Photosynthesis - No Sun No Sugar, No Water No Oxygen
Recall that one of the Laws of Physics states that "Energy can neither be created or destroyed, but
it can change forms." Photosynthesis demonstrates this law of physics. Primary producers are the
basis of the ocean, forest and grassland ecosystems that fuel our planet. The "fuel" is the
carbohydrates built from glucose. In order for terrestrial and aquatic plants and algae to build
glucose a source of energy is needed. This energy is energy stored in the form of ATP and
NADPH. These two energy-rich molecules in turn derive their energy from converted solar energy.
Photosynthesis consists of two major steps. The Light a Reaction is the conversion of sunlight's
energy into ATP and NADPH while the "dark reaction" or Calvin Cycle is the synthesis of glucose
from CO2, a precursor
5-C molecule which is recycled, ATP and NADPH.
The Light Reaction - In the membrane-bound chloroplasts is another set of membranes called
the thylakoids. They contain the pigments and metabolic pathways which will capture the
sunlight's energy and synthesize ATP, NADPH and split water (H2O) into Oxygen gas, a free
electron (e-) and a hydrogen proton ( H+) (note: in prokaryotes these functions occur on internal
membranes).
There are two pigment systems or photosystems which capture photons from the sunlight,
Photosystem II then Photosystem I. Chlorophylls are just one of the pigments and biomolecules
involved in the conversions. Sunlight striking the thylakoid membranes contains photons which
are initially captured by pigments in Photosystem II. The excited pigments pass the electron from
pigment to pigment temporarily "exciting" the pigment to higher energy level. [not required BIO
102 - These pigments include lutein, zeaxanthin, B-carotene, lycopene and chlorophyll b.] Eventually
the energy of the photon reaches a variant of Chlorophyll, P680.
P680 accepts the electron and becomes highly excited and releases an electron to a series of
pigments called the Electron Transport Chain (ETC). [not required BIO 102 – The ETC consists of
pigments plastoquinone, cytochromes and plastocyanin]. P680 needs to replace the lost electron. It
does so by stealing an electron from water! H2O is split into a Hydrogen proton (+), Oxygen gas
(O2) and a free electron (-). The OEC or Oxygen Evolving Complex performs the splitting of water.
The free electron replaces the electron lost when P680 was excited by the photon's energy. The
Oxygen gas is released to the air and water. All the Oxygen on planet earth is made this way. No
water, no plants, no oxygen! It is that simple. (WATER ESSAY).
The ETC accepted P680’s released electron. The energy of that excited electron is used to pump
+
hydrogen ions (H ) across the thylakoid membrane creating an area of higher concentration of
hydrogen ions inside the thylakoids. Think of this like blowing up a balloon. There is stored
potential energy in an inflated balloon waiting to be released. Similarly, there is stored potential
+
energy in the buildup of H on the other side of the membrane inside the thylakoids.
The electrons which have passed through the ETC will serve as the "make-up" electron for
Photosystem I. Like Photosystem II, Photosystem I consists of pigments which gather photons
from sunlight, pass it from pigment to pigment and eventually to P700. Like P680, P700 releases a
high energy electron and needs to replace the lost electron. It does so by accepting the electron
from the ETC (an electron that can be traced back to P680 and then to water).
The electron that P700 releases will not be passed to an ETC. It will be used to convert NADPH+
to a more energy rich NADPH. NADPH is one of the two energy rich molecules which will be used
in the Calvin Cycle. [not required BIO 102 – Ferrodoxin and FNR (ferredoxin-NADP reductase)
facilitate this electron transfer].
What about the so-called balloon of hydrogen ions (i.e. the buildup of hydrogen ions on the
inside of thylakoids). The potential energy of this gradient will be converted to chemical energy in
the form of ATP. Integrated into the thylakoid membrane is the ATP pump. It is a means by which
the hydrogen ions can equilibrate with the lower concentration on the other side of membrane
(I.e., akin to releasing pinch on balloon). The ATP pump is an physical portal consisting of
enzymatic surfaces. Each time a hydrogen ion enters the portal which is very much like a
department store revolving door, the portal rotates. When the portal rotates it exposes the
+
catalytic surface and an ATP is made from ADP + P. The potential energy stored in the H
gradient is converted to chemical energy in the third P of ATP. One by one, ATP's are synthesized.
As long as sunlight is striking the chloroplasts, ATP (and NADPH) will be synthesized. Hence it is
called the Light Reaction. The ATP and NADPH will be used to form glucose from its precursors.
The Calvin Cycle or Dark Reaction
The Calvin Cycle or Dark Reaction are the light-independent reactions of photosynthesis that
convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma,
the fluid-filled area of a chloroplast outside of the thylakoid membranes.
CO2 is combined with an existing 5-Carbon molecule [not required BIO 102 – the 5-Carbon
compound is RuBP (Ribulose biphosphate) ; note: Ribulose is 5-C sugar in DNA]. The resulting 6Carbon molecule is split into two 3-Carbon molecules. Energy present in the third ATP bond of
ATPs and in NADPH is used to make the 3-Carbon molecules more energy dense. Some (2 of
every 12) of these 3-Carbon molecules are converted into 6-Carbon glucose. Some (10 of every
12) are recycled into six 5-Carbon (RuBP) (note: 10 x 3-Carbons or 30 Carbons converted into 6 x
5-Carbons, also 30-Carbons). Some ATP is also used for this step. It takes six turns of the Calvin
cycle to produce one net glucose molecule. This makes sense since six turns brings in 6 carbons
in form of CO2 . All the ADP + P and NADP+ produced in Calvin Cycle are then reused in Light
Reaction.