Download Chapter 21 Nutrition & Digestion

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 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