BIOL 2108 Chapter 33

Animal Nutrition

Animal Diets

• An animal’s diet provides:

  • Chemical energy, which is converted into ATP powers cellular processes

  • Organic building blocks, such as organic carbon and organic nitrogen, which synthesize a variety of organic molecules

  • Essential nutrients, are required by cells and must be obtained from dietary sources; there are four classes:

    • Essential amino acids: animals can synthesize 10 of their required 20 amino acids from molecules in their diet, the remaining amino acids (essential amino acids) must be obtained from food in prefabricated form

    • Essential fatty acids: fatty acids are converted into a variety of cellular components, essential fatty acids cannot be synthesized by animals but can be synthesized by plants

    • Vitamins: organic molecules required in the diet in small amounts; grouped into fat-soluble and water-soluble vitamins

    • Minerals: simple inorganic nutrients, usually required in small amounts; ingesting large amounts can impair health

Dietary Deficiencies

• Deficiencies in essential nutrients can cause deformities, disease, and death

  • A diet with insufficient amounts of amino acids is the most common type of malnutrition among humans

  • Malnutrition can arise when a diet does not provide enough chemical energy

    • An undernourished individual will:

      • Use up stored fat and carbs

      • Break down its own proteins

      • Lose muscle mass

      • Suffer protein deficiency of the brain

      • Die or suffer irreversible damage

Food Processing

• Food processing can be divided into four stages:

  • Ingestion: the act of eating or feeding; strategies for extracting resources from food differ widely among animals

  • Digestion: breaking down food into molecules small enough to absorb; mechanical digestion, including chewing, increases the surface area of food, while chemical digestion, splits food into small molecules that can pass through membranes

    • Extracellular digestion is the breakdown of food particles outside of cells.

    • Animals with simple body plans have a gastrovascular cavity the functions in both digestion and distribution of nutrients

    • More complex animals have a complete digestive tract of alimentary canal with a mouth and an anus, with specialized regions to carry out digestion and absorption

  • Absorption: uptake of nutrients by body cells; most animals process food in specialized compartments to reduce risk of an animal digesting its own cells and tissue

  • Elimination: passage of undigested material out of the digestive system

Mammalian Digestive System

• The mammalian digestive system consists of an alimentary canal and accessory glands that secrete digestive juices through ducts

  • The accessory glands are the salivary glands, pancreas, liver, and gallbladder

The Oral Cavity, Pharynx, and Esophagus

• The first stage of digestion is mechanical and takes place in the oral cavity

  • Salivary glands deliver saliva to the oral cavity through ducts

  • Teeth chew food into smaller particles that are exposed to salivary amylase, initiating the breakdown of glucose polymers

  • Saliva also contains mucus, a viscous mixture of water, salts, cells, and glycoproteins

  • The tongue shapes food into a bolus and provides help with swallowing

• The throat, or pharynx, is the junction that opens to the esophagus and the trachea

  • Each time you swallow, a flap of cartilage covers your vocal cords and the opening between them to prevent choking

  • The esophagus conducts food from the pharynx down to the stomach through rhythmic cycles of contraction, or peristalsis

    • Valves called sphincters regulate the movement of material between compartments

Digestion in the Stomach

• The stomach stores food and secretes gastric juice, which converts a meal to a chyme, a mixture of food and digestive juice

  • Gastric juice is made up of HCl and pepsin, a protease, or protein-digesting enzyme, that cleaves proteins into smaller peptides

  • It has a low pH of 2, which kills most bacteria and denatures proteins

  • Mucus protects the stomach lining from gastric juice

• Coordinated contraction and relaxation of stomach muscle churn the stomach’s contents and help move material through the alimentary canal

  • Stomach contents typically pass into the small intestine 2-6 hours after a meal

Digestion in the Small Intestine

• The small intestine, named for its small diameter, is the major organ of digestion and absorption

  • The first portion, the duodenum, is where chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestinal wall

  • The pancreas secretes bicarbonate, which neutralizes the acidity of chyme and produces proteases trypsin and chymotrypsin, which are activated in the lumen of the duodenum

    • The epithelial lining of the duodenum is the source of additional digestive enzymes

  • Bile aids in the digestion and absorption of fats

    • It is made in the liver and stored and concentrated in the gallbladder

    • It also destroys nonfunctional red blood cells

Absorption in the Small Intestine

• The small intestine has a huge surface area due to villi and microvilli that project into the intestinal lumen creating a brush border that greatly increases the rate of nutrient absorption

  • Transport across the epithelial cells can be passive or active depending on the nutrient

• The hepatic portal vein carries nutrient-rich blood from the capillaries of the villi to the liver, then to the heart

• The liver regulates nutrient distribution, interconverts many organic molecules, and detoxifies many organic molecules

• Epithelial cells absorb fatty acids and monoglycerides and recombine them into triglycerides

  • These fats are coated with phospholipids, cholesterol, and proteins to form water-soluble chylomicrons

    • Chylomicrons are transported into a lacteal, a lymphatic vessel in each villus; these vessels deliver chylomicron-containing lymph to large veins that return blood to the heart

Processing in the Large Intestine

• The alimentary canal ends with the large intestine

• The small intestine connects to the large intestine at a T-shaped junction; one arm is the colon, which leads to the rectum, and the other arm is the cecum, which aids in the fermentation of ingested material

  • The human cecum has an extension called the appendix, which serves as a reservoir for symbiotic microorganisms

  • The colon recovers water that has entered the alimentary canal, what remains are the feces, which becomes solid as it moves along the colon

  • Fiber helps move food along the alimentary canal

  • Feces are stored in the rectum until they can be eliminated through the anus

  • Two sphincters between the rectum and anus control bowel movements

Evolutionary Adaptations of Vertebrate Digestive Systems

• Dental Adaptations

  • Dentition, an animal’s assortment of teeth, is key to the success of mammals

  • Nonmammalian vertebrates usually have less specialized teeth

• Stomach and Intestinal Adaptations

  • Many carnivores have large, expandable stomachs and shorter digestive systems than herbivores and omnivores, who usually have longer alimentary canals

Energy Allocation

• The flow and transformation of energy in an animal—its bioenergetics—determine nutritional need

• An animal’s energy use per unit of time is called its metabolic rate, which can be determined by monitoring an animal’s rate of heat loss, the amount of oxygen consumed, or the amount of carbon dioxide produced

Minimum Metabolic Rate

• Animals maintain a minimum metabolic rate for basic cell functions

• Basal metabolic rate, BMR, is the minimum metabolic rate of a nongrowing endotherm that is at rest, has an empty stomach, and is not experiencing stress

• The metabolic rate of a fasting, not stressed ectotherm at a particular temperature is called standard metabolic rate, SMR

Regulation of Energy Storage

• When an animal takes in more energy than is needed for metabolism and activity, excess energy is stored

  • In humans, the liver and muscle cells are used first; energy is stored as glycogen

  • When glycogen depots are full, additional excess energy is stored as fat in adipose cells

  • When fewer calories are taken in than expended, the body expends liver glycogen, muscle glycogen, and then fat

Glucose Homeostasis

• Insulin and glucagon together maintain glucose levels (pancreas in beta cells and alpha cells, respectively)

  • Insulin levels rise after carb-rich meals and glucose entering the liver through the hepatic portal vein is used to synthesize glycogen

  • When glucose concentration is low in the hepatic portal vein, glucagon stimulates the liver to break down glycogen and release glucose in the blood