Chapter 41
Animal Nutrition

An overview of key concepts:
  • 41.1) The supply of chemical energy, organic molecules, and essential nutrients through an animal's diet
  • 41.2) Ingestion, digestion, absorption, and elimination; the main stages of food processing
  • 41.3) The mammalian digestive system is formed of organs specialized for sequential stages of food processing
  • 41.4) Diet correlates with the evolutionary adaptations of vertebre digestive systems
  • 41.5) An animal's energy balance is contributed to by homeostatic mechanisms

Important vocabulary and a detailed summary of chapter 41:

Section 41.1) The supply of chemical energy, organic molecules, and essential nutrients through an animal's diet
  • Nutrition: Food taken in, taken apart, and taken up
      • The Kodiak bear catches and consumes a salmon. Skin and muscles, among other parts of the fish, are chewed into pieces, broken down by acid and enzymes in the bear's digestive system, and finally absorbed as small molecules into the body of the bear.
- Diet
  • Herbivores: Creatures whose diets consist mainly of plants or algae
      • Cattle, dining primarily on grasses and other forages.
        A guide to foods containing various nutrients common in a human diet
        A guide to foods containing various nutrients common in a human diet
  • Carnivores: Creatures whose diets consist mainly of other animals
      • Sharks, different species preying upon anything from plankton to bottlenose dolphins.
  • Omnivores: Creatures that regularily consume both plants or algae and other animals
      • Cockroaches, consuming woods as well as susceptible animals.
- Nutrition
  • Essential Nutrients: Materials that an animal cell requires but cannot synthesize
      • Vitamin C for humans and other primates.
    • Essential Amino Acids: Amino acids that cannot be synthesized and must be obtained in prefabricated form
      • Proteins in animal products, such as meat and eggs, provide all essential amino acids in proper proportions.
    • Essential Fatty Acids: Fatty Acids that cannot be synthesized and must be obtained through diet
      • Linoleic acid is required by humans, obtained through the consumption of seeds, grains, and vegetables.
  • Vitamins: Organic molecules with diverse functions that are required in the diet in very small amounts
      • Vitamin B2 is converted in the body to FAD, a coenzyme used in many metabolic processes, including cellular respiration.
  • Minerals: Inorganic nutrients that are usually required in small amounts- from under 1 mg to about 2,500 mg per day
      • Vertebrated require large quantities of calcium and phosphorus for building and maintaining bone.
- Dietary Deficiencies
  • Undernourishment: The result of a diet that constantly supplies less chemical energy than the body requires
      • In human populations, a number of things can disrupt the food supply, such as in sub-Saharan Africa, where approximately 200 million cannot eat enough food.
  • Malnourishment: The long-term absence from the diet of one or more essential nutrients
      • Cattle, deer, and other herbivores may develop fragile bones if they graze on plants growing in soil that lacks phosphorus.

Section 41.2) The main stages of food processing
- Feeding Mechanisms
  • Substrate feeders: Animals that live in or on their food source
      • The leaf miner caterpillar lives on and eats through various leaves.
  • Fluid feeders: These animals suck nutrient-rich fluid from a living host
      • The mosquito penetrates animal skin and sucks a blood metal with its hollow needle-like mouthparts.

Bulk feeders: These animals eat relatively large pieces of food. In this example, a Rosy Boa consumes a mouse whole
Bulk feeders: These animals eat relatively large pieces of food. In this example, a Rosy Boa consumes a mouse whole


Suspension feeders: Animals that sift small food particles from the water
Suspension feeders: Animals that sift small food particles from the water



















- Stages
  • Ingestion: The act of eating
      • What did you have for dinner yesterday? Remember that?
  • Digestion: Food is broken down into molecules small enought for the body to absorb
      • Fruit flies and humans convert proteins in their food to the same 20 amino acids, from which they assemble specific proteins.
  • Absorption: The animal's cells take up small molecules.
      • Amino acids and simple sugars can be absorbed.
  • Elimination: Undigested material passes out of the digestive system
      • Waste is expelled.

The four stages of food processing
The four stages of food processing



















- Digestive Compartments
  • Intracellular digestion: The hydrolysis of food inside vacuoles
      • After a cell engulfs solid food by phagocytosis or liquid food by pinocytosis.
  • Extracellular digestion: The breakdown of food in compartments that are continuous with the outside of the animal's body
      • Animals can devour much larger sources of food than through phagocytosis.
  • Gastrovascular cavity: Digestive compartment with a single opening, aiding digestion as well as the distribution of nutrients throughout the body
      • Hydras use tentacles to push prey into its gastrovascular cavity, where digestive enzymes are excreted and food particles are absorbed.
  • Complete digestive tract/alimentary canal: Digestive tube extending between two openings, a mouth and an anus
      • Food moves in a single direction and this tube can be organized into specialized compartments that carry out digestion and nutrient absorption in a stepwise fashion.

Concept 41.3) Organs specialized for sequential stages of food processing form the mammalian digestive system
The human digestive system (left); A schematic diagram of the human digestive system (right)
The human digestive system (left); A schematic diagram of the human digestive system (right)

  • Peristalsis: Pushes food through the alimentary canal
      • Alternating waves of contraction and relaxation in the smooth muscles lining the canal move food along, even while lying down.
  • Sphincters: Used to regulate the passage of material between specialized compartments
      • At junctions between specialized compartments, the muscular layer forms these ringlike valves
  • Oral cavity: Ingestion and the initial steps of digestion occur here
      • Mechanical digestion begins as teeth cut, and salivary glands produce saliva.
    • Salivary glands: Delivers saliva through ducts to the oral cavity
      • The presence of food stimulates this nervous reflex.
    • Amylase: Hydrolyzes starch and glycogen into smaller polysaccharides and the disaccharide maltose
      • This is an enzyme in saliva
    • Bolus: The ball-shape that the tongue forms food into before swallowing
      • During swallowing, the tongue pushes the bolus to the back of the oral cavity and into the pharynx.
  • Pharynx: the throat region
      • Opens to two passageways: the esophagus and the trachea.
  • Esophagus: Connects to the stomach, containing both striated and smooth muscle
      • Striated muscle is active during swallowing, where smooth muscle functions in peristalsis, both helping move the bolus to the stomach.


(above) A walk through the swallowing of food. (Factual as well as aesthetically pleasing)

- Digestion in the Stomach
  • Stomach: A few nutrients are absorbed from the stomach into the bloodstream, but its main purpose is food storage and continued digestion
      • Located just below the diaphragm in the upped abdominal cavity.
  • Chyme: The mixture of ingested food and gastric juice
      • Allows chemical digestion to be carried out.
  • Gastric juice: Mixes with food through a churning action
      • The stomach excretes this digestive fluid.
    • Hydrochloric acid: Disrupts the extracellular matrix that binds cells together in meat and plant material
      • Due to high HCA, the pH of gastric juice is so low that proteins denature, increasing exposure of their peptide bonds.
    • Pepsin: A protease, or protein-digesting enzyme
      • Works best in a strongly acidic environment, breaking peptide bonds.
    • Pepsinogen: Inactive form of pepsin
      • Chief cells release pepsinogen into the lumen.
    • Mucus: Secreted by stomach lining to protect against self-digestion
      • A viscous and slippery liquid mixture of glycoproteins, cells, salts, and water.
- Digestion in the Small Intestine
  • Small intestine: Most enzymatic hydrolysis of macromolecules from food occurs here
      • Chyme passes into the small intestine after chemical digestion.
    • Duodenum: Here, chyme mixes with digestive juices from the pancreas, gallbladder, liver, and gland cells of the intestinal wall itself
      • This forms the first 25 cm or so of the small intestine.
  • Pancreas: Aids chemical digestion by producing an alkaline solution rich in bicarbonate as well as several enzymes
      • The bicarbonate neutralizes the acidity of chyme and acts as a buffer. Pancreatic enzymes are activated only when safely located in the extracellular space within the duodenum.
  • Bile: Relied upon to digest fats and other lipids in the small intestine
      • Contains bile salts, which act as detergents that aid in digestion and absorption of lipids.
      • Made in the liver.
      • Stored and concentrated in the gallbladder.
- Absorption in the Small Intestine
  • Villi: Finger-like projections in the large folds of the alimentary canal lining
      • Nutrients in lumen passes through the lining.
    • Microvilli: Microscopic appendages located on the apical surface of the epithelial cells of a villus
      • These are exposed to the intestinal lumen, and give the intestinal epithelium a brush-like appearance.
      • Transport can be either passive or active, with active e transport across the epithelial cells allowing much more absorption of nutrients than would be possible with passive diffusion alone.
  • Lacteal: A vessel at the core of each villus, lactels are a part of the lymphatic system, and are filled with lymph
      • Lymph, containing chylomicrons, passes into larges vessels of the lymphatic system and eventually into large veins that return the blood to the heart.
    • Chylomicrons: Water-soluble globules created with fats coated in phospholipids, cholesterol, and proteins.
      • Not absorbed through the bloodstream, these products of fat digestion are absorbed by epithelial cells and recombined into triglycerides.
      • From the liver, blood travels to the heart and then to other tissues and organs.
- Absorption in the Large Intestine
  • Large intestine: Includes the colon, cecum, and rectum
      • The small intestine connects to the large intestine at a T-shaped junction.
  • Colon: A major function is to recover water that has entered the alimentary canal as the solvent of digestive juices
      • with no biological mechanism for active transport of water, water absorbtion in the color occurs by osmosis.
    • Feces: Wastes of the digestive system
      • Becomes increasingly solid as they are moved along the colon by peristalsis.
  • Cecum: Important for fermenting ingested material, especially in animals that consume large amounts of plant material
      • A pouch connected to from the large intestine, as is the colon.
    • Appendix: A finger-like extension of the human cecum
      • Has a minor and dispensable role in immunity.
  • Rectum: The terminal portion of the large intestine, where feces are stored until they can be eliminated

A great video as well as extra information on digestion

Concept 41.4) Evolutionary adaptations of vertebrate digestive systems correlate with diet

A carnivorous lion
A carnivorous lion

Two omnivorous humans
Two omnivorous humans

A herbivorous deer
A herbivorous deer


- Carnivores
  • Pointed incisors and canines that can be used to kill prey and rip or cut away pieces of flesh. Jagged premolars and molars crush and shred food.
  • Large expandable stomachs to compensate for the amounts of meat that must be consumed in response to the long periods of time they can go without food.
- Herbivores
  • Teeth with broad, ridged surfaces that grind tough plant material.
  • Longer alimentary canals relative to body size in comparison with carnivores, allowing more time for digestion of vegetation containing cell walls, and greater surface area for nutrient absorption.
  • Microorgimisms help herbivores digest plants in a mutualistic relationship
      • The most elaborate adaptations for herbivorous diet have evolved in animals called ruminants, including deer, sheep, and cattle.
- Omnivores
  • Relatively unspecialized dentition. Humans, from midline to the back along one side of the jaw, posses two blade-like incisors for biting, a pointed canine for tearing, two premolars for grinding, and three molars for crushing.
  • Longer alimentary canals relative to body size in comparison with carnivores, allowing more time for digestion of vegetation containing cell walls, and greater surface area for nutrient absorption.

Concept 41.5) Homeostatic mechanisms contribute to an animal's energy balance
- Energy Sources and Stores
  • Nearly all of an animal's ATP generation is based on the oxidation of energy-rich organic molecules such as carbohydrates, proteins, and fats, in cellular respiration.
  • When more energy-rich molecules are taken in than can be broken down, the excess is converted into storage molecules. Excess energy from the diet is stored as glycogen. When fewer calories are taken in than are expended, glycogen is oxidized.

Talking about humans, but applicable to all cases of overnourishment, a video on obesity

- Overnourishment and Obesity
  • Overnourishment: The consumption of more calories than the body needs for normal metabolismCauses obesity, the accumulation of fat, which contributes to a number of health problems.
      • Lepatin is a product of adipose cells, rising when levels of body fat increase, cuing the brain to suppress appetite and vice versa.
- Obesity and Evolution
  • Fat hoarding may have had an advantage in our evolutionary past, early humans leading a feast-famine existence.



Recent scientific articles:
Additional outside information: