Chapter 21Nutrition & Digestion

Overview:

Obtaining and processing food

Human Digestive System

Diets

Nutrition

• Animals obtain and process nutrients in a variety of ways

• Humpback whales eat small fishes and crustaceans called krill– This painting shows how

the whales corral their food using “bubble nets”

Getting Their Fill of Krill

• Humpback whales strain their food from seawater using large, brushlike plates called baleen– When they feed, they take in large amounts of

seawater in which the fish and krill live– They must filter out the water in order to get a meal

• In a typical day, a humpback whale’s digestive system will process as much as 2 tons of fish and krill– They store the excess energy they harvest in the form

of blubber– In about 4 months, a humpback whale eats, digests,

and stores as fat enough food for an entire year

• Animal diets are highly varied– Herbivores are plant-eaters– Carnivores are meat-eaters– Omnivores eat both plants and

other animals

Animals ingest their food in a variety of ways

OBTAINING AND PROCESSING FOOD

• Omnivores

– Ingest both plants and animals

– Some animals are suspension feeders, consuming particles from water

• Herbivores

– Feed mainly on plants

• Carnivores

– Mainly eat animals that eat plants

– Some are fluid feeders, sucking liquids

• Ingestion

The Four Stages of Food Processing

– Is another word for eating

• Digestion– Is the breakdown of food to small molecules

• Absorption

– Is the uptake of the small nutrient molecules by the body’s cells

• Elimination– Is the disposal of undigested materials from the food

we eat

• Mechanical digestion

Digestion: A Closer Look

– Begins the process– Involves physical processes like chewing

• Chemical digestion

– Is the breakdown of food by digestive enzymes

• Chemical digestion

– Proceeds through hydrolysis reactions

• Hydrolases

– Are enzymes that catalyze digestive hydrolysis reactions

• In animals, chemical digestion is contained safely within some kind of compartment

Digestive Compartments

• Food is digested in compartments housing hydrolytic enzymes

• Most animals have a specialized digestive tract

• Relatively simple animals have a sac with a single opening– This is called a gastrovascular cavity– Example: hydra

• In most animals, the digestive compartment is an alimentary canal

– This is a tube running from mouth to anus– This tube is divided into specialized regions that

process food sequentially

• When food is swallowed, it is moved through the alimentary canal by peristalsis– Peristalsis is rhythmic muscle contraction in the walls

of the digestive tract– Ringlike sphincter muscles regulate the passage of

food

The human digestive system consists of an alimentary canal and accessory glands

HUMAN DIGESTIVE SYSTEM

• The teeth break up food

• Saliva moistens it

• Salivary enzymes begin the hydrolysis of starch

• The tongue pushes the chewed food into the pharynx

Digestion begins in the oral cavity

The Pharynx• The pharynx

– Connects the mouth to the esophagus– Also opens to the trachea

• The swallowing reflex moves food from the pharynx into the esophagus– At the same time, food is kept out of the trachea

The food and breathing passages both open into the pharynx

• During swallowing a reflex tips the epiglottis to close the windpipe entrance

The Esophagus• The esophagus

– Is a muscular tube– Connects the pharynx

to the stomach– Moves food down by

peristalsis

• Peristalsis in the esophagus moves food boluses into the stomach

The Stomach• The stomach

– Can store food for several hours– Churns food– Mixes food with

gastric juices, which are acidic

• The stomach mixes food with gastric juice

– The gastric juice contains pepsin, which begins the hydrolysis of protein

• New evidence suggests that a spiral-shaped prokaryote causes gastric ulcers – Helicobacter pylori growth erodes protective mucus

and damages the stomach lining

Connection: Bacterial infections can cause ulcers

– Are erosions of the stomach lining

The Small Intestine• The small intestine

– Is the longest part of the alimentary canal– Is the major organ for chemical digestion and

absorption

Chemical Digestion in the Small Intestine• In the small intestine, hydrolases break down

food to monomers

• Alkaline pancreatic juice neutralizes stomach acids– Its enzymes digest polysaccharides, proteins, nucleic

acids, and fats

• Bile emulsifies fat droplets for attack by pancreatic enzymes– It is made in the liver and stored in the gall bladder

• Enzymes from the walls of the small intestine complete the digestion of many nutrients

• The lining of the small intestine is folded and covered with tiny, fingerlike villi

• Nutrients pass through the epithelium of the villi and into the blood– The blood flows to the liver– The liver can store nutrients and convert them to

other substances the body can use

• The intestinal wall

– Contains villi and microvilli– Has a large surface area for absorption

Absorption of Nutrients• Although food has been ingested

– It is not technically “in” the body yet

– It must be absorbed

• The duodenum

– Is the first part of the small intestine– Receives digestive agents from several organs

• The pancreas

– Secretes juice that neutralizes stomach acids

• The liver– Secretes bile, which

helps digest fats

• The jejunum and ileum

– Are parts of the small intestine– Are specialized for absorption

The Large Intestine (and Beyond)• The large intestine

– Is shorter, but wider, than the small intestine

• The colon

– Makes up most of the length of the large intestine– Absorbs water from the alimentary canal– Produces feces, the waste product of food

• The rectum

– Is the last 15 cm (6 inches) of the large intestine

• The anus– Regulates the opening of the rectum

• Undigested material passes to the large intestine, or colon– Water is absorbed – Feces are

produced

The large intestine reclaims water

• Herbivores and omnivores generally have longer alimentary canals than carnivores– Plant matter is more difficult to digest than meat– Nutrients in vegetation are less concentrated than in

meat

Adaptations of vertebrate digestive systems reflect diet

DIETS AND DIGESTIVE ADAPTATIONS

• Some mammals house cellulose-digesting microbes in the colon or cecum

– The cecum is a pouch where the large and small intestines connect

– Examples: horses and elephants

• Other mammals re-ingest their feces to recover nutrients– Examples: rabbits and some rodents

• An animal’s diet provides– fuel for its activities– raw materials for making the body’s own molecules– essential nutrients that the body cannot make

Overview: A healthful diet satisfies three needs

NUTRITION

• Once nutrients are inside cells, they can be oxidized by cellular metabolism to generate energy– This energy is in the form of ATP

Chemical energy powers the body

Calories• Calories are

– A measure of the energy stored in your food– A measure of the energy you expend in daily

activities

• A calorie is

– The amount of energy required to raise the temperature of a gram of water by 1ºC

• A kilocalorie is– One thousand calories– The unit listed on food labels

• The energy a resting animal requires each day to stay alive is its basal metabolic rate (BMR)

Metabolic Rate• The metabolic rate of an organism is the rate of

energy consumption per day

• More energy is required for an active life

– Excess energy is stored as glycogen or fat

• The human body tends to store excess fat molecules instead of using them for fuel

• A balanced diet includes adequate amounts of all nutrients

Connection: Body fat and fad diets

• Fad diets are often ineffective and can be harmful

• The eight essential amino acids that adults require must be obtained from food– They are easily

obtained from animal protein

– They can also be obtained from the proper combination of plant foods

Connection: Vegetarians must be sure to obtain all eight essential amino acids

• Most of these vitamins function as coenzymes

Connection: A healthful diet includes 13 vitamins

– Are organic molecules required in the diet for good health

– Mostly function as assistants to enzymes

• Vitamins

• Minerals are elements other than carbon, hydrogen, oxygen, and nitrogen – They play a variety of roles in the body

Connection: Essential minerals are required for many body functions

– Are inorganic substances required in the diet

• Food labels provide important nutritional information about packaged foods

Connection: What do food labels tell us?

NUTRITIONAL DISORDERS• Nutritional dysfunction can cause severe

problems

• Malnutrition is a dietary deficiency of one or more of the essential nutrients

Malnutrition

– Protein deficiency is an example

• Undernutrition

– Is caused by inadequate intake of nutrients

• Obesity

Obesity

– Is an inappropriately high ratio of weight to height

• To some extent, a tendency toward obesity is inherited

• Choice of diet may reduce the risk of cardiovascular disease and cancer

Connection: Diet can influence cardiovascular disease and cancer

• A sound diet supplies

– enough raw materials to make all the macromolecules we need

– the proper amounts of prefabricated essential nutrients

– enough kilocalories to satisfy our energy needs