Photosynthesis

1
Photosynthesis

• Chapter 9

2
Photosynthesis The Big Picture

• Source of BOTH matter and energy for most living
  organisms
• Captures light energy from the sun and converts
  it into chemical energy
• Synthesized organic molecules from inorganic
  molecule
• BOTTOM LINE Makes FOOD

3
Definitions

• Autotroph
• Organisms that make their own food (energy-rich
  organic molecules) from simple, inorganic
  molecules
• Photoautotroph
• Organisms that make their own food through
  photosynthesis obtain energy from the sun
• Type of autotroph
• Heterotroph
• Get carbon and energy by eating autotrophs or one
  another

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Photoautotrophs

• Capture sunlight energy and use it to carry out
  photosynthesis
• Plants
• Some bacteria
• cyanobacteria
• Many protistans
• algae

5
Linked Processes

• Photosynthesis
• Energy-storing pathway
• Releases oxygen
• Requires carbon dioxide

• Aerobic Respiration
• Energy-releasing pathway
• Requires oxygen
• Releases carbon dioxide

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Photosynthesis Equation
LIGHT ENERGY
6H2O 6CO2
6O2 C6H12O6
Water
Carbon Dioxide
Oxygen
Glucose
In-text figurePage 115
7
Chloroplast Structure
Inner and outer membranes
Stroma
Granum (Grana)
Thylakoid
Figure 7.3d, Page 116
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Two Stages of Photosynthesis

• Light dependent reactions
• Converts light energy into chemical energy (ADP?
  ATP)
• Gathers e- and H from water (NADP ? NADPH)
• Occurs in thylakoid membranes
• Light independent reactions (Calvin-Benson Cycle)
• Reduces CO2 to synthesize glucose using energy
  and hydrogens (i.e. ATP and NADPH) generated in
  the light dependent reaction
• Occurs in Stroma
• Notice that these reactions do not create NADH,
  but rather NADPH

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Two Stages of Photosynthesis
sunlight
water uptake
carbon dioxide uptake
ATP
ADP Pi
LIGHT-INDEPENDENT REACTIONS
LIGHT-DEPENDENT REACTIONS
NADPH
NADP
glucose
P
oxygen release
new water
In-text figurePage 117
10
Electromagnetic Spectrum

• Shortest Gamma rays
• wavelength X-rays
• UV radiation
• Visible light
• Infrared radiation
• Microwaves
• Longest Radio waves
• wavelength

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Visible Light

• Electromagnetic energy with a wavelength of
  308-750nm
• Light energy is organized into packets called
  photons
• The shorter the wavelength the greater the energy
  carried by the photons

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Properties of Light

• White light (from the sun) contains all of the
  wavelengths of light
• When light hits matter, it can be reflected
  (transmitted) or absorbed
• White substances reflect all light
• Black substances absorb all light

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Pigments

• A substance that absorbs light
• We see the color that is transmitted by pigment
• The absorbed color disappears into pigment

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Plant pigments

• Plant use a variety of pigments during
  photosynthesis
• Chlorophylls a and b
• Carotenoids
• Anthocyanins
• Phycobilins
• The main photosynthetic pigment is Chlorophyll a

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Chlorophylls

• Chlorophyll a absorbs red and blue light, and
  reflects green light (what we see)
• Note The colors that are absorbed are used for
  photosynthesis

chlorophyll a
Wavelength absorption ()
chlorophyll b
Figure 7.7Page 120
Wavelength (nanometers)
Figure 7.6a Page 119
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Effect of Light on Pigments

• What happens when light hits pigments?
• The color disappears, but the energy does not
• Absorbing photons of light excites electrons
  (e-), thus adding potential energy
• Ground state normal pigment
• Excited state pigment absorbing light (e-
  excited)

Photon of light
Atom in pigment Ground state
Atom in pigment Excited state
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Photosystems

• In thylakoid membrane, pigments are organized in
  clusters called photosystems
• These clusters contain several hundred pigment
  molecules
• Two types of photosystems
• Photosystem I P700 (absorbs light at 700nm)
• Photosystem II P680 (absorbs light at 680nm)

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Reaction Center Chlorophyll

• One of the pigments in each photosystem is known
  as the reaction center chlorophyll (RCC)
• if any pigment within the photosystem gets hit by
  a photon, the energy is transferred to the RCC
• The RCC will then transfer its excited e- into an
  electron transport chain

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Pigments in a Photosystem
reaction center
Figure 7.11Page 122
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Light Dependent Reactions

• Location the thylakoid membranes
• Function to generate ATP (energy!) and NADPH
  (reducing power!) that will be used in the light
  independent reaction
• Two processes
• Non-cyclic electron flow
• Generates ATP and NADPH
• Cyclic electron flow
• Generates only ATP

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Noncyclic Electron Flow

• Two-step pathway for light absorption and
  electron excitation
• Uses two photosystems type I and type II
• Produces ATP and NADPH
• Involves photolysis - splitting of water

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Machinery of Noncyclic Electron Flow
H2O
second electron transfer chain
photolysis
e
e
ATP SYNTHASE
NADPH
first electron transfer chain
NADP
ATP
ADP Pi
PHOTOSYSTEM I
PHOTOSYSTEM II
Figure 7.13aPage 123
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Steps of Non-cyclic electron flow

• Photosystem II gets hit by a photon electron of
  RCC gets excited
• The excited (high energy) e- gets picked up by an
  electron carrier and taken into an electron
  transfer chain (ETC)
• The excited e- provides energy to pump protons
  (H) into the thylakoid (tiny space)
• Through chemiosmosis, ATP is generated

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Chemiosmotic Model of ATP Formation

• Electrical and H concentration gradients are
  created between thylakoid compartment and stroma
• H flow down gradients into stroma through ATP
  synthase
• The energy driven by the flow of H powers the
  formation of ATP from ADP and Pi

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Chemiosmotic Model for ATP Formation
Gradients propel H through ATP synthases ATP
forms by phosphate-group transfer
H is shunted across membrane by some components
of the first electron transfer chain
Photolysis in the thylakoid compartment splits
water
H2O
e
acceptor
ATP SYNTHASE
ATP
ADP Pi
PHOTOSYSTEM II
Figure 7.15Page 124
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Non-cyclic electron flow Photolysis

• While Photosystem II gets hit by light, etc.,
  water is split
• H2O ? ½ O2 2H 2e-
• This process is called photolysis
• The H are pumped into the thylakoid to create
  the proton gradient
• The e- replace the excited e- that was taken away
  from the RCC

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Non-Cyclic Electron FlowThe saga continues

• Photosystem I gets excited at the same time as
  photosystem II
• Its excited e- gets taken into a second electron
  transfer chain that attaches the excited e- and
  the leftover H to NADP to make NADPH
• NADP H e- ? NADPH

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Non-cyclic electron flow

• The electron hole in photosystem I is then
  filled with the used up, low energy e- from
  photosystem II
• Now everything is back to normal, and we can
  start all over again

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Energy Changes in Non-cyclic electron flow
second
transfer
chain
e
NADPH
e
first
transfer
chain
Potential to transfer energy (volts)
e
e
(Photosystem I)
(Photosystem II)
1/2O2 2H
H2O
Figure 7.13bPage 123
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Non-cyclic electron flow Summary

• After two excited photosystems, two ETCs and the
  splitting of water, both ATP and NADPH are
  generated!!!

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Cyclic electron flow

• The light independent reactions require more ATP
  than NADPH
• Cyclic electron flow is like a short cut to
  making extra ATP
• Involves only Photosystem I

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Cyclic electron flow

• Photosystem I gets excited
• Excited e- is carried into the first ETC energy
  goes to pump H into thylakoid compartment
• Chemiosmosis powers formation of ATP
• The same e- (now low energy) replaces itself in
  the electron hole in Photosystem I

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Cyclic electron flow
H2O
second electron transfer chain
photolysis
e
e
ATP SYNTHASE
NADPH
first electron transfer chain
NADP
ATP
ADP Pi
PHOTOSYSTEM I
PHOTOSYSTEM II
Figure 7.13aPage 123
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Light dependent reactionsSummary

• Non-cyclic electron flow
• Generates ATP and NADPH
• Uses both photosystems (P680 and P700) and both
  electron transport chains
• Involves photolysis
• Cyclic electron flow
• Generates ATP only
• Uses only P700 and only one ETC

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Light-Independent Reactions

• Synthesis part of photosynthesis
• Can proceed in the dark
• Take place in the stroma
• Also called Calvin-Benson cycle, or Calvin Cycle,
  or Dark Reactions

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Calvin-Benson Cycle

• Three Phases
• Carbon Fixation
• Reduction
• Regeneration of RUBP

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Calvin-Benson Cycle Carbon Fixation

• Capturing atmospheric (gaseous) CO2 by attaching
  it to RuBP, a 5-carbon organic molecule
• This process forms two 3-carbon molecules
• The enzyme that catalyzes this process is called
  Rubisco

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Calvin Benson CycleReduction

• The captured CO2 has very little energy and no
  hydrogens
• In order to make sugar, energy and hydrogens need
  to be added to the molecules formed by Carbon
  fixation
• ATP and NADPH (made in the light dependent
  reactions) break down to form ADP and NADP and,
  in the process, transfer energy and hydrogens to
  the 3-carbon compounds formed by carbon fixation,
  resulting in sugar formation

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Calvin-Benson CycleRegeneration

• Some of the sugar created by reduction leaves the
  Calvin cycle, and is used to build up glucose and
  other organic molecules
• The rest of the sugar is used to remake
  (regenerate) RuBP
• This process requires ATP (which was made in the
  light dependent reactions)

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Calvin-Benson CycleSummary

• The cycle proceeds 6 times to form each molecule
  of glucose
• In the process, ATP and NADPH is used up
• 6CO2 are converted into C6H12O6 - glucose

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The C3 Pathway

• In Calvin-Benson cycle, as described, the first
  stable intermediate is a three-carbon PGA
• Because the first intermediate has three carbons,
  the pathway is called the C3 pathway

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Photorespiration in C3 Plants

• On hot, dry days stomata (holes in the leaf)
  close to prevent evaporation of water
• As a result, within the leaf oxygen levels rise,
  and Carbon dioxide levels drop
• Rubisco attaches RuBP to oxygen instead of carbon
  dioxide
• Results in a VERY wasteful process known as
  Photorespiration uses up ATP without generating
  sugar

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C4 and CAM Plants

• To avoid photorespiration, plants that live in
  hot, dry climates evolved mechanisms to separate
  carbon fixation from the Calvin Cycle
• The CO2 that enters the Calvin cycle is derived
  from the breakdown of previously synthesized
  organic acids
• In this way, the enzyme that catalyzes the
  reaction that attaches CO2 to RuBP is not exposed
  to atmospheric oxygen

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C4 and CAM Plants

• C4 plants (grasses) do carbon fixation in a
  different location (cell type) than the Calvin
  cycle
• CAM plants (succulents and Cacti) do carbon
  fixation at a different time (night) that the
  Calvin cycle (day)

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Summary of Photosynthesis
LIGHT-INDEPENDENT REACTIONS
Figure 7.21Page 129