Photosynthesis
Chapter 7
Photo means light; synthesis
means “to put together”. Plants
make glucose from CO2, H2O
and some other elements,
mainly N, P, and K. The
chemical formula is opposite the
one for respiration:
6CO2 + 6H2O + light  C6H12O6
+ 6O2.
Biochemists use “heavy water”
with an isotope of oxygen, 18O
rather than 16O, and traced the
radioactive elements passing
through the plant and determined
that plants split heavy H2O
producing heavy O2.
Photosynthesis
takes place in 2
parts. The first
part powers ADP
ATP using
energy from light
units called
photons. This is
called
phosphorylation.
The first reaction also reduces an
electron carrier, NADP+ to
NADPH. Phosphorylation and
reduction of NADP+ are called the
light dependent reaction.
Light in photons travels in a
straight line . They hit a surface
and bounce off at an angle.
Photons have different amounts
of energy, called wavelengths.
Wavelengths are measured in
nanometers (millionths of
millimeters). The ones we can see
are called the “visible spectrum”.
The visible spectrum goes from u
high energy wavelengths which
are very short between crests, 390
nm. At the low energy end are
infrared wavelengths at 800 nm.
Plants have pigments that use all
most wavelengths except green
and yellow which are reflected
rather than being absorbed.
A photon is pure energy and has
no mass. Blue wavelengths have
about 2x as much energy as red.
Light intensity depends on the
number of photons received. A
molecule that absorbs a photon
boosts an electron to a higher
energy level.
High energy ultraviolet
wavelengths of light are
absorbed by the ozone layer in
the atmosphere. Red
wavelengths are absorbed by
H2O vapor and CO2. When
molecules absorb photons and
electrons kicked to higher energy
levels the electrons are passed to
other molecules..
The light absorbing pigments of
plants are mainly chlorophyll a
and to a lesser extent chlorophyll
b. Yellow and orange carotenoids
also absorb some light. Pigment
molecules form a complex that
transfers light to chemical energy.
Some wavelengths do not
promote photosynthesis, those
longer than 680 nm. But a
combination of 680 and 700 nm. at
the same time will. Photosynthesis
is more efficient using light of 2 or
more wavelengths because it
consists of 2 sets reactions called
Photosystem I and Photosystem II.
The chlorophyll at Photosystem I
absorbs light at 700 nm., deep red
and is called P700. The
chlorophyll at photosystem II
absorbs light at at 680 nm., red,
and is called P680. The
photosystems are within the
chloroplasts inside the thylakoids
(stacks of disks).
Excited electrons of photosystem
I can take either of 2 paths. The
first is called cyclic
photophosphorylation and
produces ATP. The second is
noncyclic photophosphorylation
and makes NADPH that provides
electrons to make sugar.
Noncyclic
photophosphorylation occurs
when photosystem I receives
electrons from photosystem
II. Excited electrons from
photosystem II all take one
path to the electron hungry
photosystem I.
Cyclic Photophosphorylation
When P700 absorbs a photon its free
energy increases and it becomes a
powerful electron donor. Excited
P700 gives up an electron and loses
most of the energy it got from the
photon. P700 can gain another
photon. The electron acceptor is a
cytochrome complex that passes the
electron on to another acceptor.
Passage of electrons through the
cytochrome complex result in a
proton gradient. Protons move
from a space outside the thylakoid
(stroma) into the thylakoid space.
An electron transport chain pumps
protons out of the stroma and into
the thylakoid space.
Inside the thylakoid membrane is
ATP synthase that produces
ATP’s. Protons flow down the
gradient ATP is made in the
stroma and later will be used to
make glucose.
Noncyclic Electron Flow
Energy of the excited electrons of
P700 is used to make NADPH an
electron carrying coenzyme that
chloroplasts use to build glucose.
It provides most of the reducing
power for glucose synthesis.
After 2 P700 have given up 2
excited electrons to form NADPH
it is oxidized and needs an
electron. Photosystem II provides
electrons to P700 Chlorophyll of
photosystem II, absorbs available
light at 680nm and is called P680.
Photo-excited P680 is a more
powerful electron donor than
excited P700.
Electrons move through
photosystem II’s electron transport
chain from excited p680 to oxidized
P700. By regenerating P700
photosystem II supplies electrons to
photosystem I. Photosystem II
produces ATP as protons flow down
the gradient produced by the
electron transport chain.
If an electron from excited P680
is transferred to oxidized P700,
P680 must be regenerated. The
electron that regenerates P680
comes from water. H2O is split
by light in the light dependent
reaction. The splitting of water
by light is photolysis.
Photosystem II takes electrons
from water, separates H’s from O.
Oxygen is released as a waste
product.
Photosynthetic prokaryotes
evolved about 3 billion years ago
producing O2 in the atmosphere.
Today 50-70% of atmospheric O2
comes from marine algae.
The 2 photosystems
1.Split water to Hydrogen and oxygen
2.Produce ATP
3.Produce NADPH
ATP and NADPH remain in the
chloroplasts where they contribute to
the synthesis of sugar (light
independent reaction).
Reducing NADP+ to NADPH
requires 2 electrons therefore
photosystem I and II must each
absorb 2 photons . To move
electrons along the paths requires
2 electrons so it takes 4 electrons
to make NADPH.
The electons flow from
photosystem II to photosystem I
and supply energy to pump
protons across the thylakoid
membrane . The protons flow
back through ATP synthase to
produce of ATP. Energy stored in
NADPH and ATP make glucose (or
other carbohydrate).
Plants transport sugar from cell to
cell. Hydrogen for glucose comes
from water. Carbon and oxygen
come from CO2. To synthesize
glucose from ATP and NADPH also
takes many small steps.
Discovery of the isotope carbon – 14
allowed Melvin Calvin to determine
what compounds were involved in
synthesis. Calvin used paper
chromatography where pigments
were dotted onto paper and put in a
solvent. The solvent separated the
pigments by molecular weight into
beta carotene, xanthophyll,
chlorophyll a and chlorophyll b.
Calvin identified the radioactive
C14 compounds by placing the
paper on photographic film which
it developed.
Part I – Capturing carbon.
CO2 and a 5-carbon sugar, ribulose
biphosphate catalyzed by an
enzyme, Rubisco produce a 6Carbon compound that immediately
breaks into 2, 3-carbon molecules
called phosphoglycerate. It is
unstable and becomes 2, 3-carbon
molecules of glyceraldehyde
phosphate.
These may become glucose, fat,
or amino acids but some will
replenish RuBP in 7 steps and
requiring ATP. Rubisco is a slow
enzyme so it makes up over half
of the protein in the chloroplasts.
Photosynthesis in is subject to
drought and other environmental
problems. The carbohydrates, etc.
produced by plants form the trunk,
leaves, stems, etc. 3 major factors
that affect photosynthesis are
available water, CO2, and photons of
appropriate wavelength. Intensity of
light (number of photons striking a
leaf per day) is important.
In hot, dry weather plants
close their stomata to
conserve water but it also
limits their CO2 uptake. If
O2 in a plant builds up,
CO2 decreases initiating
photorespiration where
plants use their sugar.
This occurs because
Rubisco requires a lot of
CO2.
When CO2 levels are low Rubisco
converts ribulose biphosphate
into 1 phosphoglycerate molecule
and a 2 Carbon compound which
is a waste product. During
photorespiration plants waste half
of the carbohydrates they have
made. Photorespiration produces
no ATP or NADPH.
Some plants have a biochemical
pathway that allows them to
maintain high CO2 even when
stomata are closed. The first
reaction does not make
phosphoglycerate , rather it
makes a 4 carbon compound –
oxaloacetic acid (OAA), which
cells convert to carbohydrate
and CO2 which enters the
Calvin cycle.
Credit: © Inga Spence/Visuals Unlimited
Corn 4-5 leaf stage.
304561
Ethanol in gasoline
comes fro corn and
sugar cane. Cacti use a
modified C4 pathway ,
CAM (crassulacean acid
metabolism). They open
stomata at night and
store CO2 until the next
day. This requires a lot
of ATP so they grow very
slowly.