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Nutrition and Digestion in Animals. Diets of animals vary:. Herbivores eat autotrophic organisms (e.g., plants, plant-like protists ) Carnivores eat other animals Omnivores eat both autotrophic and heterotrophic organisms

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diets of animals vary
Diets of animals vary:

Herbivores eat autotrophic organisms (e.g., plants, plant-like protists)

Carnivores eat other animals

Omnivores eat both autotrophic and heterotrophic organisms

Detritovores eats wastes or bodies of dead organisms (AKA a decomposer)


Suspension-feeders sift small food particles from their environment

Substrate-feeders live on or in a host food source

Fluid-feeders suck nutrient-containing food from a host

Bulk-feeders consume relatively large pieces of food (whole or torn into pieces)


Like plants, animals require nutrients to: supply energy (carbohydrates, lipids, and proteins), provide the building blocks for structures, and provide the essential nutrients necessary for all of the activities of the organism. Essential nutrients are materials that an animal needs but cannot synthesize.

essential amino acids
Essential amino acids

Humans (post fetal) require eight amino acids in their diet.

Meat and other animal products provide all of the needed amino acids

Those who avoid animal products can combine various other foods in order to obtain all of the essential amino acids.

essential fatty acids
Essential fatty acids

Animals produce most of the fatty acids that they need. One exception is the essential fatty acid, linoleic acid that is used to make the phospholipids of cell membranes.


Vitamins are necessary for an animal’s health. Different vitamins have different functions and thus deficiencies of them cause various symptoms or disease conditions. Occasionally too much of a given vitamin (especially the fat-soluble vitamins) can cause toxicity symptoms. See Table 41.1 for the water-soluble and fat-soluble vitamins that are necessary for human health and know two water-soluble and two fat -soluble vitamins as well as their sources, major functions, and deficiency symptoms.


These are inorganic (unlike vitamins) and are needed in relatively small amounts. However, deficiencies can be devastating for an animal and thus they are necessary. See Table 41.2 and pick two minerals (know their major sources, functions. and deficiencies).

the main stages of food processing
The main stages of food processing

Ingestion is the process of eating

Digestion is the breaking down of large food particles (too large to be absorbed) into smaller particles (see below)

Absorption is the process whereby nutrients are transported into the cells of the animal

Elimination of undigested wastes

the oral cavity
The oral cavity

is where digestion begins [mechanical by chewing and chemical of carbohydrates (amylases) and lipids (lipases)]. Food is also moistened and lubricated by saliva reducing abrasion (and damage) and making the food easier to swallow. A bolus (a ball of chewed food) is formed by chewing, and by the movement of the tongue

the pharynx
The pharynx

opens into the esophagus and trachea (which is blocked by the epiglottis when swallowing occurs)


Peristalsis (by smooth muscles) pushes the bolus through the esophagus and then into the stomach.

the stomach
The stomach

is found just below the diaphragm.

The bolus enters the stomach after passing through the cardiac sphincter.

The stomach functions in chemical and mechanical digestion.

Gastric juice containing HCl (from parietal cells), and pepsinogen (from chief cells) is produced by cells of the stomach. This acid converts a small amount pepsinogen into an active form, pepsin. Then pepsin takes over the conversion process.

The cells of the stomach are protected by mucus that is also secreted by cells of the stomach.

The HCl also helps to break down extracellular matrix and thus helps with the digestion process.

Gastrin is a hormone that regulates the production of gastric juice.

The liquefied food that is produced in the stomach is called chyme. The pyloric sphincter regulates the passage of the chyme into the small intestine.

the small intestine
The small intestine

is composed of three sections (duodenum, jejunum, and ileum).

A base that is secreted by the pancreas neutralizes the acid from the stomach.

Fats are emulsified by bile salts (from the liver via the gallbladder) so that they can be digested and absorbed.

Some of the fats are not absorbed into the blood but instead are absorbed into the lymphatic system.

Most of the digestion (chemical) of proteins, carbohydrates and fats occurs in the jejunum.

In addition most of the absorption of these compounds occurs in the jejunum.

the large intestine
The large intestine

functions in the concentration of wastes and in water absorption. In humans the appendix is vestigial.


Accessory organs

of the digestive system

the salivary glands
The salivary glands

produce and secrete saliva (containing salivary amylases and lipases)

the pancreas
The pancreas

secretes digestive enzymes (e.g., pancreatic amylase) and also the bicarbonate that neutralizes the acid in the chyme.

the liver and gall bladder
The liver and gall bladder

The liver produces bile (for fat emulsification) and also functions in metabolism

The gall bladder stores and concentrates the bile