70 Human Energy Storage and Expenditure

Chemical Potential Energy

We have learned that when you jump, bend a paper clip, or lift an object you transfer kinetic energy, potential energy, or thermal energy to the objects, but where did that energy come from and what form was it in before? Plants use photosynthesis to convert electromagnetic energy in sunlight to chemical potential energy into organic molecules in the food we eat. During cellular respiration, organic molecules are oxidized with the release of carbon dioxide, water, and energy used to form ATP molecules[1][2]. The body uses the molecule ATP to power cellular functions, including muscle contraction. To learn more about these processes consider taking courses in human anatomy and physiology, general biology, cell biology, molecular biology, and biochemistry. The release of chemical potential energy during the ATP cycle is shown in the following animation:

Everyday Example: No work and all heat

Hold an object up in the air. Keep holding. Do you eventually get tired? Why? You are certainly applying a force, but the object hasn’t moved any distance, so it would appear that you  really done anywork. Why should you get tired when you aren’t doing any work? The animation in the previous video provides the answer, you haven’t done any useful work, but you have work on a microscopic scale to transfer chemical potential energy to thermal energy. The ATP cycle occurs repeatedly just maintain muscle tension, even if the muscle does not actually move a noticeable distance. The ATP cycle continues to use up  potential energy even if you aren’t doing any useful work. Where does that energy go? Into thermal energy If you hold the object long enough, you might even begin to sweat! If using stored energy without doing useful work seems pretty inefficient, you’re right. In fact the efficiency of the body in such a situation is zero!

The digestive and metabolic process is essentially oxidation of food so it requires oxygen just like oxidation of fuel in an engine requires oxygen.  Therefore, we can determine the the actual chemical potential energy consumed during different activities by measuring oxygen use. The following table shows the oxygen and corresponding energy consumption rates for various activities.

Energy and Oxygen Consumption Rates for an average 76 kg male
Activity Energy consumption in watts Oxygen consumption in liters O2/min
Sleeping 83 0.24
Sitting at rest 120 0.34
Standing relaxed 125 0.36
Sitting in class 210 0.60
Walking (5 km/h) 280 0.80
Cycling (13–18 km/h) 400 1.14
Shivering 425 1.21
Playing tennis 440 1.26
Swimming breaststroke 475 1.36
Ice skating (14.5 km/h) 545 1.56
Climbing stairs (116/min) 685 1.96
Cycling (21 km/h) 700 2.00
Running cross-country 740 2.12
Playing basketball 800 2.28
Cycling, professional racer 1855 5.30
Sprinting 2415 6.90
A person is measuring the amount of oxygen in blood and metabolic rate using a pulse oxymeter. The pulse oxymeter is strapped to the person’s wrist, and the index finger is inside the clip.
A pulse oxymeter is an apparatus that measures the amount of oxygen in blood. Oxymeters can be used to determine a person’s metabolic rate, which is the rate at which food energy is converted to another form. Such measurements can indicate the level of athletic conditioning as well as certain medical problems. (credit: UusiAjaja, Wikimedia Commons)

Food Calories

For historical reasons we often measure thermal energy in units of calories  (cal) instead of Joules.  There are 4.184 Joules per calorie. We measure chemical potential energy stored in food with units of 1000 calories, or kilocalories (kcal) and we sometimes write kilocalories as Calories (Cal) with with capital C instead of a lowercase c. For example, a bagel with 350 Cal has 350 kcal, or 350,000 cal. Converting to Joules, that would be 350,000 \,\bold{cal} times (4.184 \,\bold{J}/bold{cal}) =1,464,400\, \bold{J} in the bagel.

Everyday Example: Walking MPB (miles per bagel)

According to the chart above walking at 5 km/hr, or about 3 mph, requires 280 Watts (J/s) of power. Let’s convert that to miles you walk to expend the energy contained in a bagel. First let’s find the energy per hour.

    \begin{equation*} (\frac{280\, \bold{J}}{1\,\bold{s}}) (\frac{3600\,\bold{s}}{1\,\bold{hr}}) = 1,008,000\,\bold{\frac{J}{hr}} \end{equation}

Now to bagels per hour:

    \begin{equation*} (\frac{1,008,000\,\bold{J}}{1\,\bold{hr}})(\frac{1\, bagel}{1,464,400\, \bold{J}}) = 0.7\, \frac{bagel}{\bold{hr}} \end{equation}

With a walking speed of 3 miles per hour:

    \begin{equation*} (\frac{3\,\bold{mi}}{1\,\bold{hr}})(\frac{1\,\bold{hr}}{\bold{0.7\, bagel}}) = 4.3\,\bold{mpb} \end{equation}

You can walk about 4.3 miles per bagel. For comparison, a gallon of gasoline has 90x more chemical potential energy than a bagel[3] but at 20-40 mpg a  typical car can only drive about 5-10 times as many miles on a gallon of gas than you can walk on a bagel . In terms of miles per usage of chemical potential energy, you are about 9x more efficient than a car!

Reinforcement Exercises

Burning Calories

We often talk about “burning” calories in order to lose weight,  but what does that really mean scientifically?. First, we really we mean lose mass because that is the measure of how much stuff is in our bodies. Second, our bodies can’t just interchange mass and energy — they aren’t the same physical quantity and don’t even have the same units. So how do we actually lose mass by exercising?  we break down the fat molecules into smaller molecules and then break bonds within those molecules to release chemical potential energy, which we eventually convert to work and exhaust heat.  The atoms and smaller molecules that resulting from breaking the bonds combine to form carbon dioxide and water vapor (CO2 and H2O) and we breath them out. We also excrete a bit as H2O in sweat and urine. The process is similar to burning wood in campfire — in the end you have much less mass of ash than you did original wood. Where did the rest of the mass go? Into the air as CO2 and H2O. The same is true for the fuel burned by your car. For more on this concept see the first video below. The really amazing fact is that your body completes this chemical process without the excessive temperatures associated with burning wood or fuel, which would damage your tissues. The body’s trick is to use enzymes, which are highly specialized molecules that act as catalysts to improve the speed and efficiency of chemical reactions, as described and animated in the beginning of the second video below.




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