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3.3
Photosynthesis
E X P E C TAT I O N S
Describe the energy transformations that occur in photosynthesis.
Describe the role of enzymes in metabolic reactions in chloroplasts.
Investigate and explain how the structure of molecules can influence
metabolic rate.
Compare the structure and function of chloroplasts and mitochondria.
Photosynthesis is one of the most important
chemical processes on Earth. Photosynthesis
involves the use of energy from light to form
carbohydrates. Organisms that manufacture their
own food (autotrophs), such as plants, algae,
cyanobacteria, and photosynthetic bacteria, do so
through photosynthesis. Autotrophs form the base
of the food chain for virtually all communities of
heterotrophs, which must eat to obtain nutrients.
The process of photosynthesis also produces the
oxygen found in the atmosphere. Oxygen is used
by organisms for many processes, such as aerobic
cellular respiration. The overall equation for
photosynthesis is as follows:
6CO2 + 6H2O + energy → C6H12O6 + 6O2
carbon
water
glucose oxygen
dioxide
The process of photosynthesis is believed to
have originated in bacteria. Some of these bacteria
were able to produce oxygen. Other bacteria were
able to carry out a different form of photosynthesis
but did not produce oxygen. In 2000, biochemists
led by Dr. Carl Bauer at the University of Indiana
found that non-oxygen-producing species (purple
and green bacteria) are the most ancient
photosynthetic bacteria. The oxygen-producing
cyanobacteria that exist today (see Figure 3.19)
evolved from a non-oxygen-producing bacteria
called heliobacteria.
As you may have noticed, the overall equation
for photosynthesis is exactly the opposite of the
equation for aerobic cellular respiration. This does
not mean, however, that the reactions follow the
same course in reverse. Photosynthesis requires
structures and metabolic processes similar to those
used in mitochondria: electron transport chains,
dissolved enzymes, and a membrane-enclosed
space for chemiosmosis.
Figure 3.18 The process of photosynthesis is one of the
most important of all life processes. Green plants use
carbon dioxide and water to produce oxygen and food.
Figure 3.19 Cyanobacteria eventually gave rise to the
structures that carry out photosynthesis in the algae and
green plants of today.
Chapter 3 Cellular Energy • MHR
83
leaf cross section
mesophyll cell
opening for C O2
to enter leaf
chloroplast
vacuole
nucleus
cell wall
granum
lamella
thylakoid stroma
Structure of Chloroplasts
In plant cells, photosynthesis occurs within
chloroplasts. Chloroplasts have a double membrane
and contain membrane pockets called thylakoids
(see Figure 3.20). Thylakoids occur in stacked,
parcel-like structures called grana (singular granum),
which are held together by support structures
called lamellae. The stroma, a thick, enzyme-rich
liquid, fills the interior of each chloroplast.
Mesophyll cells in the leaves of plants are
specialized for photosynthesis and contain
numerous chloroplasts. These cells provide the
chloroplasts with the two important ingredients
necessary for photosynthesis — carbon dioxide and
water. Gas exchange (oxygen and carbon dioxide)
occurs through pores on the underside of leaves,
and water is delivered via veins that extend to the
roots of the plant.
Within the grana, solar light energy is captured
by the thylakoids. This energy is used to form
ATP molecules, which fuel the production of
carbohydrates. These carbohydrate molecules are
then used to synthesize glucose — the molecules
used in cellular respiration. The thylakoid
membrane in the chloroplast is the site of ATP
production, using chemiosmosis and complex
structures functionally similar to those found in
mitochondria.
Stages of Photosynthesis
As the previous section suggests, there are two main
stages of photosynthesis: the photo and synthesis
stages. The first stage of photosynthesis converts
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MHR • Unit 1 Metabolic Processes
inner
membrane
outer
membrane
Figure 3.20 Structure
of a chloroplast
solar energy into chemical energy. The second stage
uses this energy to produce PGAL, which is then
used to form glucose (see Figure 3.21). The photo
reactions require light and are called lightdependent reactions. The synthesis reactions do
not require light directly, and are called lightindependent reactions. However, light seems to be
important in activating enzymes in both the photo
and synthesis reactions.
sunlight
photo reactions
A
synthesis
reactions
organic
molecules
B
Figure 3.21 Two stages of photosynthesis. The first stage
(A) consists of reactions that require light energy. The
second stage (B) involves the synthesis of glucose molecules.
As light strikes the leaf of a plant, the energy is
captured by pigments in the chloroplasts. These
pigments, known as chlorophylls, absorb various
wavelengths of visible light (see Figure 3.22). The
two most important types of chlorophyll are
chlorophyll a and chlorophyll b. Photosynthesis is
most active at light wavelengths of about 400 nm
to 450 nm and 650 nm to 700 nm. The colour of
chlorophyll, green, is a result of the absorption of
mainly blue and red parts of the visible light
spectrum. In the following MiniLab, you will
extract chlorophyll from leaves and examine the
colour and properties of both types of chlorophyll.
B Chlorophylls a and b
absorb certain
wavelengths of
visible light.
chlorophyll a
yellow and green
transmitted
visible spectrum
prism
Relative light absorption
A Visible light represents only
a small segment of the
electromagnetic spectrum.
chlorophyll b
solution of
chlorophyll
white light
400
450
500
550
600
650
700
Wavelength (nm)
Figure 3.22 The relative absorption of light by chlorophylls a and b.
MINI
LAB
Photosynthetic Pigments
In this lab you will investigate the colours of chlorophylls,
the photosynthetic pigments found in many plants. You will
need to produce a concentrated extract of chlorophylls.
The materials you will require are: 15 g fresh or frozen
spinach, 50 mL isopropyl alcohol, food blender, 100 mL
beaker, funnel, filter paper, strong light source (for example,
slide projector).
2. What wavelengths of visible light are absorbed by the
chloroplasts?
3. When you viewed the solution, the effect you saw is
called fluorescence. What colours and wavelengths
were produced?
Using the food blender, grind the spinach with 50 mL of
isopropyl alcohol. Filter the extract through several layers
of filter paper in the funnel. Then, in a darkened room, shine
a strong beam of light at a sample of the filtered extract.
Observe the colour of the chlorophylls by viewing the
sample at a slight angle and then at a right angle to the
beam of light. Describe the appearance of the chlorophylls
as viewed from both angles.
Analyze
1. What colours of the visible light spectrum are absorbed
by chlorophyll as part of the photo reaction?
Photosynthetic pigments
Chapter 3 Cellular Energy • MHR
85
Light energy is absorbed by a network of
chlorophyll molecules known as a photosystem
(see Figure 3.23). These chlorophyll molecules are
known as antenna pigments because they collect
and channel energy. This energy causes electrons
in the chlorophyll molecules to become energized.
Energy from these electrons is passed from one
chlorophyll molecule to another in the photosystem.
Eventually the energy reaches the reaction centre,
a specific chlorophyll a molecule. Only one in 250
chlorophyll molecules forms a reaction centre. A
unit of several hundred antenna pigment molecules
together with a reaction centre is called a
photosynthetic unit. The large number of antenna
pigment molecules in each photosynthetic unit
allows the reaction centre to be supplied with the
greatest possible amount of energy. Once the energy
has reached the reaction centre, an electron
acceptor receives the energized electron. Energy
from these electrons is used to move H+ ions into
the thylakoid interior for ATP production.
ADP
+
ATP
n s p o rt s y st e m
Pi
e−
n tr a
reaction-centre
chlorophyll a
electron
acceptor
tr o
chlorophyll
molecules
electron
acceptor
sun
ele
c
light
photosystem 700. After the electron acceptors
receive the energized electrons from the reaction
centre, the electrons flow through an electron
transport system. Here the electrons are passed
from one electron carrier to another. Some of these
carriers are cytochrome molecules. As the electrons
pass through the system, they release energy that is
used to phosphorylate ADP molecules to produce
molecules of ATP. This process is called cyclic
photophosphorylation, because after the ATP
molecules are produced the electrons are cycled
back into the photosystem. Only ATP molecules
are produced by photosystem 700.
energy of electron
Photosystems
e−
reaction–
centre
chlorophyll a
pigment
complex
photosystem 700
Figure 3.24 The cyclic electron pathway. In photosystem 700,
Figure 3.23 A photosystem works by passing light energy
from one molecule of chlorophyll to another.
Cyclic Electron Pathway
There are two types of photosystems. Photosystem
700, which absorbs light 700 nm in wavelength,
is used by some photosynthetic bacteria. This
photosystem contains molecules of chlorophyll a,
which is found in cyanobacteria and all
photosynthetic eukaryotes (such as green plants).
Figure 3.24 shows how electrons pass through
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MHR • Unit 1 Metabolic Processes
electrons are recycled after their energy is used to form ATP
molecules.
WEB LINK
www.mcgrawhill.ca/links/biology12
To find out more about how photosynthetic bacteria perform
photosynthesis, go to the web site above, and click on Web
Links. Note the types of bacteria that use photosynthesis to
produce ATP. Prepare a chart that compares and contrasts the
methods by which bacteria and plants perform photosynthesis.
Non-cyclic Electron Pathway
Photosystem 680 absorbs light 680 nm in wavelength.
The shorter the wavelength of light, the higher its
energy. Therefore, photosystem 680 is more powerful
than photosystem 700 because photosystem 680
can capture higher-energy light. In addition to
chlorophyll a, photosystem 680 contains molecules
of chlorophyll b. It also contains molecules of
chlorophyll c, chlorophyll d, and accessory pigments
such as carotenes, xanthophylls, and anthocyanins.
The pigments other than chlorophyll a aid in
absorbing wavelengths of light not absorbed by
chlorophyll a. Green plants, algae, and cyanobacteria
(unlike other bacteria species) use both photosystems
680 and 700 to carry out photosynthesis. In this
case, electrons from photosystem 680 are shunted
to photosystem 700, as shown in Figure 3.25. The
energy from electrons in photosystem 680 is used
to produce ATP molecules. These electrons then
move to photosystem 700 where, after becoming
energized, they are taken up by NADP+
(nicotinamide adenine dinucleotide). After NADP+
accepts two electrons and a hydrogen ion (H+ ), it
becomes the coenzyme NADPH. The production
of NADPH and ATP are endothermic reactions,
which require an input of energy. The ATP and
NADPH molecules are then used in the synthetic
steps to produce glucose.
After ATP molecules are produced by
photosystem 680, electrons that have passed
through the electron transport system are not
cycled back into photosystem 680. This type of
ATP production is called non-cyclic
photophosphorylation. However, photosystem 680
requires electrons to keep the photosystem
operating. After photosystem 680 transfers an
electron to the electron acceptor, photosystem 680
captures an electron from a Z enzyme. This enzyme
is responsible for splitting water molecules into
hydrogen ions and oxygen molecules and
sun
electron
acceptor
energy of electron
sun
NADP +
e−
electron
acceptor
NADPH
H+
e−
ele
e
−
c tr
on
tra
nsp
ort
sys
tem
ADP
+
Pi
e−
ATP
reaction–centre
chlorophyll a
e−
reaction–centre
chlorophyll a
photosystem 700
photosystem 680
e − H+
Z enzyme
H2O
CO2
2 H+
1
O
2 2
Figure 3.25 The non-cyclic electron pathway. Electrons
from water move from photosystem 680 to photosystem 700
and then to NADP+ . The ATP and NADPH molecules that are
C16 H12 O6
synthesis reactions
produced by these reactions fuel the synthesis reactions
that form glucose.
Chapter 3 Cellular Energy • MHR
87
channelling electrons to the electron acceptor
(see Figure 3.25). This process is called photolysis
because light energy is required to split bonds
within the water molecule. All of the oxygen that
we breathe, and all the oxygen in Earth’s
atmosphere, has been generated through the
photolysis stage of photosynthesis.
In addition to passing electrons from water to
chlorophyll molecules, the Z enzyme that performs
photolysis also donates a hydrogen ion from the
same water molecule to the reaction-centre of
photosystem 680. This hydrogen ion joins the
electron in its journey along the electron transport
chain. The electron–hydrogen ion combination
supplies energy to an electron transport chain
comprised of cytochrome enzymes. This chain of
enzymes in turn drives a proton pump, similar to
the one you learned about in chemiosmosis in the
mitochondrion. The photosynthetic proton pump,
like proton pumps in the electron transport chain
of the mitochondrion, moves H+ ions out of the
stroma, into a membrane-enclosed space, as
illustrated by Figure 3.26. Just as the inner membrane
of the mitochondrion contains an ATP synthase
complex that opens to the matrix, the thylakoid
membrane of the chloroplast contains an ATP
synthase complex where H+ ions flow through to
the stroma and energize the phosphorylation of
ADP. This process is called photophosphorylation.
ELECTRONIC LEARNING PARTNER
To learn how the intensity and wavelength of light can
affect ATP/NADPH production in chloroplasts, go to your
Electronic Learning Partner now.
The thylakoid space serves as a reservoir for
hydrogen ions. Every time the Z enzyme splits
water to form two hydrogen ions, the thylakoid
space receives them. Whenever photosystem 680
donates an electron to the electron transport system,
giving up energy along the way to drive the proton
pump, hydrogen ions move in from the stroma.
A hydrogen ion gradient is formed when the
thylakoid space contains more hydrogen ions than
the stroma. The movement of hydrogen ions across
the thylakoid membrane releases energy that is
used in ATP synthesis. This gradient forces the
hydrogen ions through the ATP synthase complex
that resides on the membrane of the thylakoid
body. This movement of hydrogen ions provides
the energy required to join ADP and Pi in the
chemiosmotic synthesis of ATP.
photosystem II
light
photosystem I
cytochrome
complex
antenna
complex
stroma
light
antenna
complex
+
H
e
NADP + + H+
NADP
reductase
−
e−
e
NADPH
proton
pump
−
Z enzyme
Figure 3.26 Within the thylakoid
H+
H2O
H+
1
O
2 2
thylakoid
membrane
+
synthesis
reactions
+
2 H
H+
H+
ATP
+
H
thylakoid space
H+
ATP synthase
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MHR • Unit 1 Metabolic Processes
ADP + Pi
membrane, enzyme complexes
pump hydrogen ions from the
stroma into the thylakoid space.
This process forms a hydrogen
ion gradient. As hydrogen ions
flow down the gradient and back
into the stroma through the ATP
synthase complex, ATP molecules
are formed. As you can see in
this diagram, photosystem 680
is also called photosystem II.
Photosystem 700 is also called
photosystem I.
membrane are used during the synthesis reactions
to produce organic molecules from carbon dioxide.
ATP and NADPH molecules are formed on the
thylakoid membrane by means of the ATP synthase
complex and the NADP reductase complex,
respectively (see Figure 3.26). The ATP and NADPH
The electrons from photosystem 680 energize
electrons that travel through the electron transport
chain in the thylakoid to pump protons. The
electrons lose energy after they move through the
electron carriers. At photosystem 700, the electrons
are re-energized by light energy. These two electrons
now move along the final carriers of the electron
transport chain to the NADP reductase complex.
Here, two electrons are transferred to NADP+ ,
which also combines with a hydrogen ion to form
the reduced NADPH, as shown in Figure 3.26.
Both chloroplasts and mitochondria use
chemiosmosis to produce ATP. These organelles
also rely on an electron transport chain to power
proton pumps and move electrons to an electron
acceptor that removes them (such as water or
NADPH). The proton pumps create the hydrogen
ion gradient that both organelles use to make ATP.
Chloroplasts and mitochondria even share the basic
construct of an ATP synthase complex, which is
remarkably similar in both structures. However,
there are some differences in the way that
phosphorylation occurs in the two organelles,
as summarized in Table 3.3.
Table 3.3
Differences in phosphorylation between mitochondria
and chloroplasts
Oxidative phosphorylation
in mitochondria
Electrons in the electron
transport chain are supplied by
the oxidation of food molecules.
food energy → ATP molecules
Photophosphorylation
in chloroplasts
Electrons in the electron transport
chain are extracted from water
during photolysis and passed
on to pigment molecules (driven
by captured solar energy) to
donate them to the chain.
light energy → ATP molecules
The Calvin Cycle
In photosynthesis, both the NADPH and the ATP
produced by the photo reactions in the thylakoid
Inner membrane pumps
hydrogen ions from the matrix
to the intermembrane space.
Intermembrane space serves
as proton reservoir.
Thylakoid membrane pumps
hydrogen ions from the stroma
to the thylakoid interior.
Thylakoid interior pools ions.
ATP synthase resides between
the membrane and the matrix,
producing ATP molecules as
hydrogen ions move back into
the matrix from the
intermembrane space.
ATP synthase bridges the
thylakoid membrane and its
interior, producing ATP
molecules as hydrogen ions
move into the stroma.
3 CO2
Metabolites of the Calvin Cycle
3 RuBP
C5
6 PGA
C3
carbon
fixation
RuBP
ribulose bisphosphate
PGA
3-phosphoglycerate
PGAP
1,3-bisphosphoglycerate
PGAL
glyceraldehyde-3-phosphate
6 ATP
3 ADP + 3 Pi
Calvin cycle
These ATP
3
molecules
ATP
were produced
by the photo
reaction.
6 ADP + 6 Pi
reduction
re-formation
of RuBP
6 PGAP
C3
5 PGAL
C3
6 NADPH
6 PGAL
C3
Figure 3.27 The Calvin
6 NADP +
There is a net gain
of one PGAL.
1 PGAL
C3
These ATP and
NADPH molecules
were produced by
the photo reactions.
Glucose phosphate and
other organic compounds
cycle produces one
molecule of PGAL for
every three molecules of
CO2 that enter the cycle.
PGAL is used to form
glucose and other
organic compounds.
Chapter 3 Cellular Energy • MHR
89
molecules formed then leave the thylakoid
membrane and enter the stroma, where a series of
enzymes perform synthesis reactions in the Calvin
cycle. The Calvin cycle is named after biochemist
Melvin Calvin. In the late 1940s, Calvin led a
team of researchers to determine the steps of this
synthesis reaction.
Every photosynthetic plant uses the Calvin cycle
to form PGAL. PGAL is then used to synthesize
many different molecules. Using PGAL as the
building block, plants can synthesize amino acids
and fatty acids. Other molecules that can be formed
from PGAL include fructose phosphate, glucose,
sucrose, starch, and cellulose. Although plants
synthesize these molecules, not every plant uses
the same metabolic pathway.
The Calvin cycle has three distinct stages, as
shown in Figure 3.27, on the previous page:
1. Stage 1: carbon fixation
2. Stage 2: reduction
3. Stage 3: re-formation of RuBP (ribulose 1,5
bisphosphate)
These three stages will now be discussed.
Stage 1: Carbon Fixation
Carbon fixation is the initial incorporation of
carbon into organic molecules. To eventually build
complex molecules, such as glucose, plants must
first attach carbon to smaller carbon-containing
molecules. They do this by taking carbon dioxide
from the atmosphere and attaching it to RuBP,
ribulose bisphosphate, as shown in Figure 3.28. A
six-carbon molecule is the product of this reaction,
but this molecule is extremely unstable and
immediately splits into two molecules of threecarbon PGA (phosphoglycerate). The enzyme RuBP
H2C
H2C
O
C
O
HC
HC
H2C
O
OH
RuBP
carboxylase
O
P
−
O
H2C
+ H+
O
C
O
P
OH
HC
RuBP
(Ribulose
1,5 bisphosphate)
+ H+
O
C
CO2 + H2O
P
OH
HC
P
OH
O
−
PGA
(phosphoglycerate)
Figure 3.28 In the Calvin cycle, C3 fixation produces two
three-carbon PGA. The reaction is catalyzed by the enzyme
RuBP carboxylase.
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MHR • Unit 1 Metabolic Processes
carboxylase catalyzes this reaction, as shown in
Figure 3.28. This reaction is called C3 fixation
because it produces two three-carbon molecules of
PGA. This molecule then passes into the next stage
of the Calvin cycle. C3 fixation is used by plants,
such as rice, wheat, and oats, which occur mainly
in temperate regions.
To form a molecule of glucose (C6H12O6 ), six
carbon atoms must be fixed. Figure 3.27 shows
that nine molecules of ATP are required to fix the
three carbon atoms in the PGAL that is available
to be used for glucose production. Therefore,
18 molecules of ATP are needed to fix the six
carbon atoms required to form a glucose molecule.
In addition to carbon fixation, RuBP carboxylase
oxidizes RuBP with O2 to form CO2 by a process
called photorespiration. Photorespiration creates
an inefficiency in the carbon fixation process, since
both the oxidation of RuBP and carbon fixation are
catalyzed by the same enzyme — RuBP carboxylase.
Both oxygen and carbon dioxide compete to bind
with RuBP. The Calvin cycle is an ancient process
that developed in an atmosphere with little
free oxygen.
The rate of reactions in the Calvin cycle
increases with temperature to about 25°C. Reaction
rate levels out and declines when temperatures
approach or exceed 37°C. At warmer temperatures,
RuBP carboxylase is mainly involved in oxidizing
RuBP, and very little carbon fixation occurs. Thus,
plants that live in warmer climates have developed
a different approach to fixing carbon. For example,
C4 fixation is used by plants, such as sugarcane and
corn. In these plants, the Calvin cycle takes place
in bundle-sheath cells, as shown in Figure 3.29.
Plants that use C4 fixation form the four-carbon
oxaloacetate and malate in parenchyma cells. The
malate moves into the bundle-sheath cells and a
carbon is removed as CO2 . Inside the bundlesheath cells, there is a greater concentration of CO2
and a lower concentration of oxygen than in
parenchyma cells at the surface of the leaf. This
difference in concentration allows CO2 to have a
greater opportunity to bind with RuBP carboxylase.
As a result, the plant can fix sufficient amounts of
carbon to produce glucose in the Calvin cycle.
In tropical climates, where the temperature often
exceeds 28°C, food crops such as corn and sugarcane
are commonly grown. Crops that use C3 fixation,
however, do not survive well in tropical climates
because they fix relatively less carbon and form
fewer glucose molecules. Thus, the types of crops
that can be grown in warmer climates are limited
mainly to plants that use C4 carbon fixation.
WEB LINK
www.mcgrawhill.ca/links/biology12
Deserts plants, such as cacti and aloe vera, use a third method
of carbon fixation called CAM (crassulacean-acid metabolism)
fixation. Why do desert plants require a different method of
carbon fixation? What processes are involved in CAM fixation?
To find out more about CAM fixation, go to the web site above,
and click on Web Links. Prepare an illustrated information
handout that explains the process of carbon fixation in desertdwelling species of plants.
Stage 2: Reduction
In the second stage of the Calvin cycle, the stroma
performs the necessary enzymatic reactions that
reduce PGA to form PGAL. This happens in two
stages. First, ATP molecules donate phosphate
groups to the PGA molecules, converting them to
bisphosphoglycerate, or PGAP molecules (see
Figure 3.31). Secondly, an NADPH molecule, which
was produced during the photo reactions, donates
a hydrogen ion and two electrons to PGAP. This
reduces PGAP to glyceraldehyde phosphate, or
PGAL — the building block for anabolic processes
including the synthesis of glucose. The oxidized
NADP+ can return to the thylakoid membrane to
be reduced again.
ATP
ADP + Pi
PGA
PGAP
NADPH
PGAL
NADP + + H+
Figure 3.31 During the reduction stage, PGAP is reduced to
become PGAL.
CO2
phosphoenolpyruvate (PEP)
oxaloacetate
CCC
CCCC
AMP
parenchyma cell
ATP
pyruvate
malate
CCC
CCCC
pyruvate
malate
CCC
CCCC
bundlesheath
cell
A
CO2
Calvin
cycle
sugar
B
Figure 3.30 Wheat (A), and corn plants (B) have
Figure 3.29 Carbon fixation in C4 plants
adapted in different ways to their climates to
circumvent the problem posed by active-site
competition in the fixation of carbon dioxide.
Chapter 3 Cellular Energy • MHR
91
Stage 3: Re-formation of RuBP
Recall from Figure 3.27 and Figure 3.28 that RuBP,
ribulose bisphosphate, is required in the carbon
fixation stage of the Calvin cycle. RuBP is used to
produce PGA, needed for the reduction stage of the
cycle. Because PGAL is needed to reform RuBP, the
majority of PGAL molecules, do not contribute to
glucose production. The Calvin cycle reactions
must occur twice to create one molecule of glucose.
This is because for every three times that the
Calvin cycle reactions occur, five PGAL are used to
re-form three RuBP, ribulose bisphosphate, as
shown in Figure 3.32. Notice from Figure 3.27 that
5 three-carbon PGALs contain the same number of
carbon atoms as 3 five-carbon RuBPs.
5 PGAL
3 RuBP
3 ADP + 3 Pi
3 ATP
Figure 3.32 Re-formation of RuBP. As five molecules of
PGAL become three molecules of RuBP, three molecules of
ATP become three molecules of ADP + Pi .
To summarize the synthesis reactions of the
Calvin cycle:
■
Stage 1: Carbon fixation, which takes carbon
atoms from atmospheric carbon dioxide
molecules and incorporates these atoms
into organic molecules.
■
Stage 2: Reduction, which involves the
formation of PGAP and its reduction to PGAL.
■
Stage 3: Re-formation of RuBP, which uses
most of the PGAL molecules formed in the
reduction stage to produce RuBP. This is then
used to form more PGA in the Calvin cycle.
Glucose: The Ultimate Food Source
After glucose is produced in the synthesis reactions,
plant cells can use glucose for glycolysis, followed
by aerobic respiration in the mitochondria. The
products and intermediary molecules of aerobic
respiration provide the carbon-based molecules
necessary to build amino acids, as well as the
precursors to nucleic acids and lipids. However,
there are many other ways that plants use glucose,
for example,
the conversion of glucose to starch,
the formation of cellulose from glucose, and
the conversion of glucose to sucrose.
THINKING
LAB
Metabolic Rate and
the Structure of Molecules
O
O
O
O
O
Background
The molecule used by plants, such as corn and potatoes,
to store energy is called starch. Starch is a large polymer
composed of about 1000 glucose molecules. The starch is
formed through condensation reactions, which link together
individual glucose molecules. The starch molecule may
contain side-branches, as shown in the illustration. Before
a plant can use starch in aerobic cellular respiration, the
starch must be broken down into individual glucose
molecules. Recall that glucose is the molecule that first
enters glycolysis. The rate at which glucose is available to
be used in respiration can affect how quickly the cell will
carry out metabolic processes. In other words, the availability
of glucose can determine a plant’s metabolic rate.
You Try It
1. What reaction is needed to break down a starch
molecule into individual glucose molecules?
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MHR • Unit 1 Metabolic Processes
O
O
O
O
O
O
O
CH2
O
O
O
O
O
O
O
A starch molecule
2. If sweet corn contains mainly glucose molecules and
starchy corn contains mainly starch molecules, in which
type of corn would you expect cells to have a slower
metabolic rate? Explain briefly.
3. Discuss how the structure of molecules might affect
metabolic rate.
4. How could you determine if corn is sweet or starchy
without tasting it?
5. What is the energy storage molecule in animal cells?
These three ways will now be described.
Autotrophs, such as green plants, produce a
molecule used for energy storage, called starch.
The starch is a large, branched polysaccharide
composed of hundreds of glucose molecules linked
by condensation reactions. Plants convert glucose
to starch in the stroma. During peak hours of bright
daylight, plants may produce more starch than they
can use. This starch is stored in cells and is ready
to be broken down into glucose for use in cellular
processes. In the Thinking Lab on page 92, you will
consider how the structure of starch can influence
metabolic processes.
In another series of reactions, plants may form
another kind of polysaccharide that is the building
block of cell walls — cellulose. PGAL is first
exported from the chloroplast into the cytoplasm
where condensation reactions take place to link
glucose molecules.
The formation of sucrose (the transport sugar in
plants) also occurs in the cytoplasm. In order for
glycolysis and cellular respiration to take place in
the cytosol and mitochondria of plants, glucose is
required. Because plants cannot move glucose
molecules through the phloem (vascular tissue that
transports organic material), they convert PGAL to
glucose in the cytoplasm of leaf mesophyll cells.
Glucose and fructose are then converted to sucrose.
Sucrose is a molecule of fructose covalently
bonded to a molecule of glucose. After sucrose
is formed it is actively transported to the phloem,
and then moved to locations in the plant that
metabolize glucose.
Photosynthesis Versus
Aerobic Cellular Respiration
Both plant and animal cells have mitochondria and
carry out aerobic cellular respiration. However,
only plants use photosynthesis. The cellular
organelle for photosynthesis is the chloroplast,
while the cellular organelle for aerobic cellular
respiration is the mitochondrion. Figure 3.33
compares the processes of photosynthesis and
respiration. Both processes have an electron
transport chain located on membranes in the
chloroplast and mitochondrion. ATP is produced
on these membranes through the process of
chemiosmosis. In photosynthesis, water is oxidized
and oxygen is produced. In aerobic cellular
respiration, oxygen is reduced to form water.
Reactions in the chloroplast and mitochondrion
are catalyzed by enzymes. These enzymes help
to reduce CO2 to glucose in the chloroplast and
oxidize glucose to CO2 in the mitochondrion.
photosynthesis
H2O
aerobic cellular respiration
O2
membranes
ADP
O2
ATP
chloroplast
NADPH
CO2
H2O
mitochondrion
NADP +
enzymes
C16 H12 O6
NAD +
C16 H12 O6
NADH
CO2
Figure 3.33 Photosynthesis versus cellular respiration. How are products
produced by one organelle used by the other organelle? The organelles shown are
not drawn to scale, a chloroplast is about four times larger than a mitochondrion.
Chapter 3 Cellular Energy • MHR
93
SECTION
1.
2.
94
K/U
REVIEW
Draw a chloroplast and label the key structures.
K/U Explain how plants capture solar energy.
Why does photosystem 680 provide some adaptive
advantages over photosystem 700?
3.
K/U Write an equation to summarize the photo
reaction of photosynthesis.
4.
K/U What are the components of a photosynthetic
unit, and what roles do they play? In which part of
the chloroplast are photosynthetic units located?
9.
K/U How many CO2 molecules need to be fixed to
produce 2 PGAL molecules that can leave the Calvin
cycle to form glucose?
10.
I In an experiment, a team of researchers uses a
heavy isotope of oxygen-18 to track the passage of
oxygen through the process of photosynthesis. What
results would you expect to find if the researchers
initiated the reaction in an environment in which
(a) the carbon dioxide contained the heavy oxygen?
(b) the water contained the heavy oxygen?
5.
What role does water play in the photo
reactions of photosynthesis?
6.
K/U List the three stages of the Calvin cycle.
For each stage, identify the energy input and
the product(s).
MC You are hired to advise strawberry producers on
ways to increase their harvest. Assume the plants are
grown under fully controlled conditions, so you can
alter the temperature, lighting, atmosphere, water
supply, and nutrients.
7.
What becomes of the atmospheric carbon
fixed by a plant cell?
(a) What conditions would you recommend to
maximize the plants’ productivity?
8.
Explain, with diagrams, how high levels of oxygen
reduce the effectiveness of the Calvin cycle. Explain
how alternative forms of carbon fixation avoid this
problem.
K/U
K/U
C
MHR • Unit 1 Metabolic Processes
11.
(b) Can you think of any mutation that would help
increase the rate or efficiency of photosynthesis?
Discuss your reasoning.