Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ch.6 Digestion

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Ch.6 Digestion


• Regardless of what an animal eats, an adequate diet must satisfy three nutritional needs

– Fuel for all cellular work

– The organic raw materials for biosynthesis

–


•

• Ingestion, the act of eating

– Is the first stage of food processing


• Digestion, the second stage of food processing

– Is the process of breaking food down into molecules small enough to absorb

–

– Mechanical digestion breaks food into smaller pieces (mouth, stomach)

– Chemical digestion breaks chemical bonds between monomers with help of enzymes


• Proteins broken down into amino acids

• Carbohydrates broken down into monosaccharides

•

• Nucleic acids broken down into nucleotides


• Absorption, the third stage of food processing

– Is the uptake of nutrients by body cells

• Transport by circulatory system delivers products of digestion to body cells is the fourth stage


Digestion

• Food molecules we ingest are generally to large to pass across the cell membrane into the cell

•

• Digestion is a hydrolysis reaction (uses water to break apart chemical bonds)

• These molecules can be used for energy or to build large molecules our body needs


Role of Enzymes during Digestion

•

• Enzymes act as organic catalysts that lower activation energy

• Allows reactions to occur at rates high enough to sustain life, but at temperatures low enough to prevent damage to cells

• Without enzymes reactions would occur too slowly to sustain life, or would require high amounts of (activation) energy that would be damaging to cells


How Enzymes Lower the EA Barrier

• An enzyme works by lowering the EA barrier

• Enzymes do not add energy to reactions, they speed up reactions by lowering EA

• This means reactants require less energy to react so consequently more of them can react

•


• The active site can lower an EA barrier by

– Orienting substrates correctly

–

– Makes it more likely that surrounding thermal energy from body will provide enough energy to break the bonds


Digestive Compartments

• Most animals process food

– In specialized compartments that must favor action of digestive enzymes (acidic) and must prevent enzymes from attacking organisms own cells


• Animals with a more complex body plan

– Have a digestive tube with two openings, a mouth and an anus

• This digestive tube

–


• The digestive tube can be organized into specialized regions

– That carry out digestion and nutrient absorption in a stepwise fashion


• The mammalian digestive system consists of the alimentary canal (mouth, oesophagus, stomach, small intestine, large intestine, rectum)

– And various accessory glands that secrete digestive juices through ducts (salivary glands, pancreas, liver)


• Food is pushed along the digestive tract by peristalsis

– Rhythmic waves of contraction of smooth muscles in the wall of the canal


The Oral Cavity, Pharynx, and Esophagus

• In the oral cavity, food is lubricated and digestion begins

– And teeth chew food into smaller particles (mechanical digestion) that are exposed to salivary amylase, initiating the breakdown of glucose polymers

–


• The region we call our throat is the pharynx

– A junction that opens to both the esophagus and the windpipe (trachea), epiglottis covers trachea when swallowing

• The esophagus

–


The Stomach

•

• Stomach also performs mechanical digestion by churning food through muscular contractions

• Gastric juice

– Is made up of hydrochloric acid and the enzyme pepsin

– Secretion is regulated by nervous system and hormones


The Stomach

• Lining of stomach is highly folded and contains many pits

•

• Gastric glands contain

– Mucus cells that secrete mucus to protect lining of stomach

– Parietal cells which secrete HCl

– Chief cells which secrete pepsinogen


• Pepsinogen is the inactive form of pepsin which digests proteins

• Pepsinogen is a zymogen which is the inactive form of an enzyme

•

• Secreting inactive enzymes helps to protect the cells that produce enzymes from digesting themselves


• The lining of the stomach

– Is coated with mucus, which prevents the gastric juice from destroying the cells


• Gastric ulcers, lesions in the lining

– Are caused mainly by the bacterium Helicobacter pylori


The Small Intestine

• The small intestine

– Is the longest section of the alimentary canal (6m)

–


Enzymatic Action in the Small Intestine

• The first portion of the small intestine is the duodenum

– Where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestine itself


Small Intestine

• Secretes protein digesting enzymes like aminopeptidase, and dipeptidase

• Aminopeptidase, dipeptidase and carboxypeptidase are exopeptidases which cut off the terminal amino acid from chain

•


• Enzymatic digestion is completed

– As peristalsis moves the mixture of chyme and digestive juices along the small intestine

Figure 41.21

Oral cavity,pharynx,esophagus

Carbohydrate digestion

Polysaccharides(starch, glycogen)

Disaccharides(sucrose, lactose)

Salivary amylase

Smaller polysaccharides,maltose

Stomach

Protein digestion Nucleic acid digestion Fat digestion

Proteins

Pepsin

Small polypeptides

Lumen of small intes-tine

Polysaccharides

Pancreatic amylases

Maltose and otherdisaccharides

Epitheliumof smallintestine(brushborder)

Disaccharidases

Monosaccharides

Polypeptides

Pancreatic trypsin andchymotrypsin (These proteasescleave bonds adjacent to certainamino acids.)

Smallerpolypeptides

Pancreatic carboxypeptidase

Amino acids

Small peptides

Dipeptidases, carboxypeptidase, and aminopeptidase (These proteases split off one amino acid at a time, working from opposite ends of a polypeptide.)

Amino acids

DNA, RNA

Pancreaticnucleases

Nucleotides

Nucleotidases

Nucleosides

Nucleosidasesandphosphatases

Nitrogenous bases,sugars, phosphates

Fat globules (Insoluble inwater, fats aggregate asglobules.)

Bile salts

Fat droplets (A coating ofbile salts prevents small drop-lets from coalescing intolarger globules, increasingexposure to lipase.)

Pancreatic lipase

Glycerol, fattyacids, glycerides


• The pancreas produces proteases, protein-digesting enzymes

– That are activated once they enter the duodenum


Pancreas

• Secretes alkaline solution to neutralize acidic chyme

• Secretes pancreatic amylase which digests most of the starch

• Secretes lipase which digests lipids

•

• Secretes trypsin and chymotrypsin as zymogens, carboxypeptidase which digest proteins

• Endopeptidases digest protein from within the protein, like trypsin, chymotrypsin and pepsin


Liver

• Secretes bile which helps to break down fat into smaller pieces (emulsify), does not digest fat

•

• After digestion and absorption by the small intestine, blood rich in nutrients travels first to the liver, where nutrients are converted into needed substances, and excess glucose is stored as glycogen


Absorption of Nutrients

• The small intestine has a huge surface area

– Due to the presence of villi and microvilli that are exposed to the intestinal lumen


• The enormous microvillar surface

– Is an adaptation that greatly increases the rate of nutrient absorption


• The core of each villus

– Contains a network of blood vessels and a small vessel of the lymphatic system called a lacteal

•


• Amino acids and sugars pass through the epithelium of the small intestine and enter the bloodstream

• After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within these cells and then absorbed into lacteals of the lymphatic system


The Large Intestine

• The large intestine, or colon

– Is connected to the small intestine


• A major function of the colon

– Is to recover water that has entered the alimentary canal

• The wastes of the digestive tract, the feces

–

– Pass through the rectum and exit via the anus


• The colon houses various strains of the bacterium Escherichia coli

– Some of which produce various vitamins (biotin, folic acid, B, K)


Osmoregulation

•

• Fluids in living organisms are solutions that have water as a solvent

– Cytoplasm

– Blood plasma

– Lymph

– Intercellular fluid


Osmoregulation

• Osmoregulation regulates solute concentrations and balances the gain and loss of water

• Excretion gets rid of metabolic wastes

•

• The relative concentrations of water and solutes in this environment must be maintained within fairly narrow limits


• Osmoregulation is based largely on controlled movement of solutes (salts) between internal fluids and the external environment

•

• Whether animals inhabit land, fresh or saltwater, their cells cannot survive a net gain or loss of water that significantly alters the concentration of dissolved solutes


Body Fluid Regulation

• Water can enter the body through:– Drink

– Food

– Metabolism

• Water is lost from body by:

– Evaporation (skin, lungs)-perspiration, ventilation

– Feces

– Excretion


• Osmoregulators expend energy to control water uptake and loss in a hyper(tonic)osmotic or hypo(tonic)osmotic environment

• Animals that have body fluids whose solute concentration is different from that of the environment must use energy in controlling water gain or loss

•


Waste Disposal

•

• Metabolism produces toxic by-products such as nitrogenous wastes from the breakdown of proteins and nucleic acids

• Animals must dispose of these wastes so they are not poisoned by them


• Nitrogenous wastes are produced by the breakdown amino acids

• Amino acids are deaminated (removal of amine group) to produce waste products containing nitrogen

•

• The type and quantity of an animal’s waste products may have a large impact on its water balance


• Among the most important wastes

– Are the nitrogenous breakdown products of proteins and nucleic acids


Forms of Nitrogenous Wastes

• Different animals

– Excrete nitrogenous wastes in different forms


Ammonia•

• Release it across the whole body surface or through the gills to be diluted by surrounding water

• Ammonia is a very toxic by-product because it raises the pH (strong base) of body fluids, and this disrupts membrane transport functions

• Ammonia is highly soluble in water and diffuses rapidly across cell membranes


• Terrestrial animals convert amino groups to less toxic compounds such as urea or uric acid

• These substances can be stored in body safely and eliminated when needed

•


Urea

• The liver of mammals and most adult amphibians converts ammonia to less toxic urea

•

• Urea is carried to the kidneys, concentrated and excreted with a minimal loss of water


Uric Acid

• Insects, land snails, and many reptiles, including birds excrete uric acid as their major nitrogenous waste

• Uric acid is largely insoluble in water and can be secreted as small solid crystals in a paste or dry powder with little water loss

•


Excretion

• Excretion is the process of removing metabolic waste products from the body

•

• Excretory systems

– Regulate solute movement between internal fluids and the external environment


Excretory Processes

• Most excretory systems

– Produce urine by refining a filtrate derived from body fluids


Vertebrate Kidneys

• Kidneys, the excretory organs of vertebrates function in both excretion and osmoregulation

• Blood circulates through kidneys 20-40X a day


• Nephrons and associated blood vessels are the functional unit of the mammalian kidney

• The mammalian excretory system centers on paired kidneys

–


• Each kidney

– Is supplied with blood by a renal artery and drained by a renal vein


• Urine exits each kidney

–

• Both ureters

– Drain into a common urinary bladder

• The urethra eliminates urine from the body


Structure and Function of the Nephron and Associated Structures

• The mammalian kidney has two distinct regions an outer renal cortex and an inner renal medulla


• The nephron, the functional unit of the vertebrate kidney consists of a single long tubule and a ball of capillaries called the glomerulus


• Key functions of most excretory systems are

Ultrafiltration, pressure-filtering of body fluids producing a filtrate

Reabsorption, reclaiming water and valuable solutes from the filtrate

Secretion, addition of toxins and other solutes from the body fluids to the filtrate


Filtration of the Blood

• Ultrafiltration occurs as blood pressure increases and forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule

• No active transport occurs in Bowman’s capsule-the work is performed by the heart as it drives blood under hydrostatic pressure into glomeruli

• The increase in blood pressure results from fluid entering the glomerulus in a larger tube (afferent arteriole) than the one it leaves from (efferent arteriole), this increases the blood pressure

•


• Filtration of small molecules is nonselective

– And the filtrate in Bowman’s capsule is a mixture that mirrors the concentration of various solutes in the blood plasma, minus large proteins and cellular components

•


Pathway of the Filtrate

• From Bowman’s capsule, the filtrate passes through three regions of the nephron

– The proximal tubule, the loop of Henle, and the distal tubule

• Fluid from several nephrons

–


Proximal convoluted tubule

• Secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate

• Active transport of salt out of nephron (75%) occurs, ~75% of water follows passively

• Reabsorption of water and salt by capillaries occurs

•

• Secretion (active transport) of excess H+, K+, uric acid, ammonia, creatinine, histamine, poisons - moves these substance from blood to nephron filtrate


• The walls of the proximal convoluted tubule contain a single cell layer which contains microvilli to increase the surface area for reabsorption


Descending loop of Henle

• Water is passively transported because of strong osmotic gradient and reabsorbed by capillaries

•

• 75% of remaining water diffuses out to be reabsorbed by capillaries (~6% remains)


Ascending loop of Henle

• ALOH is impermeable to water

• Lower thinner region of ALOH is permeable to salts, passive transport of salt occurs here-this helps maintain high solute concentration gradient along inner medulla region of kidney

• Thick upper region of ALOH actively pumps Na+ ions out to help increase conc.gradient in outer medulla (~4% of salt remains)

•


Distal tubule

• The distal tubule plays a key role in regulating the K+ and NaCl concentration of body fluids

• Na+ ions actively pumped out, distal tubule permeable to water , and water follows passively

•

• Secretion (active transport) of excess H+, poisons from blood to filtrate


Collecting duct

• The collecting duct carries the filtrate through the medulla to the renal pelvis

• Distal region permeable to urea, some passively diffuses out to maintain high osmotic gradient

•


• The collecting duct is differentially permeable to water, its permeability depends on the presence or absence of the hormone ADH

•

• When ADH is present, the collecting duct becomes more permeable to water, more water moves out by osmosis and is reabsorbed by the capillaries, resulting in decreased amounts of urine


From Blood Filtrate to Urine: A Closer Look

• Filtrate becomes urine as it flows through the mammalian nephron and collecting duct


•

• The mammalian kidney can produce urine much more concentrated than body fluids, thus conserving water

• Kidneys regulate water balance of the blood, thus maintaining blood volume and blood pressure


Solute Gradients and Water Conservation

• In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts are largely responsible for the osmotic gradient that concentrates the urine

•

• Solute concentration ranges from very low in tissue of cortex to very high in tissue of inner medulla


• Passive transport of salt ions in ascending loop of Henle and passive transport of urea in collecting duct both help to establish osmotic gradient in tissues surrounding the nephron

• Urea and NaCl form the osmotic gradient that enables the kidney to produce urine that is hypertonic to the blood


Maintenance of pH and Osmolality

• More than 99% of sodium filtered at glomerulus is returned to blood, most (67%) at the proximal convoluted tubule

•

• Aldosterone-from adrenal cortex

• Renin-from kidney

• Atrial Natriuretic Hormone (ANH)

• pH adjusted by either

– The reabsorption of the bicarbonate ions, or

– The secretion of hydrogen ions


Hormone regulation of salt reabsorption

• Angiotensinogen from liver --> renin from kidney changes angiotensinogen into angiotensin I --> converted to angiotensin II --> stimulates adrenal cortex to aldosterone --> promotes reabsorption of sodium ions --> causes reabsorption of water --> increases blood pressure


Atrial natriuretic hormone (ANH)

•

• ANH inhibits renin/aldosterone secretion, which promotes excretion of Na ions, resulting in elimination of more water, decreasing blood pressure


How kidneys change blood

• Levels of proteins are >700 mg/100ml within the plasma, and 0 in the filtrate entering the glomerulus and urine exiting the body, showing that proteins are too large to fit through basement membrane and be filtered

• Glucose levels are >90 in the plasma, and >90 in the filtrate, then drop to 0 in the urine leaving the body, showing that glucose is reabsorbed by the blood

•


Diabetes

• When normal mechanisms fail to properly regulate glucose levels in the blood (hormones insulin and glucagon)

•

• Active transport of glucose out of the nephron and reabsorption by the capillaries cannot keep up, leaving some glucose remaining in the urine

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ch.6 Digestion