Transcript You Light Up My Life - Hillsborough Community College

Where It Starts –
Photosynthesis
Chapter 6
Sunlight as an Energy Source
• Photosynthesis runs on a fraction of the
electromagnetic spectrum, or the full
range of energy radiating from the sun
Visible Light
• Wavelengths humans perceive as
different colors
• Violet (380 nm) to red (750 nm)
• Longer wavelengths, lower energy
Electromagnetic Spectrum
Shortest
wavelength
Longest
wavelength
Gamma rays
X-rays
UV radiation
Visible light
Infrared radiation
Microwaves
Radio waves
Photons
• Packets of light energy
• Each type of photon has fixed amount
of energy
• Photons having most energy travel as
shortest wavelength (blue-green light)
Pigments
• Light-absorbing molecules
• Absorb some wavelengths and
transmit others
• Color you see are the
wavelengths not absorbed
Pigment Structure
• Light-catching part of molecule
often has alternating single and
double bonds
• These bonds contain electrons
that are capable of being moved
to higher energy levels by
absorbing light
Variety of Pigments
Chlorophylls a and b
Carotenoids
Xanthophylls
Phycobilins
Anthocyanins
Chlorophylls
Wavelength absorption (%)
Main pigments in most photoautotrophs
chlorophyll a
chlorophyll b
Wavelength (nanometers)
Carotenoids
• Found in all photoautotrophs
• Absorb blue-violet and blue-green that
chlorophylls miss
• Reflect red, yellow, orange wavelengths
• Two types
– Carotenes - pure
hydrocarbons
– Xanthophylls - contain
oxygen
Xanthophylls
Yellow, brown, purple, or blue
accessory pigments
Phycobilins & Anthocyanins
Red to purple pigments
• Phycobilins
– Found in red algae and cyanobacteria
• Anthocyanins
– Give many flowers their colors
T.E. Englemann’s Experiment
Background
• Certain bacterial cells will move
toward places where oxygen
concentration is high
• Photosynthesis produces oxygen
T.E. Englemann’s Experiment
Hypothesis
• Movement of bacteria can be used to
determine optimal light wavelengths for
photosynthesis
T.E. Englemann’s Experiment
Method
• Algal strand placed on microscope slide
and illuminated by light of varying
wavelengths
• Oxygen-requiring bacteria placed on
same slide
T.E. Englemann’s Experiment
Results
Bacteria congregated where red and
violet wavelengths illuminated alga
Conclusion
Bacteria moved to where algal cells
released more oxygen – areas
illuminated by the most effective light for
photosynthesis
T.E. Englemann’s Experiment
Light-Dependent Reactions
• Pigments absorb light energy, give up ewhich enter electron transfer chains
• Water molecules are split, ATP and
NADH are formed, and oxygen is
released
• Pigments that gave up electrons get
replacements
Light-Independent Reactions
• Synthesis part of
photosynthesis
• Can proceed in the dark
• Take place in the stroma
• Calvin-Benson cycle
Photosynthesis Equation
Chloroplasts
Organelles of photosynthesis
leaf’s upper surface
photosynthetic cells
central vacuole
chloroplast
one photosynthetic cell inside the leaf
vein
stoma (gap) in lower epidermis
section from the leaf, showing its internal organization
Inside the Chloroplast
• Two outer
membranes
enclose a
semifluid
interior, the
stroma
• Thylakoid
membrane
inside the
stroma
two outer membranes
thylakoid
membrane
system
chloroplasts
see next slide
stroma
Inside the Chloroplast
• Photosystems
are embedded
in thylakoids,
containing 200
to 300
pigments and
other molecules
that trap sun’s
energy
• Two types of
photosystems: I
and II
light
harvesting
complex
electron
transfer
chain
PHOTOSYSTEM II
thylakoid
membrane
PHOTOSYSTEM I
thylakoid
compartment
Carbon and Energy Sources
• Photoautotrophs
– Carbon source is carbon dioxide
– Energy source is sunlight
• Heterotrophs
– Get carbon and energy by eating
autotrophs or one another
Photoautotrophs
• Capture sunlight energy and use it to
carry out photosynthesis
– Plants
– Some bacteria
– Many protistans
Linked Processes
Photosynthesis
• Energy-storing pathway
Aerobic Respiration
• Energy-releasing
pathway
• Releases oxygen
• Requires oxygen
• Requires carbon
dioxide
• Releases carbon
dioxide
Two Stages of Photosynthesis
sunlight
energy
CO2
(carbon dioxide)
H2O
(water)
ATP
lightdependent
reactions
ADP + Pi
lightindependent
reactions
NADPH
NADPH+
glucose
O2
H2O (metabolic water)
Excitation of Electrons
• Excitation occurs only when the quantity
of energy in an incoming photon
matches the amount of energy
necessary to boost the electrons of that
specific pigment
• Amount of energy needed varies among
pigment molecules
Pigments in Photosynthesis
• Bacteria
– Pigments in plasma membranes
• Plants
– Pigments embedded in thylakoid
membrane system
– Pigments and proteins organized into
photosystems
– Photosystems located next to electron
transfer chains
Photosystem Function:
Harvester Pigments
• Most pigments in photosystem are
harvester pigments
• When excited by light energy, these
pigments transfer energy to adjacent
pigment molecules
• Each transfer involves energy loss
Photosystem Function:
Reaction Center
• Energy is reduced to level that can be
captured by molecule of chlorophyll a
• This molecule (P700 or P680) is the
reaction center of a photosystem
• Reaction center accepts energy and
donates electron to acceptor molecule
Electron Transfer Chains
• Adjacent to photosystem
• Acceptor molecule donates electrons
from reaction center
• As electrons flow through chain, energy
they release is used to produce ATP
and, in some cases, NADPH
Cyclic Electron Flow
• Electrons
– are donated by P700 in photosystem I to
acceptor molecule
– flow through electron transfer chain and
back to P700
• Electron flow drives ATP formation
• No NADPH is formed
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
ATP and NADPH Formation
LIGHTHARVESTING
COMPLEX
photon
PHOTOSYSTEM II
sunlight
PHOTOSYSTEM I
a light-harvesting
complex has a
ring of pigment
molecules
NADPH
NADPH + H+
H+
H+
H+
A photosystem is surrounded by densely
packed light harvesting complexes.
H+
H+
H+
H+
H+
H+
H+
H+
thylakoid
compartment
thylakoid
membrane
ADP + Pi
ATP
stroma
ATP Formation
• When water is split during photolysis,
hydrogen ions are released into
thylakoid compartment
• More hydrogen ions are pumped into
the thylakoid compartment when the
electron transfer chain operates
ATP Formation
• Electrical and H+ concentration gradient
exists between thylakoid compartment
and stroma
• H+ flows down gradients into stroma
through ATP synthesis
• Flow of ions drives formation of ATP
Energy Transfers
PHOTOSYSTEM I
p700*
H+
Higher energy
photon
e-
p700
Cyclic Pathway of ATP Formation
Energy Transfers
PHOTOSYSTEM I
NADPH
p700*
PHOTOSYSTEM II
NADH+
p680*
photon
e-
p700
p680
2H2O
4H+ + O2
Noncyclic Pathway of ATP
and NADPH Formation
Calvin-Benson Cycle
• Overall reactants
• Overall products
– Carbon dioxide
– Glucose
– ATP
– ADP
– NADPH
– NADP+
Reaction pathway is cyclic and RuBP
(ribulose bisphosphate) is regenerated
Calvin-Benson Cycle
6CO2
ATP
6 RuBP
12 PGA
12
6 ADP
Calvin-Benson
cycle
ATP
12 ADP +
12 Pi
12 NADPH
4 Pi
12 NADP+
10 PGAL
12 PGAL
1 Pi
1
glucose-6-1-phosphate
Building Glucose
• PGA accepts
– phosphate from ATP
– hydrogen and electrons from NADPH
• PGAL (phosphoglyceraldehyde) forms
• When 12 PGAL have formed
– 10 are used to regenerate RuBP
– 2 combine to form phosphorylated glucose
Using the Products of
Photosynthesis
• Phosphorylated glucose is the building
block for:
– Sucrose
• The most easily transported plant carbohydrate
– Starch
• The most common storage form
The C3 Pathway
• In Calvin-Benson cycle, the first stable
intermediate is a three-carbon PGA
• Because the first
intermediate has
three carbons, the
pathway is called
the C3 pathway
stomata closed,
no CO2 uptake
RuBP
PGA
CalvinBenson
cycle
sugar
Photorespiration in C3 Plants
• On hot, dry days stomata close
• Inside leaf
– Oxygen levels rise
– Carbon dioxide levels drop
• Rubisco attaches RuBP to oxygen
instead of carbon dioxide
• Only one PGAL forms instead of two
C4 Plants
• Carbon dioxide is fixed twice
– In mesophyll cells, carbon dioxide is fixed
to form four-carbon oxaloacetate
– Oxaloacetate is transferred to bundlesheath cells
– Carbon dioxide is released and fixed again
in Calvin-Benson cycle
C4 Plants
stomata closed,
no CO2 uptake
C4
oxaloacetate
cycle
mesophyll cell
CO2
RuBP
PGA
CalvinBenson
cycle
sugar
bundle-sheath cell
CAM Plants
• Carbon is fixed twice (in same cells)
• Night
– Carbon dioxide
is fixed to form
organic acids
CO2 uptake at night only
C4
cycle
runs
at
night
• Day
– Carbon dioxide is
released and fixed
in Calvin-Benson
cycle
CalvinBenson
cycle
sugar
runs
during
day
Summary of Photosynthesis
sunlight
LightDependent
Reactions
12H2O
6O2
ADP + Pi
ATP
6CO2
6 RuBP
LightIndependent
Reactions
NADPH
CalvinBenson
cycle
NADP+
12 PGAL
6H2O
phosphorylated glucose
end products (e.g., sucrose, starch, cellulose)