NUTRITION AND BIOCHEMISTRY
OF PARASITES
n 6.1 THE PARASITIC ADVANTAGE
Parasites have the unique advantage over free-living animals in that they live entirely
surrounded by their food. The methods of feeding have been adapted to the parasites’
habitat. In most cases the food has already been digested into a soluble form available for
absorption. Hence some of the helminth groups have dispensed with a gut altogether (the
cestodes) and absorb their nutrients through the outer covering (the tegument). Others
have a gut but also absorb food through the tegument (the trematodes), while there are
some (the nematodes) that only absorb food via their mouths.
The direct uptake of soluble molecules through either a tegument or a membrane is a
physiological process similar to that of the uptake of solutes by mammalian gut epithelial
cells. The process of absorption centres round the following mechanisms:
n Simple diffusion: molecules are absorbed passively through the membrane: a process
regulated by the movement of molecules from a higher concentration into the para-
site’s cells or tissues with a lower concentration.
n Active transport: absorption whereby molecules pass into the parasite against a
concentration gradient. This process requires energy and can be inhibited by sub-
stances that interfere with, or inhibit, respiration. This system may involve a ‘carrier’
molecule and the simultaneous movement of sodium ions.
n Facilitated or mediated diffusion: movement into the parasite by molecules first ‘con-
jugated’ to a carrier molecule and then absorbed through the membrane. The solute
is released once it has crossed the membrane. The carrier is a locus on the membrane
to which the solute binds to and is then released on the other side of the membrane
where the concentration is lower.
n Pinocytosis (endocytosis and exocytosis): large molecules such as proteins are trans-
ported into the tissues within membrane-bound vesicles.
n Contact digestion: there is evidence to show that absorption of nutrients involves
enzymes. These are either intrinsic, ie of parasite origin, or extrinsic, ie of host ori-
gin. The intrinsic enzyme phosphohydrolase helps phosphate esters to be absorbed
through the tegument. Fructose 1,6-diphosphate is hydrolysed at the tegument sur-
face to release inorganic phosphate that is absorbed while the detached fructose is
unable to pass through the tegument.