Download Nutrition, Digestion, and Absorption

David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
C – Il corpo umano
Nutrition, Digestion, and
Absorption
Nutrition, Digestion, and Absorption
• What do humans require from food?
• How does the vertebrate gastrointestinal
system function?
• How is the flow of nutrients controlled and
regulated?
• What causes ulcers in the stomach?
Nutrition, Digestion, and Absorption - What do humans require from food?
Humans require organic molecules to
supply a carbon skeleton.
The acetyl group (CH3CO––) is used
to build more complex molecules.
Acetyl groups can be derived from
almost any food.
Nutrition, Digestion, and Absorption - What do humans require from food?
The acetyl group is an acquired carbon skeleton
Nutrition, Digestion, and Absorption - What do humans require from food?
The human digestive system
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
The gut has a layered plan:
• lumen – the gut cavity;
• mucosal epithelium – layer of cells
that secrete mucus, digestive
enzymes or hormones, some absorb
nutrients through microvilli;
• submucosal layer has blood and
lymph vessels, and nerves.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Two layers of smooth muscle are
outside the submucosa:
• the circular muscle layer - innermost
cells oriented around the gut,
constrict the gut;
• the longitudinal muscle layer outermost cells oriented along the
gut, shorten the gut.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Between layers of smooth muscle are
nerves that coordinate movement of
the gut.
The peritoneum is a membrane that
surrounds the gut and lines the wall
of the cavity.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Tissue layers of the gut
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Greater intestinal surface area means more nutrients absorbed
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Mechanical activity in digestion:
• the tongue pushes a bolus to the soft
palate, and initiates swallowing, food
passes into the esophagus;
• food is kept out of the trachea by the
closed larynx and the epiglottis;
• peristalsis - waves of muscle
contractions that move food toward
the stomach.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Swallowing and peristalsis (part 1)
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Swallowing and peristalsis (part 2)
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Part of the esophagus is skeletal muscle and part of
the esophagus is smooth muscle.
As food reaches the smooth muscle, the esophagus
contracts and pushes the food toward the stomach.
Nerves coordinate the muscles of the esophagus:
• contraction is preceded by an anticipatory wave of
relaxation;
• as an area contracts, the region below it relaxes so
food does not move upwards;
• as food moves down, it causes the next region to
contract.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
The lower esophageal sphincter, a ring
of muscle, prevents food from moving
backward into the esophagus.
The pyloric sphincter controls the
passage of food into the intestine.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Teeth structure:
• enamel composed of
calcium phosphate covers
the crown;
• dentine in the crown and
root;
• pulp cavity contains blood
vessels, nerves and dentineproducing cells.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Teeth have shapes adapted to specific
functions:
• incisors - used for cutting, chopping,
or gnawing;
• canines - for stabbing, gripping, or
ripping;
• molars or premolars - shearing,
crushing, or grinding.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Macromolecules are broken down by
digestive enzymes, known by the
substances they hydrolyze:
• protease - breaks bonds of amino acids;
• carbohydrase - carbohydrates;
• peptidase - peptides;
• lipase - fats;
• nuclease - nucleic acids.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Chemicals in the mouth and stomach
begin digestion:
• salivary glands secrete amylase;
• gastric pits in the stomach are lined
with three types of secretory cells
(chief cells, parietal cells, and mucussecreting cells) which protect the
stomach.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Action in the stomach (part 1)
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Chief cells secrete pepsinogen, the
inactive form of a proteolytic enzyme,
pepsin.
The low pH of the stomach converts it
to the active form.
Newly active pepsin activates other
pepsinogen molecules—a process
called autocatalysis.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Action in the stomach (part 2)
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Parietal cells secrete HCl and keep the
stomach pH below 1.
Carbonic anhydrase catalyzes the
formation of H2CO3 from CO2, which
dissociates into HCO3– and H+.
H+ ions are exchanged for K+ in the
lumen of the gastric pits.
K+ leaks back into the cells and H+ is
continually returned to the stomach.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Action in the stomach (part 3)
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Chyme is a mixture of gastric juice
and partly digested food.
The stomach walls contract and move
the chyme to the bottom of the
stomach.
The pyloric sphincter allows small
amounts to enter the small intestine.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
The small intestine has three sections: duodenum,
the initial section and site of most digestion,
jejunum and ileum, that carry out most
absorption.
The liver synthesizes bile, which flows through the
hepatic duct to the duodenum, and through a
branch to the gallbladder.
Fat entering the duodenum signals the gallbladder to
contract.
Bile is released and flows via the common bile duct
to the duodenum.
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Ducts of the gallbladder and pancreas
(Gallbladder)
Nutrition, Digestion, and Absorption - How does the vertebrate gastrointestinal system function?
Bile contains salts that emulsify fats and expose them
to lipases; enzymes that digest fats.
Micelles are small fat particles that result from the
action of bile salts.
The pancreas is an endocrine and exocrine gland.
Endocrine functions include hormone release.
Exocrine functions include secretions through the
pancreatic duct to the gut lumen, via the common
bile duct.
Nutrition, Digestion, and Absorption – How is the flow of nutrients controlled and regulated?
Digestive hormones and sites of
production:
• secretin - in the duodenum, causes
pancreas to secrete digestive juices;
• cholecystokinin - in the small intestine,
causes gallbladder to release bile,
stimulates pancreas, slows stomach;
• gastrin - released by the stomach into
the blood.
Nutrition, Digestion, and Absorption – How is the flow of nutrients controlled and regulated?
Hormones control digestion (part 1)
Nutrition, Digestion, and Absorption – How is the flow of nutrients controlled and regulated?
Hormones control digestion (part 2)
Nutrition, Digestion, and Absorption – How is the flow of nutrients controlled and regulated?
Gluconeogenesis in the liver is the conversion of
amino acids and other molecules into glucose.
The liver also converts molecules into glycogen and
controls fat metabolism through lipoproteins.
Lipoproteins produced in the liver:
• high-density lipoproteins (HDLs) remove
cholesterol from tissue and carry it to the liver;
• low-density lipoproteins (LDLs) transport
cholesterol in body;
• very-low-density lipoproteins (VLDLs) transport
triglycerides to fat cells.
Nutrition, Digestion, and Absorption – How is the flow of nutrients controlled and regulated?
Insulin is released by the pancreas during the
absorptive period, when blood glucose levels
(glycemia) rise. Insulin promotes uptake and
utilization or storage of glucose, and it acts in
skeletal muscle, adipose tissue, and in the liver.
Blood glucose levels fall in the postabsorptive
period. Insulin release is lower and glucose uptake
is slowed.
If blood glucose level is very low, glucagon is
released and causes the liver to break down
glycogen and begin gluconeogenesis.
Nutrition, Digestion, and Absorption – How is the flow of nutrients controlled and regulated?
Regulating glucose levels in blood
Nutrition, Digestion, and Absorption - What causes ulcers in the stomach?
Ulcers are sites of damage to the stomach lining.
Causes include stress and lifestyles that lead to
excess stomach secretions, like HCl.
Warren and Marshall noted that ulcer patients
always had an unknown bacterium present.
They discovered Helicobacter pylori also causes
ulcers; it survives in the stomach by an enzyme
reaction that neutralizes acid.
Antibiotics are able to cure this type of ulcer.