41.1 Nutritional Requirements

• A carbohydrate, protein, or fat is an example of a nutrient: a substance that an organism needs to remain alive.

• Food is any material that contains nutrients.

• Although all nutrients are necessary for the growth and survival of animals, essential nutrients are those that cannot be synthesized and must be obtained from the diet.

• Humans require four classes of essential nutrients:

  • Essential amino acids are amino acids that an animal cannot synthesize from simpler building blocks.

  • Essential fatty acids are fatty acids that an animal must obtain in its diet.

  • Vitamins are organic, or carbon-containing compounds at are vital for health but are required in only minute amounts.

  • Minerals are inorganic substances used as components of enzyme cofactors or structural materials.

41.2 Capturing Food: The Structure and Function of Mouthparts

• The cichlids that inhabit the Rift Lakes of East Africa a spectacular example of adaptive radiation-the diversification of a single ancestral lineage into many species, each of which lives in a different habitat or exhibits a distinct form.

41.3 The Structure and Function of Animal Digestive Tracts

• Salivary amylase, the enzyme responsible for carbohydrate digestion in the mouth, is one of 1e best-studied enzymes.

• Salivary glands in the mouth secrete amylase and also produce the slimy substance called mucus.

• Cells in the tongue synthesize and secrete another salivary enzyme, lingual lipase, which begins the digestion of lipids by breaking triglycerides, a common form of fat in the diet, into diglycerides and fatty acids.

  • Lingual lipase plays only a minor role in digestion, mostly once it has been swallowed into the stomach.

• Once a bolus of food is swallowed, it enters a muscular tube called the esophagus, which connects the mouth and stomach.

• In response to nerve signals, the smooth muscles in the esophagus con act and relax in a coordinated fashion called peristalsis.

• In an array of bird species, the esophagus has a prominent, widened segment called the crop where food can be stored and, in some cases, processed.

• The stomach is a tough, muscular pouch in the digestive tract, bracketed on both the superior and inferior ends by ringlike muscles called sphincters, which control the passage of material.

• In cattle, sheep, goats, deer, antelope, giraffe, and pronghorn-species that are collectively called ruminants-the stomach is specialized for digesting cellulose instead of proteins.

• The small intestine is a long tube 1at is folded into a compact space within the abdomen.

  • In the small intestine, partially digested food mixes with secretions from the pancreas, liver, and gallbladder and begins a journey of about 6 m (20 ft).

41.4 Nutritional Homeostasis-Glucose as a Case Study

• The illness of diabetes mellitus is a classic example of nutrient imbalance. People with diabetes mellitus have an abnormally high level of glucose in their blood because cells cannot import glucose.

• Insulin is a hormone that is secreted by cells in the pancreas when the blood glucose level is elevated.

• If the blood glucose level falls too low, as it does when an animal has not eaten for a while, other cells in the pancreas secrete a hormone called glucagon.

  • In response to glucagon, liver cells catabolize stored glycogen and produce glucose via the process of gluconeogenesis, the synthesis of glucose from non-carbohydrate compounds.

  • As a result, the glucose level in the blood rises.

• The researchers found a strong relationship between the incidence of type 2 diabetes mellitus and body mass index (BMI), a measure of obesity.

  • BMI is calculated as body mass (in kilograms) divided by height (in meters) squared.

  • A person with a BMI of 30 or higher is considered obese and has a far greater risk of developing type 2 diabetes mellitus than people with lower BMIs.