Chapter 41: Animal Nutrition

Animals are heterotrophs that require food for fuel, carbon skeletons, and essential nutrients.

Animals have basal energy requirements that must be met to maintain metabolic functions for sustaining life. In humans, the liver and muscle cells store energy in the form of glycogen. If glycogen stores are full and caloric intake still exceeds caloric expenditure, the excess may be stored as fat. When fewer calories are taken in than are expended, fuel is taken out of storage and oxidized, and weight loss may occur. Humans generally expend liver glycogen first, and then draws on muscle glucose and fat. An UNDERNOURISHED person or other animal is one whose diet is deficient in calories. An OVERNOURISHED person’s body tends to hoard fat molecules obtained from food instead of using them for fuel or biosynthesis.

An animal’s diet must supply essential nutrients and carbon skeletons for biosynthesis. An animal whose diet is missing one or more essential nutrients is said to be MALNOURISHED. There are four classes of essential nutrients: essential amino acids, essential fatty acids, vitamins and minerals.

Animals can synthesize most of the fatty acids they need. Most diets provide ample amounts, and thus deficiencies are rare.

Vitamin and mineral requirements are usually small, and vary per species.

SUSPENSION-FEEDERS sift small food particles from the water.

SUBSTRATE-FEEDERS live in or on their food source, eating their way through the food.

EARTHWORMS are also substrate-feeders. As they eat their way through the soil, they salvage partially decayed organic material consumed along with soil.

FLUID-FEEDERS suck nutrient-rich fluids from a living host. Mosquitoes & leeches harm their host, they are considered parasites. Hummingbirds and bees benefit their host plants.

BULK-FEEDERS eat relatively large pieces of food. Their adaptations include such diverse utensils as tentacles, pincers, claws, poisonous fangs, and jaws an teeth that kill their prey or tear off pieces of meat or vegetation.

An Overview of Food Processing The four main stages of food processing are ingestion, digestion, absorption, and elimination.

Intracellular Digestion vs. Extracellular digestion

Most animals have complete digestive tracts, or ALIMENTARY CANALS. Because food moves in one direction, the tube can be organized into specialized regions.

The Mammalian Digestive System

The Oral Cavity Physical and chemical digestion of food both begin in the mouth The presence of food in the oral cavity triggers a nervous reflex that causes the salivary glands to deliver saliva through ducts to the oral cavity. Saliva contains SALIVARY AMYLASE, a digestive enzyme that hydrolyzes starch (a glucose polymer from plants) and glycogen (a glucose polymer from animals.) The main products of this enzyme’s action are smaller polysaccharides and the disaccharide maltose. The tongue tastes food, manipulates it during chewing, and helps shape the food into a ball called a BOLUS. During swallowing, the tongue pushes a bolus into the pharynx

The Pharynx The pharynx is the region we call our throat. It leads to both the trachea (windpipe) and the esophagus. When we swallow, the top of the windpipe moves up and is covered by a cartilaginous flap called the epiglottis. This action ensures that when we swallow, the bolus will be guided into the entrance of the esophagus (and that food and fluids do not enter the respiratory system.)

The Esophagus The esophagus conducts food from the pharynx down to the stomach. PERISTALSIS (rhythmic waves of contraction by smooth muscle) squeezes a bolus along the narrow esophagus. The muscles at the top are voluntary (swallowing begins voluntarily), but then involuntary waves of contraction by smooth muscle take over.

The Stomach The stomach is located on the left side of the abdominal cavity, just below the diaphragm. The epithelium that lines the stomach secretes GASTRIC JUICE. Gastric juice has a high concentration of hydrochloric acid, which gives it a pH of about 2. Also present in gastric juice is PEPSIN, which is an enzyme that begins the hydrolysis of _____. Each end of the stomach is closed of from the rest of the digestive system by sphincters. They remain contracted until they are stimulated to dilate.

The Small Intestine The small intestine has a length of more than 6m in humans. It is the longest section of the alimentary canal. Most enzymatic hydrolysis of macromolecules in food occurs here. It is also responsible for the absorption of most nutrients into the blood. The Large Intestine A major function of the colon is to reabsorb water that has entered the alimentary canal as the solvent of the various digestive juices. With the help of the small intestine, about 90% of the water that entered the alimentary canal is recovered. The remaining wastes, the FECES, become more solid as they move along the colon by peristalsis. The terminal portion of the colon is the RECTUM, where feces are stored until they can be eliminated. Between the rectum and the anus are two sphincters. One involuntary and the other voluntary.

To enter the body, nutrients that accumulate in the lumen when food is digested must cross the lining of the digestive tract. The lining of the small intestine has a huge surface area, which is an adaptation well suited to the task of absorbing nutrients.

Hormones help regulate digestion

Structural adaptations of the digestive system are often associated with diet.

Symbiotic microorganisms help nourish many vertebrates: Ruminant digestion.