BIO 311C
Spring 2010
Exam 3: Friday, Apr. 16, in this classroom
The last 30 minutes of Wednesday’s lecture period will be
devoted to a review of the topics covered since Exam 2.
Lecture 30 – Monday 12 Apr.
1
Summary of the Light Reactions of Photosynthesis
noncyclic electron transport and photophosphorylation:
cyclic photophosphorylation (resulting from cyclic electron transport):
sum - when cyclic and noncyclic electron flow occur in a 1:1 ratio:
Thus, a 1:1 ratio of cyclic PHP to noncyclic PHP produces a ratio of 2
NADH : 3 ATP, the ratio required for operation of the Calvin Cycle.
3
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Overview of light harvesting, noncyclic electron transport
and noncyclic photophosphorylation in photosynthesis,
shown as the traditional "Z-Scheme“.
From textbook Fig. 10.13, p. 195
5
The Z-scheme does not attempt to show the actual position of the
components in the thylakoid membrane, but instead illustrates the
energy level of electrons transported through the system at each
component during operation of the light reactions.
*
Overview of light harvesting by PS I, cyclic electron transport
and cyclic photophosphorylation in photosynthesis.
energy of electrons
Textbook Fig. 10.15, p. 196
light
PS 2 temporarily
inactivated
6
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Summary of the Light Reactions of Photosynthesis,
Showing Initial Reactants and Final Products Stoichiometrically
Photosystem 2
Photosystem 1
The production of 1 molecule of glucose (hexose) in the Calvin Cycle requires
12 NADPH and 18 ATP, which must be generated by the light reactions of
photosynthesis. The illustration above shows the number of quanta of light
and number of substrates that are utilized in the light reactions in order to
produce the correct number of NADPH and ATP molecules.
7
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The Light Reactions of Photosynthesis (simplified)
The ratio of products of the light reactions is:
The Dark Reactions of Photosynthesis (simplified)
The ratio of reactants required in the dark reactions (Calvin Cycle) is:
9
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Relationship of the Light Reactions to
the Calvin Cycle of Photosynthesis
Products of the light reactions are reactants for the Calvin Cycle, while
products of the Calvin Cycle are reactants for the light reactions.
10
Heat is shown as a product since photosynthesis is only approximately
24% efficient, with the remaining 76% of captured energy released as
heat.
*
Illustration of the Requirement of the Light Reactions
to Drive the Calvin Cycle of Photosynthesis
energy
11
reducing
units
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The Calvin Cycle
(The "Dark" Reactions of Photosynthesis)
Calvin Cycle
The “dark reactions” of photosynthesis can operate in the light or in the dark.
The rate of this metabolic pathway is regulated by light, but light is not a
substrate.
12
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The Calvin Cycle Shown as a Cycle
(shown as four stages)
Phase 1: fixation
Phase 2: reduction
Phase 3: rearrangement
Phase 4: regeneration
Accumulation of C3, C4, C5, C6, and C7 sugar phosphates in the stroma.
Six times around the cycle produces one hexose, the form of sugar that
can be polymerized and stored as starch in chloroplasts.
Three times around the cycle produces one triose, the form of sugar that
can be transported from the chloroplast to the cytoplasmic matrix for
further metabolism.
13
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The Calvin Cycle
Textbook Fig. 10.18, p. 199
Note: This textbook illustration shows
Phases 3 and 4 (rearrangement and
regeneration from Slide 13) combined
into a single phase.
14
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The First Reaction of the Calvin Cycle; Carbon Fixation;
Assimilation of Carbon Dioxide into an Organic Molecule
Unstable intermediate
This reaction is referred to as carbon dioxide fixation.
“Fixation” refers to the capture of a gaseous substance and converting it
chemically into a stable non-gaseous form.
16
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Rubisco (also called RUBISCO)
The Enzyme that Catalyzes the First Reaction of the Calvin Cycle
"side" view
small subunits
(shown in red)
"top" view
large subunits (shown
in light and dark blue)
Rubisco consists of 16 polypeptide chains, including 8 catalytic large
polypeptide chains (large subunits) and 8 allosteric small polypeptide
chains (small subunits). Thus, it is an oligomeric protein.
17
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Is rubisco the most important enzyme on earth?
Consider the following:
- Rubisco is the most abundant enzyme on earth.
- Virtually all organic carbon in the world became organic
through the Calvin Cycle of photosynthesis.
- The carbon dioxide level in the atmosphere remains low
because of its continuous removal through the Calvin Cycle of
photosynthesis.
18
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Is rubisco one of the most interesting enzymes on earth?
Consider the following:
The 8 catalytic polypeptides of rubisco are made from DNA information
coded in the chloroplast, using 70S chloroplast ribosomes.
The 8 regulatory polypeptides of rubisco are made from DNA
information coded in the nucleus, using 80S cytoplasmic
ribosomes. The regulatory polypeptide chains then pass through
both membranes of the chloroplast envelope, into the stroma.
The synthesis and movement of the large and small subunits are
coordinated such that equal concentrations of each accumulate in
the stroma, and they bind together correctly to form functional
rubisco enzyme that contains eight large subunits and 8 small
subunits with exactly the right quaternary structure.
19
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Is rubisco an efficient enzyme?
Consider the following:
There are many times more rubisco molecules in the chloroplast than any
other enzyme.
Rubisco doesn't react very rapidly with CO2, so its activity is sluggish.
Rubisco also reacts with O2 from the atmosphere, which competes
with CO2 as a substrate. The rubisco-catalyzed combination of O2 with
a C5 molecule is a highly wasteful process called photorespiration. In
most plants (Called C3 plants), rubisco reacts with CO2 about 2/3 of the
time and reacts with O2 about 1/3 of the time.
Thus, rubisco is an inefficient enzyme. Photosynthetic cells must
compensate for its inefficiency by producing large amounts of it. That
is why rubisco and it is the most abundant enzyme on earth.
20
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Most plants in the world are characterized as C3 plants.
Cross-section of a Leaf
of a Typical C3 Plant
Vein
From textbook Figure
10.3, p. 187
Epidermis: prevents gases
from entering and leaving the
chlolroplasts except through
stomata.
Stomata
CO2
O2
Photosynthetic cells of C3 plants are all directly exposed to CO2 from the atmosphere.
In all plants, including C3 plants, carbon dioxide enters the interior of leaves only
through the stomata.
The first product of carbon dioxide fixation in C3 plants is a 3-carbon sugar phosphate (an
aldotriose phosphate). Photosynthesis of C3 plants, whereby the first product of
photosynthesis is a C3 sugar phosphate, is called C3 photosynthesis.
21
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Standard (C3) photosynthesis
CO2
CO2
Atmosphere
(low conc. of CO2)
(low conc.)
photosynthetic CO2 fixation
via Rubisco to form a C3
sugar phosphate.
chloroplast
Green Plant Cell
Photosynthesis occurs entirely in the chloroplast, and only during the daytime.
Photosynthesis is relatively inefficient because:
- The concentration of CO2 provided to the chloroplast is low.
- The rubisco enzyme is inefficient in utilizing CO2 and also uses
O2 as a competing substrate.
22
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Structure of the Interior of a Leaf of a C4 Plant
From textbook Fig. 10.19, P. 201
mesophyll cell
bundle-sheath cell
Vein
(also called a
vascular bundle)
epidermis
(sealed to prevent
gas exchange)
Stomata (not shown in this illustration, see Slide
21) allow CO2 diffusion into leaves of C4 plants,
just as in C3 plants.
Mesophyll cells of C4 plants are directly exposed to CO2 from the atmosphere.
But bundle sheath cells, where the Calvin cycle takes place, do not have
access to atmospheric CO2.
23
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C4 Photosynthesis
CO2 accumulates to a
high concentration
low CO2 concentration
C4
PS I
Cyclic
PHP
CO2
Active transport
C4
ATP
CO2
CO2
C3
Active transport
Outer (mesophyll) cell
typical
photosynthesis,
as in C3 plants
C3
Inner (bundle-sheath) cell
CO2 is pumped in this direction
The first product of carbon dioxide fixation is a 4-carbon
compound (a C4 compound).
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C4 Photosynthesis;
Occurs in some unusually fast-growing plants
1. CO2 from the atmosphere is covalently bound to
a C3 organic molecule to produce a C4 organic
molecule in a plant cell that is accessible to the
atmosphere.
mesophyll
cell
C4
2. The C4 organic molecule is transported by active
transport to an adjacent cell that is not accessible
to the atmosphere.
3. CO2 is released from the C4 molecule. The
regenerated C3 molecule is then transported back
to the original cell, and the released CO2
accumulates to a high concentration.
C3
bundle
sheath
cell
4. The Calvin Cycle converts the high-concentration
CO2 into organic molecules because rubisco
functions efficiently at high CO2 concentration.
From textbook Fig. 10.20, p. 202
25
C4 photosynthesis is more efficient than C3 photosynthesis at high
light intensities, when light is not rate limiting and CO2 concentration
becomes rate limiting.
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