Animal nutrition

Chapter 41
Animal Nutrition
Overview: The Need to Feed
Every mealtime is a reminder that we are heterotrophs
Dependent on a regular supply of food
Figure 41.1
In general, animals fall into one of three dietary categories
Herbivores eat mainly autotrophs (plants and algae)
Carnivores eat other animals
Omnivores regularly consume animals as well as plants or algal matter
Regardless of what an animal eats, an adequate diet must satisfy three nutritional needs
Fuel for all cellular work
The organic raw materials for biosynthesis
Essential nutrients, substances such as vitamins that the animal cannot make for itself
Animals feed by four main mechanisms
Figure 41.2
Concept 41.1: Homeostatic mechanisms manage an animal’s energy budget
Nearly all of an animal’s ATP generation
Is based on the oxidation of energy-rich molecules: carbohydrates, proteins, and fats
Glucose Regulation as an Example of Homeostasis
Animals store excess calories
As glycogen in the liver and muscles and as fat
Glucose is a major fuel for cells
Its metabolism, regulated by hormone action, is an important example of homeostasis
Figure 41.3
STIMULUS:
Blood glucose
level rises
after eating.
Homeostasis:
90 mg glucose/
100 mL blood
STIMULUS:
Blood glucose
level drops
below set point.
When fewer calories are taken in than are expended
Fuel is taken out of storage and oxidized
Caloric Imbalance
Undernourishment
Occurs in animals when their diets are chronically deficient in calories
Can have detrimental effects on an animal
Overnourishment
Results from excessive food intake
Leads to the storage of excess calories as fat
Figure 41.4
Obesity as a Human Health Problem
The World Health Organization
Now recognizes obesity as a major global health problem
Obesity contributes to a number of health problems, including
Diabetes, cardiovascular disease, and colon and breast cancer
Researchers have discovered
Several of the mechanisms that help regulate body weight
Over the long term, homeostatic mechanisms
Are feedback circuits that control the body’s storage and metabolism of fat
Several chemical signals called hormones
Regulate both long-term and short-term appetite by affecting a “satiety center” in the brain
Figure 41.5
The complexity of weight control in humans
Is evident from studies of the hormone leptin
Mice that inherit a defect in the gene for leptin
Become very obese
Figure 41.6
Obesity and Evolution
The problem of maintaining weight partly stems from our evolutionary past
When fat hoarding was a means of survival
A species of birds called petrels
Become obese as chicks due to the need to consume more calories than they burn
Figure 41.7
Concept 41.2: An animal’s diet must supply carbon skeletons and essential nutrients
To build the complex molecules it needs to grow, maintain itself, and reproduce
An animal must obtain organic precursors (carbon skeletons) from its food
Besides fuel and carbon skeletons
An animal’s diet must also supply essential nutrients in preassembled form
An animal that is malnourished
Is missing one or more essential nutrients in its diet
Herbivorous animals
May suffer mineral deficiencies if they graze on plants in soil lacking key minerals
Figure 41.8
Malnutrition
Is much more common than undernutrition in human populations
Essential Amino Acids
Animals require 20 amino acids
And can synthesize about half of them from the other molecules they obtain from their diet
The remaining amino acids, the essential amino acids
Must be obtained from food in preassembled form
A diet that provides insufficient amounts of one or more essential amino acids
Causes a form of malnutrition called protein deficiency
Figure 41.9
Most plant proteins are incomplete in amino acid makeup
So individuals who must eat only plant proteins need to eat a variety to ensure that they get all the essential amino acids
Some animals have adaptations
That help them through periods when their bodies demand extraordinary amounts of protein
Figure 41.11
Essential Fatty Acids
Animals can synthesize most of the fatty acids they need
The essential fatty acids
Are certain unsaturated fatty acids
Deficiencies in fatty acids are rare
Vitamins
Vitamins are organic molecules
Required in the diet in small amounts
To date, 13 vitamins essential to humans
Have been identified
Vitamins are grouped into two categories
Fat-soluble and water-soluble
Table 41.1
Minerals
Minerals are simple inorganic nutrients
Usually required in small amounts
Mineral requirements of humans
Table 41.2
Concept 41.3: The main stages of food processing are ingestion, digestion, absorption, and elimination
Ingestion, the act of eating
Is the first stage of food processing
Digestion, the second stage of food processing
Is the process of breaking food down into molecules small enough to absorb
Involves enzymatic hydrolysis of polymers into their monomers
Absorption, the third stage of food processing
Is the uptake of nutrients by body cells
Elimination, the fourth stage of food processing
Occurs as undigested material passes out of the digestive compartment
The four stages of food processing
Figure 41.12
Piecesof food
Smallmolecules
Mechanicaldigestion
Food
Chemical digestion(enzymatic hydrolysis)
Nutrient moleculesenter body cells
Undigested material
INGESTION
Digestive Compartments
Most animals process food
In specialized compartments
Intracellular Digestion
In intracellular digestion
Food particles are engulfed by endocytosis and digested within food vacuoles
Extracellular Digestion
Extracellular digestion
Is the breakdown of food particles outside cells
Animals with simple body plans
Have a gastrovascular cavity that functions in both digestion and distribution of nutrients
Figure 41.13
Animals with a more complex body plan
Have a digestive tube with two openings, a mouth and an anus
This digestive tube
Is called a complete digestive tract or an alimentary canal
The digestive tube can be organized into specialized regions
That carry out digestion and nutrient absorption in a stepwise fashion
Esophagus
Mouth
Pharynx
Crop
Gizzard
Intestine
Typhlosole
Lumen of intestine
Esophagus
Rectum
Mouth
Crop
Anus
Intestine
Gizzard
Stomach
Mouth
Esophagus
Foregut
Midgut
Hindgut
Earthworm. The digestive tract ofan earthworm includes a muscular pharynx that sucks food in through themouth. Food passes through the esophagus and is stored and moistened in the crop. The muscular gizzard, whichcontains small bits of sand and gravel, pulverizes the food. Digestion and absorption occur in the intestine, which has a dorsal fold, the typhlosole, that increases the surface area for nutrient absorption.
(b) Grasshopper. A grasshopper has several digestive chambers grouped into three main regions: a foregut, with an esophagus and crop; a midgut; and a hindgut. Food is moistened and stored in the crop, but most digestion occurs in the midgut. Gastric ceca, pouches extending from the midgut, absorb nutrients.
(c) Bird. Many birds have three separate chambers—the crop, stomach, and gizzard—where food is pulverized and churned before passing into the intestine. A bird’s crop and gizzard function very much like those of an earthworm. In most birds, chemical digestion and absorption of nutrients occur in the intestine.
Figure 41.14a–c
Concept 41.4: Each organ of the mammalian digestive system has specialized food-processing functions
The mammalian digestive system consists of the alimentary canal
And various accessory glands that secrete digestive juices through ducts
Figure 41.15
Food is pushed along the digestive tract by peristalsis
Rhythmic waves of contraction of smooth muscles in the wall of the canal
The Oral Cavity, Pharynx, and Esophagus
In the oral cavity, food is lubricated and digestion begins
And teeth chew food into smaller particles that are exposed to salivary amylase, initiating the breakdown of glucose polymers
The region we call our throat is the pharynx
A junction that opens to both the esophagus and the windpipe (trachea)
The esophagus
Conducts food from the pharynx down to the stomach by peristalsis
From mouth to stomach
The Stomach
The stomach stores food
And secretes gastric juice, which converts a meal to acid chyme
Gastric juice
Is made up of hydrochloric acid and the enzyme pepsin
The lining of the stomach
Is coated with mucus, which prevents the gastric juice from destroying the cells
Gastric ulcers, lesions in the lining
Are caused mainly by the bacterium Helicobacter pylori
Figure 41.18
The Small Intestine
The small intestine
Is the longest section of the alimentary canal
Is the major organ of digestion and absorption
Enzymatic Action in the Small Intestine
The first portion of the small intestine is the duodenum
Where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestine itself
Figure 41.19
The pancreas produces proteases, protein-digesting enzymes
That are activated once they enter the duodenum
Figure 41.20
Enzymatic digestion is completed
As peristalsis moves the mixture of chyme and digestive juices along the small intestine
Figure 41.21
Hormones help coordinate the secretion of digestive juices into the alimentary canal
Figure 41.22
Absorption of Nutrients
The small intestine has a huge surface area
Due to the presence of villi and microvilli that are exposed to the intestinal lumen
The enormous microvillar surface
Is an adaptation that greatly increases the rate of nutrient absorption
Figure 41.23
The core of each villus
Contains a network of blood vessels and a small vessel of the lymphatic system called a lacteal
Amino acids and sugars
Pass through the epithelium of the small intestine and enter the bloodstream
After glycerol and fatty acids are absorbed by epithelial cells
They are recombined into fats within these cells
These fats are then mixed with cholesterol and coated with proteins
Forming small molecules called chylomicrons, which are transported into lacteals
Figure 41.24
The Large Intestine
The large intestine, or colon
Is connected to the small intestine
Figure 41.25
A major function of the colon
Is to recover water that has entered the alimentary canal
The wastes of the digestive tract, the feces
Become more solid as they move through the colon
Pass through the rectum and exit via the anus
The colon houses various strains of the bacterium Escherichia coli
Some of which produce various vitamins
Concept 41.5: Evolutionary adaptations of vertebrate digestive systems are often associated with diet
Some Dental Adaptations
Dentition, an animal’s assortment of teeth
Is one example of structural variation reflecting diet
Mammals have specialized dentition
That best enables them to ingest their usual diet
Figure 41.26a–c
Stomach and Intestinal Adaptations
Herbivores generally have longer alimentary canals than carnivores
Reflecting the longer time needed to digest vegetation
Figure 41.27
Carnivore
Herbivore
Symbiotic Adaptations
Many herbivorous animals have fermentation chambers
Where symbiotic microorganisms digest cellulose
The most elaborate adaptations for an herbivorous diet
Have evolved in the animals called ruminants