their vital functions. This
chapter discusseshow the cells of living creaturesproduce and use
energy.Sometimeswe can better understand and appreciatewhat
we have by seeingsomeonewho is lacking our possessionsor qualities. Our Casein Point concernsa person who lacked energy.
Cnsrlt Portr:A mysterious
fatigue
Emily had been plagued by fatigue since
buth Numerous tests eliminated rhe
@ =
g = AJ # posslfrillty ot anemra or other causes of
WL*;*e
fatigue. Puzzled,,her doctor referred her
to a nearby medical research center for further evaluation. After exlensive testing, the medical researchers pinpointed the source of Emity's fatigue as a
defect in her ability to generate the central molecule
in the transmission oI energy. \Alhatis this molecule,
and what defect rnterfered. with its formation? We
wrll learn the answers to these questions in Section
23.6.
;
Extensiveexaminationand testingallowed doctorsto
diagnosethe causeof Emily'sfatigue.
704
23 Energyand Life
CHAPTER
2il.l Metobolism
AIM: To differentioteomongmetobolism,cotobolism,ond
onobolism.
Focus
Metabolism is all the chemical
reactions that occur in living
organisms.
Metabolism
meta(Greek): beyond
ballein (Greek): to cast or throw
Catabolism
cnta(Greek\
dorvn
Anabolism
ana(Greek\
up
There are three important terms that describe the chemical reactions in
living organisms: metabolism, catabolism, and anabollsm. Metabolism ls
all the chemical reactionsthat occur in liuing organisms.Yirfiially all metabolic reactions are catalyzedby enzyrnes.Catabolism is a part of metabolism-the breakdownof moleculesby an organism. Anabolism ls another
part of metabolism-the synthesisof moleculesfor cell growth and multiplication. Nutrients are also converted to their storageforms by anabolic
processes.Conversionoffatty acids to triglyceridesfor storagein fatty tissue is one example.Conversionof glucoseto glycogenfor storagein liver
and muscle cells is another.
Anabolism and catabolism are quite distinct from each other. Cells
usually employ different chemical reactions for the breakdown and synthesis of the same molecule. The reactions used to synthesizeglucose,for
example, are not the reverse of the reactions used to degrade it. Apart
from being chemically separated, catabolic and anabolic reactions are
frequently separated physically. Many important catabolic reactions
occur in the mitochondria, whereas many anabolic reactions occur in
the cytoplasm. The chemical and physical separation of anabolism from
catabolism enablescells to regulate metabolism to make it responsiveto
current needs.
21,2 Photosynthesis
AIMS: To write o chemicolequotionfor photosynthesis
indicotingthe energy-richond energy-Poorcorbon
compounds.To distinguishomong chloroplosts,
thyolkoids,and chlorophyll.
Focus
The energy of sunlight is
harnessed by plants in photoslmthesis.
Photosynthesis
Tthotos(Greek): light
synthesis(Greek): to place
together
Energyproduction by cells involvesthe catabolism of carbon compounds
that serve as nutrients-mainly sugars, fats, and amino acids. Oxidation
reactions are generally energy-producing. Oxidation reactions that are part
of cellular catabolism releasethe energy stored in the chemical bonds of
nutrient molecules, making it available to perform the work that cells must
do to stay alive.
Where do sugars,fats, and amino acids originate?Carbon dioxide in the
Earth'satmosphereis the ultimate sourceof all carbon compounds.Carbon
dioxide is an energy-poorcompound becauseit cannot be oxidizedfurther.
Animal cells discard it as a waste product. Greenplants, blue-greenalgae,
and certain bacteria, however, conduct photosynthesis-harness the
energyof sunlight, conuertit to usefulchemical energy,and usethat energyto
synthesize glucose, a more reduced molecule, from carbon dioxide. The