Cellular Energy
Photosynthesis & Respiration
Part 1: Photosynthesis
Photosynthesis
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It is the process that converts sunlight energy
into chemical energy stored in the form of
organic compounds (like glucose)
Equation:
6CO2 + 6H2O +light
 The CO2 is drawn in from the surrounding
air through the stomata, while the water
is pulled up from the roots and travels
through the xylem. Light is absorbed by
the chlorophyll.
 The glucose (carbs) will be transported
around the plant by the phloem for a
variety of uses (cell respiration, cellulose
and starch formation, etc.) and the O2
will either be eliminated into the air, or
some is use in cellular respiration.
---> C6H12O6 + 6O2
Photosynthesis
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Occurs in the chloroplast of plant cells
 The first half of the reaction occurs in the thylakoid (multiple
thylakoids form a granum); the second half happens in the
stroma (fluid)
Utilized by autotrophs (plants, algae, some bacteria)
Photosynthesis: Major Steps of the Light Dependent Reactions
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Light-Dependent Reactions (Light Reactions)
 Sunlight strikes chlorophyll a located in the membrane of the thylakoids in
Photosystem II.This excites the electrons in the chlorophyll.
 Energized electrons from the chlorophyll move through a chain of proteins
in the thylakoid membrane called the electron transport chain.
 Here they go through a series of oxidation reduction reactions, losing
energy as the move through the chain. As they lose this energy, they are
pumping H+ into the thylakoid from the stroma (fluid surrounding the
thylakoids).
 At the same time, light is hitting a different chlorophyll a location in
Photosytem I. The electrons as excited here as well and move from the
chlorophyll molecule through a series of proteins also called an electron
transport chain. (This ETC is simply in Photosystem I, not Photosystem II.)
 The electrons from Photosystem I, move through the ETC, moving closer
to the top of the membrane which is near the stroma. Eventually, they
reach the end, and each joins with an energy carrier called NADP+ and a
H+. They form a high energy energy carrier called, NADPH.
Photosynthesis: Major Steps of the Light
Dependent Reactions
At the same time, energy is from the light is being used to split water
molecules (located inside the thylakoids) apart into 2 H +, 2 electrons, and O
(photolysis), releasing oxygen (More than 1 water is needed to make O2).
Those electrons combine with the chlorophyll a in Photosystem II, to replenish
the electrons that were lost when the process started. This allows the light to
strike the molecule of chlorophyll a PSII again, allowing its new electrons can
be become excited and start the process all over again.
 The electrons that were originally in PSII, reach the end of the ETC and join
with the chlorophyll a molecule at the beginning of PSI, which allows the
second half to start all over again as well.
 (Refer back to bullet point 3) The electrons were using their energy to pull H +
into the thylakoid, and water was just split releasing H + there as well. This
makes the inside of the thylakoid positive when compared to the outside. This
gradient represents potential energy. This energy is harnessed by an enzyme
called ATP synthase. ATP synthase is located in the thylakoid membrane. It is
used to create ATP by attaching a P to ADP. The potential energy is being
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converted into chemical energy, ATP, when ATP synthase pushed a H + out of
the thylakoid into the stroma. ATP is created.
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Photosynthesis: Major Steps of the Light Independent Reactions
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Light-Independent Reactions (Calvin Cycle, Dark Reactions)
 The ATP and NADPH produced in the first step are used in this step which
occurs in the stroma of the chloroplast.
 Carbon dioxide is taken in from the air through the stomata.
 In this step, a series of enzyme assisted reactions occur to create 3-carbon
organic compounds. The incorporation of the CO2 into forming an organic
compound is called carbon fixation. The carbon in the CO2 is “fixed”
(converted) into a new molecule.
 A series of reactions will take 3 CO2 molecules and arrange them into a
variety of 3-carbon molecules along the path of the cycle.
 The energy stored in the ATP and NADPH are used to create the 3-carbon
product, converting them back into ADP and NADP+ to be used again in the
light dependent reactions.
 The key thing to note, is that at the end glucose has NOT been made. A 3carbon organic molecule has left the cycle. Two complete cycles will
release 2, 3-carbon molecules (hence the 6 carbon found on glucose.)
Some of these will combine to form glucose for cellular respiration, some
will form starches, and many can come together to form complex carbs 6
such as cellulose. They can be used to form a variety of organic material.
How are photosynthesis and respiration linked?