Learning focus
Build precise biological explanations, interpret diagrams and data, and connect structure, process, health and practical evidence.
Definition
Excretion is the removal from organisms of toxic materials and waste products of metabolism. Egestion is removal of undigested food from the alimentary canal and is not excretion because the material has not been produced by cell metabolism.
Carbon dioxide
Carbon dioxide is produced in aerobic respiration. It diffuses from cells into blood, is carried to the lungs and diffuses into alveoli for exhalation. Accumulation would alter blood pH and disrupt enzymes.
Urea
Excess amino acids cannot be stored. In the liver, their nitrogen-containing part is removed and converted into urea. Urea is toxic at high concentration and is carried in blood to the kidneys.
Water and ions
Water and ions are essential but become waste when present in excess. Kidneys adjust their loss in urine. Sweat also contains water and ions, but its main biological role is temperature control.
Transport and excretion
Excretion depends on circulation. Wastes move from cells to blood, then to lungs or kidneys. The liver produces urea and the kidneys remove it, so several organs cooperate.
Practical or data skill
Classify examples as excretion, egestion or secretion and justify each answer using whether the material is a metabolic waste.
Examination tip
Urea is formed in the liver and removed from blood by the kidneys.
Review questions and suggested answers
Question 1
Define excretion.
Suggested answer
Removal of toxic materials and waste products of metabolism from an organism.
Question 2
Why is egestion not excretion?
Suggested answer
Undigested food has not been produced by metabolic reactions in cells.
Question 3
Where is urea produced?
Suggested answer
In the liver.
