Photosynthesis Lab Report

Christina Grello
With the dramatically increasing population it is extremely important that we find a low-cost, carbon-neutral fuel. American chemist Dr. Daniel Nocera has devoted his career to finding this alternative energy source using photosynthesis as his inspiration. Photosynthesis, the conversion of solar energy into chemical energy stored in the bonds of glucose, is arguably one of the most important chemical reactions on earth. The maintenance of life on earth, our food, oxygen, and fossil fuels depend upon photosynthesis. In this process water and carbon dioxide are converted to oxygen and glucose using energy from the sun (Hoefnagels, 2012, p.90). The overall reaction can be written as: 6 CO2 (g)+ 6 H2O (l) →sunlight→ C6H12O6
(s) + 6 O2 (g)
Photosynthesis occurs in two stages, the light reactions and the dark reactions.
The light reactions occur on the thylakoid membranes of the chloroplast; here photons from the sun strike molecules of chlorophyll a, exciting the electrons into a higher energy state. In a series of reactions the energy is converted into ATP and NADPH. During this process water is split and oxygen is released as a byproduct. ATP and NADPH are then used in the dark reactions to produce glucose (Hoefnagels, 2012, p.96).
The dark reactions, also known as the Calvin Cycle, or light independent reaction, occur in the stroma of the chloroplast. During the Calvin Cycle a series of reactions reduce carbon dioxide by combining it with hydrogen to form carbohydrates, this process is referred to as carbon fixation (Hoefnagels, 2012, p.98).
In 2011, drawing inspiration from the light reactions of photosynthesis, Dr. Nocera developed the “artificial leaf”, a silicon coated sheet that is about the size of a playing card. When placed in water and exposed to sunlight the leaf splits water into hydrogen gas and oxygen gas, the gas can then be collected and used as fuel. Dr. Nocera’s process of “artificial photosynthesis” can more precisely be defined as solar-powered electrolysis of water (Owen, 2012). During the electrolysis of water energy from the sun electrochemically splits water into hydrogen gas and oxygen
gas:
2 H2O (l) →sunlight→ O2 (g) + 2 H2 (g)
One problem Dr. Nocera faced when developing his artificial leaf was the stability of water. Water is very chemically stable and its components tend to resist molecular rearrangement making it difficult to decompose (Chaplin, 2014). The stability of water is due to the bonds of the oxygen and hydrogen atoms. In water, two hydrogen atoms are bound to a central oxygen atom by a shared pair of electrons, referred to as a covalent bond (Denniston et al., p.84). In this bond only two of oxygen’s six valence electrons are used leaving two none-bonding pairs of electrons on the oxygen. These electron pairs push the two hydrogen atoms closer together, resulting in a bent shape (Denniston et al., p.114). These bonds are very strong and do not want to break easily. The partially-positive hydrogen atom on one water molecule is electrostatically attracted to the partially-negative oxygen on a neighboring molecule (Chaplin, 2014). This process is called hydrogen bonding and is the strongest form of covalent bonding resulting in a very stable molecule (Denniston et al., p.183). Dr. Nocera had to develop a way to overcome the molecular stubbornness of water, to do this he enlisted the help of a catalyst to promote the shuffling of bonds.
A catalyst is a substance that speeds up a chemical reaction by lowering the activation energy required to start the reaction. A catalyst is not consumed by the reaction and can be recovered chemically unchanged at the end of the reaction (Denniston et al., p.241). Dr. Nocera along with Matthew Kanan discovered a highly effective low-cost phosphate and cobalt catalyst (Owen, 2012).
The artificial leaf in its current form is a piece of silicon coated on one side with the cobalt-phosphate compound, and on the other with an inexpensive nickel-based catalyst. Once it is placed in water and exposed to sunlight, light penetrates the cobalt layer and a wireless electric current rises within the silicon. The two sides become electrodes causing hydrogen to bubble from one side and oxygen to bubble from the other (Owen, 2012).
When developing the artificial leaf Dr. Nocera primary focus has been to provide energy to what he refers to as the “non-legacy world” (Owen, 2012). These impoverished people have little to no access to modern fuels or to any electricity grid, the artificial leaf would provide these individuals with a little energy, allowing them to burn hydrogen till dawn at an average rate of about a hundred watts (Owen, 2012). In our standard of living this is not a lot of energy but to people living in extreme poverty this can be life changing. Electricity would empower the poor and could theoretically lead to a decline in population growth. It has been shown time and time again that as wealth has risen, the population growth in those areas has slowed.
While the artificial leaf would not directly impact my life, the indirect influence it would have on population growth in developing countries would affect me greatly. The rapid population growth is a major contributing factor to the food crisis as well as pollution and green house gases.

References Cited

Chaplin, M., 2014, Water Structure and Science, http://www1.lsbu.ac.uk/water/index.html (December 1, 2014)

Denniston, K. J., Topping, J. J., Woodrum, K. R., and Caret, R. L., 2012 General, Organic, and Biochemistry Eighth Edition, McGraw-Hill, New York City, 325 p.

Hoefnagels, M., 2012, Biology: Concepts and Investigations Second Edition, McGraw-Hill, New York City, 335 p.

Owen, D., 2012, The Artificial Lead, The New Yorker