BIO/PLS 210 Jan Smalle [e-mail] Website: Smalle Lab

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BIO/PLS 210Jan Smallejsmalle_at_uky.eduWebsite
Smalle Lab
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Photosynthesis

• Chapter 10

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Photosynthesis

• Light energy stored as chemical energy for future
  use
• Original source of energy for other organisms
• Except for a few species of bacteria, all life
  depends on the energy-storing reactions of
  photosynthesis

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Overall Photosynthesis Reaction

• 6CO2 6H2O energy ? C6H12O6 6O2

Oxygen
Glucose
Carbon dioxide
Water
7 C-O bonds 5 C-C bonds 7 C-H bonds 5 H-O
bonds 12 O-O bonds 36 covalent bonds
24 C-O bonds 12 H-O bonds 36 covalent bonds
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Location of photosynthesis in the plant cell
Photosynthesis
Cell wall
CHLOROPLAST
CHLOROPLAST
MITOCHONDRIUM
NUCLEUS
MITOCHONDRIUM
MITOCHONDRIUM
CYTOSOL
CHLOROPLAST
CHLOROPLAST
MITOCHONDRIUM
Notes 1) cytosol is the same as cytoplasm
2) not all of the plant cell structures and
organelles are shown
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Origins of chloroplasts and mitochondria

• Endosymbiosis theory
• Chloroplasts and mitochondria are of bacterial
  origin. They were taken inside the cell as
  endosymbionts.
• - Chloroplasts and mitochondria both contain DNA
  (chromosomes) that encode for some of the
  proteins needed in chloroplastic or mitochondrial
  processes.

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Chloroplasts
Light microscopic image of chloroplasts in leaf
cells
http//botit.botany.wisc.edu
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intact chloroplast
stroma
granum
stroma lammellae
Fig. 10-4, p. 152
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Chloroplasts structure
two outer membranes
thylakoids
stroma
lumen
Fig. 3-12 (a), p. 40
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stroma
Stroma lammellae
granum
Fig. 10-2a, p. 151
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Chloroplast Structure

• Double-membrane envelope
• Within the double-membrane envelope there are two
  types of internal membranes (thylakoids)
• Grana (singular, granum)
• Stroma lamellae (singular, lammellum)
• interconnect grana
• Lumen solution inside the thylakoids
• Stroma
• Thick enzyme solution outside (surrounding)
  thylakoids

Thylakoids
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Chloroplasts function

• Convert light energy into chemical energy
  (photosynthesis)
• Accomplished by proteins in thylakoids and
  stromal enzymes
• Can store products of photosynthesis
  (carbohydrates) in form of starch grains

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Division of Labor in Chloroplasts

• Green thylakoids
• Capture light
• Liberate O2 from H2O
• Form ATP from ADP and phosphate
• Reduce NADP to NADPH

Light reactions

• Colorless stroma
• Contains water-soluble enzymes
• Captures CO2
• Uses energy from ATP and NADPH for sugar
  synthesis

Dark reactions
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Light(-dependent) reactions
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Light reactions

• Are performed in the thylakoids
• 1) Harvesting of light energy
• 2) Use this energy to generate
• ATP and NADPH

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Characteristics of Light

• Two models describing nature of light
• Interpret light as electromagnetic waves
• 2) Light acts as if it were composed of discrete
  packets of energy called photons

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Characteristics of Light

• White light (visible light)
• Can be separated into component colors to form
  visible spectrum
• Visible wavelengths range from
• Red (640 740 nm)
• Violet (400 425 nm)

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Absorption of Light Energy by Plant Pigments

• Spectrophotometer
• Instrument used to measure amount of specific
  wavelength of light absorbed by a pigment
• Absorption spectrum
• Graph of data obtained
• Chlorophyll
• Does not absorb pale green and yellow light
• Predominantly absorbs blue and red wavelengths
• Wavelengths used in photosynthesis

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Absorption spectra of Chlorophyll a and b
100
80
chlorophyll b
Percent of light absorbed
60
40
chlorophyll a
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0
400
500
600
700
Wavelength (nm)
Fig. 10-5, p. 152
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Photons

• Packets of energy making up light
• Contain amount of energy inversely proportional
  to the wavelength of light
• Blue light has more energy per photon than does
  red light

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Photons

• Only one photon is absorbed by one pigment
  molecule at a time
• Energy of photon is absorbed by an electron of a
  pigment (chlorophyll a or b) molecule
• Gives electron more energy (raises the potential
  energy of the electron bringing it further away
  from the nucleus)

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Absorption of Light Energy by Plant Pigments

• Chlorophyll
• Two major types of chlorophyll in vascular plants
• Chlorophylls a and b
• Absorb much of red, blue, indigo, and violet light

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Two Photosystems involved in trapping light
energy
light-harvesting complex combination of
pigment molecules that act as light traps
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Absorption of Light Energy by Chlorophyll

• Chlorophyll molecule absorbs or traps light
  energy (absorbs a photon)
• Light energy causes electron from one of
  chlorophylls atoms to move to higher energy
  state (increased potential energy further away
  from the nucleus)
• Unstable condition
• Electron needs to move back to original energy
  level (or leave the chlorophyll a molecule)

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An extremely important element
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Pigment molecules in a light-harvesting complex
(a complex of proteins and pigments) absorb light
energy and transfer it to the reaction center,
where a special chlorophyll a molecule loses an
electron with increased potential energy. This
high energy electron enters an electron transport
chain. Pigments chl chlorophyll car carotene
light
light harvesting complex
Fig. 10-6, p. 153
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Absorption of Light Energy by Chlorophyll

• Absorbed energy transferred to adjacent pigment
  molecule in light harvesting complex
• Process called resonance high energy electron
  from one pigment molecule loses this energy and
  drops back to a more stable position (closer to
  the nucleus of the atom it belongs to). The
  energy released is taken up by an electron of the
  next pigment molecule in the light harvesting
  complex. And so on.
• The energy is eventually transferred to an
  electron of a chlorophyll a molecule in the
  photosystem reaction center
• This electron (with increased potential energy)
  leaves the chlorophyll a molecule and enters an
  electron transport chain

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NONCYCLIC ELECTRON TRANSPORT
e-
P700
-0.6
sunlight energy
Electron Transport System
NADPH
e-
P680
e-
potential to transfer electrons (measured in
volts)
0
H NADP
ADP Pi
electron transport system
sunlight energy
e-
e-
P700
0.4
Pigments from the light harvesting complex
photosystem I
released energy used to form ATP from ADP and
phosphate
0.8
photosystem II
e-
H2O
photolysis
P680 reaction center of photosystem II
P700 reaction center of photosystem I
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CYCLIC ELECTRON TRANSPORT
e-
P700
sunlight energy
Electron Transport System
NADPH
e-
P680
e-
H NADP
electron transport system
ADP Pi
sunlight energy
e-
e-
P700
photosystem I
released energy used to form ATP from ADP and
phosphate
photosystem II
e-
H2O
photolysis
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Adenosine Triphosphate Synthesis

• Photophosporylation
• Light-driven production of ATP in chloroplasts
• Two types
• Cyclic photophosphorylation
• Noncyclic photophosphorylation

Compare with oxidative phosphorylation in
mitochondria
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Adenosine Triphosphate Synthesis

• Cyclic Photophosphorylation
• Electrons flow from light-excited chlorophyll
  molecules to electron acceptors and cyclically
  back to chlorophyll
• No O2 liberated
• No NADP is reduced
• Produces H gradient that leads to energy ATP
  production (see chemiosmosis theory)
• Only photosystem I involved

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Adenosine Triphosphate Synthesis

• Noncyclic photophosphorylation
• Electrons from excited chlorophyll molecules are
  trapped in NADP to form NADPH
• Electrons do not cycle back to chlorophyll
• Photosystems I and II are involved
• ATP and NADPH are formed
• Energy contained in ATP and NADPH drives CO2
  reduction reactions of photosynthesis (see Calvin
  cycle)

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Why cyclic photophosphorylation?

• Under certain conditions, plant cells do not
  consume NADPH at a high rate (for example growth
  arrest). Less NADP is available as acceptor for
  the electrons coming from Photosystem I. Cyclic
  photophosphorylation allows the plant cell to
  continue to produce ATP (as a direct result of
  light energy capture) that can be used in
  metabolic reactions other than the Calvin cycle.

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Transformation of Light energy into increased
electron potential energy

• Light reactions and electron transport occur in
  thylakoid membranes
• Thylakoids have two interconnected photosystems
  that can trap light energy photosystem I and II
• Both photosystems have a reaction center that
  contains Chlorophyll a
• P680 is the reaction center of photosystem II and
  preferably absorbs light of 680 nm wavelength
• P700 is the reaction center of photosystem I and
  preferably absorbs light of 700 nm wavelength

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Transformation of Light energy into increased
electron potential energy

• Light energy is used to increase the potential
  energy of an electron in the reaction center
  Chlorophyll a (both in P680 and P700). This
  electron leaves Chlorophyll a (that is now
  oxidized) and is passed along an electron
  transport chain.
• For photosystem I, NADP acts as the final
  electron acceptor of this chain, forming NADPH
  and thereby storing some of the energy acquired
  by the absorption of light energy as chemical
  energy.
• For photosystem II, the oxidized chlorophyll a
  molecule in P700 of photosystem I acts as an
  electron acceptor. The electron transport chain
  between photosystem I and II is used to gradually
  decrease the potential energy of the electron and
  harvest the released energy to pump H from the
  stroma into the lumen

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Transformation of Light energy into increased
electron potential energy

• The oxidized Chlorophyll a of P680 promotes the
  photolysis of H2O.
• As a result of photolysis, oxygen and H are
  formed, and Chlorophyll a of P680 is reduced
  (receives an electron). Chlorophyll a is now
  again ready to capture light energy by increasing
  the potential energy of this electron.
• The increased H concentration in the lumen is
  used to make ATP from ADP and inorganic
  phosphate. An ATP synthase in the membrane
  captures the kinetic energy of H ions as they
  flow from the lumen (high concentration) into the
  stroma (low concentration). This mechanism for
  ATP formation is similar to the electron
  transport chain used in mitochondria.

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oxygen released
sunlight energy
photosystem II
e-
H
electron transport system
Light-dependent reactions
H2O is split
H
lumen (H reservoir)
H
photosystem I
e-
electron transport system
NADP
carbon dioxide used
ADP Pi
H
H
Light-independent reactions
sugar phosphate
water released
Stroma
Fig. 10-3, p. 151
carbohydrate end product (e.g. sucrose, starch,
cellulose)
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Dark reactionsor Light-independent reactions
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Dark reactions (light-independent reactions)

• Are performed in the stroma
• 1) Capture of CO2 to make C-C bonds
• 2) Use ATP and NADPH to make C-H
• bonds (out of C-O)
• 3) Use ATP to regenerate Ribulose
• bisphosphate needed for CO2
• capture by rubisco

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Enzymes of Light-Independent Reactions

• Ribulose biphosphate carboxylase/oxygenase
  (rubisco)
• Catalyzes first step in carbon cycle of
  photosynthesis

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The Calvin cycle (C3 pathway of
photosynthesis) PGA phosphoglyceric acid PGAL
phosphoglyceraldehyde RuBP ribulose
bisphosphate Rubisco ribulose bisphosphate
carboxylase The energy carriers ATP and NADPH
(formed by photosystems I and II) are used to
form high energy containing C-C and C-H bonds
starting from H2O and CO2. Through the Calvin
cycle, plants capture CO2 and H2O and transform
low energy containing CO and H-O bonds into the
high energy containing C-C and C-H bonds of
sugar. Rubisco is the worlds most abundant
protein!
2 X
2 X
2 X
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Enzymes of Light-Independent Reactions

rubisco Carbon dioxide ribulose
biphosphate ? 2 phosphoglyceric acid
(RuBP)

• RuBP ? 5-C sugar present in plastid stroma. When
  rubisco is present RuBP and CO2 react (no energy
  required) to form a 6-C intermediate compound
  that eventually is split in two molecules of 2
  phosphoglyceric acid.

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Using ATP and NADPH to generate high energy
containing covalent bonds PGA phosphoglyceric
acid PGAL phosphoglyceraldehyde
H
H
C
C
OH
P
PGA
H
O
C
2 X
Low energy electrons
O H
ATP NADPH
H
H
2 X
2 X
C
C
OH
P
PGAL
H
O
C
High energy electrons
H
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Using ATP and NADPH to generate high energy
containing covalent bonds PGA phosphoglyceric
acid PGAL phosphoglyceraldehyde
2 X
Energy (ATP) is also invested to regenerate RuBP
(Ribulose bisphosphate) allowing again the
capture of a CO2 molecule by the rubisco enzyme.
The phosphates eventually also allow the
formation of glucose (via several intermediates)
from PGAL without a further energy requirement
(i.e. ATP and/or NADPH) .
2 X
2 X
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C3 Pathway (Calvin pathway)

• First product PGA contains 3 Cs
• Calvin cycle (in honor of discoverer, Melvin
  Calvin)
• Key points
• CO2 enters cycle and combines (catalyzed by
  Rubisco) with RuBP produced in stroma
• 2 molecules of PGA are produced
• Energy contained in NADPH and ATP transferred
  into stored energy in phosphoglyceraldehyde
  (PGAL).
• Energy contained in ATP is also used to
  regenerate RuBP

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C3 Pathway

• PGAL is enzymatically converted to
  dihydroxyacetone phosphate
• Two molecules of dihydroxyacetone phosphate
  combine to form a sugar phosphate, fructose 1,6 -
  biphosphate

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C3 Pathway

• Some fructose 1,6 biphosphate transformed into
  other carbohydrates (glucose), including starch
  (reactions not part of C3 cycle)
• RuBP is regenerated using ATP (generated by
  photophosphorylation)
• Free to accept more CO2

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Photorespiration

• RuBP has oxygen added to it by the enzyme Rubisco
  instead of CO2.
• This leads to reduction in photosynthesis.
• Rubisco prefers to use CO2. However, when the CO2
  concentration is low, it will also use O2.

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Photorespiration

• When Rubisco uses O2, this will result in one
  molecule of PGA and one molecule of
  phosphoglycolate (a two-carbon molecule), instead
  of two PGA molecules (see the Calvin Cycle).
• Phosphoglycolate cannot be used in the calvin
  cycle and thus represents a loss of efficiency in
  photosynthesis.
• Photorespiration can cause up to a 25 reduction
  in photosynthesis in C3 plants.

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Photorespiration
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Photorespiration

• Photorespiration becomes a problem during hot and
  dry days.
• During hot and dry days. Plants close stomata to
  avoid water loss.
• Closing of stomata prevents CO2 uptake, resulting
  in lower CO2 concentrations in leaf cells.

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C4 Pathway

• C4 plants display higher photosynthesis rates
  compared to C3 plants under hot and dry
  conditions.
• C4 plants minimize photorespiration by increasing
  the CO2 concentration in specific leaf cells.
• A higher ratio of CO2 to O2 leads to lower
  photorespiration levels in these cells.

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Corn, a C4 plant (right), is able to survive at
a lower CO2 concentration than bean, a C3 plant
(left), when they are grown together in a closed
chamber in light for 10 days.
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The C4 pathway concentrates CO2
Interaction between the C4 cycle and the C3
cycle
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The C4 pathway concentrates CO2
In C4 plants, CO2 is first captured by PEP
carboxylase in mesophyll cells to make
oxaloacetate which is subsequently turned into
malate. This malate then diffuses into the
chloroplasts of bundle sheath cells where it
releases CO2. Thus, bundle sheath chloroplasts
contain higher CO2 concentrations compared to
chloroplasts in mesophyll cells and therefore
have higher photosynthesis and lower
photorespiration rates.
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C4 Pathway

• C4 plants employ their specific leaf anatomy
  where chloroplasts exist not only in the
  mesophyll cells in the outer part of their leaves
  but in the bundle sheath cells as well .
• CO2 is first captured by PEP carboxylase in
  mesophyll cells. This ultimately results in the
  formation of malate which diffuses into
  chloroplasts of the bundle sheath cells where it
  releases CO2 that then enters the Calvin cycle.

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However!

• The C4 pathway requires additional ATP for CO2
  fixation.
• Thus, C4 plants only grow better than C3 plants
  under hot and dry environmental conditions.

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Productivity

• Only about 0.3 to 0.5 of light energy that
  strikes leaf is stored in photosynthesis
• Genetic engineering to improve this yield?
• - engineering improved Rubisco for more
  efficient CO2 fixation?
• - engineering more efficient CO2 uptake?
• some plant species have specialized biochemical
  pathways and/or cellular organizations to
  increase CO2 concentrations in photosynthesizing
  cells (C4 pathway, CAM). Genetic engineering
  allows to transfer these mechanisms to
  agriculturally important species that have less
  efficient CO2 uptake.
• Not covered in this course

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SUMMARY Transforming Light Energy into Chemical
Energy
Transforming CO2 and H2O into food Light energy
is captured to make ATP and NADPH via the action
of photosystems I and II. This ATP and NADPH
is used via the Calvin cycle to transform the low
energy containing C-O and H-O bonds of CO2 and
H2O into the high energy containing C-C and C-H
bonds of sugar. In other words Light energy
from the sun is used by plants to increase the
potential energy of electrons in the bonding
orbitals of covalent bonds. This is done by
replacing oxygen in C-O and H-O bonds by carbon
or hydrogen, leading to the production of O2 and
carbohydrates (sugars, starch, etc).
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• Consumption of photosynthesis products
• Agriculture
• Annual accumulation of light energy as C-H and
  C-C bonds (FOOD).
• 2. Fossil fuels
• Accumulation of light energy as C-C and C-H bonds
  over millions of years (accumulation of
  photosynthesis products over millions of years).
• 3. Energy intensive agriculture
• use of fossil fuels to increase agricultural
  yields (fertilizer and pesticide production,
  irrigation, harvest, storage, transportation,
  etc). Use of photosynthesis products of the past
  to increase FOOD yields (present photosynthesis
  productivity).
• How do we maintain present levels of food
  production when fossil fuel sources become
  depleted?