almost all life on Earth is
solar-powered plants algae and some
bacteria capture light energy from the
Sun and convert it to chemical energy in
a series of reactions called
photosynthesis these organisms produce
carbohydrates from simple building
blocks like water and carbon dioxide
from the environment and in the process
they release oxygen photosynthesis
nourishes almost the entire living world
photosynthesis is a set of chemical
reactions in which light energy is
converted to chemical energy light
energy enables the movement of electrons
from molecules that donate electrons to
molecules that accept electrons but
which molecules water is the first
electron donor the carbon in carbon
dioxide is the ultimate electron
acceptor carbon dioxide combines with
other molecules to form carbohydrates
such as a three carbon sugar called g3p
carbohydrates are used to make other
organic molecules that plants use to
grow and as a source of energy to fuel
their lives an important byproduct of
photosynthesis is oxygen we're going to
zoom in on a cross section of a leaf to
have a closer look at the center of
action for photosynthesis
a leaf has many different types of cells
such as mesophyll cells epidermal cells
and vascular bundles most cells in the
middle of a leaf contain large numbers
of chloroplasts pigments and
chloroplasts make these cells green
chloroplasts are the organelles where
photosynthesis occurs
carbon dioxide from the air enters a
leaf through small pores called stomata
on the outer cell layer oxygen formed
during photosynthesis also exits the
plant through the stomata the plant
transports organic molecules produced in
its leaf cells to other cells via the
plumbing system found in vascular
bundles let's take a closer look at a
chloroplast the organelle where
photosynthesis takes place
photosynthesis consists of two sets of
chemical reactions the light reactions
and the calvin cycle they occur in
different regions of the chloroplasts
chloroplasts contain stacks of membrane
lined discs called thylakoids surrounded
by a watery clear fluid called stroma
the light reactions are carried out by
molecules in the thylakoid membranes and
the Calvin cycle reactions by molecules
in the stroma let's explore these
regions and their functions in more
detail in the thylakoid membranes the
light reactions transform light energy
to chemical energy light energy drives
the formation of ATP molecules from adp
end of NADPH molecules from nadp+ and
electrons along the way water molecules
are split and oxygen is formed which can
be released into the atmosphere in the
stroma the Calvin cycle reactions use
the chemical energy of ATP and NADPH to
combine carbon dioxide from the air with
organic molecules to form new molecules
like the sugar g3p ADP and nadp+
are recycled and may be used again in
the light reactions a plant increases
its biomass
through the formation of these new
organic
molecules the thylakoid membranes
contains specialized molecules that work
together to perform the light reactions
light is absorbed by protein pigment
complexes called photosystems there are
two photosystems photosystem 1 and
photosystem 2 the photosystems transform
light energy to chemical energy by
exciting and then shuttling electrons
from molecule to molecule in a chain
light fashion on the thylakoid membrane
this process is called an electron
transport chain let's take a closer look
to see how this process works first
photons of light hit chlorophyll a light
absorbing pigment in photosystem 2
electrons in the chlorophyll are excited
to a higher energy level the excited
electrons are passed to an electron
carrier meanwhile water splits and
releases electrons these electrons
replace those lost at photosystem 2 the
byproduct of this reaction is oxygen
which is eventually released into the
air the other products are protons or
hydrogen ions which are released into
the inside of the thylakoid or lumen the
excited electrons move to the cytochrome
complex some of the energy from the
electrons is used by the cytochrome
complex to transport additional protons
into the lumen the second electron
carrier a protein inside the lumen
receives the electrons and passes them
to photosystem 1 these electrons have
now lost most of the energy they gained
from light in photosystem 2 photons of
light hit chlorophyll in photosystem 1
and excite the electrons again the
electrons are then passed to the third
electron carrier finally these electrons
are either recycled or they interact
with an enzyme and nadp+
the final electron acceptor of the light
reactions to form nad
PAH some of the energy from light is now
stored in the reduced molecule NADPH
some of the energy released from the
transfer of electrons established a
proton gradient across the thylakoid
membrane protons that accumulated in the
lumen diffuse into the stroma through an
enzyme called ATP synthase ATP synthase
uses the potential energy of the proton
gradient to combine ATP with inorganic
phosphate to form ATP in this way the
potential energy is transformed into
chemical energy stored as ATP ATP and
NADPH now have stored energy from the
light reactions this energy can be used
in the calvin cycle this light driven
electron transport chain is usually
continuous in the presence of sunlight
it encompasses a series of chemical
reactions that involve light absorption
energy conversion and electron transfer
carried out by the photosystems and
other enzymes on the membrane of the
thylakoids the calvin cycle takes place
in the chloroplast stroma the watery
clear fluid surrounding the thylakoids
it's helpful to think of the calvin
cycle in three phases fixation reduction
and regeneration in phase one inorganic
carbon in the form of carbon dioxide
from the air is incorporated into
organic molecules a process known as
carbon fixation three molecules of
carbon dioxide react with three
molecules of ribulose bisphosphate rubp
to produce six molecules of a three
carbon molecule called 3-pga
the enzyme rubisco catalyzes this
reaction in the second phase the organic
molecules except electrons a process
known as reduction the six molecules of
3-pga
use six molecules of ATP
six molecules of NADPH which store
energy from the light reactions to
generate six molecules of g3p the g3p
molecules contain more electrons and are
higher in potential energy than 3-pga
one molecule of g3p exits the cycle it
can be used to make other organic
molecules in Phase three the
regeneration phase a large set of
reactions use the other five molecules
of g3p and energy from three molecules
of ATP to produce three molecules of
rubp with rubp reformed the process can
start again notice that in the calvin
cycle the energy from ATP and NADPH
produced in the light reactions is used
to generate one g3p molecule from three
carbon dioxide molecules in this process
the electrons lost from NADPH are
accepted by the carbons from carbon
dioxide molecules which are the ultimate
electron acceptors of photosynthesis g3p
the net product from the calvin cycle
can be used to generate other organic
molecules such as sucrose or starch
sucrose produced by leaf cells is
transported through the vascular bundles
to other parts of the plant like stems
and roots leaf cells can also sometimes
form starch for long-term energy storage
over all the molecules generated by
photosynthesis provide fuel and building
materials that allow a plant to grow
globally photosynthesis produces an
estimated 150 billion metric tons of
carbohydrate per year and is responsible
for the oxygen in our atmosphere making
it one of the most important chemical
processes for life on Earth
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