Digestive System Ch. 25. Food for Energy and Growth Carbohydrates are obtained primarily from cereals, grains, and breads on the average, carbohydrates

Digestive SystemCh. 25

Food for Energy and Growth

• Carbohydrates are obtained primarily from cereals, grains, and breads on the average, carbohydrates contain 4.1

calories per gram the body uses carbohydrates for energy

• Dietary fats are obtained from oils, margarine, and butter and are abundant in fried foods, meats, and processed snack foods fats contain 9.3 calories per gram the body uses fats to construct cell membranes,

to insulate nervous tissue, and to provide energy


• Proteins can be obtained from many foods, including poultry, fish, meat, and grains proteins have 4.1 calories per gram proteins are used for energy and as

building materials for cell structures, enzymes, hemoglobin, hormones, and muscle and bone tissue


• In wealthy countries, being significantly overweight is common this is due to habitual overeating and

high-fat diets, in which fats constitute over 35% of the total caloric intake

the standard measure of appropriate body weight is the body mass index (BMI), estimated as your body weight in kg, divided by your height in meters squared

Are you overweight?

66% of Americans are overweight with a BMI of 25 or more

Types of Digestive Systems

• Heterotrophs are divided into three groups on the basis of their food sources herbivores eat plants exclusively carnivores are meat eaters omnivores eat both plants and animals

• Single-celled organisms, as well as sponges, digest their food intracellularly

• All other animals digest their food extracellularly, within a digestive cavity


• A gastrovascular cavity is found in cnidarians and flatworms this cavity has only a

single opening that serves as both a mouth and an anus

there is no specialization within this type of digestive system because every cell is exposed to all stages of digestion


• The alimentary canal is a digestive tract with a separate mouth and anus this permits specialization and the transport of

food is one way• physical forces, such as chewing and grinding, break

the ingested food into smaller fragments • chemical digestion involves hydrolysis reactions

that liberate the subunits of food• the products of digestion are absorbed into the blood• any molecules in the food that are not absorbed by

the animal are excreted through the anus

One-way digestive tracts

Vertebrate Digestive Systems

• In humans and other vertebrates, the digestive system consists of a tubular gastrointestinal tract and accessory organs in general, carnivores have shorter intestines

for their size than herbivores Herbivores have long, convoluted small

intestines because they ingest a large amount of plant cellulose, which resists digestion

the tubular gastrointestinal tract has a layered structure

The human digestive system

The layers of the gastrointestinal tract

The Mouth and Teeth

• Many vertebrates have teeth, and chewing (mastication) breaks up food into small particles and mixes it with fluid secretions

The Mouth and Teeth

• Inside the mouth, the tongue mixes food with a mucous solution called saliva saliva moistens and lubricates food so

that it is easier to swallow saliva also contains a hydrolytic enzyme

called amylase• this enzyme initiates the breakdown of

starch into the disaccharide maltose

The Mouth and Teeth

• Food is moved by the tongue to the back of the mouth for swallowing

• The soft palate is raised, closing off the nasal cavity, and muscles push the food past the larynx food is prevented from going into the respiratory tract

by the epiglottis

Figure 31.10 The human pharynx, palate, and larynx.

The Esophagus and Stomach

• The esophagus is a muscular tube that connects the pharynx to the stomach the upper third is enveloped in skeletal

muscle for voluntary control of swallowing the lower two-thirds is surrounded by

involuntary smooth muscle rhythmic waves of contractions, called

peristalsis, propel food towards the stomach

Peristalsis


• The movement of food from the esophagus into the stomach is controlled by a ring of circular smooth muscle, called a sphincter contraction of the sphincter prevents food in the

stomach from moving back into the esophagus in humans, stomach contents can be brought

back out during vomiting the relaxing of the sphincter may lead to acid

reflux, which is when stomach acid moves into the esophagus

• this produces a burning sensation known as heartburn


• The stomach is a saclike portion of the digestive tract the stomach contains an extra layer of

smooth muscle for churning food gastric juice is released by gastric glands

in the lining of the stomach• parietal cells secrete hydrochloric acid (HCl)• chief cells secrete pepsinogen

– pepsinogen requires a low pH to be activated into pepsin, a protease that begins the digestion of proteins

The stomach and gastric glands


• Gastric juice has a pH of 2, much more acidic than the 7.4 pH of blood the low pH helps to denature protein,

keep pepsin active, and kill most bacteria

active pepsin hydrolyzes food proteins into short chains of polypeptides that are not fully digested until the mixture enters the small intestine

chyme is the name for the mixture of partially digested food and gastric juice


• Overproduction of gastric acid can occasionally eat a hole through the wall of the stomach, called a gastric ulcer normally the stomach epithelial cells are

protected by alkaline mucus susceptibility to ulcers is increased by

an infection of the bacterium Helicobacter pylori

The Small and Large Intestines

• The small intestine is the true digestive vat of the body only relatively small portions of chyme

are introduced into the small intestine at one time• this allows time for acid to be neutralized

and enzymes to act in the small intestine, carbohydrates,

protein, and lipids are broken down and absorbed into the bloodstream


• While some enzymes necessary for digestion are secreted by the cells of the intestinal wall, most are made in the pancreas the pancreas is an exocrine gland,

meaning it secretes through ducts the pancreas sends it products via a

duct that empties into the first part of the small intestine, the duodenum


• Much of the food energy the vertebrate body harvests is obtained from fats fat digestion involves bile salts that are

secreted into the duodenum by the liver bile salts act like detergents and combine

with drops of fat to form microscopic droplets• this process is known as emulsification• this increases the surface area for the enzyme

lipase to work on in order to breakdown the fat


• The small intestine also includes jejunum where digestion

continues ileum where water and

digested products are absorbed

• The lining of the small intestine is folded into ridges, which are covered with fine projections called villi (singular, villus) each of the cells covering the

villus is covered by a field of projections called microvilli


• The large intestine has a wider diameter than the small intestine no digestion takes place here only about 6% to 7% of fluid absorption

occurs here• some water, sodium, and vitamin K

the main function of the large intestine is to compact and store undigested material as feces

Accessory Digestive Organs

• The pancreas secretes fluid through the pancreatic duct into the duodenum the fluid contains a host of enzymes

• trypsin and chymotrypsin digest proteins•pancreatic amylase digests starch• lipase digests fats

the pancreas also secretes bicarbonate, which neutralizes the HCl from the stomach


• In addition to being an exocrine gland, the pancreas is also an endocrine gland it produces hormones

in the islets of Langerhans

• the two most important pancreatic hormones are insulin and glucagon


• The liver is the largest internal organ of the body the liver produces bile and stores it in

the gallbladder where it is concentrated • if the bile duct becomes blocked, a

gallstone forms

the arrival of fatty food in the duodenum triggers a neural and endocrine reflex that stimulates the gallbladder to contract

Accessory Digestive

• The liver removes toxins, pesticides, carcinogens, and other poisons by converting them into less toxic forms excess amino acids that may be present

in the blood are converted to glucose• an amino group (-NH2) is removed from the

amino acid to become ammonia (NH3)

• NH3 then combines with CO2 to form urea, which then goes to the liver

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Digestive System Ch. 25. Food for Energy and Growth Carbohydrates are obtained primarily from cereals, grains, and breads on the average, carbohydrates