13 2 Nutrients and Food Acquisition All physiological systems in animals rely on a supply of oxygen water salts and nutrients We have spent several weeks learning about how animals acquire deliver and regulate the supply of oxygen water and salts to the cells in their bodies Now we will turn to the acquisition delivery and regulation of nutrients Nutrients is a vague term that encompasses the substances necessary for cellular growth and function that are derived from the diet This includes vitamins minerals amino acids fatty acids and carbohydrates The digestive system of an animal is specialized for acquiring delivering and regulating the delivery of nutrients to the cells of the body Digestion can be broken down into assimilation which includes acquisition of nutrients digestion and absorption and egestion or excretion of undigested food Assimilation takes place along the gastrointestinal GI tract Like other systems of tubes in the body blood vessels nephrons the GI tract comprises many different cell types including secretory cells absorptive cells called enterocytes muscle cells and neurons Unlike other systems the GI tract is contiguous with the external environment Recall that during development of all animals with true tissues excludes Porifera gastrulation creates multiple cell layers In all Bilateria the result is three layers ectoderm mesoderm endoderm The endoderm folds inward forming a tube that bisects the animal s forming body This tube with openings at either end becomes the gastrointestinal tract The overall process of digestion is illustrated in the figure below The energy content of an animal s diet must match the metabolic demands of that animal The amount of energy contained in a particular molecule is described in units of kilocalories per gram One kilocalorie is the amount of energy needed to increase the temperature of one kg of H2O by 1 C Protein and carbohydrates have roughly 4 kcal g fat has roughly 9 kcal g Each of these macromolecules has potential energy in the form of chemical bonds which is converted into usable chemical energy ATP The macromolecular composition of an average human body by weight is shown below The caloric energy of each macromolecule is affected by its number of chemical bonds as well as where it enters the ATP producing process of cellular respiration Not all of this caloric energy will be used to make ATP Some food is indigestible or unmetabolizable and the energy it contains will be lost in the feces or urine Some energy is spent on the process of digestion itself as cells involved in digestion increase their metabolic rate during this process and thus consume more ATP than they otherwise would This energy expenditure is the specific dynamic action or the thermic effect of food Vitamins are a diverse group of unrelated molecules that have a wide range of functions Many vitamins act as cofactors for enzymes or other proteins For example vitamin A retinol is used to make the retinal chromophore that is bound to opsin in the photoreceptors of the retina Many vitamins are obtained directly from the diet or derived from dietary precursors but some are obtained from bacteria that live in the GI tract such as one form of vitamin K The 13 vitamins necessary in humans are shown in the table below Vitamins can be categorized based on polarity as either fat soluble or water soluble as this affects how they are metabolized Fat soluble vitamins are non polar and can diffuse directly across cell membranes Because of their direct access to cells high amounts of these vitamins in the diet can have adverse effects on physiology However they can also accumulate inside cells so consistent acquisition of fat soluble vitamins is less important than acquisition of water soluble vitamins Water soluble vitamins on the other hand are polar and cannot diffuse directly into cells Their access to cells depends on specific membrane transport proteins that absorb the vitamins Because they dissolve in water they are readily excreted and do not build up inside cells This prevents high levels from accumulating but it also requires that the animal take in fairly regular amounts Minerals are metallic elements that are part of protein structure such as Ca P Fe Cu and Zn Like water soluble vitamins most minerals are absorbed by specific transport proteins in the cells that line the GI tract Many of the nutrients that are ingested by an animal are eaten in the form of larger macromolecules that must be processed Animals have digestive enzymes many of which are released into the lumen of the GI tract that use H2O to hydrolyze chemical bonds and break macromolecules in ingested food into smaller units that can be absorbed by the enterocytes These digestive enzymes include proteases break down proteins into shorter polypeptides lipases break down triglycerides phospholipids into fatty acids amylases break down polysaccharides into oligosaccharides and nucleases break down DNA from ingested cells into nucleotides In many animals digestion is also aided by symbiotic organisms like bacteria fungi and photosynthetic organisms For example reef building corals require light because they are animals that are symbiotic with algae Populations of algae live in the gastrodermis of each coral polyp These photosynthetic organisms use light energy to fix CO2 from the water storing it as carbohydrates These photosynthetic products are passed directly to the animal cells in each polyp There are three main types of symbionts that assist with digestion Enterosymbionts live within lumen of GI tract often in an enlarged region called the cecum alternative spelling caecum Exosymbionts actively cultivated outside the animal s body Endosymbionts grow in interstitial spaces or within host s cells Amino acids are used to build new proteins Some of the 20 standard amino acids found in living cells can be synthesized by the animal These are called non essential amino acids About half of these must be obtained from the diet and are thus called essential amino acids Diets deficient in any of these essential amino acids especially during development can lead to defects and slow growth as they are necessary for forming enzymes cytoskeletal proteins motor proteins etc Proteins vary in the profile of amino acids they contain which makes some dietary proteins more complete than others Animal tissue provides a more complete source of essential amino acids than does plant tissue as some plants lack