photosynthesis,veganarians, a-bombs, & Dr. Pimentel

an earlier post led me to read a little of Dr. Pimentel’s arguments against the ethanol subsidy (and i do agree burning food in our cars is idiocy) and one point he emphasizes is that adding up all the energy from sunlight available for photosynthesis leaves us far short of the energy needed to run our car culture.
However I wonder if sunlight is in fact the only energy input in a green plant’s final product. say if i’m given a- bomb components and decide to go green and assemble my bomb in a solar factory, my final product obviously has much greater potential energy than the sunlight i consumed. do the atoms and molecules plants assemble also carry enough potential energy that it needs to be considered along with solar energy in the net energy of the final product?

The energy from a fission or fusion bomb comes from the release of binding energies in the nucleus and conversion of some components into other, resulting in a net release of high energy photons (x-rays and gamma rays) and energetic neutrons that are absorbed by other atoms. Except for natural decay, we don’t normally see these kind of nuclear energy releases on terrestrial conditions, but they are obviously many orders of magnitude more energy than are normally seen in the chemical reactions we are more familiar with. To give a concrete example, a typical energetic yield for a beta decay is ~1MeV, or 96.5x10[SUP]6[/SUP] kJ/mol. In comparison, pure TNT has a yield of about 620 kJ/mol. So there is about a four or five order of magnitude difference in energetic yield per mass or volume between nuclear reactions and a highly energetic chemical reaction.

It should go without saying that the energy derived from the combustion of plants or plant-derived material is all chemical in nature, which involves only exchanges of or sharing electrons, and essentially all of this energy is either directly derived from the Sun via photosynthesis, or comes from other solar-powered processes, including the hydrological cycle which purifies and delivers water to the plants. Basically every energy source on Earth that is not directly derived from geothermal processes has converted and stored energy from the Sun.


given all that, the question is, does the energy in a molecule of sugar come from the sunlight that fell on the plant, or was the energy of that sunlight used in assembling the components of the sugar and the energy we can extract from the sugar was already in the components? and once the molecule is assembled does it contain more energy than that of the sunlight used in the immediate process of assembling it, as in the a-bomb (Ivymike was my favorite A-bomb, by the way)?

No. The sugar molecule is “assembled” from carbon dioxide and water. When you metabolize it, you end up with carbon dioxide and water. Since you ended up with the same thing you started with, you cannot get any more energy back out than what you put in.

i was just a humble liberal arts major at a state university, but that doesn’t sound right. the body might break sugar into carbon dioxide and water and just utilize the energy, but we’re talking about converting plant sugars to ethanol and it seems that would be a more complex reaction. don’t the water and carbon dioxide molecules come into the process carrying energy of their own? and (the question is), does the nergy in the ethanol equal the energy from sun light only, or does it equal the energy from sunlight plus whatever was brought to the table by the components?

The “Ivy Mike” device was actually a two-stage fusion (hydrogen bomb, or just “The Bomb”) device, one of the few to use cryogenic components (as they are impractical in actual weapons). The following “Shrimp” device used in the “Castle Bravo” was a three stage (fission-fusion-fission) device that was actually weaponizable.

As Baracus has stated, all of the energy that you get out of biofuels comes from the original growth of the plants which use photosynthesis to provide energy for binding and building sugars and other hydrocarbon chains. The use of fermentation produces more chemically active chains with easily reduced hydroxyl (-OH) molecules. Other processes can combine simple chains into the more complex and energy dense hydrocarbons in petroleum, but all of his typically involves a loss of actual energy in the reactants. Water and carbon dioxide are basically energy-negative configurations compared to any terrestrial environment; in other words, to split them up (oxidize) requires more energy than you will receive out of the binding energy released.

The processes of making biofuel concentrate the available energy into a smaller volume or mass, but in no way multiply the amount of energy available for a starting mass of material. This is convenient as you would rather have a car that runs on ethanol than one that runs on hay, but doesn’t violate any laws of conservation or require any external energy sources other than that to power the conversion operations (usually to achieve a minimum thermal threshold for effective reactions).


You have a good point slow Learner. CO2 and H2O are in their lowest energy state. Converting them to sugar leaves them in a state where energy is available. Converting the sugar to alcohol uses part of the energy, but leaves the CH3CH2OH still containing useable energy. We used to use a device for transportation that could convert sugar and even starch to energy. It is called the horse.

Imma majorly hijack but I’ve wanted to tell someone my pet theory for a long time.

In the next 30 years the US will undergo a huge increase in birthrate. Here’s how it goes:

  1. Energy and water crises force the decline of large-scale commercial farming in much of the Midwest (especially the parts that use fossil or imported water.)
  2. Much of this land reverts to grassland for feeding, and in order to provide more meat we convert or start to use more other land as feed grassland.
  3. This will cause a surge in actual hat wearin, horse-ridin cowboys (I’m not sure how much ranching is done by that type nowadays but combined with peak oil it will be inevitable that extraneous motor-driven vehicle use will decline)
  4. No woman can resist a cowboy.
  5. Birthrate skyrockets!