Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Cellular respiration

  1. Jan 9, 2007 #1
    Basically Is cellular respiration what we 'turn to' when we need energy according to the basic chemical reaction

    glucose + oxygen -> carbon dioxide + water

    I noticed that water is produced as a byproduct. Does this mechanism act to cool us down also? A certain amount of exercise in summer will produce more sweat than that same amount in winter. Does that imply more cellular respiration went on during summer in though the work done was the same? Therefore more glucose and other energy reactants in our bodies is burnt. Meaning more weight loss etc. So is that why its better to do a workout in hot conditions and sweat more? And why people can loss weight simply sitting in hot saunas?

    Or is the water produced in cellular respiration too small for cooling purposes?

    Note: I am not looking for weight loss myself but just curious about how the human body works and the methods of weight loss.
    Last edited: Jan 10, 2007
  2. jcsd
  3. Jan 10, 2007 #2
    Hi Pivoxa,
    Although I am not a biologist, I actually took exercise physiology and nutrition classes many years ago and used what I learned to this day (age 40) to stay in shape. Hopefully, I can be of help.

    Actually, some of the water is exhaled and some gets recylced back into your system.

    Actually, it is because your body is expelling more water to cool down.

    The thing to remember is that there is a big difference between fat loss and weight loss. Weight loss can be in the form of muscle, water or fat. Water loss (above techniques) will only dehydrate you more quickly and cause problems.

    If what you are looking for is fat loss, the best way to do that is to create a caloric deficit without going into starvation. A caloric deficit is simply a shortage of calories which forces the body to go into its own fat stores in order to balance the energy equation. Starvation, on the other hand, is a deficiency in a vital nutrient which forces the body to cannibalize its own living tissue (muscle) in order to satisfy the deficiency.:smile:
  4. Jan 10, 2007 #3
    I realise that weight loss in terms of dehydration is undesireable. I was thinking of turning glucose and fats into water in celluar respiration and using that water to cool the body. That seems like a good idea but would it work? Does the body uaully dosen't look to celluar respiration for cooling purposes?

    The other thing is that energy is created from this process. So the body gets hotter from this reaction. So that might discourage the body to undergo this process for cooling purposes even more.

    One thought is another technique of burning fuel faster in the body is to put it in a cool room so in order for the cells to maintain body tempertures, it must undergo this reaction more often in order to heat itself up. Hence more fuel is burnt this way. That also explains people usually have larger appetites in winter.

    What do you mean by calories and caloric deficity? You mean eat low energy foods? Which really means less fatty foods?
    Last edited: Jan 10, 2007
  5. Jan 10, 2007 #4
    Cellular respiration is used for developing ATP, which is the energy your body uses. Sweat glands are responsible for the perspiration, now what mechanism is behind this I do not know off the top of my head. People loose weight in saunas because they are loosing water, dehydration. I believe the water molecule released during cellular respiration is to minuscule to bother with the cooling process.
  6. Jan 11, 2007 #5
    I agree.

    Giving a reasonable answer will be a little lengthy so please bear with me.o:)
    The definition of a calorie varies with the context in which it is used. In the realm of physics and chemistry, a calorie is the energy needed to raise the temperature of 1 gram of water by 1 degree celsius. In the realm of nutrition, a Calorie is the energy need to raise the temperature of 1 kilogram of water by 1 degree celsius. Although the latter is more relevant to this post, they are both referring to energy and basically, a caloric deficit is a state where you are burning more Calories than you are taking in. Essentially, body fat is the body's energy store house. For each lb (.45 kg) of fat that you burn, you will get roughly about 3500 Calories of energy. However, adding a pound of fat to your body requires over 4000 Calories (about 9 Cals/gram). The disparity is due to the 2nd Law of Thermodynamics. In other words, losing 2 lbs of fat in 1 week (7000 Cals/7 days) requires a 1000 Cal/day deficit. So when someone on a starvation diet loses 10 lbs (4.5 kgs) of weight in a week, much of that weight is in the form of muscle and water because it is almost impossible to maintain a 5000 Cal/day deficit.

    Creating a caloric deficit is best done by minimizing empty Calories and by increasing caloric expenditure. Empty Calories are those for which the metabolism has no use. For example, if I eat (and absorb) 50 grams of protein in one sitting when my body can only metabolize about 20 grams in that sitting, then 30 grams (120 Cals) are empty calories. You are right about avoiding greasy foods for the following reasons:

    1. Fats (9 Cals/gram) has more than double the caloric density of carbohydrates (4 Cals/gram) or protein (4 Cals/gram).

    2. Unlike carbohydrates and proteins which get metabolized first with the remainder going into storage, most fats are the opposite. Fatty acids with more than 12 carbons usually get absorbed into the lymphatic system and emptied out into the thoracic duct after which they get pumped straight to the fat cells.

    3. The necessary doses of essential fats are actually relatively small and are almost never to be found in the typically greasy junk food.

    However, it is crucial to avoid starvation because as stated in a prior post, it will reduce your lean mass which is a major factor in your basal (resting) metabolic rate (BMR). The BMR is important because it burns Calories on a 24 hour/day basis and not just when you are exercising. As I stated in a prior post, starvation is caused by nutrient deficiency (not caloric deficit) and can occur in a caloric deficit, a caloric equilibrium (maintaining fat), and even in a caloric surplus (where the person is getting fatter and fatter but the lean mass is wasting away). The latter case is happening all over the USA. Starvation can be caused by skipping meals or by eating meals that are completely out of balance. As an analogy for the latter case, if your car runs out of gas and you service the oil, it is still out of gas.

    As far as increasing the BMR, resistance exercise is the most effective because it increases the lean mass. For this reason it gives a 24 hour per day residual benefit. When combined the fact that it stimulates the body to increase or at least preserve the lean mass, it makes the body more selective about burning fat and (contrary to popular belief) is actually more effective than cardio.

    I hope that this helps.
    Last edited by a moderator: Jan 11, 2007
  7. Jan 19, 2007 #6
    Thanks for your lengthy explanation grant9076. It seems like incresing lean body mass is desirable for calory burning. This means exercising. However, when I do excercise such as running, I find that I tend to eat more than what I should after the excercise which means doing the excercise makes things even worse. High intensive excercise like running uses a lot of oxygen and glucose in cellular repiration. The decrease in glucose reasults in more hunger. Fat is burnt later in running doesn't it when the glucose storage is depleted. Walking however, directly accesses the fat storage? Although it will take longer to burn the same amount of calories as running.
  8. Jan 22, 2007 #7
    You brought up some good points but there are some things that I need to clarify further. Because biology is such an extremely complicated topic, it takes a lot of information to just scratch the surface (Biologists, feel free to jump in and help out). I will try to explain why resistance exercise is about twice as effective as cardio, and why nutrition is about twice as effective as both forms of exercise combined.

    Eating more after exercise is quite natural and often necessary. This is because it is easier to enter a starvation state which depletes your lean mass and lowers your resting metabolism. The main discriminator is in what you eat. A post workout meal should have at least a source of high quality protein (a lean source of the essential amino acids), a source of carbohydrates, and a source of fiber (i.e. grilled chicken, brown rice, broccoli). The fiber acts as a filler and also as a diluting agent which reduces the rate at which the caloric nutrients enter your system. The protein, in addition to providing the essential amino acids, delays gastric emptying and will also slow the rate at which calories are entered into your system. By not inundating the system with caloric nutrients, there is less chance of them turning into empty calories. Also, it is for this reason why it is better to eat many small meals than a few big ones.

    As far as exercise is concerned, your post highlighted some of the limitations of cardio. As I mentioned before, resistance exercise is the only type that really increases the lean mass. Resistance exercise is any exercise that forces your muscles to contract against a significant resisting force. Although the most popular type is weight lifting, it can include many other types. However, unless you are running up steep hills, distance running is not likely to qualify. Also, depleting your glycogen during distance running is not recommended because anyone who has "hit the wall" during an endurance event can tell you that it is absolute suffering and agony. Also, an average person burns about (roughly) 90 to 120 Calories per mile depending on the size of the person. As you mentioned, walking will burn the calories at a slower rate but will also take longer to cover the same distance so the amount of Calories burned will stay about the same. Also, this is less than what can be replaced by a candy bar. As a result, the best exercise programs focus on boosting the resting metabolic rate because the extra calories burned during the exercise is less than the extra calories burned 24 hours per day for seven days a week. The residual effect means that someone who does a lot of resistance exercise is still getting benefits from exercises that he/she did a year ago.

    Also, there is another interesting point about resistance exercise. Most people know that the primary source of energy during a weightlifting set is supplied by ATP and anaerobic glycolysis. However, a weightlifter is usually breathing heavily after each set and the reason for this is a process called oxidative phosphorylation. Phosphorylation is the process of putting the third phosphate group back onto the ADP to turn it back into ATP. In addition, some of the pyruvate (from glycolysis) is converted back into glucose in a process called gluconeogenesis. Because these are both non-spontaneous reactions, they need energy and have to be coupled with spontaneous reactions to occur. Much of this energy is supplied by the reduction of oxygen and by the oxidation of NADH which is in turn supplied by the burning of fat in the citric acid (krebs) cycle. In other words, a reasonable amount of fat is burned during resistance exercise.

    Note: The gluconeogenesis that happens between workout sets uses a different source of pyruvate from the type of gluconeogenesis that results from low carb starvation diets. In the first case, the pyruvate is being provided by spent glucose. However, in the latter, the pyruvate is being supplied by de-aminated/de-carboxylated amino acids.
    Last edited by a moderator: Jan 22, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook