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Biochemistry · Problem type
Urea from protein nitrogen
Use this for “Someone breaks down 85 g of protein in a day. How much urea is made, and how much ATP does the urea cycle spend?”
Chart
This is an example. Type your own values to start a new problem.
mP
Protein broken down
example · g
fN
Nitrogen share of protein
example · %
mN
Nitrogen
calculated · g
nU
Urea made, in moles
calculated · mol
mU
Urea made
calculated · g
ATP
ATP equivalents spent
calculated · mol
Formulas
mN
=
mP
×
fN
100
11.2
=
70
×
16
100
nU
=
mN
28.02
0.3997
=
11.2
28.02
mU = 60.06 × nU
24.01 = 60.06 × 0.3997
ATP = 4 × nU
1.599 = 4 × 0.3997
Assumptions
•
Protein is about 16% nitrogen by mass, and all of the nitrogen broken down leaves as urea.
•
Each urea, CO(NH₂)₂ at 60.06 g/mol, carries two nitrogens, 28.02 g per mole of urea.
•
The urea cycle spends 3 ATP per urea, one split to AMP, so it costs 4 ATP equivalents.
Step-by-step
WE KNOW
mP = 70 g, fN = 16%
FIND
nitrogen (mN), urea made, in moles (nU), urea made (mU), ATP equivalents spent (ATP)
Step 1 · Find nitrogen (mN)
Use mN = mP × fN ÷ 100
Take the nitrogen share of the protein’s mass.
mN
=
70
×
16
100
mN
=
1,120
100
mN = 11.2 g
Step 2 · Find urea made, in moles (nU)
Use nU = mN ÷ 28.02
Each urea carries two nitrogens, 28.02 g per mole of urea.
nU
=
11.2
28.02
nU = 0.3997 mol
Step 3 · Find urea made (mU)
Use mU = 60.06 × nU
Multiply the moles of urea by its molar mass.
mU = 60.06 × 0.3997
mU = 24.01 g
Step 4 · Find ATP equivalents spent (ATP)
Use ATP = 4 × nU
Each urea costs 3 ATP, one split to AMP and pyrophosphate: 4 ATP equivalents.
ATP = 4 × 0.3997
ATP = 1.599 mol
Check
Put the numbers back in. Both sides should match.
11.2
=
70
×
16
100
✓
0.3997
=
11.2
28.02
✓
24.01 = 60.06 × 0.3997 ✓
1.599 = 4 × 0.3997 ✓
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Glycogen: made or broken down?
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