What
is Photosynthesis?
Photosynthesis is the process by which plants, some bacteria,
and some protistans use the energy from sunlight to produce sugar, which cellular
respiration converts into ATP, the "fuel" used by all
living things. The conversion of unusable sunlight energy into usable chemical
energy, is associated with the actions of the green pigment chlorophyll.
Most of the time, the photosynthetic process uses water and releases the oxygen
that we absolutely must have to stay alive. Oh yes, we need the food as well!
We can write
the overall reaction of this process as:
6H2O
+ 6CO2 ----------> C6H12O6+ 6O2
Most of us
don't speak chemicalese, so the above chemical equation translates as:
six molecules
of water plus six molecules of carbon dioxide produce one molecule of sugar
plus six molecules of oxygen
Fig 1. Diagram of a
typical plant, showing the inputs and outputs of the photosynthetic process. (Image from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates and WH Freeman ,
used with permission.)
Plants are the
only photosynthetic organisms to have leaves (and not all plants have
leaves). A leaf may be viewed as a solar collector crammed full of
photosynthetic cells.
The raw
materials of photosynthesis, water and carbon dioxide, enter the cells of the
leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf.
Fig 2. Cross section
of a leaf, showing the anatomical features important to the study of
photosynthesis: stoma, guard cell, mesophyll cells, and vein. Image from Purves
et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman , used with permission.
Water enters
the root and is transported up to the leaves through specialized plant cells
known as xylem (pronounces zigh-lem). Land plants must guard against
drying out (desiccation) and so have evolved specialized structures known as
stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass
through the protective waxy layer covering the leaf (cuticle), but it can enter
the leaf through an opening (the stoma; plural = stomata; Greek for hole)
flanked by two guard cells. Likewise, oxygen produced during photosynthesis can
only pass out of the leaf through the opened stomata. Unfortunately for the
plant, while these gases are moving between the inside and outside of the leaf,
a great deal water is also lost. Cottonwood trees, for example, will lose 100
gallons of water per hour during hot desert days. Carbon dioxide enters
single-celled and aquatic autotrophs through no specialized structures.
Fig 3. Pea Leaf Stoma,
Vicea sp. (SEM x3,520). This image is copyright Dennis Kunkel at ,
used with permission.
White light is
separated into the different colors (=wavelengths) of light by passing it
through a prism. Wavelength is defined as the distance from peak to peak (or
trough to trough). The energy of is inversely porportional to the wavelength:
longer wavelengths have less energy than do shorter ones.
Fig 4. Wavelength and
other saspects of the wave nature of light. Image from Purves et al., Life:
The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman , used with permission.
The order of
colors is determined by the wavelength of light. Visible light is one small
part of the electromagnetic spectrum. The longer the wavelength of visible
light, the more red the color. Likewise the shorter wavelengths are towards the
violet side of the spectrum. Wavelengths longer than red are referred to as
infrared, while those shorter than violet are ultraviolet.
Fig 5. The
electromagnetic spectrum. Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates and WH
Freeman , used with permission.
Light behaves
both as a wave and a particle. Wave properties of light include the bending of
the wave path when passing from one material (medium) into another (i.e. the
prism, rainbows, pencil in a glass-of-water, etc.). The particle properties are
demonstrated by the photoelectric effect. Zinc exposed to ultraviolet light
becomes positively charged because light energy forces electrons from the zinc.
These electrons can create an electrical current. Sodium, potassium and
selenium have critical wavelengths in the visible light range. The critical
wavelength is the maximum wavelength of light (visible or invisible) that
creates a photoelectric effect.
Chlorophyll and Accessory Pigments
A pigment is
any substance that absorbs light. The color of the pigment comes from the
wavelengths of light reflected (in other words, those not absorbed). Chlorophyll,
the green pigment common to all photosynthetic cells, absorbs all wavelengths
of visible light except green, which it reflects to be detected by our eyes.
Black pigments absorb all of the wavelengths that strike them. White
pigments/lighter colors reflect all or almost all of the energy striking them.
Pigments have their own characteristic absorption spectra, the absorption
pattern of a given pigment.
Fig 6. Absorption and
transmission of different wavelengths of light by a hypothetical pigment. (Image
from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates and WH Freeman ,
used with permission.)
Chlorophyll is
a complex molecule. Several modifications of chlorophyll occur among plants and
other photosynthetic organisms. All photosynthetic organisms (plants, certain
protistans, prochlorobacteria, and cyanobacteria) have chlorophyll a.
Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory
pigments include chlorophyll b (also c, d, and e in algae and
protistans), xanthophylls, and carotenoids (such as beta-carotene).
Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red
wavelengths, and little from the intermediate (Green-Yellow-Orange)
wavelengths.
Fig 7. Molecular model
of chlorophyll. (The above image is from http://www.nyu.edu:80/pages/mathmol/library/photo.)
Fig 8. Molecular model
of carotene. (The above image is from http://www.nyu.edu:80/pages/mathmol/library/photo.)
Carotenoids and
chlorophyll b absorb some of the energy in the green wavelength. Why not so
much in the orange and yellow wavelengths? Both chlorophylls also absorb in the
orange-red end of the spectrum (with longer wavelengths and lower energy). The
origins of photosynthetic organisms in the sea may account for this. Shorter
wavelengths (with more energy) do not penetrate much below 5 meters deep in sea
water. The ability to absorb some energy from the longer (hence more
penetrating) wavelengths might have been an advantage to early photosynthetic
algae that were not able to be in the upper (photic) zone of the sea all
the time.
Fig 9. The molecular
structure of chlorophylls. (Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates and WH
Freeman , used with permission.)
The action
spectrum of photosynthesis is the relative effectiveness of different
wavelengths of light at generating electrons. If a pigment absorbs light
energy, one of three things will occur. Energy is dissipated as heat. The
energy may be emitted immediately as a longer wavelength, a phenomenon known as
fluorescence. Energy may trigger a chemical reaction, as in photosynthesis.
Chlorophyll only triggers a chemical reaction when it is associated with
proteins embedded in a membrane (as in a chloroplast) or the membrane
infoldings found in photosynthetic prokaryotes such as cyanobacteria and
prochlorobacteria.
Fig 10. Absorption
spectrum of several plant pigments (left) and action spectrum of elodea
(right), a common aquarium plant used in lab experiments about photosynthesis. (Images from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates and WH Freeman ,
used with permission.)
The thylakoid
is the structural unit of photosynthesis. Both photosynthetic prokaryotes and
eukaryotes have these flattened sacs/vesicles containing photosynthetic
chemicals. Only eukaryotes have chloroplasts with a surrounding membrane.
Thylakoids are
stacked like pancakes in stacks known collectively as grana. The areas
between grana are referred to as stroma. While the mitochondrion has two
membrane systems, the chloroplast has three, forming three compartments.
Fig 11. Structure of a
chloroplast. (Image from Purves et al., Life: The Science of Biology, 4th
Edition, by Sinauer Associates and WH Freeman ,
used with permission.)
Photosynthesis
is a two stage process. The first process is the Light Dependent Process (Light
Reactions), requires the direct energy of light to make energy carrier
molecules that are used in the second process. The Light Independent Process
(or Dark Reactions) occurs when the products of the Light Reaction are
used to form C-C covalent bonds of carbohydrates. The Dark Reactions can
usually occur in the dark, if the energy carriers from the light process are
present. Recent evidence suggests that a major enzyme of the Dark Reaction is
indirectly stimulated by light, thus the term Dark Reaction is somewhat of a
misnomer. The Light Reactions occur in the grana and the Dark Reactions
take place in the stroma of the chloroplasts.
Fig 12. Overview of the
two steps in the photosynthesis process. (Image from Purves et al., Life: The
Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman , used with permission.)
In the Light
Dependent Processes (Light Reactions) light strikes chlorophyll a in such a way
as to excite electrons to a higher energy state. In a series of reactions the
energy is converted (along an electron transport process) into ATP
and NADPH. Water is split in the process, releasing oxygen as a
by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the
Light Independent Process (Dark Reactions).
In the Light
Independent Process, carbon dioxide from the atmosphere (or water for
aquatic/marine organisms) is captured and modified by the addition of Hydrogen
to form carbohydrates (general formula of carbohydrates is [CH2O]n).
The incorporation of carbon dioxide into organic compounds is known as carbon
fixation. The energy for this comes from the first phase of the photosynthetic
process. Living systems cannot directly utilize light energy, but can, through
a complicated series of reactions, convert it into C-C bond energy that can be
released by glycolysis and other metabolic processes.
Photosystems are arrangements of chlorophyll and other
pigments packed into thylakoids. Many Prokaryotes have only one photosystem,
Photosystem II (so numbered because, while it was most likely the first to
evolve, it was the second one discovered). Eukaryotes have Photosystem II plus
Photosystem I. Photosystem I uses chlorophyll a, in the form referred to as
P700. Photosystem II uses a form of chlorophyll a known as P680. Both
"active" forms of chlorophyll a function in photosynthesis due to
their association with proteins in the thylakoid membrane.
Fig 13. Action of a
photosystem. (This image is from the University of Minnesota page at http://genbiol.cbs.umn.edu/Multimedia/examples.html.)
Photophosphorylation is the process of converting energy from a
light-excited electron into the pyrophosphate bond of an ADP molecule. This
occurs when the electrons from water are excited by the light in the presence
of P680. The energy transfer is similar to the chemiosmotic electron transport
occurring in the mitochondria. Light energy causes the removal of an electron
from a molecule of P680 that is part of Photosystem II. The P680 requires an electron,
which is taken from a water molecule, breaking the water into H+
ions and O-2 ions. These O-2 ions combine to form the
diatomic O2 that is released. The electron is "boosted" to
a higher energy state and attached to a primary electron acceptor, which begins
a series of redox reactions, passing the electron through a series of electron
carriers, eventually attaching it to a molecule in Photosystem I. Light acts on
a molecule of P700 in Photosystem I, causing an electron to be
"boosted" to a still higher potential. The electron is attached to a
different primary electron acceptor (that is a different molecule from the one
associated with Photosystem II). The electron is passed again through a series
of redox reactions, eventually being attached to NADP+ and H+
to form NADPH, an energy carrier needed in the Light Independent Reaction. The
electron from Photosystem II replaces the excited electron in the P700
molecule. There is thus a continuous flow of electrons from water to NADPH.
This energy is used in Carbon Fixation. Cyclic Electron Flow occurs in some
eukaryotes and primitive photosynthetic bacteria. No NADPH is produced, only
ATP. This occurs when cells may require additional ATP, or when there is no
NADP+ to reduce to NADPH. In Photosystem II, the pumping to H ions
into the thylakoid and the conversion of ADP + P into ATP is driven by electron
gradients established in the thylakoid membrane.
Fig 14. Noncyclic
photophosphorylation (top) and cyclic potophosphorylation (bottom). These
processes are better known as the light reactions. (Images from Purves et al., Life:
The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman , used with permission.)
The above
diagrams present the "old" view of photophosphorylation. We now know
where the process occurs in the chloroplast, and can link that to chemiosmotic
synthesis of ATP.
Fig 15. Chemiosmosis as
it operates in photophosphorylation within a chloroplast. (Images from Purves et
al., Life: The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman , used with permission.)
Halobacteria,
which grow in extremely salty water, are facultative aerobes, they can grow
when oxygen is absent. Purple pigments, known as retinal (a pigment also found
in the human eye) act similar to chlorophyll. The complex of retinal and
membrane proteins is known as bacteriorhodopsin, which generates electrons
which establish a proton gradient that powers an ADP-ATP pump, generating ATP
from sunlight without chlorophyll. This supports the theory that chemiosmotic
processes are universal in their ability to generate ATP.
Carbon-Fixing
Reactions are also known as the Dark Reactions (or Light Independent
Reactions). Carbon dioxide enters single-celled and aquatic autotrophs
through no specialized structures, diffusing into the cells. Land plants must
guard against drying out (desiccation) and so have evolved specialized
structures known as stomata to allow gas to enter and leave the leaf.
The Calvin Cycle occurs in the stroma of chloroplasts (where would it
occur in a prokaryote?). Carbon dioxide is captured by the chemical ribulose biphosphate (RuBP). RuBP is a 5-C chemical. Six molecules of
carbon dioxide enter the Calvin Cycle, eventually producing one molecule of
glucose. The reactions in this process were worked out by Melvin Calvin (shown
below).
Fig 16 . Melvin Calvin (The above image
is from http://www-itg.lbl.gov/ImgLib/COLLECTIONS/BERKELEY-LAB/PEOPLE/INDIVIDUALS/index/BIOCHEM_523.html,
Ernest OrlandoLawrence Berkeley National Laboratory.) " One of the new
areas, cultivated both in Donner and the Old Radiation Laboratory, was the
study of organic compounds labeled with carbon-14. Melvin Calvin took charge of
this work at the end of the war in order to provide raw materials for John
Lawrence's researches and for his own study of photosynthesis. Using carbon-14,
available in plenty from Hanford reactors, and the new techniques of ion
exchange, paper chromatography, and radioautography, Calvin and his many associates
mapped the complete path of carbon in photosynthesis. The accomplishment
brought him the Nobel prize in chemistry in 1961. (The preceding information
was excerpted from the text of the Fall 1981 issue of LBL Newsmagazine.)
Citation Caption: LBL News, Vol.6, No.3, Fall 1981 Melvin Calvin shown with
some of the apparatus he used to study the role of carbon in
photosynthesis."
Fig 17 . The first steps
in the Calvin ccycle. (Image from Purves et al., Life: The Science of Biology,
4th Edition, by Sinauer Associates and WH Freeman ,
used with permission.)
The first
stable product of the Calvin Cycle is phosphoglycerate (PGA), a 3-C
chemical. The energy from ATP and NADPH energy carriers generated by the
photosystems is used to attach phosphates to (phosphorylate) the PGA.
Eventually there are 12 molecules of glyceraldehyde phosphate (also known as phosphoglyceraldehyde
or PGAL, a 3-C), two of which are removed from the cycle to make a glucose.
The remaining PGAL molecules are converted by ATP energy to reform 6 RuBP
molecules, and thus start the cycle again. Remember the complexity of life,
each reaction in this process, as in Kreb's Cycle, is catalyzed by a different
reaction-specific enzyme.
Some plants
have developed a preliminary step to the Calvin Cycle (which is also referred
to as a C-3 pathway), this preamble step is known as C-4. While most C-fixation
begins with RuBP, C-4 begins with a new molecule, phosphoenolpyruvate (PEP), a
3-C chemical that is converted into oxaloacetic acid (OAA, a 4-C chemical) when
carbon dioxide is combined with PEP. The OAA is converted to Malic Acid and
then transported from the mesophyll cell into the bundle-sheath cell,
where OAA is broken down into PEP plus carbon dioxide. The carbon dioxide then
enters the Calvin Cycle, with PEP returning to the mesophyll cell. The
resulting sugars are now adjacent to the leaf veins and can readily be
transported throughout the plant.
Fig 18 . C-4
photosynthsis involves the separation of carbon fixation and carbohydrate
systhesis in space and time. (Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates and WH
Freeman , used with permission.)
The capture of
carbon dioxide by PEP is mediated by the enzyme PEP carboxylase, which has a
stronger affinity for carbon dioxide than does RuBP carboxylase When carbon
dioxide levels decline below the threshold for RuBP carboxylase, RuBP is
catalyzed with oxygen instead of carbon dioxide. The product of that reaction
forms glycolic acid, a chemical that can be broken down by photorespiration,
producing neither NADH nor ATP, in effect dismantling the Calvin Cycle. C-4
plants, which often grow close together, have had to adjust to decreased levels
of carbon dioxide by artificially raising the carbon dioxide concentration in
certain cells to prevent photorespiration. C-4 plants evolved in the tropics
and are adapted to higher temperatures than are the C-3 plants found at higher
latitudes. Common C-4 plants include crabgrass, corn, and sugar cane. Note that
OAA and Malic Acid also have functions in other processes, thus the chemicals
would have been present in all plants, leading scientists to hypothesize that
C-4 mechanisms evolved several times independently in response to a similar
environmental condition, a type of evolution known as convergent evolution.
Fig 19 . Photorespiration. (Image from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates and WH Freeman ,
used with permission.)
We can see
anatomical differences between C3 and C4 leaves.
Fig 20 . Leaf anatomy of
a C3 (top) and C4 (bottom) plant. (Images from Purves et al., Life: The
Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman , used with permission.)
Plants may be
viewed as carbon sinks, removing carbon dioxide from the atmosphere and
oceans by fixing it into organic chemicals. Plants also produce some carbon
dioxide by their respiration, but this is quickly used by photosynthesis.
Plants also convert energy from light into chemical energy of C-C covalent
bonds. Animals are carbon dioxide producers that derive their energy from
carbohydrates and other chemicals produced by plants by the process of
photosynthesis.
The balance
between the plant carbon dioxide removal and animal carbon dioxide generation is
equalized also by the formation of carbonates in the oceans. This removes
excess carbon dioxide from the air and water (both of which are in equilibrium
with regard to carbon dioxide). Fossil fuels, such as petroleum and coal, as
well as more recent fuels such as peat and wood generate carbon dioxide when
burned. Fossil fuels are formed ultimately by organic processes, and represent
also a tremendous carbon sink. Human activity has greatly increased the
concentration of carbon dioxide in air. This increase has led to global
warming, an increase in temperatures around the world, the Greenhouse Effect.
The increase in carbon dioxide and other pollutants in the air has also led to acid
rain, where water falls through polluted air and chemically combines with
carbon dioxide, nitrous oxides, and sulfur oxides, producing rainfall with pH
as low as 4. This results in fish kills and changes in soil pH which can alter
the natural vegetation and uses of the land. The Global Warming problem can
lead to melting of the ice caps in Greenland and Antarctica, raising sea-level
as much as 120 meters. Changes in sea-level and temperature would affect
climate changes, altering belts of grain production and rainfall patterns.
After
completing this chapter you should be able to:
- Study the general equation for
photosynthesis and be able to indicate in which process each reactant is
used and each product is produced.
- List the two major processes of
photosynthesis and state what occurs in those sets of reactions.
- Distinguish between organisms
known as autotrophs and those known as heterotrophs as pertains to their
modes of nutrition.
- Explain the significance of the
ATP/ADP cycle.
- Describe the nature of light
and how it is associated with the release of electrons from a photosystem.
- Describe how the pigments found
on thylakoid membranes are organized into photosystems and how they relate
to photon light energy.
- Describe the role that
chlorophylls and the other pigments found in chloroplasts play to initiate
the light-dependent reactions.
- Describe the function of
electron transport systems in the thylakoid membrane.
- Explain the role of the two
energy-carrying molecules produced in the light-dependent reactions (ATP
and NADPH) in the light-independent reactions.
- Describe the Calvin-Benson
cycle in terms of its reactants and products.
- Explain how C-4 photosynthesis
provides an advantage for plants in certain environments.
- Describe the phenomenon of acid
rain, and how photosynthesis relates to acid rain and the carbon cycle..
1. The organic
molecule produced directly by photosynthesis is: a) lipids; b) sugar; c) amino
acids; d) DNA
2. The
photosynthetic process removes ___ from the environment. a) water; b) sugar; c)
oxygen; d) chlorophyll; e) carbon dioxide
3. The process
of splitting water to release hydrogens and electrons occurs during the _____
process. a) light dependent; b) light independent; c) carbon fixation; d)
carbon photophosphorylation; e) glycolysis
4. The process
of fixing carbon dioxide into carbohydrates occurs in the ____ process. a)
light dependent; b) light independent; c) ATP synthesis; d) carbon
photophosphorylation; e) glycolysis
5. Carbon
dioxide enters the leaf through ____. a) chloroplasts; b) stomata: c) cuticle;
d) mesophyll cells; e) leaf veins
6. The cellular
transport process by which carbon dioxide enters a leaf (and by which water
vapor and oxygen exit) is ___. a) osmosis; b) active transport; c. co-
transport; d) diffusion; e) bulk flow
7. Which of the
following creatures would not be an autotroph? a) cactus; b) cyanobacteria; c)
fish; d) palm tree; e) phytoplankton
8. The process
by which most of the world's autotrophs make their food is known as ____. a)
glycolysis; b) photosynthesis; c) chemosynthesis; d) herbivory; e) C-4 cycle
9. The process
of ___ is how ADP + P are converted into ATP during the Light dependent
process. a) glycolysis; b) Calvin Cycle; c) chemiosmosis; d)
substrate-level phosphorylation; e) Kreb's Cycle
10. Once ATP is
converted into ADP + P, it must be ____. a) disassembled into components
(sugar, base, phosphates) and then ressembled; b) recharged by chemiosmosis; c)
converted into NADPH; d) processed by the glycolysis process; e) converted from
matter into energy.
11. Generally
speaking, the longer the wavelenght of light, the ___ the available energy of
that light. a) smaller; b) greater; c) same
12. The section
of the electromagnetic spectrum used for photosynthesis is ___. a) infrared; b)
ultraviolet; c) x-ray; d) visible light; e) none of the above
13. The colors
of light in the visible range (from longest wavelength to shortest) is ___. a)
ROYGBIV; b) VIBGYOR; c) GRBIYV; d) ROYROGERS; e) EBGDF
14. The
photosynthetic pigment that is essential for the process to occur is ___. a)
chlorophyll a; b) chlorophyll b; c) beta carotene; d)
xanthocyanin; e) fucoxanthin
15. When a
pigment reflects red light, _____. a) all colors of light are absorbed; b) all
col;ors of light are reflected; c) green light is reflected, all others are
absorbed; d) red light is reflected, all others are absorbed; e) red light is
absorbed after it is reflected into the internal pigment molecules.
16. Chlorophyll
a absorbs light energy in the ____color range. a) yellow-green; b)
red-organge; c) blue violet; d) a and b; e) b and c.
17. A
photosystem is ___. a) a collection of hydrogen-pumping proteins; b) a
collection of photosynthetic pigments arranged in a thylakjoid membrane; c) a
series of electron-accepting proteins arranged in the thylakoid membrane; d.
found only in prokaryotic organisms; e) multiple copies of chlorophyll a
located in the stroma of the chloroplast.
18. The
individual flattened stacks of membrane material inside the chloroplast are
known as ___. a) grana; b) stroma; c) thylakoids; d) cristae; e) matrix
19. The
fluid-filled area of the chloroplast is the ___. a) grana; b) stroma; c)
thylakoids; d) cristae; e) matrix
20. The
chloroplast contains all of these except ___. a) grana; b) stroma; c) DNA; d)
membranes; e) endoplasmic reticulum
21. The
chloroplasts of plants are most close in size to __. a) unfertilized human
eggs; b) human cheek cells; c) human nerve cells; d) bacteria in the human
mouth; e) viruses
22. Which of
these photosynthetic organisms does not have a chloroplast? a) plants; b) red
algae; c) cyanobacteria; d) diatoms; e) dinoflagellates
23. The
photoelectric effect refers to ____. a) emission of electrons from a metal when
energy of a critical wavelength strikes the metal; b) absorbtion of electrons
from the surrounding environment when energy of a critical wavelength is
nearby; c) emission of electrons from a metal when struck by any wavelength of
light; d) emission of electrons stored in the daytime when stomata are open at
night; e) release of NADPH and ATP energy during the Calvin Cycvle when light
iof a specific wavelength strikes the cell.
24. Light of
the green wavelengths is commonly absorbed by which accessory pigment? a)
chlorophyll a; b) chlorophyll b; c) phycocyanin; d) beta carotene
25. The function
of the electron transport proteins in the thyakoid membranes is ___. a)
production of ADP by chemiosmosis; b) production of NADPH by substrate-level
phosphorylation; c) pumping of hydrogens into the thylakoid space for later
generation of ATP by chemiosmosis; d) pumping of hydrogens into the inner
cristae space for later generation of ATP by chemiosmosis; e) preparation of
water for eventual incorporation into glucose
26. ATP is
known as the energy currency of the cell because ____. a) ATP is the most
readily usable form of energy for cells; b) ATP passes energy along in an
electron transport chain; c) ATP energy is passed to NADPH; d) ATP traps more
energy than is produced in its formation; e) only eukaryotic cells use this
energy currency.
27. Both cyclic
and noncyclic photophosphorylation produce ATP. We can infer that the purpose
of ATP in photosynthesis is to ____. a) supply hydrogen to the carbohydrate; b)
supply carbon to the carbohydrate; c) supply energy that can be used to
form a carbohydrate; d) transfer oxygens from the third phosphate group to the
carbohydrate molecule; e) convert RuBP into PGA
28. The role of
NADPH in oxygen-producing photosynthesis is to ____. a) supply hydrogen to the
carbohydrate; b) supply carbon to the carbohydrate; c) supply energy that can
be used to form a carbohydrate; d) transfer oxygens from the third phosphate
group to the carbohydrate molecule; e) convert RuBP into PGA.
29. The dark
reactions require all of these chemicals to proceed except ___. a) ATP; b) NADPH;
c) carbon dioxide; d) RUBP; e) oxygen
30. The first
stable chemical formed by the Calvin Cycle is _____. a) RUBP; b) RU/18; c) PGA;
d) PGAL; e) Rubisco
31. The
hydrogen in the carbohydrate produced by the Calvin Cycle comes from ___ a.)
ATP; b) NADPH; c) the environment if the pH is very acidic; d) a and b; e) a
and c
32. The carbon
incorporated into the carbohydrate comes from ___. a) ATP; b) NADPH; c) carbon
dioxide; d) glucose; e) organic molecules
33. C-4 photosynthesis
is so named because _____. a) it produces a three carbon compound as the first
stable product of photosynthesis; b) it produces a four carbon compound as the
first stable produc of photosynthesis; c) it produces four ATP and four NADPH
molecules for carbon fixation.; d) there are only four steps in this form of
carbon fixation into carbohydrate.
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