Summary

Nutrition

• Essential organic nutrients are organic compounds that animals must obtain from other organisms because the animals are biochemically unable to synthesize them. About ten of the standard amino acids required for protein synthesis are essential in most sorts of animals. Omega-3 and omega-6 fatty acids are essential in many animals, and vitamins are essential.
• Proteins often present particular nutritional problems because nitrogen can be in short supply in ecosystems and essential amino acids may be unavailable in the amounts needed. Because animals employ a “just in time” strategy for protein synthesis (rather than storing amino acids for future use), the essential amino acids must be eaten together in the amounts they are needed if they are to be used. A shortage of any one essential amino acid can cause wastage of available supplies of others.
• Vitamins are very diverse in their chemical structures. What they have in common is not their chemical nature but the fact that they are organic compounds required in small (often minute) amounts. Most are used as substructures in molecules of critical importance for animal function. The water-soluble vitamins, notably the B vitamins, are essential for most or all animals. The lipid-soluble vitamins, such as vitamins A and K, are more specialized in their functions and not as universally required.
• Minerals are also essential nutrients. Metal atoms occur in approximately 40% of proteins. Minerals are also required as constituents of body fluids and skeletons. More than 20 chemical elements are required for the construction of animal bodies.
• Structural carbohydrates such as chitin, cellulose, and hemicelluloses are the most abundant organic compounds on Earth, but many animals lack enzymes required to digest them and thus are unable (on their own) to tap those carbohydrates for nutritional value.

Feeding

• One mode of feeding is for animals to target and ingest individual food items, such as when orcas or cone snails catch fish. Toxic compounds, such as venoms and secondary compounds, often play roles as weapons or defenses in feeding of this sort.
• Suspension feeding is a second major mode of feeding. In this mode, an animal feeds on living or nonliving food items, suspended in water, that are individually tiny in comparison with the animal and that typically are collected in numbers rather than being individually targeted. Suspension feeding permits animals to feed lower on food chains and thus gain access to higher food productivity. Many of the largest and most productive animals on Earth are suspension feeders.
• Symbiotic associations with microbes are a third major mode of feeding in animals. Some animals maintain symbiotic associations with algae; warm-water reef corals, which are symbiotically associated with dinoflagellate zooxanthellae, are prominent examples. Other animals, notably species in hydrothermal-vent communities, are symbiotically associated with chemosynthetic autotrophs. Many herbivorous animals, both vertebrate (e.g., ruminants) and invertebrate (e.g., lower termites), maintain specialized symbiotic associations with heterotrophic, anaerobic fermenting microbes.
• Fermenting microbes most commonly provide three sorts of nutritional advantages to their animal hosts. They synthesize vitamins and essential amino acids. They break down structural polysaccharides such as cellulose so that the host can gain food value from them. They permit waste nitrogen to be recycled for use in animal protein synthesis.
• Apart from specialized symbioses with heterotrophic microbes, biologists now recognize that large assem-blages of bacteria—termed the gut microbiome—are found in the guts of all mammals and probably most other animals. The functions of these assemblages are only starting to be understood.

Digestion and Absorption

• Digestion is the process of splitting food molecules into smaller parts that an animal can take into its living tissues and distribute throughout its body. Absorption (assimilation) is the process of taking organic compounds into the living tissues of an animal from the gut lumen or from other places outside those tissues. In vertebrates, arthropods, and some other groups, digestion precedes absorption. However, absorption precedes digestion in certain other animals, such as certain bivalve molluscs.
• Digestion is carried out mostly by hydrolytic enzymes, each of which catalyzes the splitting of specific types of chemical bonds. The enzymes may be intraluminal, membrane-associated, or intracellular. Extracellular digestion is carried out by intraluminal and membrane-associated enzymes, whereas intracellular digestion is carried out by intracellular enzymes.
• The absorption of relatively simple hydrophilic compounds—such as monosaccharides, amino acids, and water-soluble vitamins—usually requires transporter proteins in the cell membranes involved, and it occurs by either facilitated diffusion or secondary active transport. The absorption of the hydrophobic fatty acids and monoacylglycerols produced by lipid digestion can occur to a large extent by simple diffusion across cell membranes. However, complexities arise in the absorption of these compounds because (being hydrophobic) they need to be emulsified in the aqueous solutions outside and inside cells while being absorbed. SCFAs are a special case because they are water- and lipid-soluble.
• In vertebrates, the midgut is typically the most important site of digestion and absorption. This is true because (1) the apical membranes of midgut epithelial cells are richly populated by membrane-associated digestive enzyme proteins and by transporter proteins, and (2) pancreatic and biliary secretions enter the midgut.
• An animal’s digestive and absorptive capabilities are major determinants of the nutritional value of foods because ingested organic compounds can be used only to the extent that they can be digested and absorbed.