Nutrient cycles (A-level only): The role of saprobionts in decomposition.

The idea

Saprobionts, also called saprobiotic or saprophytic bacteria and fungi, feed on dead organic material and on faeces and urine. They cannot ingest solid food, so they use extracellular digestion: hydrolytic enzymes, including proteases, lipases, carbohydrases and nucleases, are secreted out of the cell onto the substrate, the enzymes hydrolyse the polymers there, and the soluble products, amino acids, sugars, fatty acids and glycerol, are absorbed across the cell surface membrane by diffusion and active transport. The nitrogen in those amino acids is surplus to the saprobiont's needs, so it is deaminated and released as ammonia, which dissolves to give ammonium ions, and that step is ammonification. Phosphate is released in the same way from nucleic acids and phospholipids.

What earns the marks

• Saprobionts feeding on dead organisms, faeces or urine.
• Secreting enzymes onto the material, that is extracellular digestion.
• Naming a hydrolytic enzyme, or the hydrolysis of a named polymer.
• Absorbing the soluble products across the cell surface membrane.
• Releasing ammonia or ammonium ions, and phosphate, back into the soil.

Common mistakes and how to put them right

• Saying saprobionts eat or ingest dead material, when they digest it outside the cell and absorb the products.

• Treating saprophyte as though it meant a plant, when the organisms are bacteria and fungi.

• Describing decomposition as rotting without naming enzymes, hydrolysis or the ions released.

• Confusing saprobionts with mycorrhizae, which are mutualistic with living roots rather than feeding on dead tissue.

• Omitting the absorption step, so the products of digestion never enter the organism.

Exam tips

• The word examiners look for is extracellular: enzymes are secreted onto the material, then the products are absorbed.
• Name at least one enzyme and one ion released; a general 'they break down dead things' rarely scores more than one mark.
• Saprobiont, saprobiotic and saprophyte are all accepted terms, so use whichever you remember.