Digestion, 51
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Nutrition, and
Absorption
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Nutrient Requirements
Animals are heterotrophs: They must obtain their
nutrition by eating other organisms.
Most plants, some bacteria, and some protists are
autotrophs: They trap solar energy through
photosynthesis and use that energy to synthesize
all of their components.
Heterotrophs depend on the
organic synthesis carried out by
autotrophs.
The Food Chain
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Measuring energy
A calorie is the amount of heat needed to raise the
temperature of 1 g of water by 1C.
1 kilocalorie (kcal) = 1,000 calories
Nutritionists refer to the kilocalorie as the Calorie (Cal),
which is capitalized to distinguish it from the single
calorie.
Scientists are abandoning the calorie in favor of the
International System of Units measure of energy called
the joule (1 joule = 0.239 calories).
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Nutrient Requirements
An animals metabolic rate is the measure of the
overall energy needs that must be met by the animals
food.
Fats, carbohydrates, and proteins are the
components of food that provide that energy.
Basal metabolic rate is the metabolic rate resulting
from all of the essential physiological functions that take
place in a resting state.
Physical activity adds to the basal energy requirement.
Figure 51.2 Food Energy and How We Use It
...the time it takes to burn off these foods
Time it takes a person with a bmr of 1800 kcal/day to utilize energy
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Energy Stores
Most animals store fuel molecules that can be released
as needed between meals.
Carbohydrates are stored in liver and muscle cells as
glycogen. The total glycogen store is about one days
energy requirements.
The most important form of stored energy in animals is
fat. Fat has more energy per gram than glycogen, and it
is stored with little associated water, making it more
compact.
Protein is not used to store energy, but it can be
metabolized as a last resort.
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Nutrient Requirements
Animals that do not take in enough food to meet their
energy requirements are undernourished.
They must metabolize molecules of their own body to
provide the energy they need.
The first storage compounds to be metabolized in this
state are glycogen and fat, so that protein loss is
minimized for as long as possible.
Eventually, a starving animal must use its own proteins
for fuel.
This impairs body functions and eventually leads to
death (also: remember diabetes mellitus).
Figure 51.3
The Course of Starvation
Protein
Fat
Glycogen
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Nutrient Requirements
Animals that take in more food than is necessary to meet
their energy demands are overnourished.
Excess nutrients are stored as increased body mass.
Glycogen reserves are built up, and then additional
carbohydrates, proteins, and fats are converted to body
fat.
Some species use seasonal overnourishment to survive
periods when food is unavailable.
In humans, however, overnourishment can be a serious
health hazard, increasing the risk of high blood pressure,
heart attack, diabetes, and other disorders.
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Digestion
Humans digest their food outside of their body:
Food is taken into a body cavity (the lumen of the
gastrointestinal tract or gut) that is continuous
with the outside environment, where it is acted on by
enzymes secreted by exocrine glands.
These enzymes break the food down into nutrient
molecules that are absorbed by the cells lining the
cavity.
The gastrointestinal tract:
The
Epithelial cells face the outside world
~60% of the recognized major human cell types are classified as epithelial cells
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Protein digestion
For example, proteins are digested in the gut into amino
acids before being used by the body:
Macromolecules are not readily absorbed by the cells
of the gut, but the smaller monomers are.
A protein that functions well in one species might not
function well in another.
Foreign proteins entering the body directly from the
gut would be attacked by the immune system.
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Animals must obtain their essential aa from food
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Digestion
Most animals have a tubular gut with a mouth that takes
in food and an anus for waste excretion.
The tubular gut has different regions that are specialized
for different functions.
All regions within the tubular gut are outside of the body
of the animal; only by crossing the plasma
membranes of the epithelial cells lining the gut do
nutrients enter the body.
Figure 51.7 Compartments for Digestion and Absorption (Part 1)
Figure 50.8 Compartments for Digestion and Absorption (Part 2)
Figure 51.8 Greater Intestinal Surface Area Means More Nutrient Absorption (Part 2)
Surface area is increased by villi
and microvilli
Text
Figure 51.8 Greater Intestinal Surface Area Means More Nutrient Absorption (Part 3)
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Digestion
The parts of the gut that absorb nutrients have evolved
large surface areas to maximize nutrient absorption.
Vertebrates have a gut wall that is richly folded, with
individual folds bearing fingerlike projections called villi,
which in turn have projections called microvilli.
Together, villi and microvilli present an enormous surface
area for the absorption of nutrients.
Movie: camera tour down the gastrointestinal tract
Figure 51.10 Tissue Layers of the Vertebrate Gut
/ Serosa
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Structure and Function of the Vertebrate Gut
The vertebrate gut has four major layers of tissue
throughout its length.
The cavity of the gut is called the lumen.
Starting from the lumen, the first layer of tissue is the
mucosa.
The mucosa consists of epithelial cells that have
secretory and absorptive functions.
Just outside the mucosa is the second layer of cells, the
submucosa, which contains blood and lymph vessels
that carry absorbed nutrients to the rest of the body.
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Structure and Function of the Vertebrate Gut
There are two layers of smooth muscle cells external to
the submucosa.
The innermost layer is the circular muscle layer and
has cells that are oriented around the gut.
The longitudinal muscle layer is the outermost layer
and has cells that are oriented along the length of the gut.
A fibrous coat called the serosa surrounds the gut.
All abdominal organs are covered and supported by a
tissue called the peritoneum.
Polarized Cells
Apical
Epithelial Cells
Budding
Yeast
Basolateral
Migrating Cell
Neuron
Photoreceptor
Cell
Maximizing the small intestine surface area for absorption
Structure of a villus
Human jejunum; circular muscle, villi, crypts lined by columnar epithelium. LM.
small intestine = duodenum, jejunum and ileum
Villi
Folds
H&E Stain
Human ileum: mucosa composed of villi, crypts lined by simple columnar epithelium.
Goblet cells
Jejunum, dog (central part of small intestine) H&E Stain
Jejunum (central part of small intestine)
Jejunum (central part of small intestine)
Mouse small
intestine
(transmission
EM)
BB = brush border
TJ = tight junction
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Hydrolytic enzymes
Hydrolytic enzymes break down protein, carbohydrate,
and fat macromolecules into their simplest monomeric
units.
Digestive enzymes are classified according to the
substances they hydrolyze:
Carbohydrases hydrolyze carbohydrates.
Proteases hydrolyze proteins.
Lipases hydrolyze fats.
Nucleases hydrolyze nucleic acids.
The salivary glands
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Hydrolytic enzymes
The salivary glands secrete mucus and amylase
Mucus: mucins (a family of large, heavily glycosylated
proteins) and inorganic salts suspended in water
Amylase is a carbohydrase that hydrolyses starch into
maltose
Starch: a glucose polymer
Maltose: a glucose dimer
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Functions of saliva
Lubrication and binding:
binds food into a slippery bolus that (usually) slides easily through the
esophagus without inicting damage to the mucosa.
Coats the oral cavity and esophagus, and food basically never directly
touches the epithelial cells of those tissues.
Solubilizes dry food:
e.g. pretzels.
Oral hygiene:
The oral cavity is almost constantly ushed with saliva, which washes
away food debris keeps and the mouth relatively clean.
Saliva also contains lysozyme, an enzyme that lyses many bacteria and
prevents overgrowth of oral microbial populations.
Initiates starch digestion:
alpha-amylase digests starch into maltose.
Amylase is not present, or present only in very small quantities, in the
saliva of carnivores.
Salivary
gland
Mucous cells
Serous
cells
secrete
mucus
secrete
amylase
Serous cells
Mucous cells
Swallowing
Figure 51.11 Swallowing and Peristalsis (Part 2)
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Gastric acid
Hydrochloric acid (HCl) maintains a pH of 1 3 in the
stomach fluid.
Stomach's interior can secrete 2 to 3 liters of gastric fluid
per day.
Functions:
to kill microorganisms that are taken in with food
to hydrolyze acid-labile substances
to activate pepsin
Activating a Zymogen: Pepsin
The endopeptidase pepsin is the major enzyme produced by the
stomach.
Pepsin is secreted by cells in the gastric glands as a zymogen
called pepsinogen.
Newly formed pepsin activates other pepsinogen molecules in a
process called autocatalysis.
Figure 51.12 The Stomach (Part 1)
Chief
Figure 51.12 The Stomach (Part 2)
Parietal cell
Zymogenic cell
Parietal cells secrete HCl
Zymogenic cell (transmission EM)
Parietal cell (transmission EM)
Stomach (scanning EM)
Pit
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Mucus and stomach ulcers
Mucus secreted by the stomachs mucous cells coats the
walls of the stomach and protects them from being eroded and
digested by HCl and pepsin.
When walls of the stomach are exposed directly to HCl and
pepsin, an ulcer can result (peptic ulcer).
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Until recently, peptic ulcers were
thought to be caused by stress until it
was discovered that the bacterium
Helicobacter pylori can infect the
stomach and cause the disease.
In Western countries the prevalence of
Helicobacter pylori infections roughly
matches age (i.e., 20% at age 20, 30%
at age 30, 80% at age 80 etc).
Prevalence is higher in third world countries.
In 1994, the National Institutes of Health
endorsed antibiotics as the standard treatment
for peptic ulcers.
Helicobacter pylori
The Nobel Prize in Physiology or Medicine 2005
"for their discovery of the bacterium Helicobacter pylori and its
role in gastritis and peptic ulcer disease"
Barry J. Marshall
J. Robin Warren
.
...watch documentary (~8 minutes)
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Helicobacter pylori
urease converts
urea to bicarbonate
and ammonium ions
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Structure and Function of the Vertebrate Gut
The muscles in the walls of the stomach contract to
churn its contents and mix them with the stomach
secretions.
This mixture of gastric juice and partly digested food is
called chyme.
Peristaltic contractions of the stomach push the
chyme toward the bottom end of the stomach and into
the beginning of the intestine through the pyloric
sphincter.
The small intestine
Adult human: more than 6 m long, 2 to 3 cm in diameter
inner surface area of about 550 m2 (tennis court).
The small intestine is much longer than the large intestine
(typically 4-5 times longer) but has a smaller diameter.
Three structural parts:
duodenum (0.25 m)
jejunum (2.5 m)
ileum (3.5 m)
- site of most digestion
- site of 90% of the
absorption of nutrients
}
Human ileum: mucosa composed of villi, crypts lined by simple columnar epithelium.
Figure 51.14 The Ducts of the Gallbladder and Pancreas
Accessory organs = liver and pancreas
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Structure and Function of the Vertebrate Gut
The liver and the pancreas provide many of the
specialized enzymes required for digestion.
The liver produces bile, which aids in fat digestion.
Bile is secreted from the liver and flows through a
branch of the hepatic duct to the gallbladder, where it is
stored until it is needed.
When fat enters the duodenum, bile is squeezed into the
common bile duct, where it flows into the duodenum.
Figure 51.15 The Digestion and Absorption of Fats (Part 1)
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Fat absorption
Lipases break fats down into diglycerides,
monoglycerides, and fatty acids, which are able to pass
through the plasma membrane of microvilli.
Once in the cells they are resynthesized into watersoluble lipoproteins called chylomicrons. These pass
to the lymph vessels in the submucosa.
They enter the blood through the thoracic ducts.
Figure 51.15 The Digestion and Absorption of Fats (Part 2)
FAs & monoglycerides
TG + cholesterol + PL
Chylomicron
(a lipoprotein)
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Bile acids (bile salts)
Bile acids are emulsifiers (surfactants)
they have lipophilic end and a lipophobic end.
they disperse fat droplets into micelles.
this greatly increases the surface area of the fats that
are exposed to lipases.
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Structure and Function of the Vertebrate Gut
Bile acids are synthesized from cholesterol.
~ 90% of excreted bile acids are reabsorbed (by active transport in
the ileum) and recycled via the liver: enterohepatic circulation.
~10% of bile acids are lost in the feces. This is the major pathway
for elimination of cholesterol.
Cholic acid
Animation: Fat Digestion
Watch movie on Gauchospace
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Lipoproteins
Lipoproteins transport fats in
the aqueous circulatory system.
They consist of a core of fat and
cholesterol covered by
apolipoproteins that make them
water-soluble.
The largest lipoproteins are the
chylomicrons.
Lipoproteins are classified
according to their densitythe
more fat, the lower its density.
Low-Density Lipoprotein.
Approximately 22 nm (220 ) in
diameter
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Lipoproteins
Chylomicron - carry triacylglycerol (fat) from the intestines
to the liver and to adipose tissue.
Very low density lipoproteins (VLDL) - carry (newly
synthesized) triacylglycerol from the liver to adipose tissue.
Low density lipoproteins (LDL) - carry cholesterol from
the liver to cells of the body. Sometimes referred to as the
"bad cholesterol" lipoprotein.
High density lipoproteins (HDL) - collects cholesterol from the
body's tissues, and brings it back to the liver. Sometimes
referred to as the "good cholesterol" lipoprotein.
LDL to HDL is a risk factor for atherosclerotic heart disease.
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http://www.youtube.com/watch?v=lEc-Rsv9pMc&feature=related
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The exocrine pancreas
The pancreas is a large gland that lies
just beneath the stomach and functions
as both an endocrine and exocrine gland.
The exocrine tissues of the pancreas
produce a number of digestive enzymes,
released as zymogens.
Trypsinogen is activated in the duodenum by enterokinase,
which is produced in the cells lining the duodenum.
Active trypsin can activate other trypsinogen molecules by
autocatalysis.
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The exocrine pancreas
The pancreas also produces a secretion rich in
bicarbonate ions, which neutralize the pH of the
chyme from the stomach.
This process is essential because intestinal enzymes
function best at a neutral or slightly alkaline pH.
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Absorption of nutrients
Only the smallest products of digestion can be absorbed
through the mucosa (epithelial lining) of the small intestine.
The final digestion that produces these absorbable
products takes place at the microvilli (apical plasma
membrane).
Membrane-bound peptidases on microvilli that cleave
peptides into tri- and dipeptides and individual amino acids.
Intestinal epithelial cells also produce maltase, lactase,
and sucrase, which cleave common disaccharides into
monosaccharides.
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Finally: The large intestine
Peristalsis pushes the contents of the small intestine into
the large intestine, or colon.
The colon absorbs water and ions, producing semisolid
feces from indigestible material.
Too much water absorption results in constipation and too
little water absorption results in diarrhea.
Large populations of bacteria live in the colon, including
Escherichia coli, which synthesizes vitamin K and biotin
that are absorbed across the wall of the colon.
Prolonged intake of antibiotics can lead to vitamin
deficiency because the antibiotics kill the normal intestinal
bacteria.