Chapter 21 Nutrition Digestion Overview Obtaining and

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

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 – Proceeds through hydrolysis reactions • Hydrolases – 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 Pharynx • 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

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 • 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 – 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

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

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

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 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 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