Saprotrophic Nutrition: Digest Outside, Then Absorb
Definition
Saprotrophic nutrition
Saprotrophic nutrition is a mode of heterotrophic nutrition where organisms derive energy and nutrients by feeding on dead or decaying organic material.
Some fungi and bacteria feed on dead organic matter by external digestion.
They secrete enzymes such as proteases, lipases and cellulases onto the dead matter.
The enzymes break the matter into soluble products, which are then absorbed across the cell membrane.
This makes saprotrophs the main decomposers, recyclers that return nutrients to the ecosystem.
Note
Saprotrophs do not ingest solid food the way animals do.
Their digestion happens entirely outside the cells.
The Process: Secrete, Digest, Absorb
Enzyme secretion: the organism releases digestive enzymes onto the dead matter.
Proteases break proteins into amino acids.
Amylases and cellulases break carbohydrates into simple sugars.
Lipases break lipids into fatty acids and glycerol.
External digestion: these products are small and soluble, so they can cross the cell membrane.
Absorption: the organism absorbs the products and uses them for energy and growth.
Common Mistake
A common mistake is to say saprotrophs eat their food like animals.
Their digestion is external, and they absorb the soluble products directly.
Examples: Fungi and Bacteria as Saprotrophs
Fungi such as mushrooms grow on decaying wood and break down cellulose with secreted enzymes.
Moulds such as Rhizopus grow on bread and fruit.
Bacteria such as Bacillus and Pseudomonas decompose plant material in soil.
Example
Fungi and bacteria colonise a fallen leaf and secrete enzymes that break its cellulose into glucose.
They absorb the glucose for growth and leave the remaining nutrients in the soil.
Uses: Saprotrophs Put to Work
Composting uses fungi and bacteria to turn organic waste into nutrient-rich compost.
Bioremediation uses saprotrophic bacteria to break down pollutants such as oil.
Example
In oil-contaminated soil, Pseudomonas putida secretes enzymes that break hydrocarbons into simpler compounds.
Active recall
What does external digestion mean in saprotrophs?
Name three enzymes a saprotroph secretes and what each breaks down.
Why are saprotrophs called decomposers?
Give one named example of a saprotrophic fungus and one bacterium.
