Posted by Azrel1013 at 24-116-119-142.cpe.cableone.net on March 10, 2004 at 11:47:25:
In Reply to: smart and WRONG posted by Runshouse21 on March 10, 2004 at 11:27:09:
: The majority of your body heat and your energy comes from breathing.
: Thats why you are tired after you eat because it takes more energy to digest the food than the food actually gives you. Eating gives your body the materials it needs not the fuel.
Breathe all you like. :)
The living cell can get the energy it needs to survive by absorbing energy from light to produce carbohydrates, or by eating carbohydrates that it find in its environment, and then burning the carbohydrates using atmospheric O2 to CO2 and H2O. This process, called respiration, is very complicated, but also very efficient. About 38% of the free energy in the nutrients can be utilized by the organism.
When fuels are burned to produce heat, and the heat is used to produce mechanical power, the efficiency is limited by the requirement that the overall entropy change caused by the heat transfers must not be negative. The most efficient engine possible is one operating reversibly between a hot temperature T and a cold temperature T'. Heat Q is absorbed at T, heat Q' is rejected at T', and the difference Q - Q' is available as work (ordered energy). The entropy principle requires that Q/T - Q'/T' = 0 if the engine is perfect, so the thermal efficiency is ç = T/(T - T'), which is always less than unity if T' > 0.
In respiration, we are not limited by this condition, and can extract all the free energy if we do it reversibly. The reactions of respiration cannot be reversible, because they must be spontaneous. Body warmth represents an irreversibility as well. This increase in entropy causes a decrease in the free energy F = U - TS, so not all the free energy in food can be realized. Nevertheless, respiration is very efficient, and this efficiency means that it must be broken up into a long chain of small steps. This chain has the same end effect as burning in air, but is extremely involved.
Nutrients can usefully be assigned to three classes. Fats are esters of fatty acids, carboxylic acids with long hydrocarbon chains, and alcohols. Esters with glycerol produce triglycerides, since glycerol has three OH groups. The hydrocarbon chains may be saturated, monounsaturated or polyunsaturated, and so are the fats they make. Fats are as close to fuels like petroleum as they can get, and provide 9.3 kcal/gm of energy. Fats are insoluble, and so can be stored safely in the body for times of need. The second class is protein, made up of 20 different kinds of amino acids. Amino acids have the COOH (carboxyl) group on one end, and the NH2 (amide) group on the other. A carboxyl on one acid and an amide group on another can fuse to make a peptide link, and proteins are long chains of amino acids. Most proteins are catalysts for reactions, or enzymes. The amino acids contain nitrogen and sulphur, making up a tool kit for the reactions of life. Protein supplies 4.1 kcal/g when it is burned, and the leftover N and S are very valuable. A kilocalorie, kcal or Cal, is the heat required to warm 1 kg (1 litre) of water by 1 °C. It is the usual calorie in diet books.