Photosynthesis

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Photosynthesis

• Chapter 10

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Photosynthesis
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Photosynthesis

• Process where organisms capture energy from
  sunlight
• Build food molecules
• Rich in chemical energy
• 6CO2 12H2O
• C6H12O6 6H2O 6O2

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Photosynthesis

• Captures only 1 of the energy of the sun
• Uses it to provide energy for life

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Photosynthesis

• Autotrophs
• Producers
• Make own organic molecules
• Heterotrophs
• Consumers

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Photosynthesis

• Plant leaves - Chloroplasts

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Chloroplasts

• Thylakoids
• Internal membranes of chloroplasts
• Membranes of thylakoids contain chlorophyll
• Green pigment that captures the light for
  photosynthesis
• Grana
• Stacks of thylakoids

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Chloroplasts

• Stroma
• Semi-liquid substance
• Surrounds the thylakoids
• Contain enzymes
• Make organic molecules from carbon dioxide

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Chloroplasts
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Fig. 10-3b
Chloroplast
Outer membrane
Thylakoid
Intermembrane space
Thylakoid space
Granum
Stroma
Inner membrane
1 µm
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Chloroplasts

• Photosystem
• Cluster of photosynthetic pigments
• In membrane of thylakoids
• Each pigment in system captures energy
• Photosystem then gathers energy
• Energy makes ATP, NADPH and organic molecules

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Stoma (Stomata)

• Opening on the leaf
• Allows exchange of gases.

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NADP

• Nicotinamide Adenine Dinucleotide Phosphate
• Coenzyme
• Electron carrier
• Reduced during the light-dependent reactions
• Used later to reduce carbon in carbon dioxide to
  form organic molecules
• Photosynthesis is a redox reaction

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Photophosphorylation

• Addition of phosphate group to ADP
• Light energy

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Photosynthesis

• Occurs in 3 stages
• 1. Capturing energy from the sun
• 2. Energy makes ATP
• Reducing power in NADPH
• 3. ATP and NADPH
• Power synthesis of organic molecules from carbon
  dioxide

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Photosynthesis

• Light dependent reactions
• First 2 steps of photosynthesis
• Take place in presence of light
• Light-independent reactions
• Formation of organic molecules
• Calvin cycle
• Can occur /- light

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Experimental history

• Jan Baptista van Helmont
• Plants made their own food
• Joseph Priestly
• Plants restored the air

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Experimental history

• Jan Ingenhousz
• Suns energy split the CO2 into Carbon Oxygen
• Oxygen was released into the air
• Carbon combined with water to make carbohydrates

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Experimental history

• Fredrick Forest Blackman
• 1. Initial light reactions are independent of
  temperature
• 2. Second set of dark reactions are independent
  of light
• Dependent on CO2 concentrations temperature
• Enzymes must be involved in the light-independent
  reactions

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Experimental history

• C.B. van Neil
• Looked at the role of light in photosynthesis
• Studied photosynthesis in Bacteria

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C.B. van Neil

• CO2 2H2S ? (CH2O) H2O 2S
• CO2 2H2A ? (CH2O) H2O A2
• CO2 2H2O ? (CH2O) H2O O2

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C.B. van Neil

• O2 produce from green plant photosynthesis comes
  from splitting the water
• Not carbon dioxide
• Carbon Fixation
• Uses H from spitting of water to reduce carbon
  dioxide into organic molecules (simple sugars).
• Light-independent reaction

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Photosynthesis

• 1. Occurs in the chloroplasts
• 2. Light-dependent reactions use light to reduce
  NADP and manufacture ATP
• 3. ATP and NADPH will be used later in the
  light-independent reactions
• Incorporate carbon dioxide into organic molecules

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Fig. 10-5-4
H2O
CO2
Light
NADP
ADP
P

i
Calvin Cycle
Light Reactions
ATP
NADPH
Chloroplast
CH2O (sugar)
O2
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Sunlight

• UV light from sun
• Important source of energy when life began
• UV light can cause mutations in DNA
• Lead to skin cancer

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Light

• Photon
• Packets of energy
• UV light has photons with greater energy than
  visible light
• UV light has shorter wavelengths
• X-Rays have shorter wavelengths then UV more
  energy.

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Light

• Visible light
• Purple has shorter wavelengths
• More energetic photons
• Red has longer wavelengths
• Less energetic photons

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Spectrum
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Spectrum
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Absorption Spectrums

• Photon of energy strikes a molecule
• Lost as heat or absorbed by the molecule
• Depends on amount of energy in the photon
  (wavelength)
• Dependent on the atoms available energy levels

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Absorption spectrum

• Specific for each molecule
• Range efficiency of photons it is capable of
  absorbing

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Pigments

• Molecules that are good absorbers of energy in
  the visible range
• Chlorophylls Carotenoids
• Chlorophyll a b absorb photons in the
  blue-violet red light

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Pigments

• Chlorophyll a main pigment of photosynthesis
• Converts light energy to chemical energy
• Chlorophyll b carotenoids are accessory
  pigments
• Capture light energy at different wavelengths

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Pigments
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Pigments

• Chlorophyll b

Chlorophyll a
Carotenoids
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Chlorophyll structure

• Chlorophyll located in the thylakoid membranes
• A porphyrin ring with a Mg in the center
• Hydrocarbon tail
• Photons are absorbed by the ring
• Excites electrons in the ring
• Absorbs photons very effectively

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Chlorophyll structure
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  sentation\10_07LightAndPigments_A.html

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Carotenoids

• Two carbon rings attached by a carbon chain
• Not as efficient as the Chlorophylls
• Beta carotene (helps eyes)
• Found in carrots and yellow veggies

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Photosystems

• Captures the light
• Located on surface of the photosynthetic membrane
• Chlorophyll a molecules
• Accessory pigments (chlorophyll b carotenoids)
• Associated proteins

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Photosystems

• Consists of 2 components
• 1. Antenna (light gathering) complex
• 2. Reaction center

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Photosystem

• 1. Antenna complex
• Gathers photons from the sun
• Web of Chlorophyll a molecules
• Tightly held by proteins in the membrane
• Accessory pigments carotenoids
• Energy is passed along the pigments to reaction
  center

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Photosystems

• 2. Reaction centers
• 2 special chlorophyll a molecules accept the
  energy
• Chlorophyll a than passes the energized electron
  to an acceptor
• Acceptor is reduced (quinone)

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Photosystem
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Fig. 10-12
STROMA
Photosystem
Photon
Primary electron acceptor
Light-harvesting complexes
Reaction-center complex
e
Thylakoid membrane
Pigment molecules
Special pair of chlorophyll a molecules
Transfer of energy
THYLAKOID SPACE (INTERIOR OF THYLAKOID)
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2 photosystems

• Photosystem I (older)
• Absorbs energy at 700 nm wavelength
• Generates NADPH
• Photosystem II (newer)
• Absorbs energy at 680 nm wavelength
• Splits water (releases oxygen)
• Generates ATP
• 2 systems work together to absorb more energy

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Photosynthesis (Process)

• Light dependent reactions
• Linear electron flow
• Energy transfer
• Thylakoid membranes

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Light dependent reactions

• Photosystem II (680 nm)
• Light is captured by the pigments
• Excites an electron (unstable)
• Energy is transferred to the reaction center
  (special chlorophyll)
• Passes the excited electron to an acceptor
  molecule

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Light dependent reactions

• PS II is oxidized
• Water splits (enzyme)
• Water donates an electron to the chlorophyll
• Reduces PS II
• Oxygen (O2) is released with 2 protons (H)

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Light dependent reactions

• Electron is transported to PS I (700 nm)
• Electron is passed along proteins in the membrane
  (ETC)
• Protons are transported across the membrane
• Protons flow back across the membrane through
  ATP synthase
• Generate ATP

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Light dependent reactions

• At the same time PS I received light energy
• Excites an electron
• Primary acceptor accepts the electron
• PS I is excited
• Electron from PS II is passed to PS I
• Reduces the PS I

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Light dependent reactions

• PS I excited electron is passed to a second ETC
• Ferredoxin protein
• NADP reductase catalyzes the transfer of the
  electron to NADP
• Makes NADPH

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Fig. 10-13-5
Electron transport chain
Primary acceptor
Primary acceptor
4
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Electron transport chain
Fd
Pq
e
2
e
8
e
e
NADP H
H2O
Cytochrome complex
2 H
NADP reductase

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NADPH
O2
1/2
Pc
e
e
P700
5
P680
Light
Light
1
6
6
ATP
Pigment molecules
Photosystem I (PS I)
Photosystem II (PS II)
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Fig. 10-UN1
H2O
CO2
Primary acceptor
Electron transport chain
Primary acceptor
Fd
Electron transport chain
NADP H
H2O
Pq
NADP reductase
O2
NADPH
Cytochrome complex
Pc
Photosystem I
ATP
Photosystem II
O2
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Enhancement effect
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Enhancement effect
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Fig. 10-17
STROMA (low H concentration)
Cytochrome complex
Photosystem I
Photosystem II
Light
4 H
NADP reductase
Light
3
Fd
NADP H
NADPH
Pq
Pc
e
2
e
H2O
O2
1/2
1
THYLAKOID SPACE (high H concentration)
4 H
2 H
To Calvin Cycle
Thylakoid membrane
ATP synthase
STROMA (low H concentration)
ADP
ATP
P
i
H
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Fig. 10-16
Mitochondrion
Chloroplast
CHLOROPLAST STRUCTURE
MITOCHONDRION STRUCTURE
H
Diffusion
Intermembrane space
Thylakoid space
Electron transport chain
Inner membrane
Thylakoid membrane
ATP synthase
Stroma
Matrix
Key
ADP P
i
ATP
Higher H
H
Lower H
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Photosystems

• Noncyclic photophosphorylation
• 2 systems work in series
• Produce NADPH ATP
• Replaces electrons from splitting water
• System II (splits water)works first then I (NADPH)

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Photosystems

• When more ATP is needed
• Plant changes direction
• The electron used to make NADPH in PS I is
  directed to make ATP

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Calvin Cycle

• Named for Melvin Calvin
• Cyclic because it regenerates its starting
  material
• C3 photosynthesis
• First organic compound has 3 carbons

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Calvin cycle

• Combines CO2 to make sugar
• Using energy from ATP
• Using reducing power from NADPH
• Occurs in the stroma of the chloroplast

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Calvin Cycle

• Consists of three parts
• 1. Fixation of carbon dioxide
• 2. Reduction-forms G3P (glyceraldehyde
  3-phosphate)
• 3. Regeneration of RuBP (ribulose 1, 5
  bisphosphate)

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Calvin Cycle

• 3 cycles
• 3 CO2 molecules
• 1 molecule of G3P
• 6 NADPH
• 9 ATP

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Fixation of carbon

• Carbon dioxide combines with ribulose 1, 5
  bisphosphate (RuBP)
• Temporary 6 carbon intermediate
• Two three carbon molecules called
  3-phosphoglycerate (PGA)
• Ribulose bisphosphate carboxylase/oxygenase
  (Rubisco) is the large enzyme that catalyses the
  reaction

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Reduction

• Phosphate is added to 3-phosphoglycerate
• 1,3 Bisphosphoglycerate
• NADPH reduces the molecule
• Glyceraldehyde 3-phosphate (G3P)

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Regeneration

• 5 molecules of G3P are rearranged to make 3 RuBP
• Uses 3 more ATP

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Fig. 10-18-3
(Entering one at a time)
Input
3
CO2
Phase 1 Carbon fixation
Rubisco
3
P
P
Short-lived intermediate
6
P
3
P
P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
ATP
6
6 ADP
3 ADP
Calvin Cycle
P
6
P
3
ATP
1,3-Bisphosphoglycerate
6
NADPH
Phase 3 Regeneration of the CO2 acceptor (RuBP)
6 NADP
P
6
i
P
5
G3P
P
6
Glyceraldehyde-3-phosphate (G3P)
Phase 2 Reduction
1
P
Glucose and other organic compounds
Output
G3P (a sugar)
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Fig. 10-UN2
3 CO2
Carbon fixation
3 ? 5C
6 ? 3C
Calvin Cycle
Regeneration of CO2 acceptor
5 ? 3C
Reduction
1 G3P (3C)
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Calvin Cycle

• 3 carbon dioxide molecules enter the cycle
  combine with RuBP
• Generates 3 molecules more of RuBP one G3P
  (glyceraldehyde 3-phosphate)
• G3P now can be made into glucose other sugars

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Calvin Cycle

• Enzyme mediated
• 5 of these enzymes need light to be more
  efficient
• Net reaction
• 3CO2 9 ATP 6NADPH
• G3P 8Pi 9ADP 6NADP

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G3P

• G3P (glyceraldehyde 3-phosphate)
• Converted to fructose 6-phosphate (reverse of
  glycolysis)
• It is made into sucrose
• This occurs in the cytoplasm
• Intense photosynthesis
• G3P levels rise so much some is converted to
  starch

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Fig. 10-21
H2O
CO2
Light
NADP
ADP
P

i
Light Reactions Photosystem II Electron
transport chain Photosystem I Electron
transport chain
RuBP
3-Phosphoglycerate
Calvin Cycle
ATP
G3P
Starch (storage)
NADPH
Chloroplast
O2
Sucrose (export)
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Photorespiration

• When hot the stoma in a leaf close to avoid
  loosing water
• Carbon dioxide cannot come in.
• Oxygen builds up inside
• Carbon dioxide is released
• G3P is not produced

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Photorespiration

• Occurs when Rubisco oxidizes RuBP (starting
  molecules of Calvin cycle)
• Oxygen is incorporated into RuBP
• Undergoes reactions that release CO2
• Carbon dioxide oxygen compete for the same
  sight on the enzyme
• Under conditions greater than the optimal 250C
  this process occurs more readily

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C4 Photosynthesis

• Process to avoid loosing carbon dioxide
• Plant fixes carbon dioxide into a 4 carbon
  molecule (oxaloacetate)
• PEP carboxylase (enzyme)
• Oxaloacetate is converted to malate
• Then taken to the stroma for the Calvin cycle
• Sugarcane and corn

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CAM

• Another process to prevent loss of CO2
• Plants in dry hot regions (cacti)
• Reverse what most plants do
• Open stoma at night to allow CO2to come in
  water to leave
• Close them during the day.

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CAM

• Carbon fix CO2 at night into 4 carbon chains
  (organic acids)
• Use the Calvin cycle during the day.

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Fig. 10-20
Sugarcane
Pineapple
C4
CAM
CO2
CO2
Mesophyll cell
Night
CO2 incorporated into four-carbon organic
acids (carbon fixation)
1
Organic acid
Organic acid
Bundle- sheath cell
Day
CO2
CO2
Organic acids release CO2 to Calvin cycle
2
Calvin Cycle
Calvin Cycle
Sugar
Sugar
(a) Spatial separation of steps
(b) Temporal separation of steps