Lesson 4 of 7 / Nutrition in humans
The liver processes more than glucose
Why does the liver have so many different jobs?
I can explain the liver's roles in glucose storage, fat digestion, amino acid metabolism, urea formation and substance breakdown.
I can state the five specified liver roles.Syllabus
Syllabus K224 / K225, Topic 4(d). State the five specified liver roles.
About 6 min
Make a guess. You are not marked.
You eat far more protein than your body needs. What happens to the extra amino acids?
Show the answer
The liver removes their nitrogen and makes urea.
Excess amino acids cannot be stored. The liver deaminates them; the kidneys then excrete the urea.
The key idea
The liver stores and processes nutrients, produces bile, converts excess amino acid nitrogen into urea, and breaks down alcohol and hormones.
Metabolism means the chemical reactions in cells. Deamination is removal of the nitrogen-containing amino group from an amino acid.
Explore the idea
Name the change and its destination
The reverse conversion also occurs: glycogen is broken down to glucose when needed.
Original process summary. Each route highlights one liver role; the liver performs several roles at the same time.
Explanation
The liver helps control the supply of glucose. It turns glucose into glycogen to store it, and turns glycogen back into glucose when needed. This keeps glucose available between meals.
The liver also makes bile. Bile is stored in the gallbladder and released into the duodenum, where it emulsifies fat so lipase can digest it faster. Bile does not digest fat itself.
Amino acids are used to make proteins, but the body cannot store extra amino acids. In the liver, extra amino acids are deaminated: the part containing nitrogen is removed and turned into urea. Urea is a less toxic waste.
Urea goes into the blood and is carried to the kidneys, which remove it in urine. Remember: the liver makes urea, and the kidneys excrete it. The rest of the amino acid can be used in respiration.
The liver also breaks down alcohol and hormones. It can only break down alcohol slowly, so drinking too much can still cause harm. Because the liver has so many jobs, liver damage affects many parts of the body.
Step by step
- 1
Separate nutrient routes
Glucose can become glycogen; amino acids can become proteins or be deaminated if in excess.
- 2
Track the nitrogen
The liver removes amino groups and forms urea. Blood transports it to the kidneys for excretion.
- 3
Include the other roles
Remember bile production, alcohol breakdown and hormone breakdown as distinct liver functions.
Worked example
An excess supply of amino acids
Amino acids are absorbed in excess of the amount needed for protein synthesis. Explain what happens to their nitrogen and name the organ that removes the resulting waste from blood.
One way to explain it
The excess amino acids are deaminated in the liver. Their nitrogen is converted into urea, which enters the blood. The kidneys remove urea from blood and excrete it in urine.
Why this answer works
- Place deamination and urea formation in the liver.
- Name blood as the transport medium.
- Assign removal into urine to the kidneys.
Watch out for this
A student says: "The kidneys turn amino acids into urea." What is wrong with this?
Show the answer
The liver forms urea after amino acid deamination. The kidneys remove the urea from blood and excrete it.
Check your understanding
Which statement correctly distinguishes liver and kidney functions?
Show the answer
The liver forms urea; the kidneys remove it from blood into urine.
Formation and excretion occur in different organs, connected by transport in blood.
Exam-style questions
Check your understanding
A person eats far more protein than they need. What happens to the excess amino acids?
Show the answer
The liver deaminates them and turns the nitrogen into urea
The amino group is removed and made into urea, which the kidneys excrete.
Quick recall
Why does the liver have so many different jobs?