EXCRETION

MRS GREN

WHAT IS EXCRETION?

• The removal of waste products from an organism (excretory products)• Waste products of metabolism (incl. Respiration)• Toxic materials• Substances in excess

EGESTION VS. EXCRETION

• Egestion is: - the passing out of food that has not been digested, as faeces (through the anus)

• Humans cannot digest cellulose, but it is not an EXCRETORY product. Why?

• Excretion is the removal of waste products, because cellulose is not digested, it goes straight through the alimentary canal and out through the anus.

• Not involved in any metabolic reactions.

NITROGENOUS WASTE (UREA, AMMONIA, URIC ACID)• Nitrogenous waste is formed from excess proteins and amino acids.• This excess waste cannot be stored in animals, so surplus is broken

down to a nitrogen-containing excretory product.  In mammals – UREA.• Urea is a toxic substance formed in the liver and removed by the

kidneys.

UREA 

• We eat proteins --> enzymes in stomach, duodenum, and ileum break them down

• Proteins --> amino acids, absorbed.  Absorbed food taken to liver• From liver, some amino acids continue to other parts of the body• If excess, must be gotten rid of• (Excreting amino acids would be wasteful, because they contain

energy)

IN THE LIVER

• In the liver, enzymes split the extra amino acids• The parts containing energy are kept --> turned into carbohydrates and stored

The other parts (containing nitrogen) ammonia are turned into urea. DEAMINATIONA metabolic reaction that takes place in the liver, in which the nitrogen-containing part of amino acids is removed to form urea, plus the release of energy from the remainder of the amino acids. 

UREA

• This urea dissolves into the blood plasma, taken to kidneys for removal, excretion.

• NOTE. A small amount is excreted as sweat

WHAT IS THE LIVER FOR?

• Deamination• Makes plasma proteins (blood clotting)• Controls glucose levels• Stores carbohydrates• Makes bile (aids digestion of lipids)• Breaks down old RBC• Breaks down toxic substances – alcohol• Stores certain vitamins, and potassium• Makes cholesterol – make and repair cell membranes