Photosynthesis
• Photosynthesis is the process of converting
light energy to chemical energy stored in
carbon compounds.
– Plants, algae, cyanobacteria, and some
protists produce organic compounds from
inorganic materials and light energy and are
classified as photoautotrophs.
– Chloroplasts are the site of photosynthesis.
• Photosynthesis like cellular respiration is
performed through a series of redox
reactions with glucose being reduced
(gaining charge) through the oxidation of
water.
Chemical Equation
• 6CO2 + 12H2O + light energy g C6H12O6 + 6 O2 + 6H2O
• 6CO2 + 6H2O + light energy g C6H12O6 + 6 O2 (simplified formula)
– CO2 + H2O g [CH2O]n + O2 (reduced formula)
• the reduced formula is necessary to visualize that the sugar molecule is built one carbon
at a time
– Production of O2 is from the splitting of water (not CO2)
• confirmed by C.B. van Neil using the isotope O-18
– Carbon in the glucose comes from CO2
– Hydrogen in the new water and in the water come from reactant
water
– Oxygen in the glucose and the new water come from the CO2 in the
reactants
• Chloroplasts - site of photosynthesis
– primary pigment is chlorophyll
(green)
• found in the mesophyll - tissue of the
leaf's interior
– contains 30-40 chloroplasts
– stroma - fluid filled compartment
surrounded be 2 membranes
– thalakoids - stack of membraneous sacs
that separate the stroma from other
compartments (place where chlorophyll
resides)
– grana - stack of thalakoids
• CO2 enters through holes in the
leaf's surface called stomata
• veins carry water to the leaves and
export sugar to various parts
Structures
• The light reaction of photosynthesis
takes advantage of the photoelectric
effect (ability of photons to exert inertia
on electrons) and the photosynthetic
pigment chlorophyll. There are 3 main
pigments (2 types of chlorophyll) used in
plants. The action spectrum describes
which wavelength of light works best for
each (studied by looking at CO2 uptake
and O2 production).
– Chlorophyll a
• blue-green in color
• absorbs violet and red best
• reflects blue & green
– Chlorophyll b
• yellow-green in color
• absorbs blue and red best
• reflects yellow & green
– Carotenoids
• various shades of red, orange, and yellow
• called accessory pigments
• absorbs violet & blue-green
The Light
Reaction
• Photosystems – light harvesting complex
containing the pigments where light
energy is harvested
– When a photon is absorbed by a pigment
it causes an e- to move from its ground
state to an excited state.
• The electron (because it is unstable in its new
configuration) immediately returns to its
ground state releasing the stored energy.
– photon energy is transferred from
pigment to pigment until it reaches the
reaction center
– The energy is then harvested where it is
transferred to an electron acceptor. NADP+
• the reaction center is composed of a
chlorophyll a molecule & a primary electron
acceptor
– chlorophyll a boosts its e- to an excited state
where it is oxidized as the primary electron
acceptor removes it
– this is the 1st step in changing light energy to
chemical energy (glucose)
Photosystems
•
•
•
•
The thylakoid membranes are populated by two types of photosystems (PS I & PS II). The two work
together to produce ATP & NADPH (8 steps in the non-cyclic flow of electrons)
A photon strikes a pigment in PS II & is transferred until in reaches P680 in the reaction center.
The excited electron is captured by an electron acceptor.
Water is split
–
–
–
•
The excited e- is passed through an electron transport chain (ETC) to PS I
–
•
•
plastoquinine (Pq) g cytochrome complex g plastocyanin (Pc)
The exergonic "fall" of electrons provides energy for the synthesis of ATP (via proton-motive force &
ATP synthase)
PS I absorbs a photon and donates an e- from P700 to its electron acceptor
–
•
•
2 H+ are produced (reduced)
2 e- are given back to the P680
O2 is produced from the oxidized O
2 electrons from PS II fill in the space (electron hole)
Electrons captured by P700 are transfered down a 2nd ETC through the protein ferredoxin (Fd)
Enzyme NADP+ reductase transfers the 2 e- to NADP+ to NADPH (requires 2 e-)
–
NADPH is then transferred to the Calvin Cycle
• Occasionally the system may
skip PSII and operate under
cyclic flow. In cyclic flow, ATP
is generated but no NADPH is
produced for transfer into
the Calvin Cycle