Download 2l.I Metaholism 2t.2 Photosynthesis

704
23 Energyand Life
CHAPTER
2l.I Metaholism
AIM: To differentioteomong metobolism,cotobolism,ond
onobolism.
Metabolism is all the chemical
reactions that occur in living
organisms.
Metabolism
meta (Greek): beyond
ballein (Greek): to cast or throw
Catabolism
cata(Greek): down
Anabolism
ana(Greek): up
There are three important terms that describe the chemical reactions in
living organisms: metabolism, catabolism, and anabolism. Metabolism is
all the chemical reactions that occur in ltuing organisms.Virtually all metabolic reactions are catalyzed by enzymes. Catabolism is a part of metabolism-the breakdown of molecules by an organism. Anabolism ls another
part of metabolism-the synthesisof moleculesfor cell growth and multiplication. Nutrients are also converted to their storage forms 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 reactionsused 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.
2t.2 Photosynthesis
AIMS: Towrite o chemicolequotionfor photosynthesis
indicotingthe energy-richond energy'Poorcorbon
compounds.Todistinguishamong chloroplosts,
thyolkoids,ond chlorophyll.
Focus
The energy of sunlight is
harnessed by plants in photoslmthesis.
Photosl'nthesis
photos (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.
\,Vheredo sugars,fats, and amino acids originate?Carbon dioxide in the
Earth'satmosphere is the ultimate source of all carbon compounds. Carbon
dioxide is an energy-poor compound becauseit cannot be oxidized further.
Animal cells discard it as a waste product. Green plants, blue-green algae,
and certain bacteria, however, conduct photosynthesis-harness the
energyof sunlight, conuertit to usefulchemical energy,and usethat energyto
synthesizeglucose, a nlore reduced molecule, from carbon dioxide. The
23.2 Photosynthesis 705
processesthat occur in photosynthesis are often summarized by a single
equation.
6CO2 +
Carbon
dioxide
(more oxidized)
6HrO -!4!:!9q
Water
c6H12o6 +
Glucose
(more reduced)
6C,,
Oxygen
Reoxidation of glucose back to carbon dioxide and water releasesthe
energy expended in forming the chemical bonds of the glucose molecule.
c6H12o6
Glucose
(more reduced)
Figure
25.1
Transmission
electronmicrograph
of a chloroplast.
Porphlrin
ring
CH,
tl
CH
F]
H
CH.
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CH.
tzc",
zzc'.
lzc"
rzc
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\
/
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-CHz
CH:
? cI t_C\/,cCH:
/c-N.. ,'N-C'
tt\'t\
;,
H
,rMc \ t _C//CH
,2
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,'C:T
H
N-C.
.c--CH.
, ,,/z\c/\/ -z
C
C
C
,a t l I
H
t 'lIn c r c-c:o
CHz
QH, I
I
COOCHs
oocH3
CC
/ /"\\ / /,t\
/1 f
CHz
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)-c:o
o-c:
I
H-C_H
H-C
I
_-E
C-CH.
ur13
I
H-C-H
I
H-C-H
I
H-C_H
I
-CH:
H-C-CH.
+
6o,
Oxygen
+
6COz
+
Carbon
dioxide
(more oxidized)
6HzO +
Energy
Water
Glucose is an energy-rich molecule and a potential source of chemical
energy for any organism that obtains it.
The equation for photoslmthesis is deceptively simple. Molecules of
carbon dioxide and water do not simply bang together in sunlight and pop
out molecules of glucose and oxygen. To carry out photosynthesis, Ceri
must be able to absorb light energy and convert it to chemical energy.For
this purpose,photosynthetic eukaryotic cells- thoseof green plants are the
most familiar example-contain special organelles, rhe chloroplasts,
shor,rmin Figure 23.1. chloroplasts can carry out this conversion. Located
within the chloroplasts are a large number of connected disks called
thyalkoids. The thyalkoids contain the molecules that constitute thelight
system of photosynthesis.Responsibility for trapping light energy and converting it to useful chemical energy rests with the light system. chlorophyll-the pigment that giues green plants their color-is a molecule of
chief importance to the light system. Its molecular structure is depicted in
Figure 23.2. chlorophyll initially absorbs light energy and passesit on to
other molecules of the light system. The light systems of prokaryotic cells
that are capable of conducting photosynthesis-certain bacteria and the
blue-green algae-are located on the interior of the cell mernbranes.
The importance of light to life on Earth through photoslnthesis in
plants cannot be overemphasized.However, researchersare beginning to
find that healthyhuman beings respond to light inways unimaginabte only
a few yearsago.The cure rate of certain cancersmay be improved by a treatment involving light. SeeA closer Look phototherapies, for a discussion of
some connections between light and health.
I
H
H-- C - H
I
..
H
FI-C-H
-
I
H
H-- C - H
I
-CH=
F{-C-CH3
I
,.
PRAGTIGE
EXERCISE
2I.I
(a) Identify the structural feature that is common to vitamin 812 (see
Figure 21.3on page 650),hemoglobin, and chlorophyll. (b) lVhat metals
are essentialto each of these molecules?
T-C-H
I
rI-C-H
I
.I-C-H
I
H{-c-cH3
-
I
H]-C-H
-
I
H
Figure25.2(left)
Themolecular
structure
of chlorophyll.
Themagnesium
ion is essential
for chlorophyll'slight-trapping
function,Noticethe similarity
betweenthe structure
of heme
(seeSec.I8.l l) andthe porphyrin
ringof chlorophyll.
706
23 Energyand Life
CHAPTER
Phototherapies
You may recall that the transformation of steroids
in the skin by sunlight is important in the formation of vitamin D2, but medical specialists and
researchersare just beginning to recognizemany
of the human body's responsesto light. Several
conditions, including some forms of disturbed
sleep, "'1etlag," and seasonal affective disorder
(SAD),appearto be relatedto the biological cycles
of wakefulness and sleep called the circadian
rhythms. A small portion of the hypothalamus, a
symptomsof
the distressing
oftenrelieves
Phototherapy
region at the base of the brain, supervises our
disorder.
affective
SAD,seasonil
bodies' clocks, telling us when it is time to eat,
sleep, and wake up. This small portion of the
than the tired, run-dornrnfeeling many people
hlpothalamus, called the suprachiasmatic nuhave during the winter months. Preliminary
cleus, responds to light and darkness to affect
resultsindicate that light treatmentalso may be
body temperature fluctuations, hormone release, effective against some nonseasonal forms of
blood pressure, heart rate, and the sleep-wake depression.Light treatments are also important
cycle. Light treatment is proving useful in the
in the treatment ofjaundice, oryellowing, in newtreatment of severalhuman conditions related to
;;r;;, ;r ;" *lr t.. i" a cr*"t l-oof, uypetthe circadian rhlthm. More than half of all Amerbilirubinemia, in Chapter 25.
icans over age 65 suffer from disturbed sleep,
Recent studies indicate that synchronizing
waking too early in the morning and becoming
cancer treatment with the body's internal
sleepy early in the evening. Experts suggestthat
rhyhms gives chemotherapy drugs a stronger
this sleep disorder is causedby a speedingup of
effect, perhaps as much as doubling their power
the body's circadian rhi,thms in older people. To
to fight tumors. Some chemotherapy drugs work
combat this troublesome shift, researchersare
better at night; others seem more potent when
exposingolder patients to bright light in the early
given during the day. Light is also being pressed
evenlng to delay their sleep-wake cycles.]et lag
into serviceagainstcertain blood cell cancersby a
similarly affectspeople who fly by jet acrossintertechnology called photopheresls.In photopherenational time zones and shift workers, such as
sis,mixtures of normal and cancerousLrloodcells
nurses, who rotate shifts. The body's sleep-wake are removed from the patient and treated with a
cycle is set to have night during the day in the new
drug such as psoralen.
location or on the new shift. A person respondsby
feeling exhausted during the day and wide awake
o
at night until the cycle is reset. This usually happens after a few days in the new location, but
researchers suggest it can be speeded up by
spending the first 2 days after a trip in sunlight.
Psoralen
Exposureto bright light is also said to exert a posThe psoralen is selectivelyabsorbed by the canitive effect on SAD (seefigure), the name given to
cerous cells. The interaction of light with the
a form of depression experiencedby some peopsoralen cross-linksthe DNA in the cancerous
ple. SAD people may feel fine in summer but
cells, making it impossible for the cells to reprobecome depressed during the winter months,
duce. The treated blood is then returned to the
when the days are short and sunshine may be
scarce.SAD is an emotional illness more severe patient.
23.3 The Energyand Carbon Cycle
707
2t.t Theenergyond corhon cycle
AIM: To describethe energyond corboncycle.
Focus
Photosynthesis is a key step in
the energy and carbon cycle.
Environmentalists are extremely
troubled over the rapid destruction of vast areas of the world's
rain forests.What do you think
are some of their concerns?Do
you share these concerns?
Life depends on the energyand carbon cycle(Fig.23.3). In the energy arrd
carbon cycle, the photosynthetic organisms in the Earth'sforestsand oceans
produce glucoseand use it as a sourceof chemical energy and to build the
carbon skeletonsof carbohydrates,fats, amino acids, aid other biological
molecules.Animals obtain these substancesby eating plants, by eatinginimals that eat plants, or by a combination of both.
Animals and plants in need of energy unleash the energy stored in the
chemical bonds of these carbon nutrients by oxidizing themback to carbon
dioxide and water. The energy is used for the work that plant and animal
cells must do, and the carbon dioxide and water can be recycled in photosynthesis.Plant life on Earth could probablysurvive without animals. However, animal life could never survive without plants, since without photosynthesis there would be no new supply of the crucial carbon compounds
needed by animals for energy production.
.., PRACT|CE
EXERCTSE
25.2
::
Explain why our existencedepends on photosynthesis.
'"*tt'*#**u
err"rgyno-
Water
Atmospheric CO2
(oxidized carbon)
*:.=+
\g
"i" \N*RdosYnthesis
rffi
*
€
Glucose
(reduced carbon)
€.€=
e
ft$
Figure25.5
The energyand carbonrycle.
oxidationofglucoseand
othercarboncompounds
byplantsandanimals