Absorption is the passage of molecules and ions from the gut lumen into the gut cells, thus traversing the cuticle (if present) and the cell plasma membrane. Absorption depends on the permeability of those barriers and on the concentration ratio of a compound in gut lumen and inside gut cells. The permeability of cuticles is variable, whereas that of the plasma membrane is greater for water (a fact not well understood) and for hydrophobic compounds, reflecting the ease with which they solubilize in the lipid bilayers characteristic of cell membranes. Thus, absorption of hydrophilic compounds requires special devices (transporters) to help the molecules find their way through the cell membrane. These transporters are transmembrane proteins that bind the molecule to be transported in a membrane face and, after suitable conformational changes, deliver the molecule from the other face. A uniporter is a transporter that carries a single solute, whereas symporters and antiporters are transporters that carry two solutes into the same and opposite directions, respectively. To transport molecules against a concentration gradient, the process must be energized by coupling with ATP hydrolysis or with cotransport of another molecule down its concentration gradient. Transporter-mediated absorption may be inhibited by molecules resembling those of the transported solute, and its velocity attains a maximum (transporter becomes saturated) at a high solute concentration. This behavior is not observed in the case of simple diffusion, exemplified by lipid absorption.
The study of gut absorption in insects is difficult because of the small size of these animals, which frequently hinders the use of methods developed for studying vertebrates. Absorption sites in insect guts are identified by feeding groups of insects with known dye solutions and then dissecting insects at different periods of time. If the insect is large enough, absorption studies can go further, using gut sections mounted as a sac and measuring the rates at which compounds traverse it under different conditions.
Tracer studies showed that lipid is absorbed more heavily in the anterior than in the posterior midgut of insects, thus following the tissue distribution of fatty acid binding proteins. These proteins are thought to facilitate fatty acid uptake by cells, by decreasing their diffusion back from cells to the gut lumen and by targeting them to specific metabolic pathways. The fatty acids acetate and butyrate are absorbed in significant amounts by the hindgut of insects utilizing cellulose.
Water absorption in the midgut occurs associated with midgut fluid fluxes, but in large amounts it is characteristic of insects feeding on dilute diets, of blood feeders, and also of insects in which salivation is important (e.g., grasshoppers, seed-sucker bugs). Water absorption in the hindgut is part of the water conservation mechanism that is important in all terrestrial insects. Frequently, special cell aggregates (rectal pads) are involved in this process. Water uptake is thought to depend on salt being pumped into spaces enclosed by the basolateral infoldings (Fig. 3A, C) of the absorbing cell. This creates an osmotic pressure that moves water into these restricted spaces. The resulting hydrostatic pressure drives water into the hemolymph, with salts being absorbed from the water on its way out. In insects feeding on dry diets in dry habitats (e.g., T. molitor, D. maculatus), hindgut water absorption is improved by the cryptonephridial system, which consists of an association of Malpighian tubules and rectal pads. Less sophisticated forms of this system are found in other insects.
Ions, Amino Acids, and Sugars
In insects, as is usual for all animals, most nutrient absorption occurs in the midgut through symporters, with ions being cotransported down the concentration gradient. Favorable ion gradients are maintained by ion pumps. The most ubiquitous of these pumps is the ATP-driven Na+, K+-antiporter (Na+-K+-ATPase) localized in the midgut cell basal membrane. Another important ion pump is the H+, K+-ATPase found in the goblet cell (Fig. 3B, E) microvillar membranes of lepidopteran larvae. Those pumps maintain cell Na+ and cell K+ low in insects with Na+-rich and K+-rich diets, respectively. The midgut of lepidopteran larvae contains K+-dependent symporters for amino acids, consistent with the K+-rich plant diets of these larvae. Such symporters are relatively insensitive to amino acid sizes and shapes, but have narrow specificities towards charge. Thus, there are usually symporters for neutral, acid, and basic amino acids. Insects with high-Na+ diets seem to have midgut amino acid symporters dependent on Na+, as shown in cockroaches. The absorption of glucose was shown in several insects to depend on a difference of concentration between midgut lumen and cells. This transport increases as the concentration of luminal glucose increases (no saturation) and is not inhibited by molecules similar to glucose, at least in the range of concentrations tested. This finding led to the speculation that, in insects, glucose is absorbed by simple diffusion. Nevertheless, this is probably false, since a hydrophylic molecule is not expected to pass through membranes without the help of a transporter.
The primary urine produced in Malpighian tubules contains salts and amino acids, and passes into the hindgut together with food remains. Salts are absorbed in the hindgut by means of special pumps, like those for chloride and calcium. Amino acids, at least in locusts, are absorbed in the hindgut through a Na+-dependent amino acid symporter.
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