cellular respiration
...to burn (glucose is in paper, wood and marshmallows while oxygen is required for fires to burn), whereas the first two products are what we use to put fires out (carbon dioxide is used in many fire extinguishers). So our cellular respiration is similar to burning sugar (in a tightly controlled way) to get energy. Also note that this equation (respiration) is the reverse of photosynthesis. III. Steps in breaking down glucose. The first step is breaking the glucose molecule (a 6-carbon chain) into 2 3-C chains. (This step is called Glycolysis) C-C-C-C-C-C + 2ADP + 2P + 2NAD+ --> C-C-C + C-C-C + 2ATP + 2NADH Note that this reaction requires no oxygen. NAD+ is an electron carrier that is empty, whereas NADH is a full electron carrier. NADH has more energy than NAD+, just as ATP has more energy than ADP + P. The ATP from glycolysis are only 2% of energy in glucose. So where did the energy go? 1. 3% lost as heat 2. 2% in 2 ATP 3. 16% in NADH 4. 79% still in 2 C-C-C The energy that is now stored in NADH requires oxygen in order to convert it to ATP. The next steps require oxygen (called aerobic respiration) and produce the remaining 34 ATP. Even then, this total (36 ATP) represents only 37% of the energy in glucose. What happened to the rest? (Lost as heat, so this is explained by which Law of Thermodynamics? Second) IV. What do organisms do if they lack oxygen? They still get some energy from glycolysis, so they use that pathway. Problem: They quickly run out of NAD+, but they need that to run glycolysis. So they have to convert NADH back to NAD+. Yeast do this by converting the C-C-C to ethanol, producing CO2 in the process. What is this process called? (Alcohol fermentation: the CO2 released is what carbonates beer.) Animals do this by converting the C-C-C to lactic acid (called lactic acid fermentation). Problems with fermentation: You only get a little energy from glucose (alcohol burns, so you know that there is still energy in it). The byproducts are toxic to the organisms. Yeast pickle themselves in alcohol, while lactic acid burns and causes cramps in animals (like me and you!). So let us revisit our good friend the marshmallow. If we eat the marshmallow, how much of the energy will we be able use in our cells? (At best 37%, and that's if all of the glucose we eat crosses the gut wall and is delivered to our cells.) The remaining 63% is lost as heat within our body, but it is not WASTED, since it helps to maintain our internal temperature. Remember that some researchers believe that many large dinosaurs were warm-blooded, not because they possessed the capacity to regulate their body temperature (homeostasis), but rather because they were so large that the heat from their metabolism could not readily escape!! V. Frequently asked questions A. Why does our heart beat faster when we are scared? 1. Because of the release of adrenaline (proximate, or mechanistic hypothesis). 2. To prepare us to fight or run, which requires a lot of oxygen (ultimate or evolutionary hypothesis). (We'll discuss the difference between these two different types of hypotheses at length next Wednesday and Thursday.) B. If yeast uses fermentation to cause bread to rise, why don't they produce alcohol as well? 1. They do, but the small amount of alcohol produced is burned off in the oven. 2. The CO2 is what causes bread to rise! C. Do commercial breweries still use yeast to carbonate beer? 1. Not most breweries. If they do, there is a little yeast residue at the bottom of your beer. So most breweries filter the beer to remove the yeast, then force CO2 into the bottles or keg. D. How do we get rid of the lactic acid in our cells? 1. The bloodstream carries the lactic acid to the liver, which uses O2 (when it is available) to detoxify it. That is why you continue to breathe a while after you sprint or do other anaerobic activities. E. I'm a swimmer. How can I reduce my need for O2 during a race? 1. Some competitive swimmers breathe deeply BEFORE the race. Hyperventilation causes an increase in O2 and a decrease in CO2 just before a race. -------------------------------------------------------------------------------- VI. Sample Exam Questions Dieters rave about a celery diet, saying that you actually LOSE energy eating celery. How could this be true? (Select all that are reasonable.) a. A substance in the celery causes your body to lower its temperature b. The celery is mostly water, and water has no energy. c. The non-water part of celery is mostly cellulose, which we cannot digest. d. There are a few sugars in celery that can be digested, but the body uses more energy PROCESSING the celery than it acquires FROM the celery. Answer: A is not reasonable, since a decrease in your body temperature would require LESS energy, not more. Fevers require more energy than normal temperatures. B is reasonable, since water has no energy (you use it to PUT OUT fires). Diets usually recommend drinking large amounts of water, since it fills your stomach but does not provide any calories. C is reasonable, because we cannot digest cellulose. Again, it fills your stomach and thus makes you feel full, but in actuality, you are not getting any calories from it (cellulose). D is also reas...