Extracellular digestion
Extracellular phototropic digestion is a process in which saprobionts feed by secreting enzymes through the cell membrane onto the food.
During extracellular digestion, food is broken down outside the cell either mechanically or with acid by special molecules called enzymes.
For fungi to gain their energy needs, they find and absorb organic molecules appropriate to their needs, either immediately or following some form of enzyme diminution outside the thallus.
The fungi that utilize a variety of energy sources usually absorb the simplest compounds first, then the more complex.
On depletion of primary sources of glucose, enzymes to degrade more complex molecules such as cellulose and starch, are then released.
As a consequence, fungi specifically target the breakdown of the cellulose in their environment, and do not waste energy on the unnecessary formation of enzymes for degradation of molecules that may not be present.
Because of the huge range of potential food sources, fungi have evolved enzymes suitable for the environments in which they are usually found.
Further, utilization of a common and abundant substrate has led many fungi to evolve a range of highly specific degradative enzymes.
Some enzymes are actively excreted through the plasma membrane, where they diffuse through or act in the cell wall.
Enzymes are secreted from the cells bordering this cavity and poured on the food for extracellular digestion.
In this case the digestive enzymes are released into a cavity that is continuous with the animal's external environment.
Specializing occurs when the digestive tract or alimentary canal has a separate mouth and anus so that transport of food is one-way.
The ingested food may be stored in a specialized region of the digestive tract or subjected to physical fragmentation.
Chemical digestion then occurs, breaking down the larger food molecules of polysaccharides and disaccharides, fats, and proteins into their smallest sub-units.
Chemical digestion involves hydrolysis reactions that liberate the sub unit molecules—primarily monosaccharides, amino acids and fatty acids—from the food.
These products of chemical digestion pass through the epithelial lining of the gut into the blood, in a process known as absorption.
All free-living species exhibit a distinct and separate mouth and anus, and in all species, food must be moved through the digestive tract by muscular activity rather than cilia activity since the lumen of the fore gut and hind gut is lined with cuticle.
The absorbed nutrients enter the circulatory system for distribution throughout the body or are stored in the digestive glands for later use.
[11] The initial components of the gastrointestinal tract are the mouth and the pharynx, which is the common passage of the oral and nasal cavities.
