Photosynthesis
Lecture 7
Fall 2008
Photosynthesis
Photosynthesis
• The process by which light energy from the
sun is converted into chemical energy
1
Photosynthesis
Inputs
• CO2
2
– Gas exchange occurs through stomata
– Stomata (stoma) – small pore
– CO2 in, O2 out
• H2O
– enters roots from soil
• Energy from sunlight
• Also need minerals from soil
– e.g. potassium (K), nitrogen (N), and
phosphorous (P)
• Also need O2 from soil
The Chloroplast
Fig.10.3
3
The Chloroplast
Structure
• Outer membrane
• Inner membrane
• Stroma
– Thick fluid inside inner membrane
• Thylakoids
– Membrane bound sacs
– Interconnected
– Photosynthetic pigments
embedded in membranes
– Thylakoid space
• Interior of the thylakoid
• Grana (granum)
Fig.10.3
– Stacks of thylakoids
– Large surface area
4
Photosynthetic prokaryotes
• Infoldings of plasma membrane allow for specialized
functions
• Endosymbiosis of photosynthetic prokaryote led to
chloroplast
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6
• Read Tracking Atoms through
Photosynthesis: Scientific Inquiry, pgs.
187-188
Photosynthesis & Cellular Respiration
Cellular respiration
• Redox reactions move electrons (and hydrogen)
from glucose to oxygen
• “fall” of electrons
• Produces energy in the form of ATP
Photosynthesis
• Redox reactions move electrons (and hydrogen)
from water to carbon dioxide to form glucose
• Electrons moved “uphill”
• Requires large initial investment of energy
(sunlight)
• Produces energy in the form of glucose
molecules
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8
Photosynthesis Overview
Two metabolic stages of
photosynthesis
Each process occurs in
a specific area
• Light Reactions
– In the thylakoids
• Calvin cycle
– In the stroma
Fig. 10.5
98
Photosynthesis Overview
• Light Reactions
– Convert solar energy to
chemical energy
• ATP & NADPH
– Water split
• Calvin cycle
– Synthesizes sugar from
CO2
– Uses the ATP & NADPH
produced in the light
reactions
Fig. 10.5
10
The Nature of Sunlight
Electromagnetic energy
(electromagnetic radiation)
• Radiation = emission of
energy in the form of
electromagnetic waves or
photons
– Wavelength - distance
between the crests of
two adjacent waves
– Photon - discrete packet
of energy
• Electromagnetic spectrum
– Range of wavelengths
of electromagnetic
energy
•Gamma rays
– Short waves
– High energy
•Radio waves
– Long waves
– Low energy
Fig. 10.6
11
The Nature of Sunlight
• Sunlight radiates the full
spectrum
• Our atmosphere filters
out much of the
spectrum
• Visible light
– Passes through
atmosphere
– Light that humans can
see with our eyes (colors)
– Wavelengths that powers
photosynthesis
Fig. 10.6
The Nature of Sunlight
When light meets matter:
• Reflected
– Wavelengths “bounce back” from matter
• Transmitted
– Wavelengths pass through matter
• Absorbed
– Wavelengths “disappear” into matter
Pigments
• Chemical compounds that absorb certain wavelengths of
light
– We only see “color” of wavelength that is reflected or transmitted
– If a pigment absorbs all wavelengths, then we see black
– If a pigment absorbs wavelengths from 380 to 550, what color
would we see?
12
Which Wavelengths are Used in Photosynthesis:
The Scientific Method at Work
Question:
• Which wavelengths are used in photosynthesis?
Observations:
• Photosynthetic organisms use visible light from
the sun
• Visible light comes in many wavelengths
• By using a prism, light can be separated into its
wavelengths
• Unicellular algae are photosynthetic organisms
• Bacteria tend to gather in areas of high oxygen
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Which Wavelengths are Used in Photosynthesis:
The Scientific Method at Work
Hypothesis:
• Algae will photosynthesize when exposed to its
ideal wavelengths
Predictions:
• If the algae photosynthesize in response to a
particular wavelength, then O2 will be released in
that area
• If O2 is produced in an area, then aerobic
bacteria will gather in that area
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Which Wavelengths are Used in Photosynthesis:
The Scientific Method at Work
Methods:
• Algae placed in strip
on microscope slide
• Bacteria add to slide
• Light shown through a
prism onto slide
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Which Wavelengths are Used in Photosynthesis:
The Scientific Method at Work
Results?
Conclusions?
Fig. 10.9
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Why are Leaves Green?
• Inside chloroplasts are
photosynthetic pigments
– Pigments – chemical
compounds that absorb
certain wavelengths of light
• Chlorophyll a absorbs
blue-violet and red light
• Given that info – why are
leaves green?
Fig. 10.7
18
Photosynthetic Pigments in Chloroplasts
Chlorophyll a
– Required for photosynthesis
– Absorbs blue-violet and red light
Accessory pigments
• Pigments other than chlorophyll a
• Broadens the spectrum of light that can be
absorbed & used for photosynthesis
• Used as a “sunscreen” – protection against
UV radiation
• Coloration – attract pollinators to flowers,
attract fruit dispersers to fruit
Photosynthetic Pigments in Chloroplasts
Accessory pigments
• Chlorophyll b
Why would so
many different
pigments
evolve?
– Present in plants, some
algae
– Absorbs blue and orange
light
• Carotenoids
– Present in plants, algae,
cyanobacteria
– Absorb blue-green light
• Chlorophyll c
– Present in some algae
Fig. 10.9
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Photosystems
• The structure of thylakoids and position of
pigments critical to function
• Pigments arranged into photosystems
– Photosystem – reaction center plus light harvesting
complexes within the plasma membrane
Fig. 7.10
21
Photosystems
Reaction center complex
• Protein complex with:
– Special chlorophyll a molecule (2)
– Primary electron acceptor
Light harvesting complex
• Protein complex with many
photopigments (chl a, b,
carotenoids)
– Able to harvest light over broader
spectrum
How light is “harvested”
• Photon absorbed by a pigment
molecule
• Energy transferred from one
pigment molecule to another
• Energy ultimately passed to chl
a in reaction center
Fig. 10.12
22
Light Energy
What happens when energy from photon
arrives at the reaction center?
• Electron from chl a is excited
What is an “excited” electron?
• Electron receives energy and move to
an ‘”excited” state
– Higher orbital – more potential
energy
• Unstable position, so electron falls
back down to ground state
Process releases energy:
• Heat
• Light - florescence
If there is a molecule to receive the
electron, it retains its high energy and
does not fall to the ground state
Fig. 10.11
Photosystems
What happens when energy arrives at
the reaction center?
• Electron from chl a is excited
• Electron passed to the primary
electron acceptor
– Redox reaction
• Two paths for electrons, depending
on photosystem type:
– Creates NADPH
– Gets passed to electron transport chain
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Photosystems
Two types:
• Photosystem 2 or PSII
– Water-splitting photosystem
• Photosystem 1 or PS1
– NADPH-producing photosystem
Fig. 10.13
25
PS1 (NADPH-producing photosystem)
• Electrons from reaction center chl
a excited
– P700
• Passed to primary electron
acceptor
• Primary electron acceptor passes
electrons ferredoxin (FD)
• FD transfers electron to NADP+
– NADP+ reduced to NADPH
– Requires NADP+ reductase
– 2 electrons
• NADPH will take electrons to the
Calvin cycle
– Energy to produce sugar
Fig. 10.13
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PS1 (NADPH-producing photosystem)
Problem: If the electrons
from the reaction center
chl a get passed on to an
electron acceptor, how do
they get replaced?
Fig. 10.13
Photosystems
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Which is the stronger electron acceptor?
1) the reaction center chl a in NADPH-producing
photosystem (PS1)
Or
2) the primary electron acceptor in the water-splitting
photosystem (PS2)
Fig. 10.13
28
Photosystems
Electrons from water-splitting photosystem (PS2)
“pulled” down electron transport chain by the
reaction center chl a in the NADPH-producing
photosystem (PS1)
Fig. 10.13
PS2 - Water-splitting photosystem
• Electrons from reaction center chl a
excited
– P680
• Passed to primary electron acceptor
How do the electrons get replaced?
• Take electrons from H2O
– Water-splitting step
– Requires enzyme
– O2 forms & 4H+
What is the strongest electron acceptor
(oxidizing agent) in these
photosystems?
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Fig. 10.13
30
PS2 - Water-splitting photosystem
• Primary electron acceptor passes electron to
electron transport chain
• Replaces electron in P700 chl
• Entire process (PS2 – PS1) called linear
electron flow
Fig. 10.13
Electron Transport Chain
ETC composed of many protein complexes embedded in
the thylakoid membrane
• Plastoquinone (Pq)
• Cytochrome complex
• Plastocyanin (Pc)
What benefit is gained from being in the thylakoid
membrane?
Fig. 10.17
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Electron Transport Chain
• Electrons provide by primary electron acceptor
of PSII
• Electrons ”fall” down chain
– Pulled by P700 chl
• Produces energy at each step
Fig. 10.17
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Electron Transport Chain
• Transfer of electrons activates transfer of H+
• H+ moved from stroma, across the thylakoid
membrane, and into the thylakoid space
• Creates a concentration gradient of H+ across
the thylakoid membrane
Fig. 10.17
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ATP Production
ATP synthase
– Complex of proteins built into the inner
membrane of the thylakoid
Chemiosmosis
• Concentration gradient of H+ harnessed
to do cellular work
• Proton-motive force
– The thylakoid membranes are not freely
permeable to H+
– Path down concentration gradient is through
ATP synthase
– As H+ travels through ATP synthase, it
causes turbine-like structures to turn,
activating enzymes
– Enzymes generate ATP from
ADP + P = phosphorylation
– Photophosphorylation
• Using energy from light to excite electrons that
go down the ETC
Fig. 9.14
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ATP Production
ATP synthase
• H+ ions enter through half-channel
on stator
• Enter binding sites on rotor
– Changes conformation, rotor spins
• One rotation, H+ exits through halchannel
• Rotor spin causes rod to spin
• Spinning rod activates catalytic
sites on knob
• ATP produced from ADP + P
– Read Fig. 9.15 Inquiry
Fig. 9.14
Cyclic Electron Flow
• FD passes electron to cytochrome complex
• Produces ATP
Fig. 10.15
36
Light Reactions Summary
Light Reactions
• Convert solar energy to
chemical energy
– ATP & NADPH
• Requires H2O
• Produces O2 as “waste
product”
Fig. 10.5
37
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The Calvin Cycle
Calvin cycle
• Synthesizes sugar from CO2
• Anabolic
Inputs
• ATP
• NADPH
• CO2
Output
• Organic compound - G3P
(glyceraldehyde 3phosphate)
– Used to make glucose and
other compounds
Fig. 10.5
39
The Calvin Cycle
3 Phases:
• Carbon Fixation
• Reduction
• Regeneration of
RUBP
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The Calvin Cycle
Carbon fixation
• CO2 enters one at a time
• Attached to ribulose bisphosphate (5-carbon sugar) to
become 6-carbon intermediary
– Enzyme: rubisco (RuBP carboxylase)
– Most abundant enzyme
• Splits into 2 molecules of 3-phosphoglycerate
Fig. 10.18
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The Calvin Cycle
Reduction
• 3-phosphoglycerate
phosphorylated by ATP
– 1,3 bisphosphoglycerate
• 1,3 bisphosphoglycerate
reduced by NADPH
– G3P (glceraldehyd-3phosphate)
– High potential energy
• One molecule of G3P is
output
Fig. 10.18
42
The Calvin Cycle
Regeneration of
RuBP
• 5 molecules of
G3P rearranged
into 3 molecules
of RuBP
• Requires ATP
After the Calvin Cycle
What happens to the G3P produced in
photosynthesis?
Transport to other cells
1. Produces glucose & fructose
2. They combine to form sucrose
3. Sucrose transported to other cells
• If growing cell
– Sucrose broken down to glucose & fructose & used
in cellular respiration & growth
•
If storage cell
– Sucrose converted to starch
Starch production in photosynthetic cell
• Starch broken down overnight to supply cellular
respiration
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