Debbie Pham
College Biology I
October 18, 2011
1610 Section 035
Writing #2
Have you ever wondered what makes the world go round? How did we come
to be “alive”? What keeps us alive? Now these are very big general questions and we
need to look smaller and focus more on specifics to eventually see the EMERGENCE
of the answers to these big questions.
All of our “life energy” begins with a ray of light, the sunlight to be more
exact. Now, we as humans cannot eat or drink sunlight, but plants do. Plants don’t
literally open up and chew on sunlight or sip down sunlight. There are little
structures called chlorophyll that can turn the sunlight into sugars that eventually
we eat directly or indirectly eat animals that have eaten the plants. There are many
parts and organizations to chlorophyll that work together and eventually have the
emergent property to change sunlight and water, two organic substances, to
chemical energy, sugars/glucose which the plant and we can use that keeps life
going as we know it today and everyday.
The photosystems of the chlorophyll consists of a reaction-center, which is a
type of protein complex that is surrounded by light-harvesting complex. The lightharvesting complexes are pigment bound molecule proteins that transfer the energy
of photons to the reaction center. A primary electron acceptor in the reaction center
accepts excited electrons and is reduced as a result. The solar powered transfer of
an electron from a chlorophyll molecule to the primary electron acceptor is the first
step of the light reactions. Now the second step is made possible two different
photosystems: Photosystem II and Photosystem I both located in the thylakoid
membrane of chlorophyll. During the light reactions, there are two possible routes
for electron flow after sunlight excitement: linear electron flow and cyclic electron
flow. I will explain the two routes and show how everything works together to
eventually be able to make sugars that sustain life on earth.
Liner electron flow is the primary pathway that involves both photosystems
and produces ATP and NADPH using sunlight. A photon hits a pigment and its
energy is passed among pigment molecules until it excites P680 and from there, the
excited electron is transferred to the primary electron acceptor and makes P680+.
P680+ is a very strong oxidizing agent. Enzymes split the H20 that the plants take
up and the electrons are transferred from the hydrogen atoms to P680+ reducing it
back to just plain P680. Oxygen is released as a by-product of this reaction. Now,
the electrons are very excited still and begin to “fall” down an electron transport
chain from the primary electron acceptor of PS II to PS I. Energy released by the fall
drives the creation of a proton gradient across the thylakoid membrane. The
diffusion of H+ across the membrane drives ATP synthesis. In Photosystem I, like in
Photosystem II, transferred light energy excites P700, which loses an electron to an
electron acceptor. P700+ accepts an electron passed down from Photosystem II via
the electron transport chain. After arriving at Photosystem I the electrons must be
re-excited and when it does, each electron “falls” down another electron transport
chain from the primary electron acceptor of PS I to the protein ferredoxin. The
electrons are then transferred to NADP+ and reduce it to NADPH. The electrons of
the NADPH are available for the reactions of the Calvin cycle and this process also
removes an H+ from the stroma.
The second option for electron flow is the cyclic flow. It uses only
Photosystem I and produces ATP, but not NADPH and no oxygen is released. The
cyclic electron flow generates surplus ATP, satisfying the higher demand in the
Calvin Cycle.
The Calvin Cycle builds sugar from smaller molecules by using ATP and the
reducing power of electrons carried by NADPH. Carbon enters the cycle as CO2 and
goes through 3 phases: Carbon fixation, Reduction, Regeneration of the CO2
acceptor (RuBP), the eventually leaves as a sugar named glyceraldehyde 3phosphate (G3P). For the synthesis of 1 G3P, the Calvin cycle must take place three
times, fixing three molecules of CO2. That G3P is then used to make glucose and or
other organic molecules. The glucose is now chemical energy that the plants use for
cell functions and eventually we eat either directly or indirectly to sustain our own
cell functions as well.
The definition of emergent property is any unique property that "emerges"
when component objects are joined together in constraining relations to "construct"
a higher-level aggregate object, a novel property that unpredictably comes from a
combination of two or more simpler constituents.
Now, I’ve broken down the constituents and steps that make photosynthesis
happen within plants and you now have an individual understanding of what each
Photosystem both II and I, each protein in the two election transport chains, each
H2O input and O2 released, each electron, and excited photon. All of these systems
and components alone could not function alone effectively to give us the result of
sugars. The electrons themselves are needed from the H2O input in the light
reactions in Photosystem II. Think about the single proteins that make up the
electron chain. Without the others there would not be a step to step falling that
would best utilize the energy of the falling electron. The Calvin Cycle could not keep
transforming the CO2 from the air to usable Glucose if not for the light reactions that
created ATP and NADPH that constantly fuel the Calvin cycle.
The analogy that I love and has stuck with me is the story of the bicycle. The
wheels or even the handlebars alone are almost useless if not for the rest of the bike
such as the proteins in photosynthesis. Even if you do have all the parts, they still
render useless if not put together in the right order or place such as the
Photosystems II and Photosystems I of the chloroplasts.
This might be a far stretch, but this has helped realize a new perspective of
life. Sometimes we might feel small and insignificant, but even the smallest of things
such as chloroplasts belonging in the cell of plants can be the reason why life can be
on earth. So if you ever feel small, just remember that in the right “system” along
with the right “components” you can make a difference with your own emergent
properties.