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CONCEPruAL LIFE SCIENCE
Energy Capture by Plants
lNTRODUCTION
Green plants produce food for humans and other animals by means ofthe process
ofphotosynthesis. Without green plants, life as we know it would soon cease to exist.
Green plants absorb light energy :from the Sun and convert it to stored chemical energy.
Because ofthis, green plants are the basis of the food chain.
The Sun is a star. It is the nearest star to the Earth and it is the principal
component of the Solar System. The Sun gives off electromagnetic energy•
. Electromagnetic energy consists ofradiation in the form ofwaves. These waves have
different wavelengths. The shorter the wavelength, the more energy the radiation has.
The various types of electromagnetic radiation form the Electromagnetic Spectrum that is
illustrated in Figure 6-1.
Weyelength (meters)
Shortest
WeYelength
Increeslng
Weyelength
Longest
Weyelength .
I
t.· 1211
t.·
t.· 1O
t.· e
t'·'7
t.·
t.·~
t.·'4
t.·
t.··2
t.·
t.· 1
t. o
t. 1
t. z
t. •
Gemmere,s
X.,.e,s
Highest energy-much more
energy then light
UItreYlolet
Visible Light
Infrered, he.
• More energy then light
Microweyes
• Less energy then light
• Less energy then light
TeleYlslon en.d FM redlo
Short weye redlo
AM redlo
Long weye redlo
Lowest energy-much less
energ, then light
Figure 6-1. The electromagnetic spectrum. Light is an ~xample of electrom~c
energy. Heat and ultraviolet radiation are also examples ofelectromagnetic radiation.
Physicists explain that the waves ofelectromagnetic radiation, such as light,·are
made up oflittle particles called photons. Photons are little bundles ofenergy. The green
plant contains chlorophyll,Which has the ability to absorb photons oflight. The plant
uses red and blue light. It absorbs these wavelengths. It does not use green light. That is
why it looks green~ The plant reflects green light because it does not use it. .
>
6-2
When light is absorbed by chlorophyll, the chlorophyll gets excited and expels an
electron. For each pair ofthese electrons, which contain electrical energy, the plant can
make molecules that store chemical energy. These molecules are known as ATP and
NADPH. The abbreviations are identified as foUows:
ATP = Adenosine Triphosphate,
NADPH = Nicotinamide Adenine Dinucleotide Phosphate.
The processes that produce these molecules are called Cyclic and Non-eyclic
Photophosphorylation.
Basically photophosphorylation means making ofATP (phosphorylation) using
energy from light (photo). ATP is a widely encountered energy-storage molecule in
living systems.
Definition ofphotosynthesis
Photosynthesis is the capture and use ofJight energy to produce organic materials
from inorganic raw materials.. In photosynthesis, light energy from the Sun is trapped
and' converted to chemical energy. The chemical energy is used to change CO2 into
glucose, a simple sugar. Details ofthese two components follow below.
ilght energy
.. I CO 2 + 12 H2 0
., C.H 12 0. + • 02 ~ •.H 2 0
Figure 6-2. General reaction ofphotosynthesis. The C~ entering at the left is converted
to glucose, which has the fannula Ct,}IJ206. Oxygen is the waste product of
photosynthesis. The oxygen results from photolysis of water, a process that replaces the
electrons ofnon-cyclic photophosphorylation.
Light energyfrom the Sun is trapped and c01lVertedto chemical energy
Chlorophyll is located in the grana ofthe chloroplast. The primary locale of
photosynthesis in green plants is in the leaves. Leaftissue contains layers of cells that ...
have large numbers ofchloroplasts to absorb solar energy.
In the light reaction ofphotosynthesis, light energy from the Sun is.trapPed by .
chlorophyJI and converted to chemical energy in
two fonDs ofATP and NADPH.
Cyclic photophosphorylation produces only ATP.. Non-eyclic photophosphorylation. .
. prpduces both ATP and NADPH. In order to complete tbeprocess,a source of electrons
iSilecessary.Green planis use H20 and extract the electrons nom it using aJaCtion .
called photolysis ofwater.'
..
.
the
','
...
, .'.
~
, 6-3
Cyclic photophosphorylation
In this process, light causes a pair ofelectrons to be released from a pair of
chlorophyll a molecules. These electrons travel a cycle where some of their energy is
used to make ATP. The electrons will return to the chlorophyll a molecules from which
they came. This is why the pathway is cyclic.
Cyclic Photophosphorylation
v
"
~ Fd
2"
7"\
ADP
AT;
.PI
\2.­
\
Cytochrome II
ADP.PI~2.·
2 molecul•• or ~
chioroph)'U -
ATPc-1
Cylochr. .e
t:
2.
PHOTOIYlTEM I
Figure 6-3. Cyclic photophosphorylation.
The pair of electrons in Photosystem I receives energy from red light. The
electrons lose some oftheir energy as they pass to postulated molecule X. As they
continue through the cycle, energy is lost each time they move from one molecule to
another.. Sometimes, some ofthe energy is retained in the fom of ATP. At the end of
the cycle, the electrons,have lost all oftheir energy and theyretum to chlorophyll Q.
X. The current explanation predicts that there is an unknown molecule that
receives the two electrons from chlorophyll a before they are sent to ferredoxin.
Ferredoxin (Fd). Ferredoxin is a special electron-transferring molecule that .
accepts electrons from molecule X. Ferredoxin easily adds electrons to other molecules.
Cylochromes. Cytochromes are enzymes that transfer electrons to other
substances. In certain cases the energy is not completely lost but some of it is used to
makeATP.
Phosphorylation. Phosphorylation is the process of making adenosine
triphosphate (ATP). The cell begins with the diphosphate (ADP) and adds an inorganic.
phosphoryl group (Pi) to it. These phosphate comPounds are derived from phosphoric
acid. Every ATP stores 7,400 calories ofenergy when it is made. Cyclic
photophosphorylation does not make NADPH, so non-cyclic photophosphorylation is .
necessary.
6-4
Non-cycHc photophosphorylation
The process ofnon-cyclic photophosphorylation involves three systems. These
are as follows:
• Photosystem 1
• Photosystem 11
• Photolysis ofWater
We have already been introduced to Photosystem 1as .it is also part ofthe cyclic
photophosphorylation process. All three systems are required to produce NADPH.
Photosystem J
Photosystem J sends two electrons to NADP+ in the non-cyclic
photophosphorylation process. The electrons begin travelling the tim part ofthe cyclic.
pathway. They make one molecule of ATP during this passage. In non-cyclic
photophosphorylation, however, the electrons are sent from ferredoxin to NADP+ to .
produce NADPH. The electrons do not retmn to the chlorophyll Q molecules of
Photosystem 1. Photosystem II is used to replace the electrons that did not return to
Photosystem 1.
Photosystem 11
Photosystem II replaces the electrons lost by PhotosysteJn 1. Photosystem 11 has
both chlorophyll Q and b and it responds to both red and blue light. After leaving
Photosystem 11, the electrons travel to Coenzyme Q (an eleetron-transfening enzyme)
and then the other part ofthe cyclic pathway back to Photosystem I. The electrons do not
retUrn and must be replaced. During this pathway, another ATP molecule is made.. Both
photosystems make ATP when they are operational.
.
Photolysis ofwater
Photosystem 11 gets new electrons by photolysis of water. "Lysis" in biology
implies something being broken down, getting destroyed or exploding. In this case, the
chloroplast has a way ofdestroying the water molecule and making it into three
components. The first component consists of protons (H). These protons·are used as
part ofthe reaction that makes NADPH. The second component consists ofelectrons.
The electrons replace the electrons lost by Photosystem D. The third component is
oxygen gas «h). Oxygen gas is released by the plant into the atmosphere through the
stoinates. Oxygen is the waste product ofphotosynthesis.
..
6-S
CJclic Photophosphorylation
)(
2"
,. Fd
()
ADP
+PI
r
NOIKJcnc Photophosphorrilltion
2.- • JlADp.
JlADPY • Y·
. ATP
H 0 ~ 2H~ + 2" + 112 O2
2 Photo.,sls ~fWllter
Cytochrome b
.
.~••
AD P
+P
I j f2 •
Blue
ATP
e
Coenzyme Q 2 •
&
~ chl_&
Red
2 molecules of f
•
Cytochrome c
chi II, t: tit d light
chiorop..,11 2.
.
PHOToaTaTEM II
PHOTOIYITEM I
Figure 6-4.
N~n-cyclic
2.
photophosphorylation.
. Photosystem I sends electrons to reduce NADP+. Then, Photosystem II sends
replacement electrons to Photosystem I. Finally, photolysis ofwater replaCes the
electrons lost by Photosystem II. Therefore, water is the ultimate sautee ofelectrons fOr
photosynthesis. The oxygen released by the plant comes frOm photolysis of water.
Oxygen is the waste product of photosynthesis.
Summary of light reaction
.
The light reaction of photosynthesis comprises all ofthe light-dependent or
photochemical reactions. For each pair ofelectrons, the plant makes one NADPH and
two ATP molecules. These molecules are used to produce glucose sugar during the
carbon-fixation reactions ofphotosynthesis. Photolysis of water provides the electrons
used to produce the NADPH and to carry the eleetJ'ical energy necessary to make ATP.
Oxygen is released from photolysis ofwater as a waste product.
The chemical energy is used to change C02 into glucose, a simple sugar
The production ofglucose from carbon dioxide (C~) takes place all the time,
whether the Sun is shining or not. As it is the only part ofphotosynthesis that works
without light, it is sometimes called the dark reaction ofphotosynthesis. The dark
reaction ofphotosynthesis contains the carbon-jixation reactions. These reactions are
localized in a pathway known as the Calvin-Benson Cycle.
The Calvin-Benson Cycle
Overall reaction
6RuDP+6 CO2 + IBATP+ 12NADPH -+ 6 RuDP+ 1 Gluc:ose+ IBADP+ 18Pi+ 12NADr
6-6
•
~H20P03
C
=°
H-~-OH
_.. . . ,;,\" ,
...,...
..
2 ••I.nl.. If
~
~.,
.~.n'. (pGA)
RI••I•••
DI,•••, ••,.
H- ~-OH
I
CO
2
•
CHZOPO,
2 NADPH
t........ ADP .PI
~ATP
2 HADP.
CH 0H
I 2 .
C=O
H-~-OH
P•••••••
I
H-C-OH
I
(I
RI••I•••
•
2 ••I.ul•• If
re.dl...)
.I~c.n....~...
r
t.,
••••••••
cPGAL)
••Iu•••
..c. I tu....
a.
CHZOPO,
.
.
Figure 6-6. The Calvin-Benson cycle. The cycle uses ATP and NADPH produced by the
photochemical reactions. This energy is used to incorporate C02 into Ribulose­
Diphosphate (RuDP). The resulting molecules PGA and PGAL lead into nine more
reactions. For every six turns ofthe cycle, one molecule of glucose results.
The Calvin-Benson cycle constitutes the essence ofthe dark reactions of
photosynthesis. This pathway is of a typ~ known as anabolic. Anboljc pathways build
things. In this case, the molecule being built is glucose. Glucose is built by reduction of
C(h. This means that the Calvin-Benson pathway is also a reductive pathway.
Reductive, anabolic P~\Y8ys are commonly known as biosynthesis..
Study guide for photosynthesis
A.
General considerations
I.
Photosynthesis requires light, chlorophyll and C(h.
2.
The overall equation ofphotosynthesis is:
3.
4.
B.
The raw materials for photosynthesis are H20 and CO2.
The waste product ofphotosynthesis is~, which comes from the
breakdown ofwater.
The photochemical Oight) reactions
1.
Light is absorbed by chlorophyll
L
Red and,blue light are absorbed.
b.
Green light is not absorbed, it is reflected.
~
l
,
6-7
2.
3.
4.
5.
6.
c.
. The process of photosynthesis occms in the chloroplasts ofplant cells.
Water is broken do~ by photolysis to replace the electrons in the non­
cyclic pathway.
&.
The H is incorporated into the glucose sugar.
b.
The 0 is released as a waste product.
The light energy is converted.to electrical energy.
The electrical energy is converted to chemical energy and is stored in the
form of ATP and NADPH.
The photochemical reactions occur only in the light.
The carbon-fixation (dark) reactions
.
1.
The carbon-fixation reactions of photosynthesis occur all ofthe time,
whether there is light or not.
2.
These reactions do not occur without C~.
3.
These reactions use the energy stored in ATP and NADPH to convert C~
to glucose.
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