Transcript Table of Contents

50&51
Nutrition, Digestion, Absorption,
and Nitrogen Excretion
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Adaptations for Feeding
• Heterotrophic organisms can be classified
by how they acquire their nutrition.
 Decomposers, mostly protists and fungi,
absorb nutrients from dead organic
matter.
 Detritivores, such as earthworms and
crabs, actively feed on dead organic
material.
 Predators are animals that feed on living
organisms:

Herbivores prey on plants.

Carnivores prey an animals.

Omnivores prey on both.
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Digestion
• Food is taken into a body cavity that is continuous
with the outside environment, where it is acted on
by enzymes secreted by the animals.
• These enzymes break the food down into nutrient
molecules that are absorbed by the cells lining the
cavity.
• Most animals have a tubular gut with a mouth that
takes in food and an anus for waste excretion.
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Digestion
• The anterior end of the gut consists of the mouth
and the buccal cavity (mouth cavity).
• Food is usually broken up here into smaller
fragments by structures such as teeth.
• Stomachs are storage chambers that enable
animals to ingest large amounts of food and
digest it at leisure.
• The next section of the gut is called the midgut,
or intestine. Most materials are digested and
absorbed here.
• Specialized glands secrete some digestive
enzymes into the intestine, and cells in the gut
wall itself secrete other digestive enzymes.
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Digestion
• The hindgut recovers water and ions and stores
undigested wastes (feces).
• A muscular rectum near the anus assists in the
expulsion of feces (defecation).
• Many species have colonies of endosymbiotic
bacteria within their hindguts.
• These bacteria obtain nutrients from the food
passing through the host’s gut and contribute to
the digestive processes of the host.
Figure 50.10 The Human Digestive System
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Digestion
• The parts of the gut that absorb nutrients have 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.
Figure 50.9 Greater Intestinal Surface Area Means More Nutrient Absorption (Part 3)
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Structure and Function of the Vertebrate Gut
• The vertebrate gut has four 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.
• Cells of the mucosa 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.
• There are two layers of smooth muscle cells external to the
submucosa.
Figure 50.11 Tissue Layers of the Vertebrate Gut
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Structure and Function of the Vertebrate Gut
• When food enters the mouth it is chewed and
mixed with the secretions of salivary glands.
• When the food makes contact with the back of the
mouth, the reflexive action of swallowing is
initiated.
• Swallowing involves muscles propelling food
through the pharynx (where the mouth cavity and
the nasal passages join) and into the esophagus
(the food tube).
Figure 50.12 Swallowing and Peristalsis (Part 1)
Bolus – a rounded, semisolid mass of food that is either
being swallowed or passing through the digestive tract.
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Structure and Function of the Vertebrate Gut
• Carbohydrate digestion begins in
the mouth, where amylase is
secreted with saliva and mixed
with the food as it is chewed.
• Secretions of the stomach kill
microorganisms that are taken in
with food and begin the digestion
of proteins.
• An endopeptidase called pepsin
is the major enzyme produced by
the stomach.
• Initially, pepsin is secreted by cells
in the gastric glands in its inactive
form called pepsinogen.
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Structure and Function of the Vertebrate Gut
• Hydrochloric acid (HCl) maintains a pH of 1 – 3 in
the stomach fluid, which activates the conversion
of pepsinogen to pepsin.
• Mucus secreted by the stomach mucosa 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.
Figure 50.13 The Stomach (Part 1)
Figure 50.13 The Stomach (Part 2)
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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.
• Peristaltic contractions of the stomach push the
digested food toward the bottom end of the
stomach and into the beginning of the intestine
through the pyloric sphincter.
• When this partially digested food leaves the
stomach, then it is called chyme.
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Structure and Function of the Vertebrate Gut
• In the small intestine, the digestion of
carbohydrates and proteins continues, and the
digestion of fats and the absorption of nutrients
begins.
• The small intestine has three sections: The
duodenum is the initial section and is the site of
most digestion.
• The jejunum and the ileum carry out 90 percent
of the absorption of nutrients.
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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 stored until it is needed.
• When fat enters the duodenum,
bile is squeezed into the
common bile duct, where it
flows into the duodenum.
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Structure and Function of the Vertebrate Gut
• Bile is an emulsifier –
a substance that
prevents oil droplets
from aggregating.
• Bile emulsifies fats
and greatly increases
the surface area of the
fats that are exposed
to lipases.
• The small fat particles
surrounded by bile
molecules are called
micelles.
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Structure and Function of the Vertebrate Gut
• 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, such as trypsinogen.
• 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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Structure and Function of the Vertebrate Gut
• The mechanisms by which cells lining the intestine
absorb nutrient molecules are diverse and not completely
understood.
• Carrier proteins actively transport many inorganic ions
into cells.
• Carrier proteins also transport amino acids, glucose, and
galactose, in conjunction with diffusion of sodium ions.
• The process of fat absorption does not involve carrier
proteins.
• Lipases break fats down into diglycerides,
monoglycerides, and fatty acids, which are able pass
through the plasma membrane of microvilli.
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Structure and Function of the Vertebrate Gut
• 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.
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Structure and Function of the Vertebrate Gut
• Intestinal bacteria produce gases such as
methane and hydrogen sulfide as by-products of
anaerobic metabolism.
• A large percentage of the material in feces
consists of cell walls of dead bacteria.
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Excreting Nitrogenous Wastes
• Fats and carbohydrates break down into water
and CO2, which are easily eliminated.
• Proteins and nucleic acids contain nitrogen.
Breakdown of these produces nitrogenous waste.
• Ammonia is the most common nitrogenous waste
product and is highly toxic.
• It must be quickly eliminated or converted into
less toxic molecules such as urea and uric acid.
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Figure 51.3 Waste Products of Metabolism
ex. fishes
ex. mammals,
amphibians
Most species produce more
than one nitrogenous waste.
Different developmental
stages may have different
forms of nitrogen excretion.
ex. insects,
reptiles, birds
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Vertebrate Excretory Systems
• The kidney is the major excretory organ of
vertebrates.
• The functional unit of the vertebrate kidney is the
nephron.
• Each human kidney has about a million nephrons.
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• Nephrons have three main
parts:
 The glomerulus is a
ball of capillaries that
filters plasma.
 The renal tubules
receive and modify
filtrate.
 Peritubular capillaries
carry substances to and
from the renal tubules.
Vertebrate Excretory Systems
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Vertebrate Excretory Systems
• The two capillary beds of the nephron—the
glomerulus and the peritubular capillaries—lie in
series between the arteriole and the venule.
• An afferent arteriole supplies blood under
pressure to the glomerulus; the blood leaves
through an efferent arteriole.
• The renal tubule begins with Bowman’s capsule
which encloses the glomerulus.
• Cells of the capsule that come into direct contact
with the glomerular capillaries are called
podocytes. They have fine projections that wrap
around and cover the capillaries.
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Figure 51.8 A Tour of the Nephron
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Vertebrate Excretory Systems
• The glomerulus filters the blood to produce a fluid
that lacks cells and large molecules (renal filtrate).
• The walls of the capillaries, the basal lamina of
the capillary endothelium, and the podocytes of
Bowman’s capsule all participate in filtration.
• The force that drives filtration in the glomerulus is
the pressure of the arterial blood.
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Vertebrate Excretory Systems
• The composition of fluid entering the nephron is
similar to that of blood plasma, except it lacks the
plasma proteins.
• As the fluid moves down the renal tubule, it is
concentrated and altered to form urine.
• The cells of the tubule control the composition of
the urine by actively secreting and resorbing
specific molecules.
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The Mammalian Excretory System
• Humans have two kidneys, which filter blood,
process the filtrate into urine, and release the
urine into a duct called the ureter.
• The ureter of each kidney leads to the urinary
bladder, where urine is stored until it is excreted
through the urethra.
• Two sphincter muscles surrounding the base of
the urethra control the timing of urination.
• One is smooth muscle, controlled by the
autonomic nervous system. When the bladder is
full, a spinal reflex relaxes this sphincter.
• The other sphincter is skeletal muscle and is
under voluntary control.
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Figure 51.9 The Human Excretory System (Part 1)
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The Mammalian Excretory System
• The ureter, renal artery, and renal vein enter the
kidney on its concave side.
• Subunits of the kidney called renal pyramids
include all the functional units.
• The renal pyramids make up the medulla of the
kidney and are surrounded by tissue called the
cortex.
• Each kidney has about a million nephrons, which
are its basic functional units.
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The Mammalian Excretory System
• The first section of a renal tubule (closest to the
glomerulus) is called the proximal convoluted
tubule. These lie in the cortex.
• The renal tubule then dives into the medulla and
makes a loop called the loop of Henle.
• The tubule of the loop of Henle makes a hairpin
turn within the medulla and continues back up to
the cortex.
• When it reaches the cortex again it is called the
distal convoluted tubule.
• Distal convoluted tubules join to form collecting
ducts.
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Figure 51.9 The Human Excretory System (Part 2)
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The Mammalian Excretory System
• The organization of blood vessels in the kidney
parallels the organization of the nephrons.
• Arterioles branch from the renal artery and travel
into the cortex.
• An afferent arteriole carries blood to each
glomerulus. The efferent arteriole also gives rise
to the peritubular capillaries.
• Most peritubular capillaries are in the cortex, but a
few run down into the medulla, parallel to the loop
of Henle and the collecting ducts.
• In the medulla region, these capillaries form the
vasa recta.
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The Mammalian Excretory System
• The proximal convoluted tubule has specialized
cuboidal cells with thousands of microvilli which
greatly increase the surface area for resorption of
ions and molecules.
• The cells have many mitochondria to produce the
ATP needed to operate the transport systems.
• They actively transport Na+ and other solutes,
such as glucose and amino acids, out of the
tubule lumen, and water follows by osmosis.
• Almost all glucose and amino acid molecules are
resorbed.
• The peritubular capillaries take up the water and
solutes.
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The Mammalian Excretory System
• Humans can produce urine that is four times more
concentrated than their blood. Some mammals
produce urine even more concentrated.
• The loop of Henle functions as a countercurrent
multiplier system to increase the solute potential
of the surrounding tissue fluid.
• The tubule fluid in the descending limb of the loop
flows in the opposite direction from that of the
ascending limb.
• The system creates a solute concentration
gradient in the renal medulla.
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Figure 51.10 Concentrating the Urine
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The Mammalian Excretory System
• When the fluid enters the collecting duct, it is the
same concentration as blood plasma, but the
composition is different.
• The major solute in the duct is now urea. As fluid
flows down the collecting duct, it loses water
osmotically.
• Some urea also leaks out of the collecting duct to
the surrounding tissue, which increases the
osmotic potential of the tissue.
• The urea diffuses back to the loop of Henle. The
recycling of urea contributes to the ability to
concentrate urine.