Photosynthesis Explained
By Brandon Stiller
Photosynthesis takes place in plants and other organisms like cyanobacteria, algae, and some other
protists, but we are going to focus on the photosynthesis that takes place in plants!!
Photosynthesis uses energy directly from the sun, CO2 and H2O to make carbohydrate (food) and other
organic molecules.
Overall equation: 6CO2 + 6H2O + light→ C6H12O6 + 6O2
Notice the equation is almost the direct opposite of CR, but it is hella more complicated than that.
Photosynthesis takes place in the chloroplasts of plants. Here is chloroplast structure plus the reactions or
photosynthesis. Stroma contains enzymes involved in CO2 fixation, starch granules, DNA and ribosomes,
and many ions and cofactors.
Light Dependent Reactions: PSN Part 1.
The reactions that take place in the grana (stacks of thylakoids) produce the ATP and NADPH that will be
used to make carbo. molecules in the stroma of the chloroplasts. NADPH is another coenzyme that is very
similar to NADH. NADPH is not found in animals.
So how are ATP and NADPH made? Structures in the thylakoids capture light energy to do this!!!!!
Components of the Thylakoids. PSI and PS II, mobile carriers, ETC, and ATP Synthase.
Unlike CR which uses NADH and FADH2 to provide e- to the ETC, photosynthesis uses the suns energy to
add energy to the e-s, and get them moving on a ETC. Photosystems (PS) are the light trapping part of
the thrylakoid membrane. They contain the pigment chlorophyll as well as protein. Photosystems absorb
the energy of a photon from the sun, which provides energy to move one or two of the PSs e- off of it, and
onto the plant version of the electron transport chain.
Each photosystem has a special absorption spectrum. PS I=P700. PS II=P680. Absorption of photons is
very rapid (10-15 s)!! When photons are absorbed, two things can happen:
1) e- gets excited the fall down to correct orbital and release heat and light…called fluorescence
(this does not result in photosynthesis!); or
2) excited e- move onto e- carriers. If e- move on, photosynthesis occurs:
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❊Cyclic e- transport→ Making ATP only because of a “short circuit” (not in book)
Involves only photosystem I (it has carriers on both sides of PS I. e- travels in a loop. Could be a
different PS I (Chlorophyll and proteins), just a PS I.
In cyclic e- transport, just like in the CR ETC, protons are pumped across membrane and then the gradient
fuels ATP synthesis. This is the way ATP is produced in plants that have access to light. When there is no
light, how do they get ATP? Think.
❊Non-cyclic e- transport→ Making ATP AND loading NADPH. Focus on this!!
Instead of e- returning to a PS, it is lost to NADPH just like when the Krebs cycle rips e- off what was once
glucose in CR. Non-cyclic e- transport is also where O2 is produced!!
Why are 2 PSs needed? NADPH has a higher free energy that one PS can provide. So too are needed to
get enough energy in the e- that are moving around.
Anyway, now the organism has ATP and NADPH, but no sugar in case it has to do CR!!! So plants need
the...
Light Independent Reactions (AKA “Dark” Reactions). → No direct light needed.
These reactions do not actually have to be in the dark, in fact, they happen all day long. They are called
dark reactions, or light independent reactions because they don’t need photons for energy. The energy is
already in the NADPH that have been loaded during the light reactions. The Calvin Cycle takes ATP, the E
in NADPH, and uses it, along with the e- that NADPH carries, to make carbohydrates!!!!!
In the Calvin cycle (look on other diagram below), the overall Calvin reactions are: 3CO2, 9ATP, and 6
NADPH are used to make a three carbon sugar. The CO2 is attached to the starting molecule of the Calvin
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cycle, ribulose bisphosphate (RuBP), by the enzyme, RuBP carboxylase, also called RUBISCO. Then
through a series of reactions RuBP is regenerated and glyceraldehyde phosphate is produced, and it can
be used to make different carbos or other molecules!! Word.
In the Calvin cycle, NADPH is unloaded and ATP is used. Plants get this stuff from the light reactions, or
CR (just the ATP). The Calvin cycle is important because this is the only way carbon that was used up in
CR can be returned the usable form. Plants can then use sugars for CR and animals can eat plants and
use it to do their CR.
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Photorespiration.
Photorespiration is when a plant adds an O2 to RuBP in the place of the CO2 it should be adding.
Respiration refers to the fact that O2 is added, and it is usually done in the presence of light because that
is when plants tend to have enough ATP and NADPH to make sugar in the Calvin cycle. Because they are
taking CO2 out of the air that go into their stoma, the CO2 concentration goes down, and O2 can be added
in what it essentially a mistake. Photorespiration is much more common in places that are hot and dry
because the plants have to close stomata to decrease water loss.
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Solutions for Photorespiration
❊C4 plants. An ingenious way for some plants to avoid photorespiration.
Most photosynthesis is called C3 photosynthesis because CO2 is bound to RuBP and forms the 3 carbon
PGA intermediate. This works fine for most plants, but in areas with lots of sun and heat, plants would
photorespire too much. C4 plants have evolved to circumvent this, and allow PSN to be more efficient.
C4 plants hook CO2 up to a three carbon molecule in the first step of fixation with an enzyme that does not
have bond O2, thus forming a 4 carbon intermediate, and no poison. This works because the enzyme that
fixes CO2 for the first time cannot bind to O2, thus eliminating the chance for photorespiration to occur!!
This intermediate is then changed to malate, and moved from the mesophyll cell to the bundle-sheath cell,
where the CO2 is removed from malate, allowing it to be added to the normal calvin cycle, dramatically
increasing the chance that RUBISCO actually gets the CO2 into the Calvin cycle.
❊Crassulacean Acid Metabolism: CAM plants.
Take in CO2 at night, not the day so less water is lost. Instead of using two cells to ensure C fixation,
CAM plants use their vacuoles!!! CO2 is fixed onto organic acids at night when it is cool and when less
water is lost. During the day, the CO2 is released from the vacuoles while the stomata are closed, and it
goes into the Calvin cycle.
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