6
WHERE IT STARTS-PHOTOSYNTHESIS
Pastures of the Seas
SUNLIGHT AS AN ENERGY SOURCE
Properties of Light
Pigments-The Rainbow Catchers
WHAT IS PHOTOSYNTHESIS AND WHERE DOES
IT HAPPEN?
Two Stages of Reactions
A Look Inside the Chloroplast
Photosynthesis Changed the Biosphere
. LIGHT-DEPENDENT REACTIONS
Transducing the Absorbed Energy
Making ATP and NADPH
A CASE OF CONTROLLED ENERGY RELEASE
LIGHT-INDEPENDENT REACTIONS: THE SUGAR
FACTORY
DIFFERENT PLANTS, DIFFERENT CARBON-FIXING
PATHWAYS
Interactive Exercises
Pastures of the Seas [pp.92-93]
6.1. SUNLIGHT AS AN ENERGY SOURCE [pp.94-95]
((:0)
For additional practice, use the interactive vocabulary exercises linked with your BiologyNow CD-ROM.
Selected Words: photons [p.94], accessorlj pigment [p.94], T. Englemann [p.95]
Boldfaced Terms
[p.92] autotrophs _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.92] heterotrophs _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.92] photosynthesis _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.94] pigments _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.94] chlorophyll a
Where It Starts-Photosynthesis
71
[p.94] carotenoids _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.95] xanthophylls
[p.95] phycobilins _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.95] anthocyanin _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
Fill in the Blanks
Many different kinds of photosynthetic bacteria, protistans, and plants are among the "self-nourisning"
organisms, or (1)
[p.92]. (2)
[p.92] are not self-nourishing; they cannot
make their own food. Plants can do something that humans can never do: they make food by the process of
(3) _ _ _ _ [p.92].
Visible light has wavelengths between (4) _ _ _ _ _ [p.94] and (5) _ _ _ _ _ _ [p.94]
nanometers. These are the wavelengths of visible light that drive (6) _ _ _ _ _ _ [p.94]. Light is made
of (7) _ _ _ _ _ _ [p.94], or individual packets of electromagnetic energy traveling in waves.
(8) _ _ _ _ _ _ [p.94] are a class of molecules that absorb (7) with particular wavelengths.
Certain pigments are molecular bridges from (9)
(10)
[p.94] to photosynthesis. One of them,
[p.94], is the most abundant type of pigment in plants, green algae, and a number of
photoautotrophic bacteria. It is best at absorbing maiuly red and (11)
Chlorophyll b absorbs light at slightly different wavelengths. It is an (12)
[p.94] wavelengths.
[p.94] pigment,
which means that it enhances the efficiency of photosynthesis reactions by capturing additional wavelengths. All chlorophylls reflect or transmit (13)
parts that are rich in chlorophylls appear (14)
[p.94] wavelengths, which is why plant
[p.94] to us.
Other accessory pigments include the (15) _ _ _ _ _ _ [p.94]. These absorb blue-violet and bluegreen wavelengths, so they reflect (16)
carotene is a carotenoid that colors (17)
(18)
[p.94], orange, and yellow wavelengths. Beta[p.94] and many other plant parts orange.
[p.95] are yellow, brown, purple, or blue accessory pigments. (19) _ _ _ _ __
[p.95] are red or blue-green.
Carotenoids, xanthophylls, and (20) _ _ _ _ _ _ [p.95], a red-purple pigment, become apparent in
the leaves of (21) ,
[p.95] species as their chlorophyll content declines in autumn.
Such photosynthetic pigments do not work (22)
[p.95]. Organized arrays of them work
together and harvest energy from the (23) _ _ _ _ _ _ [p.95]. Start thinking about the structure of the
chlorophyll molecule. Here, alternating single and double (24) _ _ _ _ _ _ [p.95] bonds share electrons.
And th~se are the electrons that, when excited by inputs of (25)
going.
72 Chapter Six
[p.95], get photosynthesis
!i
6.2. WHAT IS PHOTOSYNTHESIS AND WHERE DOES IT HAPPEN? [pp.96-97]
Selected Words: photosystems I and II [p.96]
!i
Boldfaced Terms
[p.96] light-dependent reactions _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _~
[p.96] light-independent reactions _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
I
II
JI
I
[p.96] thylakoid membrane _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.96] photosystems _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
il
11:1
Fill in the Blanks
11
1. In the spaces below, supply the missing information to complete the summary equation for photosynthesis:
+
12
0, + C6H ,20 6 + 6
CO, -->
[p.96]
2. Supply the appropriate information to state this equation for photosynthesis in words: (a) _ _ _ _ __
molecules of water plus six molecules of (b)
(in the presence of pigments,
enzymes, and visible light) yield six molecules of (c)
plus (e)
(d)
plus one molecule of
1
I, '
1 '1
1
II
',I',i,'
;",1,
i
i:1
1,1:
"I
"
"I
:1,
molecules of water. [p.96]
!;,I
',I:
Labeling
"I,",'
These diagrams show the location of the light-dependent and light-independent reactions within a chloroplast. Replace numbers on the diagram with the missing words. [Refer to text Figure 6.7, p.97.]
,I",.,
1,:1
;'1
'jl
3
membranes
4
7
6
complex
I
,il
,
1:1
'II
'i'
I'i,1
10
3.
4.
7.
9.
~
5.
8. _ _ _ _ _ _ _ _ __
; 10.
; 11. _ _ _ __
li'll,
,
"Ii"i
'IrOilII
I
!
Where It Starts-Photosynthesis 73
ii!j
n"Ii
H
12. Explain how Earth's atmosphere, once devoid of all oxygen, accumulated oxygen. Then explain the
evolutionary significance of oxygen. [pp.96-97]
6.3. LIGHT-DEPENDENT REACTIONS [pp.98-99]
6.4. A CASE OF CONTROLLED ENERGY RELEASE [p.100j
Selected Words: fluorescent light [p.98j, photosystem-a reaction center [p.98], photosystem II [p.98],
photolysis [p.98j, photosystem I [p.99], "noncyclic" and "cyclic" pathways [p.99j
Boldfaced Terms
[p.9S] electron transfer chains
[p.9S] ATP synthases _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
Labeling
To label items on the sketches below, select from the sketch the number that best fits each description, and
enter that number in the proper blank. [Refer to Figure 6.10, p.100.]
!
16
@~(17
~',
11 " ' .
1
>-
91
OJ
Qj
10
C
O~
<D
Qj
.c
0
12
G
~
13
1
8~
1. ___ electron transfer chain, noncyclic pathway
2. ___ electron transfer chain, cyclic pathway
NADPH
4. _ _ hydrogen to be used in ATP formation, cyclic pathway
5. ___._ ejected electron, photosystem II
74 Chapter Six
15
~~
§
OJ
I
3.
14
7
NADW
6. _ _ cyclic pathway of ATP formation
7. _ _ ejected electron, photosystem I
8. _ _ ejected electron, cyclic pathway
9. _ _ photosystem II
.,
10. _ _ photosystem I, cyclic pathway
11. _ _ photosystem I, noncyclic pathway
12. _ _ noncyclic pathway of ATP and NADPH formation
13. _ _ photon, cyclic pathway
14. _ _ photon, photosystem I
15.
water
16. _ _ hydrogen to be used in ATP formation, noncydic pathway
17. _ _ photon, photosystem II
Fill in the Blanks
The numbers in the illustration represent information missing from the following narrative, which
describes how ATP and NADPH form in chloroplasts via the noncyclic pathway of photosynthesis.
[Refer to text Figure 6.9, pp.98-99.]
18
LlGHTHARVESTING
COMPLEX
20
sunlight
25
I
compartment
27
stroma
Embedded in._photosynthetic membranes are light-harvesting clusters (complexes) of pigments and proteins. Like antennae, pigments in the clusters catch (18)
[p.98] energy and then pass it to
adjoining clusters. Eventually, the energy reaches a (19)
[p.98], or reaction center, which
contains proteins and pigments. Many light-harvesting clusters surround each. Photon energy causes
(20)
(21)
[p.99] to lose electrons. It replaces them by pulling electrons from
[p.99] molecules, which then split into (22)
[p.99] and hydrogen ions.
Oxygen leaves the cell as 02' Electrons from photosystem II enter an electron transport chain, which also
Where It Starts-Photosynthesis
75
moves (23) _ _ _ _ _ _ [p.99] from the stroma into the thylakoid compartment. (24) _ _ _ _ __
[p.99] continue on to (25)
[p.99]. (26)
[p.99] concentration
and electric gradients build up across the thylakoid (27)
[p.99]. The force of these gradients
propels H+ through ATP synthases, driving (28)
the loss of (29)
[p.99] formation. Photon energy triggers
[p.99] from photosystem I. Through an intermediary molecule, the electrons
are transferred to NADP+, which also picks up H+ and thereby becomes (30) _ _ _ _ _ _ [p.99].
6.5. LIGHT-INDEPENDENT REACTIONS: THE SUGAR FACTORY [p.I01]
Selected Words: ATP and NADPH [p.I01], RuBP [p.I01], PCA [p.I01], PCAL [p.I01], sucrose [p.I01]
Boldfaced Terms
[p.IOI] Calvin-Benson cycle _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.IOI] rubisco _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.IOI] carbon fixation _ _ _- - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Identification
Identify each part of the diagram, using abbreviations where appropriate. Complete the exercise by entering
the letter of the correct function description in the parentheses following each labeL [All are from p.IOI.]
l. - - - ( )
2. - - - ( )
3. - - - ( )
4. - - - ( )
.. ..
~
.... 1
-A
6 RuBP
5. - - - ( )
6. - - - ( )
......
122
\
Calvin-Benson
cycle
-
7. - - - ( )
1 07
~
......
125
·:r~
12\il
t~a
12 ADP +
12 p.
12c1~
12 NADP+
1 Pi - - {
1~
GlfiGiifiG 6
a. Donates hydrogens and electrons to PCA molecules
b. Cets phosphate groups from ATP, priming them for synthesis reactions that regenerate RuBP
c. Rubisco attaches the carbon atom of CO, to RuBP, which starts the Calvin-Benson cycle; this compound
forms from splitting of an intermediate compound
d. Cets a phosphate group from ATP, plus hydrogen and electrons
e. In air spaces inside a leaf; diffuses into photosynthetic cells
f. Formed by combination of two of twelve PCAL molecules
g. Donates phosphate groups to PCA
76 Chapter Six
6.6. DIFFERENT PLANTS, DIFFERENT CARBON-FIXING PATHWAYS [pp.102-103]
Selected Words: three-carbon PGA [p.102], four-carbon oxaloacetate [p.102]
Boldfaced Terms
[p.102j stomata _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _--,.'::-,-
[p.102j C3 plants _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.102j C4plants _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.102j CAM plants _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
Choice
For questions 1-15, choose from the following:
a. C3 plants
b. C4 plants
c. CAM plants
1. _ _ Succulents such as cacti, which have juicy, water-storing tissues and thick surface layers [p.102]
2. _ _ The three-carbon PGA, the first stable intermediate of the Calvin-Benson cycle [p.102]
3. _ _ Four-carbon oxaloacetate is the first to form [p.102]
4. _ _ They open their stomata and fix carbon at night [p.102]
5. _ _ Two types of photosynthetic cells occur in these plants [p.102]
6. _ _ Evolved independently over millions of years in many lineages [p.102]
7. _ _ With CO2 levels riSing, these plants may again have the edge [p.102]
8. ___ Do not grow well in hot, dry climates without steady irrigation [p.102]
9. _ _ These plants survive prolonged droughts by closing stomata even at night [p.102]
10. ___ In the first cell type, the carbon-fixing enzyme ignores O2, no matter how much there is; CO2 is
deposited in bundle sheath cells [p.102]
11. _ _ CO2 is fixed by repeated turns of a type of C4 cycle, then it enters the Calvin-Benson cycle the
next day [p.102]
12. _ _ A basswood tree [p.103]
13. ___ These plants lose less water and make more sugar than the C3 plants can when days are dry
F[p.102]
14. ___ When the oxygen level is high, rubisco uses oxygen instead of CO2 in an alternative reaction
that yields only one molecule of PGA [p.102]
15. ___ Common in grasses, com, and other plants that evolved in the tropics [p.103]
Where It Starts-Photosynthesis
77
Self-Quiz
Are you ready for the exam? Test yourself on key concepts by taking the additional tests linked with your
BiologyNow CD-ROM.
__ 1. Plants need _ _ _:-_ _ and
_ _ _ _ _ _ as the raw materials with
which to carry on photosynthesis. [p.75]
a. oxygen; water
b. oxygen; CO,
c. CO,; H,O
d. sugar; water
e. CO,; NADPH
_
2. Chlorophyll is found _~_ _ _ _ ,
[p.75]
a. on the outer chloroplast membrane
b. inside the mitochondria
c. inside the stroma
d. in the thylakoid membranes
e. on the membranes of the stroma
__ 3. The cyclic route functions mainly to
_ _ _ _ . [p.75]
a. make NADPH
b. makePGAL
c. set up conditions for making A TP
d. regenerate RuBP
e. break down CO,
__ 4. During the noncyclic pathway, the origin
of the electrons passed to NADP+ is
~~~~_. [pp.76-77]
a. CO,
b. glucose
c. sunlight
d. water
e. ATP
__ 5. Two products of the light-dependent
reactions are required to drive the
chemistry of the light-independent
reactions. They are
and
~_ _ _ _ . [pp.77-78]
a.O,;NADPH
b.CO,;H20
c. O2; inorganic phosphate
d. ATP;NADPH
e. RuBP;PGA
78 Chapter Six
__ 6. ATP synthases are involved in
_ _ _ _. [p.76]
a. creating electron transport chains
b. photosystems
c. attaching inorganic phosphates to ADP
during the cyclic route
d. the noncyclic pathway
e. allowing H+ flow to attach inorganic
phosphate to ADP
__ 7. The Calvin-Benson cycle uses
---:::-::-_ _. [p.78]
a. CO,
b. hydrogen and electrons from NADPH
c. phosphate group transfers from ATP
d. an enzyme known as rubisco
e. all of the above
__ 8. In the Calvin-Benson cycle, RuBP is
regenerated by
. [p.78]
a. rearrangement of PGAL molecules
b. the combination of two PGAL molecules
c. PGA receiving phosphate groups from
ATPs
d. glucose entering other reactions that
form carbohydrates
e. rubisco attaching carbon atoms to RuBP
__ 9. C4 plants have an advantage in hot, dry
conditions because
. [p.79]
a. their leaves are covered with thicker
wax layers than those of C3 plants
b. their stomates open wider than those of
C3 plants, thus cooling their surfaces
c. special leaf cells possess a means of
capturing CO, even in stress conditions
d. they carryon normal photosynthesis at
very high oxygen levels
e. they can carryon carbon fixation at night
~
10. The process of securing carbon from the
environment by incorporating it into a
stable organic compound is called
~,-----_ _ _. [p.101]
a. fluorescence
b. photolysis
c. photorespiration
d. the C4 cycle
e. carbon fixation
Chapter Objectives/Review Questions
1. List the types of organisms that are included in "pastures of the seas." What benefits do these organisms provide? How do industrial wastes, fertilizers in runoff, and raw sewage affect these photoautotrophs? [pp.92-93]
2. Name and describe the pigments associated with photosynthesis in various organisms. State which .•
colors of visible light are absorbed by chlorophyll a, chlorophyll b, and carotenoids. [pp.94-95]
3. List the major stages of photosynthesis and briefly tell what happens in each. [pp.96-97]
4. State the general equation for photosynthesis. [p.96]
5. Why has photosynthesis changed the biosphere? [pp.96-97]
6. Locate and name the sites of the major stages of photosynthesis. Begin with a green leaf and finish with
chloroplast structure (as in Figure 6.7 of the text). [pp.96-97]
7. Describe the structure of a photosystem; relate this to photon energy. [p.98]
8. In response to the H+ flow through
, inorganic phosphate becomes
attached to a molecule of ADP dissolved in the stroma, thus forming ATP. [p.98]
9. List the components of an electron transfer chain and describe how the chain functions. [p.98]
10. The two energy-carrying molecules produced in the light-dependent reactions are
and
_ _ _ _ _ _ . Why are these molecules necessary for the light-independent reactions? [p.99]
11. Explain how the cyclic and noncyclic reactions proceed; be sure to use the correct terms. [p.100]
12. Be able to describe the Calvin-Benson cycle and include the key molecular components. [p.101]
13. Describe the mechanism by which C4 plants thrive in stressful hot, dry conditions. Contrast this with
the CO,-capturing chemistry of C3 plants. [p.102]
14. Describe the mechanisms that allow CAM plants such as cacti to survive desert conditions; tell how
this chemical mechanism differs from that of C4 plants. [p.102]
Media Menu Review Questions
Questions 1-3 are drawn from the following lnfotrac College Edition article: "Study Shows Microscopic
Plants Keep Planet Warm, Offers New Considerations for Iron Fertilization Efforts in Oceans." Ascribe
Higher Education News Service, November 6, 2002.
1. The Frouin-Iacobellis study uses satellite imagery to show that
, which are said to
inhabit three-quarters of Earth's surface, have a fundamental
influence on the planet by
capturing and absorbing the sun's radiation.
2. An older view holds that ocean "fertilization" would
global warming. More phytoplankton would draw more
from the atmosphere and therefore reduce
global warming.
3. Frouin and Iacobellis contend that iron fertilization would increase the amount of phytoplankton in the
ocean and that this would contribute to
the ocean by absorbing more radiation.
Questions 4-6 are drawn from the following lnfotrac College Edition article: "Light of Our Lives," Norman
Miller. Geographical, January 2001.
4. The answer to the question "Why green?" probably lies in the
origins of today's
photosyn~etic organisms.
5.
wavelengths of light do not penetrate much below five meters in seawater, so the
ability to absorb some energy from the
wavelengths that penetrate deeper would
have conferred a significant advantage on early photosynthetic algae that were unable to remain
continuously in the upper levels of the ocean.
6. If early ocean-living algae wanted to make best use of the energy of sunlight available, they would
choose as their main tool a pigment that absorbed the
violet and blue wavelengths and
also the
red and orange ones. The pigment color that best strikes this balance is
_ _ _ _ _ _ , and its name is _ _ _ _ _ _.
Where It Starts-Photosynthesis 79
Questions 7-10 are drawn from the paper "Why Study Photosynthesis?" Devens Gust. Found at
photoscience.la.asu.edu/photosyn
7. List some of the ways in which photosynthetic activity directly affects our lives and our planet.
8. List some reasons why you should study photosynthesis.
9. A process called
is kind of a "short circuit" of photosynthesis that wastes much of the
plant's photosynthetic energy. Can you relate this to C3 and C4 plants as described in the textbook?
10. Describe one way in which photosynthesis is being harnessed for medical purposes.
Integrating and Applying Key Concepts
1. A scientist once proposed that human cells be injected with chloroplasts extracted from living plant
cells. Speculate about that possibility in terms of changes in human anatomy, physiology, and behavior.
Could this experiment be successful? Why or why not?
80 Chapter Six