Transcript You Light Up My Life

Where It Starts –
Photosynthesis
Lecture 8 - Autumn 2007
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 (deep blue sea - blue
light travels the deepest because it has the most
energy)
Pigments
• Light-absorbing molecules
• Absorb some wavelengths and
transmit others
• Color you see are the
wavelengths not absorbed
Variety of Pigments
Chlorophylls a and b
Carotenoids
Etc.
Chlorophylls
Main pigments in most photoautotrophs
Wavelength absorption (%)
- Organisms which make their own food
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 - simple elegance illuminate a long strand of photosynthetic algae with a
spectrum of light, and watch were the mobile bacteria
accumulate!
Light-Dependent Reactions
1. Pigments absorb light energy, give up ewhich enter electron transfer chains
2. Water molecules are split, ATP and
NADH are formed, and oxygen is
released (waste product - lucky for us!)
3. Pigments that gave up electrons get
replacements
Light-Independent Reactions
• Synthesis part of photosynthesis
• Can proceed in the dark
• Take place in the stroma (special
structures inside the chloroplast)
• Calvin-Benson cycle
Photosynthesis Equation
- remember forever
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 (mostly)
• Heterotrophs
– Get carbon and energy by eating autotrophs
(plants) and/or one another (cannibalism too)
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
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 some energy
loss - heat and atomic motion
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
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
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)