Nutrition and Digestion
Chapter 34
Nutrients
• All foods contain nutrients needed for survival
• Nutrition describes the process of acquiring
and processing nutrients in useable form
• 3 part process:
– Ingestion – taking in of food
– Digestion – the breakdown of food
– Egestion – elimination of undigested food
• 2 major modes: Autotrophic & Heterotrophic
Nutrients
• 2 categories of nutrients:
• Micronutrients – vitamins & minerals.
Needed in small quantities
• Vitamins can be water soluble or fat
soluble
• Macronutrients – fats, proteins, carbs, &
water. Needed in large quantities
Energy Obtained from Nutrients
• Most energy used by cells is derived from
carbohydrates and fats
• The energy in nutrients is measured in
calories
– A calorie is the amount of energy required to
raise 1 gram of water 1 degree Celsius
– Food calorie content is measured in units of
1000 calories (kilocalories), also known as
Calories (capital C)
Energy Obtained from Nutrients
• The average human burns 70 Calories per
hour at rest, and up to 20 Calories per
minute during exercise
Lipids
• Lipids are essential nutrients
• Lipids are diverse and contain long chains
of carbon atoms and are insoluble in
water
• Specialized lipids can be synthesized by
some animals, others require essential
fatty acids (lipid building blocks) from
their food
Lipids
• Three principal types of lipids
– Triglycerides (fats used as an energy source)
– Phospholipids (component of cell
membranes)
– Cholesterol (component of cell membranes,
sex hormones, and bile)
Animals Store Energy as Fat
• When an animal’s diet provides excess
energy, the excess is stored as fat
– 3600 Calories are stored in each pound of fat
• Fats are excellent energy storage
molecules
– A concentrated energy source compared to
proteins and carbohydrates
– Hydrophobic lipids avoid excess water
accumulation in body
Animals Store Energy as Fat
• Animals that maintain elevated body
temperatures use fats to insulate their
bodies
Carbohydrates
• Carbohydrates are used as an energy
source, but can have structural uses as
well
• Carbohydrates are broken down into
sugars (usually glucose) which are used
to make energy
Amino Acids and Proteins
• Proteins are digested into amino acid subunits,
which can be used to make new proteins
• Proteins have many functions
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Enzymes
Cell membrane receptors
Oxygen transport molecules
Structural proteins
Antibodies
Muscle proteins
An energy source
Amino Acids and Proteins
• Humans can synthesize 11 of the 20
amino acids needed to make proteins
• Those amino acids that cannot be
synthesized must be obtained in the diet
(essential amino acids)
• Protein deficiency can result in a number
of debilitating conditions, including
kwashiorkor
Minerals & Vitamins
• Minerals are elements that are important
to animal nutrition
• Minerals can only be obtained in the diet
• Vitamins are organic compounds
required in small amounts by animals
• Vitamins must usually be obtained in the
diet
Vitamins
• Some vitamins function as antioxidants
that bind to free radicals (damaging
molecules) and help prevent them from
damaging DNA
– Examples: vitamins C and E
• Human vitamins are grouped into two
categories
– Water-soluble vitamins
– Fat-soluble vitamins
Water-Soluble Vitamins
• Include vitamin C and the B-vitamin
complex
• Water-soluble vitamins dissolve in the
blood plasma and are not appreciably
stored in the body
– Must be replenished by diet
• Most help promote chemical reactions that
supply energy or synthesize biological
molecules
Fat-Soluble Vitamins
• Include vitamins A, D, E, and K
• Fat-soluble vitamins can be stored in fat
and may accumulate over time
Water
• The human body is two-thirds water
– Principal component of blood, interstitial fluids, cytosol
– Required for metabolic reactions
• The average daily adult water requirement is
about 10 cups (2500 mls)
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Increases with exercise, heat, or low humidity
50% obtained from solid food
40% obtained by drinking fluids
Cellular respiration generates 10%
An Overview of Digestion
• Digestion is the process that physically
and chemically breaks down food
• This is accomplished by digestive
systems
– Take in food and break complex molecules
into simpler forms that can be absorbed
– Material that cannot be absorbed is expelled
An Overview of Digestion
•
All digestive systems perform five tasks
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Ingestion: food brought into digestive tract through
an opening (mouth)
Mechanical breakdown: physical breakdown of food
into smaller pieces
Chemical breakdown: digestive chemicals and
enzymes break large food molecules into small
subunits
Absorption: small subunits transported out of
digestive system
Elimination: indigestible materials expelled from
body
Digestion within Single Cells
• Sponges are sedentary filter-feeders with
no specialized digestive system
• Sponges rely on individual cells to digest
food (intracellular digestion)
• Digestion takes place in collar cells lining
water-filled sponge chambers
Digestion within Single Cells
• Sponge collar cells engulf microscopic
food particles in water, forming food
vacuoles
• Food vacuoles fuse with lysozomes,
whose digestive enzymes break down
food molecules
• Small food molecules are absorbed by
cytoplasm, indigestible material is
expelled from the cell (and sponge)
The Simplest Digestive System
• A sac with one opening forms the simplest
digestive system
• Larger organisms evolved an internal
chamber in which large chunks of food are
broken down by enzymes outside of cells
(extracellular digestion)
• Hydra have a gastrovascular cavity with
a single opening at one end
The Simplest Digestive System
• Hydra have a single opening at one end
– Opening acts as both a mouth and anus
– Food captured by stinging tentacles that
escort it into the gastrovascular cavity
– Cells lining gastrovascular cavity release
enzymes and break down food
– Cavity cells absorb nutrients and food
particles, then intracellular digestion occurs
– Indigestible material expelled out mouth
Digestion in a Tube
• Tubular digestive tracts allow animals to
eat frequently
– Incoming food does not interfere with
outgoing wastes
• Most animals have a tubular system, with
a mouth at one end and an anus at the
other
• Specialized regions along the tube
optimize food breakdown
Digestive Specializations
• Different animals have specialized tubular
digestive tracts to optimize nutrient
absorption
– Carnivores: eat other animals
– Herbivores: eat only plants
– Omnivores: eat both animals and plants
Annelid Digestive System
• Mouth – ingest food. Swallows through a
muscular pharynx
• Esophagus – carries food to crop
• Crop – thin walled food storage organ
• Gizzard – muscular digestive organ that
grinds food
• Intestine – finishes chemical digestion &
absorbs digested nutrients
• Anus – expulsion of wastes (casts)
Insects
• Similar to Annelids
• Possess a stomach between gizzard &
intestine
• Aids in digestion of food
Digestive Specializations
• Ruminant animals
– Herbivores that are able to break down
cellulose and extract nutrients from tough
plant matter
– Practice rumination: regurgitate and rechew
food (cud)
• Ruminant animals
– Have elaborate digestive systems that house
microorganisms which break down cellulose
(using cellulase enzymes)
Digestive Specializations
• Intestinal length is correlated with diet
– Because cell walls are difficult to digest,
herbivore intestines are long to allow more
time for nutrient absorption
– Carnivore intestines are relatively short
because proteins are easy to digest
Digestive Specializations
• Teeth evolved to accommodate different
diets
– Incisors: used for biting
– Canines: used for tearing
– Premolars: used for grinding
– Molars: used for crushing and chewing
– Omnivores, carnivores and herbivores
emphasize different tooth use based on diet
Digestive Specializations
• Birds lack teeth, thus use gizzards for grinding
food
• Birds swallow food whole
• Food enters crop, which stores and moistens
food
• Food enters stomach, which secretes enzymes
• Food enters gizzard, which is muscular with a
hard lining for grinding food
• Further digestion and absorption occurs in small
intestine
The Human Digestive System
• The human digestive system is adapted
for processing a wide variety of different
foods
• As food travels through the digestive
system it is processed in many different
ways, by many different structures
The Mouth
• The mechanical and chemical breakdown
of food begins in the mouth
• Chewing causes mechanical breakdown
of food by teeth
– Incisors: used for biting
– Canines: used for tearing
– Premolars: used for grinding
– Molars: used for crushing and chewing
– 32 teeth in the human mouth
The Mouth
• Salivary glands produce saliva to start the
chemical breakdown of food
• Functions of saliva
– Contains amylase: breaks down starches
into sugars
– Contains bacteria-killing enzymes and
antibodies
– Lubricates food: helps with swallowing
– Carries food molecules to taste buds on
tongue for food quality identification
The Mouth
• The tongue pushed food into the
pharynx, which connects the mouth with
the esophagus
• The epiglottis folds over the larynx,
which keeps swallowed food from entering
the trachea
• Muscular contractions push food into the
esophagus
The Esophagus
• Swallowing forces food into a muscular
tube called the esophagus
• The esophagus forms waves of muscular
contraction (peristalsis) that carry food to
the stomach
The Stomach
• Food enters the stomach, which is an
expandable muscular sac
• The stomach connects to the small intestine, but
the pyloric sphincter muscle keeps food from
entering it
• The stomach has three major functions
– Food storage and gradual release into the small
intestine
– Mechanical food breakdown by using churning
contractions
– Chemical food breakdown by stomach gland
secretions
The Stomach
• Stomach glands produce several
secretions
– Gastrin: a hormone; stimulates hydrochloric
acid (HCl) secretion by stomach cells
– HCl: makes stomach fluids very acidic
– Pepsinogen: inactive enzyme that is
converted into active pepsin when exposed
to HCl; break proteins into shorter peptides
– Mucus: protects stomach wall from acids
The Stomach
• Food in stomach is gradually converted to
chyme (a mixture of partially digested
food and stomach secretions)
• Peristaltic waves propel food toward small
intestine
• Pyloric sphincter opens and lets chyme
enter small intestine in small amounts
The Small Intestine
• Most digestion occurs in the small intestine
• The small intestine is a narrow tube about 6-7
meters long
• Food is digested and absorbed with the aid of
digestive secretions from three sources
– The liver
– The pancreas
– The cells of the small intestine
The Liver and Gallbladder
• The liver has many functions
– Fat and carbohydrate storage
– Regulates blood glucose levels
– Synthesizes blood proteins
– Stores iron and other vitamins
– Converts ammonia into urea
– Detoxifies harmful substances
– Produces bile, a liquid that is stored in the
gallbladder
The Liver and Gallbladder
• Bile
– Is a mixture made of bile salts, water, other
salts, and cholesterol
– Bile salts emulsify large fat globs into
microscopic particles
The Pancreas
• The pancreas is a leaf-shaped organ attached
to the duodenum of the small intestine
• The pancreas secretes pancreatic juice, which
is released into the small intestine
• Pancreatic juice components
– Water
– Sodium bicarbonate: neutralizes acidic chyme
– Digestive enzymes
• Amylase: breaks down carbohydrates
• Lipases: breaks down lipids
• Proteases: breaks down proteins and peptides
Completion of Digestion
• Some small intestinal cells have enzymes
that complete the digestion process
– Peptidases: complete breakdown of peptides
into amino acids
– Disaccharidases: break down disaccharides
into monosaccharides
– Lipases: digest lipids
Absorption in the Small Intestine
• Most absorption occurs in the small
intestine
• The small intestine is well adapted for
absorption of nutrients
– Has numerous folds with fingerlike
projections (villi) on its surface to increase
absorptive surface area
– Each villus cell has many microvilli that
increase surface area even more
Absorption in the Small Intestine
• Nutrients are absorbed into intestinal cells
by energy-requiring mechanisms, and
then diffuse from the cell to the
bloodstream
• Water is absorbed by osmosis
• Each villus contains capillaries and a
single lacteal (lymph capillary)
– Fats are absorbed into the lacteals, which
then enter lymphatic capillaries which lead to
the bloodstream
The Large Intestine
• The large intestine is about 1.5 meters
long and three inches wide
• A mix of water, undigested nutrients, and
fiber enters the large intestine
• The large intestine has two parts
– Colon: most of its length
– Rectum: the terminal compartment
– Appendix –finger-like projection off the
colon (vestigial)
The Large Intestine
• The large intestine contains bacteria that
consume unabsorbed nutrients and
synthesize vitamins
• The large intestine absorbs vitamins,
leftover water, and salts
• Remaining semisolid material is feces
– Transported by peristaltic contractions to the
rectum, which expands and stimulates
defecation
Control of Digestion
• Digestion is controlled by the nervous system
and hormones
• Nervous control of digestion
– Nervous system responds to sensory input, such as
sight, smell, and taste
• Nervous control of digestion results in:
– Secretion of saliva
– Secretion of HCl and gastrin (which stimulates further
acid secretion)
– Acid secretion is regulated by negative feedback
Control of Digestion
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Hormonal control of digestion
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Three hormones are released by the small intestine
as acidic chyme enters it
Secretin: stimulates pancreas to release
sodium bicarbonate to neutralize acidity
Cholecystokinin: stimulates pancreas to
release enzymes and gallbladder to release
bile
Gastric inhibitory peptide: inhibits acid
production and stomach peristalsis