Name ______________________
Date __________________________
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-
ΔH = 42.7 kJ/mol
a. What does it mean for a reaction to be “catalyzed” by an enzyme? Explain in your
own words. ________________________________________________________
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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. __________________
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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.__________________
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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? _______________________________________________________
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4. Calculate the overall enthalpy of the breakdown of alcohol. Make sure to include all three
steps and show your work!
ΔH = ____________
a. Is the overall breakdown of alcohol exothermic or endothermic? ______________
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b. The first step in the breakdown of ethanol to acetaldehyde is endothermic. Give
an explanation for why this reaction actually occurs. _______________________
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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?
# ATP moles: ___________
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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 of 1 mole of
ethanol?
# 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?
Moles ethanol: ___________
# 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?
# 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. _________________________
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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. ___________________
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b. Very few ADH enzymes, many ALDH enzymes.
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c. Many ADH enzymes, very few ALDH enzymes.
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d. Many ADH enzymes, many ALDH enzymes.
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