Transcript Concept 6.5 During Photosynthesis, Light Energy Is

Pathways that Harvest and
Store Chemical Energy
Chapter 6 Hillis
Chapter 6 Pathways that Harvest and Store
Chemical Energy
• Key Concepts
• 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy
Metabolism
• 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large
Amount of Energy
• 6.3 Carbohydrate Catabolism in the
Absence of Oxygen Releases a Small
Amount of Energy
Chapter 6 Pathways that Harvest and Store
Chemical Energy
• 6.4 Catabolic and Anabolic Pathways Are
Integrated
• 6.5 During Photosynthesis, Light Energy Is
Converted to Chemical Energy
• 6.6 Photosynthetic Organisms Use
Chemical Energy to Convert CO2 to
Carbohydrates
Chapter 6 Opening Question
• Why does fresh air inhibit the formation
of alcohol by yeast cells?
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Energy is stored in chemical bonds and can
be released and transformed by metabolic
pathways.
• Chemical energy available to do work is
termed free energy (G).
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Five principles govern metabolic
pathways:
1. Chemical transformations occur in a series
of intermediate reactions that form a
metabolic pathway.
2. Each reaction is catalyzed by a specific
enzyme.
3. Most metabolic pathways are similar in all
organisms.
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
4. In eukaryotes, many metabolic pathways
occur inside specific organelles.
5. Each metabolic pathway is controlled by
enzymes that can be inhibited or
activated.
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• In cells, energy-transforming reactions are
often coupled:
– An energy-releasing (exergonic) reaction
is coupled to an energy-requiring
(endergonic) reaction.
– Two coupling molecules are the
coenzymes ATP and NADH.
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Adenosine triphosphate (ATP) is a kind of
“energy currency” in cells.
• Energy released by exergonic reactions is
stored in the bonds of ATP.
• When ATP is hydrolyzed, free energy is
released to drive endergonic reactions.
Figure 6.1 The Concept of Coupling Reactions
Figure 6.2 ATP
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Hydrolysis of ATP is exergonic:
• ATP + H2O ADP + Pi + free energy
®
•
ΔG is about –7.3 kcal/mol
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• The free energy of the bond between
phosphate groups is much higher than the
energy of the
O—H bond that forms after hydrolysis.
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Energy can also be transferred by the
transfer of electrons in reduction–
oxidation, or redox reactions.
• Reduction is the gain of one or more
electrons.
– Oxidation is the loss of one or more
electrons.
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Oxidation and reduction always occur
together.
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• It is also useful to think of oxidation and
reduction in terms of gain or loss of
hydrogen atoms:
– Transfers of hydrogen atoms involve
transfers of electrons (H = H+ + e–).
– When a molecule loses a hydrogen atom,
it becomes oxidized.
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• The more reduced a molecule is, the more
energy is stored in its bonds.
• Energy is transferred in a redox reaction.
• Energy in the reducing agent is transferred
to the reduced product.
Figure 6.3 Oxidation, Reduction, and Energy
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Coenzyme NAD is a key electron carrier in
redox reactions.
•
NAD+ (oxidized form)
•
NADH (reduced form)
Figure 6.4 NAD+/NADH Is an Electron Carrier in
Redox Reactions (Part 1)
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Reduction of NAD+ is highly endergonic:
• NAD+ + H+ + 2 e– ® NADH
• Oxidation of NADH is highly exergonic:
• NADH + H+ + ½ O2
NAD+ + H2O
®
Figure 6.4 NAD+/NADH Is an Electron Carrier in
Redox Reactions (Part 2)
Concept 6.1 ATP and Reduced Coenzymes Play
Important Roles in Biological Energy Metabolism
• Energy is released in catabolism by
oxidation and trapped by reduction of
coenzymes such as NADH.
• Energy for anabolic processes is supplied by
ATP.
• Most energy-releasing reactions produce
NADH, but most energy-consuming
reactions require ATP.
• Oxidative phosphorylation transfers energy
from NADH to ATP.
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Cellular respiration: the set of metabolic
reactions used by cells to harvest energy
from food
• A lot of energy is released when reduced
molecules with many C—C and C—H bonds
are fully oxidized to CO2.
• The oxidation occurs in a series of small
steps, allowing the cell to harvest about
34% of the energy released.
Figure 6.5 Energy Metabolism Occurs in Small
Steps
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Catabolism of glucose under aerobic
conditions (in the presence of O2), occurs
in three linked biochemical pathways:
– Glycolysis—glucose is converted to
pyruvate.
– Pyruvate oxidation—pyruvate is
oxidized to acetyl CoA and CO2.
– Citric acid cycle—acetyl CoA is oxidized
to CO2.
Figure 6.6 Energy-Releasing Metabolic
Pathways
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Glycolysis
– Ten reactions
– Takes place in the cytosol
– Final products:
• 2 molecules of pyruvate (pyruvic acid)
• 2 molecules of ATP
• 2 molecules of NADH
Figure 6.7 Glycolysis Converts Glucose into
Pyruvate (Part 1)
Figure 6.7 Glycolysis Converts Glucose into
Pyruvate (Part 2)
Figure 6.7 Glycolysis Converts Glucose into
Pyruvate (Part 3)
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
• Steps 6 and 7 are Energy
examples of reactions
that occur repeatedly in metabolic
pathways:
Energy Investment phase
Fig. 9.9a
Fig. 9.9b
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Oxidation–reduction (step 6): exergonic;
glyceraldehyde 3-phosphate is oxidized and
energy is trapped via reduction of NAD+ to
NADH.
• Substrate-level phosphorylation (step 7):
also exergonic; energy released transfers a
phosphate from 1,3-bisphosphoglycerate to
ADP, forming ATP.
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Pyruvate Oxidation
– Occurs in mitochondria in eukaryotes.
– Products: CO2 and acetate; acetate is
then bound to coenzyme A (CoA) to form
acetyl CoA.
– NAD+ is reduced to NADH.
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• As pyruvate enters the mitochondrion, a
multienzyme complex modifies pyruvate to
acetyl CoA which enters the Krebs cycle in the
matrix.
– A carboxyl group is removed as CO2.
– A pair of electrons is transferred from the remaining
two-carbon fragment to NAD+ to form NADH.
– The oxidized
fragment, acetate,
combines with
coenzyme A to
form acetyl CoA.
Fig. 9.10
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
• Citric Acid Cycle Energy
– Eight reactions
– Occurs in mitochondria in eukaryotes
– Operates twice for every glucose
molecule that enters glycolysis
– Starts with Acetyl CoA; acetyl group is
oxidized to two CO2
– Oxaloacetate is regenerated in the last
step
Figure 6.8 The Citric Acid Cycle
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Final reaction of citric
acid cycle:
The Krebs
cycle
consists
of eight
steps.
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Cells transfer energy from NADH and FADH2
to ATP by oxidative phosphorylation:
– NADH oxidation is used to actively
transport protons (H+) across the inner
mitochondrial membrane, resulting in a
proton gradient.
– Diffusion of protons back across the
membrane then drives the synthesis of
ATP.
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• When NADH is reoxidized to NAD+, O2 is
reduced to H2O:
• NADH + H+ + ½ O2 ® NAD+ + H2O
• This occurs in a series of redox electron
carriers, called the respiratory chain,
embedded in the inner membrane of the
mitochondrion.
Figure 6.9 Electron Transport and ATP
Synthesis in Mitochondria
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Electron transport: electrons from the
oxidation of NADH and FADH2 pass from
one carrier to the next in the chain.
• The oxidation reactions are exergonic,
energy released is used to actively
transport H+ ions across the membrane.
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Oxidation is always coupled with reduction.
• When NADH is oxidized to NAD+, the
reduction reaction is the formation of
water from O2.
®
• 2 H+ + 2 e– + ½ O2
H2O
• The key role of O2 in cells is to act as an
electron acceptor and become reduced.
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• ATP synthase uses the H+ gradient to drive
synthesis of ATP by chemiosmosis:
– Chemiosmosis: Movement of ions across
a semipermeable barrier from a region of
higher concentration to a region of lower
concentration.
• ATP synthase converts the potential energy
of the proton gradient into chemical energy
in ATP.
Figure 6.10 Chemiosmosis
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• ATP synthase is a molecular motor with two
subunits:
– F0 is a transmembrane domain that
functions as the H+ channel.
– F1 has six subunits. As protons pass
through F0, it rotates, causing part of the
F1 unit to rotate.
• ADP and Pi bind to active sites that become
exposed on the F1 unit as it rotates, and
ATP is made.
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• ATP synthase structure is similar in all
organisms.
– In prokaryotes, the proton gradient is set
up across the cell membrane.
– In eukaryotes, chemiosmosis occurs in
mitochondria and chloroplasts.
• The mechanism of chemiosmosis is similar
in almost all forms of life.
Fig. 9.16
© , as
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• Chemiosmosis can be demonstrated
experimentally.
• A proton gradient can be introduced
artificially in chloroplasts or mitochondria
in a test tube.
• ATP is synthesized if ATP synthase, ADP, and
inorganic phosphate are present.
Figure 6.11 An Experiment Demonstrates the
Chemiosmotic Mechanism
Concept 6.2 Carbohydrate Catabolism in the
Presence of Oxygen Releases a Large Amount of
Energy
• About 32 molecules
of ATP are produced
for each fully oxidized glucose.
• The role of O2: most of the ATP is formed by
oxidative phosphorylation, which is due to
the reoxidation of NADH.
• Some bacteria and archaea use other
electron acceptors.
– Geobacter metallireducens can use iron
(Fe3+) or uranium, making it potentially
useful in environmental cleanup.
Concept 6.3 Carbohydrate Catabolism in the
Absence of Oxygen
Releases a Small Amount of Energy
• Under anaerobic conditions, NADH is
reoxidized by fermentation.
• There are many different types of
fermentation, but all operate to regenerate
NAD+.
• The overall yield of ATP is only two—the
ATP made in glycolysis.
Concept 6.3 Carbohydrate Catabolism in the
Absence of Oxygen
Releases a Small Amount of Energy
• Lactic acid fermentation:
– End product is lactic acid (lactate).
– NADH is used to reduce pyruvate to lactic
acid, regenerating NAD+.
• Occurs in many microorganisms and
complex organisms, including vertebrate
muscle during exercise when O2 can not be
delivered to the muscle fast enough.
Figure 6.12 Fermentation (Part 1)
Concept 6.3 Carbohydrate Catabolism in the
Absence of Oxygen
Releases a Small Amount of Energy
• Alcoholic fermentation:
– End product is ethyl alcohol (ethanol).
– Pyruvate is converted to acetaldehyde,
and CO2 is released. NADH is used to
reduce acetaldehyde to ethanol,
regenerating NAD+.
• Occurs in certain yeasts and some plant
cells under anaerobic conditions.
Figure 6.12 Fermentation (Part 2)
Concept 6.4 Catabolic and Anabolic Pathways
Are Integrated
• Metabolic pathways are linked. There is an
interchange of molecules into and out of
the pathways for synthesis and breakdown.
• Carbon skeletons (molecules with
covalently linked carbon atoms) can enter
catabolic or anabolic pathways.
• These relationships comprise a metabolic
system.
Figure 6.13 Relationships among the Major
Metabolic Pathways of the Cell
Concept 6.4 Catabolic and Anabolic Pathways
Are Integrated
• Catabolism:
• Polysaccharides are hydrolyzed to
glucose, which enters glycolysis.
• Lipids break down to fatty acids and
glycerol. Fatty acids can be converted to
acetyl CoA.
• Proteins are hydrolyzed to amino acids
that can feed into glycolysis or the citric
acid cycle.
Concept 6.4 Catabolic and Anabolic Pathways
Are Integrated
• Anabolism:
• Many catabolic pathways can operate in
reverse.
• Gluconeogenesis—citric acid cycle and
glycolysis intermediates can be reduced
to form glucose.
• Acetyl CoA can be used to form fatty
acids.
• Some citric acid cycle intermediates can
form nucleic acids.
Concept 6.4 Catabolic and Anabolic Pathways
Are Integrated
• Amounts of different molecules are
maintained at fairly constant levels in the
metabolic pool.
• This is accomplished by regulation of
enzymes—allosteric regulation and
feedback inhibition.
• Enzymes can also be regulated by altering
the transcription of genes that encode the
enzymes. This is slower than feedback
inhibition.
Concept 6.4 Catabolic and Anabolic Pathways
Are Integrated
• ATP and reduced coenzymes link
catabolism, anabolism, and photosynthesis.
• Cellular respiration and photosynthesis are
linked by their reactants and products and
by the energy “currency” of ATP and
reduced coenzymes.
Concept 6.4 Catabolic and Anabolic Pathways
Are Integrated
• In cellular respiration glucose is oxidized:
®
• glucose + 6 O2
6 CO2 + 6 H2O + chemical
energy
• In photosynthesis, light energy is converted to
chemical energy:
®
• CO2 + H2O + light energy
carbohydrates + O2
Figure 6.14 ATP, Reduced Coenzymes, and
Metabolism
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• Photosynthesis (anabolic) involves two
pathways:
– Light reactions convert light energy into
chemical energy (in ATP and the reduced
electron carrier NADPH).
– Carbon-fixation reactions use the ATP
and NADPH to produce carbohydrates.
Leaf Structure & Orientation
•
•
•
•
•
•
Palisade layer (mesophyll)
Spongy mesophyll
Vascular bundle
Stomata
Plant hormones allow leaf to orient to light
Plant habit determines leaf arrangement
X section of typical leaf
http://1.bp.blogspot.com/_FYcMaoa9aqQ/SU0G0J5hEsI/AAAAAAA
AAIE/8NrNKUHT2Qw/s400/leaf.gif
Figure 6.15 An Overview of Photosynthesis
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• Light is a form of electromagnetic radiation;
it is propagated as a wave but also behaves
as particles (photons).
• The amount of energy in the radiation is
inversely proportional to its wavelength.
Figure 6.16 The
Electromagnetic
Spectrum
• When light meets matter, it may be reflected,
transmitted, or absorbed.
– Different pigments absorb photons of different
wavelengths.
– A leaf looks green
because chlorophyll,
the dominant pigment,
absorbs red and blue
light, while transmitting
and reflecting green
light.
Fig. 10.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• Photons can be absorbed by specific
receptor molecules, which are raised to an
excited state (higher energy).
•
•
•
•
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
Pigments: molecules that absorb
wavelengths in the visible spectrum
Chlorophyll absorbs blue and red light; the
remaining light is mostly green.
Absorption spectrum—plot of light energy
absorbed against wavelength.
Action spectrum—plot of the biological
activity of an organism against wavelength.
Figure 6.17 Absorption and Action Spectra
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• In plants, two chlorophylls absorb light
energy— chlorophyll a and chlorophyll b.
• Accessory pigments absorb wavelengths
between red and blue and transfer some of
that energy to the chlorophylls.
Figure 6.18
The
Molecular
Structure of
Chlorophyll
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• The pigments are arranged into lightharvesting complexes, or antenna systems.
• A photosystem spans the thylakoid
membrane in the chloroplast
– It consists of multiple antenna systems
surrounding a reaction center.
Figure 6.19 Photosystem Organization
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• When chlorophyll (Chl) absorbs light, it
enters an excited state (Chl*), then rapidly
returns to ground state, releasing an
excited electron.
• Chl* gives the excited electron to an
acceptor and becomes oxidized to Chl+. The
acceptor molecule is reduced.
®
• Chl* + acceptor
Chl+ + acceptor –
• The reaction center has converted light
energy into chemical energy.
Concept 6.5 During Photosynthesis, Light Energy Is
Converted to Chemical Energy
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• The electron acceptor is the first carrier in an
electron transport system in the thylakoid
membrane.
• The final acceptor is NADP+, which gets
reduced:
®
• NADP+ + H+ + 2 e–
NADPH
• ATP is produced chemiosmotically during
electron transport (photophosphorylation).
Figure 6.20 Noncyclic Electron Transport Uses
Two Photosystems
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• Photosystem II
• When Chl* gives up an electron, it is
unstable and grabs an electron from H2O,
which splits the H—O—H bonds.
• 2 Chl* + H2®
O 2 Chl + 2 H+ + ½ O2
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• The excited (energetic) electron is passed
through a series of thylakoid membranebound carriers to a final acceptor at a lower
energy level.
• A proton gradient is generated and used by
ATP synthase to make ATP.
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• Photosystem I
• When Chl* gives up an electron, it grabs
another electron from the last carrier in the
transport system of Photosystem II.
•
This electron ends up reducing NADP+ to
NADPH.
Concept 6.5 During Photosynthesis, Light
Energy Is Converted to Chemical Energy
• ATP is needed for carbon-fixation
pathways. The noncyclic light reactions
would not provide enough ATP.
• Cyclic electron transport uses only
photosystem I and produces only ATP.
•
An electron is passed from an excited
chlorophyll, through the electron transport
chain, and recycles back to the same
chlorophyll.
Figure 6.21 Cyclic Electron Transport Traps
Light Energy as ATP
Source of O2 released
• Science as a process and technology
• Originally thought to be from CO2
• Van Neil studied sulfanogens and deduced that H
source could vary, but H was required
• CO2 + 2H2S -> CH2O + H2O + 2S
• Labeled 18O in water & it was released in air
• Labeled 18O in CO2 & it bound in sugar & H2O
Junction between light and Calvin
• Light rx in thylakoids of chloroplasts
• Calvin cycle rx occur in stroma
• As NADP+ and ADP contact thylakoid
membranes, they pick up e- and phosphate
and then transfer the energy and functional
groups to the Calvin cycle
Concept 6.6 Photosynthetic Organisms Use Chemical
Energy
to Convert CO2 to Carbohydrates
• Calvin cycle: the energy in ATP and NADPH
is used to “fix” CO2 in reduced form in
carbohydrates
– Occurs in the stroma of the chloroplast.
– Each reaction is catalyzed by a specific
enzyme.
• The cycle is composed of three distinct
processes.
Figure 6.22 The Calvin Cycle
Concept 6.6 Photosynthetic Organisms Use Chemical
Energy
to Convert CO2 to Carbohydrates
• 1. Fixation of CO2:
– CO2 is added to ribulose 1,5bisphosphate (RuBP).
– Ribulose bisphosphate
carboxylase/oxygenase (rubisco)
catalyzes the reaction.
– A 6-carbon molecule results, which
quickly splits into two 3-carbon
molecules:
3-phosphoglycerate (3PG)
Figure 6.23 RuBP Is the Carbon Dioxide
Acceptor
Concept 6.6 Photosynthetic Organisms Use Chemical
Energy
to Convert CO2 to Carbohydrates
• 2. 3PG is reduced to form glyceraldehyde 3phosphate (G3P).
Concept 6.6 Photosynthetic Organisms Use Chemical
Energy
to Convert CO2 to Carbohydrates
• 3. The CO2 acceptor RuBP is regenerated
from G3P.
– Some of the extra G3P is exported to the
cytosol and is converted to hexoses
(glucose and fructose).
– When glucose accumulates, it is linked to
form starch, a storage carbohydrate.
Fig. 10.17.3
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Concept 6.6 Photosynthetic Organisms Use Chemical
Energy
to Convert CO2 to Carbohydrates
• The C—H bonds generated by the Calvin
cycle provide almost all the energy for life
on Earth.
• Photosynthetic organisms (autotrophs) use
most of this energy to support their own
growth and reproduction.
• Heterotrophs cannot photosynthesize and
depend on autotrophs for chemical energy.
Overview Photosynthesis
Answer to Opening Question
• Pasteur’s findings:
• Catabolism of the beet sugar is a cellular
process, so living yeast cells must be
present.
• In air (O2), yeasts use aerobic metabolism
to fully oxidize glucose to CO2.
• Without air, yeasts use alcoholic
fermentation, producing ethanol, less CO2,
and less energy (slower growth).
Photorespiration
• O2 builds up due to splitting of H2O by
photosystem II
• RuBP can accept O2 instead of CO2
• Splits into 1 3C and 1 2C molecule
• 2C molecule goes to peroxisome which
breaks down so it can enter
mitochondrion releasing CO2 from
Krebs no ATP made
Photorespiration conditions
• there is insufficient water and stomata close
(guard cells become flaccid)-prevents
excessive water loss
• CO2 levels drop below 50 ppm, rubisco
accepts O2
Photorespiration vs Photosynthesis
process
• Rubisco combines w oxygen; CO2 and O2 are
competitive inhibitors of each other's reactions.
• Instead of producing 2 3C PGA molecules, only one
molecule of PGA is produced and a toxic 2C molecule
called phosphoglycolate is produced.
• The phosphate group is removed, leaving glycolic acid.
• Then the glycolic acid is converted to glycine in
peroxisomes.
• Glycine is transported to mitochondria where it is
converted to serine and used as building block in other
molecules.
Chloroplast to peroxisome to
mitochondrion
Results
• photorespiration pathway is an enzymatic one
that is not coupled to any electron transfer
system
• It does not generate ATP.
• It uses oxygen and it produces carbon dioxide.
• It uses a sugar-phosphate as its primary fuel.
Why this is bad
• Instead of producing a sugar, a toxic molecule
is produced and sugar is consumed.
• Energy is expended to convert
phosphoglycolate to usable monomer.
• CO2 is greatly reduced in the leaf.
C3 plants
• First stable intermediate has 3C
• Agriculturally important rice, wheat,
soybeans
C4 Plants
• Before Calvin rx, incorporate CO2 into
4C cmpd
• Used by thousands species, corn and
sugar cane, agricult. Grasses
• Works well in hot, arid climates
• Leaf anatomy spatially segregates
Calvin cycle from initial incorporation
of CO2
• Bundle sheath cells Calvin occurs here
• Mesophyll cells
How C4 Plants work
• CO2 is added to PEP
(phosphoenolpyruvate) to form
oxaloacetate
• PEP carboxlylase adds CO2 to PEP
• No affinity for O2
• Convert oxaloacetate to malate 4C
• In bundle sheath, malate releases CO2
which rubisco picks up to make sugar
Fig. 10.18
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CAM plants
• Conserves H2O in day
• Stomata open night, CO2 take up and fixed
in organic cmpd
• crassulacean acid metabolism
• Stored in vacuoles of mesophyll
• Daytime, light rx supply ATP and NADPH for
Calvin cycle
• CO2 released from organic acids and picked
up by rubisco
Fig. 10.19
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Review
• Photosynthesis makes 160 billion
metric tons carbohydrate/yr
• Light rx use photons to produce ATP
and transfer e- from water to NADP+
to make NADPH
• Make 3 C sugar G3P
• Sucrose is transplant form of carb in
most plants
• Use about 50% of what is made-rest
fuels food chain
Figure 6.24 Products of Glucose Metabolism