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Photosynthesis
Overview of
Photosynthesis
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PS is the process
whereby light energy is
converted to chemical
energy
Photoautotroph:
autotroph which uses
light energy synthesize
energy rich organic
compounds
Raw materials needed:
sunlight, carbon
dioxide, water
Products: glucose,
oxygen gas
Where does
Photosynthesis Occur?
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PS occurs in leaves
of plants (stems of
cacti)
Tremendous
surface area for PS
Anatomy of a Leaf
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Waxy layer on surface
of leaf minimizes water
loss due to
evaporation—called
cuticle. Cuticle is
produced by the upper
epidermis.
Pallisade Mesophyll:
group of tightly packed
cells rich in
chloroplasts; where
most PS occurs in leaf
Spongy Mesophyll
Anatomy of a Leaf
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Vascular Tissue: xylem
conducts water,
phloem conducts
sugars
Stomata: microscopic
pores used in gas
exchange;
stoma=single pore;
carbon dioxide enters
stomata and oxygen
gas exits the stomata;
primarily on underside
of leaf
Anatomy of a Leaf
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Guard cells
regulate the size of
the opening
Chloroplasts
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Primary site of PS
Double membrane
Stroma: fluid-filled
region where Calvin
cycle (carbon fixation)
occurs; where ‘dark
reactions’ occur
Thylakoids: disk-like
structures containing
chlorophyll---where
‘light reactions’ occur;
organized into stacks
of disks (like pancakes)
called grana
Stages of
Photosynthesis
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1) light reaction; 2)
dark reaction (aka
‘light independent
reaction’
Light reactions
occur at thylakoids,
dark reactions in
stroma
Overview of the
Process
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Photon of light excites
electrons in chlorophyll
a
The activated
chlorophyll molecule
passes the excited
electrons down an
electron transport
chains, producing ATP
and the electron carrier
NADPH
Energy from ATP and
NADPH can be used to
power carbon fixation
Overview of the
Process
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So…the light
reactions transform
light energy into
ATP and NADPH
which can then be
used to power
carbon fixation in
the stroma
The Light Reactions
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Pigments: light
absorbing compounds
clustered in units on
thylakoids
Chlorophyll a: primary
pigment of
photosynthesis
Accessory pigments
include all pigments
other than chlorophyll
a. Accessory pigments
include chlorophyll b,
carotenoid pigments
The Light Reactions
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All pigments within a
unit can “capture” light,
but most cannot excite
the electrons
The high energy
electrons are passed
from one pigment
molecule to another
with energy being lost
at each pass.
Eventually, the electron
reaches the reaction
center (primary
electron acceptor).
The Light Reactions
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Two types of reaction
centers: Photosystem I
(PS I) and PS II. Each
has a chlorophyll a
molecule that accepts
a particular wavelength
of light.
P680 is the reaction of
PSII (max absorption
at wavelength of 680
nm)
PS700 is reaction
center of PSI
The Light Reactions
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The light energy is
used to form ATP and
NADPH. The process
of forming ATP using
light energy is called
photophosphorylation (using light, ADP,
and phosphates to
produce ATP)
Two types: Noncyclic
Photophosphorylation
(most plants) and
cyclic
photophosphorylation
Noncyclic
Photophosphorylation
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P680 in PSII captures
light and passes
excited electrons down
ETC to produce ATP
P700 in PSI captures
light and passes
excited electrons down
an ETC to produce
NADPH
A molecule of water is
split by sunlight,
releasing electrons,
hydrogen, and free
oxygen (photolysis)
Noncyclic
Photophosphorylation
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Hydrogen ions (from
splitting of water
molecules) are
pumped into a
thylakoid compartment,
creating an
electrochemical
gradient. As H+ then
flow thru ATP
synthases into the
stroma, the energy is
used to produce ATP.
Noncyclic
Photophosphorylation
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Chemiosmosis:
electrochemical
gradient of H ions
used to synthesize
ATP
Cyclic Phosphorylation
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P700 in PSI
captures light and
passes excited
electrons down
ETC to produce
ATP
NO NADPH
produced however
Water is not split by
sunlight
The Dark Reactions
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Aka ‘light independent
reactions’, meaning
light is not required for
carbon fixation
Occur in stroma of
chloroplasts
Carbon dioxide is
‘fixed’ to produce
glucose
Primary source of
carbon is atmospheric
carbon dioxide
The Dark Reactions
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An inorganic
compound (carbon
dioxide) is
converted into
energy rich glucose
(energy rich organic
compound—a
carbohydrate)
The Dark Reactions
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Carbon fixation is
endergonic process
Reactions require
ATP and NADPH.
ATP delivers
energy by
transferring P
groups, NADPH by
donating H ions and
electrons
The Calvin Cycle
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Aka ‘Dark Reactions’
Carbon dioxide enters
Calvin cycle and
combines with RUBP
(ribulose
bisphosphate—a 5-C
compound). The
product is an unstable
6-C compound, which
splits into two
molecules of PGA (3C; phosphoglycerate).
Because most plants
form PGA, called C3
The Calvin Cycle
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PGA is phosphorylated
to make PGAL. Some
of this is used to make
phosphorylated
glucose (w/ P stuck on,
glucose is primed for
making starch or
sucrose). The
remainder is recycled
to make more RUBP.
The C4 Pathway
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Corn, sugar cane,
plants in hot, dry
climates
A different way to fix
carbon dioxide
Carbon dioxide
combines with PEP
(phosphoenolpyruvate) to form
oxaloacetate (a 4-C
compound)
The C4 Pathway
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Oxaloacetate has a
carbon dioxide
molecule cleaved; in
essence, original
carbon dioxide used to
make oxaloacetate is
fixed, along with the
carbon dioxide cleaved
from oxaloacetate.
So… these plants are
very efficient at using a
small amount of
carbon dioxide, fixing it
twice.
The C4 Pathway
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Stomata close on hot,
dry days, so the plant
gets less CO2
If oxygen builds up in
leaf, and there is little
carbon dioxide,
photorespiration
occurs (fixed carbon
dioxide is wasted)
With C4 pathway, plant
can get by with much
smaller amount of
carbon dioxide since it
is fixed twice
The CAM Pathway
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Cacti, succulents
Keep stomata closed
during day to conserve
water
May fix carbon dioxide
formed from cell
respiration
Open stomata only at
night, perform carbon
fixation at night