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How is Alcohol Metabolized?
In order to answer this question, first you must know what makes alcohol different from
other foods and beverages. The answer? It’s the chemical makeup of alcohol! All types of
alcohol contain ethanol, or C2H5OH. The picture to the left shows what the actual compound
looks like. The ethanol molecule is a molecule that
belongs to a class of compounds called alcohols (which is
why we call drinks with this type of molecule alcohol).
All alcohols have a hydroxide molecule (-OH) attached to
one of the carbon atoms in the molecule, and it is this
chemical makeup that makes the breakdown of alcohol so
much different from the breakdown of all other foods and beverages, since most foods are made
up of solely carbohydrates, lipids, and proteins which do not have a hydroxide molecule. Ethanol
is a miscible, volatile, colorless liquid that is highly flammable. It is this miscibility of the
ethanol that allows it to be used in many different substances such as fuel for cars, hand sanitizer,
and, of course, alcohol! In this activity you will explore how alcohol is metabolized by the body,
and you will be able to see all of the thermodynamics behind these chemical reactions.
1. The breakdown of ethanol to acetaldehyde is catalyzed by an enzyme called alcohol
dehydrogenase (ADH). Ethanol is converted to acetaldehyde via this chemical reaction:
C2H5OH → C2H4O- + H2
ΔH = 42.7 kJ/mol
a. What does it mean for a reaction to be “catalyzed” by an enzyme? Explain in your
own words. A reaction is catalyzed by an enzyme when the enzyme actually
reduces the amount of energy required to change one substance to another. This is
usually done by the enzyme forcing the reactant into a configuration that is
desirable for changing it into the product. Once the reactant is in this
configuration, the product can easily be formed and the reaction occurs more
quickly than it originally would have.
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b. Based on the enthalpy (ΔH) of this reaction, is this reaction endothermic or
exothermic? In your own words, describe what this means. This reaction is
endothermic, which means that the reaction requires energy in order for the
products to be formed.
2. After the ADH converts ethanol to acetaldehyde, another enzyme called acetaldehyde
dehydrogenase (ALDH) further breaks down the acetaldehyde into acetate as shown here:
C2H4O→C2H3O-
ΔH = -215.1 kJ/mol.
a. Based on the enthalpy (ΔH) of this reaction, is this reaction endothermic or
exothermic? In your own words, describe what this means. This reaction is
exothermic, which means that the reaction actually releases energy when the
products are formed. No outside forces are necessary for this reaction to occur.
3. The next step in the breakdown of alcohol is for the body to break down acetic acid into
carbon dioxide (CO2) and water (H2O) via the Krebs cycle. The overall reaction that
occurs in the Krebs cycle is this:
CH3COOH
2CO2 + 3H2O ΔH = -1325.3 kJ/mol
a. Based on the enthalpy (ΔH) of this reaction, is this reaction endothermic or
exothermic? exothermic
4. Calculate the overall enthalpy of the breakdown of alcohol. Make sure to include all three
steps and show your work!
-1325.3 kJ/mol + (-215.1 kJ/mol) + 42.7 kJ/mol = -1497.7 kJ/mol
ΔH = -1497.7 kJ/mol
a. Is the overall breakdown of alcohol exothermic or endothermic? Exothermic
b. The first step in the breakdown of ethanol to acetaldehyde is endothermic. Give
an explanation for why this reaction actually occurs. Overall, you get a great
amount of energy being released from the breakdown of ethanol. So the body puts
in a small amount of work at the beginning to breakdown ethanol in order to get a
large return of energy in the end.
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c. ATP is a biological molecule that is created when food is metabolized and is
broken down by the body to be used as energy. If the creation of ATP requires
30.5 kJ/mol, how many moles of ATP molecules can be created per mole of
ethanol breakdown?
1497kJ / mol
 49 ATP
30.5kJ / mol
# ATP moles: 49 moles ATP
d. Whenever an ATP molecule is broken down into ADP, it releases 7 Calories/mol.
Based on the number of moles ATP molecules you calculated in the previous
question, how many Calories do you gain from the breakdown of1 mole of
ethanol?
49 moles ATP x 7 Calories/mole ATP = 343 Calories
# Calories: 343 Calories
e. A 1.5 oz. shot of 80 proof (40%) alcohol contains 0.6 oz. of ethanol, or 17.01 g of
ethanol. The molecular weight of ethanol is 46.07 g/mol. How many moles of
ethanol are in one 1.5 oz. shot? How many Calories do you gain from the
breakdown of one 1.5 oz. shot of 80 proof alcohol?
17.01g ethanol
1mol ethanol

 0.37mol ethanol per 1.5 oz shot
1.5oz shot
46.07 g ethanol
343 Calories/mol ethanol x 0.37 mol ethanol = 127 Calories
Moles ethanol: 0.37 moles
# Calories: 127 Calories
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5. One pound of fat is equivalent to 3500 Calories. How many 1.5 oz. shots of 40% alcohol
would a person have to drink in order to gain one pound?
3500 Calories/127 Calories per shot = 27.5 shots
# shots:27.5 shots
6. Many people who drink a lot of alcohol gain weight. Based on the possible number of
Calories you calculated above, explain why this happens. Every drink contains 127
calories, and it takes 27.5 drinks for a person to gain one pound. But when people drink,
they tend to drink more than one night per week. If a person average 4 drinks per night
for 3 nights every week, then he/she would approximately gain a pound every two weeks.
That is 26 pounds per year! Many people also have the occasional beer with dinner, etc.
adding to the amount of Calories they gain from drinking
7. The Krebs cycle breaks down acetic acid at a steady rate, while the rate of breakdown of
ethanol and acetaldehyde are based on the number of ADH and ALDH enzymes in the
body. For each of the cases below, determine what the rate limiting step is, and match
each case to the effect it would have: high BAC level, high level of alcohol toxicity, no
adverse effects.
a. Describe, in your own words, what a rate limiting step is. A rate limiting step is
the step in the reaction that requires the most time/energy in order to proceed, thus
the speed of this step overall determines the speed of the whole reaction.
b. Very few ADH enzymes, many ALDH enzymes. Rate limiting step is breakdown
of ethanol to acetaldehyde. This would cause high BAC levels.
c. Many ADH enzymes, very few ALDH enzymes. Rate limiting step is breakdown
of acetaldehyde into acetate. This would cause high level of alcohol toxicity.
d. Many ADH enzymes, many ALDH enzymes. Rate limiting step is breakdown of
acetate into CO2 and H2O by the Krebs Cycle, since the Krebs Cycle occurs at a
fixed rate. This would cause no adverse effects.
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