Plant Metabolism
Chapter 10
Outline
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Introduction
Enzymes and Energy Transfer
Photosynthesis
Respiration
Additional Metabolic Pathways
Assimilation and Digestion
Introduction
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Photosynthesis - converts light energy to
usable form
Respiration - releases stored energy
• Facilitates growth, development and reproduction
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Metabolism - sum of all interrelated
biochemical processes in living organisms
Animals rely on green plants for O2, food,
shelter and other products
Enzymes and Energy Transfer
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Enzymes regulate metabolic activities
• Anabolism - forming chemical bonds to build
molecules
−Photosynthesis
• Catabolism - breaking chemical bonds
−Cellular respiration
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Photosynthesis-respiration Cycle involves transfer of
energy via oxidation-reduction reactions
Enzymes and Energy Transfer
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Oxidation-Reduction Reactions
• Oxidation - loss of electron(s)
• Reduction - gain of electron(s)
• Oxidation of one compound usually coupled with
reduction of another
• H atom lost during oxidation and gained during
reduction
• O usually final acceptor of electron
Photosynthesis
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Energy for most cellular activity = adenosine
triphosphate (ATP)
• Plants make ATP using light as energy source
− Takes place in chloroplasts and other green
parts of organisms
6CO2 + 12H2O + light  C6H12O6 + 6O2 + 6H2O
− Many intermediate steps to process, and
glucose not immediate 1st product
Photosynthesis
• CO2 reaches chloroplasts in mesophyll cells by
diffusion (stomata -> leaf interior)
• Use of fossil fuels, deforestation, and other human
activities add more CO2 to atmosphere than is
removed
− Has potential to cause global increases in
temperature
− May enhance photosynthesis
Photosynthesis
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Less than 1% of all H2O absorbed by plants used
in photosynthesis
• Most transpired or incorporated into plant materials
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H2O source of e- in photosynthesis and O2
released as by-product
If H2O in short supply or light intensities too high,
stomata close and reduce supply of CO2 available
for photosynthesis
Photosynthesis
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~40% of radiant energy received on earth visible
light
• Violet to blue and red-orange to red wavelengths
absorbed
• Green light reflected
• Leaves absorb ~80% of visible
light reaching them
• Light intensity varies with
time of day, season, altitude,
latitude, and atmospheric
composition
Visible light passed through prism
Photosynthesis
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Plants vary considerably in light intensities needed
for optimal photosynthetic rates
Temperature and amount of CO2 can be limiting
Photosynthesis
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If light and temps too high: ratio of CO2 to O2 inside leaves
may change
• Accelerates photorespiration - uses O2 and releases
CO2
− May help some plants survive under adverse
conditions
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If light intensity too high: photooxidation - results in
destruction of chlorophyll
If H2O in short supply or light intensities too high: stomata
close and reduce supply of CO2 available for
photosynthesis
Photosynthesis
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Several types of chlorophyll molecules
• Mg end captures light
• Lipid tail anchors into thylakoid membrane
• Most plants contain chlorophyll a (blue-green color)
and chlorophyll b (yellow-green color)
− Chlorophyll b transfers energy from light to
chlorophyll a
Chlorophyll a
molecule
Photosynthesis
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Other photosynthetic pigments include carotenoids
(yellow and orange), phycobilins (blue or red, in
cyanobacteria and red algae), and several other types of
chlorophyll
Ca. 250-400 pigment molecules grouped in lightharvesting complex = photosynthetic unit
• Two types of photosynthetic units work together in
light-dependent reactions
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Two phases of photosynthesis:
• Light-dependent reactions
• Light-independent reactions
Photosynthesis
Major Steps of Photosynthesis
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Light-Dependent Reactions:
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Thylakoid membranes of chloroplasts
H2O split apart, releasing e- and H+; O2 released
e- pass along e- transport system
ATP produced
NADP reduced to NADPH (used in light-independent
reactions)
Photosynthesis
Major Steps of Photosynthesis
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Light-Independent Reactions:
• Stroma of chloroplasts
• Utilize ATP and NADPH to form sugars
• Calvin Cycle
− CO2 combines with RuBP (ribulose bisphosphate)
and combined molecules converted to sugars
(glucose)
− Uses ATP and NADPH produced during lightdependent reactions
Photosynthesis
A Closer Look: Light-Dependent Reactions
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Each pigment has own distinctive pattern of light
absorption = absorption spectrum
When pigments absorb
light, energy levels of eraised
• Energy from excited ereleased when drops
back to ground state
• In photosynthesis,
energy stored in
chemical bonds
Photosynthesis
A Closer Look: Light-Dependent Reactions
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Two types of photosynthetic units: photosystem I
and photosystem II
• Photosystem II before photosystem I
• Both produce ATP
• Both photosystem I and photosystem II needed to
produce NADPH and O2 as result of e- flow
Photosynthesis
A Closer Look: Light-Dependent Reactions
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Photosystem I = chlorophyll a, small amount of
chlorophyll b, carotenoid pigment, and P700
• P700 = reaction-center molecule which uses light
energy
• Remaining pigments = antenna pigments
− Gather and pass light energy to reaction center
• Fe-S proteins - primary e- acceptors, first to receive efrom P700
Photosystem II = chlorophyll a, B-carotene, small
amounts of chlorophyll b, and P680
• Pheophytin (Pheo) - primary e- acceptor
Photosynthesis
A Closer Look: Light-Dependent Reactions
Photosynthesis
A Closer Look: Light-Dependent Reactions
• Photolysis - H2O-splitting, Photosystem II
– Light photons absorbed by P680, boosting e- to higher
energy level
– e- passed to acceptor molecule, pheophytin, then to
PQ (plastoquinone), then along e- transport system to
photosystem I
– e- extracted from H2O replace e- lost by P680
– 1 O2, 4 H+ and 4 e- produced from 2 H2O
Photosynthesis
A Closer Look: Light-Dependent Reactions
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e- Flow and Photophosphorylation
• e- transport system consists of e- transfer molecules
• Photons move across thylakoid membrane by
chemiosmosis
• Phosphorylation - ATP formed from ADP
Photosynthesis
A Closer Look: Light-Dependent Reactions
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Photosystem I
• Light absorbed by P700, boosting e- to higher energy
level
• e- passed to Fe-S acceptor molecule, Fd (ferredoxin),
then to FAD (flavin adenine dinucleotide).
• NADP reduced to NADPH
• e- removed from P700 replaced by e- from photosystem
II.
Photosynthesis
A Closer Look: Light-Dependent Reactions
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Chemiosmosis
• Net accumulation of H+ in
thylakoid lumen occurs from
splitting of H2O molecules and etransport
• H+ gradient gives ATPase in
thylakoid membrane potential
to move H+ from lumen to
stroma
• Movement of H+ across
membrane = source of energy
for ATP synthesis
Photosynthesis
A Closer Look: Light-Independent Reactions
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Calvin Cycle
• 6 CO2 combine with 6 RuBP (ribulose 1,5bisphosphate) with aid of rubisco
• Results in 12 3-C molecules of 3PGA (3phosphoglyceric acid)
• NADPH and ATP supply energy and e- reducing 3PGA
to GA3P (glyceraldehyde 3-phosphate)
• 10 of 12 GA3P restructured, using 6 ATP, into 6 5-C
RuBP
• Net gain of 2 GA3P -> converted to carbohydrates or
used to make lipids and amino acids
The Calvin Cycle
Photosynthesis
A Closer Look: Light-Independent Reactions
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Photorespiration - competes with C-fixing role of
photosynthesis
• Rubisco fixes O2 instead of CO2
• Allows C3 plants to survive under hot dry conditions
Dissipates ATP and accumulated e-, prevents
photooxidation
• When stomata closed, O2 accumulates and
photorespiration more likely
• Produces 2-C phosphoglycolic acid (processed in
perioxisomes)
− Forms CO2 and PGA -> reenter Calvin cycle
− No ATP formed
Photosynthesis
A Closer Look: Light-Independent Reactions
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C4 Pathway - produces 4-C compound instead of 3-C PGA
during initial steps of light-independent reactions
• C4 plants - tropical grasses and plants of arid regions
• Kranz anatomy
− Mesophyll cells with smaller chloroplasts with welldeveloped grana
− Bundle sheath
cells with large
chloroplasts
with numerous
starch grains
Photosynthesis
A Closer Look: Light-Independent Reactions
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C4 Pathway
• CO2 converted to organic acids in mesophyll cells
• PEP (phosphoenolpyruvate) and CO2 combine, with
aid of PEP carboxylase
• Form 4-C oxaloacetic acid
instead of PGA
• PEP carboxylase converts
CO2 to carbohydrate at
lower CO2 concentrations
than does rubisco
− No photorespiration
Photosynthesis
A Closer Look: Light-Independent Reactions
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C4 Pathway
• CO2 transported as organic acids to bundle sheath
cells, released and enters Calvin cycle
• CO2 concentration high in
bundle sheath = little
photorespiration
• C4 plants photosynthesize
at higher temps than C3
plants
− Costs 2 ATP for C4
photosynthesis
Photosynthesis
A Closer Look: Light-Independent Reactions
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CAM Photosynthesis - similar to C4 photosynthesis as 4-C
compounds produced during light-independent reactions,
however:
• Organic acids accumulate
at night (stomata open)
• Converted back to CO2
during day for use in
Calvin cycle (stomata
closed)
– Adaptation to limited
H2O supply and high
light intensity habitat
Respiration
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Respiration - release of energy from glucose
molecules broken down to individual CO2
molecules
• Initiated in cytoplasm and completed in
mitochondria
• Aerobic respiration needs O2
C6H12O6 + 6O2 6CO2 + 6H2O + energy
Respiration
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Anaerobic respiration and fermentation carried on in absence of O2
• Release less energy than aerobic respiration
• Fermentation equations:
−C6H12O6 2C2H5OH + 2CO2 + 2ATP
−C6H12O6 2C3H6O3 + 2ATP
Respiration
Major Steps of Respiration
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Glycolysis - 1st phase
• In cytoplasm
• No O2 required
• Glucose converted to GA3P (glyceraldehyde 3phosphate)
• 2 ATP molecules gained
Respiration
Major Steps of Respiration
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Citric Acid (Krebs) Cycle - 2nd stage
• In fluid matrix of cristae in mitochondria
• High energy e- and H+ removed
• NADH, FADH2 , and small amount of ATP produced
• CO2 produced as by-product
Electron transport - 3rd stage
• In inner membrane of mitochondria
• NADH and FADH2 donate e- to e- transport system
• Produces ATP, CO2 and H2O
Respiration
A Closer Look
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Glycolysis
• 3 Steps:
– Phosphorylation - glucose becomes fructose 1,6bisphosphate
– Sugar cleavage - fructose 1,6-bisphosphate split into
2 3-C GA3P (glyceraldehyde 3-phosphate) molecules
– Pyruvic Acid Formation - H+, energy and H2O
removed leaving pyruvic acid
• Before citric acid cycle, pyruvic acid loses CO2 and
converted to acetyl CoA
• No O2 = anaerobic respiration and fermentation
− H+ released during glycolysis transferred back to
pyruvic acid, creating ethyl alcohol or lactic acid
Respiration
A Closer Look
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Citric Acid (Krebs) Cycle
• Acetyl CoA combines with oxaloacetic acid (O.A.),
producing citric acid
• Each cycle uses 2 acetyl CoA, releases 3 CO2 and
regenerates O.A.
O.A. + acetyl CoA + ADP + P + 3NAD + FAD 
O.A. + CoA + ATP + 3NADH + H+ + FADH2 + 2CO2
• High energy e- and H+ removed, producing NADH,
FADH2 and ATP.
Respiration
A Closer Look
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e- Transport and Oxidative Phosphorylation
• Energy from NADH and FADH2 released as H+ and epassed along e- transport system
• H+ build up outside mitochondrial matrix =
electrochemical gradient
• Chemiosmosis couples transport of H+ into matrix with
oxidative phosphorylation = formation of ATP
• O2 = ultimate e- acceptor, producing H2O as it combines
with H+
• Produces net gain of 36 ATP and 6 CO2 and H2O
Respiration
Factors Affecting the Rate of
Respiration
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Temperature
• Increase from 20o C to 30o C, respiration rates double
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H2O
• Medium in which enzymatic reactions take place
• Low H2O content - respiration rate reduced
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O2
• Reduction in O2 - respiration and growth rates decline
Additional Metabolic Pathways
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Other processes contribute to growth development,
reproduction and survival
• Includes production of sugar phosphates, nucleotides,
nucleic acids, amino acids, proteins, chlorophylls,
cytochromes, carotenoids, fatty acids, oils, and waxes
Secondary Metabolism - metabolic processes not
required for normal growth and development
• Enable plants to survive and persist under special
conditions
− Colors, aromas, poisons - give competitive edge
 Codeine, Nicotine, Lignin, Salicin, Camphor,
Menthol, Rubber
Assimilation and Digestion
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Assimilation - conversion of organic matter
produced in photosynthesis to build protoplasm
and cell walls
• Sugars transformed into lipids, proteins, or other
carbohydrates, such as sucrose, starch and cellulose
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Digestion - conversion of starch and other
insoluble carbohydrates to soluble forms
• Nearly always hydrolysis process
Review
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Introduction
Enzymes and Energy Transfer
Photosynthesis
Respiration
Additional Metabolic Pathways
Assimilation and Digestion