Nutrition and Digestion
CHAPTER 21
OBTAINING AND PROCESSING FOOD
1. Feeding can be by:
a. Absorption
- Eg in a few parasitic worms
b. Ingestion
2. Animals ingest their food in a variety of ways.
3. Ingestive feeding can be categorised by type of
food:
a. Herbivores
i. Plant-eaters
ii. Eg deer, sea urchins
b. Carnivores
i. Meat-eaters
ii. Eg lions, spiders
c. Omnivores
i. Eaters of both plants and other animals
ii. Eg humans, cows
4. Ingestive feeding can be categorised by the size &
location of food:
a. Suspension feeders
i. Ingest small organisms such as
microscopic protists, plants, & animals
suspended in water
ii. Eg clams, scallops
b. Substrate feeders
i. Ingest by burrowing into their food
ii. Eg earthworms, caterpillars
c. Fluid feeders
i. Obtain nutrients from plant sap (eg
aphids) or animal fluids (eg mosquitoes)
d. Bulk feeders
i. Consume larger prey whole or in pieces
- Use tentacles, pincers, claws, fangs,
jaws, teeth, etc.
ii. Eg humans & most animals
5. Food processing occurs in 4 stages:
a. Ingestion
- Act of eating
b. Digestion
i. Breaking down of food into molecules
small enough for the body to absorb
ii. Involves mechanical & chemical
breakdown
c. Absorption
i. Taking up of the digested food (small
nutrient molecules)
ii. NB: Food does not actually enter the body
until it is absorbed.
d. Elimination
- Release of undigested material
6. Food is digested in compartments housing
hydrolytic enzymes.
a. In relatively simple animals (e.g. Hydra), the
digestive compartment is a gastrovascular
cavity
- Sac with a single opening.
b. Most animals have a specialized digestive
tract.
- The digestive compartment is an
alimentary canal
1) A tube running from mouth to anus
2) Divided into specialized regions that
process food sequentially
3) Mouth-pharynx-esophagus-(crop)-(giz
zard)-stomach-intestine-anus
HUMAN DIGESTIVE SYSTEM
1. The human digestive system consists of:
a. Alimentary canal
- Mouth, oral cavity, tongue, pharynx,
esophagus, stomach, small intestine,
large intestine, rectum, anus
b. Accessory/digestive glands
i. Salivary glands, pancreas, liver, gall
bladder
ii. Secrete digestive enzymes into the
cavities with which they are associated
2. Once food is swallowed, the rhythmic muscle
contractions (peristalsis) squeeze the food toward
the stomach and through the rest of the alimentary
canal
3. Ringlike sphincter muscles control the passage of
food from cavity to the next.
- The pyloric sphincter regulates movement of
food from the stomach to the small intestine.
4. Total digestive process takes about 5-6 hours.
Digestion
1. Oral cavity
a. Saliva
i. Contain lubricants (mucus), buffers,
antibacterial agents, & salivary amylase
(hydrolyses starch)
ii. Release triggered by meal time & the
sight & smell of food
iii. Other functions:
1) Help keep teeth clean
2) Dissolve food so it can be tasted,
3) Aid in the formation of bolus
b. Teeth break up food (mechanical digestion)
i. Incisors for biting
ii. Canine for tearing
iii. Premolars & molars for grinding &
crushing food
c. Tongue
i. Tastes & manipulates the food, & shapes
the food into a bolus.
ii. Pushes bolus into the pharynx
2. Pharynx
a. Passage for food & air
b. When a bolus of food passes into the pharynx,
the swallowing reflex is triggered.
- The esophageal sphincter muscle relaxes,
the tracheal opening closes off by the
epiglottis, & the food passes into the
esophagus.
3. Esophagus
a. Squeezes food along to stomach
- Peristaltic action
b. Esophageal muscles are arranged in 2 layers
of smooth muscle:
i. Circular
ii. Longitudinal
4. Stomach
a. Stores food (up to 2L)
- Empties food slowly (after 2 to 6 hours)
b. Mixes bolus with gastric juice
- Forms acid chyme
c. Gastric juice contains:
i. Pepsin (begins hydrolysis of proteins)
ii. Mucus (protects stomach lining, also for
lubrication)
iii. HCl (converts pepsinogen to pepsin;
provides proper pH for the action of
pepsin)
iv. NB: Gastric juice secretion stimulated by
senses and food in the stomach.
5. Small intestine (6m)
a. Major organ of chemical digestion and nutrient
absorption
- First 25 cm is called duodenum
b. Digestion made possible by:
i. Alkaline pancreatic juice
1) Neutralizes stomach acid
2) Its enzymes digest polysaccharides,
proteins, nucleic acids, and fats.
ii. Bile
1) Made in the liver
2) Stored in the gall bladder
3) Contains salts to make fats more
soluble (make it easier for by
pancreatic enzymes to hydrolyse)
iii. Enzymes from the walls of the small
intestine
- Complete the digestion of many
nutrients.
iv. Site of food digestion (chemical)
1) Carbohydrates
a) Mouth, esophagus, duodenum
b) Amylase
2) Proteins
a) Stomach, duodenum
b) Pepsin, trypsin, peptidase
3) Fats
a) Duodenum
b) Lipase
c. Absorption of nutrients
i. The lining of the small intestine is
1) Folded
2) Covered with tiny fingerlike villi
- Increase absorptive surface
ii. Nutrients pass through the epithelium of
the villi and into the blood
(monosaccharides & amino acids) or
lymph (fatty acids & glycerol)
iii. Blood from the small intestine passes to
the liver
1) Store nutrients (eg glycogen)
2) Convert them to other substances the
body can use
6. Large intestine (colon) -60 in, 1.5m
a. Reclaims water
i.
About 7L of undigested material passes
each day
ii. 90% of water absorbed back to the blood
b. Houses some bacteria
- Eg E. coli (produces some essential
vitamins - B, K)
c.
Production of faeces
- Remaining undigested material is
compacted by peristalsis & stored in the
rectum until it is defecated as faeces
7. Appendix
a. A gland connected to the cecum
b. Has a minor immune system function
DIETS & VARIATIONS IN DIGESTIVE SYSTEMS
1. Herbivores and omnivores
a. Have longer alimentary canals than carnivores
b. Allow more time & surface area for digesting
plant material
2. Most herbivores rely on cellulose-digesting
microbes in the colon or cecum
3. Some animals reingest their faeces to recover
nutrients.
4. Ruminant mammals
a. E.g. cows, sheep, deer
b. Have an elaborate, 4-chambered stomach
NUTRITION
1. A healthful diet satisfies 3 needs:
a. Fuel to power all body activities
b. Raw materials needed to make the body’s own
molecules
c. Essential nutrients (substances the body
cannot make itself)
2. Chemical energy powers the body
a. Once nutrients are inside cells, they can be
oxidized by cellular metabolism to generate
energy in the form of ATP.
b. Energy content of food (carbohydrates, fats,
&, when these are in short supply, proteins) is
measured is measured in kilocalories (kcal)
- The amount of energy required to raise
1kg of water 1 degree C
3. Basal Metabolic Rate (BMR)
a. The amount of energy required to maintain
cellular metabolism in a resting individual
b. Average BMR for adults is 1300-1800 kcal per
day
4. Various levels of activity add to a human’s caloric
requirements, & various food supply these
requirements
- More energy is required for an active life
5. Metabolic rates are determined by measuring the
rate of oxygen consumption.
- Each litre of oxygen consumed liberates 4.83
kcal from food
6. Liver & muscle store energy in the form of
glycogen.
7. Excess energy is stored as fat
- The liver can make fat from carbohydrates &
proteins even if there is little fat in the diet
8. A balanced diet includes adequate amounts of all
nutrients.
Essential Nutrients
1. Characteristics:
a. Need to be taken in
b. Cannot be produced in the body
c. Some are produced in the body, but some raw
materials need to be taken in
2. Essential fatty acids
a. Eg linoleic acid
- Needed to make some of the
phospholipids of cell membranes
b. NB: most diets furnish ample amounts of
essential fatty acids, & deficiencies are rare
3. Essential amino acids (8)
a. Methionine, valine, histidine, threonine,
phenylalanine, leucine, isoleucine, lysine
b. Easily obtained from
i. Animal protein
ii. A proper combination of plant food
4. Essential Vitamins (13)
a. Required in smaller amounts
b. Most serve as coenzymes or parts thereof,
that are reused in metabolic reactions or in a
variety of roles in maintaining cellular health
c. 2 groups:
i. Water-soluble
- B complex (8), C
ii. Fat -soluble
- A, D, E, K
d. Most function as coenzymes
- Help enzymes catalyse a metabolic
reaction
e. Extreme deficiencies of each vitamin cause
specific set of symptoms
f. Source:
- Fruits and vegetables, dairy products
5. Essential minerals
a. Chemical nutrients other than carbon,
hydrogen, oxygen, and nitrogen
i. Ca, P, S, K, Cl, Na, Mg, Fe, F, Zn, Cu,
Mn, I, Co, Se, Cr, Mo
ii. NB: All minerals are essential
b. Play a variety of roles in the body
Health Aspects
1. Food labels provide important nutritional
information about packaged foods.
2. Diet can influence cardiovascular disease and
cancer.
- . Linked to cardiovascular disease are diets low
a
in fruits & vegetables & rich in saturated fats,
which in turn correlates with high levels of
blood cholesterol.
- LDLs
b. Linked to some forms of cancer are high
levels of dietary fat & low levels of dietary
fibre