Photosynthesis

It's a well known fact that plants need water and sunlight to make food, but how is that possible? Essentially, plants create their own food through a process called photosynthesis. Photosynthesis is made up of two different processes called the light dependent reaction (also called the light reaction) and the light independent reaction (also called the dark reaction or the Calvin cycle). Both of these reactions take place in the chloroplast (shown at the left). The light reactions specifically take place in the thylakoid. Inside each thylakoid there is a system very similar to the electron transport system. This reaction is shown in the image below labeled "Light dependent reaction". This reaction begins with energy from the sun. The plant can use this energy because they have several pigments, one of which is chlorophyll. These pigments allow the plant to absorb the sun's energy. One photon of energy enters into photosystem two (PSII) and bounces off of the walls of photosystem two. The photon then reaches the reaction center at the base of the photosystem. There, a water molecule (H2O) has broken apart into H+ and O2. When the water is broken apart, an electron is released. The photon excites the electron, giving is energy. This electron travels up to the top of photosystem two. It then descends across the system and enters photosystem one (PSI). As is descends it pumps one H+ ion from the stroma into the thylakoid lumen (the area inside the thylakoid). Once the electron is inside photosystem one, it travels upward until is reaches the electron carrier. There, it reduces NADP+ to NADPH. As this process repeats, a high concentration of H+ ions is build up in the lumen. These ions then travel through the ATP synthase one by one. As they move through, they physically rotate the synthase. This creates energy, and the energy converts ADP and P to ATP. 

As a review, the inputs of the light dependent reaction are as follows:

1 H2O

Light

and the outputs are:

1 O2 (final product)

2 ATP (used in the Calvin Cycle)

1 NADPH (used in the Calvin Cycle)

Light dependent reaction

The next component of photosynthesis is the Calvin Cycle. The calvin cycle creates G3P, or PGAL, which the plant uses to make glucose. It takes three molecules of CO2 to create one molecule of PGAL, therefore this explanation will be describing the cycle in terms of three molecules of CO2. The Calvin Cycle begins with CO2, in this instance three molecules. These molecules of CO2 combine with three molecules of RuBP to form a six carbon molecule. The enzyme rubisco assists in this joining. This is a brief transition phase, and soon these molecules split into three carbon molecules, for a total of six three carbon molecules. These molecules must be rearranged and gain phosphates. Therefore, six ATP oxidize to form ADP and six NADPH oxidize to form six NADP+ and phosphate. Next, the three carbon molecules each lose a carbon. These carbons form a PGAL, and the unused substances are rearranged. The rearranging takes energy, and three ATP oxidize to become 3 ADP. The result is three Rubisco and the cycle continues. When two PGALs are created, they combine to form glucose.

As an overview, the inputs of the Calvin cycle (when one pyruvate is created) are:
3 CO2
9 ATP
6 NADPH

And the outputs are:

9 ADP
6 NADP+
6 P
1 Pyruvate

The equation for photosynthesis is: 6CO2 + 6H2O -> C6H12O6 + 6CO2


This autotrophic system of plants is amazing. Plants take in CO2 and water, two substances that are abundant on our planet, and they convert them to food and oxygen. This phenomenon is present every plant, and even some species of bacteria. It is an essential factor to maintaining the delicate balance of life on Earth. Although we have not yet studied this in class, I believe that the amount of light will assist in photosynthesis and the amount of water. Light would increase the amount of photosynthesis because light is used to excite the electron in the light reaction and water would assist because if there is a lack of water a light reaction cannot occur and therefore there will be no energy to use in the dark reaction.


Sources:

http://en.wikipedia.org/wiki/Photosynthesis

Images:

http://dft.ba/-anQ

http://micro.magnet.fsu.edu/primer/java/photosynthesis/

http://www.daviddarling.info/images/Calvin_cycle.jpg