Cestodes are not able to synthesise cholesterol. In order to overcome this deficiency
solutions of mono-olein and sodium taurocholate are absorbed. The tapeworm H. dimin-
utahas an uptake site for a short-chain (C 2 –C 8 ) and a separate one for long-chain
(C 14 –C 24 ) fatty acids. This system can only function in the presence of the host’s bile
salts. H. diminutacannot synthesise long-chain fatty acids but can lengthen the chain of
absorbed fatty acids by adding acetate units.
Parasites that are closely related phylogenetically do not necessarily have similar
feeding mechanisms. The absorption and uptake of nutrients are often determined by the
nature of the habitat, for example Fasciola hepaticaand Schistosoma mansoniare both
trematodes but one lives in the bile duct and the other in mesenteric blood vessels.
However, F. hepaticaabsorbs amino-acids by passive diffusion whereas S. mansoniappar-
ently uses an active transport system to absorb amino-acids.
Feeding mechanisms are often adapted to their immediate environment as illustrated
by the nematode Ancylostoma duodenale(a hookworm). Like all hookworms A. duodenale
is a gut-dwelling nematode that is equipped with biting mouth parts and apparently can
secrete an anti-clotting agent to prevent the host’s blood from clotting before it can be
swallowed.
n 6.2.2 ENERGY STORAGE
All parasites need a supply of energy to maintain and regulate their metabolism. It
has been established that many parasitic helminths can function for limited periods under
anaerobic conditions. All require a source of reduced organic compounds and a mechan-
ism for the release and capture of energy (see Fig. 6.1). In most helminths the energy is
stored as glycogen and, in the protozoans Plasmodiumand Trypanosoma, carbohydrate is
the main energy store whereas in Entamoebait is probably glycogen.
The parasites investigated from a biochemical point of view indicate that they conform
to most of the established biochemical pathways. All of the parasites observed oxidise
glucose by the same glycolytic pathway as that encountered in free-living organisms.
The oxidation pathway that produces phosphoenolpyruvate (PEP) is common to nearly
all parasites but the conversion of PEP to pyruvate and further degradations are not
common. Some follow the mammalian patterns, other do not. In mammals PEP is con-
verted to pyruvate and then normally passes to the tricarboxylic acid cycle. This cycle is
far less active in parasites and when it does operate, it does so at a very low level and
appears to be far less important to parasites than to mammals. One of the reasons for
this is that parasites do not need to expend much energy in their search for food.
6.2.2.1 Energy for reproduction
The helminths have, in general, complex sets of reproductive organs that produce
numerous eggs with protective sets of membranes and shells. In many of the nematodes,
the females give birth to live larvae. Adult cestodes, trematodes and nematodes have a
high fecundity rate and some of the metacestodes reproduce asexually. Most energy
production, whether it be from an aerobic or anaerobic source, is required to fuel the
reproductive process. In addition there are added demands such as the production of hatch-
ing enzymes and penetrating enzymes.
n Many of the passive stages such as cysts, metacercaria and eggs require the appro-
priate external stimulus before they develop any further.
n Schistosome eggs need a temperature lower than of the host, together with both light
and water in order to hatch.NUTRITION AND BIOCHEMISTRY OF PARASITES