Chapter 21 Nutrition & Digestion Overview: Obtaining and processing food Human Digestive System Diets Nutrition

Getting Their Fill of Krill 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”

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

Animals ingest their food in a variety of ways OBTAINING AND PROCESSING FOOD Animals ingest their food in a variety of ways Animal diets are highly varied Herbivores are plant-eaters Carnivores are meat-eaters Omnivores eat both plants and other animals

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

The Four Stages of Food Processing Ingestion 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

Digestion: A Closer Look Mechanical digestion Begins the process Involves physical processes like chewing Chemical digestion Is the breakdown of food by digestive enzymes

Chemical digestion Hydrolases Proceeds through hydrolysis reactions Are enzymes that catalyze digestive hydrolysis reactions

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

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

Digestion begins in the oral cavity The teeth break up food Saliva moistens it Salivary enzymes begin the hydrolysis of starch The tongue pushes the chewed food into the pharynx

The food and breathing passages both open into the pharynx Connects the mouth to the esophagus Also opens to the trachea The food and breathing passages both open into the pharynx The swallowing reflex moves food from the pharynx into the esophagus At the same time, food is kept out of the trachea

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

Connection: Bacterial infections can cause ulcers New evidence suggests that a spiral-shaped prokaryote causes gastric ulcers Helicobacter pylori growth erodes protective mucus and damages the stomach lining Are erosions of the stomach lining

Chemical Digestion in 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

Nutrients pass through the epithelium of the villi and into the blood The lining of the small intestine is folded and covered with tiny, fingerlike villi The intestinal wall Contains villi and microvilli Has a large surface area for absorption 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

Absorption of Nutrients Although food has been ingested It is not technically “in” the body yet It must be absorbed

The duodenum The pancreas The liver 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) 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

The large intestine reclaims water Undigested material passes to the large intestine, or colon Water is absorbed Feces are produced

Adaptations of vertebrate digestive systems reflect diet DIETS AND DIGESTIVE ADAPTATIONS Adaptations of vertebrate digestive systems reflect diet 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

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

Overview: A healthful diet satisfies three needs NUTRITION Overview: A healthful diet satisfies three needs 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

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

Calories Calories are A calorie is A kilocalorie is 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

Connection: Body fat and fad diets The human body tends to store excess fat molecules instead of using them for fuel A balanced diet includes adequate amounts of all nutrients

Fad diets are often ineffective and can be harmful

Connection: Vegetarians must be sure to obtain all eight essential amino acids 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: A healthful diet includes 13 vitamins Most of these vitamins function as coenzymes Vitamins Are organic molecules required in the diet for good health Mostly function as assistants to enzymes

Connection: Essential minerals are required for many body functions Minerals are elements other than carbon, hydrogen, oxygen, and nitrogen They play a variety of roles in the body Are inorganic substances required in the diet

Connection: What do food labels tell us? Food labels provide important nutritional information about packaged foods

NUTRITIONAL DISORDERS Nutritional dysfunction can cause severe problems

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

Connection: Diet can influence cardiovascular disease and cancer Choice of diet may reduce the risk of 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