Photosynthesis
• Photosynthesis is
the process of
converting carbon
dioxide, water and
light energy into
oxygen and high
energy sugar
molecules.
• Plants, algae, and
some bacteria can
use sugar molecules
produced during
photosynthesis to
make complex
carbohydrates such
as starch or
cellulose for food.
• The process of
photosynthesis consists
of two basic stages:
light-dependent
reactions and lightindependent reactions.
• The light-independent
reactions are also
called the Calvin cycle.
• Photosynthesis takes
place inside an
organelle called a
chloroplast.
• The chloroplast is an
organelle with a double
membrane that contains
stack of sac-like
membranes called
thylakoids.
• The thylakoid
membrane contains
within itself a green
pigment called
chlorophyll.
• Pigments are
substances that
absorb light.
• Light-dependent
reactions take place
inside the thylakoid
membrane.
• Light-independent
reactions take place
in the stroma, which
is the region outside
the thylakoid
membrane.
• In the light-dependent phase, sunlight hits the
leaf of the plant where it is absorbed by the
pigments in the leaf.
• There are several pigments in plant leaves,
but the main one used in photosynthesis is
chlorophyll, the green pigment.
• Chlorophyll is stored in the chloroplasts of the
plant cell.
• When light hits the
chlorophyll,
electrons absorb the
energy, become
excited, and leave
the chlorophyll
molecule.
• Carrier molecules transport the electrons,
which follow an electron transport chain.
• Electron acceptor molecules pick up the
electrons in a series and pass them from one
molecule to another.
• As this occurs, energy is released, and ATP
is formed.
• The final electron acceptor is NADP+.
• Splitting a molecule of water replaces the
electrons released from the chlorophyll.
• These electrons, now available, combine with
the NADP+ to form NADPH.
• The next stage of photosynthesis uses the
NADPH, while oxygen leaves as an end
product of the reaction.
• The end products of the
light-dependent
reactions are ATP,
oxygen, and NADPH.
• The ATP and NADPH
will be used in the lightindependent reactions,
and the oxygen will be
released into the
atmosphere.
• The next phase, the
light-independent or
carbon fixation
reactions, used the
ATP formed during
the light-dependent
reaction as an
energy source.
• In this phase,
carbon from carbon
dioxide, and NADPH
are used to form
glucose.
• To accomplish this, a five-carbon sugar uses
a carbon atom from carbon dioxide to create
a six-carbon sugar.
• Glucose is the end result, after several
conversions have taken place.
• The glucose can then be used as food to
enter cellular respiration, or it can be
converted to other carbohydrate products
such as sucrose or starch.