the digestive system and metabolism muse 2440 lecture #11 digestion ii 6/15/10

The Digestive System and Metabolism

Muse 2440lecture #11digestion II6/15/10

The Pancreas

Pancreatic Enzymes Pancreatic alpha-amylase

A carbohydrase

Breaks down starches

Similar to salivary amylase

Pancreatic lipase Breaks down complex lipids

Releases products (e.g., fatty acids) that are easily absorbed

The Pancreas

Pancreatic Enzymes Nucleases

Break down nucleic acids

Proteolytic enzymes Break certain proteins apart

Proteases break large protein complexes

Peptidases break small peptides into amino acids

70% of all pancreatic enzyme production

Secreted as inactive proenzymes

Activated after reaching small intestine

The Liver

Is the largest visceral organ (1.5 kg; 3.3 lb)

Lies in right hypochondriac and epigastric

regions

Extends to left hypochondriac and umbilical

regions

Performs essential metabolic and synthetic

functions

The Liver

Anatomy of the Liver

Is wrapped in tough fibrous capsule

Is covered by visceral peritoneum

Is divided into lobes

The Liver

Figure 24–19a The Anatomy of the Liver.

The Liver

Figure 24–19b, c The Anatomy of the Liver.

The Liver

Hepatic Blood Supply

1/3 of blood supply Arterial blood from hepatic artery proper

2/3 venous blood from hepatic portal vein,

originating at Esophagus

Stomach

Small intestine

Most of large intestine

The Liver

Hepatocytes Are liver cells Adjust circulating levels of nutrients

Through selective absorption and secretion

In a liver lobule form a series of irregular plates arranged like wheel spokes

Many Kupffer cells (stellate reticuloendothelial cells) are located in sinusoidal lining

As blood flows through sinusoids Hepatocytes absorb solutes from plasma And secrete materials such as plasma proteins

The Liver

The Bile Duct System

Liver secretes bile fluid

Into a network of narrow channels (bile canaliculi)

Between opposing membranes of adjacent liver

cells

The Liver

Right and Left Hepatic Ducts

Collect bile from all bile ducts of liver lobes

Unite to form common hepatic duct that leaves the

liver

Bile Flow

From common hepatic duct to either

The common bile duct, which empties into duodenal ampulla

The cystic duct, which leads to gallbladder

The Liver

The Common Bile Duct

Is formed by union of Cystic duct

Common hepatic duct

Passes within the lesser omentum toward

stomach

Penetrates wall of duodenum

Meets pancreatic duct at duodenal ampulla

The Liver

Figure 24–21 The Gallbladder and Bile Ducts.

The Liver

The Physiology of the Liver

1. Metabolic regulation

2. Hematological regulation

3. Bile production

The Liver

Metabolic Regulation

The liver regulates:

1. Composition of circulating blood

2. Nutrient metabolism

3. Waste product removal

4. Nutrient storage

5. Drug inactivation

The Liver

Hematological Regulation

Largest blood reservoir in the body

Receives 25% of cardiac output

Coordination of Secretion and Absorption

Secretin Is released when chyme arrives in duodenum Increases secretion of bile and buffers by liver and

pancreas

Cholecystokinin (CCK) Is secreted in duodenum

When chyme contains lipids and partially digested proteins

Accelerates pancreatic production and secretion of digestive enzymes

Relaxes hepatopancreatic sphincter and gallbladder Ejecting bile and pancreatic juice into duodenum


Gastric Inhibitory Peptide (GIP)

Is secreted when fats and carbohydrates enter

small intestine

Vasoactive Intestinal Peptide (VIP)

Stimulates secretion of intestinal glands

Dilates regional capillaries

Inhibits acid production in stomach


Gastrin Is secreted by G cells in duodenum

When exposed to incompletely digested proteins

Promotes increased stomach motility

Stimulates acids and enzyme production

Enterocrinin Is released when chyme enters small intestine

Stimulates mucin production by submucosal glands of duodenum


Figure 24–22 The Activities of Major Digestive Tract Hormones.


Intestinal Absorption

It takes about 5 hours for materials

to pass from duodenum to end of ileum

Movements of the mucosa increases

absorptive effectiveness

Stir and mix intestinal contents

Constantly change environment around epithelial

cells

The Large Intestine

Functions of the Large Intestine

Reabsorption of water

Compaction of intestinal contents into feces

Absorption of important vitamins produced by

bacteria

Storage of fecal material prior to defecation

The Large Intestine

Figure 24–23a The Gross Anatomy and Regions of the Large Intestine.

The Large Intestine

Histology of the Large Intestine

Lack villi

Abundance of mucous cells

Presence of distinctive intestinal glands

Are deeper than glands of small intestine

Are dominated by mucous cells

The Large Intestine

Three Vitamins Produced in the Large Intestine

1. Vitamin K (fat soluble):

Required by liver for synthesizing four clotting factors,

including prothrombin

2. Biotin (water soluble):

Important in glucose metabolism

3. Pantothenic acid: B5 (water soluble):

Required in manufacture of steroid hormones and some

neurotransmitters

Digestion

Essential Nutrients

A typical meal contains

Carbohydrates

Proteins

Lipids

Water

Electrolytes

Vitamins

Digestion

Figure 24–26 A Summary of the Chemical Events in Digestion.

Digestion

Vitamins are organic compounds required

in very small quantities

Are divided in two major groups:

Fat-soluble vitamins

Water-soluble vitamins

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Introduction to Metabolism

Cells break down organic molecules to

obtain energy

Used to generate ATP

Most energy production takes place in

mitochondria


Metabolism

The Nutrient Pool

Contains all organic building blocks cell needs

To provide energy

To create new cellular components

Is source of substrates for catabolism and

anabolism


Metabolism

Catabolism

Is the breakdown of organic substrates

Releases energy used to synthesize high-energy

compounds (e.g., ATP)

Anabolism

Is the synthesis of new organic molecules


Metabolism

In energy terms

Anabolism is an “uphill” process that forms

new chemical bonds


Metabolism

Organic Compounds Glycogen

Most abundant storage carbohydrate A branched chain of glucose molecules

Triglycerides Most abundant storage lipids Primarily of fatty acids

Proteins Most abundant organic components in body Perform many vital cellular functions


Metabolism

Figure 25–2 Nutrient Use in Cellular Metabolism.

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 24.3

Stage 1 Digestion in GI tract lumen to absorbable forms.Transport via blood totissue cells.

Stage 2 Anabolism (incorporation into molecules) and catabolism of nutrients to form intermediates within tissue cells.

Stage 3 Oxidative breakdown of products of stage 2 in mitochondria of tissue cells. CO2 is liberated, and H atoms removed are ultimately delivered to molecular oxygen, formingwater. Some energy released isused to form ATP.

Catabolic reactionsAnabolic reactions

Glycogen

PROTEINS

Proteins Fats

CARBOHYDRATES

Glucose

FATS

Amino acids Glucose and other sugars Glycerol Fatty acids

Pyruvic acid

Acetyl CoA

Infrequent CO2

NH3

H

Krebscycle

Oxidativephosphorylation

(in electron transport chain)

O2

H2O

Overview of metabolic processes


Carbohydrate Metabolism

Generates ATP and other high-energy

compounds by breaking down

carbohydrates:

glucose + oxygen carbon dioxide + water



Glucose Breakdown Occurs in small steps

Which release energy to convert ADP to ATP

One molecule of glucose nets 36 molecules of ATP Glycolysis

Breaks down glucose in cytosol into smaller molecules used by mitochondria

Does not require oxygen: anaerobic reaction

Aerobic Reactions Also called aerobic metabolism or cellular respiration Occur in mitochondria, consume oxygen, and produce ATP


Via oxidativephosphorylationVia substrate-level

phosphorylation

MitochondrionMitochondrialcristaeCytosol

KrebscycleGlucose

Glycolysis

Pyruvicacid

Electron transportchain and oxidativephosphorylation

Chemical energy (high-energy electrons)

1 During glycolysis, each glucose molecule is broken down into two molecules of pyruvic acid in the cytosol.

2 The pyruvic acid then enters the mitochondrial matrix, where the Krebs cycle decomposes it to CO2. During glycolysis and the Krebs cycle, small amounts of ATP are formed by substrate-level phosphorylation.

3 Energy-rich electrons picked up bycoenzymes are transferred to the elec-tron transport chain, built into the cristae membrane. The electron transport chain carries out oxidative phosphorylation, which accounts for most of the ATP generated by cellular respiration.

Chemical energy



Glycolysis

Breaks 6-carbon glucose

Into two 3-carbon pyruvic acid

Pyruvate

Ionized form of pyruvic acid

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 24.4a

Enzyme

Catalysis

Enzyme

(a) Substrate-level phosphorylation



Figure 25–3 Glycolysis.



Mitochondrial Membranes Outer membrane

Contains large-diameter pores

Permeable to ions and small organic molecules (pyruvic

acid)

Inner membrane Contains carrier protein

Moves pyruvic acid into mitochondrial matrix

Intermembrane space Separates outer and inner membranes

The TCA Cycle



The TCA Cycle (citric acid cycle) The function of the citric acid cycle is

To remove hydrogen atoms from organic molecules and

transfer them to coenzymes

In the mitochondrion Pyruvic acid reacts with NAD and coenzyme A (CoA)

Producing 1 CO2, 1 NADH, 1 acetyl-CoA

Acetyl group transfers From acetyl-CoA to oxaloacetic acid

Produces citric acid


Krebs cycle

NAD+

NAD+

GDP +

NAD+

FAD

NAD+

NADH+H+

Cytosol

Mitochondrion(matrix)

NADH+H+

FADH2

NADH+H+

Citric acid

(initial reactant)

Isocitric acid

Oxaloacetic acid

(pickup molecule)

Malic acid

Succinic acidSuccinyl-CoA

GTP

ADP

Carbon atom

Inorganic phosphate

Coenzyme A

Acetyl CoA

Pyruvic acid from glycolysis

Transitionalphase

Fumaric acid

NADH+H+

CO2

CO2

CO2

-Ketoglutaric acid

Electron trans-port chain and oxidativephosphorylation

Glycolysis Krebscycle



The TCA Cycle

CoA is released to bind another acetyl group

One TCA cycle removes two carbon atoms

Regenerating 4-carbon chain

Several steps involve more than one reaction or enzyme

H2O molecules are tied up in two steps

CO2 is a waste product

The product of one TCA cycle is

One molecule of GTP (guanosine triphosphate)



Summary: The TCA Cycle

CH3CO - CoA + 3NAD + FAD + GDP + Pi + 2 H2O

CoA + 2 CO2 + 3NADH + FADH2 + 2 H+ + GTP



Oxidative Phosphorylation and the ETS

Is the generation of ATP

Within mitochondria

In a reaction requiring coenzymes and oxygen

Produces more than 90% of ATP used by

body

Results in 2 H2 + O2 2 H2O



The Electron Transport System (ETS)

Is the key reaction in oxidative

phosphorylation

Is in inner mitochondrial membrane

Electrons carry chemical energy

Within a series of integral and peripheral proteins


Glycolysis Krebscycle

Electron trans-port chain and oxidativephosphorylation

EnzymeComplex I

EnzymeComplex III

EnzymeComplex IV

EnzymeComplex II

NADH+H+

FADH2

Fre

e e

nerg

y r

ela

tive t

o O

2 (

kcal/

mol)



Oxidation and Reduction

Oxidation (loss of electrons)

Electron donor is oxidized

Reduction (gain of electrons)

Electron recipient is reduced

The two reactions are always paired



Energy Transfer

Electrons transfer energy

Energy performs physical or chemical work (ATP

formation)

Electrons

Travel through series of oxidation–reduction reactions

Ultimately combine with oxygen to form water

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 24.4b

ADP +

Membrane

High H+ concentration inintermembrane space

Low H+ concentration in mitochondrial matrix

Energyfrom food

Protonpumps

(electrontransport

chain)

ATPsynthase

(b) Oxidative phosphorylation



Coenzyme FAD

Accepts two hydrogen atoms from TCA cycle:

Gaining two electrons

Coenzyme NAD

Accepts two hydrogen atoms

Gains two electrons

Releases one proton

Forms NADH + H+



Figure 25–5b Oxidative Phosphorylation.



ATP Generation and the ETS Does not produce ATP directly

Creates steep concentration gradient across inner

mitochondrial membrane

Electrons along ETS release energy As they pass from coenzyme to cytochrome

And from cytochrome to cytochrome

Energy released drives H ion (H+) pumps That move H+ from mitochondrial matrix

Into intermembrane space



Chemiosmosis

Also called chemiosmotic phosphorylation

Ion channels and coupling factors use kinetic

energy of hydrogen ions to generate ATP



Summary: ATP Production

For one glucose molecule processed, cell gains 36

molecules of ATP

2 from glycolysis

4 from NADH generated in glycolysis

2 from TCA cycle (through GTP)

28 from ETS



Figure 25–6 A Summary of the Energy Yield of Aerobic Metabolism.



Gluconeogenesis

Is the synthesis of glucose from noncarbohydrate

precursors

Lactic acid

Glycerol

Amino acids

Stores glucose as glycogen in liver and skeletal

muscle



Glycogenesis

Is the formation of glycogen from glucose

Occurs slowly

Requires high-energy compound uridine

triphosphate (UTP)



Glycogenolysis

Is the breakdown of glycogen

Occurs quickly

Involves a single enzymatic step


Lipid Metabolism

Lipid Catabolism

Enzymes in cytosol convert glycerol to pyruvic

acid

Pyruvic acid enters TCA cycle

Different enzymes convert fatty acids to

acetyl-CoA (beta-oxidation)


Lipid Metabolism

Beta-Oxidation

A series of reactions

Breaks fatty acid molecules into 2-carbon fragments

Occurs inside mitochondria

Each step

Generates molecules of acetyl-CoA and NADH

Leaves a shorter carbon chain bound to coenzyme A


Lipid Metabolism

Figure 25–8 Beta-Oxidation.


Lipid Metabolism

Free Fatty Acids

Are an important energy source

During periods of starvation

When glucose supplies are limited

Liver cells, cardiac muscle cells, skeletal

muscle fibers, and so forth

Metabolize free fatty acids


Lipid Metabolism

Lipoproteins Are lipid–protein complexes

Contain large insoluble glycerides and cholesterol

Five classes of lipoproteins Chylomicrons

Very low-density lipoproteins (VLDLs)

Intermediate-density lipoproteins (IDLs)

Low-density lipoproteins (LDLs)

High-density lipoproteins (HDLs)


Lipid Metabolism

Chylomicrons

Are produced in intestinal tract

Are too large to diffuse across capillary wall

Enter lymphatic capillaries

Travel through thoracic duct

To venous circulation and systemic arteries


Protein Metabolism

The body synthesizes 100,000 to 140,000

proteins

Each with different form, function, and structure

All proteins are built from the 20 amino acids

Cellular proteins are recycled in cytosol

Peptide bonds are broken

Free amino acids are used in new proteins


Protein Metabolism

Amino Acid Catabolism

Removal of amino group by transamination

or deamination

Requires coenzyme derivative of vitamin B6

(pyridoxine)


Protein Metabolism

Transamination

Attaches amino group of amino acid

To keto acid

Converts keto acid into amino acid

That leaves mitochondrion and enters cytosol

Available for protein synthesis


Protein Metabolism

Deamination

Prepares amino acid for breakdown in TCA

cycle

Removes amino group and hydrogen atom

Reaction generates ammonium ion


Protein Metabolism

Figure 25–10a Amino Acid Catabolism.


Protein Metabolism

Figure 25–10b Amino Acid Catabolism.


Protein Metabolism

Ammonium Ions

Are highly toxic, even in low concentrations

Liver cells (primary sites of deamination) have

enzymes that use ammonium ions to

synthesize urea (water-soluble compound

excreted in urine)


Protein Metabolism

Figure 25–10c Amino Acid Catabolism.


Protein Metabolism

Three Factors Against Protein Catabolism

Proteins are more difficult to break apart than

complex carbohydrates or lipids

A byproduct, ammonium ion, is toxic to cells

Proteins form the most important structural

and functional components of cells


Protein Metabolism

Figure 25–11 Animation.


Protein Metabolism

Figure 25–12 A Summary of the Pathways of Catabolism and Anabolism.


Absorptive and Postabsorptive States

Five Metabolic Tissues

Liver

Adipose tissue

Skeletal muscle

Neural tissue

Other peripheral tissues


Absorptive and Postabsorptive States

The Absorptive State Is the period following a meal when nutrient

absorption is under way

The Postabsorptive State Is the period when nutrient absorption is not under

way

Body relies on internal energy reserves for energy demands

Liver cells conserve glucose Break down lipids and amino acids


Nutrition

Fat-Soluble Vitamins

Vitamins A, D, E, and K

Are absorbed primarily from the digestive tract

along with lipids of micelles

Normally diffuse into plasma membranes and lipids

in liver and adipose tissue


Nutrition

Vitamin A A structural component of visual pigment retinal

Vitamin D Is converted to calcitriol, which increases rate of

intestinal calcium and phosphorus absorption

Vitamin E Stabilizes intracellular membranes

Vitamin K Helps synthesize several proteins, including three

clotting factors


Nutrition

Vitamin Reserves

The body contains significant reserves of fat-

soluble vitamins

Normal metabolism can continue several

months without dietary sources


Metabolic Rate

Calories Energy required to raise 1 g of water 1 degree Celsius

is a calorie (cal)

Energy required to raise 1 kilogram

of water 1 degree Celsius is a Calorie (Cal)=

kilocalorie (kcal)

The Energy Content of Food Lipids release 9.46 Cal/g

Carbohydrates release 4.18 Cal/g

Proteins release 4.32 Cal/g


Metabolic Rate

Basal Metabolic Rate (BMR)

Is the minimum resting energy expenditure

Of an awake and alert person

Measured under standardized testing conditions

Measuring BMR

Involves monitoring respiratory activity

Energy utilization is proportional to oxygen

consumption


Metabolic Rate

Hormonal Effects

Thyroxine controls overall metabolism

T4 assay measures thyroxine in blood

Cholecystokinin (CCK) and adrenocorticotropic

hormone (ACTH) suppress appetite

Leptin is released by adipose tissues during

absorptive state and binds to CNS neurons that

suppress appetite

the digestive system and metabolism muse 2440 lecture #11 digestion ii 6/15/10

Documents

bile fluidinto

gallbladderejecting

eitherthe common bile

bile ducts of liver

hepatic ducts

liver lobule

pancreatic juice

hepatic portal vein