Learning focus
Build precise biological explanations, interpret diagrams and data, and connect structure, process, health and practical evidence.
Amino-acid assimilation
Amino acids absorbed into blood reach the liver through the hepatic portal vein. They may be used to build proteins. Assimilation means uptake and use by cells, not merely absorption into blood.
Why excess cannot be stored
Unlike carbohydrate and lipid, excess amino acids are not stored as a dedicated reserve. Their nitrogen-containing part must be removed before the remaining carbon skeleton can be used in metabolism.
Deamination
Deamination is removal of the nitrogen-containing part of amino acids in the liver. This leads to formation of urea. Urea is less harmful than ammonia but remains toxic enough to require excretion.
Transport to kidneys
Urea dissolves in plasma and is carried from liver through the hepatic vein, vena cava and heart to the kidneys. It is filtered into nephrons and leaves in urine.
Integrated reasoning
A complete explanation links digestion and absorption of protein, amino-acid transport to liver, protein synthesis, deamination of excess, urea transport and kidney excretion. This integrates human nutrition, transport and excretion.
Practical or data skill
Create a flow diagram from dietary protein to urea in urine. Label digestion, absorption, assimilation, deamination, transport and excretion.
Examination tip
Deamination occurs in the liver; filtration of urea occurs in the kidneys.
Review questions and suggested answers
Question 1
Define deamination.
Suggested answer
Removal of the nitrogen-containing part of amino acids in the liver.
Question 2
What waste product is formed?
Suggested answer
Urea.
Question 3
Why are some amino acids assimilated?
Suggested answer
They are used by cells to make proteins.
