In Part 1 (see my previous blog), I described the “food chain” and some energetic aspects of it. Now, I’d like to continue with more details and include entropic aspects as well.
Chlorophyll is the pigment in leaves that absorbs light. Typically, chlorophyll absorbs only visible light, mostly in red and blue wavelengths, and tends to reflect the green wavelength; this gives plants their familiar green color. Photosynthesis can be divided in two major steps: the oxygen-producing step (photophosphorylation) and the carbon fixation step that eventually produces glucose (also called the Calvin-Benson cycle, depicted below).
The first step, photophosphorylation, is a light-enabled reaction in which water is consumed and oxygen and molecules of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) are produced. To this day, we don’t completely understand all the steps and chemicals involved in making this happen. That said, we’ll limit ourselves to a brief description of how ATP and NADPH carry energy. Each molecule of ATP can store a large amount of solar energy within its chemical bonds. Likewise, an NADPH molecule can carry excited electrons, which is another way to store energy. Together, ATP and NADPH serve as the primary energy carriers in living plant cells.
A very schematic and simplified representation of the photosynthetic process. ATP, ADP, NADPH, and NADP+ are energy carriers. While photophosphorylation needs light, the Calvin‑Benson cycle can run in darkness.
The second stage (or Calvin-Benson cycle) can work without light. Using the energy of the ATP and NADPH molecules and carbon dioxide from the atmosphere, this cycle creates chemical reactions in the cell that eventually form glucose, a simple sugar.
The overall chemical reaction of photolysis can be expressed as:
6H2O + 6CO2 + hν → C6H12O6 + 6O2
where hν stands for the incoming sunlight needed to drive the reaction.
On the left side are six water molecules and six carbon dioxide molecules. Altogether, they are less complex than the single sugar molecule and six oxygen molecules on the right side. Therefore, we expect the entropy to decrease during photosynthesis, which is driven by the energy in sunlight. Indeed, quantitative calculations  show that the entropy change for the overall reaction is negative and can be calculated as ΔS = -262 J/K (for one mole  of glucose, which is 180 grams). Because the entropy decreases, this reaction cannot proceed spontaneously and therefore must be driven by an external energy source – which is, of course, sunlight.
The reverse of the photosynthetic reaction occurs when the plant needs energy. This process is called “respiration of glucose,” and is in fact the combustion of glucose under well-controlled conditions in the plant cell. The products of that reaction are water and carbon dioxide, as expressed below:
C6H12O6 + 6O2 → 6H2O + 6CO2
How much energy will this reaction deliver? A lot! For example, burning 180 grams of glucose (about 40 sugar cubes) will generate almost 3000 kJ of energy – enough to allow a human weighing 75 kg (165 pounds) to climb a mountain about 4000 meters (13,200 feet) high. Impressive, isn’t it? At the molecular level, the aerobic (oxygen-based) respiration of glucose produces energy that is stored in molecules of ATP. This happens by adding a phosphate group (PO4–) to ADP. Per molecule of glucose, 32 molecules of ATP are created and together they store about 1100 kJ of energy that can be used to drive other reactions in the cell. In the case of anaerobic respiration (where little or no oxygen is available), the energy efficiency is much less, since only two ATP molecules are formed for each combusted glucose molecule.
A sort of artificial “photosynthesis” technology is possible with photovoltaic (solar) cells. These devices use a rather elegant process to convert sunlight into electrical energy. The electrical current generated by the solar cell can then be used to split water into hydrogen and oxygen, and the resulting gases can be used in fuel cells to produce electrical power and water. On paper, this looks like a very attractive energy conversion technology, without the hazards or pollution of nuclear and fossil fuel-based power plants.
(Adapted from The Second Law of Life with permission from William Andrews Publishers)
The calculation is rather simple, thanks to scientific tables that provide the standard entropy for many chemical compounds. (The term “standard” means 1 atmosphere of pressure and a temperature of 298.15 K. The standard entropy (in J/K per mole) is 70 for (liquid) water, 214 for carbon dioxide, 205 for oxygen, and 212 for glucose. Thus, the total entropy change for the reaction without incoming sunlight works out as: ΔS
= (products) – (reactants) = (212 + 6×205) – (6×214 + 6×70) = -262 J/(K mol). The decrease in entropy has to be balanced by a similar increase somewhere else in the universe. This is indeed the case, because the incoming sunlight is accompanied by an entropy increase (For instance, see W. Yourgrau and A. van der Merwe, Proc. Nat. Ac. Sci
., Vol 59, p. 734 (1968). If the reaction involved water vapor rather than liquid water, the entropy decrease would be much larger (972 J/(K mol), as the standard entropy of water vapor is 189 J/(K mol).
 One mole represents 6.02 x 1023 atoms or molecules (for glucose that is 180 gram).